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Proc. of Bangladesh Geotechnical Conference 2009

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GEOTECHNICAL PROBLEMS IN DESIGN AND CONSTRUCTION OF CIVIL ENGINEERING WORKS IN BANGLADESH

A. M. M. Safiullah

Professor of Civil Engineering, BUET and President, Bangladesh Society for Geotechnical Engineering (BSGE),

e-mail: [email protected] INTRODUCTION Development of geotechnical engineering practice in Bangladesh has been very slow. Although some major civil engineering works in Bangladesh such as Kaptai Hydraulic Dam, Coastal Embankment Project, Teesta Barrage, Jamuna Multipurpose Bridge Project, Multipurpose Cyclone Shelter Project, involved intensive geotechnical engineering works, many of the critical aspects of these works are designed and constructed by expatriate consultants and contractors. As a result there had been a setback in the development of local skill and expertise in this area. Perhaps one of the earliest geotechnical investigation facilities in Bangladesh was developed by the then EPWAPDA (or CB & I Department) at the Hydraulic Research Laboratory in Dhaka. There were a few private organizations involved in geotechnical investigations. Private sector organizations like Swiss Boring took a leading role in expanding soil exploration and analysis work in the early days. After liberation of Bangladesh in 1971, there was a tremendous pressure for civil construction works that required ground investigation. Although importance of appropriate ground investigation in design and construction of civil engineering works was recognized, no Code of Practice was proposed and developed for this country. In absence of such Code of Practice large irregularities in investigation practice have developed. There has been a mushroom growth of soil exploration firms whose quality of work remains questionable. Despite this, soil reports lacking credibility have been used in many critical civil engineering works for design and construction. Although there is no survey of present situation, it appears to the author that inaccurate reports and wrong interpretations of soil conditions have often leaded to construction problems, delay in construction, excess cost, ill performance of structures, failure and litigation. This paper deals with some aspects of geotechnical engineering that are relevant to our local condition and is unique to special geological and soil conditions of this country. BENGAL BASIN: SOIL CONDITIONS The Geological Survey of Bangladesh has prepared a geological map of Bangladesh, which shows various land formations (Alam et al, 1990). Three factors have significantly affected the land formation in Bangladesh: climate and hydrologic conditions, fluctuation of sea levels during post glaciation and interglaciation stages, and the neotectonic activities. Works by Brammer (1971) and Bakr (1976) have drawn attention to close links between the geology and geomorphology of the basin. They suggested a land classification based on works of the sediment depositing rivers. Based on study by Bangladesh Transport Survey (1974), Hunt (1976) grouped various land systems into six Soil Units as shown in Figure 1. Distribution of soils in Bangladesh is complex and are usually heterogeneous both in vertical and horizontal direction. Soils consist of wide varieties of material ranging from gravel, poorly graded sand to silt and clay. In general there is a predominance of silt-sized materials. Some of the plastic varieties of these soils are shown in the Plasticity Chart in Figure 2. It can be seen that these soils lie both above and below A-line and cover a wide range of plasticity and liquid limits. For brevity soils from only three units out of six shown in Figure 1 will be described. These are: (1) the Raised Alluvial Terrace deposits, (2) the Alluvial Flood Plain Deposits, and (3) the Estuarine and tidal Flood Plain deposits. The first unit is usually described as Terrace deposit and the other two units are recent deposits. These constitute about more than 80 percent of land surface of Bangladesh. Figure 3 presents sample bore logs in Alluvial Flood Plain Deposits, Raised Alluvial Terrace Deposits, and Estuarine and Tidal Flood Plain Deposits.


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Figure 1: Major Soil Formations of Bangladesh

Figure 2: Plasticity Chart for Bangladesh Soils

Fine materials (silt and clay) characterize alluvial floodplain Deposits at the surface and are underlain by coarse materials (fine to medium sand). The Raised Alluvial Terrace Deposits or Pleistocene deposits comprise of a relatively homogeneous clay known as Madhupur clay (in the east) and Barind clay (in the west). The thickness of this layer varies but is underlain by sandy soil.


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Figure 3: Soil Boring Logs for Three Major Deposits.

