ScienceDirect. Earth Pressure and Settlement Analysis of Trench Ducts Backfilled with Controlled Low Strength Materials

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1 Available online at ScienceDirect Procedia Engineering 142 (2016 ) Sustainable Development of Civil, Urban and Transportation Engineering Conference Earth Pressure and Settlement Analysis of Trench Ducts Backfilled with Controlled Low Strength Materials Huang Li-Jeng a, *, Wang Her-Yung b, Sheen Yeong-Nain b, Le Duc-Hien c a Associate Professor, Department of Civil Engineering, National Kaohsiung University of Applied Science, 807, Taiwan, R.O.C. b Professor, Department of Civil Engineering, National Kaohsiung University of Applied Science, 807, Taiwan, R.O.C. c Lecturer, Dept of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam Abstract Recently trench ducts for allocating conveying pipes for various hydraulic or electric lines are tremendously constructed due to rapid growth of life and communication demand. Graded sands and concrete are usually in conventional excavation and backfill operation. Controlled low-strength material (CLSM) has been proposed as a suitable substitute for this construction. However, exact and reliable analysis is required to assure the stress and settlement limitation. This paper presents static analysis of trench duct backfilled with sustainable materials and conventional materials using boundary element (BE) method. The Young s moduli of CLSM are obtained from laboratory tests for two different binder mixtures, CLSM B-130/30% and CLSM B-8-/30%, respectively. Two-dimensional planar strain is employed in the BE formulation of static analysis and comparison study of 4 kinds of backfill materials, i.e., graded sand, CLSM B130-30%, CLSM B130-80%, and concrete. Emphasis is put on the lateral pressure on the side wall, settlement at the top of trench duct, vertical displacement along the centerline of duct and top of pipe cover, induced by three wheel surcharges: concentrated, strip and uniform lane loads. Convergence tests of BEM results are first conducted. Numerical results show that the settlement and lateral pressure of CLSM backfills are acceptable to assure the applicability of CLSM as a suitable sustainable material employed for trench duct design and backfill construction The Authors. Published by Elsevier by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review ( under responsibility of the organizing committee of CUTE Peer-review under responsibility of the organizing committee of CUTE 2016 Keywords: Controlled low strength materials; sustainable materials; trench ducts; Excavation and Backfill; Boundary Element Analysis * Corresponding author. Tel.: ; fax: address: ljhuang@kuas.edu.tw The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of CUTE 2016 doi: /j.proeng

2 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) Nomenclature B top B bottom B pc H H b Q 0 q 0 u z x z x Top width of trench duct Bottom width of trench duct Width of pipe covers in trench duct Height of trench duct Height of marked belt in trench duct Concentrated load Strength of uniformly strip load or lane load Vertical displacement (settlement) Coordinate Coordinate Lateral pressure along the side of duct 1. Introduction Highly developed city and urban with growing life and communication demands lead to an increasing need of rapid construction (excavation and backfill) of trench duct and pipe systems to provide water supply, electric power lines, control cables, etc. Trench duct is an ideal design for projects requiring underground wiring distribution [1]. In the same time excavation and backfilling techniques are also developed rapidly. Backfill performs the following important functions: (a) serves as wall support and slope stabilization, (b) provides an artificial roof for underground construction, (c) fills the excavated space and voids, (d) disposes of waste soils, (e) serves as subsidence and rockburst control, etc. However, analytical solutions of the backfill problems are not so easy due to complicated domain shape and boundary conditions and thus experimental techniques [2] and numerical approaches such as finite element methods (FEM) [3, 4] and/or boundary element methods (BEM) [5] are usually employed for stress and displacement analysis in geotechnical applications. Recently an effective rapid backfill technique of excavated trench ducts had been achieved by using the controlled low strength materials (CLSM). CLSM is a kind of flowable fill defined as self-compacting cementitious material that is in a flowable state at the initial period of placement and has a specified compressive strength of 1200 psi or less at 28 days or is defined as excavatable if the compressive strength is 300 psi or less at 28 days [6]. The special features of CLSM include: durable, excavatable, erosion-resistant, self-leveling, rapid curing, flowable around confined spacing, wasting material usage and elimination of compaction labors and equipments, etc. Literature reviews showed that on-site residual soil after pipeline excavation may be an alternative source for fine constituent in production of soil-based CLSM, effectively used as backfill around buried pipelines [7]. The authors also conducted some preliminary experimental studies on engineering properties of CLSM [8] and stress-strain relationship of CLSM [9, 10]. The authors further investigate the static and elasto-dynamic analyses of excavation zone backfilled with CLSM for retaining walls [11, 12] and bridge abutments [13-14]. The paper is aimed at the comparison of static analysis of trench duct backfilled with CLSMs of two different binder mixtures (B130/30% and B80/30%), conventional graded sands and concrete using BEM. Three loading cases defined in ASSHTO specification will be considered, i.e. concentrated, uniformly strip and uniformly lane loads. 2. Numerical Analysis of the Trench Duct 2.1. Problem Description A typical trench duct backfilled with graded sand or CLSM is shown in Fig. 1. Different backfill materials will be investigated as follows: Compacted Soil: E ; CLSM (B80/30%): E ;

