Large Diameter Steel Pipe Field Test Using Controlled Low Strength Material And Staged Construction Modeling Using 3-D Nonlinear Finite Element Analysis

Abstract

Using finite element methods and field tests, an extensive study on selected steel pipes is devised for the Integrated Pipeline Project in Dallas-Fort Worth, Texas. The project integrates Tarrant Regional Water District (TRWD) existing pipelines to the Dallas system along 150 miles. Field test of buried pipes, monitoring the pipe in different trench conditions is extremely valuable in predicting the displacement of the pipe during construction stages and during its lifetime. The field instrumentation monitors the circular displacements and strains of a buried steel pipe with an outside diameter (O.D.) of 84 in. and 0.375 in. thickness in three different trench profiles using controlled low strength material. For the first case, the trench width is the O.D. plus 36 in., and CLSM is used as embedment up to 30% of the O.D. In the second case, the trench width is the O.D. plus 36 in., and CLSM is used as embedment up to 70% of the O.D. In the third case, the trench width is O.D. plus 18 in., and CSLM is used as embedment up to 70% of the O.D. From the CLSM to the top of the pipe, ordinary local soil is used and compacted to 95% standard proctor density. Approximately 5ft. of backfilling is added for all cases. For pipe structural monitoring, strain gages are attached inside and outside of the pipe to obtain the circumferential strain, and displacement transducers are installed to record both vertical and horizontal diameter displacement. The deflections of the steel pipes are effectively measured in each of the construction stages: the CLSM embedment, the soil compaction, and during the load of the 5ft. soil on top of the pipe. In addition, the buried pipes are monitored for long term deflection (around 350 days). The tests provide guidance for the finite element modeling and are an important study in predicting the structural performance of buried steel pipes. The finite element analysis developed is a three dimensional (3D) nonlinear finite element model of steel pipe coupled with CLSM and compacted soil. The finite element model consists of the pipe and soil interaction during the staged construction of embedment and backfill. The geometry of the model is created based on the pipe's undeformed shape. Due to the effect of the loads during staged construction (i.e. subsequent layers of soil being added as embedment and/or backfill), the geometry of the pipe distored for each layer of soil around it. The model also consideres the at rest lateral pressure and the lateral effect of soil compaction on the pipe-soil structure. Different trench conditions are modeled by varying in the in-situ soil stiffness for the trench wall, and trench width. The finite element model developed simulates the load-deformation results of a buried steel pipe in different trench conditions and was verified by the field test results. After developing the nonlinear finite element model for steel pipes and verifying the results with field test, the model can be used to predict pipe performance under varying backfill and loading conditions. All parameters that are modified in the models are part of the scope of the pipeline project, and are field situations that might be observed in the construction of the Integrated Pipeline.