Response of sabkha to laboratory tests: A case study

Engineering Geology, 33 (1992): 111-125 Elsevier Science Publishers B.V., Amsterdam

l 1l

Response of sabkha to laboratory tests: A case study O m a r S a e e d B a g h a b r a A 1 - A m o u d i a, S a h e l N . A b d u l j a u w a d " , Z a g h l o u l R. E 1 - N a g g a r b a n d Rasheeduzzafar" aDepartment of Civil Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia bDepartment of Earth Sciences, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (Received February 6, 1992; revised version accepted August 24, 1992)

ABSTRACT A1-Amoudi, O.S.B., Abduljauwad, S.N., E1-Naggar, Z.R. and Rasheeduzzafar, 1993. Response of sabkha to laboratory tests: A case study. Eng. Geol., 33: I 11-125. Sabkha is a saline, evaporative flat soil that forms under arid climates. It is generally associated with saturated watertables that are very close to the ground surface. There are typically two major types of sabkhas; coastal and continental or inland. The presence of brines in the sabkha matrix and the crystallization of diagenetic minerals therein can lead to the highly variable mechanical properties of such a soil. This investigation was carried out to evaluate the engineering properties of this salt-laden and water-sensitive sabkha soil. Several laboratory tests were conducted, including compaction, permeability, unconfined compression, direct shear, triaxial, CBR, specific gravity measurements and grain-size distribution analysis. The investigation also focussed on the effect of distilled water and/or sabkha brine on the properties of this unusual type of soil.

Introduction The large, salt-encrusted, evaporative flats situated along the coasts or farther inland o f m a n y arid and semi-arid countries are k n o w n by the Arabic word " s a b k h a " . A coastal sabkha is typically bordered on the seaward side by a semirestricted lagoon, and on the landward side by a desert (i.e., sand dunes) or rock outcrops. Such are t h o u g h t to be the result o f depositional ofllap, at least in their seaward parts (Kinsman, 1969). A shoreline regression o f a b o u t 1 to 2 m/y has prevailed in the western, southwestern and southern coastal plains o f the A r a b i a n G u l f during the past 4000-5000 years (Butler, 1969; Evans et al., 1969). This has enhanced the development o f coastal sabkhas, particularly in the subtidal, intertidal and supratidal environments o f this regressing landCorrespondenceto: Omar Saeed Baghabra A1-Amoudi, Department of Civil Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. 0013-7952/92/$05.00

locked sea. Coastal sabkhas are usually stark, saltencrusted and virtually flat. Their surfaces dip very gently seaward at imperceptible rates, and do not normally exceed a few centimeters to one and a half meters in elevation above the mean high-water level (E1-Naggar, 1988). In contrast to these coastal saline flats, continental sabkhas usually develop as deflation surfaces, from which the wind has removed the dry sediment particles, parallel to the watertable, at levels that are controlled by the dampness o f the sediments (Fookes et al., 1985); and the base level o f deflation has to lie just above the capillary fringe in the sediments (Johnson et al., 1978). The rate o f evaporation in inland sabkhas is supposedly higher than in coastal ones due to the more arid conditions. Consequently, the g r o u n d w a t e r table plays a m o r e substantial role in the development o f such types o f sabkhas which are usually less-developed in extent and predominantly tectonically a n d / o r topographically controlled (EI-Naggar, 1988). Such inland salt flats have been c o m m o n l y included

© 1992 - - Elsevier Science Publishers B.V. All rights reserved.

112

under the term "playa", and hence some authors (e.g., Illing and Taylor, 1967; Smith, 1980; Fookes et al., 1985) suggested that the term "sabkha" be restricted to coastal salt flats only. However, "these two salt flats share more similarities than differences, and it might be inaccurate to distinguish between them by calling one a sabkha while the other a playa" (Kinsman, 1969). In this paper, a comprehensive laboratory investigation of a surficial sabkha soil from eastern Saudi Arabia (the Ras-Ghar sabkha) has been undertaken, with particular emphasis on its response to laboratory tests, as well as to the effect of both distilled water and sabkha brine on sabkha properties.

Geology of the Ras Al-Ghar sabkha deposit Along the Arabian Gulf coasts, sabkha soils extend intermittently for more than 1700 km with average inland extensions of about 20 km. These soils normally have loose, rather porous and permeable, sandy to gritty textures. The outer surface is usually composed of hygroscopic salts, which can render the normally stable sabkha surfaces unstable when dampened. The sedimentary features, mineralogical composition and the chemistry of the interstitial brines in such coastal sabkhas vary greatly both in horizontal and vertical directions (Butler, 1969). Horizontal variations are expected to be related to the proximity from the shoreline (Akili and Torrance, 1981), while vertical variations represent successive stages in the development of the sabkha cycle. This lateral and vertical variability of composition, structure and texture, added to the presence of highly concentrated brines, are the distinguishing features that have given the sabkha soil its hazardous geotechnical properties (Shehata et al., 1990; A1-Amoudi et al., 1992). The Ras A1-Ghar salt flat is a small part of the A1-Riyas Sabkha which is one of the most important sabkha deposits developed along the western shores of the Arabian Gulf. It starts about 30 km N N E of the Dhahran town and extends for about 70 km N N W of the coastal town of A1-Jubail, varying in width between a few km and more than 12 km.

O M A R S A E E D B A G H A B R A A L - A M O U D I ET AL.

