Jurnal Full Paper Teknologi CHALLENGES IN DESIGN AND Article history Received CONSTRUCTION OF DEEP EXCAVATION FOR 18 January 2016 Received in revised form KVMRT IN KUALA LUMPUR LIMESTONE 8 March 2016 FORMATION Accepted 18 March 2016 Yean-Chin Tana*, Chee-Meng Chowa, Kuan-Seng Kooa, Ramli *Corresponding author Nazirb [email protected] aG&P Geotechnics Sdn Bhd, Kuala Lumpur Malaysia (www.gnpgroup.com.my) bFaculty of Civil Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Graphical abstract Abstract The Klang Valley Mass Rapid Transit (KVMRT) Sungai Buloh - Kajang Line project is the first Mass Rapid Transit (MRT) project in Malaysia. The KVMRT Project when completed will cover a distance of 51km and comprises of 31 passenger stations. This paper covers the challenges in design and construction of deep excavation works for three underground stations, namely Tun Razak Exchange (TRX) station, Cochrane Station and Maluri Station, as well as one portal (South Portal) all located in Kuala Lumpur limestone formation. The Kuala Lumpur Limestone formation exhibits notorious karstic features with irregular bedrock profiles, variable weathering condition, cavities and slime zones. This paper presents the design principles of temporary earth retaining system together with vertical rock excavation to the final depth of the station in karstic limestone formation. The unique experience (design and construction) gained from this project will be a useful reference for similar excavation works, especially in karstic limestone formation. Keywords: Mass Rapid Transit (MRT); deep excavation; limestone Abstrak Projek Klang Valley Mass Rapid Transit (KVMRT) Sungai Buloh – Kajang merupakan projek pengangkutan rel bandar (MRT) pertama di Malaysia. Apabila keseluruhan projek siap, jajaran MRT ini akan merangkumi jarak sepanjang 51km dan 31 stesen penumpang. Kertas ini akan membentangkan cabaran merekabentuk dan pembinaan kerja-kerja pengorekan dalam untuk tiga stesen bawah tanah, iaitu Stesen Tun Razak Exchange (TRX), Stesen Cochrane dan Stesen Maluri, dan juga sebuah portal (South Portal) yang terletak di kawasan dengan Kuala Lumpur limestone (batu kapur). Kuala Lumpur limestone mempunyai ciri-ciri karstic dengan profil batu yang tidak seragam, keadaan luluhawa yang berubah-ubah, rongga dan juga zon lendir. Kertas ini juga membentangkan prinsip- prinsip rekabentuk untuk tembok penahan sementara dengan pengorekan batu menegak sehingga paras akhir stesen di kawasan karstic limestone. Pengalaman unik (rekabentuk dan pembinaan) yang diperolehi dari projek ini amatlah berharga sebagai rujukan masa hadapan untuk kerja-kerja serupa yang melibatkan pengorekan dalam, terutamanya di kawasan karstic limestone. Kata kunci: Mass Rapid Transit (MRT); pengorekan dalam, limestone (batu kapur) © 2016 Penerbit UTM Press. All rights reserved 78: 8–5 (2016) 97–107 | www.jurnalteknologi.utm.my | eISSN 2180–3722 | 98 Yean-Chin Tan et al. / Jurnal Teknologi (Sciences & Engineering) 78: 8–5 (2016) 97–107 1.0 INTRODUCTION f) Ground movement associated with excavation Due to scarcity of land, especially in urban areas, the need for basements to optimize the use of land has The details are presented by Tan & Chow, 2008 (4). resulted in increasing depth of basements being This paper update some of recent development for constructed. In this paper, the approximate division the design and construction of deep excavation in between shallow and deep excavation is based on Malaysia. The analysis and design flowchart for deep 6m which is guided by the definition used by CIRIA [1] excavation works are summarised in Figure 1 on trenching practice and Puller, 1996 [2]. The design of retaining walls and support systems for deep basement construction requires careful analysis, 3.0 SMALL STRAIN STIFFNESS OF SOILS AND ITS design and monitoring of performance. This is NUMERICAL CONSEQUENCES because the risk associated with the works is high and high profile failures involving deep excavation (e.g. It is well understood that soil stiffness decays non- Nicoll Highway, Singapore and Shanghai Metro, linearly with strain (Figure 2). The maximum strain at China) have highlighted the need for proper design which soils exhibit almost fully recoverable behaviour and construction control. A recent study by Moh & is found to be very small. The very small-strain stiffness Hwang, 2007 [3] has listed 43 failures since 2001 related associated with this strain range, i.e. shear strains s ≤ 1 to MRT works of which 8 failures were related to x 10-6, is believed to be a fundamental property of all retaining walls and strutting works and some of the types of geotechnical materials including clays, silts, failures have resulted in death, collapsed buildings sands, gravels, and rocks [6] under static and dynamic and economic losses in millions. Some of the loading [7] and for drained and undrained loading recommendations by Moh & Hwang, 2007 [3] include conditions [8]. having a proper risk management program For practical purposes, small-strain