Dewatering

Dewatering is the process of removing water from the construction site, utilizing berms to keep surface water away, and pumps to drain the subsurface of the site and lower the water table so that the work can be performed in a dry site. When building in an urban environment, the CM/GC often encounters subsurface water conditions given that the major cities around the world were originally founded near bodies of water to facilitate access and transportation. The specifications, local building codes, and project safety usually require that excavation and foundation work be performed in a dry site. In addition, given the limited amount of space to build on in an urban environment, buildings tend to be taller and have several sub grade levels, thus having a greater tendency to encounter subsurface water.

Water tables are usually found below grade level. Dewatering is usually accomplished by pumping the water from the site, from sump pits at the bottom elevation of the site. In order to ascertain the level of the water table and the type of sub-grade soil and rock conditions that exist at the site, borings need to be done at the beginning of the project. This will indicate the existing water table elevation. The PM needs to be aware that if the construction site is near a body of water that is affected by the tides, then the water table may vary depending on the tidal conditions and whether the site is in the zone of influence of the tidal body of water. Exhibit 6-40 is a diagram of a typical well point installation.

Methods for Dewatering

Depending on the type of soil encountered, two different types of approaches are generally utilized. The first approach is utilized when sand and silt are being excavated. Sand and silt have a tendency to seep into the sump where the water is being pumped,

Monitoring well Discharge to

\ 7 storage pond storage pond

Pump

Water-tight sheathing

Monitoring Pump Discharge to well storage pond

Water table during well point dewatering

Original water table

Pipe

Exhibit 6-40

Well point dewatering.

Screen and possibly soften and shift the soil in the surrounding area on which the foundations of the building may bear. In order to stop groundwater from entering the site, draw down sumps can be created around the site to depress the entire water table in the surrounding area. This process utilizes vertical pipes with screened openings at the bottom to keep soil in place and allow water to enter. The water has to be discharged away from the site to prevent it from potentially re-entering. Discharge from such wells is often regulated by various jurisdictions. The PM may be required to obtain permits for the project to allow for the legal discharge of the water. Piezometers, which measure the water elevation in the surrounding area, are also utilized to monitor the draw down curve of the surrounding area and its water table. Sometimes, when excavating to a lower elevation at the site, a series of rings of well points may be required to properly draw down the water table of the site and the adjacent areas. The lowering of the water table by well points in the surrounding area can have an adverse effect on neighboring buildings. It can cause consolidation or settlement of the surrounding soil under the adjacent building foundations. It can also expose untreated wood pilings that were previously immersed in subsurface water to air and possible decay. It is recommended that a geotechnical engineering consultant be retained to determine the subsurface conditions and the appropriate method of dewatering. Exhibit 6-41 indicates water being pumped utilizing well points monitoring wells in the surrounding area.

Another approach is to erect a watertight barrier creating a bathtub effect around the entire site. See the section on shoring and sheeting. This only works if the watertight barrier walls go down to the bottom edge of an impermeable layer of subsoil material to prevent water from seeping under the walls. A slurry wall is often utilized in this situation, and makes a good watertight barrier. Sheet piling is sometimes used; however, it has a tendency to leak around the joints in the piling. The hydrostatic pressure

Exhibit 6-41

Dewatering draw down curve.

Building

Monitoring wells

Pump

Pump

Original grade

Original water table

Draw down curve of lowered water level after pumping of the water surrounding the site increases as the water table is lowered at the construction site, and therefore the watertight barrier system must withstand the additional lateral pressure with a strong system of bracing and tiebacks.

Once the water table is lowered and the foundation is put in place at the site, the CM/GC must be careful as to when and at what rate the water table can be raised to its original elevation. The new foundation is like a watertight bathtub, which will experience hydrostatic uplifting pressure once the water table is raised. The building has to have a sufficient weight to counteract this uplifting force, which has to be properly engineered and sequenced.

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