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Document Type |
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Article In Conference |
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Document Title |
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ENGINEERING PROPERTIES OF SALT BEARING SOIL الخصائص الهندسية للتربة ذات الأملاح العالية |
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Document Language |
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Arabic |
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Abstract |
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ENGINEERING PROPERTIES OF SALT BEARING SOIL
ABSTRACT
Subsoil in desert regions is a product of temperature changes,
wi nd and rai nfall.
High temperature climate increase evaporation and reduce
moisture in soil resulting in development of salina soil.
Depending upon some geological considerations salt precipitations
in hot climate regions are invariably high.
Infl uence of sal t content on apparent
properties of desert soil is evaluated.
salt bearing soils are investigated.
1aboratory observed
Some properties of
Foundation problems arise due to various cOl1centrations of
sodium chlorides in desert soil samples are pin pointed and
discussed.
INTRODUCTION
Deve 1opment of bu i It up sal ina soi 1 is assoc i ated with the
inherent nature of some of the local physical envi ronment and
climatic condition of arid and extremely arid regions.
Soil in hot temperature cl imate, such as Saudi Arabia, would
be the best represent at i ve example for study of soi 1 cond ition
in arid and extremely arid regions.
The Arabian Peninsula is bisected by the tropic of cancer,
and can be taken as a model for many other arid or desert
regions, (Oweis and Rowman 1981; Higginbottom and Fookes
1982). It is divided into various topographic units depending
of the geological featlJres. These are the terrain and
mountainous regions dominated by limestone formations; the
associated foothill alluvial fans, the dunes and the plains,
(Figure 1).
Geo-technical problems associated with desert cOl1ditions are
of a different nature in each of the units and in general can
be summarized by one or more of the followings:
i) the presence of cavities in the limestone formation.
i i) the presence of weathered 1ayers ald vari abi 1i ty of
foundation materials.
iii) presence of hydrated gypsum in poorly drained areas.
277
ENGINEERING PROPERTIES OF SALT BEARING SOIL
~.S. STIPHO, ~.Sc. Ph.D.
Page 2
iV) presence of inland and costal salt bearing plains.
v) the presence of weak .cementation due to soluble salt
crystallization in upper part of the profile.
vi) the soil minerological content and their expansive
behavior has other affects.
Fookes 1978, Tomlinson 1978, Oweis and ~owman 1981, ~l Say~ri
and J. Zotl 1978, studied the geological nature and outlined
the common problems which arise in high temperature climates
and in particular the Middle East regions. [)akhil and
Al-Gahtani 1982, studied the affects of air-soil salt pollution
on the behavior of concrete in the climate, temperature
and conditions of the Eastern Province of Saudi ~rabia.
In this paper evaluations and geotechnical problems arise due
to high precipitation and hydration of salt on the subsoil
are investigated. The effect of sodium chlorides on some of
the engineering properties of salt bearing soil is singlerl
out.
NATURE OF SALT PRECIPITATION IN SOIL
Cl imatic condition in desert region is characterized by a
constantly hot prevailing wind with low precipitation.
Summer ai r temperture in coastal and central p~rts of the
peninsula frequently reaches 45-~O°C, Daily variations of
15°C in temperature are not uncommon. Temperatures in the
winter average 20°C; and could drop to well below DoC in many
of its regions. Such climate condition enhance evaporation
(124 cm/year; Dakhil &Al-Gahtani 1982) bringing the evaporation
precipitation ratio to 1:30 in many places. Precipitation
distribution in the Arabian Peninsula is shown in
Figure 2.
While rainfall in desert regions in very low, it occurs with
high intensity creating flash floods which can c~use significant
damages to property. It also acts to wash anrl tr~nsport
top soi 1 wi th salt deposits from cons iderabl e depth ~nrl
deposit them in other places causing high salt concentrations
at t he end of the hor i zonta1 fl ow path.
In the vertical direction, rain ·tlater leaches out sollJble
sillts from top layers and transport them to oeeper parts of
the profile where percolation enos. High temperature climate
enhance moisture evaporation glvlng rise to salt jJrecipitation
at or near the surfilce of the profile, ilnd the Sillt
278
ENGINEERING PROPERTIES OF SALT BEARING SOIL
~.S. STIPHO, B.Sc. Ph.D.
Page 3
circle in soil starts again.
Generally high salt precipitation in desert regions is due to
one or more of the following:
i) The rise of salina water into the top layers of profile
due to evaporation.
ii) The induction of sea water.
iii) The deposition of salty aeolian dust from sea area.
iv) Ground water table movements.
