causes of land subsidence in the kingdom of saudi arabia

Natural Hazards 16: 57–63, 1997. 57c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

Causes of Land Subsidence in the Kingdomof Saudi Arabia

AMMAR AMIN and KHALID BANKHERFaculty of Earth Sciences, King Abdul Aziz University, PO Box 1744, Jeddah 21441, Saudi Arabia

(Received: 2 July 1996; in final form: 4 March 1997)

Abstract. The occurrence of land subsidence in the Kingdom Saudi Arabia is either natural or man-made. Natural land subsidence occurs due to the development of subterranean voids by a solutionof host rocks in carbonate and evaporite terrains, over many areas of Saudi Arabia. Man-inducedland subsidence is either due to the removal of groundwater in the agricultural areas or to wettingof unstable soils. Therefore, earth fissures and a lowering of the ground surface in unconsolidatedsediments took place in alluvial plains and volcanic vent terrains. Unstable soils include Sabkhasoils and loess sediments. These types of soils occur in coastal plains, desert areas and volcanicterrains. When this soil is wetted either during agricultural activities, waste disposal or even duringa rain storm, subsidence takes place due to either the removal of salts from the Sabkha soil or therearrangement of soil particles in loess sediments.

Key words: land subsidence, karst, salt diapers, loess sediments, Sabkha soil

1. Introduction

Subsidence is defined as the differential sinking of the Earth’s surface with respectto surrounding terrain. It is a result of ground collapse due to the development ofsubsurface voids or reduction in subsurface materials, either naturally or manmade(Wittaker and Reddish, 1989). However, land subsidence in the Kingdom of SaudiArabia happens in different areas associated with karst terrains, salt diapers, with-drawal of groundwater, and unstable soils. This paper presents the different causesof land subsidence that exist in the Kingdom.

2. Genereal Geology

The coastal area of western Saudi Arabia is mainly covered by a narrow strip ofsediments whose age range from Tertiary up to Quaternary. The eastern coast ismainly covered by deposits of Quaternary ages consisting of Sabkhas and areasof sand dunes (Abu Talib and Egeli, 1981). The Arabian shield consists of aPrecambrian complex of volcanic and sedimentary layered rocks which have beenaffected by deformation movements and regional transformations associated withintrusions that led to the formation of metamorphic rocks like schists and gneisses(Al Shanti, 1993). The Arabian Shelf rocks overlie the shield rocks and appear inthe form of arcs or belts made up of successive formations of sandstone, shale,

58 AMMAR AMIN AND KHALID BANKHER

limestone, and evaporites of various ages from Cambrian to Quaternary. These rocksincrease in thickness and gently dip towards the east. They are locally covered bybelts of sand dunes in northern, central and southern Saudi Arabia (Powers et al.,1966). Figure 1 shows a generalized geologic map of the Arabian Peninsula.

3. Land Subsidence Exist

Natural subsidence in the Kingdom involves the development of subterraneanvoids by a solution of host rocks in carbonate, evaporite and salt diapers (karstphenomenon) or the collapse of unstable soils in Sabkha soils and loess sediments(hydrocompaction phenomenon). Man-induced land subsidence occurs due to theremoval of groundwater from deep aquifers or the casual addition of water to unsta-ble soil. Figure 2 shows areas of potential ground collapse due to karst phenomenaand unstable soils.

3.1. NATURAL SUBSIDENCE

3.1.1. Ground Collapse over karstic Rock Terrains

Karst is a geomorphologic feature that is formed in limestone, gypsum, anhydrite,halite, or other soluble rocks by widening existing fractures or cavities through thedissolving action of water . This chemical weathering forms sinkholes that manifestthemselves as natural land subsidence phenomena. The formation of subterraneanvoids in such rocks, i.e. limestone by solution action, where water causing a solutionof limestone carries dissolved carbon-dioxide and other chemicals and associatedproducts picked up whilst percolating through surface deposits, produces chemicalweathering properties. The acidity of the water arises mainly from the dissolvedcarbon-dioxide and reacts with the calcium carbonate of limestone dissolving itprogressively to form a bicarbonate. This chemical process creates the conditionsof favouring subsequent collapse or widening by washing down of overburdenmaterials thereby resulting in sinkhole depressions occurring at the surface (White,1988). Karst phenomena exists over large areas in the central and eastern parts ofSaudi Arabia (Figure 2) forming sinkholes and collapsed dolines associated withcarbonate and evaporite formations of the Arab, Hith, Sulaiy, Umm ar Radhuma,Rus, Dammam, and Dam formations (Powers et al. 1966; Al Sayari and Zotl, 1978;Jado and Johnson, 1983; Grosch et al., 1987; Edgell, 1990; Shehata et al., 1990).Most of karst features controlled by joint systems (Grosch et al., 1987). These karstfeatures were developed during the Pliocene or Pleistocene time where rainfall washigh (Al Sayari and Zotl, 1978; Jado and Johnson, 1983).

