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Complex geosynthetic liner system for the 70 Ranch Raw Water Reservoir

A geosynthetic liner system is custom made for a difficult site on the eastern Colorado plains.

Features | April 1, 2022 | By: Ronald K. Frobel and Ray Peebles

FIGURE 1 North end of reservoir showing slope panels transitioned into bottom panels and staged cover soils placement on bottom panels

Irrigation water in the western United States has always been a major concern for farming and ranching, especially in drought years. Water can be both scarce and expensive, and planning for future water storage and accessibility based on projected use is of paramount importance for irrigation districts. These considerations were of particular interest to the United Water Conservation District (UWCD) (today called the United Water and Sanitation District), Greenwood Village, Colo., when planning for storage and future use of water in Weld County, just to the northeast of Denver. To this end and as part of the overall water use plan, the UWCD initiated investigation and selection of a site for a storage reservoir in 2012. Based on feasibility studies and access to off-season stream water from the South Platte River, the UWCD selected a 250-acre (101-ha) site east of Kersey, Colo., at a location known as 70 Ranch. Engineers began site investigations and the preliminary design for the reservoir in 2013. They completed site geology and geotechnical investigations in 2014, and the design engineer provided design alternatives for the reservoir around the same time (Civil Resources LLC 2014).

Geotechnical considerations and alternatives

Many pond and reservoir sites can be excavated into native soils that are sufficient to retain water or are in an area where clay deposits allow for a compacted clay liner (CCL) with acceptable and economical water loss. Seepage rates can also determine whether the cost of water collected versus lost is economical. However, the 70 Ranch Raw Water Reservoir site was a challenge due to the geotechnical investigation that revealed limited available cohesive site soils and strata that could hold water. Additionally, the quality and porosity of bedrock was considered highly variable.

The extensive site investigation included numerous borings and test pits throughout the proposed reservoir site, as well as a proposed embankment dam centerline alignment and slurry wall alignment. The investigation encountered native soils consisting of sand, silty sand, clay and gravel at depths ranging from ground surface to 100 feet (30.5 m) below ground surface. Bedrock was encountered at depths ranging from 40 to 100 feet (12.2 to 30.5 m). Site investigators found small clay lenses, and the bedrock varied from relatively porous sandstone to siltstone but with no significant clay strata.

Based on seepage analysis performed on typical cross sections and composite samples, the loss of water due to seepage was deemed excessive when considering use of existing site soils and the cost of water lost per acre-foot. Thus, alternative methods of seepage control were investigated and proposed in the preliminary design report. To summarize, planners evaluated four alternative liner options for water storage cost and performance for the reservoir. The four alternatives were as follows:

  • Below-grade slurry wall and upper geosynthetic liner
  • Full geosynthetic liner
  • Full-depth slurry wall
  • Slurry wall and compacted clay liner

Design engineers evaluated the preceding alternatives with pros and cons for each as well as estimated cost to the owner per acre-foot of storage. Due in part to the highly variable and, in most cases, unknown porosity of the bedrock areas, the slurry wall concept had questionable efficient seepage control and was deemed costly to construct. Depth to bedrock for a slurry wall would be at maximum depth for trencher methods. Additionally, native materials and bedrock were both considered at the low end for use as seepage barriers. The full geosynthetic liner alternative was originally based on an exposed liner and estimated 20-year life. However, a covered liner system would extend liner life and protect the system from environmental and external mechanical damage while reducing long-term cost per acre-foot of storage volume.

Geosynthetic liner system solution

The 70 Ranch Raw Water Reservoir’s location and geotechnical investigation into site soils, soil/bedrock strata and possible alternatives resulted in choosing to line the entire reservoir impoundment with a cost-effective geomembrane, protection geotextiles, cover soils, and upper slope wave/ice impact and environmental protection using rock-filled geocells. The goal was to provide a final geosynthetic liner system design that would be cost-effective to the owner and provide an extended design life over that of a fully exposed liner. Additionally, on-site sandy soils could be utilized to provide a protective bottom cover and midslope bench cover/ballast layer.

Reservoir design using geosynthetics and site soils

Although most of the reservoir footprint was excavated to grade, the north and northwest sections were predominately filled embankments that needed to be designed as an embankment dam with spillway over the two sections. The dam required approval by the Dam Safety Branch of the Colorado Division of Water Resources. Work crews constructed the embankment in lifts with site soils, and the geomembrane was the primary upstream seepage barrier on the upstream face. The embankment upstream slope was 4H:1V.

The reservoir surface area is 170 acres (69 ha) and irregular in shape. The top of berm elevation is 4,575 feet (1,395 m), and work crews constructed the bottom at 4,530 feet (1,381 m). Engineers designed a midslope bench 20 feet (6.1 m) wide at elevation 4,552 feet (1,387 m). Engineers designed for a maximum water surface elevation at 4,572 feet (1,394 m), resulting in a 3-foot (0.9-m) freeboard. Based on site soils types and characteristics, the interior slopes of the reservoir were set at 4H:1V. This slope inclination provides stability of the underlying site soils and allows for placement and stability of upper slope mechanical protection using rock-filled geocells.

