Sumps and ditches

a. Open excavations, An elementary dewatering procedure involves installation of ditches, French drains, and sumps within an excavation, from which water entering the excavation can be pumped (fig. 2-1). This method of dewatering generally should not

‘For convenience, symbols and unusual abbreviations are listed in the Notation (app B).

SUMP PUMP

 

Sumps and ditches

(Modifiedfrom “Foundation Engineering,"G. A. Leonards, ed., 1962, McGraw-Hill Book Company. Used with permission of McGraw-Hill Book Company.)

Figure 2-1. Dewatering open excavation by ditch and sump.

be considered where the groundwater head must be lowered more than a few feet, as seepage into the ex­cavation may impair the stability of excavation slopes or have a detrimental effect on the integrity of the foundation soils. Filter blankets or drains may be in­cluded in a sump and ditch system to overcome minor raveling and facilitate collection of seepage. Dis­advantages of a sump dewatering system are slowness in drainage of the slopes; potentially wet conditions during excavation and backfilling, which may impede construction and adversely affect the subgrade soil; space required in the bottom of the excavation for drains, ditches, sumps, and pumps; and the frequent lack of workmen who are skilled in the proper con­struction or operation of sumps.

b. Cofferdams. A common method of excavating below the groundwater table in confined areas is to drive wood or steel sheet piling below subgrade ele­vation, install bracing, excavate the earth, and pump out any seepage that enters the cofferdammed area.

(1) Dewatering a sheeted excavation with sumps and ditches is subject to the same limitations and seri­ous disadvantages as for open excavations. However, the danger of hydraulic heave in the bottom of an ex­cavation in sand may be reduced where the sheeting can be driven into an underlying impermeable stra­tum, thereby reducing the seepage into the bottom of the excavation.

(2) Excavations below the water table can some­times be successfully made using sheeting and sump pumping, However, the sheeting and bracing must be designed for hydrostatic pressures and reduced toe support caused by upward seepage forces. Covering the bottom of the excavation with an inverted sand and gravel filter blanket will facilitate construction and pumping out seepage water.

2-4. Wellpoint systems. Wellpoint systems are a commonly used dewatering method as they are appli­
cable to a wide range of excavations and groundwater conditions.

a. Conventional wellpoint systems. A conventional wellpoint system consists of one or more stages of wellpoints having 1% or 2-inch-diameter riser pipes, installed in a line or ring at spacings between about 3 and 10 feet, with the risers connected to a common header pumped with one or more wellpoint pumps. Wellpoints are small well screens composed of either brass or stainless steel mesh, slotted brass or plastic pipe, or trapezoidal-shaped wire wrapped on rods to form a screen. They generally range in size from 2 to 4 inches in diameter and 2 to 5 feet in length and are constructed with either closed ends or self-jetting tips as shown in figure 2-2. They may or may not be sur­rounded with a filter depending upon the type of soil drained. Wellpoint screens and riser pipes may be as large as 6 inches and as long as 25 feet in certain situa­tions. A wellpoint pump uses a combined vacuum and a centrifugal pump connected to the header to produce a vacuum in the system and to pump out the water that drains to the wellpoints. One or more sup­plementary vacuum pumps may be added to the main pumps where additional air handling capacity is re­quired or desirable. Generally, a stage of wellpoints (wellpoints connected to a header at a common eleva­tion) is capable of lowering the groundwater table about 15 feet; lowering the groundwater more than 15 feet generally requires a multistage installation of wellpoints as shown in figures 2-3 and 2-4. A well­point system4s usually the most practical method for dewatering where the site is accessible and where the excavation and water-bearing strata to be drained are not too deep. For large or deep excavations where the depth of excavation is more than 30 or 40 feet, or where artesian pressure in a deep aquifer must be re­duced, it may be more practical to use eductor-type wellpoints or deep wells (discussed subsequently) with turbine or submersible pumps, using wellpoints as a