Cases and solutions

Sinkhole hazards along the eastern Dead Sea shoreline area, Jordan: a geological and geotechnical consideration S.A. Taqieddin ´ N.S. Abderahman ´ M. Atallah

Abstract For the last four decades, the level of the Dead Sea has been subjected to continual variation which, among other important factors, has led to the occurrence of much subsidence and many sinkholes in the southern Dead Sea area. Sinkhole activities occurred repetitively and were observed in open farms, across roads, near dwellings and near an existing factory, thus causing a serious threat to the locals and farmers of the area and their properties. This paper presents the main results from detailed geological and geotechnical studies of this area. Aerial photo interpretation and borehole drilling aided these studies. Parallel geophysical investigations (vertical electrical sounding and seismic refraction) and hydrological and hydrogeological studies were made by others in the same area to also investigate this phenomenon. It was found that sinkholes are aligned to and follow old water channels and are concentrated parallel to the recent shoreline of the Dead Sea. The development of subsurface cavities is associated mainly with the variation in the level of the Dead Sea over the four past decades, the presence of regional salt intrusion under the surface of salt beds, the fluctuation of the water table and continuous dissolution and the active tectonism of the area. Moreover, this work showed that the area is still under active sinkhole hazards and other parts of the area will be inevitably affected by sinkholes in the future.

Received: 1 July 1999 / Accepted: 11 October 1999 S.A. Taqieddin ()) Department of Civil Engineering, Jordan University of Science and Technology, PO Box 3030, Irbid 22110, Jordan e-mail: [email protected] Tel.: +962-2-7095111 Fax: +962-2-7095111 N.S. Abderahman ´ M. Atallah Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan

No practical engineering solution to this problem is feasible. Key words Dead Sea level ´ Geological ´ Geotechnical tectonism ´ Salt intrusion ´ Sinkholes

Introduction Sinkhole is a self-explanatory geomorphological term denoting a localized depression in the surface of the ground that has been developed by the washing out and dissolution of underlying earth material (rock or soil). Sinkholes result from a variety of causes, but the occurrence of numerous sinkholes in a restricted area is commonly associated with a substantial repeated lowering or rising of the water table which may be natural, manmade, or a combination of both (Newton and others 1973). Groundwater pumping and the extraction of oil, gas, coal and other ores from variable depths have been known for many decades to be followed by subsidence in the surface of the Earth and the creation of sinkholes. In many cases, limestone is particularly vulnerable to the effect of solvents and weathering, forming an irregular and highly plastic residual stratum. Some of the disintegration of the limestone takes the form of vertical erosion, forming the sinkholes or caverns (Krynine and Judd 1957; Cernica 1995). Sinkholes also occur in many areas underlain by other carbonate or salt rocks. They are usually roughly circular to elliptical in plan and in diameter are as large as 100 m or more. They usually result from the dissolution of underlying limestone, gypsum and salt, or the washing out of silt and sand. Sinkholes occur in many regions of the world. However, most sinkholes occur in limestone country; a smaller percentage is due to dissolution of dolomite, salt and gypsum, with perhaps the least number found in rock salt. Usually, the continuous slow dissolution of subsurface limestone by acidic water and humic acid in soil forms cavities near the surface or hidden at depth by an arch of soil or rock. These cavities become evident at the surface either directly or by the collapse of the soil cover or other material above the cavity. Environmental Geology 39 (11) October 2000 ´  Springer-Verlag

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Natural sinkholes have been known in many regions of the world, from time immemorial and those due to human underground activity for many decades. Only since World War II have they received widespread attention. Remarkable developments in road building, dam construction and the rapid expansion of urban areas have led to serious troubles in different areas. A remarkable example is the dewatering of the famous gold-bearing reefs of Johannesburg that began in 1960, approximately 65 km to the west of the city. A thick bed of dolomite and dolomitic limestone (up to 1000 m) overlies the goldbearing strata. The pumping program dried up traditional springs, then started to lower the entire ground surface within the compartment. Between 1962 and 1966 eight sinkholes larger than 50 m in diameter and deeper than 30 m had appeared, together with many smaller ones. In 1962, a large sinkhole suddenly developed under the crushing plant adjacent to one of the mining shafts; the whole plant disappeared and 29 lives were lost. In August 1964, a similar occurrence took the lives of five people as their home dropped 30 m into another suddenly developed sinkhole. Sinkholes will still occur catastrophically; however, the ªDecember Giant,º 97 m long, 90 m wide and 36 m deep that developed without warning in the isolated woods of Shelby County, Alabama, on 2 December 1972, is a vivid example of this phenomenon (Legget and Hatheway 1988). Sinkhole activity along the eastern coast of the Dead Sea can be dated back a few decades. The occurrence of about 30 sinkholes in the years 1992±1993 brought attention to this problem. Following the formation of these sinkholes in areas adjacent to the Lisan Peninsula (Fig. 1), an intensive study was conducted. The purposes of this study were: 1 To survey the area where sinkholes have occurred and to predict potential sinkhole sites. 2 To correlate the formation of sinkholes with the geology and hydrology of the area and the fluctuations in the level of the Dead Sea. 3 To trace the history and the progress of sinkhole development. 4 To recommend remedies if feasible.