The surface of this sabkha is generally even, with only a few, scattered elevations that do not exceed 10 m above the ground surface. Such elevations are usually covered with wind-blown sands, and are composed of marly limestones that belong to the Miocene Hadrukh or Dam Formations. Sand accumulations around such Miocene elevations indicate that the direction of the prevailing wind is N25°W (Johnson et al., 1978). This sabkha deposit is of Holocene age and represents the remnants of a marine regression that followed a middle Holocene high sea level in the region (approximately 2000-5000 years before present). Evidence of such a relatively high sea level during the middle Holocene time is given by a number of raised beaches, as well as by the vast coastal sabkhas that fringe the Arabian Gulf coast with inland extensions of more than 100 kin. Such deposits were laid down in old bays, which were gradually filled up by aeolian sands and by shallow, restricted marine sediments (silts, clays, carbonates, gypsum, anhydrite and other evaporite salts). These are commonly intercalated by algal mats or layers of shell detritus, depending on the level of the groundwater table and on the capillary fringe. Such algal mats and shell fragments have not yet been absolutely dated for A1-Riyas sabkha by the use of the ~4C method. Similar to many Holocene sabkhas that fringe the Arabian Gulf coast, the A1-Riyas Sabkha is built on top of Pleistocene aeolian sands. The direct interfingering with and transition to subrecent and recent deposits in the tidal flat area took place during the middle Holocene high sea stand (Hotzl et al., 1978). The texture of the sabkha surface varies with the seasons. During the dry periods it ranges from a very smooth crust to a faint polygonal pattern with flat irregular units 10-15 cm across, separated by gypsiferous sandy rims ( 1 - 2 m m high and 5 10 mm wide), to a gypsiferous crust, 5-10 mm thick. Occasionally, a thin discontinuous crust of halite is found, but is soon covered and/or dispersed by blowing sand. The sabkha surface is strongly hygroscopic and hence becomes wet and muddy during humid periods. The watertable in the region is generally between

113

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY

0.30 and 1.15 m below the ground surface, depending on the precipitation rate. Occasional, nonuniform drops of 10-15 cm in the watertable are attributed to both a vertical and lateral inhomogeneity of the sabkha soil, which results in both a non-uniform permeability within the sabkha body and varying rates of evaporation from its surface. The sabkha sediments are distinctly layered, with a l-cm crust of fine-grained sand, lightly cemented with halite (NaCI) and gypsum (CASO4'2 H20 ) as well as possible traces of sylvite (KCI) and epsomite (MgSO4"7 H20). Below the crust, several layers of poorly sorted, fine- to medium-grained sand and gray calcareous mud (5-10cm in thickness) alternate. This lime-mud usually contains appreciable quantities of calcite (CaCO3) and dolomite [CaMg(CO3)2], and is followed downward by somewhat thicker layers of fine to very coarse, poorly to fairly sorted sand. The sand consists mainly of well-rounded, frosted quartz grains with minor cherty grains. Thin bands (i-3 cm thick) of brown to gray mud, of algal mats and other shell layers and/or nodular to sugary anhydrite (CaSO4) veins (5-10 cm thick) are occasionally intercalated with the sand layers. Anhydrite veins can also be found a few cm above the watertable, and range in substance from thick plastic mud to hard rock. Prismatic gypsum crystals (0.5-10cm long) and halite cubes (5 cm across) are usually found clustered in a layer, just above, and extending several cm below the watertable. The sabkha sediments (about 5-8 m thick) are lightly to moderately cemented, especially just above and below the watertable. Gypsum is the principal cement, but both carbonates and halites are sometimes also involved in the process of cementation. Lenticularity is pronounced in the various sabkha layers, and hence both lateral and vertical variations (in composition, texture, structure, permeability, etc.) are observed only over very short distances. In the extreme eastern part of the Ras AI-Ghar sabkha, a salt lens intercalates the succession. This lens increases steadily in thickness from its western outcrop limits to more than 6 m at Ras Al-Ghar in the east. Here the salt lens is covered by a layer,

about 1.5-2 m thick, consisting of very loose carbonate sand (of fine to medium grain size) with salt crystals and pockets, which overlies more than 12 m of loose to very dense, gray, calcareous, fineto medium-grained sand with numerous shell fragments. Further to the west, the salt lens disappears completely, and the upper 1.5-2 m sabkha overlies a layer of soft, salt-rich clay. These are underlain by weakly cemented, sandy, silty clays and sandy, clayey silts with low plasticity (with intercalating cemented sand layers), and are followed downward by silty clays and clayey silts (with occasional lenses of cemented silty sand) that grade downward into silty clays for more than 30 m. Apparently, the salt lens was formed by evaporation of salt ponds (salina) within the furthest extremities of the seaward limits of the sabkha as remnants of the regressing sea (in the form of restricted marches or lagoons).

Experimental program The first phase of our experimental program was to survey certain potential locations in the Eastern Province of Saudi Arabia including Ju'aymah, Jubail, Ras Tanura and Ras Al-Ghar (Fig. 1).

,

SAUDI

ARABIA

"~ \

~

Fig. 1. Vicinitymap showingthe project location.

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114

Visual observation of the layers above the groundwater table was conducted. The salt-encrusted surface, the sandy nature of the surficial layers, the presence of pockets of white materials (i.e., most probably anhydrite and gypsum) and the proximity of the groundwater table to the ground surface were the symptoms that were considered. Chemical analysis of the groundwater was also undertaken for relative comparison with literature data. A detailed experimental program was carried out to evaluate the response of sabkha to laboratory tests. Surficial undisturbed samples were obtained using PVC tubes that had sharpened penetrating ends. Classical samplers made of steel or brass were not used due to the high salt content of the sabkha which could cause a rust problem. Disturbed sabkha samples were brought to the laboratory and spread over a large area for air drying. Oven drying was not used in order to inhibit any phase transformation, particularly gypsum which is known to transform to anhydrite at high temperatures (Sonnenfeld, 1984). Afterwards, the soil was crushed gently with plastic hammers to break down aggregated particles that were cemented together, allowing the soil to pass ASTM sieve No. 10 (2 mm mesh), in general accordance with ASTM D 421. The soil was then thoroughly homogenized and kept in plastic drums for testing. Sieve analysis tests were conducted using both dry (ASTM D 422) and wet techniques. Two more wet sieve analyses were conducted, once using methylene chloride (Russell, 1974) and once in the sabkha brine, because neither of these liquids can cause any salt dissolution (A1-Amoudi, 1992). The Atterberg limit tests were not conducted because the soil is nonplastic. The specific gravity of three disturbed samples was conducted using vacuum only, in general accordance with ASTM D 854. Oven drying was carried out at 70°C until constant weights were attained. Six, 9.7-cm diameter, undisturbed samples were tested under constant (ASTM D 2434) and variable-head permeability conditions. The head was 177 cm for the former while the head differential was 100cm for the latter (i.e., h l = 1 5 0 c m and h z = 5 0 c m ) , both of which were kept constant during the tests. Distilled water was used for testing

