5
CHAPTER II
LITERATURE STUDY
2.1
Introduction
The minimum requirement for a site investigation are that of desk study and
walk-over survey carried out by a competent personnel, who has been carefully briefed
by the leading professionals (e.g. architect and engineer) on the forms and locations of
construction proposed at the site. This approach will be satisfactory where routine
construction is being carried out in well-known and relatively uniform ground
conditions. The desk study and walk-over survey are intended to (Clayton C.R.I.,
Simons N.E. and Matthews M.C., 1995):
(a)
Confirm the presence of the anticipated ground conditions, as a result of
the examination of geological maps and previous ground investigation
records
(b)
Establish that the variability of the sub-soil is likely to be small
(c)
Identify potential construction problems such as requirements of special
plants and machineries
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(d)
Establish the geotechnical limit states (e.g. slope instability and excessive
foundation settlement) which must be incorporated in the design
(e)
Establish the likelihood of presence of hard materials in project site
Items (a), (b), (c), (d) & (e) are scopes of work related to the sub-strata materials
such as excavations, ground treatments and other activities to ensure the success of the
construction.
2.2
Extent and depth of site investigation
Most of the information collected during site investigation will relate to subsurface deposits of rocks and soils. The general objective is to build up a threedimensional picture of the site, which extends laterally and vertically to include all of
the strata likely to be affected by the changes in loading brought about by the proposed
structure.
In order to achieve adequate coverage of the investigation, the spacing of the
points of investigation needs to be based on considerations of both the type of structure
(e.g. narrow or wide; low or tall; heavy or light, etc.) and the nature of the ground
conditions (e.g. rock or soil; firm or soft; homogeneous or stratified, etc.). Borings
should be sunk at strategic points related to the layout, near to locations where
settlement is to be limited. In uniform and homogeneous conditions, boreholes may be
located up to 100m apart; in conditions of lateral or vertical variability the spacing is
accordingly reduced, to as close as 5m in severe cases if the presence of hard material is
anticipated at shallow depth.
7
The depth of investigation is related mainly to the types of material present and
their susceptibility to the structure to be built. Strata likely to be subjected to an increase
in vertical effective stress of 10 percent or more should be included in the investigation.
Another simple, but perhaps not so reliable, rule of thumb is to carry out borings down
to at least 1.5 times the breadth of the foundation; care is required in interpreting this
guideline where multiple foundations are to be installed.
Where rock strata are encountered, it is a good practice to continue all borings
for at least 1.5m into sound unweathered rock. In all cases, a small number of borings
should be taken down significantly deeper, so that no unexpectedly weak layers are
missed. This procedure is also important where presence of hard materials is anticipated
in the area. Samples in the form of cores may be acquired for further assessments (field
and laboratory) on the sample. Perhaps this is essential for acquiring optimum
information on the sub-strata materials so that detailed and informative contract
documentation could be prepared (Witlow Roy, 1995).
2.3
Methods of site investigation
There are various methods of field investigation that include those discussed in
the following sections depending on the number of methods by which the ground can be
investigated, using geophysical techniques. Whilst these methods can be extremely
valuable for ground investigation purposes, there are not in everyday use.
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2.3.1
Trial pits
In cohesive soils and soft rocks, above the water table, trial pits are often
preferable to borings; they are easily dug with a mechanical excavator, or even by hand,
and they have the advantage of exposing the succession of strata for easy visual
examination. The main disadvantage is that they are limited to depths of 2-3 m; perhaps
a little deeper by additional hand-digging. Samples may be taken by hand from the
bottom and sides of the pit. Trial pits are often particularly useful in soils containing
boulders or cobbles, for ground water observations and for locating buried pipes and
services. For verifying the presence of mass body of hard materials, this method may
not be feasible (Witlow Roy, 1995).
2.3.2
Hand auger
The hand auger (also called a post-hole or Iwan auger) is a very simple handtool used for drilling into soft soils down to a maximum of 5-6 m. The usual form
consists of a 100mm diameter half-cylinder clay auger, which is attached, through a
series of 1 m extension rods, to a cross-piece, that may be turned manually at the
surface (Fig. 1.1(a)).To obtain samples, the clay auger is replaced with a 38mm sample
tube attached to a sliding percussion link (Fig. 1.1(b)). By raising the extension
rod/percussion link assembly and forcing it downwards, the sample tube is driven into
the ground at the bottom of the hole. The cross-piece is then rotated to shear off the
bottom end of the sample and the sample tube driven upwards using the percussion link.
