Three slides at the same slope, 35m high, on clay shale are analysed. Geological and geotechnical features are shown. Back analysis from the last two slips and sensitivity analysis were done. Good predictions and mistakes are shown

Guimarães, R. B.

English

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in Geotechnical Engineering (1984) - First International Conference on Case Histories in Geotechnical Engineering 08 May 1984, 10:15 am - 5:00 pm Failure Slope in Clay Shales in Aratu Bay - Brazil R. B. Guimarães Universidade Federal da Bahia-Brazil Follow this and additional works at: https://scholarsmine.mst.edu/icchge  Part of the Geotechnical Engineering Commons Recommended Citation Guimarães, R. B., "Failure Slope in Clay Shales in Aratu Bay - Brazil" (1984). International Conference on Case Histories in Geotechnical Engineering. 17. https://scholarsmine.mst.edu/icchge/1icchge/1icchge-theme3/17 This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Failure Slope in Clay Shales in Aratu Bay .. Brazil R. B. Guimaraes M Sc Professor of Civil Engineering, Universidade Federal da Bahia-Brazii SUMMARY Three slides at the same slope, 35m high, on clay shale are analysed.Geological and geo-technical features are shown. Back analysis from the last two slips and sensitivity analysis were done.Good predictions and miskates are shown. INTRODUCTION During the construction of the road system of Aratu harbour in state of Bahia, there were several slides in shales and residual soils along it. One of them, the most interesting and challenging, took place in a cut slope 35m high that failed three times and was re-design-ed twice. The former was on may 78, when the slope was cut to enlarge the highway passage. At the top of the slope there is a railway terminal, the projection of which would cross the highway line design, fig.l. This slip has happened at the upper half portion of the slope with a volume lower than 20 000 cubic meters. The railway was left intact. The first re-design had recommended a flatten-ing, of the slope, an embankement compaction upon a drainage blanket at the top, fig.2,and some superficial drainage measure indications. At the beginning of the flattening a long crack appeared crossing the railway. The design was changed and the slope was more flattened. During the work two minor slides (2 000 cubic meters each) occurred. One of them near the crest and the other, with a shallow and circular shape, at midheight. At the and of the work, a large dark spot, indicating presence of water, appeared at a midheight point of slope. Two weeks later, on september 28, 1978 the second slide occurred with a volume of 400 000 cubic meters.In spite of a bad whether, there were no heavy raining day during the last month. The cracks were covered with clay and compact-ed by hand because it was inaccessible to machinery. Levelling and a detailed geological exploration were been done when, on january 79 --after two raining days (81 and 70 milimeters), the third and last failure occurred. The ap-pearence of the slide was quite different from the others. One point near the top had been carried away more than 40 meter horizontally and down a few meters. It appeared as if the 695 the whole slope had been lifted up by an ex-plosion and moved downhill. The harbour authorities, few months later, decided to modify the highway line design, by loosing Borne area and by stabilizing only the broken soil-rock mass, that is still in place, until today (1983) • Fig. 1 TN EB LEVEL ~0 Sketch of the slope situation First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduFig. 2 •• •• ~· . x .. "0 AINAGE B NKET •• i -, 10 -~ --·· zo •• 10 •• 100 IZO ---- BEFORE SEPT 78 (m} Typical cross section of the former re-design GEOLOGICAL SETTING Ara~u harbour is located in a region called Reconcavo, composed of magmatic and sedWEntary rocks as conglomerate, siltstone sandstone, shales. The most important geological attribute is the fault presence with a general strike Nl0°E and Nl50E. The west side of this fault is formed by sedimentary rocks from jurassic and cretaceous; the east side: pre cambrian embasement of gneiss. Covering these two groups of rocks are the clayey sand and gravely sand of Barreiras formation from tertiary. Its ho-rizontal and subhorizontal layers indicate no tectonic effects since its deposition. All of these sedimentary formations has been form-ed in a typical non marine enviromental. The slope foccus of current analysis is sedi-mentary from !lhas form~tion. Upon this were in the past Sao Sebastiao formation 1 000 to 2 OOOm thick or more