O M A R S A E E D B A G H A B R A A L - A M O U D I ET AL.

the first three samples. For the remaining three samples, sabkha brine was used. The brine was used to simulate groundwater movement. The viscosity of the brine was determined according to ASTM D 455 while its density was determined using a hydrometer. A standard Proctor test ASTM D698) of 24.5 N hammer weight and 0.305 m drop height was conducted using distilled water and sabkha brine as lubricating agents, respectively. The brine was used because distilled and fresh water may be economically unfeasible. Moisture contents were determined first at the oven-drying temperature of 70°C until constant weights were obtained; thereafter, the temperature was raised to I10°C and again the constant dry weights were recorded. It should be mentioned that the soil became spongy around the optimum moisture content (Wop0 and it was very difficult to compact the soil on the wet side of Wopt for both liquids used. Two series of consolidation tests were conducted on four undisturbed samples in accordance with ASTM D 2435. The samples were initially loaded with an overburden pressure (8.4 kPa), thereafter, two samples were flooded with distilled water, the other two were flooded with sabkha brine. Load increments were added until a maximum pressure of 1810 kPa was reached after which unloading proceeded. Reloading was then resumed until the test termination at a pressure of 1810 kPa. Three undisturbed samples were retrieved and tested by an unconfined compression test. Two more series of samples were prepared to study the effect of moisture content on sabkha strength. In the first series, distilled water was used, while in the second, sabkha brine was used. Unconfined compression (qu) tests were performed on 5 × 10 cm samples at a constant loading rate of 0.5 mm/min. Direct shear tests were conducted in general accordance with ASTM D 3080 at a loading rate of 0.75 mm/min. The normal stresses were 109, 218 and 435 kPa, respectively. Isotropically consolidated-drained triaxial tests (ASTM D 2850) were conducted on similar specimens to those of the unconfined compression tests. Six undisturbed specimens were tested under six different confining pressures, namely 13.8, 34.5, 68.9, 103.4, 137.9 and 206.8 kPa. The reason for

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY

testing many samples is the variability of sabkha, which may cause variation in results (Al-Amoudi et al., 1991). California bearing-ratio (CBR) tests were conducted in compliance with ASTM D 1883, at a penetration rate of 1.27 mm/min and the load and penetration readings were taken until a penetration of slightly more than 13 mm was obtained. Eight undisturbed specimens were retrieved from the field in the special CBR moulds and covered with nylon sheets to prevent any moisture loss. Four were tested at their natural state on the same day that they were obtained. The other four were soaked in distilled water under a surcharge of 5 kg, according to the standard procedure, for four days, with a gage attached to the specimens to measure any expansion due to water flooding. Experimental results and discussion

Site selection • Of the four potential sites two were rejected; Ras Tanura, because of its proximity to a petrochemical unit and Jubail, because of the low level of the groundwater table, which was not in evidence even at a depth of some 1.5 m. Further, the soil there was relatively clean, suggesting that a recent fill had been laid out, as confirmed later. The subsurface investigation carried out at both Ju'aymah and Ras A1-Ghar confirmed that both locations represented true sabkhas according to the characteristics mentioned earlier. The chemical analysis of the groundwater for both sites and average values reported by Johnson et al. (1978), as well as an analysis of a typical seawater from the Arabian Gulf for relative comparison, are presented in Table 1. These data indicate that the Ras Al-Ghar site corresponded to those character, istics (Johnson et al., 1978) that represent typical eastern Saudi sabkhas, and to those in other Arabian Gulf countries (Butler, 1969).

Grain-size analysis The results of a grain-size distribution analysis are depicted in Fig. 2, which clearly reflect the large variations between the different techniques

[ 15

used. While dry sieving and wet sieving using distilled water resulted in 2% and 32% passing sieve No. 200, respectively, the wet sieving using methylene chloride and sabkha brine resulted in an almost similar percentage of about 13% passing sieve No. 200. These different classes of performance clearly show the potential salt content in sabkhas and the need for a procedure to consider this fact. The results also indicated the nonsuitability of the ASTM D 422 procedure. The methylene chloride and sabkha brine gave reasonably similar results, which indicates that these procedures are more accurate in the determination of grain analysis than standard ASTM procedures (A1-Amoudi, 1992). Incorrect determination of grain-size analysis may lead to a misleading interpretation of the results, for example soil classification. Table 2 presents the soil classification of sabkha according to the A A S H T O and USCS, These data again indicate that there is no clear difference between the use of methylene chloride and sabkha brine in classifying the sabkha; both yielded similar results.

Specific gravity Specific gravity results were consistent; the three tests yielded 2.74, 2.72 and 2.72, with an average value of 2.73. This value is within the range of the specific gravity of sabkha soils of eastern Saudi Arabia (Abu-Taleb and Egeli, 1981). The value is not, however, within the range of typical sands or silty sands as proposed by Bowles (1978). This may probably be attributed to the conjoint effect of low oven temperature (70°C) at which the specific gravity was determined, and the high salt content of the sabkhas.

Permeability Comparison between the properties of distilled water and sabkha brine is shown below:

Distilled water Sabkha brine

/220 10.09 19.43

/2T 9.73 19.07

Gs 0.997882 1.207

where: /220 = dynamic viscosity at 20°C (millipoise);

116

OMAR SAEED BAGHABRA AL-AMOUDI ET A L

TABLE 1 Chemical analysis of sabkha brines and seawater in mg/ml (i.e., parts per thousand) Ions

Ju'aymah brine

Ras A1-Ghar brine

Averages of ref. + +

K F U P M beach sea water

Na + Mg 2+ K+ Ca z+ Fe z + Sr 2 + C1 Br(SO4) 2 (HCO3)

41.4 4.53 1.40 1.34 Trace 0.031 76.8 0.22 8.43 0.114

78.8 10.32 3.06 1.45 Trace 0.029 157.2 0.49 5.45 0.087

79.8 7.33 2.69 1.86 --** --** 158.2 --** 5.24 0.056

20.7 2.30 0.73 0.76 --** 0.0! 3 36.9 0.121 5.12 0.128

7.2

6.9

6.7

8.3

152,000

208,000

pH Conductivity*

--**

46200

*Microsiemen. **Not reported. + +From Johnson et al. (1978).