This method may also be employed to detect the presence of hard materials (e.g. stiff
clay, dense sand and rock body) or changes in the compactness of sub-strata materials.
9
It is not suitable for penetration through harder materials thus, it is not appropriate for
the determination of depth or thickness of such materials.
Fig. 1.1
Hand auger and sampler: (a) Iwan auger (b) 38mm sample tube
(after Witlow Roy, 1995)
2.3.3
Probing
A wide range of dynamic and static penetrometers are available, with different
types being used in different conditions of sub-strata materials. However, the objective
of all probing is the same, namely to provide a profile of penetration resistance with
depth, in order to give an assessment of the variability of in-situ materials on site.
Probing is carried out rapidly, with simple equipment. It produces simple results, in
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terms of blows per unit depth of penetration, which are generally plotted as blowcount/depth graphs.
One of the most common types of probing is Mackintosh Probe. The
Mackintosh prospecting tool consists of rods which can be threaded together with barrel
connectors and which are normally fitted with a driving point at their base, and a light
hand-operated driving hammer at their top (Fig. 1.2). The tool provides a very
economical method of determining the thickness of soft deposits such as peat.
The driving point is streamlined in longitudinal section with a maximum
diameter of 27mm. The drive hammer has a total weight of about 4kg. The rods are
1.2m long and 12mm dia. The device is often used to provide a depth profile by driving
the point and rods into the ground with equal blows of the full drop height available
from the hammer: the number of blows for each 150mm of penetration is recorded.
When small pockets of stiff clay are to be penetrated, an auger or a core tube can be
substituted for the driving point. The rods can be rotated clockwise at ground level by
using a box spanner and tommy bar. Tools can be pushed into or pulled out of the soil
using a lifting/driving tool. Because of the light hammer weight the Mackintosh probe is
limited in the depths and materials it can penetrate (Clayton C.R.I., Matthews M.C. and
Simons N.E., 1995).
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Fig. 1.2
Mackintosh probe (after Clayton C.R.I., Matthews M.C. and Simons
N.E., 1995)
2.3.4
Percussion Rig Boring
The equipment for Percussion Rig Boring (Wash Boring) consists of a derrick,
power-winch (Fig. 1.3(a)) and a set of drilling tools. A percussion method is used,
whereby the tool assembly is raised by the winch to about 1 m above the bottom of the
hole and then allowed to fall under its own weight, thus driving the cutting tool into the
soil. When the tool becomes full of soil, it is raised to the surface, where disturbed
samples may be taken from its contents. The most usual borehole diameter is 150mm,
but others up to 300mm can be drilled; the maximum depth of exploration, although
dependent on soil type to some extent, is around 50-60 m.
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In compact cohesion-less soils, or where boulders or cobbles are encountered,
the chisel (Fig. 1.3(d)) is used to break up hard materials; fragments and slurry are then
removed using the bailer.
In wet conditions and in loose soils, and for very deep holes, a casing must be
installed near the surface. This usually consists of steel tubes, screwed together in as
many lengths as appropriate, and jacked or knocked into the drilled hole as drilling
proceeds. They can be hauled out after completion of drilling or left in place if further
observations are required (Witlow Roy, 1995).
Fig. 1.3
Percussion drilling equipment: (a) Derrick and winch; (b) Bailer; (c) Clay
cutter; (d) Chisel (after Witlow Roy, 1995 )
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2.3.5
Core-drilling
In stiff soils and rocks power-operated core-drills are used, consisting of smalldiameter hollow tube, fitted at the lower end with a coring bit (Fig. 1.4). The core barrel
is rotated at speeds ranging between 600 and 1200 rpm, a controlled pressure applied
and water circulated through the bit. The fragments removed in the annular cut are
brought to the surface with the circulating water as the core fills the barrel. A drilling
run of 1-3 m is usually made before raising the barrel and removing the core. The more
usual standard sizes of core barrel used in site investigation range between 30 and 100
mm (hole diameter), although larger-diameter equipment is available for special uses
(Witlow Roy, 1995).