Methods of study This study was accomplished through the following activities: (1) interpretation of aerial photos taken within different years; (2) a geological survey that was done through field trips and drilling; and (3) geotechnical studies carried out to determine the physical properties of the Earth material. El-Isa and others (1995) conducted parallel studies for the purpose of collecting: 1 Information about the hydrology of the area and changes in the sea level.

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Fig. 1 Map showing the areas subjected to sinkhole formation in the southern Dead Sea basin (modified after Tapponnier 1993)

2 Geophysical exploration covering the study area by using shallow refraction seismic and resistivity techniques to locate subsurface geologic structures and subsurface anomalous features. The history of development of sinkholes in the area was determined by personal interviews with property owners and interested persons.

Geology Regional geology The area of Ghor Al Haditha is part of the most prominent structural feature in Jordan, namely the Dead Sea Transform (Rift). The Dead Sea Transform (DST) extends 1000 km and links the Taurus zone of continental collision with the Red Sea crustal spreading center, separating the Arabian plate from Sinai subplate (Fig. 2). The rift margins show many features characteristic of an extensional rift as bounding normal faults and flexures

Cases and solutions

Fig. 2 Tectonic setting of the Dead Sea±Jordan transform fault system (modified after Barazangi 1983)

(Bender 1968). Offsets of numerous rock bodies of Precambrian to Late Cretaceous age and some structures reveal a left slip of about 105 km on the DST (Quennell 1958; Freund and others 1970). The movement along the transform began in the Early Miocene age in two stages of left-lateral displacement and anticlockwise rotation of the Arabian plate. Several pull-apart basins and push-up swells developed along the transform (Garfunkel 1981). The Dead Sea basin is the largest and deepest pull-apart basin in this system; it is 80 km long and 5±15 km wide. The Dead Sea rhombic-shaped basin was formed also in two stages as a result of the overlapping of two left-lateral strike slip faults, the Wadi Araba fault in the south and the Jordan Valley fault in the north. Active seismicity in the transform area indicates that the movement is still occurring on the faults (Ben Avraham and others 1990). Evidence of active movement along the

transform faults is also deduced from physiographic features observed along the fault traces (Garfunkel and others 1981; Atallah 1992) and by displacement of late Quaternary sediments. The Dead Sea Rift in Jordan forms a major topographic depression with a width ranging between 10±25 km, extending from the Gulf of Aqaba to Lake Tiberias (Fig. 2). The Dead Sea is the lowest point of this system (409 m below sea level). The rift floor is mainly covered by Pleistocene and Holocene alluvium, playa deposits and sand dunes. The highlands east and west of the rift consists of older rocks ranging from Precambrian basement in the south to Tertiary in the north. The Dead Sea basin was filled with several kilometers of marine and continental sediments deposited since the Pliocene. These sediments started with the Usdom formation of Plio-Pleistocene age, composed of 2±4-km-