100

WetSieving(SobkhoBrine) eo-~ WetSieving(MethyleneChloride) ~__We 5 Sievin~(DistilledWater) M-i÷ DrySieving

0 1E-02

TABLE 2

]

1E-01 Groi~-~ze Dk~eter ( mm

Classification of sabkha soil according to A A S H T O and USCS Classification system

1E+{~

Fig. 2. Grain-size distribution of sabkha using sieve analysis.

/~T=dynamic viscosity at 21.5°C (laboratory temperature; (millipoise); and G s = specific gravity. which indicates the concentrated nature of the brine as shown in Table 1. Figures 3 a - f depict the constant and variable head permeability results using both distilled water and sabkha brine. The results obtained using sabkha brine in both types of permeability tests are presented in Figs. 3a-c, while those pertinent to distilled water are shown in Figs. 3d f. The use of sabkha brine in the constant and variable head

Dry sieve analysis

Wet sieve analysis using: Distilled water

Methylene chloride

Sabkha brine

AASHTO*

A-3

A-2-4

A-2-4

A-2-4

USCS: D6o (mm) D,o (mm) D3o (ram) Cut Cc +

0.37 0.18 0.21 2.1 0.7

2.0 0.0015"* 0.07** 1,300 1.5

0.28 0.050** 0.17 5.6 2.1

0.24 0.058** 0.15 4.1 1.5

Soil type

SP

SW

SW-SP

SW-SP

*Based on sieves//40 and 200. **By extrapolation of the data in Fig. 2. tCoefficient of uniformity. ++Coefficient of curvature.

permeability tests resulted in a reduction of permeability coefficients as the test was repeated. Figure 3a shows that the permeability coefficient was initially 1 . 7 8 x 1 0 - 6 m / s and reduced to 1.35 x 10 6 m / s after the test had been repeated twice. The same trend was observed with the

117

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY 22£-~ SpecimenI'1 2"0[' t1%,-016 -06

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r

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Fig, 3(b). Constant head permeability test using sabkha brine (specimen #2). 1~-(}~

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Fig. 3(c). Variable head permeability test using sabkha brine (specimen//3).

Fig. 3(f). Variable head permeability test using distilled water (specimen #6).

second sabkha specimen in which the constanthead permeability coefficient was reduced from 2.00× 10 -6 to 1.70x 10 -6 m/s after the test had been repeated twice (see Fig. 3b). Results of the

variable head test were m o r e or less consistent with the constant-head permeability test; the permeability coefficient was reduced from 1.26 x 10 - 6 to 1.24×lO-6m/s, after only two repetitions

118

(Fig. 3c). In the case of distilled water, the picture was completely different. The repetition of the test caused the permeability to increase from 2.10x 10 - s m/s to 3.15 x 10 5 m/s after four repetitions, using the constant head test (see Fig. 3d). The same trend was also observed in the case of the other specimen (Fig. 3e). The effect of distilled water on the variable-head permeability test is clearly shown in Fig. 3f. It should be mentioned that the volume of flowing water (or brine) for the constant head test was 500 cm 3, while it was only 35 cm 3 for the variable head test. Comparing the first two specimens, for which a constant-head permeability test using sabkha brine was used, the second one seems to be more permeable; however, the difference between the two was very small. It is also clear that the constant head test gave higher values of permeability coefficients than the variable head test. This is mainly due to the different mechanisms of the tests, coupled with a higher head for the former (h = 177 cm) and a lower head for the latter ( h l = 1 5 0 c m ) . For the other case, when distilled water was used, specimen #5 seems to be slightly more impervious than specimen .#4 (compare Fig. 3d with 3e). Comparison of the constant- and variable-head permeability tests indicates that the difference in the permeability coefficient between the two tests appears to be negligible although the variable head test yielded relatively lower values. It should be mentioned that the variation in the permeability coefficient with test repetitions seems to be more profound for the case of the constant-head permeability tests. This can be observed for both distilled water and sabkha brine results. A final comparison is made between the permeability coefficient to distilled water and to sabkha brine. Permeability (i.e., the permeability coefficient) to distilled water was higher than that to sabkha brine by as much as a factor of ten; the same trend was observed for both constant and variable head tests, although the difference was slightly more pronounced for the case of variable head test. The higher values of permeability and its increase with test repetition when distilled water was used is attributed to salt dissolution which causes more channels to form, thus tending to increase the permeability. Salt leaching due to test repetition has recently been observed by AI-Sanad

O M A R S A E E D B A G H A B R A A L - A M O U D I ET AL.

and AI-Bader (1990), although they reported negligible changes in the permeability coefficient. However, the increase in permeability with test repetition in the case of sabkha brine can be attributed to salt precipitation due to the time lag between the test repetitions, rather than salt dissolution due to the high salt concentration in the brine (Table 1).

