North American Society for Trenchless Technology (NASTT) NASTT’s 2016 No-Dig Show

Dallas, Texas March 20-24, 2016

TM1-T2-04

Clark Public Utilities Uses HDD for Dual Crossings of the in Southwest

Kimberlie Staheli, Ph.D., P.E., Trenchless Project Manager, Staheli Trenchless Consultants, Seattle, WA Brent Gruber, P.E, Project Manager, Murray, Smith & Associates, Inc., Vancouver, WA Brendan O’Sullivan, P.E., Technical Advisor, Murray, Smith & Associates, Inc., Portland, OR Joel Staheli, Construction Manager, Staheli Trenchless Consultants, Seattle, WA Barry Lovingood, P.E., Owner Project Manager, Clark Public Utilities, Vancouver WA

1. ABSTRACT

Clark Public Utilities (CPU) is a regional water and electricity provider located in Southwest Washington. CPU is currently developing a groundwater well field on the west bank of the East Fork of the Lewis River with no connection to their existing system. CPU proposed to cross the river with two parallel HDD crossings from the existing well field on the west bank of the river to Paradise Point State Park on the east side of the river. The first pipeline is 1,200 LF of 24-inch diameter HDPE raw water transmission main. The second pipeline is 1,200 LF of 20-inch HDPE casing pipe with bundled backwash and communication conduit carrier pipes. The paper will describe the design process, including subsurface geotechnical conditions, design of the two HDD crossings, and the permitting challenges encountered for locating the HDD exit points within a state park. The paper also presents how the contractor elected to construct the challenging parallel bores. Details of how the contractor used a gyro system as well as how the contractor was able to deal with issues during the construction of the dual crossings will also be discussed.

2. INTRODUCTION

CPU is a customer-owned, municipal corporation organized under the laws of Washington State providing electric and water service in Clark County, Washington. In the north portion of the County, the water utility serves unincorporated areas around the City of La Center and Town of Yacolt. In the central portion of the County, CPU provides water service to urban, suburban and rural service centers. CPU also manages satellite water systems serving small developments and clusters of dwellings, as well as providing wholesale water sales to adjacent public agencies. CPU has 31,897 service connections including its main regional water system and eight satellite Group A systems. CPU obtains water for its main system from production wells situated throughout the County. There are 35 active wells with a total capacity of approximately 35 million gallons per day (mgd).

CPU is currently developing the Paradise Point Water Supply System, a regional groundwater supply project designed to meet the demand for water from the growing communities in northern Clark County. The water source for this project is a well field located at the confluence of the East Fork and the North Fork of the Lewis River. The well field will be developed to a build out capacity of 10,000 gallons per minute (gpm). A water treatment plant (WTP) will be constructed east of Interstate 5 near Paradise Park Road and NW La Center Road, about one mile from the well field. Well water will be conveyed from the well field to the WTP in a 24-inch diameter raw water pipeline. Approximately 9,000 gpm of treated water from the WTP will be delivered thru a pipe network consisting of proposed 24-inch and existing 18-inch diameter water mains to the Cities of La Center, Ridgefield, and Battle Ground with each city receiving approximately 3,000 gpm.

Paper TM1-T2-04 - 1 The schedule for developing the Paradise Point Water Supply System calls for construction of the well field, transmission mains and WTP in stages through 2020. The initial construction phase included the drilling of a single production well and site grading for the well head building pads and WTP filter backwash settling ponds. The proposed Phase 1 Raw Water Transmission Main is the latest phase to be constructed and is the main topic of this paper. Future phases including the well field development, construction of the WTP, and remaining raw water transmission main and the pipeline projects will continue through 2020 as funding allows.

The Phase 1 Raw Water Transmission Main project includes final design and horizontal directional drill (HDD) installation of approximately 1,200 feet of 24-inch diameter raw water (unfiltered well water) transmission main, and a 20-inch diameter casing pipe for the bundled pullback of an 8-inch diameter WTP backwash pipeline and communication conduits between the well field and Paradise Point State Park, crossing land owned by the Washington Department of Fish and Wildlife (WDFW). Pipeline installation will continue south from the HDD termination an additional 3,300 feet, utilizing open trench construction across land owned by the Washington State Department of Transportation (WSDOT) and Paradise Point State Park, as shown in Figure 1.

