Geotechnical and Geological Engineering 16: 291–307, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
291
Technical note
Swelling behaviour of expansive shales from the middle
region of Saudi Arabia
ABDULLAH I. AL-MHAIDIB
Civil Engineering Department, College of Engineering, King Saud University, PO Box 800, Riyadh
11421, Kingdom of Saudi Arabia
Received 22 September 1997; accepted 4 May 1998
Abstract. Severe and widespread damage in residential buildings, sidewalks and pavements in various
parts of the middle region of Saudi Arabia is caused by the development of heave and swelling pressure
in the expansive shales in the region. This paper presents the problems and the geotechnical and
physicochemical properties of the tested shales. The swelling potential was determined using various
methods. Swell tests were conducted under different loading conditions and following different
procedures to quantify the amount of vertical swell and swelling pressure. The conventional onedimensional oedometer swell tests were performed using three different procedures, namely, free swell,
constant volume swell and swell overburden. In addition, swell tests were performed in the stress path
triaxial apparatus. Tests under different vertical stresses and confinements were conducted. Vertical swell
and swelling pressure obtained from the various methods were compared. The reliability of the different
methods for estimation of swelling potential is discussed.
Key words: oedometer tests, shale, swelling pressure, triaxial tests, vertical swell
Introduction
Damage to structures from the swell of foundation soils due to changes in moisture
conditions are common problems that occur frequently in many parts of the world
including vast areas of the Kingdom of Saudi Arabia. Damage inflicted on superstructures by expansive soils each year is enormous. Although there has been no
precise estimate, annually in Saudi Arabia, expansive soils are responsible for
millions of dollars worth of damage to man-made structures (Ruwaih, 1987;
Dhowian et al., 1990). Damage ranges from minor cracking of pavements or interior
finishes in buildings, which is very common, to irreparable displacement of footings
and superstructure elements.
Figure l shows the distribution of expansive soils in the Kingdom of Saudi Arabia.
Expansive soils are encountered over a large area due to geological history,
sedimentation and climatic conditions (Slater, 1983). It is estimated that expansive
soils cover an area of approximately 800 000 km2 in the kingdom (Ruwaih, 1987).
Shale formation prevails in different parts of the country including Al-Ghatt, Tabuk,
Tayma and Sharorah. The shale of the town of Al-Ghatt was chosen in this study
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Fig. 1. Map of Saudi Arabia showing the distribution of the expansive formations and the study area
(modified after Ruwaih, 1987).
because Al-Ghatt experiences some of the largest differential ground movements in
Saudi Arabia (Dhowian, 1990).
This paper presents the results of a laboratory investigation into the swelling
behaviour of an expansive shale from the middle region of Saudi Arabia. The
investigation included the determination of the geotechnical and physicochemical
properties, mineralogical composition and the swelling characteristics of the tested
shale. Experiments using various methods to determine the swelling potential of the
shale were performed to quantify the amount of vertical swell and swelling pressure.
Swell tests were conducted under different loading conditions and following different
procedures. The conventional one-dimensional oedometer swell tests were performed
using three different procedures: free swell, constant volume swell and swell
overburden. In addition, swell tests were performed in the stress path triaxial
apparatus. Tests under different vertical stresses and confinements were conducted. A
comparison of the results is presented and discussed in terms of the percentage of
vertical swell and swelling pressure.
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
293
Fig. 2. Cracking in a masonry wall due to expansive soil.
Description of the problem
Al-Ghatt is a small town in the middle region of Saudi Arabia, 270 km northwest of
Riyadh, the capital of Saudi Arabia (Fig. l). More than 400 one-story residential
buildings were constructed as part of a housing development. The buildings are made
of rigid reinforced concrete frames with hollow concrete block partitions and panels.
The problem in this town started with the development of cracks after the buildings
were occupied. The cracks extended to include slabs, beams and columns.
