PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS ♦
JUDUL:
TYPES AND MODE OF FORMATION OF DURICRUSTS
SESI PENGAJIAN: 2005/2006
KUHAN A/L BALA SUBRAMANIAM
_____________________________________________________________________________
(HURUF BESAR)
Saya
mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di Perpustakaan
Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut:
1.
2.
3.
4.
Tesis adalah hakmilik Universiti Teknologi Malaysia.
Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan
pengajian sahaja.
Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi
pengajian tinggi.
**Sila tandakan (√ )
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub di dalam
AKTA RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan
oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
_________________________________
Disahkan oleh
_________________________________
(TANDATANGAN PENULIS)
(TANDATANGAN PENYELIA)
Alamat Tetap:
15, LORONG RAJA HAJI OTHMAN,
41100 KLANG,
SELANGOR DARUL EHSAN.
MOHD. FOR MOHD. AMIN ____
_____________________________
Nama Penyelia
Tarikh: 28 APRIL 2006
Tarikh: 28 APRIL 2006
CATATAN:
* Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak
berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh
tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.
♦ Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana
secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan
penyelidikan, atau Laporan Projek Sarjana Muda (PSM).
“I hereby declare that I have read this report and in my opinion this report is
sufficient in terms of scope and quality for the award of the degree of Bachelor of
Civil Engineering.”
Signature
:
....................................................
Name of Supervisor :
Mohd. For Mohd. Amin
Date
28 April 2006
:
i
TYPES AND MODE OF FORMATION OF DURICRUSTS
KUHAN S/O BALA SUBRAMANIAM
A report submitted in partial fulfillment of the
requirements for the award of the degree of
Bachelor of Civil Engineering
Faculty of Civil Engineering
Universiti Teknologi Malaysia
APRIL 2006
ii
“I declare that this report entitled “Types and Mode of Formation of Duricrusts” is
the result of my own research except as cited in references. This report has not been
accepted by any degree and is not concurrently submitted in candidature of any other
degree”.
Signature
:
....................................................
Name
:
Kuhan s/o Bala Subramaniam
Date
:
28 April 2006
iii
To my beloved father, mother and brother
iv
ACKNOWLEDGEMENT
In preparing this thesis I was in contact with many people, researchers,
academicians and practitioners. I would like to particularly thank my supervisor, Mr.
Mohd. For Mohd. Amin. With his continuous encouragement, concern, guidance
and critics, I am able to complete this thesis.
I am also thankful to my father and mother for their constant encouragement
and spiritual support. Without them, this study would not have been possible.
I would also like to thank all the technicians of the Geotechnics and
Engineering Geology Department as well as librarians of Universiti Teknologi
Malaysia for their assistance in providing relevant literature as well as assistance
throughout the course of my study. My deepest gratitude also goes to all the
lecturers and staff members of the Civil Engineering Faculty for their support and
cooperation.
v
ABSTRACT
Duricrusts are the accumulation of sediments which are cemented together by
cementing agents. The chemical constituent of the cementing agent is the most
important component of duricrusts because they determine the type of duricrust.
Duricrusts are usually found in regions that exhibit humid and sub-humid or arid and
semi-arid environmental conditions. There are certain characteristics of duricrusts
that display a strong resemblance to clastic sedimentary rocks even tough duricrusts
cannot be classified as rocks. Similarities between duricrusts and clastic sedimentary
rocks are their material texture, mineralogy, strength and physical properties. The
mode of formation of duricrusts is what differentiates it from clastic sedimentary
rocks. This study was conducted to determine the different modes of formation of
duricrusts and come up with a classification method for duricrusts. This study was
also conducted to determine the common types of duricrusts that can be found in
Malaysia and also provide an appropriate excavation method for duricrusts.
Extensive literature reviews conducted include the geological aspects as well as
engineering properties such as rebound number (surface hardness), point-load index
strength, uniaxial compressive, tensile strength, density and seismic (P-wave)
velocity. These parameters are important in determining the degree of excavatability
and rippability of rock materials. From the analysis conducted, it was determined
that duricrusts have similar material texture, mineralogy, strength and physical
properties as certain clastic sedimentary rocks but cannot be termed as a clastic
sedimentary rock based on its geological origin and mode of formation. This is
because duricrusts suffer considerably less over burden pressure and are formed in
situ unlike clastic sedimentary rocks. Due to the similar material properties between
duricrusts and clastic sedimentary rocks, it was also concluded that mechanical
ripping can be used for the excavation of duricrusts.
vi
ABSTRAK
Duricrust adalah pengumpulan sedimen yang terikat dengan agen-agen
pengikat. Jenis-jenis duricrust ditentukan melalui bahan kimia yang menghasilkan
bahan pengikatnya. Duricrust biasanya dijumpai di kawasan-kawasan yang
menonjolkan iklim tropika lembap atau iklim kontang. Tekstur, kandungan mineral,
kekuatan dan kriteria-kriteria fizikal lain duricrust didapati menyerupai batuan
sedimen klastik. Cara pembentukan duricrust adalah kriteria yang membezakan
duricrust daripada batuan sedimen klastik. Kajian ini telah dijalankan bagi
menentukan cara-cara pembentukan duricrust dan menyediakan cara kelasifikasi
bagi duricrust. Kajian ini juga dijalankan bagi menentukan jenis duricrust yang
biasa ditemui di Malaysia serta menentukan cara pengorekan yang sesuai bagi
duricrust. Kajian literature yang menyeluruh telah dilaksanakan bagi menentukan
aspek geologi serta kejuruteraan duricrust seperti nombor pantulan (kekuatan
permukaan), kekuatan indeks beban titik, kekuatan mampatan ekapaksi, kekuatan
tegangan, ketumpatan dan halaju seismik (P-wave). Parameter-parameter ini adalah
penting bagi menentukan darjah pengorekan serta robekan bahan batuan. Daripada
analisis yang dijalankan, telah dapat dipastikan bahawa tekstur, kandungan mineral,
kekuatan serta kriteria-kriteria fizikal lain duricrust menyerupai batuan sedimen
klastik tetapi duricrust tidak boleh dikelasifikasikan sebagai batuan sedimen klastik
berdasarkan kepada cara pembentukannya. Ini adalah kerana didapati duricrust tidak
mengalami jumlah mampatan yang serupa dengan batuan sedimen klastik serta ia
terbentuk berdekatan dengan batuan asalnya (in situ). Disebabkan oleh kriteriakriteria bahan yang serupa antara duricrust dan batuan sedimen klastik, ia juga telah
dirumuskan bahawa robekan mekanikal boleh digunakan untuk mengorek duricrust.
vii
CONTENTS
CHAPTER
1
2
TITLE
PAGE
TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
CONTENTS
vii
LIST OF TABLES
x
LIST IF FIGURES
xii
INTRODUCTION
1.1
Introduction
1
1.2
Statement of Problem
2
1.3
Study Objective
3
1.4
Scope of Study
3
1.5
Methodology
4
LITERATURE REVIEW
2.1
Introduction
5
viii
2.2
Clastic Sedimentary Rocks
7
2.3
Types of Duricrusts
8
2.4
Mode of Formation of Clastic
Sedimentary Rocks
11
2.5
Mode of Formation of Duricrusts
12
2.6
Material Texture and Strength of
Duricrusts and Clastic Sedimentary
Rock
22
2.7
Duricrusts in Malaysia
24
2.8
Typical Properties of Various Rock
2.9
3
27
Engineering Properties of Duricrusts
31
METHODOLOGY
3.1
Introduction
35
3.2
Mode of Formation of Duricrusts
36
3.3
Material Texture of Duricrusts and
3.4
4
Types
Clastic Sedimentary Rocks
39
Strength and Physical Properties
40
3.4.1
Uniaxial Compressive Strength
42
3.4.2
Durability
43
3.4.3 Point-load Index Strength
44
3.4.4
Surface Hardness
45
3.4.5
Tensile Strength
47
3.4.6
P-wave Velocity
48
RESULTS AND ANALYSIS
4.1
Mode of Formation of Duricrusts
49
4.2
Material Texture
51
4.3
Strength and Physical Properties
52
ix
5
REFERENCES
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
55
5.2
Recommendations for Future Studies
56
57
x
LIST OF TABLES
TABLE NO.
2.1
TITLE
PAGE
The general classification of duricrusts
based on its cementing materials (after Mohd.
