City of Regina installs shoreline protection system
as new lagoon is constructed

Raking the concrete into the geoweb and applying a broom finish.

Stantec Consulting Ltd. was retained to design a new sewage lagoon for the City of Regina. The planned dimensions of this lagoon were about 450 m in length by 170 m in width. The exterior slopes of the lagoon cell were to be constructed at 4:1 and the interior at a 3:1 slope. The lagoon was to be oriented in an east to west direction. Due to the average winds expected in Regina and based on previous experience, a shoreline protection system was required to accommodate the wave action.

In order to adapt to the challenging elements, Stantec Consulting Ltd. recommended a solution based on a number of factors: price, installation, and flexibility. Consequently, the Geoweb Cellular Confinement System, manufactured by Presto Geosystems^® of Appleton, Wis., was selected for its performance history and ease of construction.

The City of Regina Wastewater Treatment Plant was initially constructed in the early 1960s. The first phase consisted of a primary and secondary treatment operation, complete with a series of lagoons. These lagoons contain the secondary effluent from the sewage treatment operation. All lagoons utilize the native soil, which is a Regina Clay, as the containment medium.

The original lagoons were constructed without erosion control measures in place.

Some post construction solutions created challenges for the treatment plant operations crew. The concrete rubble areas, placed on the top three metres of the slopes, were difficult to access and permitted extensive growth of vegetation. Poured in place concrete sections were expensive to install and cracked extensively. The cracked concrete sections, because they were not restrained, tended to slide downslope and the poured in place concrete also allowed growth of vegetation through the cracks.

Alternative solutions were considered when the design of Lagoon 2A was undertaken.

Geosynthetic Materials

Geosynthetics have evolved into a very diverse and important group of construction materials during the past 35 years. They are produced from a range of base polymers, most notably: polyester, polyethylene and polypropylene. These polymers are characterized by their high flexibility, low weight, high strength and outstanding durability. Through the evolution of geosynthetic products, their form and construction has been adapted to meet a number of specific functions: filtration, drainage, separation, confinement and the retention and reinforcement of soil.

Geosynthetics were first incorporated into major soil stabilization projects in the early 1970s when both woven and non-woven geotextiles were used as key components in roadbase stabilization and erosion control projects. Today, geosynthetic products include an extensive range of woven and nonwoven geotextiles, geogrids, geomembranes, geonets and geocells.

Geocells have evolved from a co-operative development effort between the Presto Products Company and the US Army Corps of Engineers. The concept behind geocells is relatively simple. High Density Polyethylene strips are ultrasonically welded at specific intervals in order to form a panel of three dimensional honeycomb cells. The standard plan dimension of cells is 203 mm x 244 mm. Installed in expanded form, these panels are then infilled with a project specific infill material to complete the system. Geocell systems are available in a variety of pre-engineered cell dimensions and depths. The selection of the appropriate configuration is site specific.

Geocells were originally developed to provide load support to military vehicles travelling over soils of low shear strength, such as sand. Additional applications that have since been developed include soil retention in gravity and geogrid reinforced retaining walls and erosion protection in channel linings and slope protection. The infill material may be a structural fill, vegetated topsoil or concrete.

The potential value of geocells as a means of providing permanent protection from the erosive forces of wind and precipitation was first recognized in the early 1980s. Today, the state of the practice for geocell systems in slope protection and channel lining applications includes the use of Kevlar^® tensile tendons to assist in distributing the down slope driving forces to the stable subgrade and pre-engineered perforations to improve drainage and infill material retention.

In addition to the geocell confinement system, a 200-gram per square metre (grab tensile strength - 670 N MARV) nonwoven geotextile was utilized immediately below the geocell, in intimate contact with the native soil. The function of this engineered fabric was to eliminate the potential for erosion of the native soil immediately beneath the infill material.

