Building Foundation Practice on Deep Layer of Soft to Medium Stiff Soil Having Low Bearing Capacity in Khulna City, Bangladesh

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1 Paper ID: GE International Conference on Recent Innovation in Civil Engineering for Sustainable Development () Department of Civil Engineering DUET - Gazipur, Bangladesh Building Foundation Practice on Deep Layer of Soft to Medium Stiff Soil Having Low Bearing Capacity in Khulna City, Bangladesh S. U. Islam 1, S. Hasan 2, M. Alam 3 and M. K. A. Molla 4 Abstract The sub-soil of southwest region of Bangladesh, especially in Khulna City Corporation area, is very thick soft to medium fine grained soil with high percentage of organic matter up to a great depth in most of the sites. The Bearing Capacity of sub-soil of this region is very low and so the buildings in this region are experienced huge amount of total and differential settlements. As a result the constructed structures lost their utility and abandoned in some cases. This paper describes the case studies of different foundation system adopted for the construction of some building in Khulna City Corporation Area. Different types of foundation have been practiced to ensure the stability of the structure in this region. The adopted foundation system ranges from continuous brick footing to pile foundation. Considering the engineering soundness, safety of building and the cost, a foundation system associated with ground improvement using sand compaction piles and mat foundation over a compacted sand layer is proposed for small to average projects. For the construction of high-rise buildings on soft to medium stiff soil, piled raft foundation is suitable. Keywords: Bearing capacity, foundation, geotechnical, mat, safety, soft soil, settlement 1. Introduction Bangladesh is a deltaic land, formed by the lower reaches of the Brahmaputra, the eastern channel of the Ganges, the Meghna and their associated tributaries which contributed very much in the formation of sub soil profiles of Bangladesh. The South-West costal region of Bangladesh contains fine grain soil with the presence of organic soil deposits (Razzaque et al. [1]). Khulna City is situated on southern region of Bangladesh. The soil in this region is formed by the alluvial deposits from different rivers (Rupsha, Vairab etc.). This region is also covered by deep forest of Sundarban. Due to tectonic forces at different times in the past, these deep forests were buried underneath. For these reasons, the soil is very soft, compressible having organic matter with low bearing capacity as well as pile capacity (Molla et al. [2]). For this reason, many infrastructure are settled by large amount. Due to this inherent limitation, the foundation design for the construction of civil infrastructure needs special caution and consideration, which leads to high cost for the preparation of sub structure in this region of Bangladesh. 2. Methodology Sub-soil investigation reports of six sites in different places in Khulna city corporation area were collected from Khulna Development Authority (KDA) and Consultancy Research and Testing Services (CRTS), Department of CE, KUET, Khulna. The sites selected for the study are shown in Fig.1. Soil boring are performed to know the classification of soil, N value at different layer and engineering properties of soil. The performance of the constructed buildings are evaluated and hence 1 S. U. Islam, Undergraduate Student, Dept. of CE, KUET, sabbir_ul_islam@yahoo.com 2 S. Hasan, Undergraduate Student, Dept. of CE, KUET, sakibce2k10@gmail.com 3 M. Alam, BSc. in Civil Engineering, KUET, malamce09@gmail.com 4 M. K. A. Molla, Professor, Dept. of Civil Engineering, KUET, kamolla@ce.kuet.ac.bd

2 425 discussed in this paper. The arrangement of various soil layer can be best shown in the form of a geologic profile or soil profile. A soil profile is a graphical representation of underground condition along a given line on the ground surface. In order to clearly show the various soil layers, the vertical scale is usually made large than horizontal scale. The reliability of a soil profile as compared to the actual soil condition depends upon the nature of the ground and spacing of the borings. If the soil conditions are erratic, the arrangement of various layers between the boring may differ considerably from the interpolation. The main focus of this study is titled as: (i) to know the foundation system adopted in different building constructed on deep layer of soft soil; (ii) to know the sub soil condition on which the building are constructed; (iii) to prepare the soil profiles and depth vs N-value graphs and (iv) To justify and recommend the suitability of the foundation which could be used in different site. Fig. 1. Location map of study sites in Khulna City 2.1. Site-1: Delta Life Insurance Co. Ltd., Sonadanga, Khulna The field and laboratory investigation reveals that the sub-soil consists of predominantly silt up to a depth of about 35 ft with very soft to medium stiff consistency. A soft to medium stiff layer exists also at a depth from 50 ft to 65 ft. This building consists 22 storey having 450 m 2 plinth area. Bearing capacity of soil in upper layer is low. Therefore, piled raft foundation is used.

