Investigation of Pile Capacity in Mongla Sub-Soil

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1 Paper ID: GE-22 International Conference on Recent Innovation in Civil Engineering for Sustainable Development () Department of Civil Engineering DUET - Gazipur, Bangladesh 460 Investigation of Pile Capacity in Mongla Sub-Soil M. R. Islam 1, M. S. Hossain 2 and M. K. A. Molla 3 Abstract Capacity is a cardinal measure for the stability of pile foundation. This study presents the pile capacity in tabular and graphical form based on a field and laboratory investigation of sub-soil in Mongla region. The sub-soil investigation reports of different sites of Mongla were collected from Consultancy Research & Testing Services (CRTS) of department of Civil Engineering, KUET and pile capacities at different depths were calculated. The analysis to determine pile capacity was performed by using allowable skin friction and allowable end bearing capacity. Variation of pile capacity with respect to length of the pile obtained by conventional method is exposed graphically. This difference can be considered as reasonable from practical point of view. The results obtained from this study can be used for the design of pile in those cases where the owners are going to use pile foundation for small project without any sub-soil investigation. Moreover, for large projects pile capacity obtained by sub-soil investigation reports can be utilized for design with confidence by comparing with these results where pile load test is not performed. Keywords: Conventional method; mongla region; pile capacity; pile load test; subsoil 1. Introduction Piles are vertical or slightly inclined relatively slender structural members which have the function of transferring load from the superstructure through weak compressible strata onto stiffer or more compact and less compressible soil or onto rock. In spite of having high expenditure of pile set-up, in practice, they are still widely used for sufficient capacity and low settlement requirements. (Donald, 2000[1]; Kim and Barker, 2010[2]). Mongla is located on southern region of Bangladesh. The soil in this region is formed by the alluvial deposits from different rivers and sea. Furthermore, the soil is very soft, compressible having organic matter with low bearing capacity as well as pile capacity (Molla and Malik, 1997) [3]. Mongla is expanding with large development projects including construction of high rise buildings, bridges, jetty, overhead water tanks etc. Due to low bearing capacity of the sub-soil of Mongla, pile foundation is used for the construction of high rise buildings and other important structures. For most of the projects other than a large project, the owner does not like to bear the cost of pile load test. In such cases the pile capacity obtained by conventional method can be used for the design of pile foundation confidently if any published research paper is available on pile capacity of Mongla sub-soil. Unfortunately, such research has not found to be done till now. Therefore, this research has been designed to satisfy the present need of pile capacity investigation in this area. It is well known that the sub-soil of Mongla is of critical nature. The existence of organic layer situated almost everywhere in this region is a very disadvantageous aspect of Mongla sub-soil. In some places the thickness of this layer is 5ft to 20ft and that in some other places is much more. At most of the places the top layer is composed of clayey silt or silty clay. The thickness of this clay and silt layer including organic layer is 35 to 40ft from ground level. Somewhere sand pockets are available within these 40ft. All the above mentioned factors have effects on the pile capacity in this area. The main objectives of this study is to determine the pile capacity of Mongla sub-soil by Conventional method and represent the test result in tabular and graphical form. 1 First Author, Department of Civil Engineering, KUET, r_islam_rahul@hotmail.com 2 Second Author, Department of Civil Engineering, KUET, kuet@gmail.com 3 Last Author, Professor, Department of Civil Engineering, KUET, kmolla@ce.kuet.ac.bd

