Knowledge Center

It's amazing what a heavy rain can do to a construction project. Prior to the rain, the site may be dry, heavy equipment efficiently moving earth, the other trades smoothly performing their work. Within hours the project can be a sloppy, mud-hole with worker efficiency cut to about 10%. In many cases, the change comes mostly from poor planning. In most areas of the world, the Construction Supervisor must remember a simple fact: IT WILL RAIN.

Good planning can minimize the damage and disruption of a heavy rain to a jobsite. Often the excavation and grading is left to the Sitework Contractor (and their Foremen is responsible to supervise and direct the heavy equipment and operators). But remember, the Construction Supervisor is responsible to complete the entire project in an efficient, timely manner. Therefore the Construction Supervisor must be continuously aware of what rain will do to the project site. It is not uncommon for the Sitework Foreman to work their heavy equipment for maximum efficiency and hope it doesn't rain.

One of the best ways to prepare for rain is to slope all grades to drain and to smooth rolled the surface prior to a rain. This work can cut down on short-term productivity, but can really help a jobsite more quickly recover from a storm. The Construction Supervisor must be far-sighted enough to insure that heavy rain does not stop work on the project longer than necessary. Daily discussions with Sitework Foremen may be required to achieve this goal.

Any time excavation is required below the existing water table on a project, the process of dewatering must be considered. Generally dewatering is required because of a high level of ground water (underground streams and springs) in areas with cohesionless (sandy or gravelly) soil or water yielding rock. In a truly cohesive soil, the water travels so slowly through the clay or silt that dewatering is not usually necessary for the relatively short time of excavation.

Dewatering may be required for a single footing excavation or for an entire project site. The most common dewatering methods are trench drains, deep wells and well points. A variety of pumps and piping types are used with these systems. Ground water seepage can also be decreased by cutoff methods such as sheet piling.

The costs for dewatering can be staggering, including equipment rental, labor and electricity (or fuel). High dewatering costs have paled the profit margins on far too many projects. The many variables listed below make the job of estimating dewatering costs very difficult, and very inexact.

1. Specific soil and rock characteristics and locations.
2. Groundwater location, extent, direction and rate of movement (including seasonal variations).
3. Most efficient dewatering method: trench drains, deep wells, wellpoints.
4. Effectiveness of cutoff structures.
5. Time period dewatering is required.
6. Costs associated with dewatering system failure (mucking out and cleaning).
7. Costs associated with over dewatering (damage to adjacent structures).
    

In dewatering, the simplest solution is often the best. On many sites dewatering can be accomplished without pumps, using gravity, by the use of trench drains or siphons. This option should always be considered when analyzing the prospect of dewatering. Obviously the option is only viable if gravity can run the water to lower ground. Trench drains can be cut with a backhoe and filled with a coarse, granular material (#4 stone for example), but care must be exercised in choosing the water outlet type and location.

Siphons present an economical, and often overlooked, method of dewatering. A siphon, by definition, uses atmospheric pressure to carry water from one elevation, up over an obstacle, to a lower elevation. The pipes in a siphon system must be airtight and some ingenuity is often required to completely fill the siphon pipe. The siphon pipe must be full for the siphon to begin. The time required for siphon dewatering is usually longer then for pumps since the force moving the water is atmospheric pressure, not electricity or gasoline.

A deep well consists of a pump, hose and a vertical well casing. The pump intake is at the bottom of the well casing (usually some crushed stone is placed down there as a filter medium). The water is pumped up the hose, out of the well casing, and to a suitable discharge location. The effectiveness of a deep well depends mainly of the permeability of the soil. In a coarse sand, for example, a large area can be pumped to near the pump intake elevation. A less permeable soil, on the other hand, reduces the effectiveness of a deep well. Since the pump is generally at the bottom of the deep well, there are no height limitations due to vacuum lift, and deep wells can lower the groundwater over 50 feet.

Wellpoints are the most complex dewatering system discussed here. The wellpoint system is composed of vertical wellpoints (commonly 1-� inches diameter) connected to a header pipe at grade and a vacuum/discharge pump at grade. The wellpoints operate on the principle of vacuum and have a maximum depth of about 15 feet. If deeper dewatering is required, successive sets of wellpoints can be installed at lower elevations.

The vertical wellpoint is actually jetted into the ground with water. On the bottom of the wellpoint there is a 2 foot long screen and valve, water jets out of this valve and creates a hole into which the wellpoint pipe can be lowered. This hole is often made a larger diameter (for example 10 inches) to allow for a coarse sand backfill to help filter the water. The wellpoint spacing depends on the permeability of the soil, but 2 feet to 6 feet centers are common. On average a single pump can handle up to 50 wellpoints.

The jetting portion of wellpoint work generally results in quite a bit of mud and muck at grade. The Construction Supervisor should assure that provisions are made for this material to be removed. While the wellpoint system usually does an excellent job of dewatering, the pipes also cause significant congestion at grade. Since the Construction Supervisor manages the project site, he should strategically place the wellpoints and headers to minimize conflicts.

Pumps are the most important, and the most troublesome, piece of equipment in most dewatering schemes. Most dewatering pumps are described by the following categories:

1. Centrifugal pumps
   a.  suction pumps
   b.  submersible pumps
2. Diaphragm pumps (or trash pumps)

Centrifugal pumps come in a great variety of sizes, from a 1-� inch to over 12 inch nominal discharge diameter. The most common sizes are 2 inch and 3 inch nominal discharge diameter. Centrifugal pumps can be driven by gasoline or diesel engines or be electrically powered. If gas or diesel, provisions must be made to keep fuel in the tank; these solutions range from assigning the responsibility to someone to special valves that feed fuel from 55 gallon drums. Special care must be used with liquid fuel on the jobsite.

Electric pumps tend to be easier to use in many ways, but safety is an important aspect here also. It is extremely disconcerting to be standing ankle deep in water, working on a pump, and realize that the guy who wired it didn't know a volt from an amp. Even though it seems trivial, get an electrician to wire dewatering pumps. Using the proper breaker, wire size and ground fault protection is essential.

Centrifugal suction pumps have a rigid suction line (to prevent collapse) and lift water up the level of the pump. Fifteen to twenty feet is the maximum height that water can be lifted by suction. Some centrifugal suction pumps are self priming, but many must be primed prior to each use, which can be labor intensive and annoying.

The centrifugal submersible pump operates in the bottom of the hole. There is no suction line, the pump must be flooded. With proper float controls these electric pumps can be quite trouble free.

Diaphragm pumps are the workhorses of construction dewatering; pumping mud, sand, small rocks, and trash (pumped  material can be up to 70% of the size of the pump intake). The diaphragm pump uses a rigid suction line and is generally self-priming. The responsibility for installing and maintaining construction dewatering pumps must be clearly assigned. Sometimes weeks of work can be ruined if the pumps malfunction for only one day. Therefore the Construction Supervisor should know who is responsible for dewatering and be updated of the status regularly.

The use of cutoff walls can also help control ground water on a construction project site. Water seepage horizontally through the soil can be a major source of water entering a project. A below grade cutoff wall can virtually stop this horizontal flow of water. Interlocking steel sheet piling, is the most common type of cutoff wall. Sheet piling functions best in course grained soils where boulder sizes are generally less then 6 inches, or where there are stratified soil layers and horizontal water flow greatly exceeds vertical. It is important to drive cutoff walls well below the inside excavation elevation.