Introduction

The term sabkha is used in the Arab countries as equivalent to the term “salt flat”. In general, sabkha is composed of sand deposits mixed with silt and clay and is intercalated with evaporites (Shehata et al., 1990b; Abu Taleb and Egeli, 1981). Ellis (1973) describes the sabkha deposits as the bottom of a closed depression, the zone of evaporation for accumulated run-off from shallow saline subterranean water, characterized by the presence of salt deposits and absence of vegetation. The salt constituent plays an important role in determining the engineering properties of the sabkha soil. During the dry seasons, the salt encrusted surface of the sabkha is strong and durable whereas when the surface is wet due to rain or storms/tides, the sabkha is soft and muddy (Akili and Ahmad, 1983; Shehata Bader and Harari 1990a).

This paper presents an assessment of the engineering properties of the coastal sabkhas of the Red Sea and Arabian Gulf in Saudi Arabia, and compares these.

Types, distribution and formation of Sabkha deposits

Sabkha deposits are classified according to their locations: inland sabkha (continental) and coastal sabkha (Kinsman and Park, 1969).

The inland sabkha is found in basins and typically surrounded by sand dunes. Their salts are related to the evaporation of the continental groundwater, which contains little carbonate but abundant gypsum and anhydrite (Akili and Ahmad, 1983). In Saudi Arabia the inland sabkhas are mainly in the northern area in Wadi As Sirhan.

The coastal sabkha stretches along the coastal plains of the Red Sea and the Arabian Gulf (Fig. 1) and is a highly friable material. Geotechnically it is the more important, as most of the residential and commercial structures are along the sea coast (Paterson and Kinsman 1981).

Fig. 1
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Distribution of sabkha deposits in the Kingdom of Saudi Arabia

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The formation of sabkha deposits, their geological/ geomorphological features and their geotechnical properties have been described by many investigators (Kinsman and Park, 1969; Butler, 1969; Ellis, 1973; Fookes and Collis, 1975; Akili and Torrance, 1981; Paterson, 1981; Abu-Taleb and Egeli, 1981, Abou Al Heija and Shehata, 1986, 1989). Shehata et al.,1990b suggest the sabkha deposits were formed as the infill of pre-existing lagoons which were separated from the sea by a barrier as the sea regressed. As the rate of evaporation along the coastal areas exceeds the precipitation, capillary rise results in the formation of a salt-rich crust (Fig. 2) comprised mainly of gypsum, anhydrite, halite and celestine.

Fig. 2
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Formation of coastal sabkhas (after Shehata et al., 1990b)

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Potential geotechnical problems of the coastal sabkha

The main potential problems in sabkha can be summarized as follows:

a):

The quick decrease of soil strength due to water flooding (Fig. 3);

Fig. 3
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Photograph showing subsidence pits in the pavement due to subsoil wetting in north Jeddah city

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b):

Variation in the soil compressibility characteristics within a short distance (both vertically and horizontally) which may lead to significant differential settlement;

c):

Under hot and humid conditions gypsum (CaSO4.2H20) undergoes alternating hydration to form anhydrite (CaSO4) and dehydration to form gypsum again — a chemical process which involves significant volume changes;

d):

Ground water movement related to evaporation precipates salts at or near the surface. In the hot temperatures, the growth and horizontal expansion of the salts causes small cracks in the pavements. In addition, where gravely soils are present, salts may accumulate in these and cause “heave” (Fig. 4),

Fig. 4
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Photograph showing salt crystals covering the ground surface of soil sabkha in the Al Salamah district

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e):

The salt-rich waters migrate to the surface by capillary rise. The high concentrations of sulphate (SO4) and chloride (Cl) are corrosive to both concrete and steel (Fig. 5).

