Geotechnical Hazards Associated with Desert

Natural Hazards 16: 81–95, 1997. 81c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Geotechnical Hazards Associated with DesertEnvironment

W. M. SHEHATA and A. A. AMINKing Abdulaziz University, Faculty of Earth Sciences, P.O. Box 1744, Jeddah, Saudi Arabia 21411

(Received: 19 November 1996; in final form: 14 April 1997)

Abstract. The aridity of the Arabian Peninsula’s deserts ranges between arid to hyperarid with hot dryclimate, scarce precipitation and sparse vegetation. These harsh environmental conditions enhancesome geomorphologic processes more than others, cause specific geotechnical problems, and increasedesertification.

From west to east, the general physiography of Saudi Arabia shows the Red Sea coastal plainsand the escarpment foothills called Tihama followed by the Arabian Shield mountains, the ArabianShelf plateau and finally the Arabian Gulf coastal plains. Sand moves by wind either as driftingsand or migrating dunes in four major sand seas, over the Arabian Shelf, and in the inter-mountainvalleys, in the Arabian Shield causing problems of erosion and deposition. Human activities in thedeserts may cause more instability to the sand bodies, enlarging the magnitude of the problem. Finesilty soil particles also move by wind, depositing loess mainly in selected areas downwind in theTihama. These loess deposits subside and may form earth fissures by the process of hydrocompactionupon wetting. The addition of water can be either natural through storms or man-made throughhuman agricultural or civil activities. Extensive sabkhas exist along the coastal plains of both theRed Sea and Arabian Gulf. The sabkha soil may also heave by salt re-crystallization or collapse bywetting. The shallow groundwater brines present in sabkhas also attack and corrode civil structures.Urbanization and excessive groundwater pumping may also deplete the fresh groundwater resourcesand may cause subsidence, ground fissuring and surface faulting as observed in some locations in theArabian Shield. Although the average annual precipitation is very low, rain usually falls in the formof torrential storms, collected by dry valley basins and causing floods to unprotected downstreamareas on the coastal plains of the Red Sea.

The desert environment, being a fragile echo system, needs to be treated with care. Intercommu-nications between different national and international agencies and education of the layman shouldhelp to keep the system balanced and reduce the resulting environmental hazards. In addition, anysuggested remedial measures should be planned with nature and engineered with natural materials.

Key words: Geotechnical hazards, Saudi Arabia, physiography, desert, arid, erosion, deposition, dunemovement, hydrocompaction, hydroconsolidation, loess, sabkha, floods, subsidence, groundwaterwithdrawal.

1. Introduction

Although deserts are known to be simply barren areas, they are scientifically definedin terms of water shortage or aridity, soil type, topography and vegetation. UNESCO(1979) presented a map showing the distribution of deserts in the world. Accordingto this map, most of the Middle Eastern countries lie within the semi-arid, arid andhyper-arid desert zones, with aridity index (ratio between annual precipitation andmean annual potential evapotranspiration) ranging between>0.03 and<0.20.

82 W. M. SHEHATA AND A. A. AMIN

Figure 1. General physiography of Saudi Arabia (modified after Anon, 1972).

Most of the geotechnical hazards associated with desert environments as citedby the Saudi Arabian environment are principally related to the aridity conditionsand the land forms inherited by these conditions. The physiography of SaudiArabia is generalized in Figure 1 as interpreted from the topographic map of theSaudi Arabian peninsula at scale 1 : 4,000,000 (Anon, 1972). From west to east thegeomorphology of Saudi Arabia (Figure 1) starts with the eastern coastal plains ofthe Red Sea and the foot-hills known as Tihamah. Sabkha areas exist as longitudinalstretches parallel to the shore-line. The coastal plains are bounded eastward by theescarpment of the Arabian Shield mountains which locally rise over 3,000 m abovesea level. The mountains are followed eastward by the central plateau of the ArabianShelf which is bounded westward by the Tuwayq escarpment and largely coveredby sand seas. The Arabian Gulf coastal region, along the west coast of the ArabianGulf, is partially covered either by extensive sabkha areas or by the Jafurah sandsea. Several steep dry valleys flow from the Arabian Shield escarpment westwardtoward the Red Sea, while other gentler slopping valleys flow eastward. Loessand loess-like material could be observed in scattered locations along some of thewesterly flowing valley courses.

