1 - Deere & Patton, (1971) Slope Stability in Residual Soils PDF [PDF]

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Fourth Panamerican Conference on Soil Mechanics and Foundation Engineering



Cuarto Congreso Panamericano de Mecánica de Suelos e Ingeniería de Fundaciones



Proceedings of the Fourth Panamerican Conference on Soil Mechanics and Foundation Engineering Memorias del Cuarto Congreso Panamericano de Mecánica de Suelos e Ingeniería de Fundaciones



VOL. I STATE-OF-THE-ART-PAPERS ESCRITOS



SOBRE El ESTADO-DEL-ARTE



San Juan, Puerto Rico June 1971



PUBLlSHED AMERICAN



BY



SOCIETY OF CIVIL ENGINEERS 345 EAST 47 STREET NEW YORK, N.Y. 10017 U.S.A.



Conference



organized



under the sponsorship



of:



Soil Mechanics and Foundations Division of the American Society of Civil Engineers Institute of Engineers, Architects Surveyors of Puerto Rico



and



Puerto Rico Section of the American Society of Civil Engineers



Congreso organizado



bajo el patrocinio



de:



La División de Mecánica de Suelos y Fundaciones Sociedad Americana de Ingenieros Civiles Colegio de Ingenieros, Arquitectos de Puerto Rico El Capítulo de Puerto Rico de la' Sociedad Americana de Ingenieros



© 1971 - American



Society



of Civil



y Agrimensores



Civiles



Engineers



de la



FOREWORD At the Third Panameriean Conferenee on Soil Meehanies and Foundation Engineering held in Venezuela in 1967, the invitation of Puerto Rico to aet as host for the Fourth Conferenee was accepted. The Fourth Conferenee was organized around the theme Performance of Foundations and Earth Structures; the formal teehnieal sessions were devoted to a penetrating examination of six speeifie topies. The Conferenee Proceedings are presented in three volumes reprodueed from author-prepared, camera-ready copy. Volume 1 contains State-of-the-Art papers on the six topies and has been published in two editions, one Spanish and one English. Volume 11 contains aeeepted papers on the six teehnieal topies and on the Business and Praetiee of Foundation Engineering. Volume 111, whieh will be distributed after the Conferenee, will contain prepared diseussions of the State-of-the-Art papers, closures by the State-of-the-Art speakers, the keynote address, and a record of the Conferenee. ]ohn A. Focht, ]r. Gabriel Fuentes, Jr. Dario Hernandez-Torres Pedro ]imenez-Quinones T. William Lambe, ex o[ficio Kenneth L. Lee F. E. Richart, Jr., ex o[ficio Miguel Santiago-Melendez Lillian Skerrett Efrahim Murati, Co-Chairman Ronald C. Hirschfeld, Co-Chairman The Organizing Committee, Fourth Panameriean Conferenee on Soil Meehanies and Foundation Engineering



CONTENTS VOL. 1



STATE OF THE ART PAPERS Page



Session



1. THE STANDARD PENETRATION TEST Victor F. B. de Mello, Brasil



.



11. SLOPE STABILlTY IN RESIDUAL SOILS D. U. Deere and F. D. Patton,



U.S.A.



87



111. THE ALLOWABLE SETTLEMENT OF STRUCTURES H. Q. Golder, Canada



IV.



EFFECTS OF FOUNDATION CONSTRUCTION ON NEARBY STRUCTURES David J. D'Appolonia,



V.



171



U.S.A.



EFFECTIVENESS OF CUTOFFS IN EARTH FOUNDATIONS ANDABUTMENTSOFDAMS Raúl J. Marsal and Daniel Reséndiz,



VI.



189



México



237



ACCURACY OF FIELD DEFORMATION MEASUREMENTS James P. Could and C. John Dunniclíff,



U.S.A.



313



\



SLOPE



University



STABILITY



IN RESIDUAL



SOILS



D. U. Deere and F. D. Patton of Illinois, Urbana, Illinois,



USA



SYNOPSIS The key to understanding the stability of slopes in residual soils lies in recognizing the roles of weathering profiles, groundwater, and relict structures. It is absolutely necessary to have a clear understandin~ of the typical weathering profile in many rock types. With this profile in mind, the investigator is prepared to recognize significant local variations and to outline exploration, monitoring, and testing programs to establish the data required for a stability analysis. However, a design based upon a stability analysis alone is economical only in very simple geologic environments or in especially important slopes where large expenditures of funds can be justified. In most cases a design method is recommended in which precedent is modified by exploration, testing, and analysis.



