Horizontal arching of earth pressures on retaining structures

Yen Chua, Cambridge University
Geotechnical Engineering Group


Where a relatively “flexible” wall panel is situated between more rigid panels, the horizontal arching mechanism transfers horizontal earth pressures acting on it to its neighbouring elements via soil shear stresses. In this research, the horizontal arching mechanism and lateral deformations of cantilever walls are investigated using centrifuge tests. A 300mm high, L-shaped (in plan view) model basement comprising separate but contiguous wall panels of different widths and stiffnesses was used for this purpose. A re-entrant corner was included as part of the basement geometry, in order that a study of the stress distribution at such corners might be made. The model basement retained dry sand poured within an external confining steel tub of 850mm in diameter. Heavy fluid was contained within the basement to generate stress conditions similar to those found in-situ after casting a wall. The fluid was then drained in-flight to simulate an excavation sequence.

A series of six tests was carried out at 45g and the panel widths and thicknesses around the model basement were varied, so that the effects of panel geometry and stiffness on horizontal arching could be studied. Displacement distributions at the re-entrant corner were investigated with the help of Particle Image Velocimetry (PIV), a programme that tracks object movements in patches through a series of still digital images. Where possible, photogrammetric calibration was then applied to establish the soil and panel movement vectors in object space

Results indicate that the magnitude of horizontal arching reduces with increasing panel width for the flexible panel. The equivalent earth pressure coefficient, K, was established through the superimposition of predicted and measured panel displacements and bending moments. Data obtained showed that the earth pressure dropped to below active values (when horizontal stress was compared to the nominal vertical stress at a given depth), represented by a new coefficient Kmin. Miniature earth pressure cells were also used to confirm these results, that horizontal arching is capable of reducing lateral pressures locally by a significant amount in favourable circumstances

Based on these results, it was deduced that the reduction of lateral pressure on a retaining wall panel of a certain width, B, and stiffness, EI, is also a function of panel displacement u. New theoretical approaches involving analogies with cavity contraction and pressures in a silo was then sought to establish quantitative relationships between the arching mechanism and panel geometry. The predictions obtained from these numerical models were found to be broadly similar to the experimental data

A new estimate of horizontal earth pressures, which can be incorporated into existing design of retaining systems (which incorporate flexible and rigid panels) was proposed, based on experimental data and validation through soil mechanics theories. The findings of this research present a preliminary approach to design for such retaining systems and need to be investigated further to develop a more concrete theory.