Compressibility and swelling characteristics of Al-Khobar ...

Abstract 

Compressibility and swelling characteristics of Al-Khobar 

Palygorskite, eastern Saudi Arabia 

Saad A. Aiban ⁎ 

Department of Civil Engineering, King Fahd University of Petroleum and Minerals,Dhahran 31261, Saudi Arabia 

Received 7 May 2005; received in revised form 25 June 2006; accepted 7 July 2006 

Available online 8 September 2006 

Expansive soils are found in different locations in eastern Saudi Arabia. The area is arid with high temperatures, highly variable 

humidity and an excessive rate of evaporation compared to the low precipitation. This resulted in the formation of water sensitive 

soils. In the present investigation, line valve buildings for a sweet water feeder (1118 mm in diameter) were constructed on a highly 

expansive material consisting mainly of brown palygorskite and gray palygorskite with thin sheets of gypsum and limestone. Block 

samples from both palygorskites were brought to the laboratory and cores as well as remolded samples were obtained from the 

blocks. The two palygorskites were found to be highly plastic and have a very high swelling potential. The liquid limit (LL) and 

plastic limit (PL) values for the brown palygorskite are 261% and 140%, respectively. The gray palygorskite has a LL of a 285% 

and a PL of 123%. The oedometer free swell tests for the two palygorskites produced an expansion ranging between 31.8% and 

42.5% for the remolded samples. However, the expansion for cores ranges between 8.3% and 19.3%. The constant volume pressure 

tests produced a stress in excess of 4240 kPa. The swell potential reached a steady state after four days while the swelling pressure 

reached a steady state in about 3 h. The paper addresses the geology of the area, the characterization of the geomaterial including 

mineralogical composition using X-ray diffraction and SEM techniques and the swelling characteristics of the material. 

© 2006 Elsevier B.V. All rights reserved. 

Keywords: Palygorskite; Expansive material; Arabian shelf; Gypsum; Swell potential; Swelling pressure 

1. Introduction 

1.1. General 

Expansive soils are encountered in many places in the 

world. These soils exhibit large expansions when 

subjected to an increase in the moisture content. 

Construction works in areas where expansive soils 

exist can suffer from serious damages regardless of the 

⁎ Tel.: +966 3 860 2272; fax: +966 3 860 1413. 

E-mail address: saiban@kfupm.edu.sa. 

Engineering Geology 87 (2006) 205–219 

0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. 

doi:10.1016/j.enggeo.2006.07.003 

www.elsevier.com/locate/enggeo 

construction type unless the swelling characteristics are 

taken into account. Damage of different structures 

caused by expansive soils is well documented in the 

literature (Chen, 1988). Such soils can cause damages to 

reinforced concrete pile caps, grade beams, walkway 

slabs, masonry walls, pavements, etc., some of which 

cannot be easily repaired. It is, therefore, essential to take 

into account the behavior of these soils during the design, 

construction and maintenance of structures intended for 

construction on or within these types of expansive soils. 

The literature reveals that expansive soils in Saudi 

Arabia are found in many locations in the eastern province 

and northwestern part of the Kingdom as reported

206 S.A. Aiban / Engineering Geology 87 (2006) 205–219 

by Dhowian (1981), Erol and Dhowian (1982), Slater 

(1983), Ruwaih (1984), Dhowian et al. (1985), Erol and 

Dhowian (1990), Abduljauwad and Al-Sulaimani 

(1993), Abduljauwad (1994), and Azam et al. (2003). 

Dhowian et al. (1985) discussed the distribution and 

types of expansive soils in Saudi Arabia. They stated that 

expansive soils in Saudi Arabia exist in three main types: 

(1) shale of various degrees of weathering, (2) 

montmorillonitic clays and (3) calcareous clays. They 

also stated that Madinah montmorillonite clays and 

Tabuk shales are highly expansive while Al-Ghatt and 

Tayma shales and Hofuf calcareous clays have relatively 

lower degrees of expansion. Table 1 gives a summary of 

the previous work related to expansive soils in Saudi 

Arabia. 

Abduljauwad (1994) conducted a thorough investigation 

on the swelling behavior of eastern Saudi Arabia 

calcareous sediments. He stated that calcareous expansive 

clays in eastern Saudi Arabia have moderate to very 

high swelling potential. A survey of damages resulting 

from expansive soils in several locations in Al-Hassa, 

Al-Qatif and nearby areas has been carried out by 

Abduljauwad (1994). The types of damage reported 

therein included the heaving of footpaths, distortion of 

door frames and cracking of ground floor block work, 

reinforced ground beams and floor slabs. The problems 

encountered due to expansive soils are mainly due to 

improper site investigation and lack of appreciation of 

the swelling potential of the underlying soils. Though 

quantifications of damages of different structures due to 

expansive soils in Saudi Arabia are not fully available, 

Ruwaih (1984) stated that building damages due to 

differential heave of the soil accounts for the greatest 

loss as compared with all other damages associated with 

construction. 