Compressibility Consolidation tests are cumbersome and time consuming to perform and, in the case of sabkhas, they are very difficult to interpret. Nevertheless, these tests are instrumental in predicting the compressibility and swell potential and, sometimes, collapse potential. The results of the two series of consolidation tests are presented in Table 3 and are shown graphically in Figs. 4a and 4b. The data indicate that there was an unexpectedly insignificant effect when the sabkha specimens were flooded with either distilled water or sabkha brine, when the samples were loaded with overburden pressure. The reason might be attributed to the fact that the profuse cementation and desiccation of these surficial sabkhas (Hossain and Ali, 1988) could not be destroyed or disrupted, in contrast with what has been shown before in the case of permeability tests. This is because the head difference in the permeability tests was more than 150 cm while here the head difference was only a few centimeters. Another very important point is that the liquid (whether distilled or sabkha brine) was not allowed to seep continuously through the specimens and, therefore, the collapse potential could not accurately be detected (Al-Amoudi, 1992). These data also show that the response of sabkha to the consolidation test is not unique; different samples showed different total changes in void ratio although the samples were retrieved from the same place (i.e., at a depth of 25 cm from the ground surface) at the same time and tested under exactly the same conditions. This could be attributed to the variability of the sabkha due to the presence of diagenetic minerals (Bush, 1973). The data pertinent to consolidation tests are quantitatively shown in Table 3. These data give rise to the following observations. First, the change in void

119

RESPONSE O F SABKHA T O LABORATORY TESTS: A CASE S T U D Y

TABLE 3 Summary of consolidation test results (Series g1 and #2) Test characteristics

Series g1

Initial void ratio Change in void ratio due to flooding Final void ratio Total change in void ratio Change in moisture content (%) Overburden pressure (kPa) tr'c, over consolidation pressure (kPa) C'c, Compression index C'~, Swelling index Overconsolidation ratio (OCR)

Distilled water

Sabkha brine

Distilled water

Sabkha brine

0.830 0.012

0.826 0.001

0.814 0.006

0.796 0.003

0.571 0.259 + 4.0 8.4 130 0.17 0.017 15

0.604 0.222 + 0.4 8.4 185 0.15 0.016 22

0.507 0.307 +3.6 8.4 190 0.18 0.016 23

0.574 0.222 +2.1 8.4 155 0.16 0.015 18

0.8,50

O~ 0.75C "..

~ 0.7~]0 0.~

%

.•. Sebkho Brine #2 -e- Distilled Water #1

0550 I i i I 6 1E+01 2

1E~2 Prmcce,kPo

4 E 1E+03 2

Fig. 4(a). Consolidation test results (Series #l). 0.850

•e SabkhoBrine#1 -~ DistilledWater#1 0.750

"" ' '1..

0.7~

-...

~ 0.650 OleO0

i" e

~

',....

0.5~

0.450 1E+01

4 6 1E+(}2 2 Prmlxe,kPo

4

Fig. 4(b). Consolidation test results (Series #2).

1£+03

Series #2

ratio due to flooding, although not as significant as anticipated, was greater when the samples were flooded with distilled water than when flooded with sabkha brine (0.009 for the former c o m p a r e d to 0.002 for the latter). Second, all samples, whether flooded with distilled water or sabkha brine, exhibited an increase in moisture during the test; the increase, however, was generally greater when distilled water was used than with sabkha brine (3.8% c o m p a r e d to 1.3%). This probably reflects the ease o f wetting the sabkha samples with distilled water c o m p a r e d to wetting with sabkha brine, as shown in the permeability tests. Thirdly, the overconsolidation pressure (tr'c) seems to be insensitive to the type o f flooding fluid, i.e., flooding with distilled water produced a'c o f 130 and 190kPa, while flooding with sabkha brine produced a'c o f 185 and 155 kPa. This is in spite o f the variation in total change in void ratio, that tended to be more in the case o f the former. Consequently, different overconsolidation ratios ( O C R ' s ) resulted. An average O C R o f 19 and 20 were produced using distilled water and sabkha brine, respectively. These high values o f the O C R are not the result o f preconsolidation but reflect primarily the desiccation and cementation o f sabkhas. Similar behavior has been reported by Hossain and Ali (1988) and others. The fourth observation is that the compression index (C'¢) and the swelling

120

index (C's) are the same for all samples, with an average C'c of 0.16 and C's of 0.016, irrespective of the flooding liquids. This indicates that these parameters are also insensitive to changes surrounding the sabkha. C's is often assumed to be 5-10% of C'c for typical soils (Holtz and Kovacs, 1981), while here it is about 10% of C'~ for both liquids. Typical values of C's range from 0.015 to 0.035; the lower values are for clays of lower plasticity and lower OCR (Leonards, 1976). It is thus seen that sabkha falls within the lower limit o f this range of values. Furthermore, the ratio C'~/ C~ for the sabkha under consideration is 10, irrespective of the type of flooding liquid, and according to Saeedy and Mollah (1990), if this ratio ranges from 8 to 12 then the soil will possess low to moderate compressibility. Therefore, it can be concluded that the Ras A1-Ghar sabkha, a typical example of the Arabian Gulf sabkhas, possesses low compressibility. However, it should be mentioned that the extensive damage to buildings and other constructional facilities in southwestern Saudi Arabia reported by Erol (1989) and Dhowtan (1991) was not due to presence of sabkha per se but rather due to the presence of a salt dome and the high organic content in that sabkha soil.

OMAR SAEED BAGHABRA AL-AMOUDI ET AL.

TABLE 4 Sand cone field test results* Property

Test #1

Test #2

Test #3

Wet density (g/cm 3) (pcf) Moisture content (%) Dry density (g/cm 3) (pcf)

1.83 (113.7) 15.3 1.58 (98.8)

1,77 (110.7) 13,8 1,56 (97.3)

1.91 (119.1) 15.0 1.66 (103.6)

Average wet density = 1.84 g/cm 3 (114.5 lb/ft 3) Average dry density = 1.60 g/cm 3 (99.9 lb/ft 3) *According to ASTM D 1556 and AASHTO T-191 with bulk specific density of sand = 1.59 g/cm 3.

130 1 Ib/ft..3 = 16 tt~rrm3 125.... •... ,.. "~ 120-

"'i.'.'".O.

~115-

.." O"

" i "'"-. "E)

110-

105/

100

In-situ density Three field density tests using the classical sandcone method were conducted at the apices of an equal-sided triangle that surrounded the area from which disturbed/undisturbed samples were taken from a depth of about 25 cm below the ground surface. The results of these tests (Table 4) clearly reflect the very low density and the inhomogeneity of these soils.

~

~

~

4

6

8

L,

i

~ ! ~ D'~lled Water #1 at 70 de0 10 12 ioist~e C~tenL I

14

10

18

Fig, 5(a). Effect of oven temperature on compaction test results using distilled water.

130 1 Ib/ff~3 = 16 kll/t~3 125-

~::::~,.