Figures 2–5 show examples of the kind of structural damage experienced in the
town. Damage to the buildings has been caused mainly by differential heave of the
foundation subsoils. The damage includes cracking of masonry walls in a diagonal
pattern (Fig. 2); the cracks are typical expansive soil cracks being wide at the top and
getting narrower at the bottom of the wall. In addition, it includes cracking of
reinforced concrete gates (Fig. 3), columns (Fig. 4) and ground beams. Figure 5
shows cracking of a road pavement in the town. Damage was attributed to the
excessive moisture content increases due to water infiltration from domestic sources
and garden watering as well as the surface cover by paving the areas around the
buildings which protect the moisture of the ground from outside dry weather.
Domestic waste water soaks into the soil as there is no public sewerage system in the
town.
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Fig. 3. Cracks in a reinforced concrete gate and adjacent wall.
Fig. 4. Cracks in a reinforced concrete column.
AL-MHAIDIB
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
295
Fig. 5. Cracks in a road pavement in the town.
A test pit was excavated in a damaged area and soil samples were obtained from
a depth of approximately 3 m and brought to the soil mechanics laboratory in the
Civil Engineering Department, King Saud University. A laboratory testing programme was designed to determine the geotechnical and physicochemical properties,
mineralogical composition and the swelling characteristics of the tested soil.
Characteristics of the investigated soil
Geotechnical properties
Soil samples were subjected to moisture content, unit weight, grain size analysis and
Atterberg limit tests using American Society for Testing Materials (ASTM) standard
procedures. Table l shows the geotechnical properties as well as the chemical
composition of the soil investigated. The grain size distribution curve, which is
shown in Fig. 6, indicated that more than 99% passed through ASTM sieve No. 200
(75 mm), from which the soil can be described as purely fine grained. The soil is
composed of predominantly clay size of approximately 80% and silt size of
approximately 20%. The soil is classed as inorganic clay of high plasticity (CH) in
the unified soil classification system.
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The shale is highly weathered and, with the arid and dry desert climate in the
town, it has relatively high swell potential. The geotechnical properties of the tested
soil show that the soil has swelling potential of high (Seed et al., 1962;
Dakshanamurthy and Raman, 1973) to very high (Van Der Merwe, 1964).
Soil composition
X-ray diffraction analysis was performed on the clay fraction of the tested soil to
identify the clay minerals. A Philips PW 1050 diffractometer was used with a PW
1373 goniometer supply operating at 35 kV and 20 mA, with CuKa radiation and an
Ni-filter. The X-ray diffraction patterns shown in Fig. 7 indicate that the clay
minerals were mostly kaolinite and some illite but no montmorillonite was present.
Quartz and calcite were found in relatively small quantities. The results of the
chemical tests (Table 1) indicated that the soil has high concentrations of K and Na.
These monovalent exchangeable cations cause more swelling with water addition
when compared with divalent exchangeable cations such as Ca and Mg which the
soil contains in lesser amounts. Furthermore, the soluble sulphate concentration is
Table 1. Geotechnical properties and chemical composition of the tested soil
Parameter
Value
Specific gravity
Natural water content (%)
Dry unit weight (kN m–3)
2.8
22
18
Consistancy limits
Liquid limit (%)
Plastic limt (%)
Plasticity index (%)
Shrinkage limit (%)
60
31
29
16
Classification
Unified soil system
AASHTO classification system
Chemical compostion
Cations (p.p.m.)
Ca2+
Mg2+
Na+
K+
Anions (p.p.m.)
SO2–
4
CL–
CO–3
HCO–3
pH
CH
A-7-6
270
50
8570
6000
53 800
240
Trace
3
8.0
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
297
53 800 p.p.m. which is believed to have a significant effect on the swelling potential
of the tested soil. The pH value of 8.0 puts the soil in the slightly alkaline range.
Laboratory swell tests
The amount of vertical swell and swelling pressure of the tested soil were obtained
using the three oedometer testing procedures: free swell, constant volume swell and
swell overburden tests as described in ASTM standard No. D4546. In addition, swell
tests were performed in the stress path triaxial apparatus. Tests under different
vertical stresses and confinements were conducted.