Fig. 1.4
Rotary coring equipment: (a) Double-tube core barrel; (b) Coring bits
(after Witlow Roy, 1995)
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2.3.5.1 State of Recovery of Core
The state of rock cores recovered is largely a function of the drilling method,
and the amount of care employed by the driller during a core run and extraction of core
from the core-barrel, and hence these factors must be considered when assessing core
recovery and fracture state. The nature and amount of core recovered from good careful
drilling can provide a valuable indication of the in-situ condition and probable
engineering behavior of the rock mass. In any core recovered there will be fractures of
natural and artificial origin. It is important that natural fracturing is distinguished from
artificial fracturing on the log. Artificially induced fractures should not be ignored since
they may assist in the assessment of rock excavation (Clayton C.R.I., Simons N.E. &
Matthews M.C., 1995).
The core recovered can be divided into five categories:
(i) solid core greater than 0.1m in length
(ii) solid core less than 0.1m in length
(iii) fragmental material not recovered as core
(iv) additional material which may have been lost from the previous core
(v) reduced length and/or diameter of core due to erosion of soft or friable
material
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Table 2.1: Methods of classifying the quality of rock cores (after Clayton, Simons & Matthews,
1995)
_____________________________________________________________________________
Category of
Classification
Definition
core considered
Remarks
_____________________________________________________________________________
totalcore
percentage of the (i) (ii) (iii) (v)
gives indication of
recovery
rock recovered
i.e. all the core
material that has been
during a single
placed in the
washed into
coring ‘run’
core box
suspension or the
presence of natural
voids
solid core
recovery
percentage of full (i) and (ii)
diameter core
recovered during
a single coring
‘run’
rock quality
percentage of
designation
constant diameter
(R.Q.D) (Deere) solid core
1964)
greater than 0.1m in
length recovered
during a single
coring ‘run’
Stability index
(Ege, 1968)
(i)
gives indication
of fracture state
can give indication
of fracture state
but does not take
changes in core
diameter into account.
The diameter of the
core should preferably
not be less than 55mm
(NWX or NWM size)
index no.= 0.1 x (i)(ii)(iii)(v)
can give indication
core loss (length
of fracture state
drilled – total
but does not take
recovery x 10ֿ²
changes in core
+
diameter into account
no. of fractures
per 0.3m (1ft)
+
0.1 x broken core
(core < 7.5cm in
length)
+
weathering (graded
1-4 from fresh to
completely
weathered)
+
hardness (graded
1-4 from very hard
to incompetent)
____________________________________________________________________________
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The quality of rock recovered may be classified in terms of total or solid core
recovery or in terms of a quality index such as Rock Quality Designation (RQD) or
stability index, provided only natural fractures are considered. The definitions of these
terms are given in Table 2.1 above. Solid core recovery, RQD and stability index may
be used as criteria for a quantitative description of the fracture state of the cores. The
simplest of these is solid core recovery, particularly when contrasted with total core
recovery. The stability index is the most complicated method of assessing rock quality
and hence in most cases RQD is used in preference.
Deere et al introduced the concept of RQD as a means of classifying rock
masses. The RQD is a more general measure than fracture frequency and is based
indirectly on both the degree of fracturing and the amount of weathering in the rock
mass.
One of the most recent classifications has been advanced by Beniawaski. This
initially incorporated the RQD: the unconfined compressive strength; the degree of
weathering; the spacing, orientation, separation and continuity of the discontinuities; as
well as the ground water flow. The grades of rock quality related to RQD are given in
Table 2.2. Unfortunately, however, the RQD takes no account of the spacing,
orientation, tightness, roughness of the surface or continuity of discontinuities, or the
presence and character of infilling material.
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Table 2.2: Description of the weathering zones of the in-situ rock (after Attewell, 1993)
Weathering Zone
(material grade)
Zone 6 (Grade VI)
Descriptive terms
Material description and likely engineering characteristics
Residual soil.
Zone 5 (Grade V)
Completely weathered
Zone 4 (Grade IV)
Highly weathered
Zone 3 (Grade III)
Moderately weathered
Zone 2 (Grade II)
Slightly weathered
Zone 1 (Grade I)
Fresh
Completely degraded to a soil; original rock fabric is
completely absent; exhibit large volume change; the soil has not
been significantly transported.
Stability on slopes relies upon vegetation rooting and substantial
erosion & local failures if preventive measures are not taken.
Rock is substantially discolored and has broken down to a soil
but with original fabric (mineral arrangement & relict joints)
still intact; the soil properties depend on the composition of the
parent rock.