120-

Moisture-density relationship and unconfined compression Results of the standard Proctor tests incorporating the effects of distilled water and sabkha brine, as well as oven temperature, are illustrated in Figs. 5a and 5b. These results indicate that there is no clear difference in the moisture-density curves when using either distilled water or sabkha brine, although a minute reduction in Woptwas associated

'7 ,~-115,.~ •

110-

o S~0khaBrine 12 at 110 de! • $obkho Bri~ 12 ~ 70 deg

• 105-

-O Sobl~o Brine I1 at 110 de(

I 4

i 5

i 6

i 7

[ B

i 0i 111 9 1 MoistureC~lent, X

B.Sab~o Brine 11 at 7(] de i i 12 13 14 15 10

Fig. 5(b). Effect of oven temperature on compaction test results using sabkha brine.

121

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY

with the latter. The effect of temperature, however, is clear, since increasing oven temperature will increase the optimum water content, thereby leading to decreasing dry densities. This difference, however, is not significant (i.e., less than 41b/ f t 3 ~,~ 64 kg/m3). Comparison of the dry field density with the laboratory maximum dry density indicates that the relative compaction was about 80%, which is a typically low value. This, consequently, leads to a significantly low strength, which has been confirmed by testing three undisturbed samples in unconfined compression. The results of these tests were 14.9, 19.9 and 22.0 kPa with an average of 18.6kPa (i.e., 2.7psi). Again, such variation in strength testifies for the inhomogeneity of sabkha soil. In order to evaluate the improvement attainable by compacting sabkha, two series of samples were prepared in the laboratory at different moisture contents and were subjected to the unconfined compression (q,) test. Distilled water was used in the first series and sabkha brine in the second. The results (Fig. 6) show that compaction of sabkha can increase its strength by as much as 370% (i.e., compared to undisturbed samples) if distilled water is used, and by as much as 550% if sabkha brine is used. It also indicates the difference in terms of the strength achieved and the optimum moisture content at which the strength is maximised in the two cases. Distilled water resulted in a maximum strength (qu) of about 70 kPa at a moisture content of 7.1%, compared to a maximum qu of 103 kPa at a moisture of 6.4% in the case of sabkha brine.

1.:

__

l

-~- Distilled Water

5

"1 6

r" 7

T 8

T 9

1 10

1 11

12

Fig. 6. Unconfined compression test on sabkha using both distilled water and sabkha brine.

The difference can be attributed to the excessively high salt content in the brine. Another deduction from Fig. 6 is the difference in defining the optimum moisture content from strength and density tests (the latter was evaluated by the standard compaction test). In the strength test, the moisture content was around 7% while it was around 10-11% in the density test. This could be the reason for the spongy characteristics displayed by the sabkha when it was compacted at Wow

Shear strength The shear strength parameters of sabkha soil were determined using two techniques: the direct shear test and the isotropically consolidateddrained (CD) triaxial test. Results of the former are shown in Fig. 7, where part (a) shows the horizontal displacement versus shear stress, while part (b) shows the Mohr-Coulomb envelope using the best-fit for the normal and shear stresses at failure. In part (a), the shape of the curves is typical for loose soils; the only exception is for the highest normal load, which is similar to those of dense soils. The angle of internal friction was 36 °, while the cohesion was 50 kPa (i.e., 7 psi). Results of the CD triaxial tests are shown in Fig. 8. The strain-stress curve is shown in part (a) of this figure. It is an established fact that as the confining pressure increases the deviatoric stress should accordingly increase. The specimen that was subjected to 138 kPa confining pressure did not respond well to this fact. This could be attributed to the inhomogeneity of the sabkha soil, where the presence of variable diagenetic minerals could lead to a significant reduction in strength. Figure 9 shows the presence of gypsum that was white, very soft and intercalated with crystals of halite. The presence of these minerals, as well as others, is well documented in the literature (Bush, 1973) and will undoubtedly lead to some variability in the results. The Mohr-Coulomb best-fit, however, seems to be quite representative of all the specimens, as shown in Fig. 8(b). Comparison of the shear strength parameters developed by the use of both the direct shear test and the CD triaxial test reveals a relatively higher effective angle of friction and higher cohesion, as

122

OMAR

SAEED

BAGHABRA

AL-AMOUDI

ET AL.

40016 kg Normol LOOd

300-

o

~"

P 2oo-

~8kQ 4k9

Iot;,--

o!2

o~

' Horizontal

Displacement,

cm

Fig. 7(a). Shear stress-horizontal displacement data for the direct shear tests•

I0 I

--

20 L

30

i

40 i ___1

50

(3", p s i 60 i

70

I •60

400 -

350 '

50

300-

250 0

~O a -

200-

E p,

ff E

t50-

2O tOO-

50-

Oo

;o

~o

,;o 2&

2;0 3;0 O-,k~

Fig. 7(b). Mohr-Coulomb envelope for the direct shear test results.

determined by the direct shear test than those determined from the triaxial one (i.e., ~p'= 36 ° and c ' = 5 0 k P a compared with ~p'=34.5 ° and c'= 14 kPa). This is attributed to the confining effect provided by the shear box that tends to produce relatively higher shear strength parameters. Das (1983) reported that the friction angle ~0' determined by triaxial tests is slightly lower (0-3 °) than that obtained from direct shear tests. The last observation is the development of cohesion in spite of the cohesionless nature of this sabkha as reflected by its non-plastic characteristics. This "apparent" cohesion is the result of desiccation and cementation developed by the presence of salt in sabkha. This is supported by the results of

consolidation tests where the overconsolidation ratio was as high as 20. These results indicate that sabkha is, relatively, an excellent soil from a load-bearing viewpoint, while the unconfined compression and field density tests demonstrate the sabkha's low strength and loose nature. This anomaly can be explained by the fact that the shear strength parameters, as determined by direct shear and triaxial tests, do not reflect the actual denseness of the soil due to the existing confinement in these tests: It is thought that if the sabkha is densified or stabilized, then much higher shear strength parameters could be achieved (Juillie and Sherwood, 1983; AI-Amoudi and Asi, 1991).

