Static pullout behaviour of soil nails in residual soil

Abstract

This study investigates the axial soil nail soil-interaction of bonded passive soil nails in residual soils under quasi-static pullout force. It seeks to address three issues involved in the prediction of pullout response and distribution of axial forces along the inclusion. The three issues are; 1. Composite behaviour of bar and grout and its collective interaction with the surrounding soil. 2. Influence of operating parameters on the response of pullout behaviour and shear force-transfer mechanism along the composite inclusion 3. Feasibility of using one-dimensional pullout models for the computation of soil nail pullout response. The development of this numerical pullout model will be of practical usage in daily engineering analyses and in the interpretation of pullout tests. The first issue is related to the reduction of the composite modulus caused by the manifestation of grout cracking. The composite modulus is influenced by field conditions and has to be determined in-situ. A total of fourteen large-scaled pullout tests were conducted in the field. A test procedure known as the In-situ Grout Calibration (IGC) test was developed to determined the in-situ composite modulus. The findings; 1. Monitored data showed that the composite modulus is non-linear and reduce with increasing strain level. 2. A strain dependent composite model (SAC-Decay) was developed to represent this behaviour and was subsequently used in this study to translate the measured axial strains along the inclusion to axial forces. 3. The significance of these two developments on the local shear force-transfer mechanism can now be effectively determined and studied. 4. It was found that the manifestation and migration of grout damage increases the extensibility of the composite inclusion. This phenomenon is described as Kinematical Extensibility. 5. Field data also indicated that there is a plastic and elastic regime along the composite inclusion. The second issue deals with the investigation of operating parameters that have possible influence on the pullout behaviour. To this end, axi-symmetric and 3-D finite element analyses were conducted. The numerical experiments lead to the following findings: 1. Inclusion extensibility has a dominant influence on critical displacement. 2. The extent and magnitude of grout damage is a function of the relative stiffness between the composite inclusion and adjacent soil. 3. The replacement method of soil nail installation causes significant decrease of contact stresses at the interface. 4. The usage of elevated grouting pressure at two times the initial average overburden vertical stress gave the most significant contact stress recovery. The third issue was addressed through the development of a one-dimensional pullout model called ASONSI. The crux of the ASONSI model is its ability to capture the behaviour of bonded inclusions. The model was calibrated with the response obtained from finite element experiments and was used in the back-analysis of the field data.