In the summer of 1985, development was under way for the construction of an intersection at I-15 and Clairemont Mesa Boulevard located in San Diego, California. As part of the construction, a new MULTI-PLATE structural plate culvert was installed.
The 326-ft long, 132-in. diameter pipe was designed with variable gages (steel thicknesses) such that the inlet and outlet ends of the structure were a lighter gage than the pipe, which was under more significant fill heights. The ground sloped at a 1.5:1 ratio all the way up to the roadway elevation, which was 70 ft above the crown of the pipe at the centerline. It is common to vary the gage on structural plate pipes throughout a culvert system where fill height changes significantly (e.g. heavier gages in the center of the culvert where the fill is greater).
The MULTI-PLATE culvert performed well for several decades. However, over time, scour became noticeable at the inlet end. The pipe was built on a 2.8 percent slope, and it operated at an abrasion level 3 or 4 per FHWA’s classification system. The scour and higher abrasion levels caused pitting and slight perforations to form at the invert, as well. The increased abrasion to the invert of the inlet end came under scrutiny in 2017 and was scheduled for maintenance. Unfortunately, a huge storm event came through the area causing a critical failure to the first 40-ft section at the inlet due, in large part, to the lighter thickness of the first 40-ft section coupled with the highly abrasive bed loading.
The California Department of Transportation (Caltrans) District 11 had to react quickly and decisively. The field investigation confirmed that only the 40-ft portion of the 132-in. diameter storm drain located within the city right of way had collapsed. The collapse caused an area of slope to fail that supported the northern side of the Clairemont Mesa Boulevard. Not only would that section need replaced, but they also needed to limit the amount of impact to the remaining portion of the roadway directly above the culvert. Caltrans immediately began working on a solution to design and construct the necessary repairs.
Their initial assessment was positive. Outside of the first 40 ft, the remaining section of the culvert was still in good condition. Rather than go through a costly excavation and replacement of the entire culvert, they determined that they could tear out the front section of the inlet, replace with a concrete pipe section, and then reline the entire culvert for a complete repair to the entire storm drain. They determined the soil arching effect common to MULTI-PLATE pipe under this much fill was still performing as designed. Still, they wanted a fully structural solution with a reline pipe that could carry the intended loads and not crack if movement occurred or change properties over time. Caltrans opened the project up to bid to determine the best product solution to repair the entire length of the culvert.
After bids were received and evaluated, a contract for the work was awarded to a local contractor, S&B Engineering Inc. based in San Diego. Caltrans investigated pipe material options and determined that a steel reinforced polyethylene pipe (SRPE) product manufactured by Contech Engineered Solutions would be the best and most viable approach.
The DuroMaxx SRPE liner pipe was ideal for this solution as it adhered to the both AASHTO MP 20 and ASTM F2562 material specification and featured proven joints that would keep the grout from leaking into the pipe during the grouting stages. Not only was the lead time a critical facet in the decision-making process, but the material and installation cost-savings were also quite substantial when compared to other alternatives. The smooth interior of the liner pipe provided a pipe that was hydraulically efficient, capable of Manning’s “n” value of 0.012. The hydraulic requirements of the project allowed the use of a slightly smaller new pipe diameter of 120 in. This diameter made the sliplining process even easier.
The steel reinforced polyethylene (SRPE) liner pipe was manufactured with 80-ksi tensile strength steel reinforcing ribs, which provided the inherent strength, while the pressure rated polyethylene (PE) resin provided the durability.
They were able to reline the entire length of the 326 LF culvert with DuroMaxx SRPE liner pipe. The pipe was manufactured in 20-ft sections at the Contech manufacturing facility located in Ogden, Utah, and delivered to the site within the desired three-week turnaround time. Longer lengths are typically used but the 20-ft lengths allowed for efficient shipping. To aid in the grouting and bracing process, 2-in. diameter grout ports per piece of pipe were installed at the Ogden plant. Grout ports were placed in predetermined locations of each piece of pipe. The purpose of the grout ports was to allow the contractor to monitor the grout level during grout installation, pump grout through any of them, and to enable screw jacks to hold the liner pipe in place during grout installation using a floor beam and screw jack method that is common with segmental sliplining. Skid rails were also installed on each piece of pipe to aid in joint alignment and minimize sliplining friction.
Once submittal documents and shop drawings were approved, S&B began the installation process. They cleaned, dewatered and inspected the existing host pipe and prepared for the liner pipe to be sliplined directly into the entire length of the 326 LF culvert. An important part of the preparation work included laying two continuous and parallel plastic pipe runners along the invert of the host pipe. These runners would act as rails, which the liner pipe rested on during the pushing process. The rails insured proper liner pipe alignment and elevation while decreasing the friction between the host pipe and the new pipe. Pipe lubricant was brushed on the runners ahead of the sliplining process.
The liner pipe was pushed through the new inlet pipe and through the remaining pipe. The grout used was a standard sand-cement-water grout that had a plasticizer admixture and had a fluid unit weight of about 115 pcf. The grout unit weight produced buoyant forces applied to the liner pipe during the grouting process that had to be managed. The beams and screwjacks, along with another buoyant force countermeasure strategy, which involved grout placement in four separate lifts, were used. Staging the lifts allowed enough time for the grout to achieve initial set. The first grout lift was only 8 in. deep. After the grout was 8 in. deep at the highest end of the pipe, the contractor filled the remaining void in three additional lifts.
The project went much better than expected and was completed under budget. The new culvert will easily provide in excess of 100 years of maintenance free service.