In Saudi Arabia, the characteristics and formation of 

expansive soils vary significantly from place to place due 

to variations in the geology, sedimentation processes and 

climatic conditions. According to Fookes (1978), the 

area is characterized by its high temperature, variable 

humidity and the excessive rate of evaporation compared 

to the low annual precipitation. Therefore, the swelling 

potential of eastern Saudi Arabia is intensified by the arid 

climatic conditions and severe weathering environment. 

In addition, the soils and rocks therein are calcareous 

in nature, which further enhances the swelling 

characteristics. 

1.2. Geology of the area 

Al-Khobar city is located in the Eastern Province of 

Saudi Arabia on the Arabian Gulf coast as shown in Table 1 

Summary of the previous work on expansive soils in Saudi Arabia 

Umm As 

Sahik 

Location Al-Hassa (including Hofuf) Madinah Al-Ghatt Tabuk Tayma Al-Qatif a 

Abduljauwad 

(1994) 

Abduljauwad 

(1994) 

Ruwaih 

(1984) 

Dhowian 

et al. (1985) 

Ruwaih 

(1984) 

Dhowian 

et al. (1985) 

Ruwaih 

(1984) 

Dhowian 

et al. (1985) 

Ruwaih 

(1984) 

Dhowian 

et al. (1985) 

Abduljauwad 

(1994) b 

Ruwaih 

(1984) 

Author Dhowian 

et al. (1985) 

Silty shale Silt Green clay Gray-green 

clay 

Clayey 

shale 

Clayey 

shale 

Clayey 

silt 

Silty 

shale 

Clayey 

shale 

Greenish 

clay 

Green 

clay 

White 

clay 

Red-green 

clay 

Calcareous 

clay 

Soil Type Calcareous 

clay 

LL 60 58 69–142 82 105 69–108 65 46 54–80 61 35–85 38 30–55 71–184 58–140 

PL 24 29–47 37 39 30 21 27 25 39–83 26–46 

PI 36 33 30–95 45 66 33–56 35 25 21–47 34 22–41 13 12–24 40–127 33–93 

SL 18 28 16 21 16 23 20 

Swelling (%) 2.54–14.17 0.63–29.49 1.36–12.74 

Swell pressure (kPa) 200–1300 14–1930 57–400 

a 

This includes soils from Umm Al-Hammam, Al-Jesh and Al-Ajam. 

b 

This includes soils from Al-Khars, Al-Mubarraz, Al-Mansoriya, Al-Nathel, Al-Hamadiya, Al-Salehiya and Mahasen.

Fig. 1. Geologically, the area is part of the Arabian shelf 

which was subjected to successive transgression and 

regression cycles of the Gulf waters during the 

Pleistocene and Holocene ages (El-Naggar, 1988). In 

general, the surface rocks of the region include both 

consolidated and unconsolidated sediments. The consolidated 

sediments belong to Paleocene to middle Eocene 

age and Miocene to Pliocene age while the unconsolidated 

materials contain sediments from Quaternary age 

which include shale and claystone. The geology of the 

area has been thoroughly discussed by Al-Sayari and Zotl 

(1978), Powers et al. (1963), and others. According to Al- 

Sayari and Zotl (1978), clays are present in different 

formations in eastern Saudi Arabia including Rus and 

Dammam formations (Eocene) and Hadrukh, Dam and 

Hofuf formations (Miocene). A brief description of the 

relevant geological formations is provided in the 

following sections (Powers et al., 1963; Johnson, 1978): 

a) Rus formation (Tru): this formation is divided into 

the following three lithologic units: 

1. A 3.6 m thick white soft chalky porous limestone 

with calcarenite beds at the top. 

2. A 31.8 m thick marl and limestone: the material has 

irregular masses of crystalline gypsum, occasional 

thin limestone beds and geodal quartz at several 

levels. The material is highly variable and includes 

white compact finely crystalline, anhydrite with 

S.A. Aiban / Engineering Geology 87 (2006) 205–219 

Fig. 1. Generalized geologic map of the Arabian Gulf coastal region and its hinterland (Replotted from Johnson, 1978). 

207 

interbedded green shale and minor amounts of 

dolomitic limestone. Alternatively, it may include 

gray marl with coarsely crystalline calcite and 

interbedded shale and limestone. Such a unit is 

highly variable both in lithology and thickness. 

3. A 21.0 m limestone: the material is gray to buff 

compact commonly partially dolomitized limestone 

with minor amounts of soft limestone made porous 

by leaching of small organic remains. Quartz 

geodes occur rarely in the lower part and are typical 

of the upper part. 

b) Dammam formation: this formation is divided into 

five members: 

1. A 15 m thick Alat limestone and marl: the upper 

part is light colored chalky and porous, commonly 

dolomitic limestone. It contains abundant molds 

of mollusks and other organic remains. The lower 

part is light colored dolomitic marl. 

2. A 9.3 m thick Khobar limestone and marl: the 

material is light to dark-brown, in part dolomitic 

limestone becoming off-white soft marly limestone. 

The lower part consists of marl. 

3. A 1.0 m thick Alveolina limestone: the material 

consists of yellowish gray, microcrystalline, 

partially recrystallized, dolomitized limestone. It 

contains common specimens of Alveolina elliptica 

(Sowerby) var. flosculina Silvestri and internal 

molds of Lucina pharaonis.


