Water Control - Dewatering and Unwatering
From large-scale dewatering of sites for the installation of foundations to the removal of standing rainwater in shallow contaminated excavations (unwatering), Impact Environmental has a broad expertise in groundwater management services. The industrial nature and shallow water table of our geographic service area has created many challenges in executing our services. Foremost, is the high volume of contaminated water that must be discharged onto land underlain by aquifers and/or surrounded by recreational waterways of high quality.
It is in this area, the management of contaminated groundwater, that Impact Environmental has truly distinguished itself. The regulations pertaining to the discharge of contaminated ground or wastewaters are wide-ranging and complex. It is paramount that dewatering projects involving contaminants are managed in full accordance with all of the governing regulations. It has been our experience that many projects performed by our competitors on behalf of their clients are unwittingly absent proper permits or authorization.
Considerations that are often overlooked by other firms are a cornerstone or our service. For instance, in many cases where dewatering is necessary and the groundwater is contaminated, building slabs and foundations require the installation of a soil gas vapor barrier. A vapor barrier is necessary to prohibit the migration of contaminants from the water table into overlying interior building spaces. This problem is referred to as vapor intrusion and often results in compromised interior air quality. Such conditions are the focus of a major inter-agency initiative within the State of New York by the Department of Health and Department of Environmental Conservation. A vapor barrier system is often paired with an active sub depressurization system. Compromised interior air quality could require abatement and long term air quality monitoring.
The firm has performed a variety of projects for clients such as Jet Blue Airways, the Port Authority of New York, DeFoe Construction Corp. and New Jersey and Amoco Oil/Spartan Petroleum. The primary elements of a water control project include site investigation, data collection, interpretation, evaluation & presentation, supervision and documentation.
Excavation of materials or construction near or below the water table or near surface water bodies usually requires control of the groundwater or seepage. Control may involve isolation with cutoffs, stabilization by freezing, grouting, or other methods, or by a combination of methods. Control of groundwater and seepage usually involves the installation and operation of wells or drains. A key operation in most water control in unstable materials is the removal of water from below the ground surface in advance of excavation and maintaining the water level at a suitable depth below the working surface. These steps permit construction “in the dry,” unhampered by the adverse effects of water.
Failure to properly remove or control water during unwatering or dewatering may result in unstable slopes, unsuitable subgrade, boils, uplift of structures, safety problems, construction delays and fee escalation. Where surface and ground waters are contaminated, failure to keep an excavation in the dry will lead to chemical exposure issues for labor working within excavations or ponded areas. The exposure issues will require specialized training, environmental monitoring, medical surveillance and personal protective equipment. Additionally, water control design can create third-party liabilities by exacerbating existing contaminant plume pathways and creating human exposure issues.
Understanding what factors control water is critical for ensuring short-term stability during construction of any facility and for the long-term stability of any structure. For these reasons, the design of all jobs requires a field investigation to obtain information on the site’s various potential water sources including seasonal precipitation and runoff periods and the identification and understanding of the water bearing formations within a site. This means not only identifying and determining the depths and extent of the various water bearing formations and their water surfaces but also understanding the formation hydraulic parameters (porosity, permeability, and, when necessary, storage).
In excavations in rock, cemented granular materials, clays, and other stable materials, water removal or unwatering may be by draining to sumps and using surface pumps concurrently with or following completion of the excavation. Subsurface cutoffs such as sheet piling and slurry walls in soil or soft rock and grouting in rock are also used to control ground and surface water and for ground support. Cutoffs are seldom 100-percent effective, and supplementary dewatering or unwatering is usually required. Soils are frozen for short periods such as for a temporary excavation, especially in clayey or silty soils that have low permeability and are difficult to drain.
Unwatering methods are commonly used in soils that have high porosity but low permeability (clayey or silty soils), bedrock that has solution cavities that carry large volumes of water in isolated areas. Unwatering methods are commonly used to control surface water. Unwatering usually is performed in conjunction with dewatering to ensure control of surface water and to permit dewatering to proceed unaffected by recharge or flooding from nearby surface water.