For Mohd. Amin et al., 2005)
6
Classification of sedimentary rocks (after
Waltham 2002)
8
The general stages for the formation of calcrete
(after Klappa, 1983)
20
Typical static mechanical properties of some
common rock types (modified from Bengt
Stillborg, 1985)
27
Relative seismic (P-wave) velocity (after Bickel
& Kuesel, 1982)
28
Density of Rocks (modified after Manger and
Clark, 1966)
28
Uniaxial compressive and uniaxial tensile strengths
of rocks (after Pitts, 1984)
28
2.8
Slake Durability Classification (Gamble, 1971)
29
2.9
Classification based on point load index strength
(Broch and Franklin, 1972)
29
Classification of rock types based on unconfined
compressive strength (after McLean & Gribble,
1980)
30
2.11
Section 2904 of the Boston Building Code (1944)
33
3.1
The compressive strength by Fauzilah Ismail (2002)
42
2.2
2.3
2.4
2.5
2.6
2.7
2.10
xi
3.2
Durability index test results by Fauzilah Ismail (2002)
43
3.3
Point load test by Jerry Chua Kuo Seng (2004)
44
3.4
Rebound test results for block samples by Fauzilah
Ismail (2002)
45
Rebound test results for core samples by Fauzilah
Ismail (2002)
46
Joint wall compressive strength, JCS value by
Fauzilah Ismail (2002)
46
3.7
Tensile strength test results by Fauzilah Ismail (2002)
47
3.8
The result of P-Wave Velocity by Fauzilah Ismail
(2002)
48
Summary of test results on samples of duricrusts
54
3.5
3.6
4.1
xii
LIST OF FIGURES
FIGURE NO.
2.1
TITLE
PAGE
Classification of duricrusts (after Faniran
& Jeje, 1983)
6
2.2
Duricrusts formation (after Maignien, 1966)
7
2.3
Stages of weathering causing the accumulation
of iron or alluminium to form ferricretes or
alcretes. The profile shows the reletive
accumulation of iron or alluminium based on
time (after McFarlane, 1983)
13
Models of the formation of calcrete: (A) the
capillary rise (per ascensum) model; (B) the
precolation (per descensum) model with inputs
of carbonate from above; (C) the detrital model
in which a secondary calcrete is formed through
the solution and disintegration of an existing
calcrete horizon at a higher level in the landscape
(after Goudie, 1973)
15
Typical weathering profile of calcrete at different
locations (after Beauvais & Colin, 1993)
16
Typical weathering profile of silcrete (after
Summerfield, 1983)
17
A schematic representation of the relationship
between the formation of different duricrusts and
prevailing conditions of climate and pH (after
Summerfield, 1983)
18
Granular texture displayed by sand and gravel
size mineral grains imbedded in cement matrix
23
Duricrusts occurring as caprock in Dengkil,
Selangor
25
2.4
2.5
2.6
2.7
2.8
2.9
xiii
2.10
Secondary duricrusts in Pasir Gudang, Johor
25
2.11
The geological map of the study area (Malaysia
New Series Map, Johor Bahru, Sheet 130)
26
Schmidt hammer hardness characteristics of
some calcrete profiles on bedrock. (a) In Puerto
Rico (after Ireland, 1079); (b) In Sulawesi (after
Day and Goudie, 1977); (c) In Puerto Rico (after
Day and Goudie, 1977); (d) In Israel (after
Yaalon and Singer, 1974)
32
Close-up photographs of sandstone and duricrusts
51
2.12
4.1
1
CHAPTER 1
INTRODUCTION
1.1
Introduction
Duricrusts are the accumulation of sediments below the surface which are
cemented together by cementing agents. Duricrusts are usually found in regions that
exhibit humid and sub-humid or arid and semi-arid environmental conditions. It is
common for a duricrusts formation to reach a thickness of 1-10 m but there are cases
where the formation may reach a thickness of 50 m.
The formation of duricrusts usually occurs several meters below the surface
regarded as shallow seated strata but due to continues weathering of the loose and
less resistant overlaying sediments, duricrusts formations are easily exposed. Iron,
aluminum oxides and hydroxides, silica and calcium carbonate are some of the
cementing agent for the formation of duricrusts.
Geologically, there is significant interest about duricrusts not only due to its
origins but also due to its significant role in landscape development and its role
providing indications regarding past climates. From engineering aspects, there are
2
interests regarding the properties of duricrusts particularly their hardness and
strength which provides an indication of the degree of excavatability of duricrusts.
Formations of duricrusts in Malaysia are not as widespread as those found in
countries like Australia and Africa. In Malaysia, duricrusts have been noted in
several construction sites in Johor, Malacca and Selangor. Due to the uncertain
nature of duricrusts, problems arise during the definition and terms in tender
documents regarding excavation. As such, this study was carried out to determine
the types and modes of formation of duricrusts so that its properties with regard to
engineering interests could be obtained.
1.2
Statement of Problem
Disputes often occur between clients and contractors during the classification
of excavation materials. It is especially prominent when strongly cemented or
indurated sediments such as duricrusts are encountered. Duricrusts exhibit properties
that are similar to clastic sedimentary rocks but are not classified as rocks based on
its different mode of formation. This study is to determine the mode of formation
and classification of duricrusts to help in their classification for excavation purposes.
3
1.3
Study Objective
The objectives of this study are as follows:
i.
To understand the mode of formation and classification of duricrusts
based on geological factors.
ii.
To determine the common types of duricrusts found in Malaysia
particularly in Selangor and Johor.
iii.
To use the mode of formation of duricrusts in classifying the materials
for excavation works.
1.4
Scope of Study
The scopes that are covered in this study are as follows:
i.
Overview of duricrusts particularly on types, classification and mode
of formation.
ii.
Types of duricrusts found in Malaysia particularly in Selangor and
Johor.
iii.
Mode of formation and the respective characteristics of duricrusts
from an engineering viewpoint.
4
1.5
Methodology
In order to achieve the objectives set out, the following steps will be carried
out:
i.
Literature review on geological aspects regarding duricrusts.
ii.
Compilation of engineering properties of duricrusts related to
excavatability.
iii.
Field observations carried out on related sites in Selangor and Johor.
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction
Duricrusts are hard layers formed as a result of cementation or induration of
sediments by cementing agents. The chemical constituent of the cementing agent is
the most important component of duricrusts because they determine the type of
duricrust (Table 2.1 and Figure 2.1). Iron, aluminium oxides and hydroxides, silica
and calcium carbonate are some of the cementing agent for the formation of
duricrusts. There are certain characteristics of duricrusts that display a strong
resemblance to clastic sedimentary rocks even tough duricrusts cannot be classified
as rocks.
6
Table 2.1: The general classification of duricrusts based on its cementing materials
(after Mohd. For Mohd. Amin et al., 2005)
Cementing materials
Aluminium (Al)
Iron (Fe)
Siliceous (SiO2)
Calcium Carbonate (CaCO3)
Iron (Fe)
Aluminium (Al)
Duricrusts
Alcrete
Ferricrete
Silcrete
Calcrete
Laterite
Bauxite
Nature
Indurated
Indurated
Indurated
Indurated
Loose
Loose
Figure 2.1: Classification of duricrusts (after Faniran & Jeje, 1983)
Geologically, there is significant interest about duricrusts not only due to its
origins but also due to its significant role in landscape development and its role
providing indications regarding past climates (e.g. Faniran & Jeje, 1983; Wilson,
1983; Macias & Chesworth, 1992). Many duricrusts show evidence of replacement
of one type by another, indicating a complex history of formation under changing
conditions of climate and chemical processes.
Duricrusts commonly reach thicknesses of 1 to 10m but there have been cases
where the formations reached a thickness in excess of 50m (Summerfield, 1991).
7
Duricrusts layers are referred to as shallow seated layers and are subsequently easily
exposed to the environment due to continuous weathering of the loose and less
resistant overlaying sediments. Exposed duricrusts have a protective effect similar to
that of a resistant cap rock in sedimentary sequences, and they can produce
topographically significant landforms. Figure 2.2 shows the process associated with
the formation of duricrust.
Figure 2.2: Duricrusts formation (after Maignien, 1966)
2.2
Clastic Sedimentary Rocks
Solid debris and the dissolved mineral matter produced by the mechanical
and chemical breakdown of pre-existing rocks are the raw materials of sedimentary
rocks. These materials are then transported to depositional environments that are
away from the source of the sediments. These depositional environments include
8
meandering rivers, coastal areas, and oceans. Shales, mudrocks, siltstones and
sandstones are the types of sedimentary rocks referred to as clastic sedimentary rocks
that are comparable to duricrusts. The geological classification for sedimentary
rocks is shown in Table 2.2.
Table 2.2: Classification of sedimentary rocks (after Waltham 2002)
Clastic sedimentary rocks
RUDACEOUS: coarse grained
Grain size > 2 mm.
Conglomerate – rounded
fragments.
Breccia – angular fragments.
ARENACEOUS: medium grained
Grain size between 0.06 – 2 mm.
Sandstone & allied rocks.
ARGILLACEOUS: fine grained
Grain size < 0.06 mm.
Siltstone – quartz particles.
Shales, clays, mudstone & allied
rock.
2.3
Non-clastic sedimentary rocks
CARBONATES (Organic)
Consisting mainly of calcite.
Limestone & allied rocks.
NON-CARBONATES (chemical)
Flint & chert – nodular or banded silica.
Coal & lignite – lithified peat & plant
material.
Ironstone – any iron-rich sedimentary rock;
sand, clay or oolite texture.
Salt & gypsum – monomineralic rocks
deposited by evaporation of water.