Under desert environments, the following conditions are inevitable:

GEOTECHNICAL HAZARDS ASSOCIATED WITH DESERT ENVIRONMENT 83

1. Eolian conditions prevail and are dominated by erosional and depositionalactivities of the wind.

2. Wind deposited sediments such as sand and loess are loosely packed withrelatively low relative densities.

3. Continuous decline in fresh groundwater levels exists in several locations dueto the shortage of groundwater resources.

4. Precipitation is scarce and happens mostly after dust storms in the form oftorrential rain that may cause flash flooding.

5. Evaporation, to a great extent, exceeds precipitation that may lead to the for-mation of coastal as well as continental sabkhas or salt flats.

Accordingly, the potential geotechnical hazards that may occur under desertconditions may include erosion and deposition associated with sand drifting anddune movement; hydrocompaction of loessal soil; collapse, heave and corrosioncaused by sabkha environment; land subsidence due to excessive groundwaterwithdrawal; and flash floods. Desertification is a natural hazard enhanced underthese conditions.

2. Geotechnical Hazards

2.1. WIND EROSION AND DUNE MOVEMENT

Erosion in desert environment is caused mainly by wind action and the blastingeffect of its sand load. The rate of sand drifting is a function of the sand grain size,the wind speed, and the geomorphic conditions. Bagnold (1941) indicated that thethreshold velocity of the drifting sand increases with the increase in grain sizes forsand greater than 0.1 mm in diameter. It also increases for the smaller grain sizesdue to the cohesive action present between the grains. Naturally the rate of sanddrifting also increases with the wind speed. The sand drifting is therefore dependenton grain size, seasonal climatic changes and also on geomorphological setting. Theeolian sand existing in four sand seas, namely the Great Nufud, Al-Jafurah, Ad-Dahna and Elrub-Elkhali, covers approximately one third of the Arabian Peninsula.Within these sand seas, the wind speed is generally 4.0 m s�1 or higher during themonths May through September (Anon, 1986) as reported by the meteorologicalstations present at Dhahran, Quasumah and Al-Jawf.

Fryberger et al. (1983) studied the rate of sand drifting in different geomor-phological units in Dhahran area and confirmed the fact that the highest drift ratesoccur during the months of March through July. They indicated that the highestrate of sand drifting occur in the dune areas with an average annual drift rate of29.2 m3/m. The rate of drifting in the interdune area is 12.8 m3/m, in the sand sheetarea is 2.3 m3/m and in the sabkha area 6.5 m3/m. Irtem et al. (1989) showed thatthe amount of drifting sand depends also on the dune type. A maximum of 144t/m drifts off the crest of barchan dunes, 46 t/m in parabolic dunes and 0.1 t/m in


a sand sheets. Bader (1989, personal communication) reported a rate of 49 t/m indome dunes.

Dune movement occurs as a result of localized sand drifting. For example,barchan dunes move by sand eroded from the wind-ward face and deposited onthe slip face. Figure 2 shows the magnitude of erosion and deposition within abarchan dune in the Jafurah sand sea during its movement in a period of 18 months(Shehata et al., 1992). The measurements were performed by surveying the dunewith respect to a stationary landmark in the summer of 1987, then re-surveying it atthe end of 1988. The produced contour maps were subtracted to produce Figure 2.The rate of dune movement depends mainly on its height, wind speed and presenceof vegetation. The higher the dune, the larger the amount of sand involved in itsmovement and consequently the slower it moves. While a 60 m high barchan dune,in the Great Nufud sand sea, moves at a rate of less than 2 m/y, a 11 m highdune in the Jafurah sand sea moved at a rate of more than 12 m/y (Figure 3). Forequally high dunes the narrower they are the faster. The rate of dune movementis also dependent on dune type. Barchan dunes move faster than dome dunes andthose are still faster than the parabolic dunes (Shehata et al., 1990b). The rates ofmovement of both the parabolic dunes and dome dunes were measured by stakingthe dunes along the centerline and a few other parallel lines and calculating theirmovement from the rate of erosion or deposition along these stakes. Since thebarchan dune sand is very loose and could not be staked, its rate of movementis measured by fixing a rod in front of the dune and periodically measuring theradiating distance between the rod and the slip face.