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thick evaporite of marine origin. They are overlain by the the main laminated member which contains intercalaAmora formation of Early- Middle Pleistocene, composed tion of sand and gravel which appear to represent alluof 0.3±3 km of lacustrine and fluvial sediments. The overvial and colluvial fans; (b) middle clay member which lying rocks are the Middle-Late Pleistocene of the Samra consists of sequences of clayey silt and silty clay with formation, composed of several tens of meters of lacusthin beds of fine to medium sand; and (c) the lower trine and fluvial sediments. The Late Pleistocene lacuslaminated member characterized by the predominance trine sediments of the Lisan formation overlie them. of laminated soil that occurs almost exclusively near or These are 10±40 m thick and widely exposed along the below the water table and thus fresh and free from Dead Sea basin and Jordan Valley. The present sediments open joints typical of the dissected parts of the main are mainly lacustrine and fluvial forming the Upper claslaminated unit (Gibb and Partners 1993). tic Unit, they are 2±15 m thick and widely spread along 3 Superficial deposits composed of fluviatile and lacusthe Dead Sea basin and southern Jordan Valley. trine gravels of Pleistocene age in addition to recent alluvium and alluvial fan deposits of Wadi Ibn HamLocal geology mad and Wadi Mutaly. The Dead Sea is composed of two basins; the northern The active Dead Sea transform passes east of the sinkhole basin, which is larger and deeper, and the southern, area, its activity is indicated by sinistral shifting of recent which is shallower and smaller. They are separated by alluvial fan deposits north of Ghor Al Haditha (Khalil the Lisan Peninsula, which is underlain by an active salt 1992). Thick deposits of superficial material make it diffidiaper, up to 7 km wide and 20 km long that extends cult to trace the faults and joints in the sinkhole area, under the Dead Sea (Bender 1968). The southern basin of but a lineament map (Fig. 5) shows major lineament the Dead Sea has a large amount of clastic sediments passes through the area. (mostly gravel and silts) left by the Wadi Araba and its The beds of Lisan marl covering the salt dome locally tributaries as well as by other streams such as the Wadi exhibit a circular strike and dip gently away from the Al Hasa, the Wadi Numeira and the Wadi Al-Karak on horizontal central area. The crestal area is slightly the Jordan side (Fig. 1). affected by tensional faults and the flanks by normal The study area of Ghor Al Haditha is located on the east- faults with small throws (Bender 1968). Closed surface ern side of the Dead Sea between 31159 and 31209 north depressions with slumping along their rims give evidence and 35259 and 31339 east, to the north of Wadi Ibn of salt leaching in the crestal part of the salt uplift Hammad (Fig. 3), its southern part is covered by deposits (Fig. 3). of the alluvial fan of this wadi. The area in which the sinkholes have occurred is gently inclined wave-eroded platform, about 3 km in width east of the Lisan PeninsuDead Sea water elevation and la. This area previously formed the bed of the Dead Sea when it was submerged during a period of higher sea levgroundwater conditions els. The platform is typically covered by a thin surface cover of silty or sandy materials. Boreholes showed that Dead Sea water elevation the geological materials underlying the platform consist During 1861±1870, the level of the Dead Sea was between of laminated calcareous silts interbedded with layers of ± 396.28 and ± 396.53 m compared to Aqaba Datum. salt (halite) and gypsum (Fig. 4). The exposed walls of These elevations dominated up to 1887 where a considsinkholes also demonstrated that much of this geological erable increase in water level began to be noticed, up to sequence near the surface is composed of sand, silty sand ± 388.73 m by 1896. The levels remained constant at and gravel, in addition to some evaporate minerals (domaround ± 390 until 1930. Between 1930 and 1952, changes inantly salt and gypsum). The sinkholes were located at in the sea level occurred and a new elevation was about ± 400 m relative to the Dead Sea level, which was recorded ( ± 390 m). From 1952, the Dead Sea level con± 407 m at the time of study. The major rock units, which tinuously declined until 1976. In 1976, an elevation of were penetrated and exposed in the sinkhole area, are ± 398.9 m was recorded. This decline in the sea level with from bottom to top as follows: minor fluctuations continued up to 1992 when it reached 1 Usdom evaporite formation: in the Lisan Peninsula a new record of ± 408 m. During 1992, a sudden increase beneath a thick cover (100±150 m) of the Lisan forin water level of about 0.2 m was measured due to heavy mation sediments, halite was encountered in boreholes. rainfall and snow melt which occurred during that wet The salt body was not fully penetrated (3586 m depth). season. Since then the water level has been decreasing Geophysical studies indicated that the salt is at least and has reached a present value of ± 409 m. Tables 1 and 6000 m thick (Jad 1997). This unit is composed of 2 show the annual and, in certain years, the monthly halite, anhydrite, clay and silt. fluctuations of the water level of the Dead Sea, while 2 Lisan marl formation: this formation is, in general, a Figs. 6 and 7 show the variation of the Dead Sea water horizontally bedded sequence of laminated deposits level during the last ten decades. with bands of silty clay, clayey silts and sands. The Lisan formation has been subdivided on the basis of lithological characteristics into three distinct units: (a)

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Fig. 3 Geological map of the study area

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Fig. 4 An E±W geological cross section of the study area

Fig. 5 Tectonic map of Al-Lisan area (after El-Isa and others 1995)

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Table 1 Fluctuation in the Dead Sea level for the last 130 years, in meters below sea level/data (various sources) 1861

± 396.28

1901

± 391.38

1941

± 392.91

1981

± 400.02

1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900

± 395.33 ± 394.88 ± 394.28 ± 394.28 ± 395.33 ± 396.53 ± 396.03 ± 395.23 ± 395.28 ± 396.53 ± 396.53 ± 395.78 ± 396.83 ± 395.83 ± 395.43 ± 395.13 ± 394.93 ± 394.83 ± 395.03 ± 395.33 ± 396.23 ± 390.78 ± 396.13 ± 395.43 ± 394.63 ± 394.03 ± 393.73 ± 393.63 ± 393.43 ± 392.93 ± 392.53 ± 392.28 ± 391.63 ± 389.73 ± 388.73 ± 389.03 ± 389.28 ± 389.28 ± 390.33