Figure 1: Project site location.

3. HYDRAULICS, PIPE SIZING AND PIPE MATERIAL

Based on the build-out flows for the well field of approximately 10,000 gpm, and WTP operations, CPU engineering staff determined the pipe diameters for the raw water pipeline and the backwash pipeline to be 24-inch and 8-inch diameter, respectively. The 24-inch raw water pipeline will flow at a maximum velocity of 7.0 feet per second when operating at full build-out conditions. A pipe material evaluation was performed to determine the most cost effective pipe materials that were compatible for the proposed trenchless and open-cut installation methods for the pipelines and the pipeline design pressure criteria for the project.

A transient analysis was performed without surge protection during emergency shutdown of all pumps. The analysis assumed HDPE piping for the river crossing and ductile iron piping for the remainder of the alignment. The transient analysis indicated maximum surge pressures greater than 450 psi and minimum pressures below -14 psi causing

Paper TM1-T2-04 - 2 column separation and potential pipe cavitation. Combination surge critical, vacuum relief-air inlet and air release valves were recommended along the pipe alignment as required to mitigate the transient pressures. These mitigation measures were installed with the next phase of the raw water pipeline construction through the park road.

An analysis of soil resistivity was also performed as part of the pipe material evaluation to establish the recommend corrosion control measures suitable for the soil conditions. The analysis revealed the soils to be non-corrosive to moderately corrosive along the proposed pipe alignment connecting the well field to the WTP. The pipe used for the HDD crossing of the river was HDPE eliminating the need corrosion protection. Ductile iron pipe and fitting installations on the next phase of raw water and backwash pipeline construction will include polyethylene encasement.

During design it was determined that the 8-inch diameter backwash pipeline would be accompanied by two 2-inch diameter communication conduits. The design team considered pulling a bundle of the 8-inch and two 2-inch pipelines; however this option presented the risk of losing one or both of the communication conduits during pullback. It was determined that this risk could be eliminated if a casing was installed that would house the three pipes, presenting a lower project risk compared to the option of an HDD pulling back a bundle of the three pipe. To house the three pipes a 20-inch diameter casing was specified. In addition, the 20-inch casing pipe provided the potential benefit of providing a conduit for raw water pipeline should future wellfield development or emergencies necessitated its use.

Based upon the pipe material evaluation, the design team recommended that CPU bid both fusible PVC DR 18 and HDPE DR 9 pipe for the horizontally directionally drilled (HDD) pipeline portion of the project. Ultimately, HDPE was used for the installation of the 24-inch diameter raw water pipeline and the 20-inch casing pipe,

4. PERMITTING

One of the most challenging aspects of the project was the permitting process and designing the pipeline to meet the conditions imposed by the permitting agencies. There were a total of eight permits obtained for the construction of this project, issued by agencies that included the US Army Corps of Engineers, Washington State Department of Ecology, Washington State Department of Fish and Wildlife, Washington Department of Transportation, Washington State Parks and Clark County, Washington. Due to the site layout restrictions, the HDD rigs and staging areas were located in the wetlands area. This proved to be the most challenging conditions to permit, combined with the necessary coordination of the river crossing Joint Aquatic Resources Permit Application (JARPA) which defined an allowable HDD work window due to the East Fork Lewis River having a classification as a bearing stream. The JARPA permit work window provided minimal overlap with the allowable work start date established by State Parks in order to minimize impacts to park users, forcing the Contractor to complete work during a short time period.

