Golden Ears Forcemain and River Crossing Project

Abstract

Metro Vancouver’s Golden Ears Forcemain and Fraser River Crossing Project involves installing two new 914.4mm diameter (36”) sewer pipes under the Fraser River. Eventually, these pipes will carry wastewater from Maple Ridge and Pitt Meadows to the future Northwest Langley Wastewater Treatment Plant. The new pipes will increase the capacity of the existing system to help ensure the continued reliable and safe management of liquid waste in a growing region and provide improved seismic resiliency.  

The project includes a 127m trenchless crossing beneath the Golden Ears Bridge on- and off-ramps, approximately 600m of open-cut forcemain, and twin parallel 1,651m – 1675m long horizontal directional drilling (HDD) installations under the Fraser River.

Several trenchless construction methods were selected to minimize the environmental impact of the project, allow uninterrupted access to river users, minimize impact of vehicle and public travel across the on- and off- ramps, limit the area needed on-land and to work below existing road and utility infrastructure.

The HDD alignment was designed to be drilled as a mid-path intercept. On the north side, the HDD area was located next to an archeological site, ecological areas and close to residential areas. Surface casings were required on either side of the borepath to support drilling.

Construction of the project considered the congested urban area, ecological areas and archaeological sites, and ground conditions. The project team worked closely to address these technical, environmental and community considerations. For technical considerations, a combination of HDD, pipe ramming, auger boring and pipe jacking techniques were used for the trenchless crossings. The HDD utilized advanced drilling and steering techniques along the borepath below the watercourse.

Additionally, the multi-section, compound curve, drag section layout required advanced modelling, accurate equipment placement, and pipe restraint during pullback. Due to the Fraser River and environmental considerations, there was no ability to run a navigation surface coil. Gyroscopic steering tools were utilized to mitigate this issue. These techniques facilitated successful completion with minimal disruption to the environment, public, archaeological sites and adjacent utilities and roadways.

Introduction

Diverting flows from Maple Ridge and Pitt Meadows to the future Northwest Langley Wastewater Treatment Plant (NLWWTP) requires new twin, 914 mm pipelines to convey wastewater flows from the new Golden Ears Pump Station on the north side of the Fraser River to the wastewater treatment plant located on the south side of the River. The new conveyance system is referred to as the Golden Ears Forcemain and River Crossing (GEFRC). The project scope includes installation of 1,600 m of twin steel pipe river crossings using horizontal directional drilling (HDD) methods, 525 m of steel and high density polyethylene (HDPE) pipe using open cut construction, and 125 m of trenchless construction for road crossings, two blowdown chambers and an underground concrete air valve chamber.

The Fraser River Crossing Project is one of the longest HDD Projects of this diameter in British Columbia. Construction of the project considered the congested urban area, ecological areas and archaeological sites, and ground conditions. The project team worked closely to address these technical, environmental and community considerations.

HDD Design

Geologic Setting

The project considered the following geologic criteria when preparing construction methodologies:

• the geographic location at the confluence of the Fraser River and Pitt River and within the low-lying Fraser River floodplain;
• the geological composition of the HDD alignment below the Fraser River, with considerations for a discontinuous sand and gravel unit that was a focus of site investigations during design;
• the north bank geologic conditions; and
• south bank ground improvement activities for the NLWWTP which included thousands of stone columns and steel plates.

The majority of the HDD alignment across the Fraser River is located in a marine/glaciomarine clay layer (a silt and clay layer with high plasticity), which was considered a good drilling material in terms of borehole stability. This layer extends from the north bank to the south bank. Other soil layers closer to surface include mixtures of sand, silt, and clay, and gravel.

HDD Alignment Design

The HDD alignment was designed with the following objectives:

• the south angle was designed to reach the marine/glaciomarine clay as quickly as possible, and to avoid the layer of steel plates from the NLWWTP ground improvement program; and
• the north angle was designed to minimize the length of the bore path crossing through the sand and gravel unit, and to have an appropriate angle to facilitate the pipe breakover during pullback.

Surface casings were included at both ends of the HDD alignment to address the ground conditions. On the north side, a 50 m surface casing was proposed to provide borehole stability in the anticipated weak soils at shallow depths and to protect a nearby archeological site. On the south side, a 75 m long casing was proposed at 11 degrees to avoid the steel plates at the bottom of the rammed aggregate piers. As the surface casing lengths make them very difficult to remove following completion of the work, the casing annulus will be grouted, and the casings left in place.

A pipe stress analysis was completed to check the pipe pull force during installation, and the pipe stresses during installation, pressure testing, operation, and maintenance conditions. The analysis showed that:

• the pipe pullback was the governing load condition, resulting in a wall thickness of 18.9 mm;
• the pipe was required to be ballasted during pullback – if not, a significantly greater pipe wall thickness was required to manage the pull loads and pipe stresses; and
• due to the magnitude of the pullback loads, a large (or maxi) rig with a pull force of 1,000,000 pounds was required.

