Study on the pullout resistance of cement grouted soil nails

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Study on the pullout resistance of cement grouted soil nails

 

Author: Hong, Chengyu
Title: Study on the pullout resistance of cement grouted soil nails
Degree: Ph.D.
Year: 2011
Subject: Soil nailing -- Testing.
Soil stabilization.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Structural Engineering
Pages: xxviii, 264 p. : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2462555
URI: http://theses.lib.polyu.edu.hk/handle/200/6302
Abstract: Soil nailing is one of the most efficient reinforcement techniques for stabilizing slopes, excavations and retaining walls. Among various types of soil nails, cement grouted soil nails have been widely used in geotechnical engineering projects in Hong Kong due to the economical and technical advantages. Over the past few decades, both theoretical and experimental investigations show that the interaction mechanism between soil and soil nail is complicated, depending upon several critical parameters such as overburden soil pressure, soil dilation and grouting pressure. The thesis aims to investigate the interaction mechanism between soil and soil nail in several aspects, including the theoretical pullout response of a soil nail, effects of several critical parameters on pullout resistance of soil nails, and the behavior of cement grouted soil nails in slopes under service loading conditions and laboratory pullout test. A simple analytical model has been developed for estimating the maximum shear stress of nail-soil interface. This model is verified by results from a series of systematic laboratory tests. The Hong Kong design method is also considered in the verification part. Theoretical comparisons and parametric studies reveal that the shear stress of nail-soil interface depends on the nail radius, grouting pressure and the location of failure surface. The Hong Kong design method underestimates the test results since factors such as soil dilation and grouting pressure are all neglected. The proposed simple model also demonstrates the interactional effect between grouting pressure and overburden soil pressure on the soil nail pullout resistance as observed in laboratory tests. An analytical method has been proposed to investigate the transitional interaction behavior between soil and soil nail in the passive zone of a slope. The nail-soil interface is divided into three typical phases characterized by different stress states, including the elastic phase, elastic plastic phase, and the final pure plastic phase. Two sets of experiment data are used to examine the effectiveness of the proposed method. An extensive parametric study shows that the pullout resistance of soil nails in the passive zone depends on several key factors such as nail length and diameter, length of plastic zone, and elastic modulus of the soil nail. Main research findings are presented and discussed for better understanding of transitional interaction between soil and soil nail.
A series of field pullout experiments were carried out on ten soil nails in a natural slope of Hong Kong. These soil nails were placed at different soil depths and grouted with various pressures. Test results indicate the overburden soil pressure seems to have no apparent correlation with pullout resistance of soil nails when no grouting pressure was involved, but as the grouting pressure increased, the pullout resistance of soil nails increased substantially. Correlations between pullout resistance of soil nails and soil properties are also presented and discussed. A monitoring research program has also been conducted to study the behavior of soil nails in seven cut slopes. The temperature and strain of soil nails were all measured with fiber Bragg grating (FBG) sensors. Pullout test results show that axial forces of soil nails mostly increase after grouting work due to the established bonding stress at the nail-soil interface. As the cement grout has hardened, axial forces of soil nails appear to be time dependent, indicating that soil nails work passively in these cut slopes. In addition, it is found that the maximum tensile strains/forces of soil nails are limited compared to the yield force of the steel bar. Two optical fiber sensor technologies including the Low Coherence Interferometry (LCI) and fiber Bragg grating (FBG) technologies have been used to measure the pullout strain/stress distributions of a soil nail in laboratory pullout test. A simple and effective method is proposed for calibrating the LCI fiber sensor. Comparative studies between two sensor technologies show that the two techniques are effective for the measurement of strain/stress distributions of a soil nail. The LCI technology is suitable for the measurement of average strain distribution, while the FBG sensor provides the local strain information. Advantages and limitations of the two technologies are presented and discussed. Suggestions of further study are also proposed in the thesis.

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