123

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY

600-

~

400-

103 kPa

"~

300-

,~

200-

""

/~ .

/"

o

35kPo

. . . . . . --

o

Confining Pressure

/.~

~,% ......

tO0- ~/

207kPa

-

.--

o.;~

_

o~

14kPo

o1~

0'20

025

0.130

035

Axial Strain

Fig. 8(a). Deviatoric stress axial strain data for the CD triaxial tests. I0

20

5o

40

5o

60

70

80

9o

0-, psi Loo IrO

120

6o0.8o 500-

70

400.

c'.

14 WPo

,2 300.

40 e: 30

200

2o7k~o

2O

I00I0 o

Fig. 9. Presence of diagenetic minerals in sabkha.

....

,. . . . ,, Ioo 200

, 300

, 400

'

I 500

6oo

, 700

i 8oo

• o

0-, kPo

Fig. 8(b). M o h r - C o u l o m b envelope for the CD triaxial test results.

California-bearing ratio ( CBR) It is worth mentioning that during the soaked CBR test, settlements were surprisingly observed without any dilatation. An average of 0.025 inch (0.64 mm) compressive settlement was attained at the fourth day. A1-Sulaimi et al. (1990) have also reported negligible swelling (from 0 to 0.20%) for similar tests on remoulded samples of the cemented gatch soil. Data obtained using CBR tests conducted in the laboratory and in the field (Table 5) indicate the very low strength of the surficial sabkha layer. Naturally-existing sabkhas had very poor CBR values of only 3 to 4, and these values were reduced by as much as 50% when the sabkha was flooded with water. This indicates the susceptibility of

sabkha to collapse once inundated with water, unlike the conventional consolidation test results presented earlier. Table 5 also shows that the soaked field and laboratory tests are almost similar, while those of the natural (unsoaked) conditions vary significantly. This variation can only be attributed to the inhomogeneity of the sabkha, even within the small distances from which the present samples were obtained. An in-depth observation of the individual corrected CBR Values presented in Table 5 clearly reflects this fact.

Concluding remarks The behavior of a surficial sabkha soil from a selected site in eastern Saudi Arabia was evaluated. Several laboratory tests were conducted on disturbed/undisturbed samples in an attemptto evaluate the response of sabkha to these tests. The effect of distilled water and/or sabkha brine on the properties of sabkha was also studied. The following conclusions are derived from these laboratory investigations: (1) The self-environmental sabkha brine as well as methylene chloride seem to be the best liquids

124

OMAR SAEED BAGHABRAAL-AMOUDIET AL. TABLE 5 Summary of field and laboratory CRB test results Type of test

Sample//

CBR at 0.1 inch

CBR at 0.2 inch

Pressure (psi)

CBR*

Pressure (psi)

CBR**

Corrected CBR value +

Average CBR value

Field CBR at natural condition

I 2 3

21 43 21

2.1 4.3 2.1

26 60 47

1.7 4.0 3.1

2.1 4.3 3.1

3.2

Field CBR at soaked condition

I 2 3

23 17 8

2.3 1.7 0.8

32 25 13

2.1 1.7 0.9

2.3 1.7 0.9

1.6

Laboratory CBR at undisturbed natural condition

I 2 3 4

12.79 41.96 16.24 40.12

1.3 4.2 1.6 4.0

20.21 65.26 23.37 50.27

1.3 4.4 1.6 3.4

1.3 4.4 1.6 4.0

Laboratory CBR at undisturbed natural soaked condition

1 2 3 4

31.30 15.37 22.78 14.32

3.1 1.5 2.3 1.4

42.95 21.61 29,07 18.21

2.9 1.4 1.9 1.2

3.1 + + 1.5 2.3 1.4

3.8

1.7

Note: 1 psi = 6.9 kPa; and 1 inch = 25.4 mm. *CBR = Pressure/I 0.0. **CBR = Pressure/15.0. +This value equals to the highest of the two CBR values. ÷ +This test was deleted due to disturbance.

by which the grain-size distribution can be determined accurately. (2) Specific gravity tests must be conducted at a temperature that inhibits the phase transformation of some diagenetic minerals that typically exist in the sabkha matrix. (3) Constant- and variable-head permeability tests are very much affected by the type of liquid used. Distilled water causes the salts to be leached out, thereby increasing the permeability, while sabkha brine causes a marginal decrease in the permeability. Constant head tests give relatively increased permeability coefficients compared with the variable head tests. (4) The conventional consolidation test does not seem to reflect the anticipated collapse potential of these soils. Moreover, these surficial soils have high over-consolidation ratios which are attributed to desiccation and cementation. Swelling and compression indices seem to be unaffected by the

flooding liquid. The Ras A1-Ghar sabkha has low compressibility. (5) The field density and unconfined compression tests accurately reflect the rather loose and weak condition of these soils in their natural state. Standard compaction tests increase the density, while the strength can only be increased if and when the soil is compacted at very low moisture contents, particularly if sabkha brine is used. (6) The shear strength parameters of the sabkha (~0' and c') as determined by the direct shear test are slightly greater than those determined by the triaxial test. The apparent cohesion, in both cases, is attributed to cementation. (7) A California-bearing ratio (CBR) test indicates that sabkha is a weak soil and it is highly susceptible to collapse upon flooding with water. Flooding reduces the CBR values by as much as 50%.

RESPONSE OF SABKHA TO LABORATORY TESTS: A CASE STUDY

Acknowledgements T h e a u t h o r s w o u l d like to t h a n k M r . H a s s a n Z a k a r i a y a S a l e h f o r the a s s i s t a n c e p r o v i d e d d u r i n g the c o u r s e o f this r e s e a r c h . T h e m o r a l s u p p o r t o f the D e p a r t m e n t o f Civil E n g i n e e r i n g , K F U P M , is g r e a t l y a p p r e c i a t e d . M r . S o l a n o C r u z is a c k n o w l e d g e d for t y p i n g the m a n u s c r i p t . T h e s e n i o r a u t h o r w o u l d like to t h a n k t h e R e c t o r o f K F U P M , D r . B a k r A b d u l l a h Bin B a k r , f o r his e n c o u r a g e m e n t .