Ramp Crossing Design

The 2.1 m diameter, 127 m long ramp crossing is located below the Golden Ears Way on and off ramps. The ramp

crossing crosses several utilities, including passing 0.5 m below an active gravity sewer. This required engagement and coordination with municipalities, property owners, transportation and environmental agencies throughout the Project planning and construction phases.

The ramp crossing serves the following two purposes:

• to provide a temporary conduit for the HDD pullback alignment below the Golden Ears Way on and off ramps; and
• to provide permanent housing for the twin forcemains within the casing.

The ramp crossing geology consists of clayey soil mixed with fine sands and inorganic silts. As the anticipated soil materials were primarily fine grained with variable clay/silt constituents, groundwater inflow was not anticipated to be a significant issue. It was anticipated that the ground may ravel over time if left unsupported.  

Four trenchless construction methods were evaluated for the ramp crossing: auger boring, pipe ramming, microtunneling, and conventional pipe jacking. These methodologies were assessed against the following three performance objectives, which were considered critical to the success of the crossing:

• minimizing the potential for ground loss induced surface settlement under the Golden Ears Bridge on-ramp and off-ramp infrastructure;
• adherence to the proposed alignment and the accuracy of the drive and installation due to the proximity of nearby utilities; and
• applicability of each method with respect to the required drive diameter and length.

From the trenchless methodology evaluation, requirements for three methods were included in the contract documents: pilot tube guided auger boring, pilot tube guided pipe ramming, and microtunneling, with selection of the methodology left to the contractor. Settlement performance requirements were included in the contract documents, which contractors were to take into consideration when selecting a methodology.

The casing was required to have a certain minimum thickness to withstand external loads, including earth pressure, groundwater pressure, and live loading from the Golden Ears Bridge ramps, as well as loads generated during construction and installation. The analysis indicated that the casing installation loads were the governing load case, and that a 28.6 mm wall thickness was required. In addition, requirements were included for the contractor to check and confirm that the pipe thickness is acceptable for their anticipated means and methods.

HDD Construction Planning and Engineering

In collaboration with the Metro Vancouver Project Team, the Project Construction Team (Pomerleau/BlueFox/The Crossing Group) optimized the crossing alignment, selected equipment types and developed execution strategy. This resulted in the following outcomes:

• an increased entry angle to reduce the amount of surface casing required on the entry side;
• an increased entry angle to target a deeper alignment below the watercourse and maintain practical steering capability of the HDD method;
• an increased exit angle to gain depth of cover more quickly while balancing the pipeline lifting and available pullback geometry for the drag section; and
• confirmation that the pipe material properties would meet the long term operational requirements as well as the complex installation geometry of the pipe lift and drag section.

The specifications of the selected steel pipeline material were NPS 36 (914.4mm O.D.), 18.8mm wall thickness, Grade 483 MPa, Dual Layer Fusion Bond Epoxy with abrasion resistant overcoat.

HDD Geometry

While surface casings were included in the design, the design of the steel casing was a Contractor means and methods item. This led to the inclusion of two telescoped surface casings, a 72" casing driven to a depth of 50 meters, followed by a 60" casing telescoped to a depth of 125 meters. The casings were installed with The Tunneling Company’s hydraulic hammer. Given the proximity to residential communities, Metro Vancouver worked with the Construction Team to address questions and concerns related to project schedules and public impact mitigation.

The HDD intersect method was selected by the construction team, with considerations for the following:

• depth of cover;
• length of final ream diameter;
• desire for entry and exit surface casing isolations; and
• increased operational efficiency during pilot hole drilling through reduction of the total distance needed to be drilled by a single rig

The Crossing Company’s primary HDD rig, an American Augers DD1100, with 500,000 kilograms (1.1 million lbs) of push/pull capacity was setup on the south side of the Fraser River, within the footprint of the future wastewater treatment plant expansion. This area was also used as the temporary pipe lay down yard. Due to the width of the watercourse, a gyroscopic steering system was required for most of the crossing, which was supplemented with a Paratrack II electromagnetic steering system on the river shores.

To address the ground conditions, TCC selected special tooling for both pilot hole and reaming operations as follows:

• for the pilot hole a two-cone jetting assembly, which can advance through soft ground conditions with minimal need to pump drilling fluid, was utilized.;
• the reams were completed using a fly-cutter with a barrel reamer directly behind it; and
• the fly-cutter first opens the hole while the barrel reamer behind it pushes the cuttings into the borehole wall, resulting in a stronger hole less susceptible to sloughing.

For these crossings, TCC opened the hole in two reaming passes rather than the conventional three passes to minimize the amount of time spent reaming. With the soft ground conditions, it was important to minimize the reaming passes to avoid over-excavation caused by excessive pumping of drilling fluid. Additionally, during operations TCC ran a 10-3/4” barrel reamer in between reaming passes. The barrel reamer was used to reduce high rotary torque that could have resulted in stuck pipe. TCC utilized a customized bentonite and polymer-based drilling fluid program to improve the boreholes stability and improve hole cleaning while drilling and reaming.