2 Methodology 2.1. Study outline Seletion of Study location Collection of Test Report and primary data Collection of secondary data Data Analysis Comments on results Figure 1: Study approach 2.2. Study Location and site selection Mongla, an upazilla of Bagerhat district in the division of Khulna, Bangladesh stands on the river Pashur. The second largest sea port is situated in this upazila. It is located at 22 29ʹN and ʹE having a land area of km 2. In our study following 6 sites had been selected. Site-1 : Construction of Boundary Wall at Proposed Bangladesh Zone, Burirdanga Union (no 2) Site-2 : Electrical Sub-station, Mongla Site-3 : Rehabilitation of Jetty at Mongla Cement Factory Premises, Mongla. Site-4 : Construction of an OHT and an S.W.T.P at Mongla Port Pouroshava (Phase-II) Site-5 : At the Proposed Site (1) for the Construction of Ten Storied Zone Service Complex Building for EPZ Site-6 : At the Proposed Site (6) for the Construction of Ten Storied Zone Service Complex Building for EPZ 2.3. Capacity determination For Cohesive Soil For soil under cohesive group i.e. for clay and plastic silt, the skin friction and end bearing capacities of pile may be evaluated by the following general formula: f su = αc u..(1) q pu = N c C u (2) For Non-cohesive Soil For non cohesive soil of silt, medium sand, the skin friction and the end bearing capacities of pile may be evaluated by the following formula: f su = N/50 1.(3) q pu = 0.4N(L s /B) 4N....(4) Pile Capacity by Conventional Method Ultimate pile capacity = A s f su + A b q pu. (5) A s = Cross-sectional area of the surface of the pile = πdl (ft 2 ) f su = Ultimate skin friction, tsf A b = Cross-sectional area of the base of the pile = πd 2 (ft 2 ) q pu = Ultimate end bearing of the pile, tsf (Bowels, 1997 [4]; Singh & Chowdhury [5], 1994; and Teng, 2012 [6]). 3. Results and Discussions For each site pile capacity at different boreholes were determined and then the pile capacity for each site was calculated by averaging the pile capacities of all the boreholes. Finally the average pile capacity in Mongla sub-soil was obtained by averaging the pile capacities of all the sites.

3 Site-1 Site-1 is located at the site of Construction of Boundary Wall at Proposed Bangladesh Zone, Burirdanga Union (no 2), Mongla. The results for site-1 shown in figure-2 clarify that the average ultimate pile capacity (AUPC) increases with the increase of diameter as well as the length of pile to a length of 40 ft. and most of the cases the AUPC decreases with the increase of length after 40 ft. Figure-2: Variation of pile capacity for site-1 of different diameter pile 3.2. Site-2 Site-2 is located at Electrical Sub-station of Mongla upazilla. The results for site-2 shown in figure-3 describe that the AUPC increases with the increase of diameter as well as the length of pile to a length of 50 ft. and most of the cases the AUPC decreases with the increase of length after 50 ft. Figure-3: Variation of pile capacity for site-2 of different diameter pile The details AUPCs of various diameter obtained by conventional method of pile capacity determination for site-1 and site-2 are broadly listed in the table-1 below. Table-1: AUPC of Pre-cast Pile of site-1 and site-2 by Conventional Method Site Site-1 Site-2 Length Average Ultimate Pile Capacity (AUPC) By Conventional Method (Ton) Site-3 Site-3 is sited at the project on Rehabilitation of Jetty at Mongla Cement Factory Premises, Mongla. The results for site-3 shown in figure-4 illustrate that almost all the cases the AUPC increases gradually with the increase of diameter as well as the length of pile. Figure-4: Variation of pile capacity for site-3 of different diameter pile

4 Site-4 Site-4 is located at the project on Construction of an OHT and an S.W.T.P at Mongla Port Pouroshava. For site-4, the AUPC as shown in figure-5 increases progressively with the increase of diameter as well as the length of pile to a length of 70 ft. and almost all the cases the AUPC decreases with the increase of length after 70 ft. and further increasing of AUPC is found from 100 ft. length of pile. Figure-5: Variation of pile capacity for site-4 of different diameter pile The details AUPCs of various diameter obtained by conventional method of pile capacity determination for site-3 and site-4 are broadly listed in the table-2 below. Table-2: AUPC of Pre-cast Pile of site-3 and site-4 by Conventional Method Site Site-3 Site-4 Length Average Ultimate Pile Capacity By Conventional Method (Ton) Site-5 Site-5 is located at the proposed site (1) for the Construction of Ten Storied Zone Service Complex Building for EPZ. The results for site-5 shown in figure-6 present that the AUPC increases abruptly at a pile length of 50 ft. and almost all the cases the AUPC decreases with the increase of length after 50 ft. and further increasing of AUPC is found from 80 ft. length of pile. Figure-6: Variation of pile capacity for site-5 of different diameter pile 3.6. Site-6 Site-6 is located at the proposed site (6) for the Construction of Ten Storied Zone Service Complex Building for EPZ. The pile capacity for site-6 shown in figure-7 present that the AUPC increases

