Variation of suction pressure during caisson installation in sand

Abstract

A series of model tests was conducted in a geotechnical centrifuge to investigate the variation of suction pressure during caisson installation in dense sand. The tests were carried out for caissons of different size and surcharge (or effective weight). Finite element (FE) simulations were also conducted to study and explain the experimental results. Data from two field installations and other model tests were investigated, and compared with the results from the centrifuge model tests. The suction pressure was observed to increase relatively linearly with embedment depth, following a distinct slope that is related to establishing a critical hydraulic gradient along the inner caisson wall. The total driving force (weight plus pressure differential) required for suction installation is significantly reduced compared with the force required to install the caisson by jacking. When a larger surcharge was used, the required suction pressure decreased for a given penetration depth, but the total force increased. By contrast, the initial caisson penetration depth did not appear to have any effect on the required suction pressure. The overall behaviour and the pressure variation with depth were similar for caissons of different sizes, and wall thickness. Subsequent FE analyses appeared to explain this observation well, and a consistent pattern of behaviour was established for all the model and field data.On a effectué une série d'essais sur maquette dans une centrifuge géotechnique, pour examiner la variation de pression d'aspiration lors de l'installation de caissons dans du sable dense. Ces tests ont été effectués sur des caissons de taille et surcharge (ou poids effectif) diverses. On a égale-ment procédé à des simulations aux éléments finis pour étudier et expliquer les résultats expérimentaux. On a examiné des données provenant de deux installations sur le terrain et d'autres essais sur maquette, pour les comparer avec les résultats des essais centrifuges sur maquette. On a remarqué que la pression d'aspiration augmentait de façon relativement linéaire en fonction de la profondeur d'enfouissement, en suivant une pente distincte en rapport avec l'établissement d'un gradient hydraulique critique le long de la paroi interne du caisson. La force d'entraînement totale (poids plus différence de pression) requise pour l'installation d'aspiration est fort inférieure à la force requise pour installer le caisson au moyen de vérins. Lorsqu'on utilise une surcharge supérieure, la pression d'aspiration requise diminuait pour une certaine profondeur de pénétration, tandis que la force totale augmentait. Au contraire, la profondeur de pénétration initiale du caisson ne semblait avoir aucun effet sur la pression d'aspiration requise. Le comportement général et la variation de la pression en fonction de la profondeur étaient similaires pour des caissons de tailles et épaisseur de paroi diverses. Des analyses aux éléments finis, effectuées pas la suite, semblent bien expliquer cette observation, et on a établi une tendance régulière d...

Centrifuge model tests on installation of suction caissons in sand

“…The measured suction pressure linearly increased with penetration depth after sharp increase in the very initial stage of installation. This trend is quite similar to the previous study (Tran and Randolph, 2008). It was noted that the penetration of caisson was terminated at z/Dc = 0.76 (3.8 m) due to the lack of water pumping rate.…”

Seepage Induced Soil Failure and its Mitigation During Suction Caisson Installation in Silt

“…Based on the theoretical analysis on the container boundary effect in relation to the test locations, the minimum ratio of the distance from caisson center to the side boundary to its radius was about 4.6 in our tests, making the boundary effect as small as possible [13].Test time intervals were also kept long enough to eliminate the effect from soil disturbance. The test locations and dates are shown in Figs.…”

“…According to the test data, a skirt penetration depth ¿p = 5 cm and applied suction pressure S = -l kPa represented a typical instantaneous state during the SP process, the instantaneous ground-water flow at its skirt wall penetration depth was assumed to be steady. The distances from both the bottom and edge of the caisson to the base and side boundary were chosen to be as large as three times of its diameter [13]. The thickness of the surface layer soil was the same when the filter layer was included, and its permeability //was set to be 1/5, 1/10, and 1/20 of that of the underneath soil to represent the permeability decrease in the filter layer as it was clogged gradually by the up-moving soil particles.…”

“…Yang et al [11] found that the formation of excessive soil heave in silt was the result of seepage, which induced loosening of the soil plug and the plastic flow of silt around the caisson tip. Based on the caisson installation tests carried out in layered sandy-silty soil, Tran and Randolph [12,13] observed the formation of excessive and unstable inner-soil plug heave (up to 20% of the caisson penetration), which had a sigtiificant influence on the development of suction pressure and penetration resistance. The high suction pressure, combined with the silt's susceptibility to erosion, may create piping channels and produce soil overflow into the caisson.…”

Bearing capacity of hybrid suction foundation on sand with loading direction via centrifuge model test

“…Caisson length to diameter ratio (L/Dc) is 1. The thickness of the model was over-designed than the typical prototype caisson dimension by concern for structure failure during the tests (typically, tc/Dc = 0.3 ~ 0.4% for sand, Tran and Randolph (2008)). Hybrid foundation have same dimensions with those of the single caisson model and the mat dimension of Dm = 142.8 mm, representing a prototype of (Dm)p = 10 m. the dimension of foundation are summarized in Table 1.…”