The bottom of the reservoir is constructed with a roller-compacted site soils base, protection and a 30-mil (0.8-mm) scrim reinforced polypropylene (fPP-R) geomembrane. The bottom liner is then protected with a 12-inch (305-mm) thick layer of sandy soil excavated from the site and carefully placed by low ground pressure (LGP) equipment. The bid item area of the bottom was 5,392,397 square feet (500,970 m2).

Engineers designed the 4H:1V side slopes with roller-compacted site soils, a protection geotextile and a 45-mil (1.1-mm) fPP-R geomembrane. The upper slope section above the bench is protected with a 10 ounces per square yard (339 gsm) nonwoven geotextile and an 8-200-8 geonet composite, and is covered with 6-inch (152-mm) deep geocell infilled with 2-inch (51-mm) angular rock. The bench is protected with 3 foot (0.9 m) deep site soils on top of 10 ounces per square yard (339 gsm) nonwoven geotextile, and the bottom slope area below 4,552-foot (1,387-m) elevation is fully exposed but expected to be covered by water 90% of the time. The bid item area for the reservoir side slopes was 1,881,239 square feet (175,000 m2).

Geomembrane selection and final bid quantities for geosynthetics

Once design engineers made the decision to fully line the reservoir, they investigated the primary seepage barrier type, thickness and polymer to find a cost-effective solution. In reviewing the types of geomembranes, engineers considered many design and construction options including:

• Site geotechnical constraints 

  • Sandy soils and silty sands that are difficult to roller compact
    and maintain
  • Limited volume of available site soils for embankments/cover soils
  • Sandy soils that tend to erode rapidly and displace when it is windy
  • Minor settlement potential of base sandy soils

• Site weather 

  • Eastern Colorado plains experience high winds/wind events/rain events
  • Installation in hot summer and cold winter conditions
  • High surface temperatures that will affect wrinkling/distortion
    of material

• Construction means and methods 

  • Maintenance of smooth/dry subgrade just prior to liner placement
  • Preparation of subgrade in step sequence with liner placement
  • Cover soils placement over liner with LGP equipment
  • Cover soils staging/placement that must be sequenced with fabricated geomembrane panels 
  • Cover soils placed on liner immediately after panel placement/tie-in seams
  • Rapid large area panel tie-in seaming and non-destructive testing (NDT) construction quality assurance/construction quality control (CQA/CQC)
  • Use of LGP equipment to place a 12-inch (305-mm) sandy soil cover
  • Final CQA by electrical leak location (ELL) survey

• Operation and maintenance 

  • Large surface area producing wave and ice action on upper slopes
  • Deep reservoir at more than 42 feet (13 m) with rapid fillings/drawdown of water surface
  • Minimum operating level at elevation 4,552 feet (1,387 m)
  • Potential ice accumulation at low water surface levels in winter

Based on evaluation of the above design and construction considerations, engineers narrowed the geomembrane material options in the final bid documents to the following characteristics:

• Base polymer longevity exposed > 20 years’ life span with warranty

• Scrim reinforced for high puncture and tear resistance, high tensile strength and stability

• Factory fabrication into large custom panels using thermal fusion welds

• Minimization of field seaming by using efficient factory fabrication

• Minimum thickness of 45 mil (1.1 mm) on slopes and 30 mil (0.8 mm) at the bottom

• Efficient thermal fusion welding and NDT for field seams

• High resistance to creasing and wrinkling for factory fabrication/install 

• Testing by ELL survey for final installation CQA/CQC

• Geomembrane material options selected for lining system design concepts included the following:

  • 45- and 30-mil (0.8- and 1.1-mm) scrim reinforced polyethylene (RPE)
  • 45- and 30-mil (0.8- and 1.1-mm) scrim reinforced polypropylene (fPP-R)

The geosynthetics materials chosen and included in the bid by the general contractor for the 70 Ranch Raw Water Reservoir project were as follows:

• Primary geomembranes: 45- and 30-mil (0.8- and 1.1-mm) fPP-R geomembranes (7,696,000 square feet [716,000 m2]) 

• Panel factory fabrication for the fPP-R geomembranes:

  • 132 45-mil (1.1-mm) slope panels each 19,760 square feet (1,836 m2)
  • 180 30-mil (0.8-mm) bottom panels each 1,200 square feet (2,899 m2)

• Slope protection: 10 ounces per square yard geotextile (1,353,600 square feet [125,754 m2]) 

• Slope protection/drainage: 8-200-8 geonet composite (1,353,600 square feet [125,754 m2])

• Slope protection: 6-inch (152-mm) geocell (1,080,700 square feet [100,400 m2])

Installation

FIGURE 2 North end of reservoir showing (top to bottom): geocell, 2-inch (51-mm) stone infill, soil-covered bench, lower exposed section and finished bottom soil cover

A final bid package was sent out to prospective earthworks contractors in March 2016. The bid package included contract documents and geosynthetics specifications prepared by Civil Resources LLC (2016) of Frederick, Colo., and R. K. Frobel & Associates Consulting Geosynthetics Engineers, Evergreen, Colo. The contract was awarded to general contractors Fiori and Sons Inc., Denver, Colo. 