1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940

± 390.98 ± 390.93 ± 391.33 ± 391.23 ± 390.63 ± 390.63 ± 390.53 ± 390.33 ± 390.18 ± 390.13 ± 390.03 ± 390.18 ± 389.83 ± 389.83 ± 389.93 ± 390.03 ± 389.78 ± 389.63 ± 389.63 ± 389.73 ± 389.63 ± 389.83 ± 390.23 ± 390.33 ± 390.33 ± 390.43 ± 390.53 ± 389.83 ± 390.08 ± 309.17 ± 390.73 ± 391.45 ± 391.95 ± 392.02 ± 392.53 ± 392.86 ± 392.70 ± 392.67 ± 392.75

1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977

± 392.93 ± 392.57 ± 392.67 ± 391.76 ± 391.93 ± 392.45 ± 392.90 ± 392.03 ± 392.29 ± 392.86 ± 392.67 ± 392.78 ± 392.32 ± 392.87 ± 392.97 ± 393.18 ± 393.63 ± 394.25 ± 294.97 ± 395.47 ± 395.83 ± 396.36 ± 396.13 ± 395.98 ± 396.68 ± 396.63 ± 396.68 ± 395.95 ± 396.44 ± 396.53 ± 396.77 ± 397.67 ± 397.76 ± 398.35 ± 396.90 ± 402.00

1982 1983 1984 1985 1986 1987

± 400.67 ± 401.47 ± 402.46 ± 403.23 ± 404.00 ± 404.77

1989 1990 1991 1992 1993 1994

± 406.69 ± 407.68 ± 408.49 ± 407.30 ± 407.70 ± 408.98

Groundwater conditions Unfortunately, except for the only two wells drilled during this study, there are no groundwater wells in the study area. The data collected from Wadi Ibn Hammad, Ghor El-Haditha, Ghor El-MazraÁa wells and springs issuing into the Dead Sea indicated that groundwater flows in a westerly direction. It is necessary to mention here that the equipotential map is not constant since the water level is connected with Dead Sea level.

than 18 % which indicates that the soil has no resistance to erosion and implies that both irrigation and groundwater flow together with rainwater, causing soil erosion and playing a great role in the formation of the subsurface cavities. This is indicated by observing the springs issuing into the Dead Sea that carry fine particles of clay and silt size material to the sea, in addition to dissolved salts. Bulk density and water content were measured for all collected samples. The results of analysis are listed in Table 4.

Laboratory tests Nine soil samples were obtained from the study area; two samples were collected from the surface while the others were taken at different depths during drilling of boreOccurrence of sinkholes in the holes 1 and 2 (Fig. 8). The grain size analysis of these Dead Sea region samples (Table 3) shows that the tested materials are formed of medium-grained sand with very little silt. The History soil is poorly graded where the coefficient of uniformity was 3.15 for samples 1 and 2; 2.15 for samples 3, 4 and 5; Although sinkholes in the eastern Dead Sea shores are old, no studies prior to 1993 were conducted to explain and with an average of 2.76 for samples 6, 7, 8 and 9. this phenomenon. Sinkholes in the study area received The coefficient of fractionation was calculated at more

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Table 2 Dead Sea levels during the period 1981 ± 1994 with respect to Aqaba datum 1981 Month

Day

Jan 25 Feb 26 Mar 31 Apr 26 May 28 Jun 30 Jul 19 Aug 20 Sep 30 Oct 27 Nov 22 Dec 30 Average Maximum Minimum

1982

1983

1984

1985

1986

1987

Level

Day

Level

Day

Level

Day

Level

Day

Level

Day

Level

Day

Level

± 400.094 ± 399.864 ± 399.614 ± 399.544 ± 399.539 ± 399.794 ± 399.904 ± 400.094 ± 400.294 ± 400.394 ± 400.499 ± 400.584 ± 400.018 ± 399.539 ± 400.584

17 17 13 28 15 28 20 16 05 16 24 **

± 400.547 ± 400.534 ± 400.484 ± 400.434 ± 400.484 ± 400.680 ± 400.754 ± 400.909 ± 401.016 ± 401.154 ± 401.304 ***** ± 400.767 ± 400.434 ± 401.304

6 5 28 ** 8 22 27 ** 13 24 26 **

± 401.444 ± 401.376 ± 401.066 ****** ± 401.086 ± 401.256 ± 401.446 ****** ± 401.706 ± 401.896 ± 401.966 ***** ± 401.171 ± 401.066 ± 401.966

22 26 24 14 28 19 18 15 ** 17 21 9

± 402.126 ± 402.226 ± 402.186 ± 402.216 ± 402.228 ± 402.308 ± 402.463 ± 402.623 ****** ± 402.823 ± 402.923 ± 402.983 ± 402.464 ± 402.126 ± 402.983