Design

Stantec Consulting Ltd. worked with Armtec Construction Products to determine the stake anchor requirements satisfying a design factor of safety of 1.50 against down slope sliding of the system. To fully cover the freeboard area of the lagoon side slopes, the geocell system was designed for a 2.44 m slope length. The slope angle was 18.4° on all sides of the lagoon. The coefficient of friction between the nonwoven geotextile and the high plasticity clay was estimated to be 0.44. The design depth of concrete infill was 100 mm, matching the geocell depth.

Assuming a unit weight of 23.7 kN/m³ for concrete infill, the net downslope driving force was calculated to be 0.12 kN/m², equating to a destabilizing force of 0.29 kN/linear metre for the 2.44 m long slope. Therefore, the required resisting force of the stake anchor system had to exceed 0.29 kN/m. J-pin re-bar anchors, 610 mm x 13 mm, on a down slope spacing of 813 mm and a horizontal spacing of 610 mm, in conjunction with coated polyester tendons inserted at the same 813 downslope spacing, provided a sliding resistance of 0.4kN/m.

Construction

The initial task was to excavate the new lagoon. The preliminary excavation was completed in a period of six weeks, with the berms being constructed from material that was excavated in order to expose the lagoon bottom. Prior to geocell installation, the 3:1 internal slopes were compacted to 98 percent Standard Proctor Dry Density, using sheepsfoot compaction equipment. Since the geocell system was only to cover the slope in the wave vulnerable area, an area equal in size to the area of expanded geocell was excavated to a depth of 100 mm. This allowed the geocell to be recessed into the slope and, therefore, sit flush with the balance of the slope.

The coated polyester tendon material was field measured and cut to the required length, approximately 50 percent greater than the 6.0 m length of the geocell panels. Individual tendons were then inserted through factory-drilled holes in the collapsed geocell panels. The tendons were then tied off at strategic points along the length of the geocell panels in order to create a composite unit. Prior to placement of any geocell panels, a 200-gram per square metre geotextile was installed on the prepared slope surface in the area to be covered by the geocell system. The function of this nonwoven, needle-punched geotextile was to prevent the loss of any of the subgrade soil. The nonwoven, engineered construction fabric was retained in place by several temporary J-pins.

The initial geocell panel was placed into the previously excavated 100 mm depression and expanded. Dimensional control was established by checking the perimeter width of the panel (6.0 metres). Once this dimension was established, J shaped anchor pins, 13 mm x 610 mm were driven into the ground until the top of the J pin fully engaged the tendon. The panel was then expanded to its final 2.44 m x 6.0 m dimensions with additional J-pins installed at three cell increments on the lagoon perimeter and at 813 mm increments down the slope length. These J-pins engaged only the tendon material and were driven flush with the native soil.

Once the initial panel was installed, subsequent panels were installed in a similar manner around the entire lagoon perimeter. Adjacent panel edges were stapled together in order to maintain dimensional control of the edge cells until the placement of concrete was complete. The concrete infill material was a 10 MPa ready mix product. Location of the slope protection system relative to the top of slope allowed the concrete to be placed directly from the truck using the standard truck-mounted chute.

Filling of geocells is best done from the top of the slope down, particularly when concrete is the infill material. This permits easy raking of the concrete into the lower cells and reduces the forces attempting to drive the entire system down the slope. Vibration of the concrete in geocell systems is not required and on this project, the final surface texture was achieved using a coarse broom. The rough surface finish provides additional traction on the slope for maintenance workers.

The erosion control system for Lagoon 2A, which utilizes a multi-tendoned geocell system with concrete infill, has been in use for almost three years. Relative to the systems used on earlier lagoon slopes, results have been favorable. The initial installation cost of the system, while more than the cost of dumping concrete rubble, was similar to poured in place concrete.

In terms of performance, access to the lagoon floor for cleaning is safe and easy. Maintenance on the slopes, primarily vegetation control, has been dramatically reduced. As the geocell system is inherently flexible, cracking of the concrete infill is non-existent.