Performance of a Geogrid-Reinforced and Pile-Supported Highway Embankment Over Soft Clay: Case Study

http://www.paper.edu.cn Performance of a Geogrid-Reinforced and Pile-Supported Highway Embankment over Soft Clay: Case Study H. L. Liu1; Charles W. W. Ng, M.ASCE2; and K. Fei3 Abstract: This paper describes a case history of a geogrid-reinforced and pile-supported ͑GRPS͒ highway embankment with a low area improvement ratio of 8.7%. Field monitored data from contact pressures acting on the pile and soil surfaces, pore-water pressures, settlements and lateral displacements are reported and discussed. The case history is backanalyzed by carrying out three-dimensional ͑3D͒ fully coupled finite-element analysis. The measured and computed results are compared and discussed. Based on the field observations of contact stresses and pore-water pressures and the numerical simulations of the embankment construction, it is clear that there was a significant load transfer from the soil to the piles due to soil arching. The measured contact pressure acting on the pile was about 14 times higher than that acting on the soil located between the piles. This transfer greatly reduced excess positive pore water pressures induced in ¯ the soft silty clay. The measured excess pore water pressure ratio Bmax in the soft silty clay was only about 0.3. For embankment higher than 2.5 m, predictions of stress reduction ratio based on two common existing design methods are consistent with the measured values and the 3D numerical simulations. During the construction of the piled embankment, the measured lateral displacement–settlement ratio was only about 0.2. This suggests that the use of GRPS system can reduce lateral displacements and enhance the stability of an embankment significantly. DOI: 10.1061/͑ASCE͒1090-0241͑2007͒133:12͑1483͒ CE Database subject headings: Geogrids; Embankments; Clays; Monitoring; Three-dimensional models; Finite element method; Case reports. Introduction 1. Embankment construction can be completed within a short time period; When highway embankments are constructed over soft soils, the 2. Embankment support piles reduce total and differential structure imposes a significant load over a large area. The soft settlements significantly; and clays and other compressible soils often bear intolerably large 3. The technique is suitable for various geological conditions. settlements or fail due to insufficient bearing capacity. A variety Conventional piled embankment construction ͑i.e., piled em- of techniques can be used to solve such problems ͑Magnan 1994͒. bankments without geogrid reinforcements͒ requires closely These include techniques to modify the embankment load on the spaced piles or large pile caps to transfer most embankment loads ground ͑lightweight materials, change in embankment geometry͒, techniques to improve the ground ͑preloading, surcharging, to the piles through soil arching. In order to place the relatively staged construction, excavation and replacement, stone columns, expensive piles as far apart as possible, a relatively inexpensive lime columns͒, techniques to accelerate consolidation ͑vertical geogrid material is included at the base of the fill. This geogrid- ͑ ͒ drainage and vacuum consolidation͒, techniques to reinforce the reinforced and pile-supported GRPS system has been used in ͑ ͒ road embankment fill ͑reinforcement͒, and techniques to provide several applications. Maddison et al. 1996 described an innova- additional structural support to the embankment ͑embankment tive system of ground improvement comprising vibroconcrete support piles͒. Each alternative has its own advantages and dis- columns and a load transfer platform incorporating low-strength advantages. The benefits associated with the use of embankment geogrids. The system was used to support a 6.0 m high embank- support piles are as follows: ment constructed over highly compressible peat and clay soils. Lin and Wong ͑1999͒ illustrated the use of mixed soil and cement 1 columns in an embankment to smoothen the differential settle- Professor, Geotechnical Research Institute, Hohai Univ., Nanjing ͑ ͒ 210098, China. E-mail: [email protected] ments of a bridge’s approaches. Han and Akins 2002 reported 2Professor, Dept. of Civil Engineering, Hong Kong Univ. Science and that vibroconcrete columns and geogrids were used for widening Technology, Clear Water Bay, Kowloon, HKSAR. E-mail: cecwwng@ an existing roadway. Based on the performance investigation of ust.hk 13 pile-supported and geogrid-reinforced earth platforms, Han 3Associate Professor, Geotechnical Research Institute, Yangzhou and Gabr ͑2002͒ recommended that area ratio could be reduced to Univ., Yangzhou 225009, China. E-mail: [email protected] 10–20%, in comparison with the relative high area ratio of con- Note. Discussion open until May 1, 2008. Separate discussions must ventional piled embankments ͑50–70%͒. be submitted for individual papers. To