Construction Forecast Issue Home

March 30, 2000

Don’t let water problems drain your profit

The greatest threat to the schedule is down time while everyone decides how to deal with the dewatering problem and who will pay.

By LARRY WEST
HWA GeoSciences Inc.

Construction dewatering can become a costly issue if overlooked during project planning. Owners and engineers typically regard construction dewatering as either an insignificant consideration, or a major factor; but either reaction could be valid depending on the type of project and past experience.

If ground water issues are addressed appropriately at the investigation and design stage, construction dewatering, which involves temporarily lowering the ground water table to permit excavation and construction within a relatively dry environment, is rarely a problem.

However, when ground water issues are ignored or left solely to the devices of the contractor, dewatering can become a nightmare for the contractor, engineer and owner alike. Further, construction dewatering problems can significantly increase the cost of projects and wreak havoc with project schedules.

Construction dewatering has existed as a specialty industry since the turn of the century. Consequently, a number of well-established techniques have been developed to lower the ground water table during excavation. The geology, ground water conditions, and type of excavation all influence the selection of dewatering technology. The most common methods for dewatering include sumps, wells and wellpoints.

• Sumps provide localized, very shallow dewatering (less than 3 feet) and consist of pumping from perforated drums or casings in a gravel-filled backhoe pit. Sumps work best in tight, fine grained soils, or very coarse, bouldery deposits.
• Wells are large-diameter (greater than 6 inches) holes, drilled relatively deep (greater than 10 feet), and contain slotted casings and downhole pumps. Wells work best in soils consisting of sand, or sand and gravel mixtures, and can dewater large areas to great depths.
• Wellpoints are small-diameter (less than 6 inches), shallow wells, and are closely spaced (2 to 10 feet apart). Wellpoints effectively dewater coarse sands and gravels, or silts and clays. They have a wide range of applications. However, wellpoints use a vacuum system and their depth is limited to about 25 feet. Wellpoint systems generally cost more than either sumps or wells, and require near-continual maintenance.

A number of other dewatering techniques are available including ground freezing and electro-osmosis. However, such techniques are very costly and used only for particularly difficult dewatering applications.

The owner and design engineer should ask for guidance from their geotechnical consultant for the most appropriate way to handle dewatering design and specifications. Initially, the geotechnical investigation will indicate if ground water occurs near or above the project subgrade. If so, the owner and design engineer have several options:

• Contractor assumes all responsibility for design of dewatering system.
• Geotechnical consultant performs a preliminary dewatering evaluation based on existing data (typically, $2,500 to $5,000).
• Geotechnical consultant performs a detailed dewatering evaluation which will probably include pumping tests (typically, $20,000+).
• Engineer and geotechnical consultant design a complete dewatering system, including well spacings and depths, pump sizes, discharge piping sizes, right down to the nuts and bolts used to put the system together.

The fourth option is reserved for rare situations, such as a dewatering project that will last for several years, one that involves numerous prime contractors, or a project that presents unique dewatering challenges.

For example, under subcontract to Brown and Caldwell Engineers, HWA designed the dewatering system for the King County Renton Wastewater Treatment Plant Phase III Expansion. This major dewatering effort included four very large (200 by 300-foot) excavations which required lowering ground water by over 30 feet. The design considered potential settlement of older structures, maintenance of wetlands, and ground water contaminant transport from an old landfill adjacent to the treatment plant. The system included 54 extraction and recharge wells with dewatering discharges ranging from 1,000 to 5,000 gpm. For nearly three years of operation by Metro and three separate prime contractors, the system maintained effective ground water control throughout the entire project area. The system design allowed for retaining many of the dewatering wells, which were used in subsequent treatment plant expansion, saving the County considerable dewatering costs.

If dewatering is required, planning and preparation by the owner and contractor will prevent significant costs and schedule delays. The greatest threat to the schedule is the "down time" while everyone decides how to deal with the dewatering problem and who will pay. For instance, a typical wellpoint or deep well dewatering system can easily take three to five days to install. In addition, ground water drawdown, particularly in fine grained soils, will add days or even weeks of down time.

On the other hand, a well designed dewatering system will benefit both owner and contractor. Recently, HWA designed a construction dewatering system for Kiewit Pacific Co. to dewater two excavations, one, 1,000 feet by 2,000 feet the other 1,000 feet by 1,000 feet. The excavation was adjacent to the Green River in the City of Kent and serves as a regional storm water management facility. HWA designed the dewatering system using a numerical computer model using the owner’s data. The system included only 36 strategically placed wells. Kiewit completed the 900,000 cubic yard excavation in record time, faster than all the experts expected, and well within the City’s schedule.

Prudent up-front investigation minimizes dewatering problems. Your geotechnical consultant can help you avoid these situations, and reduce the potential for costly change orders.