Old River Crossing - Tunneling Shaft Depressurization Design

Malcom Drilling Company, Victoria Island, CA

Worked with Malcolm Drilling to value engineer an alternative shaft construction method that deviated from the “wet construction” method specified in the contract documents. The 100-ft deep launching shaft penetrated and was underlain by multiple confined aquifers, each with groundwater levels close to the ground surface. Malcolm Drilling’s proposed construction method required depressurization of the underlying confined aquifers to ensure integrity of the base of the shaft during construction. Complex alluvial subsurface conditions along with stringent drawdown and settlement restrictions required development of a three-dimensional groundwater flow model to assess the depressurization effects in the multiple soil units.

Implementation of our depressurization design resulted in groundwater levels and ground settlements within our original estimates which were considerably lower than the design team’s conservative estimates. Malcolm Drilling was allowed to proceed with its construction method, which provided the project owner with an improved final product while significantly reducing Malcolm’s construction costs.

City Creek Center- Dewatering Design

Big D Construction and Malcom Drilling Company, Salt Lake City, UT

Developed a Groundwater Control Plan and dewatering design for the City Creek Center project which was one of Salt Lake City’s largest construction project in the last 25 years. The project consisted of a mass excavation occupying one city block and extending 70 ft below grade and 25 feet below the local groundwater table. Alternating coarse and fine-grained soils in conjunction with existing building foundations and multiple shoring methods did not allow implementation of traditional deep dewatering wells.

By integrating passive groundwater control effects of the shoring, construction sequencing, and various vacuum wellpoint systems the excavation was successfully dewatered without delaying shoring and excavation production. A deep confined aquifer also required groundwater control measures as certain structural excavations extended into the aquitard. A depressurization system consisting of deep wells was implemented to control uplift pressures on the subgrade completed within the aquitard.

Sound Transit Capitol Hill Station – Dewatering and Depressurization System Design

Jay Dee Construction and DBM Contractors; Seattle, WA

Designed a Groundwater Control System to depressurize and dewater the subsurface station box during tunneling and station construction phases. The temporary station box served as a portal for launching and receiving tunnel boring machines used to construct side by side running tunnels. Temporary shoring for the station box consisted of soldier pile and lagging installed around the perimeter of the excavation (540 ft long by 100 ft wide). The excavation extended about 85 below grade which exposed a 35 ft confined aquifer with a piezometric head of about 60 ft above the bottom of the excavation. The base of the aquifer coincided with the proposed subgrade therefore requiring depressurization and complete dewatering of the confined aquifer.

A system of deep wells installed around the perimeter of the excavation provided initial depressurization and partial dewatering of the confined aquifer which allowed the excavation to progress to within 20 ft of the proposed subgrade. A vacuum wellpoint system installed through the shoring wall dewatered the remainder of the confined aquifer allowing the excavation to reach final subgrade.

Bravern Construction Dewatering System Design and Monitoring

Skanska and Malcom Drilling Company; Bellevue, WA

Worked with the shoring contractor and shoring designer to value-engineer a shoring/dewatering system for a city-block excavation that extended 90 ft below grade. Groundwater levels had to be lowered 30 feet to implement the proposed soil-nail shoring system. Strategic placement of deep dewatering wells working in conjunction with a wellpoint system at the base of the excavation eliminated wells within the interior of the excavation which were originally deemed necessary by the Project Owner’s geotechnical consultant.

Implementation of our depressurization design resulted in groundwater levels and ground settlements within our original estimates which were considerably lower than the design team’s conservative estimates. Malcolm Drilling was allowed to proceed with its construction method, which provided the project owner with an improved final product while significantly reducing Malcolm’s construction costs.

Brightwater Treatment Plant Construction – Groundwater Control Assessment and Oversight

King County Wastewater Division and Hoffman Construction, Woodinville, WA

Evaluated the groundwater control requirements to complete mass grading and deep excavations of a new 36 MGD reclaimed wastewater treatment plant situated on a 43 acre site. Designed dewatering and depressurization systems for the mass and structural excavations which were incorporated into a prescriptive dewatering specification. Dewatering/ depressurization systems included deep pumped wells, vacuum wellpoint systems, and eductor/ejector wells.

Headworks and Secondary Activated Sludge Facility – Third Party Construction Dewatering Evaluation

Orange County Sanitation District, Huntington Beach, CA

Evaluated Contractor’s designed dewatering system as well as the Contractor’s proposed supplemental systems for a 300-ft by 900-ft, 33-ft deep excavation that could not be dewatered. The evaluation identified that the initial system did not account for the limited aquifer thickness below the excavation and the Contractor’s proposed approaches would also prove ineffective. Additionally the evaluation identified excessive confined groundwater pressure from deeper aquifers that were likely contributing recharge to the excavation and would have destabilized the subgrade once exposed.