1 2300 mm Depends on Hc/D 350 mm 1150 mm OD 102 mm Thickness = 1220 mm Distinct element analysis of soil-pipeline interaction in sand under upward movement at deep embedment condition Analyse par éléments discrets de l’interaction sol sableux-oléoduc soumis à un déplacement vertical dans des conditions d’enfouissement profond S. Yimsiri Department of Civil Engineering, Burapha University, Thailand ABSTRACT: The distinct element analysis of the soil-pipeline interactions in sand under upward movements at deep embedment conditions is undertaken. Available analytical solutions provide a wide range of predicted peak dimensionless forces and there is lim- ited information regarding the transition of the peak dimensionless force from shallow to deep embedment conditions. Recently, fi- nite element analysis of soil-pipeline interactions at deep embedment conditions has been performed. In the current study, distinct element analysis is employed to reinvestigate this problem because it is considered that the distinct element analysis may give more accurate results due to its discontinuous nature which more closely simulates sand behavior. The obtained results are compared with the previously published results and it is found that the results from distinct element and finite element are consistent except for the case of dense sand at deep embedment. The possibilities of the discrepancy are discussed. RÉSUMÉ: Ce papier présente une analyse par éléments discrets des interactions sol sableux-oléoduc soumis à un déplacement verti- cal dans des conditions d’enfouissement profond. Il existe un grand nombre de solutions analytiques pour déterminer la force adi- mensionnelle limite, cependant il existe peu de solutions simultanément valables à faibles et grandes profondeurs. Récemment, une analyse par éléments finis en conditions d’enfouissement profond a été menée. Dans la présente publication, ce problème est revisité par le biais des éléments distincts dont la nature discontinue est considérée comme plus représentative du comportement mécanique des sables. Les résultats ainsi obtenus sont en accord avec les résultas publiés précédemment dans la littérature à l’exception du cas des sables denses et profond. Les causes possibles de cette divergence sont discutées dans ce papier. 1. INTRODUCTION The standard formulations of the force-displacement character- istics for soil-pipeline interactions in sand under upward move- ment are given by ASCE in the “Guideline for the Seismic De- sign of Oil and Gas Pipeline System (1984)”. According to this, the peak force per unit length F peak applied to a pipeline is ob- tained by the following equation. D N H γ F qc c peak = (1) where γ is the effective unit weight of soil, H c is the depth to the center of the pipeline, and D is the external pipe diameter. N qc is the peak dimensionless force and is a function of soil fric- tion angle and embedment ratio H c /D. These recommendations are derived from the experimental data by Trautmann and O’Rourke (1983) of a pipe with H c /D ≤ 13. Available analytical solutions give a wide range of pre- dicted peak dimensionless forces and there is limited informa- tion regarding the transition of the peak dimensionless force from shallow to deep embedment conditions. Yimsiri et al (2003) have recently presented a design chart for deep embed- ment conditions using finite element analysis. In this study, the Distinct Element Method (DEM) is employed to investigate the same problem again. Due to its discontinuous nature, it is con- sidered that DEM should better simulate the soil movement close to the pipe at large pipe displacement and, hence, may yield more accurate results to the problem. In the past, the DEM has been used mainly for the study of micromechanical behavior of sand. This study provides an example of the use of the DEM for more practical problem. The DEM analysis is firstly calibrated against large-scale tank tests data reported by Trautmann and O’Rourke (1983) to determine the micromech- nical input parameters that are not possible to be estimated from laboratory tests. By calibrating the model, the DEM analysis is extended to deeper embedment conditions. The DEM results are compared with the previous study of the same problem us- ing Finite Element Method (FEM) (Yimsiri et al., 2003). 2. DISTINCT ELEMENT ANALYSIS The pipe loading experiments were simulated using the distinct element method based on the approach by Cundall and Strack (1979). The distinct element code PFC 3D (Itasca, 1999) was employed. The code models soil particles as a collection of dis- tinct and arbitrarily sized spherical particles. The particles are treated as rigid bodies and allowed to overlap one another at the contact points. The contacts between particles are characterized through the stiffness and slip condition. The constitutive behav- ior of the particles enables the simulation of macroscale plastic- ity. No bonding between particles is employed in this study to simulate uncemented sand. Figure 1. Schematic diagram of set-up of large-scale tank test.