Sample preparation
The soil was oven dried for approximately 24 h, crushed and sieved through ASTM
sieve No. 40 (425 mm). Then it was thoroughly mixed with the amount of water
calculated as necessary to obtain the 22% initial moisture content. The mixture was
sealed in an air-tight plastic bag and placed in a moist desiccator and was allowed to
cure at room temperature for approximately 24 h. Thereafter, the 70 mm diameter, 19
mm thick oedometer samples were prepared in a purpose-made compaction unit
Fig. 6. Grain size distribution curve for the tested soil.
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containing the oedometer ring at an initial dry unit weight of 18 kN m–3. Immediately
after compaction, samples were transferred to the oedometer and the swell tests were
performed.
Free swell test
In this test, the specimen was inundated and allowed to swell freely under a seating
load of 7 kPa until primary swell was complete. Thereafter, it was loaded (after
primary swell had occurred) until its initial void ratio (height) was obtained. The
swelling pressure can be taken as the pressure that brings the sample back to its
initial height (i.e. at 0% vertical swell). Two replicate tests were conducted and the
Fig. 7.
X-ray diffraction pattern for the tested soil.
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
299
results obtained are presented in Fig. 8. The two tests led to a mean value of 19%
vertical swell and 1700 kPa swelling pressure.
Constant volume test
A soil specimen was placed in the oedometer and its vertical expansion when flooded
with water was prevented throughout the test. Sufficient load was applied to the
specimen in increments until swell pressure was fully developed in soaked conditions. The pressure developed by the specimen when saturation is completed
represents a direct measure of swelling pressure. The results of the two replicate tests
are shown in Fig. 8. The swelling pressure was found to be 960 kPa (mean of two
tests). The swelling pressure was calculated from the sum of the load increments
divided by the cross-sectional area of the sample.
Swell overburden test
In the swell overburden test, the specimen was loaded to vertical in situ overburden
pressure and inundated under this pressure until the primary swell was completed.
Five vertical pressures (35, 70, 140, 210 and 280 kPa) were applied on the samples
Fig. 8. Comparison of test results from the three oedometer tests.
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in the oedometer. For each vertical pressure, two replicate tests were performed to
ensure repeatability of test results. The total number of swell tests in this method was
ten tests. The variation of vertical swell with time is shown in Fig. 9 for the applied
pressures. The vertical swell time curves could be represented by a rectangular
hyperbola which is in agreement with the findings of Dakshanamurthy (1978). The
time/vertical swell versus time relationship can be represented by a straight line (Fig.
10). The reciprocal of the slope of the straight line for each pressure in Fig. 10 gives
the maximum vertical swell. The swelling pressure in this method was determined to
be approximately 880 kPa.
Triaxial swell test
The tests were conducted using a hydraulic triaxial stress path cell of the type
reported by Bishop and Wesley (1975). In this apparatus, it is possible to measure the
vertical swell of the sample directly, whereas in the conventional triaxial apparatus it
has generally been possible only to measure the total volume change of the sample.
Similarly to the oedometer tests, samples were prepared after approximately 24 h
curing time with an initial moisture content of 22% and initial dry unit weight of 18
Fig. 9. Variation in vertical swell with time from the swell overburden oedometer tests under different
pressures applied.
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
301
kN m–3. Preparation of the 35.5 mm diameter, 71 mm long triaxial samples consisted
of statically compacting the soil in a purpose-made vertically split mould. Thereafter,
the samples were transfered to the triaxial apparatus and the swell tests were
initiated. Details of the procedure of swell testing in stress path triaxial apparatus are
given in Al-Shamrani and Al-Mhaidib (1998). The same five vertical pressures used
in the overburden swell oedometer tests were applied to the soil samples. The total
number of swell tests was ten tests; two replicate tests for each vertical pressure were
performed.
Figure 11 shows the swell behaviour of the tested soil under the pressures applied.
