Ground Movements in Station Excavations of Bangkok First MRT
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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Geotechnical Aspects of Underground Construction in Soft Ground – Ng, Huang & Liu (eds) © 2009 Taylor & Francis Group, London, ISBN 978-0-415-48475-6 Ground movements in station excavations of Bangkok first MRT N. Phienwej Asian Institute of Technology, Bangkok, Thailand ABSTRACT: The characteristics of movements of diaphragm wall and ground in the excavation of 18 stations of the first Bangkok underground MRT line were evaluated. Three modes of deflected shapes of the walls were observed at different excavation depths, namely cantilever mode, braced modes with bulge in soft clay and bulge in stiff clay. The ratio of maximum lateral wall deflection developing with excavation depth and the ratio of ground surface settlement to excavation and the normalized zone of ground surface settlement varied with the mode of wall deflection. Back-calculation of undrained soil moduli for different soil layers were made from wall movement data of three selected stations using the 2-D linear elasto-plastic FEM analyses. The modulus values, which were higher than those commonly obtained from conventional triaxial tests, can be used as guideline for future works in Bangkok. 1 INTRODUCTION in soft and subsiding Bangkok ground. The contracts made its compulsory that full instrumentation program Deep excavation by means of strutted concrete be implemented during excavation for design verifi- diaphragm walls is often used for construction of cation and safety assurance. Evaluation on the actual multi-level building basements in Bangkok soft soil. performance at sites was performed to confirm the suf- It is superior to the excavation with steel sheet piles ficiency in the design of the support systems for the for control of ground movement to avoid damages to MRT station excavations. A comprehensive study was adjacent structures. Prior to the construction of the first made on the aspect of the wall and ground movements Bangkok MRT project, few studies were made on the (Hooi, 2003) and the salient points from the study are characteristics of the ground movement and its pre- reported herein. diction (e.g. Phienwej and Gan, 2003 and Teparaksa et al, 1999. etc.). However, it was the implemen- tation of the first Bangkok MRT subway line that provided systematic and comprehensive monitoring 2 PROJECT DESCRIPTION data on the excavation of station boxes that allowed in- depth evaluation on the characteristics of the wall and The ISP Blue Line is the first underground MRT line ground movements associated with deep excavation in constructed in Bangkok. It comprises 22-km-long twin Bangkok subsoil conditions using strutted diaphragm single-track tunnels, 18 stations and a depot. The hori- walls.The project involved the deepest ever excavation zontal alignment mainly follows the right of way of city made in Bangkok to date.The excavations of all station roads. Construction of the underground structures was boxes were fully instrumented. Monitoring data from implemented under two fast track design-built con- 18 station excavations were compiled, summarized and tracts, each having approximately the same amounts interpreted. of work. The South Contract involved construction of The construction of the first MRT underground a twin bored tunnels from the inter-city railway termi- project in Bangkok, the Mass RapidTransit Initial Sys- nal at Hua Lamphong eastwards for 5 km beneath the tem Project (MRT ISP Blue Line) was started in 1998. busy Rama IV road to the Queen Sirikit National Con- Prior to that there were public doubts on technical via- vention Center, then 4.5 km north beneath the narrow bility of the construction and operation safety of under- business Asoke road, and Ratchadaphisek road ending ground MRT in Bangkok soft soil. That pessimistic on the surface near the depot. Works of the North Con- outlook led to a call for an in-depth investigation on the tract continued for 4.5 km north along Ratchadaphisek application and performance of the excavation method road to Lad Phrao road then turned west to Chatuchak and support systems to be integrated in the excavation Park and finally terminated beneath the Bang Sue 181 railway station. The 18 stations of the project are as by approximately 23 m from the original hydrostatic follows: profile. The groundwater pumping has also created regional subsidence throughout Bangkok metropolis. South Contract North Contract At the locations of the project, a perched water table is typically encountered in Made Ground. Below this horizon, hydrostatic conditions are generally found to 1. Hua Lamphong Station 1. Thiam Ruam Mit Station a depth of approximately 8 m to 10 m depending on 2. Sam Yan Station 2. Pracharat Bumphen Station the location and thickness of First and Second Sand 3. Silom Station 3. Sutthisan Station layers. Typically, the upper few metres of sand layers 4. Lumphini Station 4. Ratchada Station underlying First Stiff Clay and Very Stiff Clay have 5. Bon Kai Station 5. Lad Phrao Station been dewatered. 