WATER/WASTEWATER STRUCTURAL STRENGTHENING
In late 2014, a new gas line lateral installed on a steep 2:1 slope faced severe erosion issues due to the effects of heavy Pacific Northwest rainfall. Within weeks of installing a conventional rip-rap drainage channel, excess runoff water bypassing the faulty channel began eroding the soil near the gas line and potentially destabilizing the bedding material around the pipe.
The design goal was to direct the water down the slope and eliminate the erosion caused by shear forces from the water flow (more than 14 lbs/ft2). Per Federal Energy Regulatory Commission (FERC) requirements, the ground surface also had to remain at the same elevation during preconstruction.
Rip-rap typically requires maintenance when used in drainage channels, riverbanks and slopes. While the solution usually creates an acceptable drainage structure, it degrades easily on slopes greater than 3:1. Rip-rap also moves over time, creating instability issues and excessive vegetation growth that can lead to flow restrictions, hydraulic capacity reductions and additional limitations. These were the major causes of the problem at this site. Possible repair solutions included:
• Poured concrete. This solution caused concerns for the asset owner, since FERC codes require the slope, with its 80-foot elevation change, be returned to its normal state after construction. The steep grade made direct cement truck access practically impossible; a large pumper truck would not have had a direct laydown area to set up.
• Turf Reinforcement Mats (TRMs). TRMs require constant maintenance to ensure vegetation is healthy and meets expected hydraulic performance values. This solution was immediately eliminated because vegetation could not be established in the drainage channel to meet the acceptable shear values prior to the next storm (which could have occurred within days or weeks after repairs were completed). There were also concerns about wildlife entanglement and mowing issues that affect TRMs when they are not soil filled.
• Articulate Concrete Blocks (ACBs). Potential issues transporting ACBs up the steep slope without causing a major disturbance around the pipe – as well as the time and expense to remove and dispose of soil off site – made this option unviable.
• Geosynthetic Concrete Composite Mats (GCCMs). GCCMs offered quick installation with limited disturbances to the area, advanced hydraulic performance capabilities, and natural blending with the surrounding vegetation.
Milliken Infrastructure Solutions evaluated the channel width, depth and several dynamic external forces on the steep slope – including water movement and other live loads – to determine how to best apply Concrete Cloth GCCM, and what kind of anchoring system would be needed to hold it in place. Less than two weeks after the initial assessment, the owner approved the repair solution, and approximately 30 portable batch rolls were used.
The ability to hydrate Concrete Cloth GCCM in the field using an overflow catchment pipe was an advantage that allowed the contractor to reduce time and resources needed for installation. The area was re-vegetated to enhance aesthetics and return the slope to its natural condition, per FERC standards.
The repair met all FERC requirements and project criteria. Long-term maintenance requirements for the slope as a result of the Concrete Cloth GCCM installation were reduced to virtually nothing, versus other active systems susceptible to degradation and displacement. This functional longevity was an important benefit to the asset owner, who will not need to request access to the right-of-way from FERC for additional repairs for the lifespan of the product.
Application: Erosion Control
Client: Transmission Pipeline Company
Location: Seattle, WA
Installation: Fall 2014