SINOPSIS La clave para la compresión de la estabilidad de taludes en suelos residuales radica en el reconocimiento de los papeles que desempeñan los perfiles de mete~rización, el agua subterránea y las e s t.r uct u ras heredadas. Es absolutamente necesario tener una clara concepción del perfil típico de meteorización en varios tipos de roca. Con este perfil en mente, el investigador está entonces capacitado para reconocer variaciones locales significativas y para delinear los programas de exploración, de prueba y de control requeridos en la obtención de la información para el análisis de estabilidad. Sin embargo, el diseño de taludes basado exclusivamente en análisis de estabilidad es económico sólamente en medios geológicos muy simples, o en casos de taludes de particular importancia donde se justifiquen gastos fuertes de capital. Para la mayoría de los casos se recomienda un método de precedente modificado por exploración) ensayos y análisis.



87



88



FOURTH PANAMERICAN CONFERENCE INTRODUCTION



Slope failures in residual soil and weathered rock are common-in humid temperate and tropical climates, particularly-during periods of intense rainfall. The profile of weathering which has developed on a rock slope over a long period of geologic time has altered the strength and permeability characteristics of the slope so as to increase its susceptibility to failure. Relict geological structural features--joints, bedding planes, and faults--which have been inherited from the original rock mass further reduce the stability of the slope. The pattern of weathering is greatly influenced by the structural features and lithology of the rock mass. In this 2?per we consider igneous and metamorphic rocks, carbonate rocks, shales with and without interbedded sandstone, and lnterbedded basalt lava flows. ---- ~he problems of slope stability in residual soils and weathered rock cannot, in practice, be separated from problems related to the often present mantle of colluvium (or slope wash and slide debris). For this reason colluvial slopes are also examined. Residual soils reach their maximum development under tropical conditions of high temperature and rainfall, but they also occur in many other parts of the world. For example, they are found in many areas where temperate and arid climates prevail and occasionally in protected areas in glaciated terranes. Geologically ancient zones of weathering or "paleosols" are also found throughout the world in association with many of the unconformities in the geologic column. These paleosols commonly have engineering characteristics similar to those of present-day residual soils. This paper includes a review of the weathering profiles found in a variety of rock types and the relationship between the weathering profiles and the slopes that develop. Common types of slope failures and their association with the weathering profile, colluvial cover, relict structures, and groundwater are examined. The effects of local and regional groundwater flow systems are reviewed as are the characteristics of relict structures and their effect on the shear strength of residual soil and weathered rock. Exploration programs are considered including the use of surface geophysical surveys, borings, borehole logging, and large diameter access openings. Presentation of the data is discussed as well as different methods of designing slopes in residual soil and their applicability to three common geologic environments. Finally, remedial measures are considered.



89



SLOPE ST ABILITY WEATHERING



PROFILES



Introduction The weathering profile is the sequence of layers of materials with different physical properties which have developed in place and which lie above the unweathered rock. The weathering profile may be formed by mechanical weathering, which is a disintegration of the original structure of the rock mass, or by chemical weathering, which is a decomposition of the original materials of the rock mass. In this paper we have concentrated on those slopes where the effects of chemical weathering predominate. However, mechanical disintegration of the' rock may also be occurring in such slopes and helping to accelerate the chemical decomposition. These mechanical processes include stress relief from unloading due to erosion and the resulting differential strains and displacements of the unloaded soil and rock. Weathering profiles can vary considerably from place to place because of local variations in rock type, rock structure, topography, rates of erosion, and groundwater conditions, and because of regional variations in climate,-particularly rainfall. Such variations make it difficult for one,to attain a broad perspective from which to view the occurrence of weathering profiles. However, as more experience has become available in recent years, we, along with a number of other authors, have found it useful to generalize and describe typical weathering profiles. Typical Igneous



Weathering Rocks



Profile



for Hetamorphic



and Intrusive



Almost all profiles of rock weathering developed over intrusive igneous rocks and metamorphic rocks may be given a three-fold subdivision of (1) residual soil, (2) weathered rock, and (3) relatively unweathered, fresh bedrock. In the profiles to follow these layers will be designated by the Roman numberals 1, 11, and 111. This sequence is shown on Fig. 1 for the igneous and metamorphic rocks. As simple as this breakdown appears, it is not always easy to apply because of the very irregular and, often, gradational contacts. The residual soil is subdivided into three zones, lA, lB, and le, which correspond to the A horizon, B horizon, and e horizon of the pedologist. The e horizon is considered by pedologists to be the parent material for the solum --the A and B horizons. They call the continuous series of soil horizons (A, B, and e, etc.) the "soil profile." A horizon, Zone IA--The surface A horizon has been defined as the zone of eluviation which has been depleted by a carrying downward of materials in suspension or solution by infiltrating water. Sandy textures often develop in the A horizon. The upper portion of the A horizon