The time/vertical swell versus time relationship is shown in Fig. 12 which did not
give a straight line. Therefore, vertical swell–time curves cannot be represented by a
rectangular hyperbola. This method gives a swelling pressure of 1000 kPa.
Comparison of results
The magnitudes of vertical swell and swelling pressure obtained from the three
methods of the oedometer test are listed in Table 2 and compared in Fig. 8. There
Fig. 10. Time/Vertical swell versus time from the swell overburden oedometer tests under different
pressures applied.
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Fig. 11. Variation of vertical swell with time from triaxial tests under different pressures applied.
appears to be no definite relationship between the values from the three methods. The
free swell test gives the highest value of vertical swell and swelling pressure and the
swell overburden method gives the lowest value of swelling pressure. The magnitude
of swelling pressure obtained from the constant volume method is in between. The
free swell method is time-consuming since the specimen passes through two phases
of deformation, complete expansion under the seating load and complete compression under different loads, whereas the constant volume method is quick. For both
methods, only one specimen is required. The constant volume method provides a
direct measure of the swelling pressure. The swell overburden method has the merit
that it follows the probable stress path that a soil may undergo in the field. However,
it requires at least three specimens, but it is less time-consuming.
The discrepancies in the swelling potential (vertical swell and swelling pressure)
are due to differences in loading and wetting conditions in the oedometer tests. The
free swell test does not represent the normal sequence of load–wetting in the field.
The soil in the field will not absorb water and swell first with the structural load
applied later, but rather vice versa. The constant volume method also does not
simulate the in situ condition, where the applied load after the structure is in service
does not change with time. In addition, it does not present the expected amount of
heave under the application of a certain load (structure load). The swell overburden
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SWELLING BEHAVIOUR OF EXPANSIVE SHALES
Fig. 12. Time/vertical swell versus time from triaxial tests under different pressures applied.
Table 2. Comparative values of swelling potential by different oedometer
methods
Maximum swelling potential
Method
Vertrical swell (%)
Swelling pressure (kPa)
Free swell
Constant volume
Swell overburden
19
–
–a
1700
960
880
a
see Table 3.
method has the important advantage of being more representative of actual field
loading and wetting conditions (El Sayed and Rabbaa, 1986). The data obtained by
this method can provide useful information for design, such as swelling pressure
corresponding to zero volume change, the expected vertical heave under the loads
imposed by the structure and the loads that could be applied to develop a certain
swelling within the tolerable limits.
The values of maximum vertical swell obtained from swell overburden oedometer
tests and triaxial tests under different applied pressures are shown in Table 3. The
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Table 3. Percentages of vertical swell from swell overburden oedometer tests and triaxial tests under
different applied pressures (mean of two tests)
Apparatus type
Vertical
pressure
(35 kPa)
Oedometer
12.67
Triaxial
3.89
Triaxial/Oedometer 0.31
Vertical
pressure
(70 kPa)
Vertical
pressure
(140 kPa)
Vertical
pressure
(210 kPa)
Vertical
pressure
(280 kPa)
Swelling
pressure
(kPa)
9.43
2.88
0.31
7.17
2.38
0.33
5.84
1.78
0.30
4.14
1.51
0.36
880.00
1000.00
0.32a
a
Mean value.
maximum vertical swell is calculated by dividing the maximum increase in height of
the sample by its initial height before it is given free access to water. The
relationships between the mean of the maximum vertical swell (two tests) and
logarithm of the applied pressure for the samples tested in oedometer (swell
overburden method) and triaxial tests are shown in Fig. 13. The relationships can be
approximated by the following equations with a correlation coefficient r 2 of
approximately 0.99:
Sp (%) 5 26.5 2 9 3 log(p)
(oedometer test)
(1)
Sp (%) 5 7.8 2 2.6 3 log(p)
(triaxial test)
(2)
Fig. 13. Comparison of vertical swell from swell overburden oedometer and triaxial tests.
