The City of Santa Cruz Water Department (SCWD) operates stream diversion structures and a 16-mile-long pipe network, collectively referred to as the North Coast System (NCS) as part of its raw water collection system. A significant portion of the network is nearing, or has exceeded, its design life and the SCWD has planned to rehabilitate or replace the NCS in a series of phases.
Most recently, Phase 3 was established as a design-tender contract to address the coastal segment of the NCS Rehabilitation Project. The project consists of replacing 18,600 lf of raw water transmission main located northwest of Santa Cruz, California along Highway 1 (CA SR 1). While the primary method of construction will be open-trench methods, the project also contains five trenchless pipe installations, four of which are horizontal auger bores and one is a horizontal directional drill (HDD). To date, the installation of the HDD undercrossing has been completed following a premature failure of the existing pipe and will be the primary subject of this article.
Phase 3 is located within land owned and operated by several entities along the Pacific coast, including: California State Parks, the California Department of Transportation, the Santa Cruz County Regional Transportation Commission, as well as several private entities. A challenging portion of the Phase 3 alignment crosses Lombardi Creek within Wilder Ranch State Park. The environmental stakes were set high, as the park has strong biological, scenic, cultural, and recreational values; it also provides habitat to many native reptile, fish and amphibian species. Furthermore, several breeding grounds of the red-legged frog, a federally-listed Threatened Species, have been documented near Lombardi Creek. In recognition of these factors, directional drilling was required as the pipeline installation method for the gulch area, as it was the lowest risk option for disturbances to native vegetation, wildlife, and terrain.
At the site of the HDD installation, Lombardi Creek was within a steep 60-ft deep gulch vegetated with coastal sage scrub and riparian species. Preliminary desktop studies suggested that the full gulch width could be traversed from west to east with a 1,300-ft HDD bore. The suitability of the subsurface conditions for this work was investigated with three exploratory borings, field geologic mapping, and a geophysical survey. Once completed, the gulch was confirmed to be a buried valley consisting of more than 100 ft of sediments and the depth to rock was unknown. The deposits consisted of mostly saturated alluvial silty sands, lean clays, and gravels. The east bank of the gulch consists of a shallow layer of marine terrace sands, overlying moderately hard mudstone and soft sandstone [classifications per Caltrans (2007)]. These formation units are roughly consistent with the west bank.
Entry and exit points of the installation were established at level locations approximately 200 to 300 ft beyond the banks of Lombardi Gulch. Final design established the entry and exit angles at 9.1 and 11.8 degrees, respectively, so that a minimum of 20 ft of overburden soils would confine the bore in the lowest portions of the gulch. Contractor’s staging area, pipe string laydown space, and pit areas were obtained with coordination with the land tenants, who have lease agreements with California State Parks to operate a Brussels sprouts farm. The work was performed to minimally impact concurrent production and harvest of the Brussels sprouts.
The design placed the bore 20 to 25 ft below the gulch in an 18 ft thick silty sand layer, which was anticipated to provide the most consistent and favorable drilling material. This layer was confined by a medium-stiff lean clay. Based on the surrounding topography and buried valley geology, the undercrossing presented risks of hydraulic fracturing and inadvertent drilling fluid returns. Design calculations confirmed this risk; however final analyses with the ultimate bore alignment were left to the contractor’s engineer, using their anticipated equipment and drilling fluid properties.
The pipeline’s operational design pressure was defined as 150 psi directly upstream of the gulch. Thus, pipe selection needed to meet these demands as well as the longitudinal tensile pipe pull stresses during installation through the bore. The final design required a 24-in. diameter fusible polyvinyl chloride (FPVC) product pipe with a 235 psi pressure class (AWWA C905).
The Lombardi Creek undercrossing was originally intended to be included with the Phase 3 bid package; however, a rupture in the buried existing pipe, just west of Lombardi creek, caused a leakage of approximately 500,000 gpd and was causing localized erosion. This unforeseen event prompted the Lombardi Gulch portion of Phase 3 to be advertised, awarded, and constructed in a fast-tracked manner. SCWD preemptively secured the pipe as an insurance against delays in fabrication or delivery after an award. The project received two bids. The low-bidder was The HDD Company at $1.27 million.
