A Sinkhole Near the Damascus Gate, Old City of Jerusalem
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A SINKHOLE NEAR THE DAMASCUS GATE, OLD CITY OF JERUSALEM Y. Arkin Geological Survey of Israel Jerusalem Abstract The area in the vicinity of the walls of the Old City of Jerusalem has been reshaped, several times, during the past centuries. Buildings have been destroyed and areas have been covered or filled-in, with rubble and building waste material. Therefore sites of national, ethnical or archaeological significance may be found in present day construction projects and may present a hazard to construction. This repeated activity has artificially changed the topography, left "underground" openings and has altered the natural drainage regime of the area. As a consequence of unusually high and concentrated rainfall, sinkholes of various sizes appeared in this ground. The present paper deals with the use of old maps, geophysics and borings to determine the configuration of the natural topography, type of fill material and depth to bedrock. On the basis of this data a mechanism of sinkhole development is proposed in order to evaluate suitable remedial measures required in areas where construction is planned or taking place. General On the 7th January 1980 a sinkhole developed in the ground, in a flat area west of the Damascus Gate, Old City of Jerusalem (Figs. 1,5). Subsidence occurred at the development site of a major road junction, forming a sinkhole of 8m diameter and over 6m deep (Fig 2) with associated concentric fractures (Fig 3) and smaller sinkholes (Fig 4) up to 5m away from the main subsidence. This type of phenomena, although considered to be an uncommon hazard in the prevailing climate of Jerusalem, caused grave concern with regard to the stability of the area in general. Particulary since the winter of 1979-80 appeared to cause an "epidemic" of sinkholes in various parts of the country after unusually high and concentrated rainfall. A detailed geotechnical survey of the area in the vicinity of the Damascus Gate was carried out to determine the type and depth of fill material, the presence of karstic openings in the limestone bedrock and man-made openings such as buried cellars, water wells and aquiducts. Each one of these types of openings was considered to be a reasonable initiator of the development of a sinkhole. The Site An area of approximately 200 x 250m located at the road junction west of the old city walls near the Damascus Gate (Figs. 1,5) was examined. The area is situated at the eastern end of a present day wide flat valley filled with natural and man-made materials. The valley drains the north western suburbs of Jerusalem (Morasha, Russian Compound, Mea She'arim, Romema) towards the old city. The present day surface and topography does not conform to the natural configuration of the valley or the natural water course. Today's surface slopes to the north-west whereas the bedrock slopes south-east towards the old city walls. 565 •,. •. '#'- tf . -.V- 45^^; *.V*. *' * #1 ?M '^•jm ;*'v-fl!SSft^ •>^fe. ™3 :•£: •T Fig.1 : Aerial view of the Damascus Gate and road junction before recostruction. Photographed 1978. 566 Fi g. 2: Sinkhole. m*% .**? Fi g.3: Surface fracture. 567 Fig.4: Secondary small sinkhole. The bedrock and the surrounding hills consist of well bedded hard, dense limestone of the Bina Fra., of Turonian Age (Arkin et. al. 1973) and is a common building stone in Jerusalem and its vicinity. The rock is a pale yellow, micritic limestone, iron stained, cut by calcite veins and contains fossil fragments. Solution features in the form of karstic caves of several cubic meters volume and associated with faults and major fractures are common. Smaller solution holes of several centimeters across are also common throughout the formation. The Damascus Gate sinkhole developed following several rainstorms of up to 90mm rainfall spread out over the winter months of December 1979 and January 1980. The repeated rainstorms, at intervals of 3-5 days did not allow the drying out of the ground as normally occurred during previous years. The Survey An examination of old maps (Figs. 6,7) of the area (Wilson, 1864; Deutschen Veren Zur Eroforschang Platestinas, 1904) and modern topographic maps (Fig. 5) showed that many changes in topography are evident, particularly the position of the deepest part