Response of buried pipelines subject to earthquake effects

Some lifelines, such as gas and oil transmission lines and water and sewer pipelines, have been damaged in recent earthquakes. The damages of these lifelines may cause major, catastrophic disruption of essential services for human needs. Large abrupt differential ground movements that result from an active fault present one of the most severe effects of an earthquake on a buried pipeline system.

Although simplified analysis procedures for buried pipelines across strike-slip fault zones that cause tensile failure of the pipeline have been proposed, the results are not accurate enough because of several assumptions involved, such as the omission of flexural rigidity of the pipe, simplification of soil resistant characteristics, etc. Note that the omission of flexural rigidity cannot satisfy equilibrium conditions for pipelines across a ‘reverse’ strike-slip fault that causes compressions in the pipeline.

This paper presents a refined analysis procedure for buried pipelines that is applicable to both strike-slip and reverse strikeslip faults after modifying some of the assumptions used previously. Based on the analytical results, this paper also discusses the design criteria for buried pipelines which are subjected to various fault movements. Parametric responses of buried pipeline for various fault movements, angles of crossing, buried depths and pipe diameters are presented.

A refined seismic analysis and design of buried pipeline for fault movement

The design of buried pipelines in areas of vertical ground movement is governed, in part, by the magnitude of the forces imposed on the pipe and the displacements at which they are developed. An experimental study of these effects, dealing in particular with the influence of soil density and depth of burial, is described. The results compare well with several published models for medium and dense sand, but measured values of uplift resistance are much lower than predicted for loose contractive sand. A simplified procedure that can be applied to the design of buried pipelines is given.

Uplift Force‐Displacement Response of Buried Pipe

The results of an experimental program to assess the response of buried pipes to lateral ground movements are presented. The effects of pipe depth, soil density, pipe diameter, and pipe roughness are considered, and test results are compared with published analytical models and experimental data. The results indicate the need to consider vertical equilibrium in predicting the horizontal response of buried pipelines, and the data agree well with several analytical models that include this effect. Pipe surface roughness was found to have little effect on response. Soil density has a large effect on displacements required to mobilize the maximum force but a relatively small effect on the value of the residual force at large displacements for depths typical of transmission pipelines. The study concludes with a simplified design procedure for predicting pipeline response to lateral ground movements.

Lateral Force-Displacement Response of Buried Pipe

The soil–pipeline interactions in sand under lateral and upward movements are investigated with particular attention to the peak forces exerted on the pipe. The analytical solutions for estimating the peak forces are summarized and it is shown that, for deep embedment condition, there is large uncertainty in the true values since the bounds established by the analytical solutions are large. In order to find the solution for the peak force and to investigate its transition from shallow to deep failure mechanism, finite element analyses of lateral and upward pipe movements are performed for different embedment conditions. Two different soil models (Mohr–Coulomb and Nor–Sand models) are used for the simulations. The accuracy of the analysis is first examined by simulating experimental tank tests. The analysis is further extended to deeper embedment ratios of as large as 100. The obtained finite element results are used to construct a design chart for deep embedded pipelines.

Lateral and Upward Soil-Pipeline Interactions in Sand for Deep Embedment Conditions

The safe and economical design of buried pipes subjected to lateral motion requires an accurate knowledge of the subgrade reaction. Too often values of the coefficient of horizontal subgrade reaction, k\dh, reported in the literature are used without understanding the author’s intentions and without thought as to whether or not the conditions in the particular application are compatible to those in the literature. A survey of the available literature discloses that the numerical values and formulations proposed were such as to underestimate the soil restraint against flexible culverts and laterally loaded piles. These values are inadequate for the safe design of buried pipes laterally displaced by settlement, thermal expansion, earthquake shaking or wave action. The results of a laboratory testing program show the complex soil-conduit interaction and the associated failure mechanisms. An analytical method is presented to determine the load-displacement curve for any size pipe embedded at any given depth. A moderate size in-situ test was performed, which confirmed that the results of the laboratory model tests could be successfully applied to in-situ conditions.

Soil Restraint Against Horizontal Motion of Pipes

Soil restraint against horizontal motion of pipes

Coefficients of Subgrade Reaction for the Design of Buried Piping

In this response to points raised by discussers, the author agrees that, for a pipeline placed in a ditch, placement and compaction of backfill material will control the early behavior of the pipe while ultimate lateral resistance is likely to depend mostly on in-situ soil characteristics. This should be taken into account to develop "hybrid" p-y curves. The author also concurs with the discusser on the need to provide the structural engineer with a range of soil restraint characteristics instead of a unique p-y curve. It is also pointed out that while it is recognized that the finite element method is quite versatile and able to easily accommodate for various boundary conditions, it is believed that the finite element method is more adapted to detailed analysis of short lengths of pipe rather than long sections due to the prohibitive costs if an adequate characterization was to be sought. It is believed that computer programs such as NEWPIPE or PIPLIN which make use of elastoplastic discrete springs, are more suitable to handle large pipeline configurations and can provide accurate analyses at a reasonable cost. Further comments are made on the possible end effects in the experiments, and on soil-pipeline interaction and for associated relationships.

Kenneth J. Nyman

Papers

Soil Restraint Against Horizontal Motion of Pipes

Soil restraint against horizontal motion of pipes

Coefficients of Subgrade Reaction for the Design of Buried Piping

Soil restraint against horizontal motion of pipes. closure