Missouri University of Science and Technology
Scholars' Mine
International Conference on Case Histories in
Geotechnical Engineering
(1988) - Second International Conference on Case
Histories in Geotechnical Engineering
Jun 1st
Foundation Failures Associated with Salt Rock and
Surrounding Coastal Plain
A. Orhan Erol
King Saud University, Saudi Arabia
Abdulmohsin W. Dhowian
King Saud University, Saudi Arabia
Follow this and additional works at: http://scholarsmine.mst.edu/icchge
Part of the Geotechnical Engineering Commons
Recommended Citation
A. Orhan Erol and Abdulmohsin W. Dhowian, "Foundation Failures Associated with Salt Rock and Surrounding Coastal Plain" ( June
1, 1988). International Conference on Case Histories in Geotechnical Engineering. Paper 14.
http://scholarsmine.mst.edu/icchge/2icchge/2icchge-session2/14
This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International
Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright
Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact
scholarsmine@mst.edu.
Proceedings: Second International Conference on Case Histories in Geotechnical Engineering, June 1-5,1988, St. Louis, Mo., Paper No. 2.39
Foundation Failures Associated with Salt Rock
and Surrounding Coastal Plain
A. Orhan Erol
Abdulmohsin W. Dhowian
Civil Engineering Department, King Saud University, Riyadh,
Saudi Arabia
Civil Engineering Department, King Saud University, Riyadh,
Saudi Arabia
SYNOPSIS: A detailed investigtion has been conducted to study the causes of severe and widespread
damages in a town situated on a salt dome. The study includes a thorough assessment of ground conditions in the coastal plain surrounding the salt dome for possible utilization of the plain as
redevelopment areas.
Major geological features in the study area comprised of collapsible
windblown sands which are underlain by salt rock in the old town and soft/loose sabkha deposits in
the proposed redevelopment zones.
It has been found that the extensive damages in the buildings
and roads are primarily due to subsidence phenomenon associated with the dissolution of salt rock.
The properties of the sabkha sediments and anticipated geotechnical problems have been described.
INTRODUCTION
A detailed investigation has been conducted
to study the causes of the severe and
widespread structural damages in the city of
Jazan, situated in a coastal plain on southwest of Saudi Arabia. In the city more than
120 buildings are damaged beyond repair.
The problem would have been more apparent if
large
tracts
of
severely
damaged
or
collapsed buildings had not been demolished.
The investigation program consisted of an
extensive
drilling
program,
laboratory
testing, field load tests, survey of damaged
buildings and, geological and geophysical
studies.
It has been found that the dramatic damages in the town is primarily associated with the unusual ground conditions
prevailing in the region.
This paper presents an assessment of ground conditions,
with reference to typical engineering geological features of the region and their
implications
in
identifying
the
major
geotechnical factors which have contributed
to the failures of the structures.
ELEVATED
r
--..
WEST .___. E.lST
SAND DUt'£5
CAPlOCU
Fig. 1
Representative cross section of
Jazan region
movement is probably due to the lower density of ·the salt as compared to the
surrounding country rocks. As the salt dome
reaches
to
the
surface,
most
of
the
overlying country rock and caprock are subsequently removed by erosional processes.
There are few outcrops of the salt body and
the thickness of the soil underlain by salt
rock varies over a range 6m to 15m in the
town. The topography of the rock surface
exhibits
considerable
variations
within
short distances.
Highly irregular surface
of the salt dome is an evidence of active
erosion of the salt through dissolution processes.
GEOLOGICAL FEATURES
The town of Jazan is situated on an elevated
terrain underlain by a salt dome measuring 4
sq.
kms in area and reaching about SOm.
above- the sea level. The salt dome is
surrounded by a coastal plain in ·the southwest corner of Saudi Arabia.
The coastal
plain which is planned as the redevelopment
area in the region, stretches north and
south for approximately 190 kms.
A schematic geological cross section of the region
is shown in Fig. 1.
An aeolian sand mantle overlies the salt
dome.