4. A 4.2 m thick Saila shale: the member consists of 

a 3.6 m dark brownish-yellow subfissile clay shale 

underlain by 0.6 m of gray-buff limestone. 

5. A 3.0 m thick Midra shale: the shale member 

consists of yellowish-brown, fissile, thinly laminated 

shale, gray marl and impure limestone. It 

contains scattered fossil shark teeth. 

c) Hadrukh formation: the thickness of this formation 

ranges between 20 m and 90 m and could 

reach 120 m in some areas. The formation is 

highly variable and contains shale, sandstone and 

marl in different colors. Inclusions of gypsum and 

chert are also present. 

d) Dam formation: The thickness of this formation 

varies considerably from 30 m to 100 m. The 

formation consists mainly of clay with minor marl 

and limestone in different colors ranging from 

green to red. 

e) Hofuf formation: this formation is 95 m thick and 

consists of the following four different members: 

(1) gray conglomerate, (2) alternating red and 

white argillaceous sandstone, (3) off-white in part 

impure sandy limestone, and (4) red and white 

conglomerate. 

1.3. Palygorskite Material 

Palygorskite, which is sometimes known as attapulgite, 

consists of double silica chains. The minerals in this 

group occur in a variety of macroscopic forms but are 

fibrous or lath-like or fine threadlike on a microscopic 

scale. Attapulgite is simply short-fibered palygorskite 

and some samples of these minerals have a fibrous texture 

and a cardboard or paper-like appearance due to the 

tangling of fibers (Grim, 1968). The palygorskite crystals 

are submicroscopic fibers whose dimensions are quite 

variable depending on their origin. The particle diameters 

range between 50 and 300 Å, while the thickness ranges 

between 50 and 100 Å and the length between 0.2 and 

5 μm (Martin-Vivaldi and Robertson, 1971; Mitchell, 

1993). The fibers are grouped together in bundles which 

are frayed or opened up at the ends (Martin-Vivaldi and 

Robertson, 1971). The structure of the fibrous mineral 

palygorskite differs from that of other layered silicates in 

lacking continuous octahedral sheets. The structure and 

morphology of the palygorskite crystals are such that a 

large number of terminal silicate tetrahedra on the ribbons 

are present at external surfaces. Broken Si–O–Si bonds 

compensate for their residual charge by accepting a 

proton or a hydroxyl and becoming Si–OH group. 

Palygorskite is formed in association with less basic 

rocks, e.g. in syenite and weathered granite (Newman 

and Brown, 1987). The presence of pyroxene or amphibole 

minerals provides a template for the formation of 

palygorskite and calcite is also necessary (Henin and 

Caillere, 1975). Palygorskite also occurs in sedimentary 

rocks. The industrial mineral resource map of Ad Dammam 

area clearly indicates the presence of Palygorskite 

(Roger and Prian, 1985). Palygorskite is abundant in the 

Arabian sea sediments ranging in age from Pleistocene to 

Cretaceous. In the younger sediments the concentration 

increases from southeast to northwest, as the Arabian 

Peninsula is approached (Weaver, 1989). 

Palygorskite is a porous, fibrous clay mineral and thus 

has a high specific surface that could reach 600 m 2 /g. 

Therefore, the material has a high adsorptive capacity. 

Besides surface water, palygorskite contains molecular or 

zeolitic water within the channels, water coordinated to 

the edge octahedral cations and the normal hydroxyl 

group of 2:1 layer silicate at the center of the ribbon 

(Newman and Brown, 1987). The water coordinated to 

the octahedral cations is called “bound”, “crystalline” or 

“coordinated” water (Newman, 1987). Sorption of water 

by partially dehydrated palygorskite apparently involves 

two processes: (1) sorption on the external surface, which 

is rapid, and (2) penetration into the channel surfaces, 

which is slow and only becomes evident when some 

zeolitic water is removed. On heating in vacuum, zeolitic 

water is lost between 100 and 300 °C together with about 

half of the coordinated water. The loss of water causes a 

partial collapse or folding of the structure (Nagata et al., 

1974; Van Scoyoc et al., 1979); at this stage, the structures 

are readily rehydrated if water vapor is readmitted. These 

minerals are formed at low temperature and pressure 

(Henin and Caillere, 1975). Wollast et al. (1968) showed 

that a sepiolite-like phase was formed in the reaction 

between silica and seawater at earth surface conditions. 

These situations are prevailing in eastern Saudi Arabia 

due to the previous transgression of the seawater. 

In most cases, when palygorskite is present in 

bedrock material, it is seen to disappear rapidly during 

weathering. This is due to the fact that palygorskite is 

known to form from highly concentrated aqueous 

solutions. On the other hand, palygorskites can be stable 

in soils and are formed in the weathering environment. 

These soils are found in caliches or carbonate precipitating 

soils or arid climates. In some profiles, palygorskite 

can comprise 100% of the clay size fraction silicate 

material (Newman, 1987). 

2. Subsurface investigations 

The main water tank farm in Al-Khobar city, eastern 

Saudi Arabia, was constructed in a mountainous area to

the west of the city. The area is about 25 m above sea 

level. A new sweet water feed pipe, 1118 mm (44 in.) in 

diameter, has recently been approved for construction to 

improve the quality of the potable water and to meet the 

demand on the existing water facility due to the continual 

expansion of the city. The new pipe runs through a series 

of line valves housed in rigid concrete structures. The 

concrete structures will have bases at about 4.5 m below 

the ground surface. 