References Abu-Taleb, M.G. and Egeli, I., 1981. Some geotechnical problems in Eastern Province of Saudi Arabia. Proc. Symp. Geotechnical Problems in Saudi Arabia, Riyadh, Vol. 1, pp. 799-81 I. Akili, W. and Torrance, J.K., 1981. The development and geotechnical problems of sabkha, with preliminary experiments on the static penetration resistance of cemented sands. Q. J. Eng. Geol., 14: 59-73. AI-Amoudi, O.S.B., 1992. Studies on soil-foundation interaction in the sabkha environment of eastern province of Saudi Arabia. Ph.D. diss., Dep. Civil Eng., King Fahd Univ. Petroleum and Minerals, Dhahran, Saudi Arabia. AI-Amoudi, O.S.B., Abduljauwad, S.N., E1-Naggar, Z.R. and Safar, M.M., 1991. Geotechnical considerations on field and laboratory testing of sabkha. Proc. Symp. Recent Advances in Geotechnical Engineering III, Singapore, Vol. 1, pp. I-6. AI-Amoudi, O.S.B. and Asi, I.M., 1991. An investigation on improvement of sabkha properties. Proc. Symp. Recent Advances in Geotechnical Engineering III, Singapore, Vol. 1, pp. 7-12. AI-Sanad, H. and AI-Bader, B., 1990. Laboratory study on leaching of calcareous soil from Kuwait. ASCE, J. Geotech. Eng., 116(12): 1797-1809. AI-Sulaimi, J.S., Mollah, M.A. and Matti, M.A., 1990. Geotechnical properties of calcrete soil (gatch) in Kuwait. Eng. Geol., 28(1/2): 191-204. Bowles, J.E., 1978. Engineering properties of soils and their measurement, 2nd ed. McGraw-Hill, New York, N.Y., 213 pp. Bush, P., 1973. Some aspects of the diagenetic history of the sabkha in Abu Dhabi, Persian Gulf. In: B.H. Purser (Editor), The Persian Gulf. Springer-Verlag, Berlin, pp. 395-407. Butler, G.P., 1969. Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf. J. Sediment. Petrol., 39(I): 70-89. Das, B.M., 1983. Advanced Soil Mechanics. Hemisphere Publ. Co., New York, N.Y. Dhowian, A.W., 1991. Secondary compression of sabkha "salina" soils. Eng. Geol., 30(2): 155-169. E1-Naggar, Z.R., 1988. Foundation problems in sabkhah deposits. In: S. Abdul-Jauwad (Coordinator), Short course

125

on foundation engineering for practicing engineer. KFUPM, Dhahran, pp. SD1-SD54. Erol, A.O., 1989. Engineering geological considerations in a salt dome region surrounded by sabkha sediments, Saudi Arabia. Eng. Geol., 26(3): 215-232. Evans, G., Schmidt, V., Bush, P. and Nelson, H., 1969. Stratigraphy and geologic history of the sabkha, Abu-Dhabi, Persian Gulf. Sedimentology, 12: 145-159. Fookes, P.G., French, W.J. and Rice, S.M.M., 1985. The influence of ground and groundwater geochemistry on construction in the Middle East. Q. J. Eng. Geol., 18: 101-128. Holtz, R.D. and Kovacs, W.D., 1981. An introduction to geotechnical engineering. Prentice-Hall, New Jersey, N.J., 733 pp. Hossain, D. and Ali, K.M., 1988. Shear strength and consolidation characteristics of Obhor sabkha, Saudi Arabia. Q. J. Eng. Geol., 21: 347-359. Hotzl, H., Kramer, F. and Maurin, V., 1978. Quarternary sediments. In: S.S. AI-Sayyari and J.G. Zotl (Editors), Quarternary Period in Saudi Arabia. Springer, Berlin, pp. 264-289. Illing, L.V. and Taylor, J.C.M., 1967. Discussion: London, Inst. Min. Metall. Trans., Sect.B., 76:883-884 (cited in Kinsman, 1969). Johnson, H., Kamal, M.R., Pierson, G.O. and Ramasy, J.B., 1978. Sabkhahs of eastern Saudi Arabia. In: S.S. Al-Sayyari and J.G. Zotl (Editors), Quarternary Period in Saudi Arabia. Springer, Berlin, pp. 84-93. Juillie, Y. and Sherwood, D.E., 1983. Improvement of sabkha soil of the Arabian Gulf coast. Proc. 8th European Conf. Soil Mechanics and Foundation Engineering, Helsinki, Vol. 2 (Session 7-12): 781-788. Kinsman, D.J.J., 1969. Modes of formation, sedimentary associations, and diagenetic features of shallow-water and supratidal evaporites. Am. Assoc. Pet. Geol. Bull. (AAPG), 53(4): 830-840. Leonards, G.A., 1976. Estimating consolidation settlements of shallow foundations on overconsolidated clays. TRB Spec. Rep. 163: 13-16. Patterson, R.J. and Kinsman, D.J.J., 1981. Hydrologic framework of a sabkha along Arabian Gulf. AAPG Bull., 65(8): 1457-1475. Russell, R.B.C., 1974. Chemical and physical properties of sabkha-type materials. TRRL, Suppl. Rep. 79UC, Crowthorne, Berkshire. Saeedy, H.S. and Mollah, M.A., 1990. Geotechnical study of the north and northwest coast of the Arabian Gulf. Eng. Geol., 28(1): 27-40. Shehata, W.M., Al-Saafin, A.K., Harari, Z.Y. and Bader, T.A., 1990. Potential sabkha hazards in Saudi Arabia. In: D.G. Price (Editor), 6th Int. IAEG Congr., Amsterdam, pp. 20032010. Smith, C.L., 1980. Brines of Wadi As Sirhan - - Kingdom of Saudi Arabia. U.S. Dep. Interior, Geol. Surv., Saudi Arabian Mission, TRI, 26 pp. Sonnenfeld, P., 1984. Brines and Evaporites. Academic Press, Orlando, Fla.