The sand has uniform grain sized
distribution, typical for windblown deposits
and its relative density varies from medium
dense to very dense depending on the depth,
water content and degree of cementation.
The salt dome is the predominant structural
feature in the coastal plain.
It protrudes
about SOm above the surrounding sabkha
flats.
Throughout the geologic time since
the formation of salt deposits and its
burial, the salt dome gradually pierced
through the overlying country rocks. This
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
TERRA~
~~;'sko"'..)
The caprocks which are formed upon the
rising salt dome come in contact with ground
237
water, appear as high ridges, as shown in
Fig. 1.
The gypsum and unhydrite hills
enclose the low lying area which is covered
by sand dune deposits.
This topographic
feature can be attributed to a long term
collapse mechanism associated with the
dissolution of salt rock by groundwater
(Al-Muhendis, 1985).
N
t
The area surrounding the elevated terrain
(salt dome) is essentially flat, and is
covered by thick sediments of soft/loose
organic saline soils which are referred to
as sabkha deposits in published literature
(Akili et al, 1981). In the sabkha terrain
three zones comprise the soil profile: i.
sabkha
crust;
ii.
compressible
sabkha
complex and; iii. sabkha base. The sabkha
crust has an average thickness of 1. Sm and
is a mixture of fine sand and silt size particles encrusted
by
precipitations of
soluble salts resulting from evaporation of
saline groundwater. The middle zone is composed of soils varying from non-plastic fine
sands to highly plastic organic clays.
Variety of materials, mutually interlayered
at random, occur in sabkha complexes without exhibiting any real stratification. The
distinct feature of sabkha deposits is their
poor mechanical properties and heterogeneous
nature. Their highly variable compressibilities coupled with significantly low bearing
resistances often give rise to serious
geotechnical problems ( Jullie et al, 1983;
Khan et al, 1981; Akili, 1981). The plasticity characteristics of the sabkhas from
Jazan region, shown in Fig. 2., reflect the
variable nature of the sediments.
~IQUID
LIMIT,
lim)H-IAIID
am-·
...._
c::::JIAIIDOUMl
r:21s&a-
•OAMAGIO
Fig. 3 The distribution of damaged
structures in the old town
~L. '1.
Fig. 2 Consistency limits of sabkha
sediments
GEOTECHNICAL PROBLEMS
Subsidence in the Salt Dome Region:
The most populated part of the city is
situated on the elevated terrain which is
underlain by the salt dome, In this section
of the city numerous buildings have been
damaged beyond repair . The distribution of
existing damaged structures are shown in
Fig.3.
Typical extent of damages observed
in the distressed buildings are illustrated
in Fig, 4.
In some heavily damaged
buildings the
ground movements
caused
Fig. 4
238
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
Damaged school building and ice
factory in the salt dome region
displacements of foundations in the order of
meters.
Excessive differential movements
have been observed even under fence walls
and pavements, as shown in Fig. 5, where the
foundation loading is not significant. Such
magnitude of ground movements can not be
attributed to compressibility of foundation
soils. Evidently the case is a typical subsidence problem arising from the dissolution
of the salt rock.
rainfall confirms that the salt dissolution
and subsequent ground subsidence is the
principal contributor to the damages.
ELAPSED TIME, WEEKS
0
~
2
4
5
6
5
E
E 10
..z:
Ill
:::£
Ill
....~
15
II.
1/)
c
20
0 OBSERVATION
PT. 6
0 OBSERVATION PT. a
25~------------------------------~
Figt.6 Ground movements observed in the
inundated site
Collapsible Nature of Windblown Sands:
Fig. 5
The study revealed that the second major
geotechnical problem in the salt dome region
arises
from collapse potential of the
windblown sand mantle.
In the study area,
the ground water table is at the boundary of
rock surface with the overlying soils.
It
has been found that the groundwater has
significantly
high
salt
concentration
because of dissolution of the salt rock.
Capillary rise of salty groundwater and its
rapid evaporation due to hot and arid climatic conditions prevailing in the region,
lead to precipitation of water soluble salts
at shallow depths.