from cretaceous too. As time goes by, erosion took place removing s. Sebastiao formation upon Ilhas, exposing it and, later in tertiary, deposition of Barrei-ras formation took place. EXPLORATORY DATA The first slide was a top failure on Barreiras formation which had not disturbed the other layers beneath ±t,from Ilhas. Hence it was possible to make measures of strike and dip in these untouched layers, that later on has been moved by the second slide. The slope can be divided into three sequences: the upper, the intermediate and the lower sequence, fig.3b. The upper sequence is formed by sandstone and siltstone with short inter-calated layers of shales, with an overburden of residual soil. The other two sequences are composed of shales with interbeded layers of siltstone and sandstone. The clay mineral predominances are: upper sequence - rich in kaolinite with some montmorillonite. Intermediate - rich in montmorillonite and some kaolinite and micaceous minerals. Lower sequence - predominance of kaolinite some montmorillonite and carboniferous materials at some levels. These information suggest a visualization of a more intense flow of water at the upper and intermediate sequence. The presence of mont-morillonite increases the possibility of shorter spacing of fractures in these zones. 696 The systems of fractures on sandstone in the upper sequence (fig.3) with the bedding,during the excavation could form joints that filled with rainning make it easy toppling failure anc rock fall. The second stage of the geological survey was done after the second slide and its results are presented in fig.4. Photo from this slide are presented in fig.5. The large discrepancy bet-weem the bed dip and strike measures, taken from the first survey stage (fig.3) and those taken from the second stage (fig.4), rise the need to emphasize the high degree of discon-tinuities in so restrict area. The last stage of the geotechnical/geological exploration was the openning of. pits on rna;y 79, whose positions are shown in fig.4. Seven were open an the slope, three had springs from slided material, another one from untouched material and the other three were completely dry. This fact reflects the difficulties to get efficient drainage in such material.SCWers (1976) wrote some remarks on similar problems. These pits were thought to be used for dewater-ing. Th; three pits on the railway terminal showed a very fractured and very weathered" zone 7.0 to 9. 5m deep, CEPED ( 19 79) . Under this zone there is a 7m thick layer of clay shale with oxidated fracture planes. This is a strong indication of water flow. Beneath these layers is a shale lightly weathered. These observations confirm the relation between expansive clay minerals a~d fractures mentioned by other authors, e.g. BJerrum (1966). They also confirm the predic-tion on water flow zone from the first explorato:ry stage. F7om the second slide, the following observa-bons, very usefull for back-analysis (next section) , were recorded: a) During drilling the water level from borehol1 8 had a sudden drawndown 1.5 meter suggesting the presence of a perched water table. b) The same borehole as transformed in water level indica tor, dropped 8. 5 meters on its water level at the time of the second slip. c) The open cracks, larger than 0,5m, were sub-vertical reaching five meters deep. d) Translational and some rotation were the nature of slopes motion. No top failure was found at any high. The toe has advanced into highway lane. e) In some cracks, seams were found. These seams were 3cm thick, of high plasticity clay(Liquid limit= 60; plasticity index= 30). f) For a long time even before the first slip, water pools have been along the toe slope. g) Sometimes from a day to another a top part of the slope had been moving down hill. The visual inspection really gave the sensation that some trees were kept away. No measures were made in order to evaluate it. First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduE'ig. 3a \ \ "" ' TN ""' EB --------KEY :]-~ -- INFEREO FAULT '-...... -.r SUB VERTICAL , "'-.. '( GENERAL DIP rc: FRACTURE SYSTEM ~ -~~~~ -~s ·-.._ Location of the inferred fault Fig. 3b 10 !! 1-l ~ EQUENCE A o• UPPER i ~ - j,INFE ~ED INTERM DIATE FAUL OWER _..,. v -I 40 •o 20 10 7 -ao 0 20 40 00 .. 