Initially, basic geological and geotechnical information 

was collected from geological maps, geotechnical 

reports and relevant publications. Site visits were made 

to select locations for the boreholes based on field 

observations. The boreholes were drilled using a truckmounted 

rig with hollow stem flight auger technique. 

The standard penetration test (SPT) was performed for 

every meter, in accordance with the ASTM D 1586 

method, and the results are all refusal for the entire 

drilled depth. The materials extracted using the auger 

and the SPT sampler show great heterogeneity of the 

profile. Brown palygorskite, gray palygorskite and 

fragments of greenish clay and crystallized gypsum 

and other materials are brought to the surface continuously. 

The preliminary test results have clearly 

indicated that some layers are highly plastic and are 

expected to have very high swell potential. Such 

findings need to be confirmed and the swell potential 

needs to be quantified in order to consider the necessary 

alternatives in the foundation design. This implies the 

execution of a detailed investigation by exposing the site 

through excavation. The site was then excavated using 

backhoe to a depth of 0.5 m below the desired 

foundation level. The excavated area was about 25 m 

long, 6 m wide and 5 m deep. The heterogeneity of the 

profile is clearly shown in the photographs in Fig. 2. The 


following observations can be made about the exposed 

surfaces (Aiban, 2005): 

1. The top meter consisted of dense, light brown, poorly 

graded sand mixed with silt and gravel. 

2. The next layer which extends for variable thickness 

consists of hard laminated brownish palygorskite. 

This layer contains a large number of discontinuities/ 

joints in all directions. Some of the joints are filled 

with crystallized gypsum. The thickness of the 

gypsum varies and reaches 100 mm in many places, 

as shown in Fig. 2. The material is brown with dark 

brown and orange spots and light grey inclusions 

which were observed frequently. 

3. At the top of the brown palygorskite layer, layers of 

light gray to light brown palygorskite were encountered 

in few places with variable thicknesses that 

could reach 600 mm. The light gray/brownish palygorskite 

is less laminated and contains fewer discontinuities 

compared to the brown palygorskite. The 

material was labeled as gray palygorskite. It was 

noticed that the brown and gray palygorskite layers 

are mixed in many places. 

3. Laboratory testing and results 

3.1. Characterization and mineralogical composition of 

the material 

Representative block samples from both palygorskite 

materials were collected and brought to the geotechnical 

laboratory at King Fahd University of Petroleum and 

Minerals (KFUPM). The samples were used for 

mineralogical composition, routine characterization 

testing and oedometer testing using both remolded and 

Fig. 2. Photographs of the exposed surfaces showing the fissures and the crystallized gypsum. 

209


Table 2 

Basic geotechnical and swelling characteristics and mineralogy of the 

collected material from Al-Khobar 

Parameter Brown material Gray material 

Liquid limit (LL) 261% 285% 

Plastic limit (PL) 140% 123% 

Plasticity index (PI) 121% 162% 

Shrinkage limit (SL) 80% 38.5 

Natural mc 47% 42% 

Mineralogical 

composition a 

Palygorskite Palygorskite (100) 

(100%) 

Average swell potential 

for remolded samples 

at a stress of 7 kPa 

Average swell potential for 

core samples at a stress of 

7 kPa and natural mc 

Average maximum 

swelling pressure for 

remolded samples at 

mc=35% 

40% (mc=35%) 

34% (mc=47%) 

mc = moisture content. 

a Obtained using the XRD technique. 

8.9% 15.2% 

33.5% (mc=35%) 

31.8% (mc=42%) 

4964 kPa 4344 kPa 

core samples. Core samples for oedometer testing were 

obtained from the blocks. These cores were identified 

and wrapped in plastic sheets and immersed in molten 

wax to prevent moisture loss. Table 2 shows some of the 

basic geotechnical characteristics and the mineralogy of 

the two types of palygorskite. It is clear from the 

plasticity tests that both materials are highly plastic and 

expected to have high swell potential when compared to 

the criteria for identifying swell potential, such as the 

one proposed by Dakshanamanthy and Raman (1973) 

Fig. 4. XRD results for the brown and the gray palygorskites. 

and shown in Fig. 3. The data in Fig. 3 clearly shows 

that both soils have an extra high swell potential. 

Therefore, most of the testing program was tailored to 

explore the swelling characteristics of the two palygorskites. 

The high shrinkage limit for both types of 

palygorskites, especially the brown one, explains the 

Fig. 3. Dakshanamanthy and Raman criterion for determining the swell potential (Dakshanamanthy and Raman, 1973).

fissuring of the material upon exposure. The fissures 

appeared right after exposure since the natural moisture 

content (mc) is less than the shrinkage limit of the 

material. These fissures produced jointed masses, as 

shown in Fig. 2. 