The evaporite contents
of the sand deposit at shallow depths is
given in Fig. 7. The data shown in Fig. 7
indicate that the soluble choloride and
sulfate contents are in the range of
(1.0-2.5) and (0.25-0.75) percent, respectively. Evidently these salts together with
the capillary stresses provide a temporary
bonding among the soil grains.
The temporary cementation looses its effectiveness
on soaking and consequently the soil is
weakened and is susceptible to collapse
(Dudley, 1970).
Damages in the fence wall and
pavement
Halite or salt rock is known as highly
soluble in water and numerous case histories
associted with the failure of structures due
to dissolution and subsequent subsidence of
salt rock has been reported in published
literature (James et al., 1980;
Durie et
al., 1964; Drescher, 1984). The ground subsidence observed in the location of damaged
structures appeared as local sinkhole type
depressions as well as linear depressions
associated with solution channels.
It is
noted that the occurrance of damaged zones
in the city is erratic. Occasionally stable
or slightly distressed zones can be found
nearby the heavily damaged buildings.
Other
field
evidences
reflecting
the
occurrance
of subsidence phenomenon were
the followings: i. highly irregular surface
of the salt body which is an indication of
solution features; ii. loss of drilling
fluid near the top of the salt rock in
boring operations.
In order to ascertain that the dissolution
of salt rock is the primary cause of the
damages, some field measurements were taken
in
the
site
of
an
deserted
factory
buildings.
The movements at the ground
level were monitored while the site was
artificially inundated.
The displacements
measured upon wetting of the site are shown
in Fig. 6.
It is interesting to note that
the
elapsed
time
of
4
weeks
which
corresponds to noticeable acceleration of
settlements in Fig. 6, coincides with a
heavy rainfall which lasted few hours in the
city.
Almost immediate response of the
ground to the artificial wetting and the
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
~ 12
.,1
~~
0
~
>
..
...a:
"'
...
•
...
0
z
..
a:
Ill
0
0
0
Cl
0
z
PERCENT SULPHATE
Fig.7
239
PERCENT CHOLOROIE
Evaporite contents in windblown sand
The results of double oedometer tests on
cemented
sand
specimens
indicated
that
collapse severity of the sand fall into
"moderate trouble" to "trouble" categories
according to the collapse classification
criteria proposed by Knight (1963).
The
studies
on
collapse
behaviour
of
the
windblown sands included in-situ plate load
tests which are performed in natural and
inundated state. The load-displacement curves obtained from plate load tests, in Fig.
8, confirm the relatively high collapse
potential of the soils predicted by the
double oedometer tests. Thus it is believed
that some of the damages in the region may
be attributed
to
the collapse of
the
windblown sands due to wetting of the ground
following
the
occupation of
the
land.
However, collapse phenomenon can not solely
account for the unprecedented magnitude of
the ground movements experienced in the
region.
The subsidence phenomenon is to be
considered as
the major contributor to
ground displacements.
tiona! problems that may be encountered on
sabkha plains (Ellis, 1973; Kinsman et al.
1971; Johnson et al. 1978).
From geotechnical engineering point of view,
sabkha sediments are highly variable in
lateral and vertical extent.
Variations
have been noted in terms of layering, soil
composition, cementation, organic content
and mineral constituent.
Lateral variation
appears to depend on proximity with respect
to shore line, whereas vertical variation
arises from development sequence of sabkha
sediments, prevailing depositional environment, and subsequent diagenesis (Akil i et
al. 1981). The borehole profiles given in
Fig. 9 from an area measu.ring 40x40 sqm in
the sabkha terrain reflect the heterogenity
of the sabkha profiles.
The heterogeneous
character of the deposits lead to difficulties and uncertainties in the analysis
of sabkha behaviour.
... 2 ...2
:z!
:z!
. .
0
~
+o
...
2
0
.~
SPT. N
0
20 40
0
2
.11.0111
~
.
~
4
6
E
:c 8
6
'!\
E
·e
\
'
ffi
\
~
....
10
12
12
14
~~
q
"'0
m
I
I
o NATURAL
I
' .......
