Dewatering Pump Considerations for Shaft Construction

Whether for tunneling, microtunneling or jack-and-bore operations, an access pit is an essential component for launching the tunneling equipment. Oftentimes, groundwater can become an issue for shaft construction, and dewatering is needed to allow excavation to the elevation required.

The art of dewatering is a special talent that comes with experience and it is important to choose a qualified dewatering contractor with knowledge of dewatering and soil stabilization techniques to achieve the task. During the initial review of the project it is important to consider the following aspects of the application:

1. Understand the overall layout of the jobsite.
2. Evaluate the soil conditions.
3. Recognize the importance of safety during the excavation.
4. Be aware of the impact the activity will have on the environment.

The Role of the Dewatering Contractor
By selecting a dewatering contractor who has experience in the type of application being performed and who is familiar with the dewatering equipment being employed, you are able to minimize the guesswork. Many pump manufacturers offer dewatering services and have dedicated personnel available to set up, operate and tear down a job properly.

Majid Tavakoli, Vice President of Applied Products at Thompson Pump and Mfg. Co. Inc., recommends selecting a dewatering contractor who has completed a minimum of five projects or has at least 10 years of experience in dewatering. A qualified dewatering contractor will ask many questions of the application before preparing a dewatering plan.

The process starts with examining the jobsite, noting the overall layout and dimensions at ground level. It is important to identify any existing underground utilities. Refer to the engineering as-built plans, if they are available, to locate underground utilities and avoid excavating too close. Also be aware of overhead power lines, trees or other obstacles that may prohibit large machinery from installing dewatering equipment.Measure and understand the dimensions and depth of the excavation and identify other equipment, such as cranes and excavators, which may be necessary.

Soil conditions are crucial to developing a dewatering plan. If possible, obtain a geotechnical report from the engineers. This report communicates site conditions and recommendations such as groundwater level, recommended dewatering level, soil boring or the characteristics of the soil and water movement, and any known contaminants. Soil conditions have a great impact on selecting the type of pump used for dewatering.

Pumping Considerations
Once the jobsite has been surveyed by the dewatering contractor, it is time to develop the dewatering plan. If electricity is available on site, then electrically driven pumps are a cost-efficient option. It is important to ask if there is an electrician available for wiring. If electricity is not accessible, then diesel-driven pumps are the best option. Regardless of the power option selected, 24-hour pump watch may be required to ensure that the pumps continue to run and keep the excavation dry.

The pumping method will be determined by the depth of the excavation. Wellpoint dewatering is the recommended method for dewatering an excavation up to 18 ft deep. Wellpoint dewatering is the process of temporarily removing groundwater from soils in a localized area to accomplish the construction. Wellpoints are a series of shallow wells installed at a pre-determined depth around the excavation. Wellpoints are installed through various techniques including jetting by high-pressure water, using a casing or punch, augers or drills, or installing by hand. The wellpoints are connected to a header pipe at the surface which is connected to a rotary wellpoint pump. Dewatering by wellpointing is considered one of the oldest methods of practical pre-drainage. Any project where the groundwater level is close to the surface is a good application for wellpoints. Rotary wellpoint pumps are specially designed to handle a high percentage of air within the water that is drawn in from the ground material.

Excavations deeper than 18 ft can be dewatered using deep wells and submersible pumps. Submersible pumps are especially good at overcoming suction lift capabilities because they push the water up from the bottom of the excavation rather than pull the water up from the surface like wet-prime and dry-prime trash pumps. Other dewatering methods include educator systems, and ground freezing methods.

It is important not to overlook the discharge method when developing the dewatering plan. Careful attention should be paid to the location at which the groundwater is being discharged to know whether any filtering or processing will be required. If filtration is required, tools such as filter bags, sediment tanks or active treatment systems may be employed. It is important to be prepared for how the water will affect the environment and have an erosion control plan. Make sure to check for any permit requirements relating to discharging pumped groundwater.

Lindsey Thorp is the Marketing Manager for Thompson Pump, a full-service manufacturer and provider of heavy-duty portable pumps, pumping equipment and engineering expertise. Contact information: lthorp@thompsonpump.com.