A Canadian creek gets geosynthetic reinforced soil walls to prevent large-debris flooding.
By Alex Strouth, Mark Pritchard, David Roche and Calvin VanBuskirk
Picturesque Fitzsimmons Creek flows through the heart of Whistler, B.C., Canada—about 80 miles north of Vancouver, and it was one of the sites for the 2010 Winter Olympics. Few visitors to Whistler realize that Fitzsimmons Creek poses a debris flood risk to the village. To address this potential danger, geosynthetic-reinforced soil (GRS) was used as a critical structural component in the construction of a debris barrier that now protects Whistler from the damaging effects of a large-debris flood.
Designed to protect
The Fitzsimmons Creek debris barrier design incorporates a GRS structure to channel debris and to support a steel arch barrier that spans across the waterway. The design allows sediment, fish, and kayakers to pass beneath the steel arches during normal flows, while trapping surges of boulders, logs, and other debris that could threaten the community during large-debris flood events. The GRS abutment walls rise vertically up to 14 meters above the final ground surface (17 meters above the foundation).
Angled GRS walls on the upstream side of the structure are positioned to absorb debris flood impact and funnel debris toward the center of the steel arch. The GRS walls protect vulnerable steel components from debris impact and resist impact and erosion from boulders and trees.
Downstream, vertical GRS walls form an abutment for the left (looking downstream) steel arch structure foundation. The GRS abutment is designed to retain stored debris and resist horizontal forces transferred from the steel arch legs during debris impact.
The GRS wall system’s flexibility allows it to accommodate abrupt changes in face alignment, slope, and footing elevation. This flexibility helped designers minimize encroachment of the abutment structure into the channel while still providing adequate bearing resistance against static and dynamic design loads. GRS flexibility also enabled on-the-fly modifications, to accommodate unexpected site conditions, without delaying construction.
How a GRS works
GRS is a term used to describe reinforcement of compacted granular soil with closely-spaced layers of geosynthetic textiles (or grids) to form a composite material of higher strength than soil alone.
When used for walls, a strong connection between the geotextile reinforcement and the wall facing is not required because the GRS facing elements are primarily a construction aid and facade. The wall facing is required only to resist the construction-induced compaction loads and active soil pressure that develops between reinforcing layers.
GRS systems are distinct from externally-supported soil retaining systems, such as mechanically-stabilized earth (MSE) that typically use stronger, but more widely-spaced, reinforcement elements connected to a rigid facing. The flexible GRS facing and reduced importance of connection between the facing and reinforcement facilitate construction and allow for a design that can be easily adapted to site conditions. Additionally, the self-stable nature of GRS is compatible with applications like debris barriers where impact and erosion forces could damage the wall facing elements.
Fitzsimmons Creek design
The GRS facing elements, reinforcement spacing, and construction sequence used at Fitzsimmons Creek have been used for numerous retaining wall, soil arch, and bridge abutment applications across Western Canada in roadway and railway applications.
Welded wire mesh forms were used as facing elements, with cobbles placed in the forms within 1meter of the face, to retain the GRS fill during construction. The open facing forms allowed simple integration of elements that pass through the face such as drains, extensometers, and tieback anchors for adjacent concrete works.
Each facing form is #4 gauge galvanized weldmesh. A high-strength woven polypropylene geotextile was placed as reinforcement at the bottom and middle of each form.
The GRS structure contains many irregular corners, variable face-slope angles, and varying foundation elevations along the walls. The flexibility and adaptability of the GRS composite system to site conditions was fully realized during construction by bending and cutting the facing elements at corners, altering the setback distance of facing elements at each level to create different slope angles, and clipping the tops of facing elements to create a horizontal lift regardless of irregular or varying foundation elevations.
A crew of three, after only a few hours of instruction, was able to assemble the facing and place the fabric reinforcement. GRS facing elements were placed, trimmed, bent and connected using hand tools, and geotextile reinforcement was rolled out, trimmed, and placed in the orientation most convenient for construction. This was possible because the geotextile reinforcement has similar strength and stiffness characteristics in both the machine direction and cross direction, and the GRS design does not require mechanical connections between adjacent sections of geotextile or between the geotextile and the facing. Fill was placed with an excavator and then compacted near the wall face.
GRS adaptability was most clearly demonstrated at the upstream end of the structure where the GRS wall tied in to a bedrock slope. The tie-in point between the GRS and bedrock was field-fit to minimize bedrock excavation and vulnerability of the tie-in point to erosion. Wire mesh facing elements were trimmed with hand tools to match the profile of the exposed bedrock, and the grouted GRS facing was anchored to the bedrock using rebar dowels.
Where the face of the GRS composite is exposed to erosion and debris impact, it is protected by grouting of the 1m-thick, cobble-filled zone. The grouted zone was constructed by sealing the welded wire facing with a minimum 8cm thickness of shotcrete, and then injecting grout into the cobbles.
The innovative use of GRS for the Fitzsimmons Creek debris barrier allowed the construction of a barrier at reduced cost, in a shorter time, with minimal environmental impact and using considerably less concrete than conventional designs.
The work highlights a number of advantages of GRS.
Design flexibility. It can be designed to create almost any shape, corner, or slope angle, and allows grouted cobble facing erosion and impact protection.
Construction adaptability. It can be easily “field fit” to adapt to site conditions, such as unexpected and irregular foundation elevations.
Ease of construction. Other than soil, the GRS system uses three components that can be hand assembled by a small crew of laborers with only a few hours of instruction.
Minimal construction footprint. Minimal laydown area and use of heavy equipment reduces environmental impact.
Cost savings. Less expensive than other concrete and steel design options that were evaluated, the GRS system construction was completed on time and within budget.
Since completion in the fall of 2009, annual inspections and instrument readings have been carried out. Performance of the GRS walls has been excellent.