\
\
. . "'D-___
0
C INUNDATED
\\
\
.110.5m
SABKHA CRUST (RELATIVELY INCOMPRESSIBLE I
l'i"iiJ ~BKHA
I
.. ....
.
....
:IE
0
u
.t'.
16
\
~ 1
c.
BASE (I NCOMPRESSIBLEI
COMRESSIBLE
SABKHA (HIGHLY COMRESSIBLE)
PLATE DIAN:
300mm
--c-...---c>----...0
Fig. 9
Fig. 8
a.
~ 10
\
•12
iii a:
......
"'"'
a:c
.......
~
·~
.....
8
60
Another distinguishing feature of sabkha
flats is the presence of highly saline
ground water table which lies within two
meters from the surface. The salts precipitate at or near the surface as a result of
evaporation and dessication and form a relatively hard crusty surfaces.
The salt
encrusted surfaces are sufficiently strong
and durable. However they become impassible
upon wetting due to rainfall or storm tides.
The salt encrustations are readily visible
with their puffy appearance as shown in Fig.
10.
Unstable nature of sabkha crusts and
their
corrosive
attack
on
foundation
materials
create
serious
constructional
problems.
Typical plate load test data
reflecting the collapsing nature
of windblown sands.
Problems with Sabkha Sediments:
Due to untolerablJ property losses and high
risks involved in building on salt dome
terrains, utilization of the sabkha plains
as residential areas have been considered.
The site investigation program was extended
into sabkha plains to determine the subsoil
conditions
in the proposed development
areas. The study revealed that sabkha soils
present series of engineering problems associated with their variable nature, poor
mechanical properties and high evaporite and
organic contents.
Although development of
sabkha deposits and their material properties are well documented in the published
literature, limited information is available
concerning the nature and range of construe-
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
Variations in sabkha subsurface
profiles
The range of undrained shear strengths
measured on sabkha deposits are shown in
Fig. 11.
The undrained shear strengths
obtained from field vane tests, which are
referred to as ·peak and remolded strengths
in Fig. 11, and various laboratory tests
240
vary over a range from 5 to 40 KPa at
shallow depths.
This magnitude of soil
strength is not adequate to support most of
the common engineering structures through
conventional
shallow
foundation
systems.
Thus
either
special
foundation
systems
and/or ground improvement techniques are to
be considered to avoid possible failures of
foundations.
Presently the phase of the
study which concerns with selection of the
most appropriate foundation methodology for
the particular ground conditions is under
consideration.
indices, Cc
vary over a wide range (i.e.
from 0.1 to 1. 0) depending on · predominant
soil type in the stratum, and correlates
well with the reciprocal of dry density.
Relatively high magnitudes of Cc clearly
reflects the compressible nature of the
sabkhas.
This observation is confirmed by
the settlements monitored in the instrumented test section where a total settlement of about 200 mm were beasured under an
embankment loading of 48.6 KPa. This magnitude of settlements under moderate loading
conditions is beyond the permissible range
for most engineering structures.
Moreover,
potential
variation
of
compressibility
characteristics of sabkha sediments may lead
to excessive differential settlements and
subsequent failures.
Additional
settlement problems are to be
due
to
potential
secondary
compress1on associated with the relatively
high organic content of sabkha sediments,
The sabkhas encountered in the region, contain appreciable organic matter (i.e.
from
3% to 9%).
anticipa~ed
• ML
1.0
.J
o CL
(e)
+ OH
o SM,SC
@ DISREGARDED
0.8
~
a;
Fig. 10
z
Appearance of sabkha crust
0.6
0
...iii
"'f
r 2a 0.932
0.,
r s Cartelotlon
c..lll cl.,,
%
UNDRAINED STRENGTH, Cu,
0
0
10 20 30
«)
110 110 70 0
OH,MH
0
u
~Po
10 20 30
«)
0.2
110 110 70 110 90
CL,ML
dO~--~---T----r---~--~---r---T--~
\
u
~
~
~
u
~
w
u
u
RECIPROCAL OF DRY DENSITY, id ,m•ll
4
E
Fig. 12
a
%
:;:
7
"'0
•
The
in-situ measurements
indicated
that
field settlements extend out on to a secondary consolidation phase within a relatively
short period of time. The settlement logtime plots given in Fig. 13 are represented
by two straight lines which are typical for
organic soils and taken to indicate primary
and secondary compression ranges (Long et
al.