FOURTH PANAMERICAN CONFERENCE



90



,- _bl.::'i~:'::.'_



ZQNE



r



Collu..,¡umor



,



c.l~~:::~er~il$ i CO::'e..rr:n



-c....



TA-



~1-ZONE I



,



I I



I



'-IA-



I



Colluvium



Ete.



A honzon



1. Re$ÍdIUlISoiI



-



.... IC



IC



l&!prolitel



T~~~



from Sapfolite te



IIA



I'}~~~~,~~. r~(~GtAl~~



'"",·U"··~''''



Weathered



Rock



11 Weathered



Rock



118



11I Unweat~ed



Rodt



Fig. 1 Typical weathering intrusive igneous rocks.



111



profile



for metamorphic



and



is normally organic. B horizon, Zone IB--The B horizon is the zone of illuviation or zone of deposition of solid materials carried down from the A horizon. The B horizon is commonly darkcolored, rich in clay-sized minerals, and leached of its original soluble constituents. The B horizon is so altered that very little indication of the parent material and no indication of the original structure of the rock mass remains. Sometimes the B horizon becomes enriched with silica, aluminum, or iron and may be cemented or susceptible to irreversible hardening as a resulto Because of these additions of materials and because they are subject to seasonal variations in moisture content, B horizons will vary considerably in their physical properties. C horizon, Zone IC--The C horizon is recognized by evidence of the original rock structure although the material is more soil-like than rock-like. The relict rock structure includes joints, faults, and minerals which have orientations identical to their original relative positions. The feldspars, however, are converted to kaolinite or other clay minerals, the micas are partly or completely degraded and altered, and most other minerals present in the parent



SLOPE ST ABILITY



91



rock except quartz are altered. The result is that the hard looking rock has the consistency of a soil and behaves in many respects like a soil. Yet the relict structures inherited from the parent rock persist and result in planes of weakness which are far more continuous and numerous than is common in transported soils. Sandy silts and silty sands predominate, and highly micaceous zones or bands are common where micas were present in the original rock. Saprolite is the term commonly applied to this zone of soil-like material which retains the relict rock structure. For purposes of distinguishing Zone IC from the layers of weathered rock below, Zone IC is defined as having less than 10% by volume of corestones. (Corestone is the term used to describe an unweathered or partially weathered rock core or remnant of a former larger joint-bounded block of unweathered rock. Corestones have also been called lithorelicts, and "floaters".) The silty and sandy sized material in Zone IC can be very compressible--particularly where it is micaceous (Sowers, 1953). Furthermore, it is often quite susceptible to surface and subsurface erosion (Oeere, 1957). The transition, Zone IIA--The weathered rock of Zone 11 is divided into two layers, an upper zone, IIA, which is a transition from the saprolite to weathered rock, and a lower zone, IIB, which is partly weathered rock. Zone IIA is characterized by the great range in physical properties of its components. These vary from soil-like materials to rock-like corestones. Corestones make up 10 to 95% by volume of the transition zone. The weathering has taken place more rapidly along the pre-existing joints and faults and along lithologic units that are more susceptible to weathering. The soil between the corestones is a medium to coarse sand which can be relatively clean, or silty and micaceous. This zone is commonly ver y permeable and water losses are often noted by drillers when they reach this zone. The transition zone is the seat of a great many engineering problems in residual soils. Its existence is the chief reason for making a distinction between the weathering profile found in the field and a list of intensity of weathering as determined, for example, by a petrographic analysis of thin sections. Zone IIA typically contains materials with many different intensities of weathering and, consequently, with a wide range of engineering properties. Partly weathered rock, Zone IIB--Zone IIB contains rock that has noticeable discoloration and some alteration along joints. In addition, alteration of feldspars and micas has begun, in some cases to a very marked degree. As the alteration advances, the rock is degraded from its original state to that of a rock with a lower strength and lower modulus but with increased permeability. The increase in permeability results from (1) volume changes in some of the grains as new mineral s form, (2) solution of some of the