5. GEOTECHNICAL CONDITIONS AT THE SITE AND ALONG THE PIPELINES

To determine subsurface conditions in the project area, a mud rotary drill (Boring B-1) collected vertical information to a depth of approximately 100 feet. This boring was completed on the southern bank of the East Fork Lewis River in the vicinity of the proposed entry point of both drills. Boring B-1 encountered approximately 35 feet of very loose to loose silty sand/sand with silt, underlain by medium dense to dense silty sand/sand with silt to the maximum depth explored (Apex, 2014). Some woody debris was encountered between EL -14 and -24 feet, and inter-bedded silt lenses were found throughout the soil column. This boring was compared to previously completed studies for the Paradise Point Well Field on the west side of the river (Columbia West Engineering, 2010) and was found to be in good agreement. Borings conducted as a part of that study also encountered loose silty sand underlain by medium dense sand and silt; however, the main difference between the west and east sides of the river is that the geotechnical conditions on the west side transitioned to the denser material approximately 15 feet deeper than on the east side of the river. Groundwater was encountered on both sides of the river within 5 feet of the ground surface, and may be as high as the ground surface elevation depending on precipitation and seasonal variance.

Paper TM1-T2-04 - 3 6. GEOTECHNICAL CONDITIONS AND COMPATIBILITY WITH TRENCHLESS METHODS

The medium dense to dense sands and silts encountered below EL -19 (eastern side) to -34 feet (western side) are highly suited to pipeline installation using HDD as they are highly stable and easily transportable by the drilling fluid, resulting in a stable borehole with minimal risk of settlement, hydro-fracture, or borehole collapse. The near-surface very loose to loose sands and silts are less desirable, but still considered feasible for HDD construction. The lower strength material will be more prone to borehole collapse, leading to the possibility of settlement and/or hydro-fracture above the borehole near the entry and exit points where depth of cover is low. These risks are expected to decrease as the bore deepens, regardless of material density, due to the increased overburden above the borehole. However, it may be difficult for the driller to steer within the softer material even where depth of cover is high. For this reason, we recommend that the bore be designed to enter the medium dense to dense material as soon as possible, and to stay within this unit while beneath the East Fork Lewis River.

In the near-surface soils, it was prudent to assume that some minor borehole collapse would occur in the vicinity of the entry and exit points. This was planned for and addressed by specification to ensure that the Contractor had appropriate materials on site to clean up or mitigate any settlement or hydro-fracture which might occur. Alternatively, the design team considered specifying the use of conductor casings to eliminate the potential for hydro fracture or settlement as it would bridge the poor soils until a greater depth of cover was encountered. Conductor casings are oversized steel casing pipes that isolate drilling fluids and allow circulation to the entry pit, supporting the borehole while preventing soil collapse or blowouts at the surface. Conductor casing may be installed using a pneumatic hammer or auger bore depending on near-surface conditions. Once installed, drilling commences from within the conductor casings. It was recommended that only one conductor casing be installed, on the entry side, if required, as the use of two casings significantly increased the complexity and cost of the crossing. This option was further investigated during the design phase of the project and deemed unnecessary based on soil conditions.

Site constraints and the depth of the competent soils would have made it extremely difficult to case the bore into the denser soils, but a casing of at least 100 feet in length would allow for sufficient overburden above the borehole as the drill exited the casing to mitigate against surface settlement and hydro-fracture. The groundwater level was not considered problematic for drilling, as drilling fluid pressures can easily be adjusted to account for a varying groundwater table. Therefore, although some mitigation was required in the upper portions of the bore, the geotechnical conditions indicated that HDD was a feasible installation technique for the pipelines. The design of the bores focused on staying within the denser soils in the portion of the alignment beneath the East Fork Lewis River where borehole stability was critical.

7. HDD DESIGN

A preliminary profile was developed based on the geotechnical information and the relative suitability of the soils. the majority of both bores were located within medium dense to dense soils below EL -19 to -34 feet. To enter into these soils rapidly, the bores entry was established at an angle of 16 degrees, transitioning after approximately 100 feet of straight section into a curve with a 1,000 foot bend radius. The base elevation of both bores is approximately -48 feet, resulting in a depth of cover of 50 feet below the deepest portion of the river. The flat section of the bore traversed beneath the river before beginning the curve back to the surface where it was designed to curve upward on a 1,000 foot bend radius to exit at an angle of 14 degrees. This resulted in a total bore length for each bore of approximately 1,150 feet. Figure 2 shows the design profile.