Types of Duricrusts
Duricrusts are the accumulation of sediments below the surface which are
cemented together by cementing agents. The chemical constituent of the cementing
agent is the most important component of duricrusts because they determine the type
of duricrust. Duricrusts are usually found in regions that exhibit humid and subhumid or arid and semi-arid environmental conditions. There are certain
characteristics of duricrusts that display a strong resemblance to clastic sedimentary
rocks even tough duricrusts cannot be classified as rocks. The types of duricrusts
that can be commonly found are as listed.
9
(a)
Alcrete
Indurated aluminium (Al) rich duricrust are known as alcrete (Macias &
Chesworth, 1992). It forms in humid to sub-humid tropics with deep weathering
profiles where rainfall totals are particularly high.
(b)
Bauxite
Another common term for aluminium (Al) rich duricrust is bauxite (Macias
& Chesworth, 1992). Bauxites are not indurated and are morphologically akin to
laterites but its aluminium concentration dominates over its iron oxide concentration.
The concentration of aluminium found in bauxites are high enough for it to become
economically extractable.
(c)
Ferricrete
Ferricrete is heavily weathered subsoil, rich in oxides of iron, aluminium, or
both. Ferricrete is the indurated form of iron (Fe) rich duricrust. It forms in humid
to sub-humid tropics with deep weathering profiles (Macias & Chesworth,1992).
Ferricrete can be found in exposures having a hard, pavement like surface. Incision
of this hard surface by streams can leave the ferricrete as resistant cap rocks on
upland mesas and plateaus. Ferricrete deposits range in thickness from a few
centimeters to tens of meters.
10
(d)
Laterite
Another form of heavily weathered subsoil, rich in oxides of iron, aluminium
or both is referred to as laterite. Laterite is the loose form of iron (Fe) rich duricrust.
It also forms in humid to sub-humid tropics with deep weathering profiles (Macias &
Chesworth,1992). Laterite can be found just below the surface as a loose, soil like
material and its deposits range in thickness from a few centimeters to tens of meters.
(e)
Silcrete
Silcretes are indurated layers of silica-enriched materials formed beneath the
surface in soils. A minimum of 95 percent of SiO2 is commonly found in silcretes.
Silcretes generally form in both humid and arid environments (Summerfield, 1983).
There have been cases where silcretes occur in humid weathering profiles close to
ferricretes and arid weathering profiles close to calcrete. Silcrete tends to form
roughly horizontal, highly resistant layers generally less than 5 m thick. When
exposed by erosion, silcretes forms highly resistant cap rocks.
(f)
Calcrete
Calcrete are widespread in arid and semi-arid areas with a current mean
annual precipitation of between 400 and 600mm (Macias & Chesworth,1992).
Calcretes have an average constituent of 80 percent CaCO3. They are a major
component of the environment possibly underlying up to 13 percent of the total land
surface (Yaalon, 1981). Calcretes are thick, massive, rock-hard accumulations that
cement gravel, sand, and fines of sediment, producing dense and impermeable layers
that resemble fresh-water limestone. The layers are a few centimeters to several
meters thick.
11
2.4
Mode of Formation of Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed by sedimentation process of
transported sediments such as clay, silt and sand in depositional environments like
meandering rivers, coastal areas, and oceans that are away from the source of the
sediments. After undergoing lithification processes, these loose sediments will be
converted into stronger sedimentary rocks (Lutgens and Tarbuck, 2003; Waltham,
2002 and McLean and Gribble, 1980):
1.
Cementation by cement matrix like siliceous (silica), ferrugineous
(iron oxides), calcareous (calcite) and clay. Cementing materials are
carried in solution by water percolating through the open spaces
between particles. The cement then precipitates onto the sediment
grains, fills the open spaces and join the particles.
2.
Recrystallisation, a small scale solution and deposition of minerals.
Confined largely to certain chemical sedimentary rocks.
3.
Compaction, restructuring and change of grain packing with
decreasing volume of pore space normally associated with high
overburden stress (i.e. thick sedimentation). Most significant in finegrained sedimentary rocks such as shale.
Sandstone consists of sand grains mostly of quartz and other minerals (mica,
feldspars and clays), set in the cement matrix. Shale and siltstone are also expected
to have similar mineral composition, except the mineral grains are relatively finer.
12
2.5
Mode of Formation of Duricrusts
Pedogenic regimes of soil forming processes are closely related to the
formation of duricrusts. Indurated zones in these pedogenic regimes form thick
weathering profiles that consist of loose sediments resulting from weathering of in
situ rocks that are associated with the formation of duricrusts. This indicates that
duricrusts are formed in situ i.e. at location where parent rocks are being weathered
to residual soils (in situ weathering deposits). The deep chemical weathering that
happens in tropical regions is the main reason for high formations of duricrusts
(Faniran & Jeje, 1983) although not confined to just this weathering environment.
For duricrusts to form there needs to be a source of its cementing matrix. The
relative accumulation of iron and aluminium oxides and hydroxides with the
dispersion of other more unstable components like silica from the weathering mantle
forms ferricretes and alcretes (Figure 2.3). This relative accumulation occurs due to
leaching caused by humid tropical regions where rainfall totals are particularly high.
A higher concentration of iron and aluminium may also occur as the elements are
transported within the weathering mantle, mechanically or in solution form from
high to low topographic positions (Summerfield, 1991).
13
Figure 2.3: Stages of weathering causing the accumulation of iron or aluminium to
form ferricretes or alcretes. The profile shows the relative accumulation of iron or
aluminium based on time (after McFarlane, 1983).
While many duricrusts generally form based on a truly pedogenic formation,
“non-pedogenic” duricrusts are those where cementing matrix has been introduced
into the host soil or sediment by “absolute” accumulation (Goudie, 1983). There are
two main forms of this accumulation process which are fluvial action and those
produced by groundwater. The fluvial process involves the deposition of cementing
matrix within channels or valleys (Lattman, 1977) or deposition from sheetfloods
(Breazeale & Smith, 1930). Throughflow water and groundwater from lateral
seepage may also cause cementing matrix to be deposited. The concentration of
cementing matrix externally derived from laterally moving groundwater is probably
the most common non-pedogenic duricrust. Accumulation may occur due to
processes such as evaporation at the capillary fringe.
14
Pedogenic duricrusts form due to the concentration of authigenic cement
essentially vertically within the soil profile primarily by “relative” accumulation
(Goudie, 1983). The in situ model of duricrust formation comes into the pedogenic
category. This model involves the relative accumulation of cementing matrix due to
changes of the suitable host materials which results in the selective removal of noncement matrix materials or by solution and reprecipitation causing a reconstitution of
the original limestone to produce case hardening (Blank & Tynes, 1965).
Reconstituted and detrital duricrusts have the unique position of being in
between pedogenic and non-pedogenic duricrusts. Weathered duricrusts are laterally
transported and redeposited, which are then reindurated by essentially in situ
processes are referred to as reconstituted and detrital duricrusts. Among the three
models, the pre descensum model is of more importance because of the realization of
the frequency and magnitude of its occurrence (Goudie, 1978;Morales, 1979).
Figure 2.4 shows models of formation of calcrete.
15
Figure 2.4: Models of the formation of calcrete: (A) the capillary rise (per ascensum)
model; (B) the percolation (per descensum) model with inputs of carbonate from
above; (C) the detrital model in which a secondary calcrete is formed through the
solution and disintegration of an existing calcrete horizon at a higher level in the
landscape (after Goudie, 1973).
16
The absolute accumulation of silica or calcium carbonate forms silcrete or
calcrete. The formation of these duricrusts require a source for the cementing agents,
a means for the accumulation of the cementing agent and a mechanism for
precipitation. These cementing agents can be obtained through the weathering of
bedrocks or sediments, inputs from rainfall or dusts, plant residues and solutes in
ground water. The cementing materials can move laterally or vertically. Vertical
movement can be further divided into upward movement due to capillary rise (per
ascensum) or downward movement due to percolation (per descensum)(Goudie,
1973). Figure 2.5 and 2.6 shows typical weathering profiles of calcrete and silcrete.
Figure 2.5: Typical weathering profile of calcrete at different locations (after
Beauvais & Colin, 1993)
17
Figure 2.6: Typical weathering profile of silcrete (after Summerfield, 1983).
Precipitation can happen due to a number of factors other than evaporation of
weathering solutions. It can happen due to pH changes, reactions with other cations
in solution and organic activities. Changes in pH seem to be of particular importance
in the formation of silcrete because it is able to explain why silcrete can be formed in
both arid and humid weathering profiles (Figure 2.7). The solubility of silica
increases dramatically once the pH level passes 9. In arid environments the pH level
is normally between the ranges of pH 8 and 10 (alkaline). The movement of silica
from a position that has a pH above 9 to a pH below 9 causes precipitation. In humid
environments highly acidic conditions are found. Aluminium becomes more mobile
then silica at a pH level below 4. This causes the aluminium to be displaced
allowing the silica to accumulate and form silcrete.
18
Figure 2.7: A schematic representation of the relationship between the formation of
different duricrusts and prevailing conditions of climate and pH (after Summerfield,
1983).
