EFFECTIVENESS OF SAND COMPACTION PILE AND PREFABRICATED VERTIC AL DRAINS IN IMPROVING SOFT SOIL FOR PAVEMENT SUBGRADE

Abstract

The present study examines the effectiveness of Sand Compaction Pile (SCP) and Prefabricated Vertical Drains (PVD) in improving properties of soft soil underlying road pavement. Subgrade improvement techniques are often used to reinforce the subsoil properties with regards to its bearing capacity, shear strength, settlement etc. At present various types of ground improvement technique aims to increase the bearing capacity of soil and reduce settlement. This research focuses on such two well-practiced subgrade improvement techniques for pavement. The study area is located at reclaimed land of Purbachol known as Jolshiri Abason, in close vicinity of Dhaka city. For the purpose of investigating the effectiveness of SCP and PVD, two interconnecting roads Purbachal 300 feet Expressway to Madani Avenue (Road-1) and Madani Avenue extension up to Shittalakha River (Road-2) were selected as a representative of reclaimed land. Physical soil properties (soil profile, specific gravity, grain size analysis) were measured at four different locations, two locations each from both the roads. Subsoil condition was almost similar before installation of ground improvement in this area. Average SPT-N values were found around 3 to 4 before subgrade improvement in all four locations up to a significant depth. Basing on soil properties and SPT-N values, suitable dimensions and parameters of SCP and PVD were designed. Physical soil properties and ground water condition directed choice of method of subgrade improvement at study area. Accordingly, SCP was installed at Road-1 due to presence of more clayey soil and PVD was conducted at Road-2 due to higher ground water table as per design. SPT test was executed again and average SPT-N values were found around 14 to 18 in all four locations. Average SPT-N value was 18 after installation of SCP at Road-1 whereas average SPT-N value was identified around 14 after installation PVD at Road-2. Subsoil improvement through SCP took less time than PVD due to soil settlement time required for the later. After installation of SCP, it took around one month to improve subsoil condition whereas, PVD took seven months for soil settlement with necessary improvement. Dynamic Cone Penetration (DCP) test was performed to check the suitability between SCP and PVD. DCP index value and average CBR (%) was slightly higher where ground iii improvement was conducted through PVD than that of SCP. DCP test results shows that PVD provides slightly more compacted ground surface than SCP at shallow depth. An economic analysis was also carried out by considering 100 square meter area to check the cost effectiveness between these two subgrade improvement techniques. It was found that, the advantage of SCP is due to sand used for construction is considerably cheaper when compared to PVD which needs geotextile and other imported materials. In comparison with SCP, PVD cost was found 1.37 times higher. From the study it can be concluded that SCP and PVD are two technically viable and costeffective solutions for soils of the study area which are weak in strength and needed treatment in order to make them suitable for construction of road pavement over them. The SCP and PVD both are suitable techniques for subsoil improvement with appropriate design. This study may guide to adopt a flexible approach for improving the poor soil conditions of any reclaimed area. However, the study was limited to two sites only and the generalization of observations needs further studies with data from various subsoil conditions.