Projects

Project Details

HDD

• Twin 2,600′ bundles under Mare Island Strait
• 12″ rec’d water, 22″ sewer, two 4″ conduits
• Two 18″ water, two 4″ conduits

Geotechnical Conditions

• Young Bay Mud over Old Bay Mud
• Claystone bedrock
• Possible contamination from Naval Yard

Challenges

• Soil to rock interface
• Hydrofracture risk in soft surficial soils
• Proximity of bridge piles for causeway

Project Overview

The City of Vallejo and the Vallejo Flood and Wastewater District collaborated on a project to install various utilities to from Vallejo to Mare Island under the Mare Island Strait.  The utilities to be installed were a 22-inch sewer force main, a 12-inch reclaimed water main, two 18-inch water mains, and four 4-inch conduits for fiber optic cables. Ground conditions included 40 to 50 feet of very soft Young Bay Mud underlain by denser Old Bay Mud and claystone bedrock.   

Preliminary design evaluated various trenchless methods to install the various utilities, including microtunneling and horizontal direction drilling.  Due to the length and required separation of the sewer and water lines, parallel twin 2,600-foot by 48-inch diameter HDD bores were recommended to install the utility bundles. The design solution used the intersect HDD method with HDD rigs on each side.  Approximately 150-feet of conductor casing was required on each end of the bores to maintain circulation and reduce hydrofracture and settlement risks in the very soft Young Bay Mud. In addition to the challenging ground conditions, BTE carefully selected alignments which did not endanger bridge piles and fender piers of the Mare Island Causeway. Soil and groundwater contamination were also significant concerns due to the Navy’s operations at the former Mare Island facility.

Bennett Trenchless Engineers helped to plan the geotechnical investigation, evaluated results and implications for construction, prepared technical specifications and drawings, conducted design calculations to evaluate and mitigate risks associated with hydrofracture, conducted settlement calculations, evaluated pipe stresses during pullback, and provided engineering services during construction, including on-site specialty construction inspection.

Project Details

HDD

• 24″ steel casing for four 6″ and one 4″ PVC conduit for electrical supply and SCADA
• 1,475′ crossing of Sacramento River levees

Geotechnical Conditions

• Loose to dense silty sand
• High groundwater

Challenges

• Levee crossings – flood control issues
• Limited work area on one side
• Moved rig to exit side for pullback
• Close proximity to established residences

Project Overview

Bennett Trenchless Engineers provided trenchless assistance for the design of a power supply crossing of the Sacramento River. The crossing provided electrical power from an existing substation in the Pocket neighborhood of Sacramento to the new South River Pump Station on the Lower Northwest Interceptor Project, west of the Sacramento River. The crossing consisted of a 1,475-foot long, 24-inch diameter steel casing pipe containing four 6-inch PVC conduits for the electrical conductors and one 4-inch PVC conduit containing fiber optics for a SCADA system.  The casing passed beneath flood control levees on both sides of the river.

The design of the crossing required special sequencing to minimize disruption to the residents in the Pocket neighborhood. The contractor was required to both drill the bore and fabricate the pipe on the west side of the river. Only after the bore was ready was the contractor allowed to mobilize the drill rig to the neighborhood on the east side of the river to pull the casing and conduits into place. The contractor was also required to contact grout the ends of the bore to prevent seepage of floodwaters along the bore annulus beneath the levees.

Bennett Trenchless Engineers provided preliminary design services, developed a risk matrix to select alignment, conducted pullback and pipe stress calculations, provided drawing edits, drafted technical specifications, prepared a Hydrofracture and Surface Spill Contingency Plan, provided planning level cost estimate, reviewed contractor’s submittals, and provided technical permitting support for California State Lands Commission lease. 

Project Details

HDD

• 18″ HDPE, DR 9 water main
• 1,700′ crossing of CA Hwy 163

Geotechnical Conditions

• Claystone
• 200-linear feet fault gouge (highly plastic clay and claystone chunks)

Challenges

• Environmentally sensitive river
• Ground pose risk of drilling fluid loss
• Pipe layout area at oblique angle to bore
• Steep topography near exit

Project Overview

As a subconsultant to GHD, Inc., Bennett Trenchless Engineers provided trenchless design services to relocate a water main crossing of the Mad River near Blue Lake, CA.  The existing water line was attached to a local railroad bridge that was no longer maintained by the rail company and was falling into disrepair.  The feasibility analysis evaluated replacement with a new pipe bridge or installation by trenchless construction methods.  HDD was the preferred construction method due to lower estimated construction cost and reduced permitting requirements.  The ground conditions included extensive near-surface gravel, cobble, and boulder deposits within the river channel; claystone bedrock for most of the bore length; and a 200-foot wide fault gauge comprised of highly plastic clay and sheared bedrock. 

The primary challenge on the project was understanding the fault gouge and its effects on hydrofracture risk.  Other challenges included minimizing impacts to the steep, densely vegetated work area at exit while accommodating the pipe layout area which was at a severe angle to the bore.  A nearby railroad ROW that roughly paralleled the river was used for pipe fabrication and layout.  The bore geometry was designed at an oblique angle across the river and incorporated a slight horizontal curve near exit to reduce the angle between the pipe layout and the bore.  BTE evaluated trenchless construction alternatives, evaluated alignments, evaluated pipe materials, conducted design calculations for hydrofracture risk and pipe stress and pull load, provided technical specifications and drawing edits, evaluated submittals, and provided on-site specialty construction inspection services.