Work crews installed geomembrane panels beginning at the north end of the reservoir with slope panels first placed down the northeast side. They then also placed large bottom panels with tie-ins to the slope panels down the northeast side of the reservoir. Once crews positioned sufficient bottom panels, they staged cover soils and cover soil placement by LGP. Several figures illustrate the transition of side slope panels to bottom panels (Figure 1), the North End of reservoir showing the slope from top to bottom (Figure 2), unfolding and positioning the fPP-R panel at the bottom of the reservoir (Figure 3) and sequential soil cover operations that required close coordination between liner install and earthworks (Figure 4). It should be noted that work crews simultaneously carried out seven construction operations on the side slopes that were staged to provide completion and cover protection of the geomembrane as it was installed. The reservoir bottom operation required four distinct operations that work crews carried out sequentially. 

ELL survey

FIGURE 3 Large scrim reinforced polypropylene (fPP-R) geomembrane panel unfolding and positioning on the bottom of the reservoir. Note soil cover material staged for immediate placement after tie-in seaming

In addition to routine CQA/CQC testing of geomembrane seams by trial welds, NDT air lance and destructive sampling, it was decided early in the project to test the completed sections using ELL survey to primarily detect damage due to soil and rock cover operations. The leak location survey company performed the ELL survey in accordance with ASTM D7007 using dipole measurements on the soil and rock-fill cover materials. A water puddle survey was conducted on the lower slope bare liner. The survey was performed as the liner system was completed and required return trips and mobilizations to complete. The ELL survey mostly found damage caused by installation equipment during the bottom liner soil cover operation. Minimal damage was found in the upper geocell/rock-covered section of the slope. Once the survey detected damage, the area was carefully excavated, and the liner repaired and re-covered. A total of 79 leak sites with minor to significant mechanical damage were detected and repaired. Figure 5 illustrates typical equipment damaged area that required repair. It should be noted that no leak sites were found in more than 25 miles (40 km) of thermal fusion field welds.

Summary

FIGURE 4 Cover soil placement by Global Positioning System (GPS)-guided low ground pressure (LGP) dozer and tracked skid-steer loader. Note minimal wrinkling/waves during placement.

The 70 Ranch Raw Water Reservoir is in a difficult area for water retention with minimal cohesive soils, overall sandy soils and bedrock, the latter susceptible to high seepage rates. The 170-acre (69-ha) reservoir was successfully lined with a geosynthetic liner system that will hold valuable irrigation water for many years in an otherwise dry region of the eastern Colorado plains.

The liner system was designed and constructed with large factory-fabricated fPP-R panels to reduce construction time in both summer and winter, reduce wrinkles for soil cover placement, allow rapid earth cover scheduling in large areas and reduce on-site CQA/CQC. The large 45-mil (1.1-mm) fPP-R panels on the slopes were sized for full-length slope placement and tie-in to the bottom 30-mil (0.8-mm) fPP-R. In addition to reduction in installation time and soil placement, on-site CQA/CQC and NDT, the entire reservoir was tested using ELL survey for final acceptance of the liner system.

The 70 Ranch Raw Water Reservoir is the largest fPP-R prefabricated geomembrane lined impoundment in North America.

Ronald K. Frobel, P.E., is owner/principal of R. K. Frobel & Associates Consulting Geosynthetics Engineers in Evergreen, Colo.

Ray Peebles is global product and market manager for Cooley Group Containment Solutions in Pawtucket, R.I.

All figures courtesy of the authors.

FIGURE 5 Electrical leak location (ELL) survey revealed significant damage from contractor equipment during cover soil placement on bottom lining system

References

Civil Resources LLC. (2014). “70 Ranch reservoir—Preliminary geotechnical investigation and design alternatives,” Frederick, Colo., March.

Civil Resources LLC. (2016). “70 Ranch dam and reservoir—Contract documents and technical specifications C 2041,” Frederick, Colo., February. 


SIDEBAR: PROJECT HIGHLIGHTS

70 Ranch Raw Water Reservoir liner

OWNER

United Water Conservation District 

LOCATION

Weld County, Colo.

GENERAL CONTRACTOR

Fiori & Sons Inc.

DESIGN ENGINEERS 

Civil Resources LLC Consulting Civil Engineers

R. K. Frobel & Associates Consulting Geosynthetics Engineers

GEOMEMBRANE/manufacturer 

Scrim reinforced polypropylene (fPP-R) geomembranes, Cooley Group Inc.

GEOMEMBRANE PANEL FACTORY FABRICATION

Environmental Protection Inc.

GEOTEXTILE PRODUCT/manufacturer

10 ounces per square yard (339 gsm) nonwoven geotextile, SKAPS Industries

GEONET COMPOSITE PRODUCT/manufacturer

8-200-8 geonet composite, SKAPS Industries

GEOCELL/manufacturer

6-inch (152-mm) geocell, Presto GeoSystems

GEOSYNTHETICS SUPPLY AND INSTALLATION

Consolidated Divisions Inc. 

ELECTRICAL LEAK LOCATION SURVEY

Leak Location Services Inc.

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