3 176 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) CLSM (B130/30%): E ; Concrete: E ; The material constants in (2) and (3) are obtained from experimental works as explained in [9] and shown in Fig. 2 (a) and (b). Selection of materials for the CLSM mixture in this study consisted of fine aggregate, type I Portland cement, stainless steel reducing slag (SSRS), and water. The experimental work was conducted on two binder content levels in mixtures (i.e. 80- and 130 kg/m3). The B80 and B130 denote for mixture series containing 80 and 130 kg/m3, respectively. Fig. 1. Schematic of a typical trench duct backfilled with CLSM or graded sand (unit: cm). Fig. 2. (a) Typical stress-strain relationship at 1-, 7-, and 28 days; (b) 28-day Elastic moduli of CLSM with different mixtures

4 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) Loading Conditions We consider three loading conditions: (a) Load Case No. 1: the vertical concentrated wheel load, Q 0 = 72.5 kn, acting on the roller of bridge deck which is located at the surface centerline of the ground surface of duct, point a. (b) Load Case No. 2: vertical uniform strip load: q 0 = 9.3 kn/m distributed from b = -10 cm to c =10 cm (bc=20 cm).. (c) Load Case No. 3: vertical uniform lane Load: q 0 = 9.3 kn/m distributed on AD= B top = 55 cm. Load Case No. 1 and 3 are based on AASHTO LRFD Bridge Design Specifications (1998). Load Case No. 1 is equivalent to the single heaviest wheel load of a common AASHTO HS20 truck (or HL-93 truck in the AASHTO LRFD version) Boundary Element Formulation The boundary element formulation for the problem can be expressed in matrix form as in [5]. 3. Numerical Results and Discussion 3.1. Convergence Tests Fig. 3 shows three different boundary element meshes for static analysis of trench duct backfilled with graded sands, coarse (57 boundary elements), medium (96 boundary elements) and fine (192 boundary elements), respectively. We can observe from Fig. 4 that three sets of numerical predictions of lateral pressure distributions (along AB in Fig. 1, element no. 1~29 in Fig. 2(b)), surface settlements (along DA in Fig. 1, element no. 66~76 in Fig. 3(b)), centerline displacements (at 14 internal points marked by x in Fig. 3) and settlements of top cover of pipes (element no. 77~82 in Fig. 2(b)) due to concentrated wheel loads agree well with one another. Convergence is rapidly reached and therefore in the following analysis we choose 96 elements in boundary element analysis for different loading cases. Only boundary nodes and data along trench duct including outer boundaries (lateral sides, top and bottom of trench duct) and inner boundaries (cover of pipes) are required for analysis. Fig. 3. Three meshes for boundary element analysis of trench ducts: (a) coarse mesh (57 elements), (b) medium mesh (96 elements), and (c) fine mesh (192 elements)

