Dharma Wijewickreme

Response of buried steel pipelines subjected to relative axial soil movement

The performance of buried steel pipelines subjected to relative soil movements in the axial direction was investigated using full-scale pullout testing in a soil chamber. Measured axial soil loads from pullout testing of pipes buried in loose dry sand were comparable to those predicted using guidelines commonly used in practice. The peak values of axial pullout resistance observed on pipes buried in dense dry sand were several-fold (in excess of 2 times) higher than the predictions from guidelines; the observed high axial pullout resistance is primarily due to a significant increase in normal soil stresses on the pipelines, resulting from constrained dilation of dense sand during interface shear deformations. This reasoning was confirmed by direct measurement of soil stresses on pipes during full-scale testing and numerical modeling. The research findings herein suggest that the use of the coefficient of lateral earth pressure at-rest (K0) to compute axial soil loads, employing equations recommended in co...

Stress Nonuniformities in Hollow Cylinder Torsional Specimens

Stress nonuniformities across the wall of hollow cylindrical torsional shear (HCT) specimens under generalized stress conditions are assessed taking into account the nonlinearity in soil behavior. The domain of stress space that results in acceptable levels of nonuniformities is thus delineated for a given specimen geometry. It is shown that previous analyses assuming linear elastic soil grossly overestimate stress nonuniformities in HCT specimens. The HCT device is thus suitable for investigating soil behavior under generalized stresses over a much larger domain of stress space than that thought earlier.

Postcyclic reconsolidation strains in low-plastic Fraser River silt due to dissipation of excess pore-water pressures.

The postcyclic reconsolidation response of low-plastic Fraser River silt was examined using laboratory direct simple shear testing. Specimens of undisturbed and reconstituted natural low-plastic Fraser River silt and reconstituted quartz powder, initially subjected to constant-volume cyclic loading under different cyclic stress ratios (CSRs) and then reconsolidated to their initial effective stresses ( σ vo ′ ) , were specifically investigated. The volumetric strains during postcyclic reconsolidation ( ev-ps ) were noted to generally increase with the maximum cyclic excess pore-water pressure ( Δ umax ) and maximum cyclic shear strain experienced by the specimens during cyclic loading. The values of ev-ps and maximum cyclic excess pore-water pressure ratio ( ru-max ) were observed to form a coherent relationship regardless of overconsolidation effects, particle fabric, and initial (precyclic) void ratio of the soil. The specimens with high ru-max suffered significantly higher postcyclic reconsolidation st...

Mechanical response of highly gap-graded mixtures of waste rock and tailings. Part II: Undrained cyclic and post-cyclic shear response

Mixing of waste rock and tailings as a homogeneous mixture (referred to as “paste rock”) has been suggested as a favourable approach to overcome deficiencies associated with traditional methods of mine waste disposal. In consideration of the current limited understanding of the fundamental mechanical response of paste rock, a laboratory research program was undertaken to investigate the monotonic and cyclic shear response of paste rock. This paper presents the findings from undrained cyclic triaxial shear testing conducted on saturated paste rock specimens reconstituted such that the tailings would “just fill” the void spaces between the coarse particles of the skeleton. During undrained cyclic loading, paste rock typically exhibited a cumulative decrease in effective stress along with a progressive degradation of shear stiffness. The material generally displayed a higher tendency for strain development under cyclic loading when compared with rock-only and tailings-only specimens subjected to similar cons...

Seismic Vulnerability Assessment and Retrofit of a Major Natural Gas Pipeline System: A Case History

The performance of pipeline systems during earthquakes is a critical consideration in seismically active areas. Unique approaches to quantitative estimation of regional seismic vulnerability were developed for a seismic vulnerability assessment and upgrading program of a 500-km-long natural gas pipeline system in British Columbia, Canada. Liquefaction-induced lateral spreading was characterized in a probabilistic manner and generic pipeline configurations were modeled using finite elements. These approaches, developed during the early part of this 10-year program, are more robust than typical approaches currently used to assess energy pipeline systems. The methodology deployed within a GIS environment provided rational means of distinguishing between seismically vulnerable sites, and facilitated the prioritization of remedial works. While ground improvement or pipeline retrofit measures were appropriate for upgrading most of the vulnerable sites, replacement of pipeline segments using horizontal directional drilling to avoid liquefiable zones were required for others.