The problems associated with sand drifting and dune movement are eithererosional where building foundations may get exposed or depositional where thedune may completely cover entire buildings (Photos 1(a) and (b)). Photo 1(a)shows a house that was completely covered by a barchan dune and was graduallyexposed. It took about two years to completely expose the house. Photo 1(b) showsa roadside coffee shop which was abandoned and left unmaintained for few years.These problems are enhanced by human activities. Overgrazing demolishes thevegetation that controls the sand drifting and human activities may destroy thedesert pavement which protects the underneath loose sand. It was noticed thathuman activities during the Kuwait war in 1991 increased the sand activities in thedownwind areas in Saudi Arabia. As remedial measures, dunes were stabilized byvegetation, by fences, by chemical spraying or by other mechanical means. Watson(1985) reviewed the different methods of sand control with special reference toSaudi Arabia. The use of vegetation and/or naturally occurring stabilizers usuallyshows better performance than man-made chemicals or fences. The design of aremedial measure is usually based on quantitive estimates of the magnitude of theaffected area, the amount of drifting sand or dune movement rate, the prevailingwind direction, and the types and rate of growth of the selected vegetations.


Figure 2. A barchan dune survey map showing the areas and magnitudes of erosion anddeposition in addition to the location of the slip face during two monitoring periods, Jafurahsand sea.

2.2. HYDROCOMPACTION OF LOESS

Loess is unconsolidated silt of eolian origin, buff in color and characterized bylack of stratification. It is porous, has low bulk density and is generally calcareous.The carbonate content depends either on the composition of the parent materialor on post-depositional solutions carrying weathering and alteration products. Themain loess mineralogical constituents are quartz, clay minerals, feldspars, micas,hornblende and pyroxene (Smalley and Vita-Finzi, 1968)

The literature of loess recognizes two major sources of loess material formedunder conditions of dry climate: glacial or periglacial areas and hot desert. In desertareas the wind-blown silt is trapped in these areas either adjacent to steppe landsor on wet soil or rock surfaces. Smalley and Krinsley (1978) showed that somedesert loess is formed near mountain areas due to weathering of igneous rockswhile typical deserts such as Sahara lack loess deposits. Loess grain size rangesbetween 20 � and 100 � with the bulk varying between about 30 � and 80 �. Forsuch grain sizes, the capillary forces are relatively high and consequently prevent


Figure 3. Rates of movement of three barchan dunes in the Jafurah sand sea.

Photo 1. (a) A house that was completely covered by a barchan dune and was exposed afterthe dune moved; (b) Deposition of drifting sand in an abandoned roadside coffee shop, Al Lith.


the accumulated material from further transportation (Cegla, 1969). The conceptof capillary forces as cause for loess accumulation explains the presence of loessin isolated patches of ‘loess islands’.

Loess structure undergoes dynamic changes upon changing its moisture content.Handy (1973) has shown that when the loess moisture content exceeds its liquidlimit, its structure instantaneously collapses under the load and the soil subsides.This process was described as hydrocompaction by Lofgren (1969) and was termedhydroconsolidation and thoroughly investigated by Bally (1988), Feda (1988),Sajgalik (1991) and Rogers et al. (1994). Therefore when an area covered with loessis subject to flooding it will appreciably subside and the soil will become closelypacked with increasing density. The magnitude of subsidence is dependent on thedegree of saturation and the thickness of the loess layer. Any variations in thesewill cause differential settlement and failure of any overlying man-made structure.In addition to hydroconsolidation, loess may fail by liquefaction, fluidization or byprogressive failure (Derbyshire et al., 1994) depending on the geomorphologicalsetting of the material.

In Saudi Arabia, very little geologic literature mentions the presence of loess(Smalley and Krinsley, 1978), although it is apt to form around desert highlands.The soil atlas of Saudi Arabia (Anon, 1984) does not include the term loess inits classification and only includes loam as one of the soil units present aroundthe desert areas. Several loess occurrences are, however, recognized in the areasto the east and southeast of the Mediterranean sea (Smalley and Krinsley, 1978;Dan, 1990; Goring-Morris and Goldberg, 1990; Yair, 1992; Pye, 1994). Foder andKleb (1994) mapped loess as a soil unit mixed with loamy soil in Hungary. It istherefore suggested that loess could be mistakenly mapped, in some locations, asfine loam as presented in the soil atlas. Field observations suggest the presenceof loess or loess-like soils in the downstream sections of the main valleys inTihama and in the southwestern part of Saudi Arabia. Geotechnical literature, onthe other hand, reports the presence of collapsible soil similar in behavior to loess inscattered locations in the Eastern Province. Figure 4 shows hydroconsolidation andcollapse in a sample from Al-Yutamah area occurring under a normal pressure of0.3 MN/m2 (Bankher, 1996). Loess occurrence and distribution in Saudi Arabia, andits engineering properties are the authors’ interest and investigations are underway.