Topography also plays a major role in the formation of duricrusts. The rate
of development of duricrusts must be higher then the rate of denudation of duricrusts
for it to be created in abundance and this requires low local relief. Topography is
also important because it determines the local drainage conditions. The formation of
silcretes is dependent on weather silica can be retained due to poor drainage while
alcrete formations require efficient drainage to encourage leaching. The constituents
of bedrock also play a major role in determining the type of duricrusts because it is
the source for cementing agents required to form duricrusts.
Another important factor in determining the mode of formation of duricrusts
is the change of climatic conditions. Studies that have been carried out, managed to
determine the climatic conditions for the formation of calcrete (see Goudie, 1973;
Faniran & Jeje, 1983; Wilson, 1983; Macias & Chesworth, 1992; Netterberg, 1983,
Summerfield, 1983) but the same cannot be said for the formation of ferricretes,
alcretes and silcretes. High chemical and mechanical resistance makes it resistant to
prevailing climate that may have changed dramatically. This causes these duricrusts
19
not to be in equilibrium showing signs of chemical, mineralogical and physical
characteristics that affect their development. Calcretes on the other hand are
extremely responsive to changing conditions. This can be noticed in the relatively
high solubility of calcium carbonate which would cause the deforming of calcrete
should the climate become humid. This contrasting resistance to environmental
climate change is why calcretes that have been exposed to the surface are dated to the
Late Cenozoic period while ferricretes and silcretes can be dated to the Early
Cenozoic period and even beyond.
There have been studies conducted regarding the role of organisms in the
formation of duricrusts. The role of organisms in the accumulation and deposition of
calcite has generally been overlooked. A certain micro flora which was found on
Israeli calcrete (nari) showed that it had capabilities of producing large amounts of
calcite (Krumbein, 1968). Also, some blue-green soil algae have calcium carbonate
precipitating bacteria which can be found in their slime sheaths (Johnson, 1967).
Klappa (1979b) also showed that many calcretes that are found in the Western
Mediterranean basin have calcified organic filaments of soil, fungi, algae,
actinomycetes and root hairs of vascular land plants. Microdium which has been
discussed in studies (Klappa, 1978; Esteban, 1974), also play an important role in
calcrete formations. Klappa (1978), states that Microdium grains constitute 43% of
the total volume of calcrete. The simulation of precipitation of calcite by gypsum
reduction also provides another indication of the role of organisms in the formation
of calcrete. Lattman and Lauffenberger (1974) hypothesized that organic
decomposition produced the hydrogen sulphide that caused precipitation of calcite by
gypsum reduction which helped form thick calcretes in Nevada.
Duricrusts have also been known to form under cold conditions. Various
forms of calcareous curst are associated with present day or Pleistocene cold-climate
landforms. This indicates that crusts are not restricted to form in arid or semi-arid
areas. Experiments carried out have shown that when water is frozen, minute
amount of bicarbonate is found in parts which freeze first. This then causes an
increase in the bicarbonate concentration in the water which remains unfrozen. This
20
causes a build up of hardness and causes fibrous calcite crystals to precipitate (Swett,
1974).
It is believed that calcrete generally forms more quickly compared to other
types of duricrusts. The rapid formation of calcretes can be shown in the
cementation of human artifacts such as gravestones. Silcrete, ferricrete and alcrete
generally form slower due to the lower concentration of silica, iron and aluminium in
surfaces and regolith waters. But studies carried out to determine the formation of
ferrecrite (iron pan) on burial mounds by Breuning-Madsen, Holst and Rasmussen
(2002) show that ferricretes have the capabilities of forming quickly given the
cementing agent (Fe) is found in abundance.
A generalized duricrust formation model is difficult to propose. This is due
to factors such as climate, host material, topography, hydrology, organism and time.
However, Klappa, 1983, gave a generalized indication regarding the formation of
calcrete (Table 2.3).
Table 2.3: The general stages for the formation of calcrete (after Klappa, 1983)
Stage 1
Stage 2
Stage 3
Preparation of host material
The formation of regolith or weathered detritus from initially
consolidated bedrock involves mechanical and biophysical
disintegration, physiochemical dissolution and biochemical
weathering. Whether this material accumulates in situ or is eroded
and deposited elsewhere depends on the rate of sediment
production versus its rate of removal. For calcrete formation to
take place within a weathered host material, sediment production
must equal or exceed the rate of removal.
Soil formation
Unconsolidated sediment or weathered detritus is transformed into
a soil by changes produced by the action of organisms and by
changes due to the movements of water through the sediment.
Vertical movements of moisture tend to reinforce the changes
produced by organisms, the general result being an anisotropy
shown by layers or horizons parallel to the land surface.
Accumulation of calcium carbonate
The soil becomes progressively enriched with calcium carbonate.
Pore spaces become lined or filled with calcite whereas relic
21
Table 2.3 (Continued)
Stage 4
grains and other minerals inherited from the host material become
progressively replaced by calcite. Characteristic structures
composed of concentrations of pedodiagenetic calcite, such as
glaebules (soil concretions) and calcitans (free and embedded
grain cutans), are formed. Biological constituents of the soil may
become calcified, thus forming biogenic carbonate structures such
as rhizoliths, calcified filaments, calcified faecal pellets, calcified
cocoons and Microcodium aggregates. The thickness and depth of
carbonate accumulation will depend on: the amount of available
calcium and carbonate ions; the depth of which calcium-bearing
waters can penetrate; the relative and absolute amounts of
precipitation and dissolution of calcium carbonate within the soil
profile; and the porosity and permeability of the host material.
Profile development
In the early stages of calcrete development, the profile is
composed of weathered materials with high porosities and
permeabilities. Vertical movements of meteoric vadose water can
take place relatively easily and the amount of water retained
within the soil profile is insufficient to supply the requirements of
all plant species. Some plants extend taproots vertically
downwards to the capillary fringe above the water table or to a
perched water table overlaying an impermeable substrate.
Accumulation of calcium carbonate from vertically elongate
glaebules and vertically oriented rhozoliths as a result of vertical
water movements and the presence of taproots respectively. Roots
extend downwards into fractures and joints within the host
material, culminating in the formation of the translation horizon.
Precipitation of calcium carbonate, without significant
cementation because of mechanical or chemical instability, forms
the chalky horizon of the pedogenic calcrete profile. The chalky
horizon is subjected to frequent periods of wetting and drying.
Precipitation and dissolution of calcium carbonate take place
concurrently. Pedoturbation (physical, chemical and biological
disturbance of soil material) precludes the formation of indurated
layers.
As the accumulation of calcium carbonate increases, porosity and
permeability of the profile decrease. Original constituents of the
host material are progressively replaced with increasing amounts
of calcite. At some point in profile development, it becomes
easier for soil water to move laterally rather then vertically. By
this stage, most plants from vertical to horizontal root systems
may reflect a change of plant species as the calcrete profile
evolves. Intertroot distances in woody species are large compared
with herbaceous species (Bowen, 1973). Large (5-20 cm),
isolated, vertically oriented rhizoliths are common in glaebular
horizons, whereas smaller (1-2 mm), branching, horizontally
oriented rhizoliths form the bulk of sheet calcrete horizons. Thus,
the development of the sheet calcrete horizon from the glaebular
horizon may be a reflection of plant succession.
22
Table 2.3 (Continued)
Stage 5
Stage 6
2.6
Induration
As accumulation of calcium carbonate increases, a point will be
reached when the soil organisms can no longer maintain viability.
The intensity of soil-forming processes diminishes and eventually
ceases to be important. Diagenetic processes, mainly cementation
and replacement lead to the fossilization and induration of the soil
profile.
Reworking
The indurated profile, if it remains at the land surface, is subjected
to further processes which will alter or destroy the profile. Soil
forming processes, governed initially by lower plant activities
(lichens, algae, fungi, bacteria) will form a protosoil. The prepared
substrate allows colonization of other plants such as mosses and
grasses. Eventually, the soil profile is able to support higher
plants. The root systems of these plants penetrate, dissolve and
fracture the indurated hardpan. Disturbance of the calcrete profile
by vegetation may form tepee structures and rhizo-breccias.
Further pedoturbation, carbonate dissolution and reprecipitation,
lead to the formation of a reworked, recemented, brecciaconglomeratic calcrete hardpan.
Material Texture of Duricrusts and Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed from fragments and their textures are
dependent on the sizes, shapes and arrangement of these fragments. These fragments
are cemented together with cement matrix much like duricrusts (see Figure 2.8).
The size of the grain of a clastic sedimentary rock is an important indicator
regarding the distance between the source of the grain and its depositional area.
Coarser particals are an indicator that the grains source is close by and finer particals
are indicators that the source is far away. Another indicator regarding the distance of
the grains from its source is the degree of roundness (McLean & Gribble, 1979). The
further away the grain travels from the source the rounder it gets due to abrasion
suffered during transport.