6. Sirikit Centre Station 6. Phahonyothin Station 7. Sukhumvit Station 7. Mo Chit Station 8. Phetchaburi Station 8. Kamphaeng Phet Station 9. Rama IX Station 9. Bang Su Station 4 STATION CONSTRUCTION The MRT stations had following features: 3 GROUND CONDITIONS – Typically,three levels of structure, with a centre plat- form that is fed by stairs and escalators between two Bangkok is situated on the southern part of the low lines of columns down the middle of the station. lying Chao Phraya plain, which extends north from the – Up to 230 m long and approximately 25 m wide, coast line at the Gulf of Thailand up to approximately excavated up to a depth of 25 m to 30 m below the 350 km and spans east-westward up to 150 km. The ground surface. flat topography plain is covered with a thick marine – The perimeters were of diaphragm walls, 1.0 m thick clay layer, which overlies a very thick series of allu- and 30 m to 35 m deep and solid in-situ reinforced vial deposits of alternating stiff to hard clay and dense concrete slabs, typically 0.9 m thick for the roof to very dense sand to gravel. The thick soft clay layer slabs, 0.7 m thick for the intermediate slabs and generally extends from the ground surface to a depth of 1.75 m thick for the base slabs, which were used 12 to 15 m. The soft clay which is known as “Bangkok as the excavation support system and permanent soft clay” has high water content (70–120%), high structures later on. plasticity, low strength and high compressibility. – There were three stacked stations, at Samyan, Silom The shallow subsoil of the upper 35 m zone is rela- and Lumphini Stations, due to space constraints tively uniform and generally consists of layers of soft caused by the existence of the foundation piles of the to medium clay, stiff to hard clay and sand. Below this long road flyover and a water transmission tunnel at shallow zone, alternating layers of stiff to hard clay and these busy intersections. dense sand exist to a great depth. Bedrock is found at – There was a side platform station at Bang Su Sta- depths more than 450 m. The typical subsoil profile tion, with two levels only, to accommodate the track for the first 50 m depth is listed as follows: alignment for future elevated extension of the line to the north. – Made Ground – The remaining stations were constructed as Centre – Bangkok Soft Clay Island Platform stations. – First Stiff Clay – Pracharat Bamphen and Sutthisan Stations incorpo- – Medium Dense Clayey Sand, Sandy Clay and Silty rated intersection road underpasses on the roofs of Clay the stations. – Very Stiff Sandy Clay/First Bangkok Sand – Silom, Lad Phrao and Phahonyothin Stations were – Second Hard Clay excavated underneath foundation piles of existing – Second Bangkok Sand road flyovers and thus, the station structures and foundation were designed to support the flyovers Maconochie (2001) summarized the general soil via cross-beam and underpinning bored piles. profile and properties at the Bangkok MRT ISP Blue Line. The variation in soil profile along the align- The top down construction technique was adopted ment was observed primarily in the Very Stiff Sandy for all station box excavations with diaphragm walls Clay/First Bangkok Sand layer and the soils immedi- and concrete slabs as the excavation support system, ately below it. Figure 1 show the soil profile along the which was later utilized as the permanent structures MRT alignment and at the stations. of the stations. The designs were made with an aid Deep well pumping in Bangkok and its environs of FEM analyses. The excavation depths and the toe has reduced the pore water pressures in the sand layers depth of the diaphragm walls of all station excavations 182 Figure 1. Soil profile along MRT alignment and at the stations. are summarized in Fig. 2. It should be noted that the The data were screened to preclude movements not ratio of the depth of the wall embedment to the depth expressly related to the excavation and support instal- of excavation was significantly different between the lation, such as diaphragm wall construction, tempo- two contracts, primarily due to the difference in the rary decking works and the initial 2.3 m excavation that design criteria adopted by the two different designers.