3.1.2. Salt Diapers

Salt diapers are the result of the intrusion of solid halite into surrounding sediments.The salt is derived from some underlying source bed, usually thousands of meters

CAUSES OF LAND SUBSIDENCE IN THE KINGDOM OF SAUDI ARABIA 59

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Figure 2. Simplified map of Kingdom of Saudi Arabia showing areas of potential groundcollapse due to karst phenomena and unstable soils (proposed by Khalid Bankher, 1994).

thick. These diapers are composed chiefly of halite, far less commonly of anhydriteor gypsum, and include only minor sandstone and mudstone. The country rockssurroundings the salt diapers are faulted and extremely disturbed by the diapiricmovement of salt (Krynine and Judd, 1957). These diapers outcrop as salt domesin the southwestern part of Saudi Arabia (Rein-Ruhr, 1973; Blank et al., 1987).Collapsed structures and cavities documented in areas of salt domes, due to disso-lution of salts by the action of rising groundwater or waste water and forming anunderground void, collapse any time without warning (Rein-Ruhr,1973; Ghazali etal., 1985; Erol, 1989).


Table I. Geotechnical properties of Sabkha deposits in AlJubail industrial area, eastern province (after Abu Talib andEgeli, 1981; Al Refeai and Al Ghamdy, 1994).

Description Maximum Minimum

Passing 0.425 mm sieve (%) 97 18Natural water content (%) 84.6 10Liquid limit (%) 84 30Plasticity index (%) 39 NPBulk density (kN m�3) 18.54 13.14Specific gravity 2.82 2.51SPT (blows/300 mm) 6.0 0Cohesion (kN m�2) 53.0 0Angle of friction (degrees) 22.0 0Inilial voids ratio 2.16 1.39Compression index (Cc) 0.95 0.39

3.1.3. Sabkha Soils

These types of soils occur in coastal plains, desert areas, and volcanic terrains.Subsidence occuring due to the wetting of Sabkha soils by rising of water tableor flooding, decrease the bearing capacity of the Sabkha soil by dissolving solublesalts such as carbonates, sulfates and chlorides which otherwise contribute tothe cohesion of the soil, following the rapid ground subsidence. This type ofproblematic soil is extensively found on the eastern coastal plains and at somelocations on the western coastal plains of the kingdom (Akili, 1981; Ghazali et al.,1985; Shehata et al., 1990; Al Refeai and Al Ghamdy, 1994). Table I gives thegeotechnical properties of Sabkha deposits. Loess soils are wind-blown sedimentswhich have low density, high void ratio, and are moisture-deficient. These sedimentsexist in wide range of desert and volcanic terrains in the kingdom (Al Refeai andAl Ghamdy, 1994). When these of soils are wetted, subsidence takes place due toas rearranging of the soil particles in bess sediments. The source of water causinghydrocompaction phenomena, rain storm and flooding as natural causes or duringagricultural activities, irrigated fields, sewer lines, or waste water as man-madecauses.

3.2. MAN-INDUCED LAND SUBSIDENCE

3.2.1. Land Subsidence Due to Withdrawal of Groundwater

This phenomenon is common due to excessive pumping of groundwater areas, butit is relatively new in Saudi Arabia . In many areas, groundwater pumping andwithdrawal has resulted in a significant negative impact on the environment caused


by lowering of the water table due to rapid drawdown. In places where lowering ofthe water table has been most severe, ground failures have occurred.

These failures are a feature of land subsidence resulting from underlying uncon-solidated sediments experiencing compaction during groundwater withdrawal,forming earth fissures due to differential compaction of unconsolidated sediments,e.g., vent-fill of ancient volcanic craters (Hall, 1983; Roobol et al. 1985; Amin,1988). For example, in the form of Tabah in the Hail region, ground failure tookplace due to subsidence occurring after increased groundwater pumping and causedthe level to drop from 15 m to more than 60 m in the vent-fill of the volcanic crater.Compaction of deeper material filling the crater resulted in the subsidence of thetop soil layer and the formation of earth fissures with a total length of 960 m. Atrial was made to measure the rate of subsidence during a period of eight monthsby Amin (1988). He found that vertical downward movement increased towardsthe center of the crater with a maximum of 1.48 mm.

4. Conclusions

This paper proposes that there is a different variety of geologic hazards that arerelated to different types of land subsidence in the Kingdom Saudi Arabia. Thesehazards include karst phenomena, salt diapers and unstable soils, which are distrib-uted over many areas in Saudi Arabia as natural subsidences. Man-induced groundfailures occurred due to excessive groundwater pumping. These collapsed featuresoccurred in arid areas of the Kingdom, due to excessive pumping in agriculturalareas and due to wetting of unstable soils, or a rise of water tables in urban cities.

References

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causes of land subsidence in the kingdom of saudi arabia

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