Key Engineering Materials Vols. 353-358 (2007) pp 3006-3009 Online available since 2007/Sep/10 at www.scientific.net Š (2007) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.353-358.3006
Application of Innovative GFRP Pipe Soil Nail System in Hong Kong Y. M. Chenga and W. B. Weib Department of Civil and Structural Engineering, the Hong Kong Polytechnic University, Hong Kong, China a
ceymchen@polyu.edu.hk, bwenbing.wei@polyu.edu.hk
Keywords: Soil nail; GFRP pipe; Laboratory and field test.
Abstract. The current method of soil nail construction in Hong Kong is both labour intensive as well as expensive and a search for new soil nail material is required. In the present pilot test, light weight high strength and high corrosion resistance GFRP pipe with Tube a’ Manchette grouting technique is used as soil nail instead of the conventional steel reinforcement. It is found that this material can be handled easily on site with difficult access. From extensive laboratory as well as field tests, it is demonstrated that this new soil nail technique has various advantages for use in Hong Kong and many developed cities. The field test results on this innovative soil nail will be discussed in this paper. Introduction Currently, high yield steel bars are commonly used as soil nails to stabilize slopes in Hong Kong and many other countries. The bond resistance between cement grout and soil is usually the governing criterion for the capacity of the steel nail, which is also confirmed by numerous field tests in Hong Kong and other countries. Corrosion protection is of paramount importance to the durability of steel soil nails installed in slopes. Provision of 2mm sacrificial steel thickness, hot-dip galvanizing or epoxy coating are the commonly used methods against corrosion of steel nail [1]. When the required stabilizing force is large, soil nails of 40 mm diameter steel reinforcement are often installed at very close spacing (1 to 1.5m in Hong Kong). Such steel bars are heavy and thus difficult to maneuver on site. The length of each 40 mm diameter steel bar that can be handled on site is limited by its weight and individual site conditions and the typical length of each segment is approximately 3-5 m (due to lack of adequate working space in Hong Kong). Couplers are often used to connect bars to the required total length. The total cost of soil nail construction including the steel nails, couplers, handling and transportation costs, drilling, and corrosion protection system in Hong Kong is hence much higher than those in many other countries. Usually, no pressure is applied during grouting of conventional soil nail (gravity flow of grout) as application of pressure with current soil nail system is difficult to be carried out. There are many reported cases in Hong Kong where shrinkage of grout has resulted in significant reduction in bond stress between cement grout and soil and remedial works are required. In views of this concern, expensive and visually unpleasing concrete grillage is commonly used in Hong Kong for loose fill slopes. In views of the various problems associated with the use of reinforcement bars as soil nails, there are various research being carried out in Hong Kong, China and many other countries. The features that are required for the soil nails in Hong Kong include: light weight and high strength; application of pressure to control the grouting zone, quality of grouting and bond strength; free from corrosion problem; acceptable cost; ease of construction - handling, joining and cutting. The glass fiber reinforced polymer (GFRP) pipes soil nail system can satisfy the above requirements. GFRP Pipe System Recently, there are rapid developments in the use of FRP for various structural purposes [2,3]. The authors have carried out research works on the use of GFRP bars as soil nails for the project at All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 24.21.134.101-02/11/10,17:55:22)
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Sanatarium Hospital in Hong Kong as bar type FRP can be found easily in the market. An innovative system of glass fiber reinforced polymer pipes has been devised by Dae Won Soil Company, Limited of Korea and is used for the present study. GFRP is a material of lightweight, high corrosion resistance, and high strength. For the present system, glass fiber reinforced polymer pipes of 37 mm internal diameter and 5 mm thick is utilized. It is fabricated by a pultrusion process during which glass fibers are drawn through die and bundled together through a resin matrix. Structural sections or pipe manufactured in this process fulfilled the requirement of a good soil nail as mentioned above. If GFRP pipe is used as soil nail, centralizers are installed on the pipe in the workshop. Afterwards, two additional layers of glass fibers soaked with resin in cross directions are installed on the surface of the pipe at 45 degrees to the longitudinal axis of the pipe to improve its lateral load resistance and shear strength capacity. Holes are drilled through the pipe at a regular spacing for grouting at a later stage. For soil nail construction, holes of 105 mm diameter are drilled on the