5 464 gradually to a pile length of 60 ft. and also the AUPC decreases with the increase of length after 60 ft. Moreover, further increasing of AUPC is found at 90 ft. length of pile. Figure-7: Variation of pile capacity for site-6 of different diameter pile The details AUPCs of various diameter obtained by conventional method of pile capacity determination for site-5 and site-6 are broadly listed in the table-3 below. Table-3: AUPC of Pre-cast Pile of site-3 and site-4 by Conventional Method Site Site-5 Site-6 Length Average Ultimate Pile Capacity By Conventional Method (Ton) Average pile capacity End bearing of the pile increases rapidly below 40ft depth from ground level. For the sub- soil of Mongla below 40 ft. depth from ground surface, the capacity of pile increases rapidly because under this depth the soil of Mongla is generally composed of gradually dense sand layer. Figure-8: Variation of average pile capacity for Mongla region of different diameter pile

6 465 The average pile capacity in Mongla sub-soil have been attained by averaging the pile capacities of all the sites. The values of average pile capacity of Mongla sub-soil are listed in the table 4. From our observation the average pile capacity for 24 diameter borehole is approximately 3 times of that for 12 diameter borehole. Furthermore, the average pile capacity for the borehole of 100 feet depth is approximately 5 times of that for the borehole of 40 feet depth. Hence, the pile capacity of Mongla sub-soil increase with the increase of diameter and the depth of the borehole. Table-4: Average pile capacity in Mongla Region Length Average Ultimate Pile Capacity By Conventional Method (Ton) Conclusions The precise prediction of maximum load carrying capacity of piles is a complex problem because the load is a function of a large number of factors. These factors include method of boring, method of concreting, quality of concrete, expertise of the construction staff, the ground conditions and the pile geometry. This study has been intended to ascertain the field performance and estimate load carrying capacities of piles. The critical feature of the Mongla sub-soil is the existence of thick compressible organic layer (the thickness of this layer varies from 10 to 40 ft.) located at a depth 5 to 20 ft. from ground surface in most of the places which affects the pile capacity to a great extent. Generally the pile capacity increases rapidly at about 40ft depth from the ground level. Below this depth the pile capacity generally increases with increase of depth and sometimes pile capacity decreases with the increase of depth because of the fact that the end bearing of pile sometimes decreases at higher depth. The results obtained from this study can be used for the design of pile in those cases where the owners are going to use pile foundation for small project without any sub-soil investigation. Moreover, for large projects pile capacity obtained by sub-soil investigation reports can be utilized for design with confidence by comparing with these results where pile load test is not performed. 5. References [1] Donald PC (2000). Foundation Design: Principles and Practices. 2th Edition, Prentice Hall Press. [2] Kim JS, Barker RM (2002). Effect of Live Load Surcharge on Retaining Walls and Abutments. J. Geotechnical and Geo-environmental Eng., 128(10): [3] Molla, S.I & Malik, A.R. (1997). "Statistical Evaluation of Bearing Capacity of Shallow Foundation and pile capacity in Khulna sub-soil Undergraduate Project Report, KUET, Khulna. [4] Bowels, J.E. (1997).Foundation Analysis & Design. Fifth Edition, McGraw Hill International Editions. [5] Singh, A. & Chowdhury, G.R. (1994)."Soil Engineering (Theory & Practice). Vol-1, Fundamental and General Principles, Third Edition, CBS Publisher and Distributors. [6] Teng, W.C. (2012).Foundation Design. Eastern Economy Edition, Prentice Hall of India Private Limited, New Delhi