In addition to the routine characterization testing, the 

X-ray diffraction (XRD) technique, the scanning electron 

microscope (SEM) and the energy dispersive spectroscopy 

(EDS) were used to obtain semi-quantitative mineralogical 

composition and chemical analysis of the material. Typical 

XRD results for both materials are shown in Fig. 4. The 

data were obtained using a JEOL JDX-3530 X-Ray 

Diffractometer (with JEOL JSX-3201 Elemental Analyzer). 

Results from the SEM and EDS analyses are shown in 

Figs. 5 and 6 and Table 3. These XRD micrographs vividly 


confirm that the material is pure palygorskite. This was 

confirmed by repeating the XRD tests using a Siemens 

D5005 X-Ray Diffractometer. 

Two groups of each material were prepared for SEM 

testing. In the first group, the samples were prepared to 

be perpendicular to the layering or sedimentation and 

these were labeled “cross-section” samples. The second 

group was intended to be parallel to the layering or 

sedimentation and these were labeled “in-plane” samples. 

In addition, some samples from each group were 

tested at their air-dried mc while some others were tested 

after being wetted with distilled water to investigate the 

effect of wetting on the microstructure. The air-dried 

samples were exposed to the laboratory environment for 

at least two weeks to ensure a constant mc. The distinct 

Fig. 5. SEM micrographs and the corresponding energy dispersive spectrum for the brown palygorskite (a and b: in-plane, c and d: cross-section). 

211


Fig. 6. SEM micrographs and the corresponding energy dispersive spectrum for the gray palygorskite (a and b: in-plane, c and d: cross-section). 

layering and sheet-like or cardboard appearance are clear 

in the cross-section samples shown in Figs. 5 and 6 

Table 3 

Chemical analysis for the brown and gray material from Al-Khobar 

Yellow material Gray material 

Element Percent 

element 

(range) 

Atomic 

percent 

(range) 

Percent 

element 

(range) 

Atomic 

percent 

(range) 

O 43.39–48.57 59.50–63.13 41.74–43.00 57.71–59 

Mg 4.30–4.69 3.88–4.06 3.92–4.09 3.53–3.94 

Al 5.87–6.39 4.69–4.96 6.34–7.44 5.19–6.04 

Si 31.08–33.38 24.28–24.72 28.40–33.96 22.31–26.74 

Cl 1.08–5.38 0.67–3.33 1.76–6.27 1.10–3.9 

K 0.82–1.32 0.44–0.74 1.11–1.42 0.63–0.78 

Ca 0–0.61 0.03–0.33 1.42–2.49 0.78–1.37 

Fe 3.78–10.66 1.41–4.16 6.86–8.70 2.69–3.45 

All values are based on at least three samples from each material. 

(plates c and d). This is also clear in the in-plane samples 

with large magnifications as shown in plate b in Figs. 5 

and 6. 

The effect of wetting is clear when comparing the airdried 

samples with the wetted ones, as shown in Fig. 7, 

whereby the wetted samples (b, d and f) exhibited 

relatively thick sheets and the spacing looks larger 

compared to the air-dried samples (a, c and e). The 

individual layers were more distinct for the wetted 

samples compared to the air-dried samples. This clearly 

indicates the effect of water in the swelling process 

which causes separation of the sheets and makes them 

thicker because of the adsorbed water. It should be 

mentioned that the SEM micrographs did not show the 

fibrous, lath-like or fine threadlike structure on the 

microscopic scale. However, the cardboard or paper-like 

appearance was clear in all samples (see Figs. 5, 6 and 7).

3.2. Oedometer testing 

The effects of molding mc and sample disturbance on 

the one dimensional expansion and compression of both 

types of palygorskite were investigated. A series of 

remolded samples and cores were prepared from both 

materials for one-dimensional oedometer swell/settlement 

potential test of cohesive soils. Remolded samples 

were prepared by grinding the material into powder 

passing the ASTM sieve No. 10, i.e. material finer than 

2 mm. The remolded samples for the oedometer testing 

were prepared at two mc values; one at their respective 

field mc and the other at a mc of 35% for both the brown 

and gray materials. The field mc was selected to simulate 

the field conditions. The 35% was selected in order 

to maximize swelling and was found to be the minimum 

molding mc at which the samples from both materials 

can be molded easily. The remolded samples were pre- 


Fig. 7. SEM micrographs for wetted and air dry samples for the gray palygorskite. 

213 

pared by mixing the powdered material with the desired 

mc and the mixture was placed in the oedometer ring 

and tamped gently and uniformly till the desired height 

was reached. The number of tamps was determined after 

many trials in order to standardize the preparation procedures. 

The samples prepared at a mc of 35% were 

compacted to produce density values that are close to the 

in-situ dry density (γ d) (1.06 g/cm 3 for the brown palygorskite 

and 1.08 g/cm 3 for the gray palygorskite). This 

was necessary to compare the two materials at the same 

density and mc. The γd for the brown palygorskite 

prepared at its field mc (47%) was about 1.08 g/cm 3 , 

while γd for the gray palygorskite prepared at its field 

mc (42%) raged between 1.0 and 1.03 g/cm 3 . These 

remolded samples from both materials were prepared to 

simulate the field density and mc. Two core samples 

from each type of palygorskite were trimmed from the 

blocks to fit properly into the oedometer ring. The


density of each core sample was determined. The dry 

density values for the two brown cores were 1.09 and 

1.05 g/cm 3 . The two gray cores had dry density values 

of 0.98 and 1.03 g/cm 3 . Two types of oedometer tests 

were performed: the free swell test and the constant 

volume test. 

