Recent Dewatering Projects Completed by VEI Contracting

The main newsletter article this month (found here) discussed the benefits of leveraging the Hydraulic Profiling Tool (HPT) to collect more and better site data used in designing and installing effective dewatering systems. This blog post highlights just a few recent examples of dewatering system projects that VEI Contracting has successfully designed, installed and operated.

Project #1: Road Reconstruction project in Guelph, Ontario

A vacuum wellpoint system was installed to temporarily lower the groundwater level for a road reconstruction project in Guelph. The groundwater was impacted with volatile organic compounds (VOCs) and the extracted water required treatment prior to being discharged to sanitary sewer. The groundwater removed with the wellpoint system was first directed into a sediment weir tank and then pumped through a filter bag vessel unit followed by granular activated carbon (GAC) vessels. Working around the network of existing underground and overhead utilities made the installation of the wellpoints more challenging than anticipated.

Over the three (3) week duration of the project, VEI extracted, treated and discharged an average of approximately 140,000 liters per day (Lpd).

Figures 1 and 2: The Vacuum Wellpoint System and Header Pipe (left) that Extracted Impacted Groundwater and Discharged it to a Sediment Weir Tank (right) Prior to Further Treatment

Figure 4: Extracted Groundwater Treatment was Completed via Filter Bags Followed by Granular Activated Carbon (GAC) Vessels Prior to Discharge to the Sanitary Sewer

Project #2: Main-Line Trunk Sanitary Sewer Rehabilitation

The project work included the rehabilitation and repair of approximately 100 meters (m) of existing main-line gravity sanitary sewer infrastructure that had settled and become partially deformed. To prevent future blockage and failure of the trunk sanitary sewer, it was necessary to complete repair and replacement works.

The hydrogeological report indicated that the invert of the sanitary sewer extended to a depth of just over 6 m below road level, into or close to an underlying confined sand aquifer with significant hydrostatic pressure acting on the base of the proposed trench excavation. Due to the upward hydraulic gradient from the confined sand aquifer and potential for uncontrolled flow and instability at the base of the trench excavation, it was necessary to lower the groundwater level along the entire proposed alignment prior to excavation works for the proposed repairs.

To better identity aquifer depth and geological characteristics, an aquifer pumping test was required. A series of five (5) initial deep wells were installed using sonic-rotary drilling techniques. Two (2) continuous soil cores were completed during the drilling program to allow visual identification of any geological interfaces and stratification. Following appropriate well development and yield testing, a constant-rate aquifer pumping test was completed to confirm aquifer parameters and the design of an appropriate and cost-effective temporary groundwater control system for the proposed rehabilitation and repair works.

Due to the pressures in the underlying confined sand aquifer, as well as the identification of some dense silt layers and geological transitions causing perched groundwater conditions just below the sanitary sewer invert elevations, the temporary groundwater control system was installed including deep wells with submersible pumps and deep 50 millimeter (mm) vacuum wellpoints installed in a pre-cut trench at an elevation within the limits of suction lift about 5 m above the trench invert. The systems were activated by noise-attenuated diesel gensets due to the proximity of nearby residences, with main-duty and standby duty vacuum wellpoint pumps. Discharge from the pumping system was clear and free of suspended solids, discharging through an open-tank weir tank to allow visual and analytical confirmation of water quality prior to discharge to the approved storm sewer receptor.

VEI staff successfully installed, operated and maintained the deep well and vacuum wellpoint pumping works. Following drawdown to below the trunk sanitary sewer invert elevations, the excavation and repair works were successfully completed within the scheduled timeframe by the contractor.

Over the five (5) week duration of the project, steady-state discharge rates reached up to approximately 900,000 Lpd following the initial drawdown period.