Foundations - Groundwater Control.

Groundwater and seepage, including the lowering of the groundwater table by pumping from wells, has been discussed previously. Groundwater can cause or contribute to foundation failure because of excess saturation, seepage pressures, or uplift forces. It has been stated that uncontrolled saturation and seepage causes many billions of dollars a year in damage (Cedergren, 1989). Common types of geotechnical and foundation problems due to groundwater are as follows (Harr, 1962; Collins and Johnson, 1988; Cedergren, 1989):

• Piping failures of dams, levees, and reservoirs
• Seepage pressures that cause or contribute to slope failures and landslides
• Deterioration and failure of roads due to the presence of groundwater in the base or subgrade
• Highway and other fill foundation failures caused by perched groundwater
• Earth embankment and foundation failures caused by excess pore water pressures
• Retaining wall failures caused by hydrostatic water pressures
• Canal linings, dry-docks, and basement or spillway slabs uplifted by groundwater pressures
• Soil liquefaction, caused by earthquake shocks, because of the presence of loose granular soil that is below the groundwater table
• Transportation of contaminants by the groundwater

Proper drainage design and construction of drainage facilities can mitigate many of these ground-water problems. In addition to the groundwater problems described earlier, drainage design and construction facilities are usually required for deep foundation excavations that are below the groundwater table.

The groundwater table (also known as the phreatic surface) is the top surface of underground water, the location of which is often determined from piezometers. A perched groundwater table refers to groundwater occurring in an upper zone separated from the main body of groundwater by underlying unsaturated rock or soil. An artesian condition refers to groundwater that is under pressure and is confined by impervious material. If trapped pressurized water is released, such as by digging an excavation, the water will rise above the groundwater table and may even rise above the ground surface. Figure 16.5 shows an example of an artesian condition, where a test pit has been excavated into a pavement and the released water has flowed out of the test pit.

Groundwater exiting a test pit excavated into a pavement.
FIGURE 16.5 Groundwater exiting a test pit excavated into a pavement.

As indicated in Table 16.5, there are many different methods of groundwater control. For example, sump pumping, such as illustrated in Fig. 16.6, can be used to lower the groundwater table in the excavation. Another commonly used method of groundwater control for excavations is the wellpoint system with suction pumps. The purpose of this method is to lower the groundwater table by installing a system of perimeter wells. As illustrated in Fig. 16.7, the system consists of closely spaced wellpoints installed around the excavation. A wellpoint is a small-diameter pipe having perforations at the bottom end. A pump is used to extract water from the pipe that lowers the ground- water table as illustrated in Fig. 16.7. Additional details on the two-stage wellpoint system and the combined wellpoint and deepwell system are presented in Fig. 16.8. It is important to consider the possible damage to adjacent structures caused by the lowering of the groundwater table at the site. For example, a lowering of the groundwater table could lead to consolidation of soft clay layers or rotting of wood piling.

 Groundwater control: example of a sump being used to lower the ground- water table.
FIGURE 16.6 Groundwater control: example of a sump being used to lower the ground-
water table. (From Peck, Hanson, and Thornburn 1974; reproduced with permission of John
Wiley & Sons.)

Groundwater control: wellpoint system with suction pump.
FIGURE 16.7 Groundwater control: wellpoint system with suction pump 
Groundwater control: two stage wellpoint system and a combined wellpoint and deepwell system
FIGURE 16.8 Groundwater control: two stage wellpoint system and a combined wellpoint and deepwell system.

TABLE 16.5 Methods of Groundwater Control

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