extend the closing date by one This paper presents a case history of a GRPS highway em- month, a written request must be filed with the ASCE Managing Editor. bankment project in which a low improvement area ratio of 8.7% The manuscript for this paper was submitted for review and possible publication on December 21, 2005; approved on February 28, 2007. This was used. The study of the embankment performance is based on paper is part of the Journal of Geotechnical and Geoenvironmental field measurements of pressures acting on the piles and soil sur- Engineering, Vol. 133, No. 12, December 1, 2007. ©ASCE, ISSN 1090- faces between piles, pore-water pressures, settlements and lateral 0241/2007/12-1483–1493/$25.00. displacements. The field measurements are compared with com- JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / DECEMBER 2007 / 1483 转载中国科技论文在线 http://www.paper.edu.cn Fig. 1. Soil profiles and properties puted results from three-dimensional fully coupled finite-element 1. A temporary double-wall casing is driven into the ground by backanalyses. a vertically vibrating driving machine. The double-wall casing consists of two concentric 8 mm thick steel pipes with different diameters. The outer and inner diameters of pipes Site Conditions are 1.016 and 0.76 m, respectively. This creates a 120 mm thick annulus between the outer and inner pipes for concret- The site is located in a northern suburb of Shanghai, China. The ing. The inner pipe is open-ended whereas the annulus is profile of the soil is as follows: there is a 1.5 m thick coarse- fitted with a temporary conical-shaped driving shoe, which grained fill overlying a 2.3 m thick deposit of silty clay; this can be detached by wet concreting pressure during concret- deposit overlies soft silty clay that is approximately 10.2 m thick. ing. During driving the casing, soil is displaced into the inner Underneath the soft silty clay is a medium silty clay layer that is pipe and outside the outer pipe. This creates a 120 mm about 2 m thick followed by a sandy silt layer. The ground water thick annulus between the outer and inner pipes for in situ level was at a depth of 1.5 m. Fig. 1 summarizes the available concreting. detailed test data, including the water content, the unit weight, 2. During in situ concreting the annulus, the casing is with- and the vane shear strength to a depth of about 24 m below drawn at a steady rate of 0.8–1.2 m/min. An appropriate ground level. The soft silty clay layer has a low to medium plas- concrete head varying from 0.3 to 0.5 m is always main- ticity, and a liquidity index IL of 1.2. Its water content ranges tained within the annulus to provide stability, whereas the between 40 and 50% and is generally close to the liquid limit. The casing is withdrawn. uppermost coarse-grained fill layer has a relatively high precon- 3. After withdrawing the double wall pipe pile, a concrete plug solidation pressure, in comparison with the underlying soft silty is constructed by replacing the top 0.5 m of soil column in- clay, which is normally consolidated or lightly overconsolidated. side the original inner pipe with concrete. This is to repair The undrained shear strength of the soft silty clay layer as mea- any possible damage to the top part of the annulus pile sured by the field vane has a minimum value of about 10 kPa at a depth of 3.8 m and increases approximately linearly with depth. caused by withdrawing the double-wall steel casing. By considering the previous installation procedures of the annulus concrete pile, it is believed that the influence of the pile installation to the adjacent ground should be similar to that of a driven Geogrid-Reinforced and Pile-Supported Embankment steel pipe pile since soil is displaced during the installation and the effects of in situ concreting on the adjacent ground should be The embankment was 5.6 m high and 120 m long with a crown minimal. This is because in situ concreting takes place inside the ͑ width of 35 m. The side slope was 1 V to 1.5 H. The fill material annulus of the double-wall steel casing i.e., concrete is not in ͒ consisted mainly of pulverized fuel ash with a cohesion of contact with the adjacent soil before the casing is withdrawn. 10 kPa, an angle of friction of 30°, and an average unit weight of The annulus concrete piles were placed in a square pattern at a 18.5 kN/m3. A cross-section view of the test embankment and the distance of three times the pile diameter ͑3m͒ from the center to locations of the instruments are shown in Fig. 2. the center of the adjacent piles. The area ratio, defined as the The embankment was supported by cast-in-place annulus con- percent coverage of the pile ͑caps͒ over the total foundation area, crete piles that were formed from a low-slump concrete with a was 8.7%, which was close to the lowest limit suggested by Han minimum of compressive strength of 15 N/mm2.

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