Projects

Project Details

Pilot Tube Guided Auger Boring

• 500′ of 18″ VCP gravity sewer
• 2 drives in densely travelled corridor

Geotechnical Conditions

• Very stiff to hard sandy fat clay
• Moderately hard sandstone

Challenges

• Densely developed urban neighborhoods
• Constrained construction window and hours
• Tight line and grade tolerances
• PTGAB in rock required drilling modifications

Project Overview

Bennett Trenchless Engineers provided design review services to Ross Valley Sanitation District for their trenchless installation of 500 feet of 18-inch VCP gravity sewer by pilot tube guided auger boring (PTGAB).  BTE identified several areas where the design could be improved to increase constructability, decrease traffic impacts, and to ensure that qualified trenchless subcontractors bid on the project.  The project had many logistical challenges including tight utility corridors; a very short construction window to minimize traffic impacts to the nearby elementary school; constrained work areas at all shaft locations; and minimizing impacts to the neighborhood. 

The largest challenge on the project was the anticipated geotechnical conditions which included very stiff to hard sandy fat clay and moderately hard sandstone.  Typically PTGAB construction is used in displaceable soil conditions.  BTE recommended additional geophysical investigations to characterize the interface between the soil and underlying bedrock; identified advancements in PTGAB methods for non-displaceable soils; researched qualified, experienced contractors; evaluated the constraints of the method; and proposed design changes to optimize the project.   BTE provided trenchless contractor pre-qualification requirements, and evaluated pre-qualification packages.  BTE also provided engineering services during construction as a subconsultant to the construction management consultant, Psomas, including submittal review,  responding to RFIs, and full-time specialty construction inspection during PTGAB work.

Project Details

Pilot Tube Guided Boring

• 19 drives totaling over 5,800 feet
• 18-24-inch VCP gravity sewer

Front Steer Guided Boring

• 7 drives totaling over 2,000 feet
• 20-inch HDPE gravity sewer

Geotechnical Conditions

• Medium stiff to stiff lean clay
• Groundwater up to 10′ above crown

Challenges

• Highly constrained work areas
• Limited work hours
• Groundwater infiltration and disposal

Project Overview

As a subconsultant to West Yost and Associates, Bennett Trenchless Engineers provided specialty construction inspection for over 5,800 feet of pilot tube guided boring for the Grayson Creek Trunk Sewer project.  The project included over 9,200 total feet of 18-inch and 24-inch gravity trunk sewer pipelines in Pleasant Hill, California.  The project was  located within residential neighborhoods and along heavily travelled corridors with multiple existing utilities.  Trenchless construction solutions were specified to minimize impacts to residents and traffic for construction of the 20-25-foot deep sewer.  Additional crossing features included a Bureau of Reclamation 96-inch storm drain with minimal clearance, two creeks, and an archaeologically sensitive area.  The trenchless construction included 19 pilot tube guided boring drives and 7 front steer guided boring drives, with a total of 35 shafts. Project challenges included highly constrained work areas with overhead utilities, short construction windows, groundwater infiltration into the shaft , and groundwater disposal.  Automated, real-time vibration monitoring was installed on houses  near launch and reception pits to assess whether  construction activities were creating vibrations that could possibly damage nearby houses.

Bennett Trenchless provided full-time on-site construction inspection services for the pilot tube construction and evaluated the contractor’s VE proposal for use of front steer guided auger boring.  

Project Details

Pilot Tube Guided Auger Boring

• 281′ of 24″ steel casing for 16″ PVC recycled water
• Caltrans ROW crossing of Hwy 101

Geotechnical Conditions

• Surficial medium dense clayey sand fill
• Soft to medium stiff silt at pipeline horizon

Challenges

• Caltrans encroachment permit
• Mitigation of settlement risk to highway
• Constrained work areas
• Minimize impacts to residences and businesses

Project Overview

Bennett Trenchless Engineers provided trenchless design services to North Marin Water District for 281 feet of pilot tube guided auger boring beneath Highway 101 in Novato, CA.  The 24-inch steel casing was installed for 16-inch DR 14 PVC water pipe to provide the sole connection between service areas on opposite sides of the highway.  Challenges included minimizing impacts to the businesses and neighborhoods near the shafts, threading the needle between multiple subsurface structures (soundwall footings, electric vault), and selection of pipe elevation to minimize risks presented by ground conditions, including settlement risk beneath Caltrans ROW.  The pipe alignment was selected within soft to medium stiff silt, above the underlying sandstone, with groundwater at approximately pipe invert. 