23
The strength of clastic sedimentary rocks and duricrusts come from the
strength of the individual mineral grain (quartz feldspar and mica) as well as the
cement matrix. Common cementing matrix such as siliceous, ferrugineous and
calcareous cementing matrixes are associated with the formation of duricrusts and
clastic sedimentary rocks. Calcareous cement matrix can be determined by exposing
it to diluted hydrochloric acid. Siliceous cement matrix is the hardest cement matrix
and therefore produces the hardest sedimentary rock or duricrust. Ferrugineous
cement matrix can be determined if the sedimentary rock or duricrust has an orange
or dark red color (Waltham, 2002).
The weakest point of these materials is the boundary between the mineral
grains and the cement matrix. Therefore, with a stronger bonding between the
cement matrix and mineral grains, the material will also be stronger. Due to this it is
believed that strength properties of duricrusts are similar to those of clastic
sedimentary rocks.
Figure 2.8: Granular texture displayed by sand and gravel size mineral grains
imbedded in cement matrix.
24
2.7
Duricrusts in Malaysia
Formations of duricrusts in Malaysia are not as widespread as those found in
countries like Australia and Africa. In Malaysia, duricrusts have been encountered at
several construction sites in Johor and Selangor (Mohd For Mohd Amin & Muhd
Zaimi Abd. Majid, 1993). Duricrusts can also be found in other states but the
problem seems prominent in Johor and Selangor because these states are the states
that are undergoing extensive construction activities.
Selangor and Johor contains both primary and secondary formations of
duricrust (Figure 2.9 and 2.10). Figure 2.11 shows a geological map of the South of
Johor (PTA M32.J69 J64 1974), which indicates the site in Johor which is located at
Bandar Baru Uda, Johor Bahru. The duricrusts found on site were varied in
thickness (1 to 3m) and were exposed to the environment due to intensive erosion
and weathering. Based on the surrounding rock masses which are granite, early
estimates regarding the type of duricrust are alcrete and ferricrete. The duricrusts are
believed to be alcrete and ferricrete because the chemical composition of the granite
stone found in the area are probably iron and aluminium.
Southern parts of Malaysia experienced semi arid climate during the
Quaternary Period (Pleistocene Epoch). Due to the high evaporation rate compared
to precipitation rate, the region was exposed primarily to mechanical weathering
rather then chemical weathering (Fauzilah Ismail, 2002). This caused the
accumulation of salts, sulfates and carbonates that dissolved into the ground during
humid climates to exist unaltered, ready to act as cementing agents to form duricrusts
(Rollings & Rollings Jr, 1997). The existence of an arid climate in Malaysia can be
proven with the findings of iron remnants found in the eastern parts of Johor. The
iron (Fe) remnants act as a common cementing agent to form ferricretes which form
in arid climates.
25
The flow of time has caused the climatic conditions of Malaysia to change
from an arid climate experienced during the Quaternary Period to a humid climate.
High precipitation caused soluble minerals to acts as cementing agents. Due to
evaporation, the cementing agent formed strong bonds with sediments to form
duricrusts. This has allowed the existence of duricrusts that are only found in humid
regions to appear in Malaysia as well.
Figure 2.9: Duricrusts occurring as caprock in Dengkil, Selangor.
Figure 2.10: Secondary duricrusts in Pasir Gudang, Johor.
26
Figure 2.11: The geological map of the study area (Malaysia New Series Map, Johor
Bahru, Sheet 130)
27
2.8
Typical Properties of Various Rock Types
Table 2.4 through Table 2.7 below show some typical properties of various
rock types that include sandstone, siltstone, mudrock and shale.
Table 2.4: Typical static mechanical properties of some common rock types
(modified from Bengt Stillborg, 1985)
Rock class
Sedimentary
rock
Metamorphic
rock
Igneous rock
Rock type
Limestone
Mudstone
Sandstone
Siltstone
Shale
Gneiss
Marble
Quartz
Basalt
Gabbro
Granite
Unconfined
compress.
strength
σc [MPa]
50 - 200
5 - 15
50 - 150
5 - 200
50 - 100
100 - 200
100 - 200
200 - 400
100 - 300
100 - 300
100 - 200
Tensile
Strength
σt [MPa]
Point load
Index
Is(50) [MPa]
5 - 20
−
5 -15
2 - 20
2 - 10
5 - 20
5 - 20
25 - 30
10 - 15
10 - 15
5 - 20
0.5 - 7
0.1 - 6
0.2 - 7
6 - 10
−
2 - 11
2 - 12
5 - 15
9 - 14
6 - 15
5 - 10
28
Table 2.5: Relative seismic (P-wave) velocity (after Bickel & Kuesel, 1982)
Type Of Rocks
Dry, loose topsoils and silts.
Dry sands, loams; slightly sandy or gravely soft clays.
Dry gravels, moist sandy and gravely soils; dry heavy silts
and clays; moist silty and clayey soils.
Dry, heavy, gravely clay; moist, heavy clays; cobbly materials
with considerable sands and fines; soft shales; soft or weak
sandstones
Water, saturated silts or clays, wet gravels.
Compacted, moist clays; saturated sands and gravels; soils
below water table; dry medium shales, moderately soft
sandstones, weathered, moist shales and schists.
Hardpan; cemented gravels; hard clay; boulder till; compact,
cobbly and bouldery materials; medium to moderately hard
shales and sandstones, partially decomposed granites, jointed
and fractured hard rocks.
Hard shales and sandstones, interbedded shales and
sandstones, slightly fractured hardrocks.
Unweathered limestones, granites, gneiss, other dense rocks.
P-wave
velocity m/s
180-370
300-490
460-910
910-1460
1460-1520
1460-1830
1680-2440
2440-3660
3660-6100
Table 2.6: Density of Rocks (modified after Manger and Clark, 1966)
Type of sedimentary rocks
Sandstone
Limestone
Dolomite
Chalk
Marble
Shale
Sand
Range of Density kg/m3
2170 – 2700
2370 – 2750
2750 – 2800
2230
2750
2060 – 2660
1920 – 1930
Table 2.7: Uniaxial compressive and uniaxial tensile strengths of rocks (after Pitts,
1984)
Rock Type
Granite
Dolerite
Basalt
Sandstones
Mudrocks
Limestones
Gneisses
Uniaxial Compress.
Strength (MN/m2)
100-250
200-350
150-300
20-170
10-100
30-250
50-200
Uniaxial Tensile
Strength (MN/m2)
7-25
15-35
10-30
4-25
2-10
5-25
5-20
29
The typical classification and description for various rock types based on the
properties and strengths are given in Table 2.8 through Table 2.10.
Table 2.8: Slake Durability Classification (Gamble, 1971)
Group name and
description
Very high durability
High durability
Medium High durability
Medium durability
Low durability
Very Low durability
% retained after one
10 min. cycle, Id1 (dry
weight basis)
>99
98-99
95-98
85-95
60-85
<60
% retained after two 10
min. cycle, Id2 (dry
weight basis)
>98
95-98
85-95
60-85
30-60
<30
Table 2.9: Classification based on point load index strength (Broch and Franklin,
1972)
Strength Classification
Is (MN/m2)
Very strong
Strong
Moderately strong
Moderately weak
Weak
Very weak rock or hard soil
>6.7
3.35-6.7
0.85-3.35
0.4-0.85
0.12-0.4
0.05-0.12
Equivalent UCS
(MN/m2)
>100
50-100
12.5-50
5-12.5
1.25-5
0.6-1.25
30
Table 2.10: Classification of rock types based on unconfined compressive strength
(after McLean & Gribble, 1980)
Descriptive terms
UCS (MPa)
Rock types
Very weak rock.
Weak rock.
Moderately weak
rock
< 1.25
1.25 – 5.0.
5.0 – 12.5
Some weakly compacted
sedimentary rocks, some very highly
weathered igneous or metamorphic
rocks, boulder-clays.
Moderately strong
rock
12.5 – 50.0
Some sedimentary rocks, some
foliated metamorphic rocks, highly
weathered igneous and metamorphic
rocks.
Strong rock
50 – 100
Some low-grade metamorphic rocks,
marbles, some strongly cemented
sandstones (silica cement), some
weathered and metamorphic rocks.
Very strong rock
100 – 200
Mainly plutonic, hypabyssal and
extrusive igneous rocks (medium to
coarse grained), sedimentary
quartzites, strong slate, gneisses.
Extremely strong
rock
> 200
Fine-grained igneous rock,
metamorphic quartzites, some
hornfelses.
31
2.9
Engineering Properties of Duricrusts
It is necessary to have a form of classification for duricrusts to predict its
excavatability. Classification suitable for excavation of duricrusts should be of both
geological and engineering significance and must be applicable in the field by
relatively untrained personnel.
Various studies have been conducted to determine the physical characteristics
of duricrusts in particular calcrete. The Schmidt Test Hammer Type N is one of the
devices that has been used with some success (Day & Goudie, 1977). With this test
the hardness of the calcrete sample was able to be determined. Studied conducted
showed that the distance of the rebound R, obtained from the hammer was highly
correlated with compressive strength (Yaalon & Singer, 1974). Studies conducted in
the Kalahari showed that hardpan and laminar calcretes had a mean R value of 42
which indicates a compressive strength of 324 kg/cm2. R values of 51-54 and
compressive strength of 600-788 kg/cm2 were obtained form samples obtained for
Israel (Yaalon & singer, 1974). Figure 2.12 shows Schmidt hammer hardness
characteristics of some calcrete profiles.