92



FOURTH PANAMERICAN CONFERENCE



more soluble constituents, and (3) increased jointing and opening of pre-existing joints due to stress relief caused by erosion of the overburden. Unweathered bedrock, Zone III--The unweathered bedrock, Zone 111, shows no alteration of feldspars and micas and very little to no trace of staining along the joints that can be attributed to weathering processes. The unweathered bedrock could, however, be a mass of highly jointed rock or, in some rare cases, hydrothermally altered rock. For practical engineering purposes the term, Zone 111 material, should probably not be applied to hydrothermally altered rock even though it may be, in fact, unweathered from surface processes. TABLE I-DESCRIPTION



OF A WEATHERING PROFILE FOR IGNEOUS ANDMETAMORPHIC PERCENT



lQNE



I RESIDUAL SOll



IA-A HORIZON



18-8



IC-C



11 WEATHERED ROCK



DESCAIPTlON



HORIZON



HORIZON (Saprolitel



IIA- TRANSITION (fromresidullsoil orUrprOlite10 pwtly -..tMred rock)



IIB-PARTlV WEATHEREO ROCK



11I UNWEATHEREO ROCK



Roo· INX Core, percentl



RECOVERV· INXCOfe)



-top son. roots, organic material zoneofluchinganclelu\liation may be porcus



AElATIVE PERMEABILITY



medium tohigh



RELATIVE STRENGTH



Iowtt>



-, medium



-chiriICteristically clay-erwidled alsoiICcumulationsol Fe. Al and Si, eeoce may be cemented -no relict structures present -relictrock structuresretained -siltygradingto sandy material -Iess than 10% core nones -often mtcececos



ROCKS



CORE



O



LOW



LOW Ihighifc:ementedl



generally 0-10%



medium



or nct eppucebre



Iow te



medium Irelic:tstructum verysignific«lt)



-highlyyariable,soil·liketo rock·like -fines commonly firoe 10 CO 75% fgenerCllly>9O%)



gener.llyl()()%



vilri,¡¡bj.e,geoer.llv 10-_



HIGH Iwale\"l0t3t"5 common)



-



..•.•.•



medium 10 Iow struc:1uresllOd reliclltructufft..e ••••••



"Notes: The descriptions provide the only retiabre means 01 distinguishmg the 10"," •• Considering only intact rock manes with no .dYeuelY orienled geologic structure.



medium lohigh



10w tt>



t



medium 10 h"o.



'*Vhigh ••



medium



.



Table 1 provides a surnmary of the features that distinguish the various zones in the weathering profile for igneous and metamorphic rocks. In addition, the approximate range of values for RQD or Rock Quality Designation (Deere et al, 1967) and percent core recovery is given for comparative purposes. However, these methods of logging and classifying rock core do not provide unambiguous determinations of the boundaries of the weathering zones. Descriptions of each zone provide the only reliable means of distinguishing one zone from another.



93



SLOPE STABILITY Comparison Profiles



of Engineering



Classifications



of Weathering



We have stated that an understanding of weathering profiles is necessary in problems of slope stability. Yet, there is little agreement on a standard description of the zones in a weathering profile. Many persons have described field geologic studies of rock weathering including Branner (1896) and Derby (1896) in Brazil, Blackwelder (1925) in California, Brock (1943) in Hong Kong, and more recently Thomas (1966) in Nigeria. Attempts to relate these descriptions to engineering problems are of relatively recent origin (see Table 11). Since the work of Vargas (1953), over a dozen different weathering profiles have been described in the engineering literature. These profiles reflect the experience of various workers in different rock types, in different climatic zones, and with different purposes. Many of the more recent classifications have been built on the earlier work of Vargas (1953), Kiersch and Treasher (1954), Sowers (1954), Moye (1955), and Ruxton and Berry (1957). Criteria for classification--Although we hesitate to add another classification to the literature, it seems appropriate to attempt some reconciliation of the various descriptions and classifications of weathering profiles which are in use for engineering purposes. The criteria we have used for this new classification are: ( 1) The descriptive terms should be as simple and easy to remember as possible. (2) The divisions should correspond closely to the chief divisions of profiles described by previous authors so as to utilize their experience and published information. (3) Conventional and traditional symbols and names should be used wherever possible. (4) Ambiguous terminology should be avoided. (For example, the use of terms such as "sound" or "solid" rock which could imply the complete absence of joints or faults.) Because we were accustomed to describing these materials simply as residual soil, weathered rock, and unweathered rock and since most other classifications did not permit this convenience, these terms became the basis for our principal divisions: 1, the upper layer of residual soil; 11, the intermediate zone of weathered rock; and 111, the unweathered rock. Study of several published profiles of others showed that it was usually possible to fit their described profiles into the scheme. Figure 2 shows two schematic profiles of young decomposed soil and of mature decomposed soil based on data given by Vargas (1953). Discussion of zone boundary criteria--One of the more