3.2.1. Free swell oedometer tests 

The one-dimensional oedometer tests are instrumental 

in predicting the compressibility, collapse and 

swelling potential of soils. The free swell test was 

performed according to the ASTM D 4546 method. The 

test consisted of either placing the cores in the 

oedometer ring or preparing the remolded samples at 

the desired mc in the ring. A seating pressure of 7 kPa 

was initially applied on the sample. The height reading 

after the application of the 7 kPa seating stress was 

labeled the initial reading (i. e. zero-reading). The 

sample was then inundated with distilled water from the 

bottom and was allowed to swell under the seating 

stress. When the swelling reading reached a stable value, 

which usually takes about four days, one dimensional 

loading (compression) and unloading were performed. 

Most of the samples were reloaded after the first cycle of 

loading/unloading. 

3.2.1.1. Free swell tests for the brown palygorskite. 

For the brown material, three sets of samples were tested 

to determine the free swell characteristics of the 

material. Each set consisted of two identical samples. 

Two sets were prepared from the powdered material at 

two different mc values (47% and 35%) and these were 

the remolded sets. In the third set, core samples were 

tested at the natural mc (47%). The free swell percent 

curves are shown in Fig. 8. The results clearly indicate 

the high swell potential for the remolded samples. The 

maximum swell percent after four days for the remolded 

samples prepared at mc values of 47% and 35% ranges 

between 31.8% and 41.5% with an average value of 

37%. On the other hand, relatively low swell potential 

values were recorded for the core samples where the 

maximum value for the two samples were 8.3% and 

9.4%. Such a low value is almost one fourth the 

corresponding values for the remolded samples at the 

same mc values. This could be attributed to the adhesion 

in the cores, which developed under pressure and was 

affected by aging. The adhesion as well as the original 

structure are lost upon remolding. Remolding usually 

destroys the structure of the sample and thus the layering 

(which usually reflects the sedimentation) is not 

retained. The SEM micrographs clearly show the cardboard 

or paper-like appearance particle orientation of 

Fig. 8. Free swell curves for the brown palygorskite. 

the cores which resulted from the sedimentation process 

and upon remolding such orientation was lost. The 

remolded samples are expected to be more uniform than 

the cores in terms of density, voids and composition. On 

the other hand, cores are not necessarily homogeneous 

and some large voids could exist and may accommodate 

some swelling. This will reduce the overall swelling of 

the cores. 

It is clear from the data presented in Fig. 8 that steady 

state for swelling was reached after a period of about four 

days. This is mainly due to the low permeability of the 

palygorskite and thus the penetration of water through the 

sample was slow. Such time seems reasonable since the 

volume increase or swelling is proportional to the volume 

of the wetted sample and thus the steady state will be 

reached when the entire sample is completely wetted. In 

addition, the results show the effect of the molding or the 

initial mc on the swelling potential. The samples prepared 

at a mc of 35% had swelling values higher than those of 

the samples prepared at a mc of 47%. The increase in the 

swell potential could be as much as 18%, which is 

attributed solely to the initial or the molding mc. 

The variations in the void ratio (e) with the applied 

vertical effective stress (σ′ v) are presented on the e−log 

σ′v curves in Figs. 9, 10 and 11 for the brown palygorskite. 

The data for the remolded samples are similar 

and the differences between duplicate samples are 

minimal. The main difference is the magnitude of the 

initial swelling upon inundation at the seating stress of 

7 kPa. The four remolded samples were prepared to have 

the same initial void ratio (e i) and thus the e i values 

ranged between 1.472 and 1.522. The samples prepared 

at a mc of 35% exhibited larger swell potential compared

Fig. 9. Oedometer test results for the remolded brown palygorskite 

prepared at a moisture content of 47%. 

to the ones prepared at a mc of 47%. For the 35% mc 

samples, the void ratio changed from ei values of 1.522 

for the two samples to e f (at the end of the free swelling) 

of 2.569 and 2.659. The corresponding values for the 

47% mc are e i values of 1.47 for the two samples and the 

ef are 2.313 and 2.218 at the end of the free swell. An 

average increase in swelling of about 17.7% based on the 

Fig. 10. Oedometer test results for the remolded brown palygorskite 



215 

Fig. 11. Oedometer test results for the core brown palygorskite tested 

at the natural moisture content (47%). 

e values was noticed due to the reduction of the initial mc 

from 47% to 35%. 

3.2.1.2. Compression loading and unloading for the 

brown palygorskite. In all oedometer tests, once the 

free swell readings approached a steady state condition, 

at a stress of 7 kPa, the samples were then loaded in the 

oedometer according to the ASTM D 4546 method. 