32
Figure 2.12: Schmidt hammer hardness characteristics of some calcrete profiles on
bedrock. (a) In Puerto Rico (after Ireland, 1079); (b) In Sulawesi (after Day and
Goudie, 1977); (c) In Puerto Rico (after Day and Goudie, 1977); (d) In Israel (after
Yaalon and Singer, 1974).
33
Information found from the Boston Building Code (Table 2.11), Section 2904
of the 1944 edition provides an indication of the engineering properties of hardpan
(also known as duricrusts). It indicates the maximum bearing pressure allowable on
hardpans for the construction of foundations above it is 10 tfs (tons per square foot).
Table 2.11: Section 2904 of the Boston Building Code (1944).
Class
1
2
3
4
5
6
7
8
9
10
11
12
13
Material
Massive bedrock without limitations, such as granite,
diorite and granitic rocks; and also gneiss, trap rock,
felsites and thoroughly cemented conglomerates such as
the Roxbury Puddingstone, all in sound condition (sound
condition allows minor cracks).
Laminated rocks, such as slate and schist, in sound
condition (minor cracks allowed).
Shale in sound condition (minor cracks allowed).
Residual deposits of shattered or broken bedrock of any
kind except shale.
Hardpan.
Gravel, sand-gravel mixtures, compact
Gravel, sand-gravel mixtures, loose; sand, coarse, compact
Sand, coarse, loose; sand, fine, compact
Sand, fine, loose
Hard clay
Medium clay
Soft clay
Rock flour, shattered shale, or any natural deposit of
unusual character not provided for herein
Allowable
Bearing
Value
Tons/sq ft
100
35
10
10
10
5
4
3
1
6
4
1
Value to be
fixed by the
Commissioner
The United States Department of Labour in its occupational safety and health
standards classifies hardpans (also known as duricrusts) as “Type A” soil under its
Regulations (Standard 29-CFR) Soil Classification-1926 Subpart P, App A. “Type
A” indicates cohesive soils with an unconfined, compressive strength of 144 kPa (1.5
tfs) or greater. However, duricrusts are not classified as “Type A” if:
(i)
It is fissured
34
(ii)
It is subject to vibration from heavy traffic, pile driving, or similar
effects
(iii)
It has been previously disturbed
(iv)
It is part of a sloped, layered system where the layers dip into the
excavation on a slope of four horizontal to one vertical (4H:1V) or
greater
(v)
It is subject to other factors that would require it to be classified as a
less stable material.
CHAPTER 3
METHODOLOGY
3.1
Introduction
The main objective of this study is to understand the types and mode of
formation of duricrusts and using this understanding to classify duricrusts for
excavation. The types and mode of formation of duricrusts will be determined by
conducting extensive literature reviews to compare the previous findings. Previous
studies regarding the excavatability of duricrusts will also be reviewed to the verify
claims regarding the similarities between duricrusts and clastic sedimentary rocks
especially in terms of material texture, mineralogy, strength and physical properties.
36
3.2
Mode of Formation of Duricrusts
Pedogenic regimes of soil forming processes are closely related to the
formation of duricrusts. Indurated zones in these pedogenic regimes form thick
weathering profiles that consist of loose sediments resulting from weathering of in
situ rocks that are associated with the formation of duricrusts. The deep chemical
weathering that happens in tropical regions is the main reason for high formations of
duricrusts (Faniran & Jeje, 1983) although not confined to just this weathering
environment.
Pedogenic duricrusts form due to the concentration of authigenic cement
essentially vertically within the soil profile primarily by “relative” accumulation
(Goudie, 1983). The in situ model of duricrust formation comes into the pedogenic
category. This model involves the relative accumulation of cementing matrix due to
changes of the suitable host materials which results in the selective removal of noncement matrix materials or by solution and reprecipitation causing a reconstitution of
the original limestone to produce case hardening (Blank & Tynes, 1965).
While many duricrusts generally form based on a truly pedogenic formation,
“non-pedogenic” duricrusts are those where cementing matrix has been introduced
into the host soil or sediment by “absolute” accumulation (Goudie, 1983). There are
two main forms of this accumulation process which are fluvial action and those
produced by groundwater. The fluvial process involves the deposition of cementing
matrix within channels or valleys (Lattman, 1977) or deposition from sheetfloods
(Breazeale & Smith, 1930). Throughflow water and groundwater from lateral
seepage may also cause cementing matrix to be deposited. The concentration of
cementing matrix externally derived from laterally moving groundwater is probably
the most common non-pedogenic duricrust. Accumulation may occur due to
processes such as evaporation at the capillary fringe.
37
Reconstituted and detrital duricrusts have the unique position of being in
between pedogenic and non-pedogenic duricrusts. Weathered duricrusts are laterally
transported and redeposited, which are then reindurated by essentially in situ
processes are referred to as reconstituted and detrital duricrusts. Among the three
models, the pre descensum model is of more importance because of the realization of
the frequency and magnitude of its occurrence (Goudie, 1978;Morales, 1979).
Precipitation can happen due to a number of factors other than evaporation of
weathering solutions. It can happen due to pH changes, reactions with other cations
in solution and organic activities. Changes in pH seem to be of particular importance
in the formation of silcrete because it is able to explain why silcrete can be formed in
both arid and humid weathering profiles. The solubility of silica increases
dramatically once the pH level passes 9. In arid environments the pH level is
normally between the ranges of pH 8 and 10 (alkaline). The movement of silica
from a position that has a pH above 9 to a pH below 9 causes precipitation. In humid
environments highly acidic conditions are found. Aluminium becomes more mobile
then silica at a pH level below 4. This causes the aluminium to be displaced
allowing the silica to accumulate and form silcrete.
Topography also plays a major role in the formation of duricrusts. The rate
of development of duricrusts must be higher then the rate of denudation of duricrusts
for it to be created in abundance and this requires low local relief. Topography is
also important because it determines the local drainage conditions. The formation of
silcretes is dependent on weather silica can be retained due to poor drainage while
alcrete formations require efficient drainage to encourage leaching. The constituents
of bedrock also play a major role in determining the type of duricrusts because it is
the source for cementing agents required to form duricrusts.
Another important factor in determining the mode of formation of duricrusts
is the change of climatic conditions. Studies that have been carried out, managed to
determine the climatic conditions for the formation of calcrete (see Goudie, 1973;
38
Faniran & Jeje, 1983; Wilson, 1983; Macias & Chesworth, 1992; Netterberg, 1983,
Summerfield, 1983) but the same cannot be said for the formation of ferricretes,
alcretes and silcretes. High chemical and mechanical resistance makes it resistant to
prevailing climate that may have changed dramatically. This causes these duricrusts
not to be in equilibrium showing signs of chemical, mineralogical and physical
characteristics that affect their development. Calcretes on the other hand are
extremely responsive to changing conditions. This can be noticed in the relatively
high solubility of calcium carbonate which would cause the deforming of calcrete
should the climate become humid. This contrasting resistance to environmental
climate change is why calcretes that have been exposed to the surface are dated to the
Late Cenozoic period while ferricretes and calcretes can be dated to the Early
Cenozoic period and even beyond.
There have been studies conducted regarding the role of organisms in the
formation of duricrusts. A certain micro flora which was found on Israeli calcrete
(nari) showed that it had capabilities of producing large amounts of calcite
(Krumbein, 1968). Also, some blue-green soil algae have calcium carbonate
precipitating bacteria which can be found in their slime sheaths (Johnson, 1967).
Klappa (1979b) also showed that many calcretes that are found in the Western
Mediterranean basin have calcified organic filaments of soil, fungi, algae,
actinomycetes and root hairs of vascular land plants. Microdium which has been
discussed in studies (Klappa, 1978; Esteban, 1974), also play an important role in
calcrete formations. Klappa (1978), states that Microdium grains constitute 43% of
the total volume of calcrete. The simulation of precipitation of calcite by gypsum
reduction also provides another indication of the role of organisms in the formation
of calcrete. Lattman and Lauffenberger (1974) hypothesized that organic
decomposition produced the hydrogen sulphide that caused precipitation of calcite by
gypsum reduction which helped form thick calcretes in Nevada.
Duricrusts have also been known to form under cold conditions. Various
forms of calcareous curst are associated with present day or Pleistocene cold-climate
landforms. This indicates that crusts are not restricted to form in arid or semi-arid
39
areas. Experiments carried out have shown that when water is frozen, minute
amount of bicarbonate is found in parts which freeze first. This then causes an
increase in the bicarbonate concentration in the water which remains unfrozen. This
causes a build up of hardness and causes fibrous calcite crystals to precipitate (Swett,
1974).