9 10 19 17 29 26 27 31 23 27 23 11

± 403.071 ± 403.077 ± 402.979 ± 402.917 ± 402.885 ± 403.033 ± 403.180 ± 403.298 ± 403.415 ± 403.566 ± 403.624 ± 403.667 ± 403.226 ± 402.885 ± 403.667

21 17 23 17 13 14 23 20 13 30 16 18

± 403.652 ± 403.719 ± 403.731 ± 403.676 ± 403.737 ± 403.845 ± 404.020 ± 404.140 ± 404.228 ± 404.389 ± 404.389 ± 404.453 ± 403.998 ± 403.652 404.453

13 5 28 22 24 24 19 24 29 28 28 27

± 404.506 ± 404.486 ± 404.507 ± 404.523 ± 404.563 ± 404.683 ± 404.705 ± 404.870 ± 405.013 ± 405.005 ± 405.143 ± 405.190 ± 404.766 ± 404.486 ± 405.190

1989

1990

1991

1992

1993

1994

Month

Day

Level

Day

Level

Day

Level

Day

Level

Day

Level

Day

Level

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

29 27 30 ** 15 25 29 28 30 ** 15 12

± 405.890 ± 405.934 ± 406.020 ****** ± 406.052 ± 406.155 ± 406.281 ± 406.410 ± 406.523 ****** ± 406.666 ± 406.686

30 26 18 7 22 30 29 25 29 30 27 30

± 406.699 ± 406.813 ± 406.845 ± 406.804 ± 406.868 ± 406.978 ± 407.134 ± 407.253 ± 407.350 ± 407.442 ± 407.534 ± 407.679

29 28 27 30 16 1 15 31 ** 30 ** 10

± 407.696 ± 407.701 ± 407.473 ± 407.512 ± 407.832 ± 407.892 ± 408.063 ± 408.257 ****** ± 408.458 ****** ± 408.490

13 29 28 15 3 22 5 5 5 5 4 31

± 408.390 ± 407.150 ± 406.680 ± 406.640 ± 406.580 ± 406.740 ± 406.820 ± 407.080 ± 407.260 ± 407.400 ± 407.440 ± 407.300

23 24 31 29 27 27 28 28 28 28 28 28

± 407.100 ± 406.800 ± 406.750 ± 406.790 ± 406.810 ± 406.950 ± 407.200 ± 407.370 ± 407.580 ± 407.680 ± 407.800 ± 407.760

28 27 28 30 28 26 31 30

± 407.670 ± 407.610 ± 407.610 ± 407.660 ± 407.770 ± 407.910 ± 408.160 ± 408.260

Fig. 6 Variation of the Dead Sea level for the last 13 decades

widespread attention after 1992, a year characterized by heavy rainfall and snow. Reports prepared by (Knill 1993; Knight 1993; Tapponnier 1993) dealt mainly with the ground conditions that occur adjacent to the southern end of the Dead Sea, west of Lisan Peninsula, in Jordan. 1244

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A brief study was carried out by these investigators after a major sinkhole suddenly appeared in October 1992 in an access road which had been constructed along the ªElevationº-404-m contour to the west of Lisan Peninsula (Figs. 9 and 10). This sinkhole had a 13-m diameter,

Cases and solutions

and Wadi Numeira which discharged into the Truce Line flood channel (Fig. 9). As a result, the Dead Sea rose from ± 407.701 m on February 27 to ± 407.512 m, contrary to the long-term trend of steady decline. On the basis of this rise in the Dead Sea and descriptions of the distribution of rainfall, it was estimated that more than 210 million m3 over24 h had occurred. The rise in the Dead Sea level caused a rise in the water table of fresh groundwater of about 14 m (El-Isa and others 1995). The platform has been influenced by the flood as can be indicated by the erosion of parts of the salt crusts and by the occurrence of new sinkholes.

Fig. 7 Variation of the Dead Sea level during the period 1981±1994 (El-Isa and others 1995)

20-m depth and was filled with groundwater to a depth of 12 m. Further sinkholes were discovered close to the road and in Ghor El-Haditha. The inspection of aerial photographs revealed a considerable number of similar sinkholes following what appeared to be a definite topographical feature. In view of their proximity to, and actual interruption of the access road alignment, it was essential to discover their cause and to minimize their hazard. A major flood occurred over a 24-h period on 22 March 1991 which was caused by a short period of intense rainfall leading to intense discharge from Wadi Ibn Hammad and Wadi Karak into the Dead Sea, and from Wadi Hasa