3 426 Avg N Value Average N Value Piled Raft Foundation Depth vs Avg N Value Soil Profile Fig. 2. Depth vs Avg N Value, soil profile and foundation of Delta Life Insurance Co. Ltd Site-2: Ladies Hall, Khulna University The sub-soil consists dominantly soft organic clay up to a depth of about 80 ft. At top, a layer of soft clay is present. Below the depth of 90 ft. the soil contains sandy clay. Even using a large number of piles neither the necessary compaction nor sufficient friction can be obtained as firm layer exists at unreachable depth. This five storied hall was safely founded by sinking it partly Compensate Foundation Depth vs Avg N Value Soil Profile Fig. 3. Depth vs Avg N Value, soil profile and foundation of Ladies Hall, Khulna University

4 427 into the ground so that the weight of excavated soil is equal to the weight of the building. This type of foundation is known as floating or compensate foundation Site-3: Quarter-1, Khulna Medical College The sub-soil consists of predominantly soft clay with silt up to maximum explored depth with very soft to medium consistency. Ground improvement technique by geotextiles is applied for the construction of the four storied quarter of Khulna Medical College. Ground improvement by geotextiles Depth vs Avg N Value Soil Profile Fig.4. Depth vs Avg N Value, soil profile and foundation of Quarter-1, KMC 2.4. Site-4: New Academic Building, KUET (very near to Khulna City) Depth vs Avg N Value Soil Profile Mat with Sand Cushion Fig.5. Depth vs Avg N Value, soil profile and foundation of New Academic Building,

5 428 Very loose silty sand layer encountered in all the boreholes at surface having relative density of 20-40%. The underlying soil layer is soft clayey silt up to about 70ft depth. A thin layer of sandy silt is encountered below 70ft depth. Beneath this thin layer, sand layer extends up to maximum explored depth (100ft). For construction of this building, planned for seven stories, mat foundation with sand cushion is used Site-5: Sheikh Abu Naser Hospital, Goalkhali, Khulna The sub-soil consists dominantly organic clay up to a depth of about 70 to 100 ft. At the top, there is a thin layer of clay are present. Below an average depth of 60 ft. the soil contains mainly silty clay layer. The building having 6 stories was constructed by using pile foundation. Depth vs Avg N Value Soil Profile Pile Foundation Fig.6. Depth vs Avg N Value, soil profile and foundation of Sheikh Abu Naser Hospital, Goalkhali 2.6. Site-6: Academic Building-1, Khulna University Very soft organic silty clay soil layer up to an average depth of 65ft is encountered in the boreholes. The underlying soil layer is medium dense sandy silt. The top soft soil layer was removed by excavation and refilled by compacted sand. At the top of the filled and compacted sand layer, several mat foundations were provided for the four storied academic building. At the top of the mat, foundation beam was provided. Despite such special precaution, the long term measurement showed that the amount of settlement of the building was very large. However, it was found that settlement is uniform. As a result the building is not damaged.

6 Conclusion and Recommendation The bearing capacity of sub-soil in Khulna City area, situated near the coast region, is low in most cases. So, the civil infrastructure suffers for very large amount of total and differential settlement. The sub-soil contains soft clay layers, loose sand, organic layer, and clayey silt at surface but firm layer exists at a depth generally unreachable. The organic layers are very much compressed and eventually contributes in large settlement for secondary consolidation. If mat foundation is used then excessive settlement occurs. Negative skin friction may occur in the case of pile foundation. So then, it is necessary to increase the pile length which results in excessive cost. Ground improvement by sand compaction pile with a granular soil layer over it can be a good alternative. After ground improvement mat foundation may be provided for small to average project. Geotextiles can also be used for ground improvement for average projects. When hard strata exist at unreachable depth and pile foundation is not suitable, then compensated foundation may be adopted. For the construction of high-rise buildings on soft to medium stiff soil, piled raft foundation is suitable. The range of bearing capacity is 0.3tsf to 0.8tsf for soft to medium stiff clay. 4. References Depth vs Avg N-value Soil Profile Mat Foundation Fig.7. Depth vs Avg N Value, soil profile and foundation of Academic Building-1, Khulna [1] Razzaque, M.A and Alamgir, M. (1999). Long term settlement observation of a building in a peat deposits of Bangladesh. International Conference on Civil & Environmental Engineering - New Frontiers and Challenges. Bangkok. Vol.2: III -85 to IV-94. [2] 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. [3] Asaduzzaman, M., Islam, M.R. and Alamgir, M. Foundation system adopted to construct building in and around KUET campus of the Bangladesh. International Journal of Advanced Structures and Geotechnical Engineering. ISSN , Vol. 03, No. 04, October 2014.