Can be excavated by hand or ripped relatively easily. Not
suitable as foundation for large structures. May be unstable in
steep cuttings and exposes surfaces will require erosion
protection.
Rock is substantially discolored and more than 50% of the
material is in degraded soil condition; the original fabric near to
the discontinuity surfaces have been altered to a greater depth; a
deeply weathered, originally strong rock, may show evidence of
fresh rock as a discontinuous framework or as core stone; an
originally weak rock will have been substantially altered, with
perhaps small relict blocks but little evidence of the original
structure.
Likely engineering characteristics are as in Zone 5.
Rock is significantly discolored; discontinuities will tend to be
opened by weathering process and discoloration have penetrated
inwards from the discontinuity surfaces; less than 50% of the
rock material is decomposed or disintegrated to a soil; rock
samples containing discoloration are noticeably weaker than the
fresh un-discolored rock; an originally weak rock will comprise
relict blocks of substantially weathered material.
Occasionally may be excavated without blasting or cutting (i.e.
by block leverage at the discontinuities); will be relatively
easily crushed by construction plant moving over it in situ, may
be suitable as rock foundation (with some reinforcements);
joints may exhibit lower strength characteristics, so rendering
side slopes unstable.
Some discoloration on and adjacent to discontinuity surfaces;
discolored rock is not significantly weaker than un-discolored
fresh rock; weak (soft) parent rock may show penetration of
discoloration.
Normally requires blasting or cutting for excavation; suitable as
a foundation rock but with open jointing will tend to be very
permeable.
No visible sign of rock material weathering; any internal
discoloration or disintegration.
Normally requires blasting or cutting for excavation; may
require minimal reinforcement in cut slope unless rock mass is
closely jointed.
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2.3.6
Weathering
Weathering of rocks is brought about by physical disintegration, chemical
decomposition and biological activity. The type of weathering which predominates in a
region is largely dependent upon climate, which also affects the rate at which
weathering proceeds. The latter is also influenced by the stability of the rock mass
concerned, which in turn depends upon its mineral composition, texture and porosity,
and the incidence of discontinuities within it.
Many rocks were originally formed at high temperatures and pressures and a
large part of the weathering process consists of an attempt to reach a new stability under
atmospheric conditions. High temperature minerals occur in the ultra basic and basic
igneous rocks. Hence such rocks tend to offer less resistance to weathering than the acid
igneous rocks which are largely composed of soda and potash feldspar, quartz and, to a
lesser extent, mica. The latter two minerals are particularly stable. Generally coarse
grained rocks weather more rapidly than do fine grained types of similar mineral
composition (Bell F.G., 1983).
2.3.7
Geophysical Methods
Certain geophysical properties, such as electrical resistance, elasticity, magnetic
susceptibility, etc., vary from stratum; a stratum boundary is therefore indicated by an
anomaly in the measurements of the particular property (Witlow Roy, 1995).
Geophysical methods do not actually measure engineering properties, hence
they provide indirect methods of soil exploration; they can be used economically to
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determine soil stratum boundaries, to locate bedrock and water table levels, to detect
organic soil areas and the presence of sub-surface cavities. In all cases, geophysical data
needs to be correlated with information gathered from borings or trial pits. The
geophysical method is one of the most common supplementary methods used for
detecting the presence of hard materials on site.
2.3.7.1 Seismic refraction method
Seismic refraction is possibly the most important and commonly used
supplementary methods in site investigation. This is because if used in the correct
manner under favorable conditions, it can provide reliable quantitative data relating to
the geological section across a site. The seismic refraction method can also provide the
necessary data for assessing the quality of rock and soil masses in engineering terms.
Seismic waves travel at different velocities through different types of material;
several factors affect the velocity of shock-wave propagation, such as density, moisture
content, and texture, presence of voids or discontinuities and elasticity. The method of
seismic refraction involves generating a sound wave in the rock or soil, using a
sledgehammer, a falling weight or a small explosive charge, and then recording its
reception at a series of geophones located at various distances from the shot-point (Fig.
1.5(a)). The time of the first sound arrival at each geophone is noted from the pen-trace
of a continuous recorder.