Soil-Water Relations
Water in soil dictates the behavior of soil such as its
effects on physical properties ihen saturated or dry- and it
speeds Chemicals reactions. When soluble salt is present in
pore soil water it gives rise to positively charged ions such
as sodium, magnisum and calcium. These ions are of exchangeable
base and can influence the properties of soil. This
could be shown by comparing clay deposits in sea water which
contains relatively large quantities of dissolved salt and
clays deposited in fresh water. It controls the matrix formation
of the soil during deposition.
Soil matrix has been found to be a flocculated type structure
with random particle orientation or dispersed type structure
with defined particle orientation depending on the ions
available and their charges (Rejurrum 1954, Scott 1963).
Such defined particle orientation wi 11 develop anisotropy
with directional variation of soil properties and I)ehavior
(Duncan and Seed 1966; Morgenstern and Tc~alenko 1967;
Kirkpatrick and Renni 1972; Stipho 1978). The subsequent
occurrence and changes of sal inity in the soi 1 profi le wi 11
influence the stability of the profile. Since all salts are
to a certain degree soluble in iater an alteration of water
content is the main factor affecting the behavior of subsoil
and the. stability of the profile.
Salt content in the i~iddle East soil is dominated by sulphate,
chlorides and carbonates of calcium, sodium and magnesilJm.
Salt in Middle I:ast regions has been found either in di ssolved
pore soil fluids or as on surface salt crusts.
279
ENGINEERING PROPERTIES OF SALT BEARING SOIL
A.S. STIPHO, B.Sc. Ph.D.
Page 4
The presence of such high salt content causes many problems
to engineering practice in the region, such as:
i) Salt attack on sub-structure materi als (attack on concrete
and its reinforcements).
ii) Salt in soil acts as corroding agent for metal elements,
pipes etc.
iii) Presence of salt in soil fluid in dissolved or a crystall
ized form, affects the nature physical behavior of
foundation.
Protection against salt attack on foundation materials, pipes
metal objects could be achieved by using sulphate resistant
cement, chemi cally neut ra 1i zed aggregates, aspha lt coat i ngs
and other chemically manufactured materials.
The affect of salt type, and concentration on the engineering
properties of subsoil is complicated and not fully understood.
This affect depends on the electrical properties of the solid
particles, minerals composition and particle size and shape;
(Kazi and Mourn 1973; Li 1978; Saxena ilnd Lastrico 1978;
St i pho 1981; Ak i 1 i 1981).
The texture of salt bearing soi I in desert region is quite
varied. In places where standing water has evaporated or a
very wet surface has dried, a thin discontinuous crust of
halite may exist which will soon be covered or dispersed by
blowing sand.
The salt bearing soil media, regardless of sand, salt or clay,
strongly hygroscopic dependi ng on the type of sal t. Such
soi 1 forms very good di rt road when dry but it becomes impossible
with high moi sture (Fookes 1978;-:- Toml inson 1978;
Al-Sayari and Zotl 1978). Saudi Arabian salt bearing plains
are generally categorized by layered sediments of fine silty
clay covered with some meters of fine sand (Akili 1981; Akili
& Torrance 1981; Jergman 1981). Survey of the salt bearing
soil in the coastal plains of the Easter" Province of Saudi
Arabia shows high percentage of the plains covered by such
soil (Figure 3 after Al-Sayari and Zotl).
Sand samples from differnt sites in Saudi Arabia were collected
and grain size distribution analyses determined and are
presented in Figure 4. .
It can be noted that all desert sand can be identified within
a narrow band as uniform medium to fine.
280
ENGINEERING PROPERTIES OF SALT BEARING SOIL
A.S. STIPHO, B.Sc. Ph.D.
Page 5
These samples were then used to study the affect of halite on
the engineering properties of the desert sand. Beside that
fine clay deposits from the inland plains of Saudi Arabia are
made available and their properties studied.
EXPERIMENTAL WORK AND TEST RESULTS
Clean washed salt free sand samples in an oven dry conditions
were prepared and packed in layers of equal thickness to preserve
uniformity. This was achieved, using raining technique.
The sample was packed in a prespex container having
two, inner and outer, compartments. Small pin holes along
the bottom sides of the inner container were made to allow
movements of sal i na water to the soil sample. t~easured salt
quantities in the range of 0-90 gm/l it dissol vedin predetermined
quantities of distilled water are used in the
tests. The saline mixture was pored in the outer compartment
of the sampl e contai nero The contai ner \~as then 1eft under
natural room temperature to allow capillary saturation of the
sample by encouraged surface evaporation. Salt precipitation
in and along the soil profile is then developed.
This procedure is believed to simulate natural conditions.