100 120 --- SECTION A A' (m) Typical slope profile assumed from surface inspection Fig. 3c 140 1110 IOr"~~~--r---r--~--r---;---o---r~ -ao 0 zo 40 00 •• -·-·-·----·--· BEFORE MAY 78 ------ BEFORE SEPT 78 --- BEFOR£ JAN 79 --- AFTER JAN 79 Typical sections from each slides 100 120 (ml 140 110 697 ANALYSIS OF THE SLIDES No shear test was carried out on slope material because there are no field and laboratory data at disposal on these shales from Reconcavo. Hence, there is none correction factor to be applied on laboratory results in order to get predictions near field behaviour. First Re-Design The second slide occurrence suggests an unde~ estimate of the inferred fault presence on first re-design studies. Slope stability estimation from block analisys, considering the dip bed-ding only measured on slope surface, leads to high factors of safety, although lower strenght parameters had been assumed, fig. 6. The election of plane failure method is due to the following field observations: a) All other failures on this material had its failure surface represented accurately by two planes. b) These planes were passed through disconti-nuities being subvertical and subhorizontal. c) Usually these discontinuities were filled with high plastic clay, Guimaraes (1980). The block analysis presented by Esu (1966),who considers discontinuities filled with water and moment equilibrium, furnished no satisfactory results. These analysis and the second slide leads to the conclusion that the existence of the fault reflects possible changes on dip bed-ding inside the shale mass. Back-Analysis Back analysis were performed based on the fol-lowing assumptions: a) The slip surface is composed of two intercept ing planes.The former is subhorizontal intercept ing the toe, the last is vertical passing~­the last top crack. The subhorizontal plane in-tercepts the vertical plane at the same water level from borehole 8 measured after the second slide, fig.7. b) Valid Mohr-Coulomb shear strength envelope. c) The state of stress is uniform for all points of the failure surface. d) Hydrostatic pressure distribution as in fig. 7. e) End restrains effects neglected. f) Possible effects of the state of tension on the slope neglected (refers to those analyzed by Chowdhury, 1976). The back-analysis results are shown in fig.8 in which a few different assumptions were consider-ed. The most probable C 1 and ~values are: effective cohesion c' = 0 - 15kN/m2 effective angle of internal friction f1 = 13-17o. In fig.7 shows the positions of field slip zone and the failure plane assumed for back analysis. Variations on this plane, making it pass through failure zone and through the water level after failure, have little effects on shear strengti1 parameters (c' and fl'). Back-analysis carried out on slided section before the last failure, show a residual fric-tion angle upper 7o and lower 13o. This range is First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduFig. 4 .. 0 GRAFICAL SCALE Geological map Fig. 5 Aerial photo sliped area 698 •• •• Fig. 6 tf@J t?·\.!:}l f;(Jj:ii Jt>:t>t>l>l ~ X CJ ,!( • ® ...... p c KEY GRAY SHALES SANDSTONE AND SILTSTONE BROWN SHALES SLUMPED MATERIAL EMBANKMENT CONTOUR LINE CONTACT CRACK FAULT DIP SPRING BOREHOLE FRACTURE VERTICAL JOINTS PIT do dh a= 2h w = 0,335a2 'rshale U = 0,088a2 T water U = O,Ol5a2 '5' water R=-; + l.h +liz R favorable stabilization Stability analysis First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edudue to the great uncertainly of water level into the cracks. The lower value is due to the hypothesis of pratically zero hydrostatic pressure into cracks. This is the lowest boundary value, probably irrealistic. Sensitivity Analysis This kind of analysis applied to slope stability consists on keeping unchanged all factors ex-cept one of them and on evaluating the con-sequences of its variations on the factor of safety. This work must be done for all relevant parameters. Doing so, it will be possible to decide which factors must be changed in order to increase the factor of safety.This is pos-sibLe, for instance, by flattening a slope,mak-ing a draw down or loading the toe. The term "sensitivity analysis" was taken from economy which has the same meaning. Sometimes "change-of-conditions" is also used, more over "sensi-tivity" alone is employed by Hoeck and Bray (1977). The importance of sensitivity analysis has been restricted for this slope because there are few parameters that could be changed on field. Height is changeless, flattening also because it is necessary to keep the railway un-thouched. Hence it was left over as stabilizing factors, dewatering and retaining walls or other methods for sustaining part of the slid-ed material. The toe loading is a doubtful stabilizing process because of the uncertainties of the residual friction angle value. If it is lower than slip plane angle, surely, loading any part of the slope