257 Journal of The Civil Engineering Forum, September 2022, 8(3): 257-266 DOI 10.22146/jcef.3467 Available Online at: https://journal.ugm.ac.id/v3/JCEF/issue/archive Determination of Downhole Dynamic Compaction Parameters using Finite Element Analysis Martin Wijaya 1,* , Ahmad Kemal Arsyad 2 , Aswin Lim 1 , Paulus Pramono Rahardjo 1 1 Faculty of Civil Engineering, Universitas Katolik Parahyangan, Bandung, INDONESIA Jl. Ciumbuleuit No.94, Hegarmanah, Kec. Cidadap, Kota Bandung, Jawa Barat 40141 2 P.T. Geotechnical Engineering Consultant, INDONESIA * Corresponding authors: mwijaya@unpar.ac.id SUBMITTED 26 November 2021 REVISED 21 April 2022 ACCEPTED 27 May 2022 ABSTRACT Downhole dynamic compaction (DDC) has been commonly used in China to stabilize collapsible soil through the application of construction and demolition waste material (CDW). DDC basically forms a column inside the soil stratum which is similar to a stone column except DDC materials are put in sequence and then compacted by using DDC hammer. Due to its attractive features such as its big diameter, feasibility of using oversized material particles, rapid and simple construction technique, it is used as one of the ground improvement methods for an airport project in Indonesia. Despite of all the advantages provided by DDC, it is difficult to obtain DDC parameters from laboratory tests as it is difficult to replicate the compaction effort induced by the DDC hammer and laboratory tests are not commonly employed for oversized materials. Hence, alternative method is required to evaluate DDC parameters. In this study, static load test is conducted to determine load-deformation curve of the DDC pile. Soil parameters are first determined through soil test data such as standard penetration test (SPT), laboratory test and also pressure meter tests. Correlation between pressure meter tests and SPT test result is also carried in order to interpret the soil parameter at the site. Axisymmetric finite element analysis is then carried by using MIDAS GTS NX in order to back analyses DDC parameters by matching the simulation curve with load settlement curve of the DDC. In this paper, it is shown that back analysis using hardening soil model for DDC material can be used to match simulation curve with the load- deformation curve. KEYWORDS DDC; Volcanic Soil; Finite Element Method; Back Analysis; Static Load Test. © The Author(s) 2020. This article is distributed under a Creative Commons Attribution-ShareAlike 4.0 International license. 1 INTRODUCTION Downhole dynamic compaction (DDC) has been used to stabilize collapsible soil (i.e. loess) in China (Feng et al., 2015). The DDC pile can be constructed using the pre-boring method which involves pre-boring a hole before filling it with DDC materials or through self-tamping method which focuses on dropping a hammer (in Figure 1) on the pile location until the hole reaches the desired depth as illustrated in Figure 2. This self- tamping method is commonly used due to its ability to cause dynamic lateral stress which densifies the surrounding soil, especially when the soil is loess collapsible soil (Feng et al., 2015). Meanwhile, the pre-boring method is mostly used when the self-tamping method is not usable (usually due to a very hard soil layer). Figure 1. The DDC Hammer (9.7-ton weight)