Lauer_Dam_Design_Summary_Report_NO_COST.pdf

Consulting Engineers and

Scientists

Lauer Dam Rehabilitation Design Summary Report

Alturas, California

Submitted to:

U.S. Department of the Interior Bureau of Indian Affairs Division of Water and Power Dam Safety, Security, and Emergency Management Branch Pacific Region

Submitted by:

GEI Consultants, Inc.

4601 DTC Boulevard Denver, CO 80237

September 2017

Andrew Lockman, P.G.

Project Manager

Chad Masching, P.E. (CO) Program Manager

Lauer Dam –Design Summary Report

GEI Consultants, Inc. i Lauer Dam Rehabilitation

Table of Contents

1.0 Introduction

1.1 Purpose

1.2 Project Background

1.3 Authorization

1.4 Project Personnel

1.5 Limitation of Liability

2.0 Project Background

2.1 Existing Dam

2.2 Dam Safety Studies and Design Development

3.0 Design Basis

3.1 Hazard Classification

3.2 Bureau of Indian Affairs Design Criteria Guidelines

3.3 Additional Design Criteria Guidelines

3.4 Operational Design Criteria

3.5 Access

3.6 Embankment

3.7 Outlet Works

3.7.1 Intake Structure

3.7.2 Outlet Works Conduit

3.7.3 Energy Dissipation Structure

3.8 Spillway

3.9 Seismic Design

4.0 Geologic Setting and Geotechnical Conditions

4.1 Regional Setting

4.2 Site Geology

4.3 Geotechnical Data

4.3.1 Clay Borrow Investigations

4.3.2 Dam Alignment Borings

4.3.3 Rockfill Borrow

5.0 Embankment Designs

5.1 Embankment Configuration

5.1.1 Crest Elevation and Width

5.2 Embankment Materials

5.2.1 Zone 1 and Zone 2 Rockfill

5.2.2 Clay Blanket

5.3 Slope Stability Analysis

5.3.1 Hydraulic Conductivity Values

5.3.2 Shear Strength Properties

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5.3.3 Loading Conditions

5.4 Excavation and Foundation Preparation

5.5 Reservoir

6.0 Hydrology and Hydraulic Designs

6.1 Inflow Design Flood and Spillway Capacity

6.2 Outlet Works Designs

6.2.1 Reservoir Evacuation Criteria

6.2.2 Intake Structure

6.2.3 Impact Structure

6.3 Spillway Modifications

7.0 Structural Design

7.1 Structural Design Components

7.2 Concrete Core Wall

7.3 Intake Structure

7.4 Trashracks

7.5 Sluice Gates

7.6 Concrete Encased Conduit

7.7 Early Warning System Building

8.0 Construction Considerations

8.1 Construction and Vehicle Access

8.2 Site Security

8.3 Water Management During Construction

8.4 Environmental and Cultural Concerns

8.5 Breach of Existing Reservoir

8.6 Excavations

8.7 Borrow Materials

8.8 Staging, Stockpile, and Material Waste Areas

9.0 Dam Operations and Maintenance

9.1 Operational Considerations

9.1.1 Embankments

9.2 Instrumentation

9.3 Reservoir Water Level

9.4 Structural Movement

9.5 Maintenance Considerations

9.5.1 Spillway

9.6 Safety Considerations

10.0 Cost Estimate

10.1 Quantities and Unit Prices

10.2 Cost Allowances

10.3 Anticipated Construction Duration

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11.0 References

List of Appendices

Appendix A Design Drawings Appendix B Opinion of Probable Construction Costs and Construction Schedule Appendix C USBR Engineering Geology Field Classification Descriptors Appendix D 2016 Borelogs and Results of Laboratory Testing on Rock Appendix E Spillway Erosion Calculations Appendix F Seepage and Stability Evaluation Appendix G Hydrology and Hydraulics Documentation Appendix H Structural Calculations Technical Memorandum

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1.0 Introduction

1.1 Purpose

This report summarizes designs for the rehabilitation of Lauer Dam, a water storage facility located near Alturas, California. Lauer Dam is operated by the Pit River Tribe and the rehabilitation project is being performed to address dam safety concerns identified by the U.S. Bureau of Reclamation (Reclamation) and the Bureau of Indian Affairs (BIA). This Design Summary Report discusses the geotechnical, hydrologic, hydraulic, and structural elements of the proposed modifications to the dam.

