Geosynthetics April/May 2022 - 26

Lining the 70 Ranch Raw Water Reservoir
Based on seepage analysis performed
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 costeffective
geomembrane,
protection geotextiles,
cover soils, and upper
slope wave/ice impact
and environmental
protection using rockfilled
geocells.
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 acrefoot
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
26
Geosynthetics | April May 2022
entire reservoir impoundment with a costeffective
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, onsite
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
Geosynthetics April/May 2022

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