Indian Geotechnical Conference – 2010, GEOtrendz December 16–18, 2010 IGS Mumbai Chapter & IIT Bombay
Interpretation of Geotechnical Report Sanjeev Gupta Senior Manager, e-mail:
[email protected]
Infrastructure /Tata Consulting Engineers, Mumbai
ABSTRACT Geotechnical engineering deals with the safe transmittal of loads due to various engineering applications to, and into, the soil or rock mass. The soil and rock mass formations are heterogeneous and generally are subjected to stresses (due to various engineering applications) which are additional to those presently existing in the earth mass from from its self weight and geological history. history. Due to these earth mass properties, properties, the amalgamation of experience, e xperience, study of what others have done in somewhat similar situations and site specific geotechnical information are required to produce an economical, practical and safe substructure design. The correct interpretation of the results obtained from field exploration and laboratory test program is one of the most important crucial parts of geotechnical engineering. Through this paper, the Author shares his experience about problems faced in various projects while reviewing reviewing the geotec hnical inv estigation reports for interpreting geotechnical geotech nical parameters required for design de sign o f structures. Autho r has made some suggestions sug gestions which could coul d be useful to resolve issues under un der such s uch circumstances. 1. INTRODUCTI INTRODUCTION ON
For any development to take place, whether within cities or outside hills or plains, dry/marsh y ground, it is essential to provide a technical criteria and guidance for its planning, design and construction. Geological and geotechnical investigations are crucial to establish various properties determining the behaviour of the soil / rocky strata at site. In such cases it becomes one of the prime responsibilities of a geotechnical engineer to make correct foundation recommendations and provide the correct and economical solution with respect to site condition. This is only possible possible by conducting proper tests and making correct interpretation of parameters obtained from various tests. 2. PURPOSE PURPOSE OF GEOTECHNICAL GEOTECHNICAL INVESTIGATION
It is necessary to review the need of investigation and the information sought there-from before the commencement of investigation in order to have the desired design parameters. Primary objective of an investigation is to determine sub-surface stratification with with an additional a dditional aim of assessing the following from an investigation: (a) The type type of foundation foundation system system (shallow (shallow or deep foundation system) with respect to th e sub-surface characteristics, location and structures.
(b) Geotechnical Geotechnical parameters parameters for for determining determining the load carrying capacity of substrata with the proposed foundation system. (c) Deformation Deformation characteristics characteristics of sub-surfac sub-surfacee strata. (d) Anticipation Anticipation of any peculiar peculiar phenomena, its effect effect during the construction and on the proposed foundation system like artesian conditions etc. (e) Ground water water level, its its seasonal seasonal fluctuations fluctuations and its effect during and after construction. (f) (f) Chemica Chemicall content content and PH values of both soil and ground water. (g) Construction Construction problems problems during excavatio excavation. n. (h) Assessment Assessment of effec effectt of the proposed development development on the adjoining structures or facilities during and after construction construction and its extent. (i) Any geote geotechnical chnical related related environmental environmental problems problems that can be caused due to the proposed development. Besides above, geotechnical investigations are also conducted to determine the cause of geotechnical nature of any failure or defects in any existing structure or system. 3. FACTORS AFFECTING AFFECTING INTERPRETA INTERPRETATION
The foundation of correct interpretation begins by proper provision of items items (tests) in the tender document, appropriate ap propriate
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to local geology with due consideration to the limitations of the various field and laboratory tests. Wherever the parameters are critical there should be provision of obtaining such parameters by using a minimum of two alternative methods. Besides the proper specification /BOQ, it is necessary to select a contractor who is a geotechnical specialist. However, sometimes, inspite of taking all precaution it is observed that poor workmanship, improper calibration of equipment and personal negligence result in reporting of unrealistic values. Major cause of misinterpretations are inexperienced supervisors, lack of inter coordination between field data (bore logs) and laboratory test data; it could be a result of improper labelling of samples while transporting to laboratory, human error while incorporating the results in the report. In most of the cases, after completion of report it becomes difficult to reach to the cause when the report is used to interpret geotechnical parameters for design of structures due to passage of time. In an important project having large quantum of geotechnical investigation, inter coordination problems can be minimised by taking the advantage of technology, that is developing digital record of field samples collected and samples tested in the laboratory which could be used later for clarifying doubts / correcting interpretation at the time of inconsistency in the report. This suggestion is made considering t hat the storage of samples may be difficult for longer duration as it requires larger space. Review of interim reports may also help in reducing inconsistency up to certain extent. Interpretation problems may exaggerate if engineer handling the geotechnical aspects is not qualified and proficient for handling the geotechnical aspects of the project assigned to him. Further the process of revising IS codes /introduction of new codes is not able to match the pace of development of new technology due to which there are lack of acceptable guidelines in some areas which also leads to conflict in interpretation. 4. SUGGESTIONS FOR IMPROVEMENT