Paper TM1-T2-04 - 4 Figure 2: Design Profile for Both HDDs

A hydro-fracture analysis was performed based on the soil and fluid mechanical properties and the cavity expansion theory (Luger and Hergarden, 1988). The cavity expansion model and its application to HDD are described in detail in Bennett and Wallin (2008). The model was developed to establish the maximum allowable pressure that can be applied to a given soil without exceeding the confining stresses in the soil allowing a cavity to expand. At a maximum pressure, a fracture occurs in the soil and the drilling fluid escapes from the expanded cavity. This maximum allowable pressure is expressed using the equation shown in Equation 1.

Equation 1: Maximum allowable pressure (Explanation of each variable in Equation 1 can be found in Staheli, 2010).

Figure 3 shows locations along the proposed alignment where hydro-fracture has a high likelihood of occurrence due to pressures created during the pilot bore. The blue line indicates the maximum allowable pressure calculated using assumed soil properties and the proposed bore geometry. The red line indicates the minimum pressure required to return drill through the native soils and return the drilling fluids and cuttings to the entry point where the excavated material is cleaned and then re-circulated for continued drilling. Where the blue and red lines are in close proximity to each other, the factor of safety (FOS) against hydro-fracture is low, and where the red line rises above the blue line, the FOS is less than 1.0 and hydro-fracture is expected to occur.

Due to the depth of the bore, the risk of hydro-fracture occurring is extremely low for this crossing, the exception being near the entry and exit locations where the minimum and maximum pressures are within less than 10 pounds per square inch (psi). This is normal for any HDD bore, but may be slightly exacerbated for this particular case due to known loose soils in these locations.

Although the drilling fluid is non-toxic and typically 97-99% water, it will still be a priority of the project to protect the project site and the East Fork Lewis River against inadvertent fluid returns. The Contractor was be required to a submit a hydro-fracture contingency plan as a part of their work plan that they are contractually obligated to follow in

Paper TM1-T2-04 - 5 case of drilling fluid release to the surface or within the river. As a part of the contingency plan, the Contractor was required to maintain drilling fluid clean up equipment on site at all times, including straw or hay bales, silt fencing, sand bags, shovels, and brooms. A vacuum truck must be available at all times to access the site within one hour of a call.

Figure 3: HDD Hydrofracture Analysis

8. HDD CONSTRUCTION (STC AND MSA)

Pilot Bore – HDD 1 The first of the two bores began on July 13, 2015. The contractor elected to use a Universal 250 x 400 drill rig with 20 foot drill pipe for the construction of the pilot bore as shown in Figure 4. The bore was guided by a gyro guidance system that was operated by SlimDrill. MiSwaco was also on site to assist with the drilling fluids to optimize the drilling mud and viscosity for the soils to be encountered during the bore.

Figure 4: Universal 250 x 400 Drill Rig

A jet assembly with a mill tooth bit was chosen by the Contractor. The downhole bit was 9-7/8 inches. The drill bit also had a 2-degree bend in the total down hole assembly (DHA) that measured 19.16 feet as shown in Figure 5.

Paper TM1-T2-04 - 6

Figure 5: Jetting Assembly with Mill Tooth Bits

Calibration of the gyro proved to be difficult on the initial pilot bore. After several attempts on the rig, the gyro was calibrated on the surface. After this was completed, the gyro still showed that the drilling rig was three degrees left of the planned bore alignment. Rather than starting and trying to “correct”, the SlimDrill operator recommended that the bore path be resurveyed to ensure that the gyro was calculated correctly.

On July 14, 2015, the survey crew reshot the elevations along the first bore path and the gyro was calibrated by 10:15 am. At this time, the pilot bore construction began. The contractor was able to install 19 (20 foot) drill pipes for a total length of 380 feet. The drilling times were rapid and averaged 7.9 feet per drill pipe or 24 seconds a foot.