Fig. 5
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Extensive corrosion in the concrete developed by salt attack in sabkha soil, north Jeddah

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Engineering properties of coastal sabkha

Sabkha along the Red Sea coast

The sabkha deposits along the Red Sea coast have been studied and evaluated especially where this deposit occurs in areas of potential urban extension. Comprehensive studies were carried out at several sites along the Red Sea coast in order to understand the behavior of the coastal sabkha e.g. in Jeddah (Ali, Kazi and Al Quahtani., 1985; Ghazali, Fatani and Khan,1986); Obhor (AIi and Hossain, 1987; Hossain and Ali, 1988); Jizan (Ghazali et al., 1986; Dhowian, Erol and Sultan., 1987) and AI-Lith (Abou Al Heija and Shehata, 1986, 1989). The results of these studies show that the sabkha is argillaceous, comprising mainly of grey silty clay with a light brown surface crust. The groundwater is highly salt-rich and is at a very shallow depth (Shehata et al.,1990b). As seen in the generalized soil profile given in Table 1, sabkhas along the Red Sea coast contain coral fragments and gypsum crystals. Table 2 gives a summary of the geotechnical characteristics of the coastal sabkha soils in the Kingdom of Saudi Arabia. The chemical analysis of the brines from the Obhor sabkha (Table 3) shows that the sabkha water is salty to brine as classified by Davis and De Wiest (1966).

Table 1 Generalized coastal sabkha profile, Red Sea Coast (after Shehata et al., 1990b)
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Table 2 Summary results of engineering properties of coastal sabkha soils in the Kingdom of Saudi Arabia
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Table 3 Chemical analysis of coastal sabkha brines
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Sabkha along the Arabian Gulf coast

Previous studies along the Arabian Gulf coast (Akili, 1975; Johnson et al., 1978; Abu, Taleb and Egeli, 1981; Shehata et al.,1990a) have revealed that the sabkha can be classified into three types:

a):

Arenaceous sabkha composed of loose, medium-grained quartz sand. This type of sabkha occurs mostly in sand dune terrain, such as found in Al Azizia, south of Dammam.

b):

Argillaceous sabkha consisting mainly of marine clays that are derived from decomposing biotic and calcareous materials, such as found in Al Fasl at Jubail.

c):

Mixed sabkha composed of both sandy and clayey deposits, such as found in Al Riyas, at Jubail.

The sabkhas are highly saline and have a puffy surface crust. The groundwater level is usually within 1 m of the ground surface. The results of the geotechnical investigations carried out at several sites along the Arabian Gulf coast are given in Table 2 . The chemical analysis for the groundwater at Al Fasl (see Table 3) shows it to be hypersaline or brine as classified by Davis and De Wiest (1966). This type of groundwater is highly likely to form salt crystals (Al Saafin and Kidwai,1984). A generalized description of the coastal sabkha is given in Table 4.

Table 4 Generalized coastal sabkha profile, Arabian Gulf Coast (after Shehata et al., 1990b)
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Comparison between sabkhas along the Red Sea and Arabian Gulf coasts

As shown in Table 5, the engineering properties of coastal sabkha along the Red Sea differ from those of the sabkha along the Arabian Gulf. The coastal sabkha along the Arabian Gulf is thicker than that along the Red Sea and has more saline and sandier constituents. It is also more porous and compressible, thus more costly to treat. Figures 6 and 7 show that the increased Total Dissolved Solids in the sabkha brines in coastal areas have a positive influence on some engineering properties such as plasticity, cohesion and compression. The sabkha along the Red Sea shows a stronger correlation between TDS and plasticity/ cohesion, although for compression the sabkha along the Arabian Gulf has the stronger relationship with TDS.

Table 5 Comparison of engineering properties of coastal Sabkhas along the Red Sea and Arabian Gulf Coasts
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Fig. 6
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Influence of TDS of Red Sea sabkha brines on the engineering properties of the sabkha soil

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Fig. 7
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Influence of TDS of Arabian Gulf sabkha brines on the engineering properties of the sabkha soil

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Conclusions

This comparative study was conducted to characterize the coastal sabkha soils in Saudi Arabia. The results indicate that the engineering properties of coastal sabkha along the Red Sea different from those of the sabkha along the Arabian Gulf. The coastal sabkha along the Arabian Gulf is thicker and has more saline and sandier constituents. In addition, it is more porous and compressible, hence it would be more expensive to develop. Further studies are required to evaluate the influence of CaCo3 and SiO2 on the engineering properties of the coastal sabkha in Saudi Arabia.