stiffness is associated with underground works and a sound probably most reliably obtained using geophysical understanding of geotechnical fundamentals to techniques which measure shear wave velocity complement the use of computer codes. Proper (Figure 3). Out of the various field and laboratory implementation of risk management programmes methods, cross-hole surveying is probably the most and the use of computer codes require sound reliable method, but also the most expensive. A understanding of the design and construction cheaper alternative would be downhole seismic considerations of underground works in order for the survey or seismic piezocone /dilatometer and as such, risk management to be effective and computer it is recommended to use a combination of the two codes used properly. As such, this paper intends to methods for in-situ measurement of shear wave highlight some of the important aspects of Malaysian velocity. experience on design of retaining walls and support The input parameters for the small-strain stiffness systems for deep basement construction to ensure a model in PLAXIS are as follows: safe and economical design. A recent successful case history involving deep a) G0 – maximum small strain-strain shear excavation works for three (3) underground stations modulus and one (1) portal in Kuala Lumpur limestone for b) 0.7 – denotes the shear strain, at which the Malaysia’s first MRT project, i.e. Sungai Buloh-Kajang shear modulus G is decayed to 70 percent of Line will also be presented. its initial value G0 The above two parameters would be able to define 2.0 DESIGN CONSIDERATIONS the entire stiffness degradation curve, for example using the Hardin-Drnevich relationship [18]. The values of G0 can be obtained from measurement of shear In this paper, a brief discussion on the planning of wave velocity from the following relationship: subsurface investigation and testing and selection of retaining walls and support systems will be presented G0 = vs2 followed by a more detailed discussion of the design of retaining walls and support systems for deep where, is mass density of soil and vs is shear wave basement excavation. The design of retaining walls velocity of soil. and support systems for deep basement excavation will cover the following aspects: a) Overall stability b) Basal heave failure c) Hydraulic failure d) Axial stability e) Finite element analysis 99 Yean-Chin Tan et al. / Jurnal Teknologi (Sciences & Engineering) 78: 8–5 (2016) 97–107 Figure 1 Flowchart for analysis and design of deep excavation works 100 Yean-Chin Tan et al. / Jurnal Teknologi (Sciences & Engineering) 78: 8–5 (2016) 97–107 Please note that the units for the above equations are in kPa. The shear strain at which the shear modulus G is decayed to 0.7G0 can be calculated form the following equation [10]: 0.385 ′ 훾0.7 = (2푐(1 + cos 2∅) + 휎 (1 + 퐾표) sin 2∅) 4퐺0 The values obtained above should also be checked against values given by Stokoe et al., 2004 [11] who proposed a linear increase of 0.7 from 0.7 1 x 10-4 for PI = 0 up to 0.7 6 x 10-4 for PI = 100. To demonstrate the effect of small-strain stiffness in Figure 2 Characteristic stiffness-strain behaviour of soil with deep excavation works, a simple comparison is made typical strain ranges for laboratory tests and structures [5]. on the analysis results of a deep excavation works modelled using PLAXIS, where one is analysed using conventional hardening soil model while another model adopted HS-Small model which incorporates the small-strain stiffness. The typical PLAXIS model of the deep excavation works with retained height of 16m using semi top-down method is shown in shown in Figure 4 while comparison of the wall deflection and bending moment of the two different models are summarised in Figure 5 and Figure 6 respectively. Figure 3 Field and laboratory methods to evaluate shear wave velocity [9]. Table 1 Typical values of maximum small-strain shear modulus [9]. Soil Type Maximum small-strain Figure 4 PLAXIS model of deep excavation using semi top- shear modulus, G0 (kPa) down method. Soft clays 2,750 to 13,750 From Figure 5, it can be observed that the deflection predicted using HS-Small model is smaller compared Firm clays 6,900 to 34,500 to conventional Hardening Soil model with maximum deflection of 37mm compared to 44mm predicted Silty sands 27,600 to 138,000 using Hardening Soil model. This represents a 16% reduction in predicted maximum deflection. Overall, Dense sands and 69,000 to 345,000 gravels the deflection predicted using HS-Small model is about 27% smaller (on average) compared to Hardening Soil model. In addition to using shear wave measurement, the Figure 6 shows bending moment induced on the maximum small strain-stiffness can also be estimated retaining wall at the final stage of excavation where using empirical correlations. Table 1 presents the the maximum bending moment predicted using HS- Small model is smaller with magnitude of 517kNm/m typical range for G0 for several generic soil types.