Project Details

HDD

• 12″ coated steel recycled water pipeline
• 2,960′ crossing of Griffith Park
• 510 ft of elevation difference entry to exit

Geotechnical Conditions

• Predominantly granodiorite and silty shale
• Landslide deposits near exit
• Groundwater

Challenges

• Severe elevation difference
• Constrained work area at exit
• 17 intermediate welds
• Limiting impacts to park usage

Project Overview

As a subconsultant to GeoPentech, Bennett Trenchless designed a 2,960-foot HDD bore for installation of a 12-inch steel recycled water pipeline to supply a new recycled water storage tank at the top of Griffith Park. The design of the HDD bore was especially challenging due to the 510-foot elevation difference between entry and exit. Ground conditions consisted of predominantly granodiorite with Monterey Formation (silty shale), with a section of potentially unstable landslide deposits near the exit point.  The available work area near the exit point was severely limited due to dense vegetation and steep topography, resulting in a maximum pipe layout area of approximately 175 feet.  The resulting design required 17 intermediate welds during pullback. 

Bennett Trenchless evaluated the feasibility of HDD, conducted hydrofracture risk analyses, provided input to planning level cost and construction schedule estimates, prepared drawings, provided technical specifications, and co-authored a Geotechnical Baseline Report for the trenchless components of the project.  Bennett Trenchless also provided engineering services during construction, including submittal review, response to RFIs, and full-time on-site specialty trenchless construction inspection.  Construction was successfully completed on the HDD bore in the late summer of 2017.

Project Details

HDD

• 12″ FPVC water main in 16″ FPVC casing
• 733′ crossing of Delta Coves Breach levees
• Intersect method used

Geotechnical Conditions

• Medium dense to dense silty sand
• Stiff silty clay
• Groundwater level above entry/exit

Challenges

• Constrained work area in residential area
• Deep low-permeability levee cores
• Stone columns beneath levees

Project Overview

Bennett Trenchless Engineers provided specialty construction management services for BKF for installation of a 12-inch FPVC water main inside a 16-inch FPVC casing beneath the water access for Bethel Island using HDD.  The project provided water services from Diablo Water District to a new waterfront residential development on Bethel Island.   The trenchless crossing passed beneath the waterway access to Sand Mound Slough for the future Bethel Island residents.  The waterway, which had levees on both sides, had a water level approximately 15 feet higher than the ground elevation outside the levees.  The levees had been built on top of 50-foot deep slurry walls to reduce permeation seepage.  The levees were further supported in the weak surficial soils with a grid of 3-foot diameter stone columns that extended approximately 20 feet below the natural ground level.  The trenchless crossing was designed beneath the stone columns and penetrated the slurry walls on both sides of the crossing.  Conductor casings were necessary on both sides of the crossing to maintain bore stability and ensure thorough annular space grouting to prevent seepage along the bore annulus.  The southern side of the crossing was located in a residential community; maintaining traffic through the work zone resulted in highly constrained work areas.

Diablo Water District requested a specialty inspector for the project to ensure that the pipeline would be installed to their specifications.  BTE provided specialty trenchless inspection services to the design engineer, BKF. BTE reviewed contractor’s submittals for the HDD components and responded to RFIs related to the trenchless work. BTE provided full-time on-site specialty inspection services for the conductor casing installation, pilot bore, reaming, pullback, testing, and annular space grouting operations. On-site inspectors monitored down-hole pressure data, pull loads and pipe stresses, and for inadvertent drilling fluid returns.  The project was successfully completed in November 2016.

Project Details

HDD

• 14- and 16-in HDPE brine intake and effluent pipelines
• Twin 1100-ft crossings of Toro Creek
• Mitigate impact to extensive 100-year flood zones

Geotechnical Conditions

• Soft clay transitioning to hard rock
• Extensive gravel and cobbles
• High groundwater

Challenges

• Constrained work areas and narrow easement
• Mitigate impact to nearby residence
• Difficult geotechnical conditions

Project Overview

BTE served as a trenchless subconsultant to Ashley and Vance Engineering, Inc. and Water Systems Consulting, Inc. for the design of a 1,100-foot HDD crossing of Toro Creek.  The 14-inch and 16-inch pipelines were influent and effluent lines for the Cayucos Wastewater Reclamation Facility.  Project challenges included very loose to loose soils on one side of the crossing and hard rock on the other.  BTE provided trenchless specifications, performed trenchless design calculations, developed cost estimates, provided a surface spill and hydrofracture contingency plan, and reviewed submittals and RFIs during the construction phase of the project.

Project Details

HDD

• 16-inch HDPE water pipeline
• 400-foot crossing of Pilarcitos Creek

Geotechnical Conditions

• Alternating layers of stiff fat clay and medium dense to dense sand with clay and clayey sand
• High groundwater

Challenges

• Permeable soils
• Mitigate impact to existing utilities
• Work area constraints

Project Overview

BTE served as a trenchless design sub-consultant to Erler & Kalinowski, Inc. (EKI) on the design of a water main replacement project in Half Moon Bay, CA.  The existing line crossed above Pilarcitos Creek, attached to the deteriorating Main Street Bridge.  CCWD relocated the 16-inch pipeline under the creek, downstream of the bridge, prior to the bridge being renovated.  During preliminary design both microtunneling and HDD were evaluated as feasible replacement method for the 400-foot creek crossing, however HDD was chosen as the trenchless construction method.  Project challenges included high permeability sands and direct groundwater recharge from the creek, and constricted work areas in the developed urban setting of downtown Half Moon Bay.