of the valley floor. The maps also revealed the location of aquiducts, water wells and old buildings (cellars) now covered and hidden. As a first approach to the problem a detailed base map was prepared on a scale of 1:5,000 including present day construction data and data from the old maps. A comparison of the above maps enabled the planning and execution of a geophysical refraction survey (Fig. 8) so that the lines chosen would provide maximum data on the thickness of the valley fill, depth to rockhead and verification of possible buried opennings. The survey provided the position and alignment of the main ancient water course and showed that the sinkhole had developed on the northern bank. The depth from the surface to rockhead in the center of the valley reached 13.5m and on the banks, up to 9m. A second (tributary) water course was revealed as flowing from south-west to north-east parallel to the old city wall and joining the main course near the Damascus Gate. Twelve auger and core drillings were carried out to verify the type of fill material and thickness. Figures 9,10,11 show examples of the material 568 Fig.5: GEOTECHNICAL SURVEY OF A SINKHOLE NEAR THE DAMASCUS GATE, OLD CITY OF JERUSALEM by YQOCOV Arkin m Fig. 5 569 1) E/3 p 0) sa •ri t fl < g h O aS!* 0) M fl > UO bO o Ht! 0c) 1 14 R S0 OJ ^D fl ËH ,Q M U O Cfl bù<u fi o N p m <H l tt^f h et !> h W 0) •p i) i 01 .al T3 •H u G (4 •H P fi 0) 0) <D M !H -n£) il <aD QJ S O > h 6 te 0 0) 01 PQ eu Q fl •d H O QJ m 0) t/3 •P 2 P S3 4) 1 0 M h> !* R l-~ en 570 S99I JO dVH SN0S11M 33tf N'INOani l»!83i«l< nij NO Na3isio 01 swmfjNoo LNOiivwaoj uNia o -z. 3 a < NI < Iff 111 < aidnvs/aaoo (w)SS3NM3IHA 3 M) •H 60 •H 571 g A301033 IVIdBiVW "1 T 1 J . , s o n . g flow t e , colour .ice n rubble geophysic loos e buildin o y rubble f d t g gree o y an ver e's'l < . s f y coars NU . Hs f m pebbles.grey-gree e gre , o 3 l , cla o f d s hol accordin *E" f 1 e e fil y o e l o . e L hol f . IJE fragments c Bas .pocket o r compacte surfac h rubble.fragment y d , |i° materia .0-06 l Subbase colou materiols Dept approximatel Roa 5 Non-cohesiv partl soil ' CM Fil cerami in sis andwvs/aaoo £8 S < a 06 0 (w)SS3NX3IHi 1 do d 1.4 <w)Hid3C] O - w m v '60-1 OIHdVMS 1!« § ^ îloEfïâAwao I A0OTO33 1V!B3iVW nu dOi| iavM .*• Ë s s II ° 3 s 11 *»3 1 !* •s O •1 fi M î! Ii 1 ii- I ALE il il: ll P- « n lui l! 1II S îi nu il. ÎL 3 ÎÎI !! 3Hd«VS/3aOD 1 1 <i § 1 î ! ! 1 î ! 1 1 .? S |UJ)SS3NX3!H1 s s S s s s s s. 2 ^ ~ ("J) Hld3a Jfil E '601 SIHdVUO >^ S.? 3 ï SIT ii §§ 'SN <^vvi l 00 •H A301033 itiaiivn n 1 J ! ™>| ™K liii ÇNIH •1 ! N ! I! I 1 iff il 1 DESCRIPTIO 1 !! % iiï •a Ë îl y il» 3~ldKVS/3aO0 fil J_ ! ! ! 1 1I I I air (uJ)SS3N)OIHl g ? s ! I II I I ["») Hid3a - '601 oiHdVaS ^\~ §§§§ îSis>SJ ^ s» Ttff •H 572 DG9 0G8 DG5 DG3 ii SURFACE T~I SINKHOLES I FILL MATERIAL Fig. 13: CROSS SECTION JOINING BOREHOLES 1-9. penetrated and its characteristics. These were compared to the section exposed in the wall of the sinkhole (Fig. 12). The fill material consists of water sensitive clay, building waste of blocks up to 30-50cm across mixed with fine and powdery material. A well developed bedding and grading of the different size materials is observed and the range in thickness of the beds, of up to 1.50m was formed due to the manner in which the material was thrown down. The coarse material forms well defined beds of extremely high permeability that act as water conduits. Bedrock in the center of the valley and along the ancient water course is covered by a gravel layer up to 1.50m thick (Figs. 10,11). The gravels consist of well rounded pebbles 5-10 cms across, unconsolidated and non-cemmented, forming a highly permeable layer. A number of karstic holes of several meters depth and cisterns (Fig. 9) were penetrated in the borings. These were found to be filled in most cases, by terra rosa soil in the former and organic clays in the latter. SURFACE SINKHOLE ROAD SURFACE INFILTRATION OF RAIN WATER // - COARSE FILL 0 1NG P°c „°o oooo0 o -< O ^O S_~o ^ A°0° GRAVELS Ooo ,0 LIMESTONE Fig. 14 : MECHANISM OF SINKHOLE DEVELOPMENT 573 A cross-section (Fig. 13) joining borehole 1-9 shows the relationship between the sinkholes, fill material, wadi gravels and rockhead. Mechanism of Subsidence Subsidence began after a wet period in which 90mm of rainfall fell in cycles of 2-3 days of rain and 3-5 days of dry weather.