Following points can help in minimising problem in the interpretation of report: (a) Test locations –Less importance is assigned for recording the co-ordinates and R.L of test location. It is common to indicate depth of bore hole with respect (w.r.t) to existin g ground level and not w.r.t R.L especially on site having flat ground, forgetting that there can be an ample gap of time for project to commence after the completion of geotechnical investigation. Ground levels may change (normally increases due to backfilling) up
Sanjeev Gupta
to the time of commencement of project, creating difficult situation for the foundation recommendations especially in absence of record about the thickness of backfill. In such cases it is worth reconfirming the depth of founding strata by excavating few trial pits at scattered location before recommending founding levels as column sizes increase with increase in their heights. Similarly it is difficult to trace the bore hole locations without proper co-ordinates. Error in coordinates will affect the design assumptions and sometime could lead to major contractual problems. Hence it may be noted that co-ordina tes and ground levels are one of the most important information r equired in the r eport. For the sites where detailed surveying is not completed and survey pillars are not stabilised coordinates can be obtained by using han d-held GPS (preferably in UTM) and levels can be taken with respect to a nearby permanent object. This information should become a part of soil investigation report. (b) Correct identification of strata- It is observed that sometimes the identification of strata is not done as per the guideline of IS 1498 and IS 11315 part 5. There are several cases where soil strata in the bore log are described to suit all types of classification example – “Brownish to greyish Medium dense clayey silty sand”. In such cases up to certain extent laborator y results can be used while making recommendations. But it is a difficult situation when the bore logs and laboratory results are very different. In such cases, recommendations are more like a guess based on other bore logs information especially in erratic soils. Sample photographs with proper identification tag, if stored could be an important tool for resolving the issue. (c) Collection of water samples: Some time water samples are collected during the drilling operation, resulting in collection of water used for drilling instead of ground water and reporting chemical content of drilling water in place of ground water. If ground water is found to have high sulphate or chloride content in any of such cases and if detected after the award of civil contract, it could lead to major contractual problem. Hence ground water samples should be collected after 48 hours of completion of drilling activities that is when water in bore holes reaches its equilibrium. Further it is advisable to use fumigated bottle for collecti on of water sample.
Interpretatio n of Geotechn ical Report
(d) Evaluating consolidation in clay: Collection of UDS sample is always topic of debate and consolidation results are very sensitive to sample disturbance. Any disturbance of sample affects the consolidation results leading to erroneous interpretation. In such cases provision of alternate item of conducting static cone penetration test is always advantageous. In hard clay expected consolidation may be negligible; UDS sample may be collected by using triple tube barrel. Preserving field moisture content of UDS /SPT samples is importan t as it also helps in obtaining cohesion by using following graphs.
(e) SPT test: There is no standard practice for bore logs showing SPT values at refusal. Reports simply indicate it as refusal. Where test could not be conducted to full depth , better practice could be to show SPT values as 70/100 or 50/100 in bore logs indicating that 70 (or 50) blows resulted in a penetration of 100mm. Although code specifies to record SPT values for three consecutive 150mm penetration, it is advisable to have a record of four consecutive 150mm penetration as it helps in reconfirming the trend of N values. Sudden variation if observed need to be substantiated (in bore log) with observations made during investigation. It will eliminate the doubt
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of typographical errors. In fractured rock reverse SPT (recording penetration against 6 consecutive 20 blows) can be helpful in obtaining shear strength of rock by using recommendations of Cole and Stroud (1997) . In reverse SPT, first 20 blows are considered as seating blows. (f) Unconfined Compression strength (UCS) of rock sample: It is observed that often UCC dry and saturated are performed at different depth in order to have more spread test result s. But it defeats the purpose of comparing effect of saturation on rock samples. Further in several cases it is found that average strength of saturated rock samples are more than average values of dry samples. In such cases one has to take decision based on his experience while recommending a value; one way is to use minimum satura ted value but sometimes, it could be too low. Hence it is suggested to test similar samples (probably from same source) for dry and saturated conditions. (g) Point load test: It is preferred in place of UCS as it is quicker. It is suggested to limit its use for rock classification and not to use it as a substitute to UCS as the correlation between UCS and point load tests are approximate, on an average UCS is 20~25times point load strength in dex but can vary up to 100 % with different rock types especially for anisotropic rock. Test is not reliable if strength is less than 1Mpa. It is noticed that guideline for sampling specified in clause no 5 of IS: 8764 are very rarely followed, as per the code recommendations are to be made out of minimum 10 test specimen. IS Code also suggests to report numerical values for water content & degree of saturation at the time of testing which is very rarely followed. All these limitations affect the accuracy of interpretation, h ence it is better to conduct UCS test and minimise the problems. (h) Pressuremeter tests: Pressuremeter tests are useful in the hard clay /soft rocks /highly weathered / highly fractured rock. Pressuremeter test has got very little significance in a hard rock whose strength is beyond the instrument capacity. Limitations of pressure meter test need to be understood while making the interpretation. Unless the soil is isotropic the value of Young’s modulus/deformation modulus (pressuremeter modulus) obtained from pressuremeter is lateral and different from vertical values needed for settlement analysis. For this reason pressuremeter modulus usually has more relevance to laterally loaded piles and drilled caissons.
