INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. A FIELD STUDY OF FACTORS RESPONSIBLE FOR QUICK CLAY SLIDES. ETUDE SUR PLACE DES FACTEURS RESPON SABLES DES GLISSEM ENTS EN ARGILES TRES SENSITIVES L. BJERRUM , T: LOKEN , S. HEIBERG, R. FOSTER. Norwegian Geotechnical Institute, Oslo, Norway. The City University, London, U. K. SYNOPSIS The paper describes the result of a field study of a 45 km^ large m arine clay area 40 km nort h­ east of Oslo. The scope of the study was to isolate , describe, and evaluate the relative im portance of the various factors and processes responsible for the o ccurrence of quick clay slides. The m ost important finding of the study is probably the establishm ent of a zone of aggression within which the risk of qu ick clay slides is greatest. It is concluded that the possib ility of preventing quick clay slides m ay exist by the con­ struction of thresholds in the stream s to reduce fu rther erosion. INTRODUCTION Within N orw ay's 40000 km^ of m arine clay area, HAUFRSFTFR KLÛFTA quick clay slides present a serious problem , each 220 century leading to loss of lives and property. In the study of this problem the Norwegian G eotechni­ cal Institute has hitherto concentrated its activit y * 180 6 on detailed investigations of individual slides. Th e (A y ............ Marin« clay real solution to the slide problem is not, however, t 140 TVS- to explain a slide which has already occurred, but 2 B e d ro c k to find som e m eans of predicting and preventing 100 ___ I__________ I__________ I potential slides. To do this, it is necessary to 10 15 20 relate our understanding of the m echanics of a slid e K ilo m e tr e s to the forces at w ork in nature which are the cause s of instability. As the first phase of such a study, Fig. 1 G eological cross section illustrating the Institute in 1967 initiated a regional m apping deposition of the m arine clay 9800 years ago. study of a 45 km^ area in the R om erike district (Fig. 2) situated about 40 km NE of Oslo. QUARTERNARY GEOLOGY This area was selected due to the fact that (I) it In com m on with the rest of Norway the area of has been the scene of num erous disastrous quick Rom erike was heavily eroded by the ice during the clay slides of which the U llensaker slide in 1953 glaciated periods. The bedrock is a granitic gneiss has been studied in great detail, (II) the quarter- of Pre-C am brian age with a tectonic structure in nary geology is w ell known and the originally sea the NE to SW direction which has been accentuated bottom plain is still intact adjacent to the northe rn by ice erosion. This has resulted in the form ation and western boundaries of the area, and (III) the of valleys and oblong knolls in the bedrock, and lower stream in the southern part of the area h'ts these can be seen clearly in the outcropping rock reached a grade of equilibrium , w hereas those in form ations. the northern part showed active erosion. After the final glaciation the m arine clays at R om e ­ In the study of the selected area all scientific rike w ere deposited in the sea in front of the disciplines relevant to the quick clay slide proble m retreating ice m asses during deglaciation about were used, nam ely geology, geom orphology, and 9800 years ago. The ice front stopped tem porarily hydrology which included erosion studies. The or even advanced slightly at H auerseter just north techniques of air and terrestrial photogram m etry of the research area, building up an outwash plain were applied. Seism ic and geoelectrical methods of sand and gravel (illustrated in Fig. 1). The w ere em ployed as well as conventional geochem ical finer particles w ere carried further south and and geotechnical methods and procedures. sedim ented as m arine clays interspersed by a few nearly horizontal 6ilt strata, which probably The study is certainly not com plete. This brief resulted from shorter periods of greater discharge review of the results to date is presented, however , of m elting water. The m arine lim it in the area has in the belief that they will dem onstrate som e of th e been established at 205 m above present sea level m ethods by which such problem s m ust be attacked. by Holtedahl (1924). Due to the isostatic land 531 BJERRUM, LOKEN, HEIBERG and FOSTER NORWAY Osloi (' QENM ARKA EEEk UFA f 1475 KLOFTA Fi g. 2 Map showi ng the r esear ch area. 1. Bedr ock area, 2. Ol d sea bot t om plain, 3. Qui ck cl ay sl i de scar s 4. Redeposi t ed qui ck cl ay sl i de masses, 5. Out croppi ng bedrock, 6. ” Fr ont of aggressi on” . 532 QUICK CLAY SUDES. Fig. 3 Exam ple on stream erosion in old sea bottom plain from an area next to the 1967 quick clay slide in TrOgstad. uplift follow ing the withdrawal of the glaciers QUICK CLAY SLIDES (Kenney» 1964) the m arine clay deposits which now have a surface elevation of 170- 185 m etres above Detailed descriptions in English of typical quick sea level cam e above the water level about 9500 clay slides have been published by G. H olm sen years ago. ( 1929), G. Holm sen (1946), P. Holmsen (1953), Bjerrum (1955b), Kenney (1967b) and Drury (1968). At the present tim e the clay deposits have consoli­ The slide always starts with a relatively shallow dated under their own weight, and, in addition, the initial slide in the slope towards the stream valle y, upper layers have been changed to a m ore or less frequently caused by brook erosion, and occurring stiff crust by drying and weathering. If a boring i s in clay which is generally w eathered and not quick. m ade in the old sea bottom plain, first a 5-7 m From this initial slip the slide develops backwards thick crust of stiff fissured clay is encountered. into the quick clay, towards the rear slope, sim ul­ Below this clay layer the norm ally consolidated taneously widening in all directions, see Fig. 4. soft m arine clay is found, extending to depths whic h The m ost characteristic feature of the slide is the can exceed 70 m . The clays which were laid down change of consistency of the clay in the process of in m arine water with a salt content of 35 g /l are sliding. As the clay becom es involved in the slide m ainly com posed of hydrous m ica, chlorite, quartz m ovem ents, and therefore rem oulded, it changes and feldspar. A description of their geotechnical to a viscous liquid. Through the opening form ed by properties has been given by Bjerrum (1954a). At the initial slide the clay slurry m oves from the som e locations where leaching has occurred and the cavity and descends down the valley carrying with i t salt concentration in the pore water has been re­ flakes of the upper dry crust. The speed with which duced, the clays have been transform ed into quick these slides occur can be illustrated by the Ullen- clays, which are characterised by the peculiar saker slide of 1953, during which about 200000 property that, when rem oulded, their consistency of liquid clay flow ed away from the slide within a changes to that of a liquid. few minutes. The number and magnitude of the quick clay slides occurring in the m arine areas can Since the m arine clay area rose above sea water be illustrated by the fact that an estim ated 50 m il l, level about 9500 years ago, at which tim e it appear ­ cu. m etres of quick clay have been rem oved by the ed as a wide, nearly horizontal, plain, it has been slides which have occurred within 25 km^ of the re­ subjected to a dram atic period of denudation lead­ search area. ing to the highly dissected landscape found today, as illustrated in Fig. 3. In the developm ent of thi s To guide the study of the factors and processes landscape the following processes have been of responsible for the quick clay slides the following governing im portance : consolidation, weathering, analysis of the conditions for their occurrence has seepage and leaching, frost action, erosion, slope been m ade : failures and slides, redeposition of slide m asses, vegetation, human activity. The occurrence of 1. The first requirem ent is obviously the existence disastrous quick clay slides, the scars of which ca n of a body of quick clay near and active erosion be seen in great num ber within the area, depended feature. As the quick clays are form ed by entirely on the relative rate at which som e or all of leaching with fresh water of m arine clays, the these processes were working.