New Slurry Displacement Method for Reconstitution of Highly Gap-Graded Specimens for Laboratory Element Testing

A new “slurry displacement” method was developed for reconstitution of cylindrical specimens of highly gap-graded soils for laboratory element testing. The method stems from a need to conduct laboratory element tests on mixtures of waste rock and tailings with specific relevance to the development of new technology and material science for mining industry. The slurry displacement method allows preparing uniform/homogeneous specimens of highly gap-graded materials in a saturated condition, thus overcoming the difficulties in the use of currently available specimen preparation techniques. The suitability of the technique to replicate specimens is demonstrated by the repeatable test results obtained from shear testing of identical specimens prepared using the method.

Mobilization of soil loads on buried, polyethylene natural gas pipelines subject to relative axial displacements

The performance of buried, medium-density polyethylene (MDPE) pipes subject to movements relative to the soil in the direction of the pipe axis is investigated by full-scale physical model testing conducted using a large soil box. A new closed-form solution was developed to account for the nonlinear material response of MDPE pipes under axial loading, and the analytical results are compared with the results obtained from the full-scale pipe pullout testing. The closed-form solution provides a rational framework to estimate the response of the pipe (i.e., level of strain, force) and the mobilized frictional length along the pipe for a known relative axial pipe movement. The approach, in turn, could be used to estimate the relative axial movements needed for pipe failure, which is an important consideration in the evaluation of field-performance of pipe systems located in areas of potential ground movement. The methodology is an important initial step towards the development of representative axial soil-spr...

Some Observations on the State of Stress in the Direct Simple Shear Test Using 3D Discrete Element Analysis

Assessment of the mobilized friction angle in a soil specimen tested in a given shear apparatus requires adequate information to establish the stress state at the instance of interest. In the most commonly used version of the direct simple shear (DSS) apparatus, where a cylindrical soil specimen is confined by wire-reinforced membrane, only the normal and shear stresses on the horizontal plane are measured. The knowledge of these stresses alone does not provide adequate information to construct the Mohr circle defining the state of stress. In this context, drained and constant volume (i.e., equivalent to undrained) discrete element simulations of a cylindrical DSS specimen were performed and the results are presented with emphasis on the DSS mobilized friction angle during shearing. It was found that planes of maximum stress obliquity rotate with the development of shear strain. This finding implies that it is not possible to calculate the friction angle accurately from typical DSS laboratory tests with unknown normal stress on the vertical plane. However, it seems that at large shear strains, the horizontal plane becomes a plane of maximum stress obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such shear strain levels.

Analytical Modeling of Field Axial Pullout Tests Performed on Buried Extensible Pipes

AbstractThe performance evaluation of buried extensible plastic pipelines, such as polyethylene pipes in the gas distribution industry, in areas prone to ground movement is a key consideration for many utility owners. Considering the relatively smaller deformation stiffness and nonlinear stress-strain response of these pipes compared with those of steel pipes, a good understanding of the basic soil–pipe interaction mechanisms forms a critical part in these evaluations. The increase in soil normal stress on the pipe attributable to soil dilation and gradual degradation of interface friction with axial pipe displacement was investigated. An analytical method was developed to estimate the pipe response when subject to axial frictional loads arising from soil movements, which accounts for these factors and the nonlinear stress-strain response of the pipe material. For a known relative axial soil displacement, the approach provides a convenient way to determine the axial force, strain, and length along which t...

Full-Scale Laboratory Testing to Assess Methods for Reduction of Soil Loads on Buried Pipes Subject to Transverse Ground Movement

A series of full-scale tests were undertaken to examine the effectiveness of the use of geosynthetic materials to reduce lateral soil loads on buried pipelines subjected to transverse ground movements. The testing program consisted of measuring lateral soil loads on steel pipes buried in trenches simulating different native soil and backfill material configurations. The effectiveness of lining the inclined surface of the trench (i.e. “trench slope”) with two layers of geotextile as a method of soil load reduction depends on the formation of good slippage at the geotextile interface. Pipes buried in relatively soft native soil can penetrate into the native soil during lateral displacement, thus causing the geotextile-lining to be ineffective as a reducer of lateral soil loads. Although there is more opportunity for slippage at the geotextile interface when the trench is in relatively stiff soil, the soil loads on the pipe seem to still increase when the pipe moves in close proximity to the trench slope; this effect is likely due to the increased normal pressures on the pipe arising as a result of the presence of the stiff trench in the vicinity of the pipe.Copyright