2.3. COLLAPSE, HEAVE AND CORROSION OF SABKHA ENVIRONMENT

Two main types of sabkha are known in the desert environments: coastal sabkha andcontinental sabkha. The two types exist under arid conditions where evaporationexceeds precipitation. The coastal sabkha is formed as broad coastal flats that getrestricted from the sea by offshore islands (Evans et al., 1969 and Bush, 1973)or by coral reef barrier. These tend to form a lagoon with very small tidal rangeand consisting of sandy, silty or clayey soil depending on the pre-existing geologicsetting. The continental sabkha, on the other hand, is formed in a closed depression


Figure 4. A consolidation test result shows the collapse of a loess soil sample upon saturation(modified after Bankhar, 1996).

in which fine soils are accumulated by washing off the surrounding formations.In Saudi Arabia, extensive coastal sabkhas exist along the Arabian Gulf and theRed Sea coasts. Continental sabkhas are also present in different locations inthe Eastern Province, in the Riyadh area and in the Northern Province. The aridconditions with evaporation exceeding precipitation in both sabkha environmentsincrease the salinization of the soil with a possible formation of salt crust. Bothtypes of sabkha are also characterized by highly saline and shallow groundwaterconditions.

The geotechnical properties of the coastal sabkha soils in Saudi Arabia andtheir potential geological hazards were investigated by Rein-Ruhr (1973), Ali et al.(1985), Ali and Hossain (1987), Dhowian et al. (1987), Hossain and Ali (1988),Abou Al-Heija and Shehata (1986, 1989) and Shehata et al. (1990a, 1990b). Onecontinental sabkha close to Ar-Riyadh was investigated by Stipho (1985). Severalother continental sabkhas are present near Hail and require thorough investigation.The sabkha soil is not homogeneous but generally loose or soft with StandardPenetration Test (SPT) values ranging from zero close to the groundwater levelto >30 at depth. The low SPT values and the presence of water soluble saltssuch as halite may cause collapse conditions to the soil upon wetting. Figure 5shows the collapse behavior of a sample from Al-Lith sabkha which is similarin behavior to the loess soil, but different in occurrence, origin and composition.The presence of anhydrite and its possible transformation to gypsum may, on theother hand, cause heave. Lutenegger et al. (1979) measured the heave pressure


Figure 5. A consolidation test result shows the collapse of a sabkha soil sample upon saturation(after Abou Al-Heija and Shehata, 1986).

caused by induced gypsum growth in shale samples in the laboratory which rangedbetween 70 and 2,100 KN/m2. The high salt content of the soil, especially thechlorides and sulfates, also severely attacks any submerged structures such asconcrete foundations, reinforced bars, and steel pipes, corroding them and causingreduction in their strength properties. During the rainy season, the sabkha soilbecomes weak and impassable because of the dissolution of the cementing salts.These problems are increased by the fact that the groundwater level is shallow andits salinity is high (Ali and Hossain, 1987; Shehata et al., 1990a). Sabkha is alsosusceptible to flooding because of its flat surface and the relatively low permeabilityof its soil.

The structure built in on sabkha soil should be carefully designed in order toavoid these problems and any other future problems that could be created afterconstruction. It was noticed that a problem of groundwater rise may occur uponthe urbanization of a sabkha location. The groundwater rise may occur because ofseveral reasons, including interception by the urbanization structures to the naturalevaporation process of the shallow groundwater, the infiltration of fresh water fromirrigation, drainage or leakage in pipelines, and the reduced permeability of thesabkha soil.

2.4. LAND SUBSIDENCE DUE TO EXCESSIVE GROUNDWATER WITHDRAWAL

Under arid desert conditions, the shortage of groundwater resources and excessivepumping may cause continuous decline in the fresh groundwater levels. Accord-ingly, the soil salinity may increase, soil fertility may decrease and consequently


desertification may increase. When the aquifer is formed of unconsolidated sedi-ments of high porosity and is interbedded with clay aquitards of low permeabilityand high compressibility, the rapid lowering of the groundwater level may alsocause subsidence and possible ground failure. Poland (1981) reported subsidenceranging between a few tens of centimeters to few meters in Georgia, Louisiana,Texas, Nevada and California. The maximum subsidence was reported west ofFresno, California which amounted to 8.8 m until 1972. Holzer (1984) reportedground failures ranging from long tension cracks or fissures to surface faults asso-ciated with land subsidence in more than 14 areas in United States. Fissures mayreach a few kilometers in length but only a few centimeters in width. However, thefissures are usually eroded by rain water into gullies 1 to 2 m wide and 2 to 3 mdeep. Surface faults commonly attain scarp height of less than 1 m but increasewith time by creep.