Upon the one-dimensional loading and unloading in the 

oedometer, the samples followed a typical soil loading 

and unloading curve. The loading parts have distinct 

virgin portions. The duplicate remolded samples 

reached almost the same void ratio (e p) at the maximum 

applied stress, which was set at a value of 1638 kPa, 

regardless of the void ratio values at the end of the free 

swell (ef). The (ep) values for the 35% mc samples were 

1.788 and 1.809, while the corresponding values for the 

47% mc were 1.678 and 1.670. The duplicate remolded 

samples exhibited consistent loading/unloading behavior 

as shown in Figs. 9 and 10. 

The cores displayed similar behavior to the remolded 

samples and the loading and unloading curves are 

similar to those of regular soils. The two core samples 

that were selected to be similar show large variations in 

the magnitude of the e values after wetting (ef) and after 

loading to a stress of 1638 kPa (ep). This clearly indicates 

that core samples cannot necessarily be assumed 

identical. For the two cores, the e i values were 1.459 and 

1.529. The corresponding e f values at the end of the free 

swell were 1.681 and 1.799. The ep values at the


maximum stress value (1638 kPa) were 1.519 and 

1.627. Upon unloading to a stress of 7 kPa, the samples 

rebound and the resulting unloading void ratio (e u) 

values are somewhat high, especially for the cores. The 

eu for the cores were 1.777 and 1.667 while the corresponding 

ef values were 1.799 and 1.681. This shows 

that wetting and loading the core samples did not change 

the swell potential significantly. On the other hand, the 

e u values for the remolded brown samples were 2.238 

and 2.32 while the corresponding e f values were 2.659 

and 2.569. Such data indicate that wetting and loading 

of remolded samples reduce the swell potential although 

remolding had already increased the swell potential 

significantly. 

3.2.1.3. Free swell tests for the gray palygorskite. 

Similar to the brown palygorskite, three sets of samples 

from the gray material were tested for the free swell 

determination. Two sets were prepared from the 

powdered material; one at the natural mc (42%) while 

the other was at a mc of 35%. The third set consisted of 

cores. The free swell curves are presented in Fig. 12. 

The data presented are similar to those for the brown 

material but the magnitude of swelling of the remolded 

material was less for the gray samples. The average 

maximum swell percentage for remolded sample prepared 

at mc values of 42% and 35% were 31.9% and 

33.6%, respectively. 

The variations of the void ratio (e) with the applied 

vertical effective stress σv′ are presented on the e−log σv′ 

curves in Figs. 13, 14 and 15 for the gray palygorskite. 

The samples prepared at a mc of 35% exhibited larger 

Fig. 12. Free swell curves for the gray palygorskite. 

Fig. 13. Oedometer test results for the remolded gray palygorskite 


swell potential compared to the ones prepared at a mc of 

42%. For the 42% mc samples, the void ratio changed 

from e i values of 1.666 and 1.59 for the two samples to e f 

of 2.462 and 2.364 at the end of the free swell. The 

corresponding values for the 35% mc are ei values of 

1.48 for the two samples and the ef are 2.313 and 2.311 at 

Fig. 14. Oedometer test results for the remolded gray palygorskite 

prepared at a moisture content of 35% (only one sample is shown since 

the curves for the second one are exactly overlaying the first one).

Fig. 15. Oedometer test results for the core gray palygorskite tested at 

the natural moisture content (42%). 

the end of the free swell. An average increase in swelling 

upon inundation of about 8% based on the e values was 

noticed due to the reduction of mc from 42% to 35%. The 

curves in Fig. 12 clearly show that duplicate gray samples 

of each remolded set give almost the same swell potential 

curves. 

On the other hand, the gray core samples showed 

swell potential values higher than those for the brown 

material and this is attributed to the type of material and 

the mc. The brown palygorskite cores were tested at a 

moisture content of 47% while the gray palygorskite 

cores were tested a moisture content of 42%. This could 

explain why the gray palygorskite produced high swell 

potential (11.2% and 19.2%) compared to the brown 

palygorskite (8.3% and 9.4%). It is also noted that the 

two gray core samples exhibited large variations in the 

swell potential readings where one has a steady state 

value of about 11.2% while the other has a value of 

about 19.2%. These deviations are expected for naturally 

occurring material in which the density, composition 

and moisture content cannot be assumed identical. 

The curves presented in Figs. 8 and 12 shows that the 

remolded brown palygorskite prepared at a mc of 35% 

show higher swelling potential (38.9% and 42.5%) 

compared to the gray palygorskite prepared at the same 

mc and produced a swelling of 33.6% for both samples. 

This is attributed to the differences in the characteristics 

of the two palygorskites; for example the shrinkage limit 


for the brown palygorskite is 80% while that of the gray 

palygorskite is 38.5%. 

3.2.1.4. Compression loading and unloading for the 

gray palygorskite. The gray palygorskite was loaded 

in the oedometer after reaching the steady state swelling 

at a stress of 7 kPa, in a way similar to that for the brown 

palygorskite. Figs. 13 and 14 show that the samples 

follow a typical soil loading and unloading curve. The 

loading parts have distinct virgin portions. The duplicate 

remolded samples reached almost the same void ratio 

(ep) at the maximum applied stress of 1638 kPa, regardless 

of the ef values at the end of the free swell. The ep 

values at the 1638 kPa stress for the 35% mc samples 

were 1.684 and 1.670, while the corresponding values 

for the 42% mc were 1.727 and 1.751. 