Design Data Requirements, Methods of Data Collection and Presentation
Adequate surface and subsurface data are essential to the proper design, installation, and operation of water control facilities. Surface information should include data on soil erosion or resistance or how erosion relates to runoff or the potential recharge of the groundwater system. Soil infiltration data from Natural Resource Conservation Service mapping should be included if available.
Subsurface data should include representative permeability, a real distribution of permeability, location and potential recharge sources or barriers, and anticipated seasonal changes in the groundwater system. When a project has a relatively high soil or rock permeability and the permeable formation extends laterally over a large area, storage (storativity, effective porosity) of the aquifer requires evaluation. The presentation of dewatering data may differ from the presentation of conventional geologic data because water control data are subject to a variety of quantitative interpretations and because water levels, flow, and water quality vary with time. Because of the potential for different interpretations, most dewatering data such as those from aquifer tests and packer tests are presented as observed field data and as interpretations. A complete description of the site, subsurface conditions, and test facilities should be given along with the data (figures 20-2 and 20-3). Where time related data are presented, the information should be in a form that will ensure maximum recognition and proper interpretation. Hydrographs that plot time versus water levels mean a lot more than a table of readings.
Monitoring during construction should include groundwater levels in excavations, areas surrounding the excavations, and off-site locations. Excavations of any substantial width (other than trenches) and a depth of more than a few feet below the anticipated water level may need groundwater monitoring instrumentation installed directly in the excavation. Excavations underlain at shallow depths by artesian conditions may need to be monitored because of the potential for blowout.
Groundwater monitoring instrumentation located within an excavation may interfere with construction, but monitoring groundwater levels within an excavation generally justifies inconveniences. Because the most difficult area to dewater is usually the center of the excavation, groundwater monitoring instruments should be located near the center. Special provisions may be necessary to ensure continued groundwater monitoring of the facilities during all stages of the excavation. In some cases, groundwater monitoring instrumentation may be incorporated into the structure. This may require that the monitoring instrument be embedded in concrete such as a wall or pier to permit continuous monitoring and simplify backfill operations. Water level monitoring instrumentation should be located in areas surrounding the excavation to monitor representative areas and specific problem areas. Groundwater levels in off-site locations are monitored to maintain a general record of conditions and to document dewatering.
Instrumentation for monitoring groundwater levels usually consists of several observation/monitoring wells or piezometers. The type of instrumentation, depth, and riser and hole diameter depends primarily on subsurface conditions, desired operating life, and type of monitoring. The design of the instrumentation should be tailored to the subsurface and data requirements so that measurements are a true indication of in place conditions. All monitoring wells should be constructed according to ASTM D-5092 and constructed for site conditions and purposes.
The discharge from dewatering facilities such as wells, well point systems, drains, and sump pumps should be monitored with flow meters to provide a record of the dewatering quantities. Data should include starting and stopping times, instantaneous rates of discharge, changes in rates, combined daily volumes, and, in some cases, water chemistry, turbidity, and biologic content. Sediment content of dewatering facilities including wells, well point systems, and drains also require monitoring. Sediment can damage pumping equipment, cause deterioration of water quality in a receiving water body, and create voids in the foundation that result in well collapse and foundation settlement. Sediment content usually is measured in parts per million by volume of water or in nephelometric turbidity units (NTU) in water taken directly from the discharge. If sediment yield increases rapidly, the facility may need to be shut down to avoid serious damage or contamination. The chemical and biologic content of water discharged from dewatering systems should be monitored by periodic collection and analysis of samples taken directly from the system discharge.
Samples of the discharge are secured for biologic and chemical analysis for f heavy metals, organics, and pesticides, total dissolved solids, conductivity, pH, and sediment content or turbidity. Testing needs to be customized to comply with permitting requirements from the City (NYCDEP Chapter 15) or from the State (SPDES Permit).
The ground surface and other pertinent points should also be monitored for settlement during dewatering activities.
A final construction report typically includes a section on water control that documents the chronology of dewatering and an evaluation of the performance of the facilities as well as compliance with the effluent water quality specifications. Problem areas and unusual events such as pump or slope failures are documented. Monitoring results, including groundwater quality, levels and discharge rates, are presented in the form of hydrographs and other similar plots or tabulated data.
Impact Environmental can perform the services necessary to design, mobilize and operate a dewatering system (type “A” systems).