1.2 Project Background

Lauer Dam is located in Modoc County in northeastern California on an unnamed tributary of the North Fork of the Pit River, approximately 15 miles north of the City of Alturas. The dam stores water for irrigating pastures and hay fields on the XL Ranch Reservation, and is classified as a High-Hazard structure due to the potential for loss of life downstream in the event of a dam failure. Lauer reservoir is fed by ephemeral runoff from a 6.2 square mile drainage basin, and has a current storage capacity of approximately 1,948 acre-feet at a normal water surface elevation (El.) of 4983.7 feet. The reservoir has a storage capacity of approximately 3,044 acre-feet at the existing dam crest of 4987.7 feet.

Lauer Dam was constructed in 1895. Little is known about construction of the dam or its appurtenant structures, but several documents prepared by Reclamation for the BIA Safety of Dams Program (SOD) program indicate the facility is in poor condition with several dam safety deficiencies related to slope stability, piping of embankment and foundation materials, and inadequate spillway capacity. The embankment slopes are over-steepened due to erosion and seepage exits at the left groin of the dam and immediately downstream of the outlet works. However, no seepage has been reported coming directly from the downstream face of the embankment.

Reclamation performed the first known dam safety work for Lauer Dam in 1981, which consisted of the preparation of a Safety Evaluation of Existing Dams (SEED) report. The first reported formal examination of the dam took place under the SEED program in December 2002. Identified dam safety deficiencies are discussed in the Deficiency Verification Analysis (DVA) prepared in 2004. A Conceptual Design and Value Engineering (VE) study performed by Reclamation in 2005 evaluated 16 alternatives to address the safety deficiencies and bring the structure into compliance with modern dam safety criteria. The option to replace the existing dam with a new embankment constructed immediately downstream was recommended by Reclamation.

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GEI was retained by BIA to advance design concepts developed by Reclamation for the new structure. The project includes the following components:

Developing designs to construct a new embankment dam immediately downstream of the existing structure that incorporates modern dam safety features.

Identifying suitable borrow material for the new embankment dam.

Excavating soils to bedrock within the footprint of the new embankment dam, and removing loose, weathered materials to provide a suitable foundation.

Providing upstream and downstream slope protection.

Designing an access road along the dam crest.

Constructing a new outlet works with separate gates to provide irrigation water and address emergency drawdown requirements.

Designing a new spillway to replace the existing rock dike.

Designing the structure to accommodate the Inflow Design Flood (IDF).

Designing an Early Warning System building.

1.3 Authorization

GEI has performed the services described in this report under the terms and conditions of the agreement (No. A15PD00643) between GEI and BIA dated August 11, 2015. This agreement has been modified to incorporate other design components, including an alternatives analysis, risk assessment, VE study, review by an Independent Technical Review Team, archaeological surveys, and Environmental Assessment (EA) Surveys. Preparation of a supplemental EA and a permit application to the U.S. Army Corps of Engineers are also part of the contract modification.

1.4 Project Personnel

The following GEI personnel were primarily responsible for performing the engineering analyses and for preparing this report:

Project Manager: Andrew Lockman, P.G.

Project Reviewer: James Niehoff, P.E Civil Engineer: Paul Eggers, P.E.

Hydraulic Engineer: Nick Miller, P.E., P.H.

Geotechnical Engineer: Chad Masching, P.E.

Staff Engineers: Mike Henrick, E.I.T Nathan Jorgensen, P.E.

Margaret Kimble, P.E.

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Independent Technical Reviewers include:

Miller Geotechnical Consultants, Inc. Debora Miller, P.E., PhD Tessara Water, LLC Tara Schutter, P.E.

Cost Estimating and Constructability Support:

Engineering Solutions, LLC Dan Hertel, P.E.

The BIA representatives assisting with the project include:

Project Manager: William Shuter, P.E.

Safety of Dams Officer: Thomas Dang, P.E.

Safety of Dams Engineer: Matthew Young, P.E., CFM

1.5 Limitation of Liability

Our professional services for preparing this Design Summary Report and associated calculations were performed in accordance with generally accepted engineering practices; no other warranty, express or implied, is made.