Development of sinkholes in the study area Aerial photo interpretation and field visits to the study area have shown that several generations of collapse and subsidence depressions have occurred. Different subsidence features and many sinkholes of different sizes and shapes have been formed during the last five decades as indicated from aerial photo interpretation (Figs. 8, 11, 12 and 13). Farmers filled some of these sinkholes with soils. However, these sinkholes have repeatedly reoccurred in a very short time. The prominent sinkholes are of different diameters, depths and sizes. They have either elliptical or circular openings and all are of conical (mushroom) shape. The bottom of all sinkholes was dry and the real depth could not be measured due to the presence of caved material at the bottom. Two of the investigated sinkholes are compound. A large sinkhole tends to ring a smaller one and a shallow sinkhole tends to include a deeper one. All sinkholes are concentrated along a belt trending NNE

Fig. 8 Locations of the two boreholes and the lithologically evaluated sinkholes (after El-Isa and others 1995)

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Table 3 Grain-size distribution of the tested samples Depths (m) Sieve no. (Mesh)

4±5

9 ± 10

14 ± 15

19 ± 20

24 ± 25

29 ± 30

34 ± 35

39 ± 40

43 ± 44

10 20 40 60 100 140 200 Pan Sample Wt. (g)

1.1 31.4 31.9 6.4 2.4 1.6 0.3 0.4 0.5 6

40.0 7.5 43.3 20.3 12.3 11.7 3.4 2.5 3.5 214.5

1.4 19.4 6.5 66.7 41.2 29.3 11.5 .9 9.9 253.3

0.2 0.8 22.9 69.6 41.8 20.3 5.2 3.7 4.1 168.6

9.1 93.6 23.4 12.1 .9 2.0 1.6 1.9 221.6

.1 2.0 57.3 39.4 20.1 4.4 2.5 2.3 205.1

0.1 20.5 107.9 85.8 37.3 6.9 3.6 2.0 264.1

0.2 0.5 5.5 69.7 55.6 38.3 8.9 5.0 4.3 188

0.7 2.0 44.9 97.9 81.8 18.5 10.5 11.5 267.4

Table 4 The water content and the bulk density of the collected samples

formed for the first time in 1991 and was filled with soil materials. Another opening formed in 1993 just east of the first one. Radial cracks surrounding the opening of Depth Minimum density Maximum density Water content this sinkhole were traced to a distance of about 1.5 m. (m) (g/cm) (g/cm) ( %) Sinkhole 4 (Fig. 18a,b) is located below a paved road to 14 ± 5 1.64 1.83 27 the east of the first sinkhole. It has an elliptical opening 10 ± 19 1.59 1.78 15 on the surface. Furthermore, other secondary openings at 14 ± 15 1.57 1.81 21 both sides of the road can be clearly identified. These 19 ± 20 1.50 1.78 21 24 ± 25 1.57 1.80 20 openings begin to subside once again. The total sub29 ± 30 1.61 1.86 24 sidence in the sinkhole is about 1.5 m and a subsidence 34 ± 35 1.56 1.86 15 of up to 60 cm was measured in the road surrounding 39 ± 40 1.59 1.93 19 this opening. Both this subsidence and the development 43 ± 44 1.57 1.83 18 of concentric cracks indicate that the subsurface in this area is still active and a new deep-seated sinkhole across the road is likely to form at any time (El-Isa and others parallel to the shoreline. The width of the belt is 1995). Actually the development of further stages of sink100±150 m (Figs. 11, 12 and 13). This may be taken to holes was observed during the fieldwork when a deep support the idea that active tectonics together with lower- opening appeared in the bottom of sinkhole 4. This new ing of the Dead Sea level may have an appreciable share opening was about 1.5 m in diameter and a depth that in the triggering of subsidence cavities and the formation was estimated to be 7 m below the floor of the first one. of sinkholes. The sinkholes (Fig. 14) show both concenSinkholes have formed in this area for a period that tric collapse and radial cracking on the surface. exceeds 30±40 years. The lack of recorded information, Sinkhole 1 (Fig. 15a,b) has a circular opening with a however, necessitates relying on information obtained by diameter of about 10 m and an approximate depth of numerous landowners. The development of sinkholes and 10 m. It has a cone-like shape with its base at the contact related features continued unabated during the fieldwork between the actual earth and the superficial deposits. It undertaken for this investigation, between August 1994 formed for the first time during the winter of 1985. and January 1995 and it is still active. Farmers filled this sinkhole twice, in 1986 and 1992, but The following description of sinkholes that developed every time it reopened (reformed) in the same place west of Lisan peninsula in the proximity of the study (Fig. 8). area is taken from the investigations conducted by Knill Sinkhole 2 (Fig. 16a,b) has an elliptical opening with (1993), Tapponnier (1993) and Knight (1993). diameter ranging between 3±4 m and a depth of 7 m. Its A major sinkhole suddenly appeared in October 1992 in diameter increases with depth. This sinkhole was formed an access road that was constructed by the Arab Potash for the first time during 1990 and was filled with soil in Company which owns the major potash factory located at 1992, but during 1993 new radial cracks appeared on the the southern end of the Dead Sea to the south of Lisan surface that were accompanied by concentric lowering peninsula. The road was constructed along the EL just east of the previous sinkhole. After a few weeks a ± 404-m contour to the west of Lisan peninsula (Fig. 9). new opening developed and the sinkhole was reopened Further sinkholes were discovered close the rood. At the just east of the previous one. time of the initial recognition of the sinkholes, it was Sinkhole 3 (Fig. 17a,b) is located at a distance of 50 m to thought to be part of an isolated situation. Inspection of the north of sinkhole 2. It has an elliptical opening with aerial photographs of this area, however, revealed that major and minor diameters of 7 and 5 m respectively there were a series of sinkholes aligned along a northand a depth of 10 m. Its size increases with depth. It easterly-directed-channel-like structure referred to as the 1246