It is believed that calcrete generally forms more quickly compared to other
types of duricrusts. The rapid formation of calcretes can be shown in the
cementation of human artifacts such as gravestones. Silcrete, ferricrete and alcrete
generally form slower due to the lower concentration of silica, iron and aluminium in
surfaces and regolith waters. But studies carried out to determine the formation of
ferrecrite (iron pan) on burial mounds by Breuning-Madsen, Holst and Rasmussen
(2002) show that ferricretes have the capabilities of forming quickly given the
cementing agent (Fe) is found in abundance.
3.3
Material Texture of Duricrusts and Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed from fragments and their textures are
dependent on the sizes, shapes and arrangement of these fragments. These fragments
are cemented together with cement matrix much like duricrusts.
The size of the grain of a clastic sedimentary rock is an important indicator
regarding the distance between the source of the grain and its depositional area.
Coarser particals are an indicator that the grains source is close by and finer particals
are indicators that the source is far away. Another indicator regarding the distance of
the grains from its source is the degree of roundness (McLean & Gribble, 1979). The
further away the grain travels from the source the rounder it gets due to abrasion
suffered during transport.
40
The strength of clastic sedimentary rocks and duricrusts come from the
strength of the individual mineral grain (quartz feldspar and mica) as well as the
cement matrix. Common cementing matrix such as siliceous, ferrugineous and
calcareous cementing matrixes are associated with the formation of duricrusts and
clastic sedimentary rocks.
The weakest point of these materials is the boundary between the mineral
grains and the cement matrix. Therefore, with a stronger bonding between the
cement matrix and mineral grains, the material will also be stronger. Due to this it is
believed that strength properties of duricrusts are similar to those of clastic
sedimentary rocks.
3.4
Strength and Physical Properties
It is necessary to have a form of classification for duricrusts to predict its
excavatability. Classification suitable for excavation of duricrusts should be of both
geological and engineering significance and must be applicable in the field by
relatively untrained personnel.
Various studies have been conducted to determine the physical characteristics
of duricrusts in particular calcrete. The Schmidt Test Hammer Type N is one of the
devices that has been used with some success (Day & Goudie, 1977). With this test
the hardness of the calcrete sample was able to be determined. Studied conducted
showed that the distance of the rebound R, obtained from the hammer was highly
correlated with compressive strength (Yaalon & Singer, 1974). Studies conducted in
the Kalahari showed that hardpan and laminar calcretes had a mean R value of 42
which indicates a compressive strength of 324 kg/cm2. R values of 51-54 and
41
compressive strength of 600-788 kg/cm2 were obtained form samples obtained for
Israel (Yaalon & singer, 1974).
Duricrusts can be classified geologically according to the types of duricrusts,
the mode of formation of duricrusts or in terms of engineering properties. The
properties that will be looked into include mechanical properties like rebound
number (surface hardness), point-load index strength, uniaxial compressive and
tensile strength. Properties like density and seismic (P-wave) velocity are also
looked into to determine the degree of compactness of duricrusts. The degree of
excavatability and rippability of rock materials can be determined with these
properties (MacGregor et al., 1994 & Pettifer and Fookes 2004). The raw data for
the comparisons were obtained form research conducted by Fauzilah Ismail (2002)
and Jerry Chua Kuo Seng (2004)
42
3.4.1
Uniaxial Compressive Strength
Uniaxial compressive strength (UCS) is the most common parameter for
studying the strength properties of materials (Jumikis, 1979). For the classification
of duricrusts, UCS provides an indication regarding the strength, weather duricrusts
will be pulverized during excavation, and resistance against loading and fracturing.
Table 3.1: The compressive strength by Fauzilah Ismail (2002)
Core Sample No.
A
B
C
D
E
F
G
H
Compressive Strength, MPa
9.06
8.86
12.74
12.95
9.97
10.96
7.36
12.20
43
3.4.2
Durability
Durability of materials is important for all applications. Since the
disintegration of materials is many and varied, the durability index provides an
indication regarding the resistance of duricrusts against slaking and the degree of
bonding of cementing agents found in duricrusts.
Table 3.2: Durability index test results by Fauzilah Ismail (2002)
Lump Group No.
A
B
C
D
E
F
Average value
Id1 % Id2 %
90.9
83.8
88.4
78.3
80.6
68.5
96.0
89.0
93.9
86.9
88.2
78.2
89.7 % 80.8 %
44
3.4.3 Point-load index strength
The point-load index provides an indication regarding the strength, weather
duricrusts will be pulverized during excavation, and resistance against loading and
fracturing.
Table 3.3: Point load test by Jerry Chua Kuo Seng (2004)
Sample
No.
1
2
3
4
E
F
Height,
D mm
28.0
27.5
30.0
27.0
30.0
28.0
Width,
W mm
50.0
50.0
50.0
50.0
50.0
50.0
Load, P
kN
0.8
0.5
0.9
0.9
1.0
0.7
De2
mm2
1782.54
1750.70
1909.86
1718.87
1909.86
1782.54
Is
MPa
0.45
0.29
0.47
0.52
0.52
0.39
F
0.77
0.76
0.79
0.76
0.79
0.77
Is50
MPa
0.35
0.22
0.37
0.40
0.41
0.30
45
3.4.4
Surface Hardness
Surface hardness provides an indication of resistance against impact and
abrasion. Every material has a range of hardness that ultimately depends on the
strength of its chemical bonds. Surface hardness provides an indication on how hard
it will be to penetrate cutting tools into duricrusts.
Table 3.4: Rebound test results for block samples by Fauzilah Ismail (2002)
Block No.
Rebound
Number
(R)
Total
Average
Value
Block
Comp.
Strength,
Mpa (α=450)
Block 1
16
25
15
22
22
15
16
18
20
18
187
Block 2
20
18
22
20
20
15
18
26
26
18
203
Block 3
16
14
18
14
18
22
20
14
24
22
182
Block 4
24
20
12
20
12
12
20
20
12
22
174
Block 5
18
20
18
22
22
16
18
22
22
26
204
Block 6
16
14
20
22
16
18
22
12
14
20
174
18.7
20.3
18.2
17.4
20.4
17.4
<10
<10
<10
<10
10.5
<10
46
Table 3.5: Rebound test results for core samples by Fauzilah Ismail (2002)
Core No.
Diameter, D
mm
Height, H
mm
Rebound
Number, R
A
B
C
D
E
F
G
H
48
48
48
48
48
48
48
48
106
108
107
106
106
92
102
95
22
23
20
14
15
22
14
18
Core Comp.
Strength, MPa
(α=-900)
16
16
14
10
10
16
10
10
Table 3.6: Joint wall compressive strength, JCS value by Fauzilah Ismail (2002)
Core Sample No.
A
B
C
D
E
F
G
H
Rebound
Number, R
22
23
20
14
15
20
14
18
Dry Density, ρ
kN/m3
18.9
18.3
18.2
18.9
19.4
20.4
19.3
19.9
Joint Wall Comp.
Strength, JCS MPa
23.99
23.99
21.38
17.38
18.62
23.44
17.78
21.38
47
3.4.5 Tensile Strength
Tensile strength is the maximum tensile stress which a material is capable of
developing against fracturing. Therefore this parameter would provide an indication
on the resistance of duricrusts against fracturing and degree of bonding of cementing
agents found in duricrusts.
Table 3.7: Tensile strength test results by Fauzilah Ismail (2002)
Sample
No.
1
2
3
4
5
Thickness
(t)
mm
m
28
0.028
28
0.028
29
0.029
26
0.026
40
0.040
Diameter (D)
mm
48
47
49
48
49
m
0.048
0.047
0.049
0.048
0.049
Penetration Load
(P)
kPa
N
1000
1.344
500
0.658
500
0.711
500
0.624
1000
1.960
Tensile
Stress (σt)
MPa
0.94
0.48
0.44
0.50
0.64
48
3.4.6
P-Wave Velocity
P-wave velocity is a useful value because the waves travel faster through
denser materials compared to porous materials. Therefore the value of P-wave
velocity provides an indication on the denseness of duricrusts. It also provides an
indication of weather duricrusts will be pulverized during excavation.
Table 3.8: The result of P-Wave Velocity by Fauzilah Ismail (2002)
Core
No.
A
B
C
D
E
F
G
H
Diameter,
mm
48
48
48
48
48
48
48
48
Length,
mm
106
108
107
106
106
92
103
95
Time (T)
μs
51.1
50.8
54.6
48.6
49.7
43.3
57.6
47.0
m/s
2074
2126
1960
2181
2133
2125
1788
2021
Velocity
km/s
2.07
2.13
1.96
2.18
2.13
2.13
1.79
2.02
f/s
6804≈6800
6975≈7000
6430≈6500
7156≈7200
6998≈7000
6972≈7000
5866≈5900
6631≈6700
CHAPTER 4
RESULT AND ANALYSIS
This chapter discusses the analysis of data obtained from literature reviews
conducted during the course of this study. The information obtained will be used to
determine the types and mode of formation of duricrusts.