One of the project team’s initial priorities was to confirm the bore alignment. Acknowledging the frac-out risks in the gulch, the contractor proposed to deepen the installation so that a minimum of 50 feet of ground cover would be obtained in the gulch. The geologic material, as encountered by borings at this location, consisted of medium-stiff clays and medium-dense clayey gravel. Interestingly, the contractor’s subsequent frac-out analysis did not show that lowering the alignment would increase the factor of safety, as drilling fluid pressures would correspondingly be larger, and thus the frac-out risk remained. Given the circumstances of the project, the proposed deepening was approved and focus was shifted to a comprehensive Frac-out and Surface Spill Contingency Plan. This document detailed procedures for monitoring, preventing, containment, cleanup and documentation procedures if spills or hydraulic fracturing occurred. Specific emphasis was placed towards preventing drilling fluid losses into Lombardi Creek.
The HDD path, as proposed by the contractor, began with an approximately 9-in. pilot-hole diameter bit, and advanced through the rock units at an average of 113 ft per day. To improve fluid circulation in the pilot bore, a mid-size reamer was pushed into the hole several times during this drilling. Progress came to a halt when the pilot bore was advancing in the critical area below the center of the gulch in the alluvial soils. Drilling fluid circulation was lost in the bore and a frac-out manifested itself to the ground surface. The project team stopped the bore and rapidly prioritized containment of the returns as well as discussed methods to minimize pressures in the hole. The returns were contained within a perimeter of silt fences, gravel bags, and wattles. Reduction in fluid circulation was immediately necessary to reduce fluid outflow through the conduit of failed ground.
To proceed with the installation, the contractor elected to mobilize the drilling rig to the exit pit and perform a mid-path intercept for the rest of the pilot drill. This change in methodology relieved the entry bore of circulation pressures until the intercept was completed. To maintain project schedule through the difficult conditions, an additional drilling rig was delivered to the site shortly after this remobilization and began push reaming the entry hole. Additional inadvertent returns surfaced at similar locations during this process once the reaming neared the location of the hydraulically-failed borehole. At this point, further efforts were directed towards containing the mud and preventing contamination of the Lombardi Creek through an installed diversion flume. A timely installation of the mitigation measures was especially critical due to the rainy climate of the installation.
To the projects benefit, the work was performed expediently and to the satisfaction of the involved parties: SCWD, The Covello Group (construction manager), the contractor, California Environmental Services (biologists), Mott MacDonald (designer), and the Department of Fish and Wildlife. Successful teamwork and a high interest enabled control of a difficult situation. With frequent coordination between those parties, the project moved forward and the bore was eventually reamed to the final diameter of 36 in. Average rate of reaming was 242 ft per day. Survey results showed that the maximum total deviation from the design alignment was 4 ft.
During drilling activities, the FPVC pipe was fused into two long pipe string segments, as the full 1,300-ft string was unable to be placed linearly on the available pipe laydown space. Joints were fused by Underground Solutions (subcontractor) at an average rate of one joint per hour. Once each string was constructed, the quality control plan required above-ground pressure testing to insure suitability for installation. The limited pipe laydown space resulted in one caveat to the testing program: a fused joint near the middle of the string was to be only tested after installation into the bore. Otherwise, the two fused pipe strings were then hydrostatically pressure tested on the ground surface and held the design operating pressure of 150 psi. The alternative use of pressurized air for pre-installation testing was discussed with the Contractor and pipe supplier, however deemed inadequate as it could neither safely reach the design operating pressure nor confirm the string suitability for pullback.
Pipe pullback was performed the day following a push and pull of a swab for the full bore length. Water was fed continuously into the pipe, filling the interior, to reduce buoyancy-induced friction in the upper crown of the bore as the first string entered the bore. The intermediate fuse was performed and the fully completed string was pulled through the bore at an observed rate of about ½-¾ feet per second with pullback loads generally kept between 48,000 and 55,000 lbs. During the pullback, drilling fluids were continuously expelled through the pre-existing frac-out conduits and surfaced directly into the containment area. Several days later, and nearly two months after the initial Contractor mobilization, the pipe string passed the in-situ hydrostatic test at 200 psi, marking a successful installation. The pipe has since been connected to the existing water main and placed into service, thereby alleviating the system of the leak. SCWD has since fully restored the site to a healthy coastal Californian ecosystem and portions of the bypassed pipe will be removed and recycled.
American Water Works Association (AWWA). C905-10: Polyvinyl Chloride (PVC) Pressure Pipe and Fabricated Fittings, 14 in. Through 48 in. (350 mm Through 1,200 mm). Denver, CO: American Water Works Association. 2012.
California Department of Transportation. Soil and Rock Logging, Classification, and Presentation Manual. Sacramento, California: Division of Engineering Services – Geotechnical Services, 2007.