Pipeline Lifting and Drag Section

To reduce friction and avoid pipe damage, the pipe needed to enter the ground at an angle equal to the exit angle of the borehole. To achieve this, the lead section of the pipeline needed to be raised into a curve, requiring the use of lifting equipment fitted with pipe cradles. Lifting equipment positioning and loading was detailed in the design drawing to limit pipe stress to 80% of specified minimum yield strength (SMYS).

In order to reduce the pull force required to install the pipelines, the use of a detailed buoyancy-control program was recommended by BlueFox.  Filling the submerged section of the pipeline with water significantly reduced the maximum pull force required for installation.  A water-filled pipe condition during pullback reduced the estimated pull force to 410,000lbs. To execute buoyancy control, TCC utilized a custom plan which involved the installation of HDPE into the longer section, and pumping of water through the HDPE into the product line as each joint of pipe was pulled in. The buoyancy control was successful in keeping pull forces low.

The congested urban environment only allowed for approximately 500m of available workspace up to the highway on- and off- ramps, with another 986m available thereafter, requiring the use of the ramp crossing mid-pull. Even with the ramp crossings, the pipe was longer than the allowable workspace, requiring a single weld mid-pullback. The pipeline drag section layout had to be designed accordingly.

Pipeline rollers, excavators, cranes, cradles and additional equipment were required throughout pipeline drag section layout, in particularly at inflection points to maintain pipe position during pullback. Due to the limited space available for entry and exit ramps into the ramp crossing, additional equipment and personnel was required to maintain pipe position as the pipe travels through the casing.  

The terrain along the drag section layout had to remain largely undisturbed, which meant that the pipeline drag section had to follow a series of horizontal, vertical, and compound curves in between existing infrastructure along the layout. This added a significant level of complexity to an already complicated project.

In order to achieve this layout, BlueFox Engineering performed a Finite Element Analysis (FEA) of the drag section layout in order to meet the topographic requirements, but also to provide direction as to strategic equipment placement in order to restrain the pipe position as it was being pulled into the HDD borehole. The pipeline pullback was performed in the following six phases:

• removal of the secondary HDD rig on the north bank after the final reaming pass was complete;
• pull of the first 70m to position the pipeline for lift, using a large bulldozer and a series of pulleys;
• lift of the front section of the pipe using six large cranes, allowing the pipe to match the exit tangent angle;
• pull of the first 150m of pipe into the borehole;
• approximately 24-hour pause as the mid weld was completed, cured and inspected; and
• pull of the remaining 1,500m of pipeline into the borehole.

The pipe pullback was executed successfully for both river crossings in September and December, 2022.

Ramp Crossing Construction

From the methods of ramp crossing construction proposed in the contract, the Construction team chose a combination of pilot tube, auger bore and pipe ramming in order to install 127m of 2133.6mm (84”) diameter steel casing. While ultimately successful, the construction of the ramp crossing realized several risks throughout.

During the installation of the ramp crossing, settlement monitoring points were utilized at regular intervals to assess the progress and impact of the pipe ramming. While the initial installations of the pilot tube and 36” casing had minimal impact, during the installation of the 84” casing sloughing was observed at the exit side, close to existing sewer infrastructure. In a risk mitigation measure, the proximal works plan was revised to include a contingency bypass system and continuous CCTV monitoring while the casing installation crossed underneath the utility.  

In addition, halfway through the installation, the auger string used to clean out the casing experienced significant reductions in efficiency due to the change in soil formation. The following responses were undertaken in an effort to prevent impacts to the critical path:

• a mini excavator was used inside the casing to manually remove internal soils;
• the installation continued until the 110m mark, where the utility monitoring points on the gravity sewer indicated that heaving was occurring;
• operations were paused until the soil plug could be reduced manually, during which time the excavated materials were found to be much firmer than previously experienced;
• once operations resumed, it was found that the previous push force of 1,500,000lbs was no longer capable of moving the assembly;
• to free the assembly of the soil friction, a hydro-hammer was mobilized to site, providing 80kJ strikes at 45bpm; and
• despite this, the assembly did not move, and two alternate means were developed: mobilization of a  much larger hammer, and open cut construction to assemble the last two lengths of casing.

The mobilization of a much larger S-150 hammer to drive the last length forward was chosen. To assist in the hammering operation, the smaller hydro-hammer was attached to the exit side and welded to the exit assembly. Field modifications were made to accommodate the S-150, as the original jacking shaft had been designed for smaller tooling. Using both hammers in tandem (a combined push/pull of ~210kJ @ 40bpm), the remaining length of casing overcame the built-up skin friction, and the installation was successfully completed.

Summary

The Fraser River Crossing Project was a unique and challenging project. Construction of the project considered the congested urban area, ecological areas and archaeological sites, and ground conditions. The project team worked closely to address these technical, environmental and community considerations

Ultimately, the flexibility and adaptability of the project team and the contractor led to a successful installation of the project, from redesign of the vertical alignment immediately before HDD drilling to the implementation of the two-stage pullback strategy.

A project summary video is available here: https://f.io/3G3b-ZH6

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