BTE made recommendations for the geotechnical investigation, reviewed the geotechnical data, and provided feasibility analysis to determine the most appropriate trenchless construction method.  Based on the selected method, BTE performed a settlement evaluation to identify the appropriate pipeline horizon to mitigate settlement risks to Highway 101.  Final design services included drafting technical specifications, providing drawing edits, providing technical assistance acquiring the Caltrans Encroachment permit, and review of construction submittals.  The project was successfully completed without any observed settlement of Highway 101.

Project Details

Pipe Ramming

• 30″ steel casing for 10″ water
• 215′ crossing of US Hwy 50

Geotechnical Conditions

• Clayey gravel
• Extensive cobbles and boulders
• Perched groundwater

Challenges

• Challenging ground conditions
• Coordination with Caltrans
• Tight work areas

Project Overview

Bennett Trenchless provided trenchless assistance for  the re-design of a critical US 50 crossing for Aerojet Corporation, after Aerojet’s initial proposal had been rejected by Caltrans.  The original design was for a 30-inch auger bored crossing at shallow depth; the revised design installed the 30-inch casing via pipe ramming at increased depth to minimize settlement and heave risks to US 50.   The geotechnical investigation conducted by Kleinfelder, Inc. indicated the presence of extensive gravel, cobbles, and small boulders  which can pose significant challenges for many trenchless installation methods.   A feasibility analysis was conducted to determine a recommended construction method that could handle the challenging ground conditions while minimizing ground movements.   The heave calculations conducted for the project used a new approach developed specifically for the challenges presented by the project. 

The revised design recommended the use of pipe ramming as the installation method.  In addition, it was recommended that the alignment be deepened to reduce the risk of ground movement, which was crucial to securing the Caltrans encroachment  permit. As a condition of the permit, Caltrans insisted that BTE serve as specialty inspectors and conduct settlement and heave monitoring to ensure protection of US 50 and existing utilities. The project was successfully completed without damage to Caltrans facilities in 2010.

Project Details

Guided Auger Boring

• 12-in cement-mortar lined and coated steel water pipeline
• 250-ft crossing of UPRR tracks
• 20-in steel casing with passive magnesium anode cathodic protection

Geotechnical Conditions

• Mixtures of sand, silt, and clay with generally medium stiff and medium dense consistencies
• Groundwater ~10 feet below pipe invert

Challenges

• Strict construction schedule requirements
• Mitigate construction impacts to community
• Mitigation of settlement risk to railroad

Project Overview

As a subconsultant to Carollo, Bennett Trenchless  Engineers provided trenchless design services to the City of Sacramento  for 250 feet of pilot tube guided auger boring beneath Union Pacific Railroad (UPRR) tracks in Sacramento, CA.  The 20-inch steel casing was installed for 12-inch cement-mortar lined and coated steel water pipeline to maintain adequate water supply and system pressure.  The only feasible launching and receiving locations for trenchless installation of the new pipeline were an industrial laundry parking lot and a community baseball field.  Challenges included strict construction schedule requirements to avoid impacts to the baseball field during  baseball season, multiple existing utilities and surface features resulting in constrained alignment alternatives, minimizing impacts on the community, and necessary coordination with UPRR to avoid delays.  The pipe alignment was selected to be above groundwater in medium stiff and medium dense mixtures of sand, silt, and clay to mitigate settlement risk to the UPRR embankment and tracks.

During preliminary design BTE reviewed the geotechnical data, performed design calculations, analyzed feasible trenchless alternatives, and provided recommendations for the most appropriate trenchless construction method.  BTE also provided final design services including drafting technical specifications, providing drawing edits, developing a settlement monitoring plan, providing technical assistance to acquire the UPRR encroachment permit, and review of construction submittals.  The project was successfully completed on schedule and without any observed settlement of the railroad tracks.