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Fig. 9 Relative locations of the two studies made in the vicinity of Lisan peninsula (modified after Knight 1993)

ªlinear channelº (Figs. 9, 10). This linear channel rises very gently to the north, extends for a distance about 2 km parallel to, and 200 m east of, the access road. The northern end of the linear channel turns to the northwest and appears to pass below the road where it dies out. The southward extension of the channel also bends to the west and passes below the access road. The maximum width of the linear channel is about 100 m towards the southern end but it narrows gradually northward. Throughout recorded history, there are observations that the southern Dead Sea basin has been dry at times, but submerged at other times. Climate and tectonic causes have been invoked and are indeed likely to account for such phenomenon.

Discussion Mechanism of sinkhole formation in Lisan peninsula This study has shown that the main causes of sinkhole formation are the fluctuations in the Dead Sea level over the last four decades and the collapse settlement due to freshwater ingress along former subsurface water flow paths. The freshwater that triggered these present features was probably the March 1992 major flood. This flood was caused by a short period of intense rainfall leading to intense discharge from Wadi Karak, Wadi Hasa, Wadi Ibn Hammad, Wadi Araba, Wadi Numeira and other small wadies. The amount of water received by the Dead Sea was estimated to be 210 million m3. This flood resulted in a rise of the water level north of the Lisan bar

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Fig. 10 Location of sinkholes in the linear channel in the Lisan peninsula within the potash project area (modified after Knight 1993)

Fig. 11 Locations of buried sinkholes, circular depressions and other circular features as interpreted from aerial photos taken in 1995

separating the Dead Sea's southern and northern basins, thus causing the rise in the water table which is possibly responsible for further dissolution of gypsum and the washing out of silt and clay fraction. This process continued and resulted in the formation of subsurface cavities. 1248

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These cavities which were developed into sinkholes could be sited below a massive halite layer presumed to exist at some 25±50 m depth and below another one at a depth of 10±15 m. These halite layers are overlain by interbedded sand/ silt and possibly clay and salt lenses or layers.

Cases and solutions

Fig. 12 The three zones of the study area according to their sinkhole risk. Levels as deduced from the geophysical, geological and geotechnical studies; zone 1 is the highest risk (after El-Isa and others 1995)

As the original sinkholes dropped some 12 m, clearly a major subsurface cavity must have existed, possibly as a horizontal ªtunnelº into which the various sinkholes collapsed at intervals in the same way as old mine workings collapse. Access to this cavity could either have been subterranean along an existing fault, or through a crack, or from the surface. With the sinkholes formed along a well-defined surface feature, it seems most probable that freshwater access to such halite was from the subsurface along water paths. Such a route would inevitably affect any soluble material in the sediments. It is therefore necessary to devise a mechanism whereby surface freshwater can (1) form large cavities at depth and (2) triggering sudden collapse settlement. We would like to describe one possible mechanism. The groundwater flow over many years allowed continuous leaching of soluble material and washing out of finegrained sediments. This process continued over many years but at different elevations that are related to the decline of the sea level. It seems that leaching is concentrated in areas with salt and fine-grained sediments. Over time the salt bodies vanished and the volume of sediments at this depth was reduced, a process that accompanied the formation and development of large cavities. This process was restricted to the freshwater table. These cavities were likely to be roofed by salt layers. During the

flood of March 1991, the sea level was raised about 0.2 m and the freshwater table was raised to about 14 m. The access of floodwater from the surface, together with the rise in the water table, has affected the salt layers below the surface. The dissolution of this layer has left the cavities without support and the irrigation in this area to grow crops has added weight to the roof material and reduced friction between the grains. Monitoring of the springs issuing directly into the Dead Sea indicates that the process of leaching is continuous, as water of these springs carry fine solids to the sea. As the process continues, many new cavities are still formed, as shown by Fig. 13, at some stage going through arching to slumping, depression and sinkhole formation.