\O I TABlE



THI$ PAPER



I



I



VARGAS



"'3



II-CQMPAAISON



MOVE· 1955



"'R~H'I TREASHERo



OF



I RUXTON' BERAV 1957



I



ENGINEERING



CLASSIFICATIONS



SOWERS



I(NILLIIoJONES



""



1954.1963



""



GNEISS



I



BASALT,



I



aUARTl CIQRnE



IIGNEOUS



I GRANITE



GRANITE



SANDSTONE



OF



&



WEATHERING



FOR



IGNEOUS



VAROAS.



I(ORZHENKQ&



SOWERS



SILVA.!.



$HWETS



"6T



1UBIO



""



""



GRAN'TE,



GNEISS



PROFILES



METAMORPHIC



GNEISS.



ROCK



SCHISTS



NON.CLAYEV ROCKS



BASALT ANO



ANDMETAMORPHIC



LOTT" "'T' "" SAUNOERS & FOOKES FOOKES



si



VARGAS



""



11IHORSWllL 1970



.



IGNEOUS.



A VAR1ETV



OF RQCKS



METAMORPH



GNEISS,



GNEISS



$CHIST, GRANnE,CLAY,



'"T1



$ANOSTONE



OTHEFiS



O



$AND$TQNE lQNE



RADE



RADE



I



1:1



CLAY



A-MORIZON



A-HORIZON MATURE



OAANITlC



GNEISSIC



THE UPPERZONE



SOIL



UPPER



SOlLS



ZONE



RES'OUAL601L



CLAYEY SO"



SOIL



'B



----



OR



TRVE



SANO LAYER



VOUNG



I



RESIOUAL



SOIL



I



H'GHL< WEATHERED



j



MATVRE



RESIDUAL



RESIDUAL



SOlL



SO" STlFF



CLAV



8



RESI~UAL



OR



HOIIIZON



SO"



CLAV-SAND



,V



COMPLETH



RESIDUAL



V



THE



DEBRIS



WEATHEREO GRANITE



"'



V COMPLETE



INTERMEDIATE



WITH



lV



COMPLETE



INTERMEDIATE



WEATHEREO



lONE



SAPAOLlTE



SAPROllTE



'C



IIA



:;o



VOUNG



VOUNG



LV



RESIDUALSOIL



WEATHEREO



C-"ORllON ISAPIIOLlTE'



RESIOUALSOIL



GNEISS



ZONE



COAESTONES



HI.GHLY



"., r;~]:'::':



WEATHEREO



WICORE



_.:2:.0~S



"'



WEATHEREO



ROCK



LAYER



THE



IIB



1



SLlGHTL



y



ZONE



RESIOUALS01L



WEATHEREO



ROTTEN



CiNEISS



MATERIAL



TRANSITION



"'



ZONE



'"



VERYALTEAEO



MODERATELY



WEATHEREO



WEATHEREO



RaCK



RaCK



2



11



WEATHEREO



SLlGHTLV



OR



SLlGHTLY



WEA1HEREO



FRACUREO



WEATHERED



O~_



ROC'



fISSUR·."



WEATHEREO



GRANITE



PARTLV



FISSUREDOR



WEATHERED



ROCK



V



"'



SOUNO



ROCK



1



ESS::;~:LLV



I



fRESH GRAN1TE



'NOTE, THESE CLASSIFtCATlONS ARE OESCI:UPTlONS OF INTENSITlES OF ROCK WEATHERING ANO WERE NOT INTENOEO TO BE WEATHERtNG



PROFILE5



I



BEOROCK



I



UNWEATHEREO RaCK



I



ftSSUREDOR FAESH



ROCK



SOLIO



CiNEISS



ROCK



FRESHROCK



SOUND



ROCK



STRATA



,.



Tllel"nl,uon10



2.



Altt,nll.llye'solshghtlyw,,"he't-dind,nlenHIV_'lh.'K!'ock



"nweall'>e,HI,ock



3."T"n""onIlflIWft!lu,IIO,I;ond