Project Details

Open-Shield Methods

• 54-in and 36-in FRP gravity sewer pipeline
• 2,900 feet of trenchless in two crossings
• 72″ and 54″ steel casing

Geotechnical Conditions

• Primarily hard clay and marlstone
• Some very loose to loose poorly-graded gravel transitioning to loose to dense clayey gravel with sand
• Groundwater encountered in some locations

Challenges

• Mixed face conditions in some locations
• Risk of over-excavation and creation of voids

Project Overview

BTE served as trenchless design sub-consultant to Kimley-Horn on the design and construction of SAWS’ E-20 sewer replacement project.  The project provides increased capacity in the E-20 sewer segment through the installation of over 13,000’ of new 54” and 36” fiberglass gravity sewer.  BTE provided trenchless design services for over 2,900’ of the pipeline to be constructed using open-shield methods.  The trenchless reaches involve installation of 72” and 54” steel casing or tunnel liner plate for containment of the gravity sewer pipe.  Ground conditions along the majority of the alignment consist of stiff to hard clay soils and marlstone bedrock which favor hand mining or open-shield pipejacking methods.  At the crossing of Salado Creek, near the downstream end of the pipeline, saturated sandy and gravelly conditions lead to the recommendation of watertight microtunneling methods.  BTE provided review of the anticipated ground conditions and provided recommendations for appropriate trenchless methods at each crossing location. 

Project Details

Auger Boring

• 20-inch water pipeline
• 350-foot crossing of Interstate 8
• 48-inch steel casing

Geotechnical Conditions

• Weak granodiorite rock with boulder-sized corestone material
• Silty sand and fill

Challenges

• Avoid impact to existing utilities
• Difficult geotechnical conditions

Project Overview

BTE served as the trenchless subconsultant to Michael Baker International for the design of a 350-foot crossing of Interstate 8 in Lakeside, CA.  BTE specified open-shield pipejacking for the installation of 48” steel pipe to serve as casing for the 20” water line.  Alternative excavation head options were allowed including use of a Robbins SBU-M. Ground conditions at the site consisted of weak granodiorite rock with floating, boulder-sized corestones of granitic rock with strengths as much as 10 times that of the surrounding material.  BTE helped to establish contractual baseline ground conditions to be used for bidding purposes and for resolution of any DSC claims.  During construction the contractor proposed to use auger boring methods in lieu of the specified method.  When the conventional auger boring methods ran into difficulties with the corestones, the equipment was augmented with a Robbins SBU-A to complete the work.  The baseline statements regarding anticipated conditions within the granodiorite were successfully used to resolve the contractor’s subsequent DSC claim.

Project Details

Auger Boring

• 36-inch steel creek bypass pipeline
• 215-foot crossing of Monterey Drive

Geotechnical Conditions

• Hard sandy fat clay over andesite
• Groundwater at casing invert
• Loose sand/soft clay behind thrust block

Challenges

• Mitigate impacts to creek in work area
• Constrained work areas
• Difficult geotechnical conditions
• Short construction season

Project Overview

As a subconsultant to BKF, Bennett Trenchless provided full design services for the auger bore installation of 215 feet of 36-inch steel pipeline under Monterey Drive.  The pipeline provided a bypass for dry-weather flows of Santa Rosa Creek to allow for rehabilitation of the existing parallel 96-inch vortex pipeline.  Trenchless construction was necessary to limit impacts to Monterey Drive, which is a major commuter thoroughfare located on an approximately 12-foot high embankment which paralleled the creek.  Bennett Trenchless conducted a feasibility study to determine appropriate trenchless methods, drafted trenchless technical specifications, provided drawing edits, and provided an estimate of construction cost and duration.   

Challenges on the project included mitigating construction impacts while working within the creek channel on both sides of the crossing, including a nearby fish ladder; challenging geotechnical conditions including a gravel/rock contact; low depth of cover on the jacking side resulting in reduced jacking capacity; protecting the concrete aprons on the embankment; and a compressed construction season in which to construct both the bypass pipeline, rehabilitate the existing pipeline, and create the gated bypass structure.  Coordination between the owner, the rehab team, and the trenchless team was critical in the project’s success.