Figure 4: Showing Natural Moisture Contents in Three Major Deposits

Figure 4 shows typical variation in moisture content with depth in Terrace and Recent (Alluvial and Estuarine) deposits. Wide variation in moisture content in Recent deposits particularly for the Estuarine and Tidal Flood Plain deposits is evident. Variation and compactness for these deposits can be observed from Standard Penetration N values in Figure 3. These show that the Terrace deposits are more compact at relatively shallow depth while there is a considerable variation in N value in Estuarine and Tidal Flood Plain deposits. As far as grain size and mineral contents are concerned. Pleistocene (Terrace) sediments are almost identical with those of Recent Flood Plain sediments. Analysis of 39 samples of Recent Sediments and 16 Terrace Sediments by Morgan and McIntire (1959) are shown in Figure 5. Despite similarity in grain size for the two types of sediments (Terrace and Recent), field differentiation between the Terrace and Recent deposit is simple. Recent deposits are typically dark, loosely compacted, and have a high water content with wide variation and may contain appreciable quantity of organic matter. Terrace deposits, on the other hand is well-oxidized and typically reddish, brown or tan, and is mottled. They commonly contain ferruginous or calcareous nodules. Water content is lower, resulting in firmer, more compact material. Organic material in the Terrace sediments is confined to surface soil profile.


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Figure 5: Grain Size Characteristics of Recent and Pleistocene Deposits

It is the Recent deposits (Alluvial and Estuarine) that provide a wide variation in compactness and strength that poses a challenge to accurate and intelligent geotechnical interpretation in the design of engineering constructions. GEOTECHNICAL INVESTIGATION FOR A SITE There are several ways geotechnical condition of a site may be evaluated. The simplest way to examine a site is to dig up a trial pit to a suitable depth to reveal soil condition and if necessary to collect undisturbed soil sample for laboratory testing. This is a very effective way of soil exploration but has the disadvantage that this cannot be performed for greater depths to which soil is usually stressed due to foundation load and in some cases when the ground water table is near the surface. The common procedure is to bore a hole and test soil conditions within the hole at several depths or to insert a rod and record its resistance as it is pushed down. Table 1 gives a list of common in-situ tests used by the geotechnical profession.

Table 1: Common in-situ Tests.

Type of Test Suitable for Not suitable for

Standard Penetration Test (SPT)

Sand Soft to firm clays

Dynamic Cone Test (DPT) Sand and gravel Clay

Static Cone Test (CPT) Sand, silt and clay

Field Vane Test (VST) Clay

Pressuremeter Test Soft rock, sand, gravel, and till Soft sensitive clays

Plate Bearing Test and Screw Plate Test Sand and Clay

Flat Plate Dilatometer Test

Sand and clay Gravel

Permeability Test Sand and gravel

The most commonly used procedure in this country is the use of a bore hole in which Standard Penetration test (SPT) is performed and soil is sampled in tubes. The borehole is usually formed by wash boring process. The author has observed that in many instances crew performing a boring operation hardly gives any attention to the stability of the borehole. Samples collected and test performed in an unstable hole can lead to serious misinterpretation of soil conditions. Factors, which are responsible for instability of a borehole, include:


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Hydrostatic conditions leading to boiling and other instability Jetting arrangement in chopping bit Adequacy in the size of bailer in uncased holes Rate of cutting

In most soils it is necessary that water in the borehole should be above the ground water level to accommodate lowering of water table during withdrawal of tools from inside the hole. It is recommended practices that while using chopping bits the water jetting should be directed upwards or sideways by reverse jetting. Direct jet action destroys soil structure and composition and defeats the purpose of soil sampling or SPT test. In cohesionless soil and in deep borings it is necessary that some drilling fluid (bentonite slurry) stabilize walls of the hole. Standard Penetration Test (SPT) One of the most widely used tests for granular formations is the Standard Penetration test abbreviated as SPT. This test is most widely performed in this country to evaluate design parameters for sandy soils. The test although named standard has many non-standard performance conditions. International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE) have recommended an international reference test procedure for this test. Some of the aspects that require attention for this test are:

Borehole formation Clogging of vent hole in the SPT spoon sampler Correction for energy during hammer blow Non-standard performance conditions Calibration of N- relations

