Performance Of Vertical Drains Research Articles

Deep-well pumping in the Bangkok Plain and its influence on ground improvement development with surcharge and vertical drains

The Bangkok subsoil that forms part of the larger Chao Phraya Plain consists of a broad basin filled with sedimentary soil deposits that form alternate layers of sand, gravel and clay. The depth of the bedrock is still undetermined, but its level in the Bangkok area is known to vary between 400 m and 1800 m deep. The aquifer system beneath the city area is very complex, and the deep-well pumping from the aquifers over the last 50 years or so has caused substantial piezometric drawdown in the upper soft and highly compressible clay layer. In this paper, the influence of the piezometric drawdown on the performance of vertical drains with surcharge on the upper clay layer is examined. Instrumented test embankments were built and the performance of large-diameter sand drains, sand wicks and prefabricated vertical drains was examined. A novel interpretation technique was adopted to evaluate the settlements from pore pressure dissipation and to compare with those measured directly from settlement measurements. Le sous-sol de Bangkok qui forme une partie de la plaine plus vaste de Chao Phraya est constitué d'un large bassin rempli de dépô ts sédimentaires qui forment des couches alternées de sable, de gravier et d'argile. La profondeur du lit rocheux reste indéterminée mais on sait que son niveau dans la zone de Bangkok se situe entre 400 m et 1800 m de profondeur. Le système aquifère sous la ville est très complexe et un pompage à puits profonds au cours des 50 dernières années a causé une baisse substantielle du niveau piézométrique dans la couche argileuse supérieure molle et hautement compressible. Dans cet exposé, nous examinons l'influence de la baisse piézométrique sur la performance de drains verticaux surchargés sur la couche argileuse supérieure. Des berges de contrôle instrumentées ont é té construites et la performance de drains de sable de gros diamètre, de mèches de sable et de drains verticaux préfabriqués a été é tudiée. Nous avons adopté une technique d'interprétation innovante pour é valuer les tassements d'après la dissipation de la pression interstitielle et pour les mesures de tassement directes.

Discharge capacity of prefabricated vertical drain and their field measurements

Discharge capacity is the most important parameter for performance of prefabricated vertical drains. However, the required discharge capacity for best performance of vertical drain is still uncertain. This paper described the single basic equation to obtain the required average discharge capacity. The estimated values were supported by the field measurement of discharge capacity with different drain spacings, different penetration length and different thickness of compressible layers. It was found that required discharge capacity is in an order of 10 −6 m/s for 100 mm width drain.

Vertical Drain Performance: Myths and Facts

A new approach is proposed for evaluating the fieid performance of vertical drains. This method involves comparing the minimum discharge capacity qw(min) of a vertical drain installation required for negligible well resistance with the discharge capacity mobilized in the field, qw(mob). A procedure has been developed for the estimation of qw(min), which has a typical value in the range Of 2 to 100 m3/yr, depending on the horizontal permeability of the soil and the maximum drainage length Of the vertical drain. A computer program, ILLICON, was developed for settlement analysis of embankments on soft ground and was used to back-calculate qw(min) from field observations of settlement and pore water pressure. The drain performance at four vertical drain installation schemes was evaluated using the proposed method. It was found that the magnitude of the compressive strain undergone by soil during preloading is a critical factor to the proper functioning of vertical drains.

Performance of vertical drains at Queenborough bypass : Nicholson, D P; Jardine, R J Geotechnique, V31, N1, March 1981, P67–90

Performance of vertical drains at Queenborough bypass : Nicholson, D P; Jardine, R J Geotechnique, V31, N1, March 1981, P67–90

Performance of vertical drains at Queenborough bypass

The bypass round the town of Queenborough on the Isle of Sheppey, Kent is constructed on soft alluvial clay up to 10 m deep. Where the bypass bridges an existing railway line the approach embankments rise to a height of 7 m above the existing marshland. Embankment construction was started in 1976, but instrument readings indicated that the initial stage of filling could not safely be constructed higher than 3·0 m. Further site investigations were made which included in situ constant head permeability tests performed by reducing the pore pressure at the piezometer cell using recently developed equipment. Analysis showed that vertical drains would be required to accelerate the consolidation and enable the embankment to be completed within the remaining construction programme. Subsequently, two types of vertical drains were assessed during trials in 1978: a 65 mm dia. Sandwick drain and a 300 mm wide AV Colbond fabric strip drain. The Paper compares the consolidation parameters and settlements predicted from laboratory and in situ tests with the field performance of the embankment with and without the vertical drains. Good agreement was found between these comparisons which confirm the predicted decrease in coefficient of consolidation as effective stress exceeded the preconsolidation pressure. La Route de Déviation autour de la ville de Queenborough sur l'Ile Sheppey, Kent, a été construite sur de l'argile alluvionnaire molle d'une profondeur atteignant 10 mètres. Là où la route de déviation passe au-dessus d'une ligne de chemin de fer existante, les remblais d'approche s'élèvent à une hauteur de 7 m au-dessus du terrain marécageux existant. La construction des remblais a commencé en 1976 mais les valeurs obtenues à l'aide d'instruments de mesure indiquaient que l'on ne pouvait dépasser sans danger 3·0m pour le remblai initial. Une analyse ultérieure a révélé qu'il faudrait prévoir des drains verticaux pour accélérer la cadence de consolidation et pour permettre de terminer les remblais dans les limites du programme de construction portant sur trois ans. Des essais de perméabilité in situ à charge constante ont été effectués en réduisant la pression interstitielle à l'extrémité du piézomètre à l'aide d'un matériel récemment mis au point. Deux types de drains verticaux ont été testés par la suite lors des essais effectués en 1978; il s'agissait du drain du type 'Sandwick' de 65 mm de diamètre, et du drain du type à bande de toile 'AV Colbond', de 300 mm de large. L'article compare les paramètre de consolidation et de tassement, déduits des essais de consolidation en laboratoire et des essais de perméabilité in situ, à la performance sur place du remblai avec et sans drains verticaux. Les comparaisons effectuées révèlent une réduction analogue du coefficient de consolidation lorsque la contrainte efficace dépasse la pression de préconsolidation.