Time-drawdown behaviour of construction dewatering systems in fine soils

Abstract

In the design of a construction dewatering system in a fine-grained soil it is necessary to consider not only the steady state flow rate that must be pumped, but also the time it will take to achieve a given drawdown. This may be weeks because the soil will usually remain saturated, and the mechanism of pore-water pressure reduction is consolidation rather than dewatering per se. If the response time is not taken into account in programming an excavation, the cost implications could be considerable. In this Paper approximate methods based on the assumption that the isochrones are parabolic are developed for the estimation of time-drawdown relationships for dewatering systems in fine soils. The parabolic isochrone analyses are compared with exact solutions where these exist, and with field data from five case studies. The parabolic isochrone approximation is shown to be suitable for flow to lines of wells, and to rings of wells, provided that the distance of influence remains small compared with the radius of the ring. Lors de la conception d'un système drainant dans un sol granulaire fin, il faut à la fois prendre en compte le débit en régime permanent qui doit être pompé et les temps mis pour atteindre un certain rabattement. Ce temps peut-être de plusieurs semaines puisque le sol reste générallement saturé et que le mécanisme gouvernant la diminution de la pression interstitielle est la consolidation plus que le drainage. Si le temps de response n'est pas pris en compte lors de la conception d'une excavation, les coûts peuvent augmenter considérablement. L'article présente des méthodes approachées, fondées sur l'hypothèse que les isochrones sont paraboliques, qui permettent d'approcher les relations temps-rabattement pour des systèmes drainant dans des sols fins. Les résultats des analyses des isochrones paraboliques sont comparés à des solutions exactes, lorsqu'elles existent, et à des données n-situ provenant de 5 cas d'étude. L'approximation des isochrones par des paraboles semble adaptée aux lignes de puits et aux cercles de puits si le rayon d'action des puits est faible devant le rayon du cercle de puits.

The application of the cumulative drawdown method in designing of groundwater lowering system - Golden Hill project

“…Therefore, this problem will be analyzed assuming confined behavior throughout. The cumulative drawdown is calculated using equation [17][18][19][20].…”

“…In this case the initial guess was twenty-two wells evenly spaced at 15 m centres (Figure 5). A spreadsheet program is then used to evaluate equation (1) for the cumulative drawdown at selected locations within the excavation [16][17][18][19][20][21]. For circular or rectangular excavations with evenly-spaced wells it is normally sufficient to determine the drawdown in the center of the excavation, because drawdown everywhere else will be greater.…”

“…During the pumping test could have yielded more if a larger pump has been used. It therefore makes sense to repeat the above calculations assuming fewer wells of greater discharge rate [18][19][20][21][22]. The results of these calculations are summarized below: It is apparent that the target drawdown can be achieved by various combinations of well numbers and yields, but that the total flow rate remains approximately constant.…”

Contributions to Géotechnique 1948–2008: Groundwater

“…This large project to control pore water pressures in a soil of such low hydraulic conductivity (about 4 3 10 À7 m/s horizontally) sparked a programme of associated research into appropriate methods of analysis for groundwater control in fine soils. Some of the results were published in Géotechnique, including papers on steady-state and time-drawdown behaviour (Preene & Powrie, 1993;Powrie & Preene, 1994).…”

Assessment of settlements caused by groundwater control

“…Effective stress settlements from groundwater control 10. Assuming that groundwater control prevents the excavation failures previously described, ground settlements result purely from the reduction in pore water pressure and increase in eective stress that occurs when groundwater levels are lowered (or drawn down) by pumping.…”