Presently SPT test results provide N values that are corrected for energy and overburden pressure. The corrected N value is correlated to angle of international friction using correlation developed by Gibbs and Holtz for medium grained soils of US origin. Many countries have developed calibration for their soils. Although some granulometric properties of Bangladesh soils are different no attempt has been made to correlate N- relations for our soils. It should be mentioned here that some sand deposits in Bangladesh contain significant percentage of mica that may invalidate the correlation suggested in many soil-engineering textbooks. With above limitations it is worth questioning whether SPT should continue to be the main form of obtaining geotechnical parameters for sandy soils in this country. Clearly this test is not recommended for clayey soils. Cone Penetration Test (CPT) The cone penetration test consists of pushing into the soil, at a sufficiently slow rate, a series of cylindrical rods with a cone at the base, and measuring in a continuous manner or at selected depth intervals the penetration resistance and/or of the local side friction resistance on a friction sleeve. In addition, the pore water pressure present at the interface between penetrometer tip and soil can be measured by means of a pressure sensor in the cone. This method has the advantage over SPT that the time required to perform this test is less, problems of borehole disturbance are completely eliminated in this test and the operators variables are minimum. But it has the disadvantage that sample collections sometimes become necessary for grain size analysis or other mechanical tests although a soil classification system is well developed from this test. Perhaps it is the best-suited test for alluvial soils of Bangladesh. Soil Sampling Evaluation of shear strength, compressibility and deformation characteristics of cohesive soils require collection of undisturbed samples for testing in the laboratory. Collecting truly undisturbed sample requires care that is seldom given by our boring crew. Table 2 shows a classification of soil samples collected by various methods after Canadian Foundation Engineering Manual (1985). Present Bangladesh practice is to collect samples from boreholes in open thin walled samplers. Therefore the samples collected by this method are between Class 2 and 3. Such samples are not recommended for shear strength or compressibility tests. Yet most of the geotechnical reports provide shear strength results from this


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type of samples. Very often these samplers are fitted to drill rod by undersized adopters that do not seal the sampler during withdrawal resulting in disturbed samples.

Table 2: Classification of Soil Samples (after CFEM 1985)

Class Quality Identification Properties that can be Measured

1. Undisturbed a-Block samples b-Stationary piston sampler

A,B,C,D.E,F ,G,H,I,J,K

2. Slightly disturbed Open tube thin-walled sampler A.B,C,D,E,F ,G,H,I 3. Substantially

disturbed Open tube thick-walled sampler, such as 'split spoon'

A,B,C,D,E,G

4. Disturbed Random samples collected by auger or in pits

A,B,D,E,G

A - Stratigraphy B - Stratification C - Organic content D - Grain size distribution E - Atterberg limits F - Relative density G - Water content H - Unit weight I - Permeability J - Compressibility K Shear strength The problems of tube sampling have been well described by Ladd and Lamb (1963). Disturbance due to sampling, transportation, extrusion, trimming, etc. generally reduce the actual effective stress in soil specimen to a value far below that existing in the ground. Ladd and Foot (1974) proposed a method of estimating in-situ undrained strength of clay called SHANSEP (acronym for Stress History and Normalized Soil Engineering Properties) to minimize the result of sampling disturbance. There has been some controversy on the effectiveness of SHANSEP method. It has been criticized that the method allows additional strength due to aging disappear and is therefore conservative while Ladd and his co-workers suggest that due to some loss in water during recompression the strength obtained is un-conservative. Little verification of this method has been done for Bangladesh soils. Problems of Silty Soils Foundation designers often use empirical correlation to design a foundation. Most empirical relations that are available in textbooks are valid either for sandy soils or for sedimented clayey soils. Uses of many of these relations for Bangladeshi soils are questionable. The soil formations in Terrace and Recent deposits of Bangladesh consist significant amount of silt-sized particles. In the soil mechanics literature very limited studies are available for behaviour of silts. Compressibility Correlations are available that relate compression index of sedimented fine-grained soils with liquid limit, natural moisture content and initial void ratio. Most widely used compression index correlation relate to liquid limit as given by Skempton (1944), Terzaghi and Peck (1967), and Nishida (1956). Figure 6 shows that for most of the Bangladeshi silty clays liquid limits do not appear to be good correlation parameter. A comparison between Figure 6(a) and 6(b) show that initial void ratio (eo ) is a better correlation parameter than liquid limit. Correlation by Serajuddin (1987) and Azzouz et al (1976) show better fit than earlier equations. It has been found by Serajuddin (1987) that initial void ratio and natural water content provides most suitable correlation with compression index for the plastic silty and clayey soils of Bangladesh. The author studied empirical correlation between Cc/(1+eo) with liquid limit for nine coastal soils of Bangladesh. The results are presented in Figure 7. At lower liquid limits good correlation exists for these soils.