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Many geotechnical engineers believe that effect of any type of disturbance on the pressuremeter results is insignificant, but it may be noted that the sensitive soil/rocks (like Phyllite) tends to expand in th e cavity when hole is opened causing considerable disturbance to the results obtained by conducting pressuremeter test. Inconsistencies in results are also observed due to equipment configuration and user technique. Despite all above limitations it is one of the effective tests for obtaining parameters of deeply located strata. Test need to be performed by the person specialised in conducting this type of test for correct interpretation. (i) Cross hole test – Often the reported value of Poisson ratio (µ) in rocky strata are very near to soil (ì are reported between 0.45~0.4) may be due to presence of filling material in the rock joints/ fractures at the testing depth which are not indicated in the report, it creates confusion at the time of interpretation. It is necessary to obtain correct value of ì as it is directly used to calculate Shear Modulus (G) and also indirectly affect deformation modulus value. Some engineers recommend boring in bore holes identified for cross holes without conducting SPT and without obtaining r ock cores which is a wrong practice as SPT helps in estimating appropriate insitu density and rock core examination helps to understand shear and compression wave velocity in a better way. Further knowledge about the stiffness of soil helps during the interpretation (through literature review). Errors in reporting shear wave velocity and compression wave velocity values were also noticed in a few cases. If such errors are noticed after a gap of time than it becomes difficult to trace the cause of error and one has to consider the values indicated for weaker strata(conservative) in the report for the design. There is a standard practice to conduct test at a regular interval. Additionally, it is suggested to conduct this test at every change of strata, as it helps in deciding the founding depth especially for the structures sensitive to earthquake like nuclear plant. Since Cross hole test is a non destructive test it is suggested to record three values at every test interval by repeating the test so as to avoid the reporting errors, as at later stage there is hardly any source to reconfirm the same. It is always
better to have the core logs as it can be helpful in interpretation of results. (j) Deformation modulus of rock: It is noticed that IS code guidelines about taking at least 10 readings over the load range to define the axial and diametric stress strain curve are not strictly followed. In one incident it was observed that the values were not reported at 50% of the ultimate stress as indicated in IS: 922 1. Hence it necessary to have stress strain curve in the report to ensure that correct interpretative values are reported. 5. CONCLUSIONS
The responsibility of planning the geotechnical work should be with a person who is qualified and proficient for handling the geotechnical aspects of the project. It is advisable to prepare specification / BOQ after site visit and after studying various available data pertaining to the site. Selection of the appropriate agency is important. For importan t and fast track projects it is better to appoint a contractor with proven capabilities. Control on the quality of data up to certain extent in projects involving large quantum of geotechnical investigation can be kept by reviewing daily bore logs and interim reports if an engineer is appointed at the time of investigation to oversee day to day work. For critical projects, the parameters should be obtained by using minimum two alternative methods so that results can be compared. It must be remembered that geotechnical engineering is a highly specialised area of civil engineering and it is incumbent upon geotechnical agencies to take all care to report the data with minimum ambiguities. The very foundations of the project depend upon these decisions. REFERENCES
Bowles, J.E. (1988). Foundation Analysis and Design, 4 th Ed., McGraw-Hill. Cole, K.W. and Stroud, M.A. (1977). Rock socketed piles at Coventry Point, Market Way, Coventry, Proc. Piles in Weak Rock, Inst. Of Civil Engrs., London IS 1892 – 1979. Code of Practice for Site Investigation for Foundations, BIS, New Delhi. IS 8764 – 1998. Method for Detrmination of Point Load Strength Index of Rocks, BIS, New Delhi. IS 9221 – 1979. Method for Determination of Modulus of elasticity and Poisson’s Ratio of Rocks, Materials in Uniaxial Compression,BIS, New Delhi. Lambe, T.W. and Whitma n, R.V. (1969). Soil Mechan ics, John Wiley and Sons, New York. Terzaghi, K. (1943). Theoretical Soil Mechanics, John Wiley and Sons, New York.
