Centrifuge modelling of active slide–pipeline loading in soft clay

Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe–soil velocities of up to 4.2 m/s. The changing density and shear strength of the samples were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature

A laboratory miniature full-flow penetrometer system for peat

Uncertainties and difficulties surrounding laboratory strength testing of peat have caused the increasing reliance on in situ testing methods, which include T-bar and ball penetrometers, to determine the undrained strength of peat for design purposes. This paper presents the results of miniature full-flow penetrometer (T-bar and ball) tests on decomposed peat samples to provide a better understanding of the interpretation of the strength parameters in this material. The tests were conducted in a pressure chamber, in which miniature T-bar and ball penetrometer tests can be performed on peat samples consolidated under specific vertical effective stresses. Penetrometer bearing factors were derived experimentally using the monotonic penetration resistance and the undrained shear strength estimated from triaxial tests. The bearing factors, expressed as the penetration resistances normalised by the undrained shear strength data (obtained from triaxial tests) compare well with those derived from plasticity solutions. In addition, the remoulded strength parameters derived from penetrometer cyclic tests are comparable with those obtained from fall cone tests. The findings reported in this paper illustrate the capability of full-flow penetrometer tests to measure the undrained strength of peat

Variable penetration rate testing for shear strength of peat – a review

This paper presents a review of the advances in the variable penetration rate testing methods on inorganic soils and its potentials to investigate the appropriate penetration rate and resistance factors for penetrometer testing of peat. A partially drained condition, which often leads to misinterpretation of test results, has been observed in peat penetration testing when the standard rate of 20 mm/s is used. Although the impact of rate-effects on penetration resistance measured with CPTu, T-bar and ball penetrometer have been investigated extensively in various intermediate soils, research is limited on how penetration rate controls drainage conditions and affects consolidation behaviour in peat. This review synthesises research developments in using variable penetration rate tests. The objective is to evaluate the transition of drainage conditions and consolidation behaviour of inorganic soils while focusing on its adaptability for peat. The review provides guidance on the investigation of the penetration rate testing in pea

Laboratory development of a vertically oriented penetrometer for shallow seabed characterization

Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in-situ test soundings to depths of 1-2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 – 30 mm/s, after embedding the VOP at 30 mm and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small scale centrifuge models and, following practical development, potentially also in the field

Bearing capacity of sandy soil treated by Kenaf fibre geotextile

Bio-based materials are widely used recently in order to introduce a more sustainable construction material. Kenaf is a type of bio-based material that can be easily obtained in a tropical country, which could be a potential material to be utilised as a geotextile material because it has good tensile strength. The geotextile could be used to improve the bearing capacity of a loose soil. This paper presents a series of small-scale physical modelling tests to investigate the bearing capacity performance of Kenaf fibre geotextile laid on and inside the sand layer. A rigid footing was used to replicate a strip footing during the loading test, and sand was prepared based on 50% of relative density in a rigid testing chamber for ground model preparation. In order to treat the soil, Kenaf fibre geotextile was laid at four difference locations which are on the soil surface and underneath the ground model surface at 50, 75 and 100 mm deep. It was found that the usage of the Kenaf fibre geotextile has improved the bearing capacity of the sandy soil up to 414.9% as compared to untreated soil. It was also found that the depth of the Kenaf fibre geotextile treated into the soil also affects the soil performance. © 2017, Springer-Verlag GmbH Germany

Interfacial friction behaviour in narrow wall paste backfill system

Understanding on the effects of interfacial friction within narrow wall is important in the design of underground stope backfill system. Laboratory scale stope model made of metal such as aluminium is normally used to simulate the actual stope paste backfill system, however, verification of such system in terms of interfacial friction behaviour is lacking. This paper presents the experimental results on the interfacial friction between backfill material and the aluminium narrow wall system as well as the internal friction of the paste backfill. Standard and modified direct shear tests are employed to investigate such behaviours under dry and saturated conditions for uncemented paste backfill (UCPB) and cemented paste backfill (CPB). For the CPB, cast in-situ and precast is also compared. The shear stress-strain behaviour is showcased in detail for every test. The general findings show that the interfacial friction angle (δ) at the backfill-aluminium interface is weaker than internal friction angle (ϕ) of the backfill itself with an average factor of 0.69 (δ/ϕ =0.69). This factor is comparable to 2/3 or 0.67 which is commonly used in the design and analyses. The results help to better understand the behaviour of the backfill system, enabling engineers to optimize the paste backfill system design

A Narrow Wall System to Capture Temperature-Stress-Strain Behavior in Paste Backfill

Placing mine tailings back underground, into mined-out stopes, is becoming increasingly used internationally, providing as it does improve ore recovery, reduced dilution of valuable ore and environmental benefits due to reduced size of surface tailings storage facilities. In recent years a number of stopes backfilled with cemented paste backfill have been instrumented with load cells and piezometers, to improve our understanding of in-situ behavior. Many of these studies have reported results that show increases in measured total stresses when there is no increase in applied load, i.e. even when the backfilling process is long completed. One explanation is that these stress increases result from expansive volume changes of the backfill as it hydrates and generates heat. This paper proposes and describes a novel laboratory apparatus called narrow wall system to investigate this hypothesis, focusing on modeling narrow stopes as these are relatively common in backfill applications. Results from the experiments agree qualitatively with the reported field observations, showing clear increases in measured pressure during periods of temperature increase. The paper concludes that the proposed narrow wall system works effectively and has been able to capture the temperature-stress-strain behavior of paste backfill. Thus, the temperature effect hypothesis has now been supported by evidences. Using the system, further studies related to geometrical or scale effects are suggested. The results are important for academics and engineer to improve backfill design in mining operations