3D Finite Element Analysis of Pile Behavior Inside the Deep Excavation in Soft Soil

A large excavation was carried out in the thick soft soil layer to construct the pile cap foundation and the basement floor for the 15 stories-building in Ho Chi Minh City. The soil profile consists of a 25-meter-thick of very soft clay with SPT value of zero laid on an 8.5-meter-thick of soft clay with SPT value of 3 and laid on a 13.9-meter-thick of medium dense fine sand with SPT value of 17. The excavation was supported by the system of 6-meter-depth SP IV steel sheet piles with the tied-back rods on the top of the wall to the 6m H steel piles installed behind the wall. Before the excavation, the spun PHC pile with 600mm diameter and 100mm thick was installed to support the superstructure. As excavation to 3.8 meter depth, four piles with the distance 4.55m, 7.15m, 10.65m and 13.25m from the wall had the top pile displacement 63.5cm, 38.86cm, 19.5cm and 11.4cm, respectively. Furthermore, all these piles were determined to be cracked by using the PIT test. Base on the collected data, back analysis was carried out by using PLAXIS 3D Foundation with Hardening-Soil soil model to determine the response of these piles during excavation. In the results, the maximum bending moment of these piles was over its ultimate value, thereby, we can analyze the reason piles group in this building was failure and use the parameters in PLAXIS for expanding the analysis to other situations. Keywords—Failure, steel sheet piles, excavation, soft clay, bending moment. I. INTRODUTION In Ho Chi Minh City, a large number of high buildings with the basement floors were constructed to supply the living houses and working offices. Ho Chi Minh City lies on a complex stratum with the soft clay layer somewhere is thicker than 30 meters. Therefore, a lot of problems were happened during the excavation process to build the basement floors for the high building. Some works have taken place the failure of inside existing piles due to the deep excavation. In 2007, all the installed piles to support the superstructure of Thao Dien building in District 2 Ho Chi Minh City were tilted during the basement excavation. A 13 stories building in District 7 has the PHC pile top move 0.6 meter when basement construction was carried out [9]. In similarly, the silo cement in Hiep Phuoc industry zone used the spun PHC pile with pile length from 33 to 35 meter through the thick soft soil layer had about 80 percent of piles under the silo were tilted in the same direction. A mumber of 2664 piles among of 7474 piles of the water treatment station in Binh Chanh District had the top horizontal displacement in the excavation process [9]. In 2011, a 15-story building in district 8 built on the ground with 25m deep of soft clay, the piles near the steel sheet pile wall was tilt and beak out during the pit foundation excavation. The maximum top pile horizontal displacement was up to 0.6 meter. Analysing the failure of pile inside deep foundation pit in soft soil is a considered problem. The deep excavation in soft soil is very complicated; the lateral displacement of soft soil produces the passive pressure on pile. It causes the pile movement and bending moment greater than capacity moment of the pile; therefore, the piles are damage. According to Kok S.T (2009), there are three common methods for researching about the effect of excavation to the nearby pile foundation: The Finite element, the centrifuge test and the field test [1,4]. Poulos & Chen (1996) did analysis by using of the finite-element method analyze the response of piles due to unsupported excavation induced lateral soil movement in clay [2]. This method was also used in back – analyzing to the responses of pile subjected to horizontal soil movements in clay, and many researchers use Finite element method for their own studies. Finite element method is easy to progress, and the result less fluctuate and appropriate with reality. However, the result from this analysis is affected by many factors that are required experience and understanding of not only geotechnical issue but also solutions and software. Recent efforts in centrifuge modeling of passive piles adjacent to unbraced excavation was done by Leung, Ong. Initially, Leung (2000) presented the results of centrifuge tests of an adjacent single pile behind an unstrutted stable and failed wall of a deep excavation in dense sand. The research also investigates the influence of head piles fixity for behind the stable wall, the pile head deflection and maximum bending moment for the free-headed pile decreases exponentially with increasing distance from the excavation. Subsequently, Leung (2003) extended the centrifuge test to pile groups, incorporating the effects of interaction factors between the piles with different head fixities. Following Leung (2000), further investigation was done for single pile behind stable wall (Ong, 2006) and instable wall (Leung, 2006) in clay [5,6,7]. Finno et al (1991) and Goh et al (2003) executed the field test. According to Goh (2003), used movement measurer for soil horizontal displacement in actual field and examine the behavior of an existing pile due to nearby excavation of a International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert. org IJERTV10IS010007 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Published by : www.ijert. org Vol. 10 Issue 01, January-2021