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Figure 6: Showing Correlation between Cc and Other Parameters

. Evaluation of Shear Strength In an effort to develop some empirical relations Safiullah (1994) investigated soils from nine locations in coastal areas of Bangladesh. The soils contained various percentage of clay (9 to 27%) and are either CL or ML type according to Unified Classification System. The soils have varying over consolidation ratios (OCR). Each of these samples were isotropically consolidated in a triaxial cell to a pressure (c') equal to 2.5 to 4 times the maximum overburden pressure (po') to attain a stress level in the virgin compression line (i.e. normally loaded state). At the end of primary consolidation a deviator stress was applied in undrained state until failure took place. Stress ratio su/c' is plotted against plasticity index (Ip) for these soils in Figure 8, su being half the deviator stress at failure. The vertical line for each of these samples indicates range in values. It is interesting to note that unlike Skempton's(1957) relation followed in most text books su/c' values increased with decrease in Ip. In an attempt to discover the effect of sampling an undisturbed block sample collected from one location was consolidated to high pressure (2.5 to 4 times po' ) and su/c' relation at various OCR was obtained. The same soil was mixed with water 1.5 times the liquid limit to form slurry and was consolidated in a Rowe cell. The reconstituted soil was tested at various OCR values. In Figure 9, the stress ratio for block sample and reconstituted soil show considerable difference in strength, particularly with increasing OCR. Since most soft clays are over consolidated at surface, considerable under estimation of shear strength is likely due to sample disturbance effect. Above demonstrate that there is a considerable need for research on sampling of silts. PROBLEMS OF CONSTRUCTION Problem with many civil engineering works begins immediately after the ground breaking. In geotechnical engineering history there has been numerous litigation cases where ground conditions interpreted during design turned out to be different at the time of construction. Often engineers design foundations on assumed soil parameters. False and fabricated soil reports are not uncommon. Whatever may be the reason for variation in soil condition from designed assumption, the result is extra cost or delay in construction time, change in design,

Figure 7: Showing Correlation between Cc/1+eoand Lw for nine coastal soils.


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Figure 8: Undrained Shear Strength Ratio on Recompression against Plasticity Index for Nine Coastal Soils

Figure 9: Comparison of Undrained Shear Strength Ratio for Undisturbed Block and Reconstituted Samples at Different OCRs.

change in construction operation, or even failure. Good geotechnical engineering practice can limit uncertain soil condition problems. Bored Pile Construction Practice A major concern for geotechnical designers in Bangladesh is the construction of cast-in-situ bored piles. At present there is no Code of Practice in force for this type of construction. This method of pile construction is very popular because piling can be done with very simple tools such as a tripod stand, wash boring equipment and tremie concreting pipes. Two operations that are critical to safe pile construction are:

Prevention of hole collapse, Quality of underwater concreting, Clean borehole bottom, and Integrity of the pile shaft

Prevention of hole collapse requires control of hydrostatic head within the borehole and use of bentonite slurry in adequate mix and density. There are several specifications for slurry to be used in piling work. Table 3 shows a slurry specification given by Federation of Piling Specialists (FPS) for cast-in-place diaphragm walling. It is important that we develop slurry specification that is consistent with our soil types and construction practice in order to provide adequate quality control during construction. Checking integrity of constructed piles is an important condition in the assessment of quality. Such checks are made by pile load test or by use of an integrity tester. During load testing it often appears that supports of reference test gauges are placed in a way that these are affected by loading and unloading from load platform. Interpretation of pile load test results is also very important. Selection of appropriate failure or performance criteria is essential in these interpretations. Ground Improvement Techniques Over last decade there has been tremendous need for extension of urban areas and engineering facilities on ground that were previously considered unsuitable. Such pressure has lead to the use of various ground improvement techniques over the world. Ground improvement techniques have limited practice in Bangladesh. Engineers often prefer to use costly pile or raft foundation rather than improve ground for shallow foundation. Perhaps this is due to our lack of experience in ground improvement technologies and lack of adequate