During the construction of drill pipe 6, approximately 120 feet from the entry point, drilling mud migrated to the surface. The Contractor and Inspector walked the alignment but did not see any further drilling fluid on the surface. Drilling continued past this point; however, the frac-out location had healed and no additional drilling mud came to the surface. During the construction of pipe 13, where the pipeline crossed the East Channel of the Lewis River, no drilling fluid migrated to the surface.

On July 15, the second day of the first pilot bore 839 feet of drilling was completed by drilling 23 drill pipe. Drilling was rapid, as with the first day until drill pipe 23 (approximately 460 feet into the bore) where the ground became significantly harder than it had been, requiring 18 minutes to complete a 20 foot drill pipe. Soil conditions appeared to remain denser than in the original surficial soils for pipes 23 through 27, where drilling times ranged between 13 and 18 minutes per drill pipe.

On drill pipe number 39, 780 feet into the bore, the gyro was having difficulty north seeking. As a result, the bore veered out of the planned alignment by 5 feet of line. The drill was encountering soft soils and the Contractor was able to “dead push” the drill pipe without any drilling fluid flow. In approximately two feet, the soils became firmer and the downhole steering was effective allowing for correction to line and grade.

On the final day of the pilot bore for the first HDD, 571 drill pipe were installed, completing the bore, removing the DHA and leaving 18 feet of drill pipe in the bore pit and 12 feet at the exit location. The last day of pilot hole production resulted in an average 10.6 minutes per foot. When the drill bit was approximately 40 feet from the exit locations, inadvertent drilling fluids appeared at the surface. Although the Contractor tried to keep mud pressures low, drilling mud continued to flow at this area due to the low overburden pressure.

The pilot bore for the first bore exited at a location that was approximately 18-inches to the left of the planned exit hole and 12-inches beyond the designed exit hole. A total of 1,410 feet of pilot hole drilling was completed in 3 days.

Paper TM1-T2-04 - 7 Reaming of Bore 1 – 20-inch The reaming of Bore 1 was performed in two stages to achieve a final bore opening of 36-inches in diameter. Stage 1 reaming utilized a 24-inch diameter reamer and progressed at an average rate of 392 feet per day and was completed in three days. Stage 2 of the reaming operation utilized a 36-inch diameter fly cutter (open-design clay reamer) and the 24-inch Stage 1 reamer as a centralizer to complete the ream in 3 days, averaging 470 feet per day. During the reaming process the drilling fluid flow to the reamer was 200 gpm and was controlled from the HDD rigs on both ends. Immediately following completion of the 36-inch ream pass, the 24-inch reamer was again repurposed and pulled back thru as a swab pass in preparation for pipe pullback.

Pullback of Bore 1 – 20-inch HDPE The pullback of Bore 1 was constructed with 20-inch HDPE, DR 9. The pullback was started on August 1, 2015 and was completed in approximately 8 hours. Fluid returns were at the exit side of the drill until drill pipe 45, approximately 900 feet in the bore, where the returns swapped to the entry side. During the pullback, the Contractor had to stop the operation briefly to allow the soil separation plant to catch up with the extremely fast rate of pipe insertion. Shut down for removal of cuttings with the separation plant typically lasted less than 20 minutes.

Pilot Bore – HDD 2 The second pilot bore began on August 4, 2015. Like the first bore, a 9-7/8-inch jetting assembly was used for drilling with a 2-degree bent sub having a total length of 78 inches. The gyro monel was 105-inches, and the sub behind the monel was 35-inches. The DHA was a total of approximately 22 feet.

During the beginning of the pilot bore, on drill pipe 4, drilling fluid escaped very close to the work area, and the Contractor attempted to use Poly Swell to heal the bore; however, the Poly Swell clogged the gyro, which took the remainder of the day to clear.

Drilling did not start in earnest until August 8, 2105 when the gyro was fixed and all equipment was ready for drilling. As with the first bore, the 20 foot drill pipes were installed rapidly through the soil formation. Steering of the drill was not difficult; however at drill pipe 18, approximately 360 feet into the bore, the Contractor reported a very soft area in the soil formation. This “soft spot” was very small and the drill recovered into the competent in less than 10 feet of drilling.