4.1
Mode of Formation of Duricrusts
There are a number of reasons which support earlier claims that duricrusts
cannot be classified as rock, specifically clastic sedimentary rock. These reasons are
as listed below:
1. Sediments that form duricrusts are sediments which are formed within zones
of weathering profiles. This indicates that the sediments are residual (in situ)
soils which defers from sediments that form clastic sedimentary rocks which
are transported and deposited into sediment basins.
50
2. The cementation process that takes place for the formation of duricrusts takes
place in situ, i.e. at the location where the parent rocks are being weathered to
residual soils.
3. Compaction does not occur during the formation of duricrusts. This is due to
the fact that the thickness of overburden sediments during the formation of
duricrusts is considerably lesser then that of sediments in a depositional
environment. The higher overburden stress found in depositional
environments makes compaction a mode of formation for clastic sedimentary
rocks and not duricrusts.
4. Climactic regimes play an important role in the formation of various types of
duricrusts due to the fact that arid climates form different types of duricrusts
compared to humid climates.
5. The formation of duricrusts is less extensive in terms of size compared to
sedimentary rocks. This is due to the fact that duricrusts occur in situ as
isolated bodies and as capping or layers between 1 to 3m thick (Mohd For
Mohd Amin et al., 2005). Sedimentary rocks on the other hand go through
the lithification process in a larger scale in depositional environments where
thick sediment layers are buried beneath subsequent layers. This allows the
sediments to go through compaction and cementation forming more extensive
and thicker sedimentary rocks.
From the deduction above, duricrusts cannot be classified as clastic
sedimentary rocks due to its mode of formation. This is because duricrusts suffer
considerably less over burden pressure compared to clastic sedimentary rocks and is
formed in situ (i.e. where the parent rock is weathered).
51
4.2
Material Texture
Through observations on site and in the laboratory as well as literature
reviews carried out, it is noticed that the material texture and cement matrix of both
duricrusts and clastic sedimentary rocks are similar. Both materials exhibit granular
texture with mineral grains imbedded in between cement matrix. The strength of
duricrusts and clastic sedimentary rocks depend on the individual strength of mineral
grains and the cement matrix that holds them together. Cement matrixes that are
related to the formation of duricrusts and clastic sedimentary rocks are Siliceous,
ferrugineous and calcareous (in terms of strongest to weakest) (Waltham, 2002).
From the research carried out, it was found that the range of strength properties of
duricrusts is similar to those of clastic sedimentary rocks.
Figure 4.1 provides an indication regarding the resemblance of material
texture between duricrusts and clastic sedimentary rocks.
(a): Sandstone
(b) Duricrust
Figure 4.1: Close-up photographs of sandstone and duricrusts
52
4.3
Strength and Physical Properties
Data used to determine the strength and physical properties of duricrusts are
those obtained from previous studies (Fauzilah Ismail, 2002; Jerry Chua Kuo Seng,
2004). These data (from Dengkil, Selangor and Pasir Gudang, Johor) can be found
in Chapter 3.
Summarized data (minimum and maximum value) obtained from various
types of tests can be found in Table 4.1. The values are compared with
corresponding values (range of values) found in Table 2.4 through to Table 2.10.
From the comparison the following deductions were made:
1. Density: Range of values is between 1920 – 2060 kg/m3. Duricrusts exhibit
similar range of density as those of sand and shale as observed in Table 2.6.
2.
Compressive strength based on Rebound Number: Range of values is
between 17.38 – 23.99 MPa. The range of the test results fall within the
boundaries of clastic sedimentary rocks as observed in Table 2.4 and Table
2.7. Table 2.10 classifies this range of value as Moderately Weak to
Moderately Strong Rock and it is typical of some weakly compacted
sedimentary rocks and sedimentary rocks.
3. Slake durability index, after first cycle Id1: Range of value is between 80.6 –
96.0 %. Duricrusts are classified as having Medium to Medium High
durability when these values were compared to Table 2.8. This is also typical
of clastic sedimentary rocks of moderate strength (ISRM, 1981).
4. Point load index Is (50): Range of values is between 0.22 – 0.41 MPa.
Referring to Table 2.4 this range of value falls within the properties of clastic
sedimentary rocks. Table 2.9 classifies this range of value as Weak to
Moderately Weak and for this strength classification; it includes some weakly
compacted sedimentary rocks (see Table 2.10).
53
5. Tensile strength: Range of values is between 0.44 – 0.94 MPa. This range of
value falls within the properties of clastic sedimentary rocks as observed in
Table 2.4 and 2.7.
6. Uniaxial compressive strength, UCS: Range of values is between 7.36 –
12.95 MPa. The range is within that of clastic sedimentary rocks when
compared to the values of clastic sedimentary rocks from Table 2.4 and Table
2.7. Table 2.10 classifies this range of value as Moderately Weak to
Moderately Strong Rock and it is typical of some weakly compacted
sedimentary rocks and sedimentary rocks.
7. Seismic (P-wave) velocity: Range of values is between 1788 – 2181 m/s.
This range of velocity is within the range exhibited by medium to moderately
hard shales and sandstones when compared to Table 2.5. It is also indicated
in Table 2.5 that hardpan (a type of duricrusts) is classified under the same
group as these clastic sedimentary rocks.
From the deductions above, it is safe to say that duricrusts exhibit similar
strength and physical properties as clastic sedimentary rocks. As such, mechanical
ripping which is an excavation method used to excavate clastic sedimentary rocks
can be used for the excavation of duricrusts.
80.6
68.5
0.22
Id1, %
Id2, %
Is(50),
(MPa)
0.44
7.36
1788
UCT,
(MPa)
UCS,
(MPa)
Vp, (m/s)
Tensile Strength
Uniaxial Compressive
Strength (UCS)
Seismic Velocity
2181
12.95
0.94
6.57
0.41
89.0
96.0
23.99
2040
Maximum
Value
* Note: Data from references are those listed in Table 2.4 through Table 2.10.
3.64
UCS,
(MPa)
Point Load Index
Slake Durability Index
17.38
qu, (MPa)
Compressive Strength
based on Rebound
Number
1820
ρ, (kg/m3)
Minimum
Value
Density
Material Properties
Laboratory tests on sample
2051
10.51
0.6
5.57
0.34
80.8
89.7
30.00
1916.25
Average
Value
1460 - 3660
10 - 200
2 - 20
1.25 – 12.5
0.1 - 7
-
-
10 - 200
1920 - 2060
Range
*Data from
references
Table 4.1: Summary of test results on samples of duricrusts
Hardpan, medium to moderately hard
shales and sandstones (Table 2.5)
Similar to clastic sedimentary rocks
(Table 2.4 and 2.7)
Moderately weak to moderately strong
rocks, similar to some weakly
compacted sedimentary rocks (Table
2.4, 2.7 and 2.10).
Similar to weak to moderately weak
rocks including weakly compacted
sedimentary rocks (Table 2.4 & 2.9).
Moderately weak to moderately strong
rocks, similar to some weakly
compacted sedimentary rocks (Table
2.4, 2.7 and 2.10).
Classified as medium to medium high
durability material and similar to
moderate strength rocks (Table 2.8).
Similar to sand and shale (Table 2.6)
Remarks
54
55
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
Based on the literature review and data collected from previous studies, the
following conclusions can be made:
1.
Duricrusts have similar material texture, mineralogy, strength and
physical properties as certain clastic sedimentary rocks but cannot be
termed as a clastic sedimentary rock based on its geological origin and
mode of formation.
2.
The difference between duricrusts and clastic sedimentary rocks is the
fact that duricrusts suffer less compaction due to its lower overburden
pressure. Duricrusts have less overburden pressure because the
thickness of overburden sediments during the formation of duricrusts
is considerably lesser then that of sediments in a depositional
environment. Duricrusts also form by the cementation of weathered
in situ sediments unlike clastic sedimentary rocks that form in
sediment basins.
56
3.
Types of duricrusts that can be found in Malaysia are calcrete,
ferricrete and alcrete. Types of duricrust are determined based on its
cementing matrix. The climactic regime of Malaysia also helped in
the duricrust identification process.
4.
Based on the comparisons carried out, it is recommended that
mechanical ripping be conducted as duricrusts fall within the class of
clastic sedimentary rocks
5.2
Recommendations for Future Studies
For future studies it is recommended that more extensive literature reviews be
conducted to further verify the mode of formation of duricrusts. This will help in
justifying the fact that duricrusts cannot be classified as clastic sedimentary rocks
based on their mode of formation.
Studies regarding the engineering properties of duricrusts should also be
looked into to obtain a more comprehensive comparison between duricrusts and
clastic sedimentary rocks. Laboratory works should also be conducted based on
samples obtained from the field to compare these results with results obtained from
previous studies. This will provide an even more conclusive result regarding the
similarities of duricrusts and clastic sedimentary rocks, specifically their material
texture, mineralogy, strength and physical properties.
Other recommendations include obtaining duricrust samples for laboratory
tests from states all over Malaysia. This will help in identifying the different types of
duricrusts that can be found in and around Malaysia.
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