will lessen the factor of safety. Thus, dewatering and retaining wall construction were the solutions recommended. To avert raining water infiltration through the railway terminal, that has been a formidable catchment area, a sealing betuminous coating was made between the residual soil and the rail way gravel blanket base. For similar reasons -a covering of compacted clay upon the failure mass was recommended. Later on, after the modification of the high-way design, it was decided not to make any re-taining wall, stabilizing only the slided mate-rial. Fig. 7 Back-analysis assumptions 699 Fig. 8 "' E ..... z =--"' 30 20 --.t_SAFE /'-- FS=l UNSAFE I o (most probable values) Back-analysis results CONCLUSIONS From 7 opened wells on slided material three were dry and one had spring only in unthouched material. This fact reflects the difficulties to make dewatering and to determine water flow on the materials, two very important things in any stability analysis. The first slide was a toppling failure. The second was a delayed base failure due to water pressure equalization. A fault presence was underestimated on the first re-design. The oppening of test pits or trenches are the best ways to sound the subsoil on this kind of material. The author states that he has been very puzzled by the slope shape after the third slide and he does not exclude the possibility of a release of the recoverable strain energy from shales mass. This strain released energy could be pos-sible due to the occurrence of second slide. Although there is other possible justification. Another explanation could be the nypothesis that the residual friction angle is lower than t!he slip plane. The failure plane position prediction using Hoek and Bray (1977), charts furnished an ex-cellent agreement with the availiable field data. Such prediction was-put in a technical memorandum, Guimaraes and Machado (1979) before any collection from evidences of plane failure position were made. Because of the occurrence of two slides assumed on the same failure plane, it was possible to estimate the shear strength parameters. First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.eduA good correlation in predominance of expansive clay minerals a number of fractures and evi-dences of water flow inside the shales masses were observed. AKNOWLEDGEMENTS The author expresses his thanks to professor Shigueimi Fujimori for his kindliness in discus sing all geological aspects and his valuable -supervision on field work and to professor Luis Edmundo Prado de Campos for the valuable discus sion on back analysis. The CODEBA- Companhia-das Docas do Estado da Bahia through its author ities for all avaliable data, to SATTE - Servi= 90 de Assessoramento de Textos Ltda. for typing this paper, their contribution is gratefully aknowledged. BIBLIOGRAPHY BJERRUM, L (1966) "Mechanism of Progressive Failure in Slopes of Overconsolidated Plastic Clays and Clay Shales". Third Terzaghi Lecture, published in: Journal Soil Mech. Fdns. Div. Am. Soc. civil Engrs. 93, 1967, N9 SMS 1-49. CEPED (1979) Progress repport may-jun 79. Aratn harbour-slide from stake 31 to 44, in portuguese, unpublished repport. CHOWDHURY, R.N. (1976) "Initial Stresses in Natural Slope Analysis". proc. specialty conf. on "Rock Engineering for foundations & slopes", ASCE, Boulder Colorado, August, Vol.I 404-414. DEERE, D.U. and PATTON, F.D. (1971) "Slope Stability in Residual Soils". Proc. 49 Cong. Pan Americana de Mec. dos Solos e Eng. de Funda9oes - State-of-the-art-Papers - Vol.I, ed. ASCE, jun, San Juan pp.87-170. ESU, F. (1966) "Short-term stability of slopes in unweathered jointed clays" Geotechnique vol.l6 n9 4 321-328. GUIMARAES, R.B. (1980) "Slope staB_ility Using Compacted Earth Wall". Proc. 2n regional seminar of Soil Mechanics and Foundations. Ed. Funda9ao Escola Politecnica, Salvador-Bahia, may 1980, in portuguese. and MACHADO, M.L. (1979) Technical Memorandum n9 1, Aratu harbour, Slide from stake 31 to 44 in portuguese, unpublished repport. HOEK, E. and BRAY, J.W. (1977) "Rock Slope Engineering". The Institution of Mining and Metallurgy. 2nd edition, London, 402p. SOWERS, G.F. (1976) "Dewatering Rock for Construction". Proc. specialty conference on "ROCK ENGINEERING for foundation & slopes". ASCE, Boulder, Colorado, August, vol.I 200-216. 700 First International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu

Failure Slope in Clay Shales in Aratu Bay - Brazil

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Failure Slope in Clay Shales in Aratu Bay - Brazil

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