slope in the present study. The GFRP pipe is installed into the hole and the top of the borehole is caulked with quick set cement to prevent leakage of grout. The annular space between the pipe and the hole is sealed with cement grout afterwards under gravity flow. A single packer system is inserted into the bore of the pipe approximately 4 hours to 8 hours later. The packer is first inflated to isolate the section to be pressure grouted. Pressured grout is then injected into the pipe, through the holes, to fracture the annular cement grout and permeate the surrounding soil. This process of soil stabilization is virtually the same as Tube a’ Manchette grouting method [4] except that GFRP pipe is used for strengthening of the soil mass and single packer is used in grouting. The present technique is hence a combination of soil improvement as well as soil reinforcing technique which is greatly different from classical steel soil nail which is soil reinforcement without any soil improvement. In general, a pressure around 10-12 bars are used for sandy soil. Lightweight, high strength and high corrosion resistance GFRP pipe would enhance the constructability of soil nails. It is particularly suitable for congested sites where access is difficult. The high corrosion resistance can eliminate the need for provision of the 2-mm sacrificial thickness on the radius of the steel bar. The pressure grout may improve the bond strength of the soil-grout interface, resulting in an increase of the tensile capacity of soil nails if the bond strength of the soil-grout interface is the limiting factor controlling the overall capacity of the nail, and a corresponding reduction in the number of soil nails. Moreover, a hollow section can increase the bending capacity of the slender member, allowing for more tolerance on non-linearity of the installed nails. Therefore, there is a potential that the system can be adopted for soil nail construction in Hong Kong. One of the advantages in the adoption of the present FRP pipe as soil nail is the possible cost saving. The installation of FRP pipes is more complicated than the conventional steel bar soil nail system, but the overall cost of the complete project can actually be reduced. The unit cost of steel soil nail is actually cheaper than the GFRP soil nail, but saving can come from the improvement of soil properties by Tube a’ Manchette grouting so that less soil nails are required. If the amount of soil nails is similar to the reinforcement soil nails, the use of GFRP is actually more expensive than the conventional reinforcement. Laboratory and field tests To evaluate the applicability of FRP pipe as soil nails, a large scale and detailed study comprises of extensive laboratory as well as field tests are carried out. For material tests, tensile tests to the vinyl ester GFRP pipe are performed and the Young's modulus of the material is determined as 22 GPa. The ultimate tensile strength and the strain at failure are determined as 387 MPa and 1.8% respectively. In the laboratory pull out tests of the pressure grouted residual soil Seoul granite, different confining pressures are applied to the samples to model the effect of different overburden stress. It is noted that the grouted zone has expanded from the initial size of 90mm to a larger size and the pull out force increase with the confining pressure. The grouting pressure and expansion of grouting zone will ensure a good quality of grouting with high bond strength which is important for a good soil nail.
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Some field borehole shear tests were performed on a silty sandy soil in Korea. The tests were performed at 0.5m, 1.0m and 1.25m from the grouted zone centre and the average increase of the cohesive strength is about 18 kPa which is a very major improvement to the soil properties. The increase of the cohesive strength at a spacing of 0.5m is about 21 kPa while the increase in cohesive strength decreases to 16.7 kPa for a spacing of 1.0m. As cohesive strength is more critical than friction angle in the stability of a slope, the enhancement in the stability of slope by the present technique will be major. From the test results, it is also found that the friction angle of soil is relatively independent of grouting which is the typical result from grouting of soil without fracturing. A research program on the use of innovative soil nail system has been jointly developed by academics and practitioners in Hong Kong and the Korean supplier to better understand the constructability and engineering performance of the FRP soil reinforcement system. A test site located to the south of Shouson Hill Road East, Hong Kong is chosen for the present study. The unit weight, cohesive strength and friction angle of the completely decomposed tuff (CDT) are determined as 20 kN/m3, 4 kPa and 35째 respectively. Four GFRP pipe test nails are tested in this site for the technical performance of this innovative soil nail system. The maximum pull-out loads of the 4 pullout tests recorded are approximately 220 kN, as tabulated in Table 1. The failure mechanism in the present study can be classified into 2 groups: (1) For the GFRP pipe where the weight of the hydraulic jack is transferred to the pipe with bending moment induced, the failure is a combined bending and shear failure. This is not a realistic failure mechanism as this condition will never happen for real soil nail. (2) For those GFRP pipes where the hydraulic jacks are supported on roller beads so that weight of jack is not applied to the FRP pipe, the failure position is the 2 external layers of FRP wrapping. Actually, this is also not the failure of the pipe but is a failure due to the testing arrangement. The jack clamped tightly at this location and torn off the skin but the FRP pipe remains intact. For use as actual soil nail, this failure mechanism will never happen for FRP pipe. For the present soil nails, the ultimate pull out resistances are so high that the maximum test loads are controlled by the limitations of the equipment setup. Even though the tests are not successful as the maximum nail capacities have not been reached due to the limitations of the equipment setup, the maximum test loads have greatly exceeded the design loads of the nails which have demonstrated the effectiveness of the grouting method in enhancing the bond strength between soil and grout. The distributions of the axial load at jack load of 200 kN is shown in Fig.1. It is noted that the use of higher grouting pressure will enhance the stiffness of the nail/soil system which can be seen clearly from the slope of the test results. Table 1 Results of grout pressure and maximum pull out loads during tests TN1
TN2
TN3
TN4
Max. test load (kN)
200
225
200
220
Grout pressure (kPa)
0
700
500
1000
Numerical modeling of pull out tests The pull out process for the 4 test nails are modeled by numerical method under the present study. In views of the relatively small size of the nail (0.105m diameter), the width of the numerical model is taken as 1.0m which is large enough so that side effects will be negligible. The axial loads and displacements from experiments and numerical analyses for soil nail 1 TN1 are both largest at nail head and decrease gradually with length. It is noticed that there is a relatively good prediction of the axial load distribution during the pull out test for TN1 (Fig.2). The predictions for TN2, TN3 and TN4 are also good and are not shown in the present paper.
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250 TN1 EXP force 200 TN3 EXP force 200
TN4 EXP force 200
150 100
EXP force 175 Force(kN) 174.5
EXP force 200 Force(kN) 198.50
150 100 50
50 0
EXP force 150 Force(kN) 150.1
200 Axial force (kN)
Axial force (kN)
200
TN2 EXP force 200
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0 0
0.5
1
1.5
Distance from nail head (m)
2
2.5
Fig.1 Distribution of axial loads for the 4 test nails at jack load 200 kN
0
0.5
1
1.5
2
Distance from nail head (m)
2.5
3
Fig.2 Distribution of axial load along TN1 from field test and numerical modeling
Summary The present study is the first large scale systematic study on the use of GFRP pipe soil nail system. It is found that GFRP possess a very high tensile strength which can be controlled easily by the use of different resin. The shear strength of the material can be controlled easily by the addition of FRP layers wrapped at 45° to the longitudinal direction. Furthermore, the use of hollow GFRP pipe has also effectively increased the second moment of area as compared with the use of bar section so that the material will be less easily damaged on site. The laboratory tests on the effect of grouting have demonstrated the effectiveness of expansion of grout hole and increase in shear strengths of soils which will provide good quality grouting and high bond strength. The field tests on the effect of Tube a’ Manchette grouting technique has demonstrated major beneficial improvement in the soil properties. Through the present large scale study, it is established that the use of light weight high strength FRP can be an innovative alternative to the classical steel bar soil nail. While most researchers and engineers are working on the use of FRP bars as soil nail, the present study has demonstrated that FRP pipe can be a competitive solution for steep slope with limited working space. References [1] Shiu, Y.K., and Cheung, W.M. (2003). Long-term durability of steel soil nails. GEO Report No. 135, Geotechnical Engineering Office, Civil Engineering and Development Department, the Government of Hong Kong Special Administrative Region, China [2] Dolan, C.W. (1993). "FRP development in the United States." Fiber-reinforced plastic (FRP) reinforcement for concrete structures: Properties and applications. A. Nanni, Editor, Elsevier, Amsterdam, the Netherlands, 129-163 [3] Dowling, N.E. (1999). Mechanical behavior of materials: Engineering methods for deformation, fracture and fatigue. 2nd Edition, Prentice-Hall, Upper Saddle River, N.J [4] Warner J. (2004), Practical Handbook of grouting: soil, rock and structures, John Wiley, USA
Progresses in Fracture and Strength of Materials and Structures doi:10.4028/www.scientific.net/KEM.353-358 Application of Innovative GFRP Pipe Soil Nail System in Hong Kong doi:10.4028/www.scientific.net/KEM.353-358.3006