Conclusion and recommendations Our conclusion is that the study area is hazardous due to the active development of sinkholes. Three zones of hazards were categorized based on the severity of formation of such sinkholes; where zone 1 represents the most hazardous one as far as the occurrence of sinkholes is concerned (Fig. 12). The mechanism underlying the occurrence of these sinkholes can be explained as follows:

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Fig. 13 Overall locations of all sinkholes detected during and after this study

1 Active tectonics, the fault system and the presence of 2 Fluctuations of the subsurface water system with time the salty groundwater conditions and underground that caused at least partial dissolution of salt bodies shallow salt bodies. Mushroom-like salt structures are created subsurface cavities that developed into sinklikely to develop from the deep regional intrusions. holes. Continuation of this process will ultimately cause These may grow to shallower depths depending on tecthe formation of new cavities and will end with comtonic and water conditions. This is evidenced by the plete collapse and the creation of such sinkholes. common occurrence of large subsiding areas including 3 The triggering of this collapse was accelerated by teca number of sinkholes in the region. tonic movements, which need to be earthquakes (El-Isa 1992). As the alluviums are quite soft and uncemented,

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Cases and solutions

Fig. 14 A photo showing one of the sinkholes in the middle of the road with radial cracking

these are easily eroded; and so the fine particles can easily be leached causing reduction of volume of subsurface material that helps to develop the cavities. This mechanism is of regional character; which makes engineering remedies very expensive and time consuming. Raising the water level of the Dead Sea, which can be achieved through the canal connection between the Read Sea and Dead Sea, may represent a visible solution to this problem. We recommend that the studied area be abandoned since no practical utilization of this area under the prevailed hazards is feasible. However, the area is being monitored where several sinkholes were developed and located.

Fig. 15 a Columnar section in sinkhole 1 in the study area, b a photo showing sinkhole 1 in the study area

Fig. 16 a Columnar section in sinkhole 2 in the study area, b a photo showing sinkhole 2 in the study area

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Fig. 18 a Columnar section in sinkhole 4 in the study area, b a photo showing sinkhole 4 in the study area

Fig. 17 a Columnar section in sinkhole 3 in the study area, b a photo showing sinkhole 3 in the study area

References Atallah M (1992) Tectonic evolution of northern Wadi Araba, Jordan. Tectonophysics 204 : 17±26 Barazangi M (1983) A summary of the seismotectonics of the Arab region. In: Clidlinsky K, Rouhban M (eds) Assessment and mitigation of earthquake risk in the region. UNESCO, France, pp 43±58 Ben Avraham Z, Brink U, Charrach J (1990) Transverse faults at the northern end of the southern basin of the Dead Sea graben. Tectonophysics 180 : 37±47

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Bender F (1968) Geologie von Jordanien ± Beiträge zur regionalen Geologie der Erde. Gebrüder Bonträger, Berlin Cernica JN (1995) Geotechnical engineering, soil mechanics. Wiley, New York El-Isa Z (1992) Seismicity of Wadi Araba-Dead Sea region. First Int Confon the Geology of the Arab world, vol 1. Cairo University, Cairo, pp 245±255 El-Isa Z, Rimawi O, Jarrar G, Abu Karaki N, Taqieddin S, Atallah M, Abderahman N, Al Saed A (1995) Assessment of the hazard of subsidence and sinkholes in Ghor Al-Haditha area. Report submitted to the Jordan Valley Authority, Ministry of Water and Irrigation, Amman, Jordan, University of Jordan Center for Consultation, Technical Services Study, Amman, Jordan Freund R, Zak I, Garfunkel Z (1970) The shear along the Dead Sea. Philos Trans R Soc Lond A267 : 107±130 Garfunkel Z (1981) Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80 : 81±108 Garfunkel Z, Zak I, Freund R (1981) Active faulting along the Dead Sea Rift. Tectonophysics 80 : 1±26

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Gibb A and Partners (1993) Storage facilities in Wadi Mallaha, Karameh dam project, phase b supplementary design project. Ministry of Water and Irrigation, Jordan Valley Authority, Amman, Jordan Jad MH (1997) Gravitational and magnetic study of the Lisan Area southern Dead Sea. MSc Thesis, University of Jordan, Amman Khalil B (1992) The geology of Araba area, Map Sheet No 3125 IV, Bull 22, Natural Resources Authority, Amman, Jordan Knight DJ (1993) Extension west of Lisan peninsular sinkholes along access road. The Arab Potash Project, Amman Knill J (1993) Report on site visit 8±11 January 1993 extension west of Lisan Peninsula sinkholes along access road. The Arab Potash Project, Amman

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