On drill pipe 25, approximately 500 feet into the bore, the north seek on the gyro failed. The Contractor elected to remove drill pipe 25, reconnect drill pipe 24, pull 24 back to the top of the rig and get a new north reading. However, the gyro was still not reporting the proper north seek reading. The contractor tripped back and removed drill pipe 23 to where the gyro numbers matched the calculated numbers, then re-drilled from drill pipe 23 to 25 without incident.

The first day of drilling was completed with the installation of 33 drill pipe, totaling 660 feet into the bore. The contractor pulled back all of the drill pipe and downhole tooling to allow the bore to sit for the one day “weekend” and secured the site.

The remainder of the second HDD Pilot bore was completed on August 10, 2015. The day began by tripping in the 33 drill pipe to get to the bottom of the hole. This was done with little difficulty and was completed in the morning. Forward drilling than began with drill pipe 34 and drilling was continued until the end of the pilot bore which was approximately 1,140 feet in length.

Reaming of Bore 2 – 24 inch The reaming of Bore 2 was performed in two stages to achieve a final bore opening of 36-inches in diameter. Stage 1 reaming utilized a 24-inch diameter reamer and progressed at an average rate of 588 feet per day and was completed in two days. During the final stage of the 24-inch reaming process, drilling mud surfaced 50 feet from the park side pit and were contained and cleaned up by the contractor. Stage 2 of the reaming operation utilized a 36-inch diameter fly cutter and the 24-inch Stage 1 reamer as a centralizer to complete the ream in 3 days. During the reaming process the drilling fluid flow to the reamer was 130 gpm and was controlled from the HDD rigs on both ends. Immediately following completion of the 36-inch ream pass, the 24-inch reamer was again repurposed and pulled back through as a swab pass in preparation of pipe pullback.

Paper TM1-T2-04 - 8 Pullback of HDD 2 The pullback of the second bore began on August 19, 2015 at approximately 1pm and was completed 20 minutes after midnight, just over 11 hours for the total pullback of the 24-inch diameter HDPE pipe. Pulling of each pipe section went very quickly; however the pulling operations had to be stopped on several occasions to allow the mud plant to “catch up” with the drilling fluid that was displaced from the bore hole. This downtime was typically only 20 to 30 minutes in duration; however, as the pull-in got closer to completion and pull forces increased, stopping for any length of time to wait on the reclaimer made the Contractor increasingly nervous.

9. CONCLUSIONS

With challenging site constraints, environmentally sensitive areas, and strict permitting conditions this project required open and transparent communication between CPU, the Engineers, and the Contractor. This project was a success because of the ability of all parties to communicate constantly and clearly during construction. The designers addressed key concerns of the permitting agencies, such as avoidance of hydro-fracture in the wetlands, proper depth beneath the river to ensure protection of the Lewis River and prediction of hydro-fracture near the entry and exit. By doing so, the designers were able to design to these critical permitting conditions, allowing the HDD process to be successful and acceptable to the permitting agencies.

9. REFERENCES

Apex (2010) Paradise Point Water Transmission Main. Geotechnical Evaluation. La Center, WA.

Bennett, R. and K. Wallin, 2008. Step-By-Step Evaluation of Hydrofracture Risks for HDD Projects, North American Society for Trenchless Technology, Proceedings of 2008 No-Dig Conference, Dallas, Texas.

Luger, H.J., and A.M. Hergarden, 1988. Directional Drilling in Soft Soil: Influence of Mud Pressures, International Society of Trenchless Technology, Proceedings of 1988 No-Dig Conference.

Staheli, K., Price, C., and L. Wetter, 2010. Effectiveness of Hydrofracture Prediction of HDD Design, North American Society for Trenchless Technology, Proceedings of 2010 No-Dig Conference, Chicago, Illinois.

Paper TM1-T2-04 - 9