The classic experiment of a yielding trapdoor in (two-dimensional) plane strain beneath a granular mass is revisited, this time in the high-gravity environment of a geotechnical centrifuge. High-speed acquisition of load and displacement data makes it possible to examine the arching behavior of geomaterials. Results indicate the formation above the trapdoor of a physical arch, which evolves from an initially curved configuration to a triangular shape and ultimately to a prismatic sliding mass with vertical sides. Simple expressions for the reduced load caused by arching are proposed. Predictions using these expressions compare favorably with test measurements, paving the way for potential application to the design of underground structures.

Investigation of Soil Arching with Centrifuge Tests

An experimental testing program was undertaken to investigate failure mechanisms induced by the active movement of a deep rectangular trapdoor underlying a granular soil. Reduced-scale models were tested under normal gravity as well as under an increased gravitational field using a centrifuge facility. Some models were used to evaluate the performance of both flexible and rigid pipes undergoing a localized loss of support. Failure mechanisms in the longitudinal direction of the models were characterized by a single, well-defined failure surface that developed within the limits of the trapdoor. However, failure mechanisms in the transverse direction of the models were characterized by multiple failure surfaces extending outside the limits of the trapdoor. Significant dilation of the soil located immediately above the trapdoor was identified in the failure of the models. The pattern of the failure mechanisms was found to be affected by the stress level and backfill density. Higher stress levels were found to lead to well-developed failure zones. The influence of backfill density was found to be more relevant in models involving flexible pipes. Pipes embedded within loose backfill were severely damaged after loss of support, while pipes embedded in dense backfill experienced negligible deformations. These results indicate that damage to pipelines caused by ground loss of support can be significantly minimized by controlling the compaction of the fill.

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Failure Mechanisms in Sand over a Deep Active Trapdoor

Serviceability design for geosynthetic reinforced column supported embankments

Classification and characteristics of soil arching structures in pile-supported embankments

Sinkhole development in a profile with a weakly cemented sand overlain by an uncemented sand and underlain by a karst limestone was studied using 51 reduced-scale, stress-correct centrifuge models. The interaction between the thickness (\iH\d\ic), cohesive strength (\ic), and unit weight (γ\i\dc) of the cemented sand layer; the thickness (\iH\i\ds) and unit weight (γ\d\is) of the uncemented sand overburden; and the diameter (\iD) of the cavity in the underlying karst formation was examined. Collapse of the cemented sand layer into the underlying opening was observed to occur either as a breakthrough plug with steeply inclined sides at an average angle of 61° to the horizontal or as a dome fallout that left behind a stable arch. The type of failure depended on the model geometry, described by \iH\i\dc/\iD. A dimensionless design chart was developed to predict failure, relating \iNγ\d\ic\iH\d\ic/\ic to \iH\i\dc/\iD for configurations without uncemented sand overburden. The overburden pressure over the cavity at collapse was less than the geostatic overburden pressure. The fraction of full geostatic stress decreased with increasing overburden depth relative to the size of the cavity opening and approached an asymptotic value of about 5%.

Modeling of sinkholes in weakly cemented sand

When the lining of a tunnel is more flexible than the surrounding soil, the pressure applied to the lining is in general far smaller than the value estimated from the weight of the overburden. Pressure applied to an underground conduit or a trapdoor is also smaller than the weight of overburden. Similarly, earth pressure applied to a retaining wall or to the timbering of an open cut, and particularly to the lower part of the wall or the cut, is sometimes smaller than the values estimated from the theory of earth pressure. Some studies of this arching action to the soil assume that the soil remains in an elastic state; others assume a plastic soil model with a Tresca yield criterion. In this Paper the stress distribution in the soil is analysed assuming that the soil is in a state of plastic equilibrium and that Mohr-Coulomb's failure hypothesis is satisfied. Results are compared with those obtained by measuring the inner air pressure of models made of vinyl tubes or bags buried in dry sand or gravel. Si l...

K. Ono

Papers

Analysis of the arching action in granular mass