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2.0 Project Background

2.1 Existing Dam

The existing Lauer Dam is a homogenous earthfill embankment that is approximately 13 feet high from the dam crest to the downstream toe, as measured at its maximum height. The dam crest is at El. 4987.7. It is approximately 830 feet long with a crest width that varies between 7 and 12 feet. The upstream and downstream slopes are over-steepened, in part due to slope erosion. The upstream face is sloped at about a 2H:1V (horizontal to vertical) along the left half of the embankment and near vertical along the right half of the embankment. A layer of riprap protects the upstream slope. The downstream embankment face has a slope of about 1.5H:1V and is protected by vegetative cover.

The dam is founded on colluvium and basaltic bedrock. The ground surface near the dam site is covered by one- to five-foot diameter, angular to sub-angular basalt slabs and blocks.

Bedrock is very near the ground surface and consists primarily of basalt flows separated by thin, interflow zones of red colored, baked sediments. Based on information from a borehole advanced in the existing dam, the embankment consists of about 2 feet of sandy silt overlying clay fill and the bedrock foundation.

Historical seepage has been noted at the downstream toe of the dam and near the outlet works. Based on construction practices that were used 120 years ago, onsite investigations, and performance monitoring, it is assumed that the embankment does not have protective features including internal filters, drainage provisions or adequate foundation treatment for seepage control.

A dike is located on the north rim of the reservoir, about one-half mile upstream from the left abutment of the dam. The dike is about 3 feet high, approximately 400 feet long, and has a crest elevation of about 4986.7 feet, which is 1 foot lower than the dam crest. The dike is composed of rockfill consisting of large size (2- to 3-foot diameter) stacked riprap.

Downstream consequences resulting from failure of the dike are considered to be minor because of its low height, and it is considered a low-hazard structure. No seepage has been reported on or downstream of the dike.

A 5-foot wide, approximately 3-foot deep breach in the rockfill dike with an elevation of

4983.7 functions as the service spillway. The breach was not engineered and was reportedly excavated by a farmer in the 1990’s. An unlined, 50-foot wide auxiliary spillway on the right abutment of the dam with an invert elevation of 4884.7 provides additional flood routing protection. The auxiliary spillway is separated from the dam by a rock wall approximately 50 feet long and 3 feet high.

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The outlet works consists of two 8-inch diameter steel pipes with slide gates at the intake and valves installed at the downstream end. An internal camera inspection of the conduits performed in 2004 identified cracking and joint separation of the pipes. The control gates for the outlet works are inoperable, making controlled releases or draw down of the reservoir unfeasible. Currently, the only releases from the reservoir are uncontrolled and occur when the reservoir level rises above the elevation of the service and auxiliary spillway.

2.2 Dam Safety Studies and Design Development

Little is known about construction of the existing dam and its appurtenant structures, but it is in poor condition with several identified dam safety deficiencies related to slope stability and potential piping of embankment materials through the foundation or along or into the outlet works conduits.

The 2004 DVA discusses three verified deficiencies and eleven assumed deficiencies that lacked enough information for verification. The 2005 Conceptual Design and VE Report recommended a new dam constructed immediately downstream of the existing structure was the best means of addressing identified dam safety deficiencies.

In 2007, Reclamation performed a site geologic investigation that included obtaining foundation data at the proposed embankment dam alignment, at the dike spillway, and evaluating the suitability of a potential clay borrow source approximately 3 miles south of the reservoir. The investigation included six test borings along the proposed dam alignment, two borings on the crest of the existing dam, four test pits excavated near the dike spillway and six test pits at the proposed borrow source. The materials in the clay borrow source were evaluated, but were not considered suitable for embankment construction due to the highly plastic nature and dispersive potential of the soils.

In 2015, GEI performed an additional borrow source investigation to identify suitable fine grained borrow material. This investigation included exploring two potential borrow sources, one in an area at the northwest end of Lauer Reservoir and another in an area west of the dam site. Test results showed materials from these borrow sources are generally less dispersive and more suitable for embankment construction than the borrow source explored by Reclamation in 2007. GEI developed 30% conceptual designs in the spring of 2016 based on the use of clay from the borrow sources.

Following the development of the 30% Conceptual Designs, a three day VE workshop was held and the following changes were made to the baseline designs based on the VE Team recommendations:

Embankment Type – the proposed clay embankment was replaced with a rockfill dam with a vertical concrete core wall as the water barrier.