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
305
Fig. 14. Ratio of triaxial vertical swell to oedometer vertical swell versus pressure applied.
where Sp is the vertical swell and p is the applied pressure (kPa). From the above
equations, the swelling pressure was determined to be approximately 880 kPa for
oedometer tests and 1000 kPa for triaxial tests at 0% vertical swell. Furthermore,
Equation l predicted the vertical swell under a pressure of 7 kPa to be approximately
19%, the same as that found by the free swell test.
The values of vertical swell from the oedometer tests are higher than those from
the triaxial tests for all the pressures applied. This may be due to many factors; the
most significant are the differences in loading and wetting conditions in both types of
equipment. The direction of swelling, lateral restraint in the apparatus and the
wetting of the sample are different in both types of equipment (A.I. Al-Mhaidib and
M.A. Al-Shamrani, submitted). The triaxial test is more representive of the field
conditions than the oedometer.
The variation in the ratio of vertical swells obtained from triaxial tests to those
obtained from oedometer tests with the applied pressure is shown in Fig. 14. The
mean value of this ratio is approximately one-third (Table 3). Previous investigators
(Gizienski and Lee, 1965; Dhowian et al., 1990; Erol, 1992) found that laboratory
results from oedometer tests overestimate the in situ heave by a factor of three. The
reason for this discrepancy may be attributed to the fact that, in the field, swelling
takes place in three dimensions rather than one dimension as imposed by laboratory
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oedometer tests. Other reasons are related to the fact that the in situ stresses on the
soil are released after the soil sample is disturbed and brought to the laboratory to be
tested. Dhowian et al. (1990) and Erol (1992) suggested that approximately one-third
of the volume changes are reflected as a surface heave, while the remainder will be
lateral. Therefore, when considering a restraint factor of one-third, swell overburden
oedometer tests predict vertical swell very well when compared with the measured
swell in the field (Dhowian et al., 1990; Erol, 1992). The results of the stress path
triaxial tests provide good prediction of the vertical swell without using any restraint
factor.
Dhowian (1990) found the percentage of swell in the field in the town of Al-Ghatt
to be in the range of 3–4% throughout the depths investigated (up to 5 m). The
vertical overburden stresses in the field are approximately 66 and 110 kPa at depths
of 3 and 5 m, respectively. The vertical swells obtained from the stress path triaxial
tests are approximately 2.8 and 2.4% under pressures of 70 and 140 kPa, respectively. Therefore, it may be concluded that the stress path triaxial tests can predict the
expected heave better than the swell overburden oedometer tests. Furthermore, the
swelling pressure from the triaxial tests was found to be approximately 1000 kPa,
close to that obtained by the constant volume oedometer test (960 kPa) which
directly measures the swelling pressure.
Conclusions
The results of the different swell tests conducted on an expansive shale from the
middle region of Saudi Arabia indicated significantly different values of swelling
potential depending on the test technique used. The swelling pressures obtained from
the three oedometer methods were different. The free swell test gave an upper bound
value for swelling pressure, the swell overburden test provided the smallest value and
the constant volume method gave an intermediate value. The different values may be
attributed to the differences in loading and wetting conditions followed in each
method. The free swell method is time-consuming but only one specimen is required.
The constant volume method is a quick test, requires only one specimen and provides
a direct measure of the swelling pressure. The swell overburden method has the
important advantage of being more representative of actual field loading and wetting
conditions. However, it requires at least three specimens in identical initial
conditions.
A comparison of the results from swell overburden oedometer tests and triaxial
tests shows that triaxial tests provide lower values of vertical swell than those from
the swell overburden oedometer tests (approximately one-third). This ratio is in
agreement with those suggested in the literature for the prediction of field heave from
the oedometer tests. Triaxial tests provide good prediction of vertical swell when
compared with field data and also provide a swelling pressure value close to that
SWELLING BEHAVIOUR OF EXPANSIVE SHALES
307
obtained from the constant volume oedometer method which directly measures the
swelling pressure. Therefore, it is concluded that the stress path triaxial apparatus
could predict the swelling potential of the tested soil better than the oedometer
methods.
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