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equipment for such work. But some ground improvement works can be very simply done by local resources provided one understands the mechanism of achieving the strength improvement. Case studies of ground improvement works are available in geotechnical literature that should inspire our engineers to promote this technique. Reference may be made of the proceedings of the Bangladesh-Japan Joint Geotechnical Seminar on Ground Improvement in 1993 published by Bangladesh Geotechnical Society in which interested cases of ground improvement are presented.

Table 3: Slurry Specification by Federation of Piling Specialists (SPS) for Cast-in-place diaphragm walling

Property Range of results at 20 C Test Method Density Less than l.l0g/ml Mud density balance Viscosity 30-90 seconds Marsh cone method Shear strength (10 min gel strength) 1.4- 10N/m2 Shear-meter PH 9.5 - 12 pH indicator

CONCLUSIONS From the foregoing discussions the following main point may be noted:

1. Bangladesh is formed predominantly by alluvial sediments which can be classified into six broad soil units. Of these the Recent Alluvial Flood Plain deposits and the Estuarine and Tidal Flood Plain deposits provide considerable challenge to geotechnical interpretation and consideration for design parameters.

2. Geotechnical correlation suggested for foundation design in many texts should be used with caution as these may not be applicable for our alluvial soil deposits. There is a need for development of new correlation consistent with our soil conditions.

3. Some ground testing and sampling procedures and techniques followed in this country need to be seriously reviewed and a Code of Practice should be established in this respect.

4. More rigorous quality control practice should be introduced for geotechnical construction works such as cast-in-situ bored piles. Specifications necessary for stable boreholes, good quality slurry concrete, and pile load test should be enforced in all such construction works.

5. Ground improvement techniques should be developed and its use encouraged meeting the challenging needs of a highly populated and expanding urban population.

REFERENCES Alam, M.K., A.K.M.S. Hasan. M.R. Khan, and J.W.Whitney (1990): Geological Map of Bangladesh. Azzouz, A.S., Krizek, R.J.,and Corotis, R.B. (1976): Regression analysis of soil compressibility. Soils and

Foundations. V.16 (2), p. 19-29. Bakr, M.A. (1977): Quartenary geomorphic evolution of Brahmanbaria Noakhali area. Geological Survey of

Bangladesh record. Vol. 1-2. Bramer, H. (1971): Soil resources of East Pakistan. Soil Survey Department. Food and Agricultural

Organizations Canadian Foundation Engineering Manual (1985): Canadian Geotechnical Society. 2nd Edition. Ladd, C.C. and Lambe, T.W. (1961): The strength of undisturbed clay determined from undrained tests. ASTM

Special Publication No. 361, p.342-371. Ladd, C.C. and Foott, R. (1974): New design procedure for stability of soft clays. J. Geotech. Engg. Divn.

ASCE, Vol. 100(7), p.763-785. Hunt, T. (1976): Some geotechnical aspects of road construction in Bangladesh. Geotechnical Engineering,

Vol.7, p. 1-23. Morgan, J.P. and McKintire, W.G. (1959): Quaternary geology of Bengal Basin, East Pakistan and India. Bull.

Geological Society of America, Vol. 70, p.319-342. Nishida, Y (1956): A brief note on compression index of soil. J of Soil Mech. Foundn. Divn., ASCE,

Vol.82(3),p.l-14. Safiullah, A.M.M. (1994): Geotechnical aspects of hazard mitigation in Bangladesh. 13th Int. Conf. On Soil

Mech. & Fdn. Engg., New Delhi, India. Serajuddin, M. (1987): Universal compression index equation and Bangladesh soils. Proc. 9th SEAGC, V.l.p.5-6. Skempton, A.W. (1944): Notes on the compressibility of clays. Quart. J. Geol. Soc. London, V.100, p.119-135, Terzaghi, K. And Peck, R.B. (1967): Soil mechanics in engineering practice. 2nd. Ed. John Wiley & Sons. Inc.

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