For geophones close to the shotpoint, the direct wave, traveling with velocity VA
in the upper layer A, arrives first. If the velocity in the lower layer is greater, then at
some distance away from the shotpoint, the refractive wave, traveling with velocity VB
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in the lower layer B and with velocity VA in the upper layer, will arrive first. The first
arrival times are plotted against distance from the shotpoint (Fig. 1.5(b)), to give two
time/distance straight-line curves of slopes 1/VA and 1/VB respectively. If the
intersection of the two slopes occurs at ‘distance’ X, it can be shown that the depth of
layer A is given by:
hA = 1/2 X[(VB – VA)/(VB + VA )]½
The seismic refraction method can be used for explorations down to about
300m, but it is a prerequisite that the wave velocity in the upper layer must be less than
that in the lower layer(s), i.e. VB > VA. This condition arises from the fact that for the
waves to be refracted along the boundary they have to arrive along a path which lies at
the critical angle ic to the boundary normal, such that sin ic = VA/ VB. In a multiplayer
series of stratum a ‘blind’ layer will occur when its wave velocity is less than that in the
overlaying layer: the direct wave will pass through a blind layer perpendicular to the
boundary and is then not refracted. Other errors may be caused due to the fact that lithological boundaries do not always correspond with boundaries between strata of different
wave velocity; also in isotropic strata there may be a difference between the vertical and
horizontal velocities (Witlow Roy, 1995).
The accuracy of velocity and depth determinations together with the chances of
actually detecting different strata (or other geological bodies) is very much dependent
on velocity contrast between different media. Providing the velocities of the layers
increase with depth, in general the greater the velocity contrast the greater the
confidence in identifying different strata and the greater the accuracy of depth
determinations (Clayton C.R.I., Simons N.E. & Matthews M.C., 1995).
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Seismic refraction method is suitable for detecting rock bodies at depth due to
its effectiveness, relatively cheap and wider coverage area compared to borehole. This
method is always used in complementary with boreholes.
In some cases once a limitation is recognized, it may be overcome, either by
modification of the refraction method by special field techniques or by integrating the
seismic data with other geophysical data or borehole data. It is important that as the
direct investigation progresses the borehole data are made available to the geophysicist
so that the geophysical interpretation may be continually updated and limitations
causing ambiguities and erroneous interpretations overcome where possible. Direct
investigations should be flexible enough to be modified by any updated geophysical
interpretations where necessary.
Seismic refraction methods are used mainly for quantitative depth
determinations and profiling velocity interfaces to obtain a more complete geological
section than can be achieved using direct methods of investigation. The refraction
method may also be used to asses the quality of rock and soil masses in engineering
terms (Clayton C.R.I., Simons N.E. & Matthews M.C., 1995).
Fig. 1.5
Seismic refraction method (after Witlow Roy, 1995)
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2.4
In-situ Tests and data collected
Type of data collected includes essential information on the degree of hardness
and strength of the materials and parameters that affect their excavatability on site.
Relevant field and laboratory strength tests provide numerical values for assessment and
comparison purposes. The tests conducted include uniaxial compression, rebound
hammer, point-load, Brazilian, and slake durability and ultrasonic velocity test.
Relevant properties of the tested materials are then analyzed and plotted on appropriate
charts in order to evaluate their degree of excavatability variations (Mohd For Mohd
Amin & Edy Tonnizam Mohamad, Universiti Teknologi Malaysia, 2003).
2.4.1
In-situ Tests
Several field index tests can be conducted on the exposed rocks in order to
assess their in-situ strengths and surface hardness. These tests include Point Load and
Rebound Hammer. The rebound number is a measure (index value) of surface hardness
of a material. If test is conducted on weak materials (e.g. clays and highly weathered
rock) rebound number will be negligible as soft surface will not induce any rebound
variations (Mohd For Mohd Amin & Edy Tonnizam Mohamad, Universiti Teknologi
Malaysia, 2003).
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2.5
Laboratory Tests
Block samples from the study can be collected for laboratory tests. Laboratory
tests give a more detailed and specific data for the purpose of assessments. Moreover,
equipments used enable direct strengths, rather than index properties, of sample to be
obtained. Tests on specific shape and size of sample helps to minimise random error
resulting from sample variations (Mohd For Mohd Amin & Edy Tonnizam Mohamad,
Universiti Teknologi Malaysia, 2003).
Types of laboratory tests usually conducted for the assessments on the degree of
excavatability of rock material on site are:

Dry Density test.

Rebound hammer (L-type) test.

Slake Durability Index test.

PUNDIT or Ultrasonic Velocity test.

Brazilian (indirect tensile) test.

Uniaxial Compression test (UCT).

Point Load Index Test.