The relative density of the prepared samples were determined
before any furt her tes t i ng. Stat i c cone res i stance for samples
with different salt precipitations, using standard cone
penetrometer was determined. The measured resistance was
divided by the relative density for each corresponding sample
and plotted against salt content as in Figure 5.
It noted that a weak cementing force is developed, and
increased with increasing salt precipitation. This cementing
force should be viewed carefully from type of salt, quantity,
solubility and stability. Such cementation can be very temporary
and leached out as a result of any water action. The
internal angle of friction (J for the desert sand tested is
found to be reduced with increasing salt content; as seen in
Figure 6. Similarly, development of some cohesion is found
with increasing salt content (Figure 7). This could be very
well due to the bonding developed by the salt crystallization
or a defined packing system.
Investigation also was made of the influence of salt preclpltation
on the shearing resistance of disturbed and undisturbed
salt bearing desert sand samples. The remolding process
acts to disturb the natural matrix of the soil. thus the
cementation bond that developed due to salt precipitation is
disturbed resulting in lower shearing strength. The
281
ENGINEERING PROPERTIES OF SALT BEARING SOIL
A.S. STIPJ:!Q_LJL~Jh.D. .
Page 6
influence of remolding and the affect of leaching on the
shearing resistance is shown in Figures 8 and 9.
Such loss of strength phenomenon is very well understood in
clay soil, known as sensitivity; but in clay the effect is
due to loss of the intact structure and its particle orientat
i on. Va ri ab 1e head permeab i 1ity tests were conducted on
i dent i ca1 samples havi ng diffe rent salt contents and the
results are depicted in Figure 10. Permeabil ity of salt
bearing desert s~nd is found decreased with increasinu salt
prec i pitat i on.
Clay specimen from the inland plains of Al-Qassim was also
made available. The soil was first oven dried and sieved;
the particles coarser than 0.164 10m (retained on sieve t-lo.
150) were disregarded. The soil is identified as light brown
silty clay with L.L. = 29% and plastic limit P.L. = 22.5%.
The specimen was then washed with excess distilled water for
several days. The sample was oven dried, pulverized, sieved
and stored for further investigation. Specimens for investigation
were prepared by mixing measured weights of the dried
powder with a pre-determined quantities of \~ater containing
various salt contents.
Consolidation behavior of clay soil samples haVing various
salt contents in pore fluid ~Iere studied. Parallel void
ratio - log consolidation pressure curves with almost same
slope during compression phase was noted (See Figure 11).
Plasticity index (P.!.) \~hich is, in general, a measure of
the c1 ay strength lias found to be i nfl uenced by the sal t content.
The plasticity index was found decreased with increasing
(NaCl) salt content in pore soi 1 fluid; this is in agreement
with results for clay soil samples obtilined from the
Eastern Province of Saudi Arabia, reported by Stipho 1981.
Although initial NaCl salt content of the soil in its natlJral
condition could not be measured, the plasticity index of soil
in this condition fits well \~ith the general trend observed
giving an initial indication of salt content around 40 gmllit
(Fi gure 12). High salt content in flare fluid of desert clays
is found to give rise to the soil shearing resistance in
direct shear. Shear box test results on soil silmples having
various salt content are sho\m in Figure 13. Salt leachin;)
from a state of as hi gh as 60 gil it. NaCl to about zero content
gives decrease of 35-45% of the measured clay strength.
Similar range of loss is reflorted by Bjurrum 1954 fur marine
cl ay.
282
14. Oweis, I. and Bowman, A" 1981, ~Geotechnical Consodirations
for Construction in Saudi Arabia", J .. of Am. Soc, of Civil
Eng., GT. 3 March, p, 319.
15. Scott, R.. , 1963, ....Principles of Soil Mechanics", Addison
Wesley.
16. Stipho, A.S. 1978, "Experimental and Theoretical Investigation
of the Behaviour of Anisotropically Consolidated
Kaolin", Ph.D. Thesis, University of Wales, U.K.
17. Stipho, A.S., 1981, "Effect of Salt Concentration on Some
of the Engineering Properties of Soil", Proc. Symp. on
Geotechnical Problems in Saudi Arabia, May, Publ. Riyadh
University.
18. Saxena, S.K. and Lastrico R.M., 1978, "Static Properties
of Lightly Cemented Sand~, J. of Am. Soc. of Civil Eng.,
GT. 12, Vol. 12, Dec. p. 1449.
19. Tomlinson, M.J., 1978, "Highway and Airfield Pavements",
Proc. of the Conf. on Eng. Problems Associated with Ground
Conditions in the Middle East, pp. 65~72. |
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Publishing Year |
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1983 AH
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Added Date |
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Tuesday, January 13, 2009 |
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