1984).
The secondary consolidation
problem which
is usually overlooked
in
routine geotechnical investigations appears
to be a serious problem in sabkha sediments
since secondary compression may reach to
significant magnitudes as evidenced by the
field data.
s
10
11 +-.~,=,n=o~Y~~r.~~r~A~K----~~~~~~--~~~~
o
Fig. 11
"!LOY~!,
R04DLO!D
Shear strength characteristics
of sabkha sediments
Laboratory and field
investigations were
carried out to study the compressibility
characteristics
of
the
sabkhas
in
the
region.
Laboratory
studies
included
numerous
oedometer tests
on undisturbed
samples.
An instrumented test embankment
was constructed to investigate the in-situ
compressibility behaviour of sabkha profiles.
The range of compression indices of
different soil types obtained from oedometer
tests are shown in Fig.
12. Compression
The comparison coefficient of consolidation,
Cv' obtained from laboratory tests with
those
determined
from
in-situ
timesettlement behaviour of the test embankment
revealed that the average ratio of field to
laboratory c~ is in the order of 55. This
241
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
Compressibility characteristics
of sabkha sediments
discrepancy can be attributed to a more
efficient
than
anticipated
subsurface
drainage system due to presence of thin permeable sublayers.
It is conceivable that
the inclusion of permeable sublayers into
the sabkha complex serve to accelerate the
consolidation processes. The comparison of
Or-----------------------~
20
60
e ao
e
.,_· 100
z
Ill
15
1
~
1-
t;j 140
C/)
a PL: 1.4 ICORN !R I
meters are essential to achieve reliable
predictions of
time-settlement behaviour
particularly
if
preloading
schemes
are
employed to improve the ground conditions.
CONCLUSIONS
A
comprehensive
subsurface
exploration
program has been undertaken to investigate
the causes of severe and widespread damages
in a town situated on the southwest coast of
Saudi Arabia.
The scope of the investigation was to identify the factors contributing to the structural damages, and to
propose an urban development scheme for new
residential areas. The existing town is
situated on a salt dome which is covered by
an aeolian mantle.
The salt dome is
surrounded by salt bearing recent sediments,
known as sabkha.
The study has revealed
that the ground conditions present the
following geotechnical and constructional
problems.
1.
The structural damages are primarily due
to subsidence phenomenon associated with
the dissolution of rock salt.
2.
Evaporative pumping processes in desert
environment are responsible for the
accumulation of evaporite minerals at
shallow depths.
The collapse potential
of the aeolian mantle · is attributed to
the weak cementation of soil grains by
the evaporites~ and to be considered as
a major geotechnical problem in the
region.
3.
Sabkha sediments are highly compressible
materials with low bearing resistances
and
serious
foundation
instability
problems are to be expected upon the
utilization of sabkha plains for urban
development.
4.
Analysis of sabkha behaviour is complicated by the composite and heterogeneous
nature of sabkha profiles with respect
to soil composition and stratification.
Relatively high organic
content of
sabkhas and existances of salt encrusted
surfaces present potential enginering
problems.
5.
Measures must be taken to protect the
construction
materials
from
highly
corrosive effect of the salts present in
sabkhas, as well as wind blown sands
overlying the salt dome.
+ PL: 2.3 [EDGE I
180 o PL: 5 I'I:ENTERI
200.__...._...._..................1...-f::----1.---L.......I....&..-L.I:::I
10
100
LOG TIME, days
Fig. 13
In-situ settlements versus logtime plots showing the secondary
consolidation effects
the observed and predicted settlements given
in Fig. 14 indicate that despite the primary
consolidation settlements can be predicted
from laboratory determined parameters within
acceptable limits, time-settlement predictions are far from being realistic.