Both land subsidence and ground fissures were reported in several places inSaudi Arabia. Roobol et al. (1985) related the ground fractures and building dam-ages, in Tabah and An’Nai villages which are built within old volcanic craters, toexcessive groundwater withdrawal. Amin (1988) investigated the problem of Tabahin detail and proved that land subsidence, ground fissures and also surface faults(Photo 2) are due to excessive pumping within the old crater. Amin and Shehata(1991) presented a model (Figure 6) showing how these features were progressive-ly developed. Amin and Bankher (1995) reviewed the land subsidence in SaudiArabia and stressed the potential hazards of excessive groundwater withdrawal.Bankher (1997) also investigated the land subsidence in western Saudi Arabia andrelated it to both excessive groundwater withdrawal and hydroconsolidation.

groundwater management and the pumping of water within a safe yield becomevery essential, especially in the areas that may undergo subsidence due to ground-water withdrawal. The management of groundwater production at the villages ofTabah and An’Nai reduced the hazard of subsidence to a great extent.

2.5. FLASH FLOODS

The average precipitation along the coasts and in the desert areas of Saudi Arabiadoes not exceed 100 mm/y, although the precipitation in the semi-arid zones alongthe higher terrain of the Arabian Shield exceeds this value. This rain in the highlandsis collected by major dry valley basins and drained both in the east and the westdirections towards the Stable Shelf and the Tihama respectively. The valleys flowingto the west are steep and less mature, and flood water may reach the Red Sea coastalarea as sheet flood. Photos 3(a) and (b) are examples of the hazards caused by aflood that was triggered by a 80 mm rain storm in February 1996 in Al-Lith area.It should be noted that the extent of flooding in any drainage basin is related to thearea, the existing types of sediment or rocks and the geomorphological parametersof the basin including its topographic characteristics. For example, if the flood waterinundates sabkha environment, the water remains stagnant for days due to the flat


Photo 2. Surface faulting associated with subsidence due to groundwater withdrawal in Tabah.

topography and the low permeability of the sabkha soil. Under more favorableconditions, the flood water may infiltrate or will be discharged into the sea.

The flood damage is sometimes extensive both in lives and properties, dependingon its magnitude. The assessment of the magnitude of floods caused by rain stormson the Arabian Shield requires the installation of flood gages and meteorologicalstations in some key valleys and monitoring them for a long period of time. Untilthen the estimates of flood potential are speculative and no quantitative measurescan be obtained and consequently no remedial measures can be suggested.


Figure 6. A model showing the progressive formation of surface faults due to groundwaterwithdrawal in Tabah (after Amin and Shehata, 1991).

3. Conclusions

The desert environment is very fragile and is highly affected by human activities.Disturbances in the balanced echo system are apt to take place, causing seriousproblems to man’s environment and consequently initiating geotechnical hazards.Urbanization, climatic conditions, and geomorphic and geologic setting are usuallycontrolling factors influencing the type of these hazards. The potential geotechnicalhazards that may occur under desert conditions may include sand drifting and dunemovement; hydrocompaction of loessal soil; collapse, heave or corrosion of sabkha


Photo 3. (a) Sheet flood along a stretch of Jeddah-Al Lith coastal highway after a 88 mm rainstorm in February 1996; (b) Undercutting of road pavement caused by flood water erosion, AlLith coastal highway.

environment; land subsidence due to excessive groundwater withdrawal; and flashfloods.

When dunes move or sand drifts, both erosion and deposition hazards may occurendangering any structures downwind. Engineering solutions preferably utilizingnatural material will then be necessary. Subsidence may occur in sabkha soil, loess,and loose soil aquifers subjected to excessive groundwater pumping. Heave andcorrosion may also occur in sabkha terrain. Engineering solutions or groundwaterproduction management will be required. Flood prediction in dry valleys requiresextensive investigation before any engineering solution is recommended.

Other hazards, either natural or man-made, including seismicity, volcanism, andslope instability may occur, but they are not restricted to the desert environments.

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