The cores displayed similar behavior to the remolded 

samples and the loading and unloading curves were 

similar to those of regular soils. The two core samples 

that were selected to be similar show large variations in 

the magnitude of the e values after wetting and after 

loading to a stress of 1638 kPa. This is expected since the 

two core samples are not necessary identical. For the two 

core samples, the ei values were 1.570 and 1.731. The 

corresponding e f values at the end of the free swell were 

1.861 and 2.346. The e p values at the maximum stress 

value (1638 kPa) were 1.665 and 1.882. 

Upon unloading to a stress of 7 kPa, the samples 

behaved in a way similar to that of the brown palygorskite. 

The core samples rebound to almost their ef values i.e. the 

eu for the cores were 1.832 and 2.289 while the corresponding 

ef values were 1.861 and 2.346. However, the 

e u values for the remolded samples (35% mc) were 2.056 

and 2.103 while the corresponding e f values were 2.311 

for both samples. This confirms the observations made for 

the brown palygorskite regarding the effect of remolding, 

wetting, loading and unloading. Careful inspection of the 

curves presented in Figs. 11 and 15, for the core samples, 

clearly show that unloading takes the samples very close 

to their initial void ratios. This indicates that prewetting 

and loading does not reduce the swell potential since upon 

unloading the sample rebound to its void ratio after the 

initial prewetting. 

3.3. Constant volume oedometer tests 

217 

The constant volume oedometer tests are instrumental 

in predicting the swelling pressure of expansive soils. 

Constant volume tests were conducted by placing the 

samples in the oedometer and applying a seating load of 

7 kPa and then restraining the vertical movement by a 

rigid reaction frame. The rigid frame reacts against the


Fig. 16. Swelling pressure curves for the remolded brown and gray 

palygorskites prepared at a moisture content of 35%. 

sample through a high capacity load cell. Upon inundation, 

the material tries to expand but the restraining 

effect from the rigid frame results in a pressure build up, 

which is measured using a high capacity load cell. The 

stress-time curves for the remolded samples are shown 

in Fig. 16, for both the brown and the gray palygorskites. 

The average maximum stress for the brown and 

gray palygorskites are 4989 kPa and 4361 kPa, respectively. 

The data show that high stresses were developed 

upon samples inundation. The maximum stress 

was reached in about 3 h which is relatively short 

compared to the four days needed to reach maximum 

swell potential. This is expected since the volume increase 

is dependent on the volume of the specimen or 

the thickness of the specimen that got wetted. Therefore, 

each part that got wetted will produce a swelling or 

expansion proportional to the height of the wetted 

portion. This swelling will accumulate at the surface of 

the specimen and will be recorded as a total height 

increase. On the other hand, the force resulting from 

swelling is related to the area and not to the thickness of 

the specimen. The specimen has a constant area and a 

certain amount of stress will result when the surface is 

wetted regardless of the wetted thickness. Thus, the time 

required to wet the top or bottom few millimeters will be 

much shorter than the time required to wet the entire 

sample thickness even if it is only 19 mm thick. Hence, 

stress develops as soon as the surfaces (top and/or 

bottom) get wetted and this is what triggers severe 

damages in rigid structures. Similar data has been 

reported by Chen (1988) who showed that the stress for 

end bearing piles reached a constant value in less than 

two days; while expansion for the end bearing piles 

under the same conditions took more than seven days to 

reach a constant value. It is, therefore, important to 

realize that for swelling stress to develop, it is not 

necessary to wet the entire layer. Furthermore, the 

curves in Fig. 16 show that the pressure reaches its 

maximum in about 3 h, after which the pressure decreased. 

The decrease in pressure was very small, however, 

it indicates that when more specimens' height got 

wetted the sample got softer and the stress did not keep 

increasing but instead it decreased. 

4. Summary and conclusions 

This investigation was carried out to determine the 

mineralogical composition and assess the swelling potential 

of Al-Khobar palygorskite. Based on the results 

reported in this study, the following concluding remarks 

can be made: 

1. The two Al-Khobar palygorskite soils have very high 

Atterberg limits and very high swelling pressure and 

swell potential. 

2. The XRD data show that the samples are pure 

palygorskite. 

3. The undisturbed samples (cores) exhibited swelling 

potential values that were much less than the corresponding 

values for the remolded samples. This 

could be ascribed to the changes of sample structure 

due to remolding. 

4. The maximum swelling pressure for the two palygorskites 

was developed in a much shorter time (3 h) 

compared to the maximum swell potential which 

took more than four days. 

5. Prewetting did not reduce the swell potential for core 

samples. 

6. Remolding increased the swell potential significantly; 

however, recompaction and wetting reduced the 

swell potential after one cycle of loading and unloading 

in the oedometer. 

Acknowledgement 

The author acknowledges the support provided by 

King Fahd University of Petroleum and Minerals. 

Thanks are extended to late Hassan Zakariya Saleh, 

Engineer Arifulla Vantilla and Engineer Walid El-Mehdi 

for their help in the experimental work. The cooperation 

of the Saline Water Conversion Corporation (SWCC), 

Al-Khobar Branch and Tekfen Construction and 

Installation Company during the field work is dutifully 

acknowledged.

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