This
observation stresses the fact that the
routine sampling and laboratory testing
methods are not adequate for heterogeneous
and complex subsoil profiles to ·represent
the in-situ conditions. Large scale loading
tests with the measurement of in-situ para-
160
e
e
PR!DICTitD I'RDN fi!LO Cv
-o--
OISEIIIV!D SETTLEICIITS (CINT!R)
PREDICTED 1'RD14 LAi.
20
3D
10
TIME IN DAYS
Fig . 14
Comparison of measured and predicted time settlement behaviour
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
ACKNOWLEDGEMENT
The study described in this paper is supported by a
research grant from King
Abdulaziz City for Science and Technology,
Saudi Arabia~ Grant No. KACST-8-106.
Part
of the field data presented is published
with the kind permission of the Ministry of
Municipal and Rural Affairs.
RBPBRBNCBS
Akili, w. 1981 •on sabkha sands of Eastern
Saudi Arabia•, symp. on Geotechnical
Problems in Saudi Arabia, Vol. 2, pp.,
775-793.
Akili,. w., J.K. Torrance, 1981 •The development and geotechnical problems of sabkha,
with preliminary experiments on the static
penetration resistance of cemented sand•
Q.J. of Bngnr. Geology, Vol. 14, No. 1,
pp. 59-74.
Al-Muhendis, 1985, :Jazan Town Planning and
Engineering
Studies•,
Structural
Unpublished Report, Ministry of Municipal
and Rural Affairs, Kingdom of Saudi
Arabia.
Drescher, G.J. 1984 •underground problems in
the subsidence area of Lueneburg city•, x.
ICSMFE, Vol. 3, PP• 83-87, Stockholm.
Dudley, J.B. 1970, •Review of collapsing
soils• ASCE, Journal of Soil Mechanics and
Foundations Division, Vol. 96, No. SM3,
PP• 925-947
Durie, R.W., F.W. Jessen 1964 •Mechanism of
the dissolution of salt in the formation
of underground salt cavities•, Soc. Pet.
Eng. J. Vol. 4, PP• 183-190.
Ellis, C.I., •Arabian salt bearing soil
(sabkha) as an engineering material•,
Transp. Rd. Res. Lab. Report. LR 523
Crowthorne, Berkshire, U.K., 19/3.
James, A.N., I.M. Kirkpatrick, 1980, •oesign
of foundations of dams containing soluble
rocks and soils•, Quarterly Journal of
Engineering Geology, Vol. 13, pp. 189-198.
Johnson, D.B., Kamal, M.R., Pierson, G.o.
and Ramsay J.B., •sabkhas of Eastern Saudi
Arabia•, In: Al-Sayyari, s.s. and zotl,
J. G. ( ed. ) , Quarternary
in Saudi
Arabia, Springer-verlag, ~-93, 1978.
Hriod
Juillie,
Y.,
D.B.
Sherwood,
1983,
•Improvement of Sabkha soils of the
Arabian Gulf Coast•,
~provement
of
Ground, Proc. of 8th European Conf. on
SMPB, PP• 781-788.
Khan, I.&., S.I. Basnain, 1981, •Engineering
properties of Sabkha soils in the Bengazi
plain
and
construction
problems•,
Engineering Geology, Vol. 17, pp. 175-183.
Kinsman, D.J.J., Park, R.K., and Patterson,
R.J., •sabkhas: studies in recent carbonate
sedimentation and diagenesis,
Persian (Arabian) Gulf, Abs. Geol. Soc.
Am., Annual Meetings, 197y:Knight, . K.,
1963,
•The
origin
and
occurrance of collapsing soils•, 3rd,
Regional Conference for Africa on Soil
Mechanics and Foundation Engineering, Vol.
1, PP• 127-130.
Second International Conference on Case Histories in Geotechnical Engineering
Missouri University of Science and Technology
http://ICCHGE1984-2013.mst.edu
Long, R. P. , w.a. Rover, 1984, • Performance
of sand drains in a tidal marsh•, Int.
Conf. on Case Histories in Geotechnical
Engineering, Vol. 3, pp. 1235-1244.