5 178 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) Fig. 4. Comparison of boundary element solutions for trench ducts backfilled with graded sand. (a) lateral pressure, (b) surface settlement of duct, (c) vertical displacement along centreline of duct, (d) vertical displacement of top surface of pipe cover Comparison Study of CLSM Backfilled with Four Kinds of Materials Four different backfill materials defined in Sec. 2.1 are employed for comparison study. Especially, two different binder mixtures for CLSM backfill are considered: CLSM-B130/30% and CLSM-B80/30%. In the following BEM analyses, 96 constant elements are adopted for trench duct pressure and settlement analyses. Mesh sizes are the same with x z H / m Load case No. 1 (concentrated wheel load) Fig. 5 shows that lateral pressure in trench duct backfilled with different materials. This can be realized that stress equilibrium is little relationship with the materials. On the contrary, the surface settlements on ground surface and top cover of pipes and centerline of duct are influenced significantly by the modulus of elasticity ( E ). In this loading case both two kinds of CLSM backfills provide good settlement resistance between graded sands and concrete. CLSM-B 130/30% backfill is stronger than CLSM-B 80/30% backfill.

6 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) Jeng, Yung, Nain, Hien / Procedia Engineering 00 (2016) Fig. 5. Comparison of results for trench ducts backfilled with 4 different materials using 96 boundary elements for case of concentrated load: (a) lateral pressure, (b) surface settlement of duct, (c) vertical displacement along centreline of duct, (d) vertical displacement of top surface of pipe cover Load case No. 2 (uniform strip load) Fig. 6 indicates the boundary element predictions on various stresses and settlements under uniform strip load acting on ground surface of trench duct. In this situation both two CLSM backfill made from different mixtures yield smaller settlement and lateral pressure than conventional graded sand. Fig. 6. Comparison of results for trench ducts backfilled with 4 different materials using 96 boundary elements for case of uniformly strip load: (a) lateral pressure, (b) surface settlement of duct, (c) vertical displacement along centreline of duct, (d) vertical displacement of top surface of pipe cover.

7 180 Huang Li-Jeng et al. / Procedia Engineering 142 ( 2016 ) Load case No. 3 (uniform lane load) The results shown in Fig. 7 explain that the CLSM backfills yield smaller settlement and lateral pressure as compared to the conventional graded sand when uniform lane load acts on the ground surface of trench duct. Fig. 7. Comparison of results for trench ducts backfilled with 4 different materials using 96 boundary elements for case of uniformly lane load: (a) lateral pressure, (b) surface settlement of duct, (c) vertical displacement along centreline of duct, (d) vertical displacement of top surface of pipe cover. 4. Concluding Remarks Static earth pressure on lateral sides and settlement of surface and in trench duct using four kinds of backfill materials (graded soil, B130/30% -CLSM, B80/30%-CLSM, and concrete) under three ASSHTO loadings (concentrated wheel load, uniform strip load, and uniform lane load) have been conducted using boundary element method. Some concluding remarks can be summarized as follows: BEM provides an efficient tool for numerical simulation of static earth pressure and settlement in trench duct; data preparation is very convenient since only boundary meshes are required in computation as compared with FEM. Both CLSM backfills made of two kinds of mixtures behave between graded sands and concrete when static earth pressures and settlements of trench ducts are considered. Thus we can adjust the mixtures to make a proper design of CLSM backfill to match the engineering requirement for special task wherein re-excavation is needed. Under three different loading conditions, the earth pressures acting on the lateral sides of trench duct are similar for soil, CLSMs and concrete backfill, but on the contrary, the surface settlements of trench duct are quite different. Considering settlement control and cost saving requirement, CLSM backfill depicts potential benefit in application in trench duct. Consideration of both lateral pressure on the wall and surface settlement from the numerical analyses using BEM, CLSM(B130/30%)( E ) shows to be a good material for trench duct construction which can be employed as an alternate design for conventional backfill using compact graded soil.

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