GEOWEB® Slope Protection System

Project Name: Mud Lake Dam

Project Location: Gardnerville, NV

Mud Lake Dam in Gardnerville, Nevada, is situated at the base of the picturesque Sierra Nevada range. This reservoir was originally built more than 100 years ago as an earthen dam using horse-drawn scrapers and steam shovels. No longer meeting the safety standards of the Dam Safety Agency of the State of Nevada, rehabilitation was deemed necessary. Colorado Lining International proposed a cost-effect alternative to the traditional reinforced concrete face dam. The proposed geosynthetic solution incorporated an impervious geomembrane with geotextile protective layers and a concrete-filled geocell armor cover.

This case study focuses on a specific geomembrane face/geosynthetic composite rehabilitation to the rockfill dam built early in the year 2000.

Design Considerations

The geosynthetic design included the following components installed on the prepared dam face, as detailed by geotechnical engineer Norm Bishop of Stone & Webster:

1. Non-woven geotextile protective under-layer (16 oz/yd²).
2. 45-mil reinforced polypropylene geomembrane
3. Non-woven geotextile protective over-layer (16 oz/yd²).
4. 75-mm (3-inch) tendoned, perforated GEOWEB® 3D Confinement system.
5. 75-mm (3-inch) poured-in-place concrete veneer facing element protecting the underlying geomembrane.

According to President of Colorado Lining International, John Heap, they chose the geosynthetic solution for three reasons. “First, it saves cost. The client saved between $600,000-$700,000 over conventional reinforced concrete methods. Second, it provided waterproof protection to the dam face; the client has a system that will conserve more water and will function for a long, long time.” The third reason, added Heap, was constructability. Custom-made geomembrane panels were precut and welded offsite, significantly reducing construction time and limiting exposure to natural degradation elements. This solution also eliminated the need for special construction equipment.

Engineers specified the GEOWEB® Slope Protection System, complete with integral tendons and the ATRA® Keys. The tendons allowed installers to anchor the GEOWEB geocells sections at the crest of the slope, secured to a deadman anchor system, and suspended over the geomembrane.

“When anchoring with traditional stakes is not possible, as in this case, integral tendons and load-transfer clips allow the geocell system to be suspended over the geomembrane without jeopardizing its integrity,” Samuel Randolph of Soil Stabilization Products Co. notes. “This method enables a system that directly protects the geomembrane from accidental puncturing and natural degradation.”

The Installation

The installation of the geosynthetic components was performed in layers.

Layer 1:  First, A 16-oz. non-woven geotextile under-layer was installed over the entire 2h:1v dam face. Rolls 4.6 – 91.4 m (15 ft x 300 ft) were pulled in place from the top of the dam and heat-welded together.

Despite winds up to 48 kilometers per hour (30 mph) slowing construction, the entire face was covered and ballasted at the top and bottom with sandbags in one 10-hour work shift.

This geotextile layer protects the geomembrane, adding puncture resistance and supporting strength across the face of the dam. Additionally, this layer allows for drainage along the face by providing a path for water to flow under the lining.

Layer 2: The second layer applied was a 45-mil reinforced polypropylene geomembrane. Custom panels were fabricated to size with staggered panels to account for length differential along the face of the dam. A total of nine panels were custom fabricated and tested before shipping to the site. On the second day of the geosynthetic installation, they began deployment of the custom geomembrane panels on the west side of the dam abutment.

A total of 8,360 m² (90,000 ft²) of material was deployed and tested in a 12-hour workday. By utilizing deployment equipment on the crest and toe of the dam face, crews placed materials quickly and efficiently. As deployment progressed, the geomembrane welding crew began field-welding the eight vertical field seams with a Hot Wedge welder and flat roller assembly. Field QA/QC consisted of trial weld samples before field welding to verify machine settings and non-destructive testing utilizing the air lance test method ASTM D-4437. All welds were subject to non-destructively and visually inspected by the geosynthetic installer and QA/QC inspector.

Layer 3: A 16-oz. non-woven geotextile layer was installed as a protective layer over the geomembrane lining. Providing an additional protective and supporting layer between the geomembrane and the cellular confinement system, this layer was also installed and welded in place in one 10-hour workday.

Pre-planning and resourcefulness by contractor, Kiewit Pacific Construction, helped expedite placement of the tendons and ATRA® clips within the geocell sections. As the geotextile/geomembrane layers were being installed, a separate crew joined GEOWEB sections for the final layer of the dam facing. Crews prefabricated geocell panels to specified lengths up to 34.7 m (114 ft) using assembly beds constructed for threading and securing tendons and load-transfer ATRA® restraint clips.

Kevlar® (aramid) tendons with a minimum break strength of 13.34 kN (3,000 lbf) were threaded through predrilled holes in the geocell sections. Restraint clips were attached to the tendons in designated cells according to the design. To ensure the pre-assembled panels would match the required length when deployed on the slope, the outer edges of the sections were stretched and placed over perimeter stakes.

Pre-assembling the geocell sections with the tendons and restraint clips in a controlled, flat environment precluded the need for construction crews to work on the steep dam face. With the bulk of the labor expended in this safer setting, the installation time of the pre-assembled sections was significantly less than the contractor’s expectations.

“This technique allowed the pre-assembled sections to be quickly deployed on the dam face without intensive labor requirements, helping to keep installation costs in line and maintain a safer working environment,” explains Randolph.

Layer 4:  After the geotextile/geomembrane layers were installed, a Schedule 80 steel pipe deadman anchor was placed along the full length of the crest anchor trench in preparation for the geocell system. The pre-assembled geocell sections were carried from stockpiles in the prefabrication area and positioned on the slope according to the correlating slope length.

The tendon ends on the up-slope end of each section were secured to the deadman anchor. Once secured, the trench was filled with concrete. The geocell sections were then expanded down the slope to their proper length and secured at the toe.

After installing the 75-mm (3-inch) depth geocell layer, concrete was placed in the geocell layer to protect the membrane/fabric layer over the dam face. They then infilled the geocell sections with a concrete mix with a 10-mm (3/8 inch) pea gravel aggregate.

On day one, the concrete infill was placed in the geocell sections on the slope’s upper area using the chutes of the concrete trucks. On day two, crews used a concrete pump to place concrete on the remaining length of the slope face. Using this method, they placed over 382m³ (500 yd.³) of concrete in just two days while cutting pumping expenses in half.

The flexibility of the geocell sections also allowed construction workers to walk within the cells and easily maneuver on the slope face while raking concrete infill.

The Results

The geomembrane provides an impermeable, waterproof barrier that will prevent water loss through the dam face. The concrete-infilled geocellular structure suspended over the liner offers stability and protection to the liner without endangering its integrity. This type of dam rehabilitation can be constructed in a relatively short schedule (6 months at this site for design, construction, and reservoir filling).

In total, 8,360 m² (90,000 ft²) of liner material and more than 100 sections or 5,574 m² (60,000 ft²) of GEOWEB geocells material plus over 382 cubic meters (500 cubic yards) of concrete were used to protect the dam face at Mud Lake Dam.

The design-build team for this project included: Kiewit Pacific, Stone & Webster, Colorado Lining International, and Soil Stabilization Products Co. associated manufacturers.

This project received Outstanding Achievement Award recognition for outstanding and innovative work with industrial/technical fabrics at the Industrial Fabrics Association International’s Expo 2001. The International Achievement Awards recognize the latest advancements in industrial technical fabric design and manufacture in 24 categories.