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Outlet Works – the initial outlet works configuration was modified by consolidating the irrigation and emergency conduits into a single, large diameter conduit with a straight alignment.

Flood Routing - The crest elevation of the embankment was increased to allow for storage of flood waters, and modifications to the dike spillway were removed from designs (note that spillway modifications were reincorporated into final designs).

These modifications were incorporated into 90% project designs with the following criteria:

The rockfill embankment has a design crest elevation of 4990.5 feet.

A concrete cutoff wall will function as the main water barrier, and will be placed at the embankment centerline. The cutoff wall has a top elevation of 4989.0 and is founded on competent bedrock.

A Parshall Flume will be placed downstream of the dam to collect and measure reservoir releases.

The outlet works will include a single 42-inch discharge conduit and two regulating gates. An 18-inch stainless steel slide gate will be utilized for controlled irrigation releases, and a 42-inch stainless steel slide gate will be used for emergency drawdown.

In support of the design change from a clay core dam to a rockfill dam, GEI performed a rockfill borrow source investigation in October 2016 that included exploring two potential rockfill borrow sources with geotechnical borings and geophysical surveys. Each potential borrow source was located downstream of the new dam alignment on opposite sides of the valley, approximately 800 feet from the embankment.

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3.0 Design Basis

This section provides a summary of the design and operational considerations for Lauer Dam. Detailed information regarding design parameters and development is provided later in this report.

3.1 Hazard Classification

Lauer Dam is classified as a High-Hazard structure based on a 2001 study that identified a population at risk of six or more people downstream who would be impacted in the event of a dam failure. The dike, approximately one-half mile upstream of the left abutment, is considered a Low-Hazard structure. Failure of the dike during a flooding event would be considered beneficial to the existing Lauer Dam because flood flows would be released downstream through the failed dike and would protect the main dam structure. Although the existing dike is anticipated to fail during an overtopping event due to poor construction, the modified Lauer Dam will have an increased crest elevation which will route floods to the dike spillway. The modified dike spillway is designed to pass the 10,000-year return interval Inflow Design Flood (IDF).

3.2 Bureau of Indian Affairs Design Criteria Guidelines

Lauer Dam is a Federal jurisdictional dam subject to the regulatory authority of the BIA SOD program. Guidance for design of dam safety modifications and operational criteria is contained in the Safety of Dams Program Handbook (BIA, 2014), and referred to herein as the BIA SOD Guidelines.

3.3 Additional Design Criteria Guidelines

Several other sources were used to establish design criteria which may not have been included in the BIA SOD Guidelines. These design criteria sources are referenced throughout the text and include publications from the following agencies:

United States Bureau of Reclamation (USBR) United States Army Corps of Engineers (USACE) Natural Resources Conservation Service (NRCS) American Concrete Institute (ACI) The International Building Code (IBC)

3.4 Operational Design Criteria

Lauer Dam currently impounds a reservoir with a storage capacity of approximately 1,948 acre-feet at the normal reservoir water surface elevation of 4983.7 feet and about 3,044 acre-

GEI Consultants, Inc. 8 Lauer Dam Rehabilitation feet at the existing dam crest elevation of 4987.7 feet. The modified dam and spillway will enable the normal reservoir water surface to be increased to 4985.5 feet, with a storage capacity of about 2,424 acre-feet. There are currently no requirements for irrigation flow volumes or durations, but the assumed operational range is between El. 4985.5 and 4978.7 for an operational drawdown range of 6.8 feet. The lower limit of this operational range corresponds to a remaining 1,642 acre-feet of storage and approximately 3.2 feet of hydraulic head.

3.5 Access

The crest of the new embankment dam will serve as the primary access road for dam maintenance and operations, and will also provide access to the spillway and land on the east side of the reservoir. The access roads for embankment construction and borrow development are considered temporary and will be reclaimed at the end of construction. The access road that will be constructed for spillway improvements will remain in place at the end of construction.

3.6 Embankment

The new rockfill embankment will be constructed using borrow materials obtained on site, and will include a vertical concrete cutoff wall near the embankment centerline to function as a water barrier. Two potential rockfill sources have been identified near the dam, and both are near the abutments downstream of the new embankment alignment, as shown in design drawing A-10. Although both potential rockfill borrow sources are considered suitable for construction, the southern borrow source (labeled Primary Rockfill Borrow Area) is preferred due to reduced requirements for clearing and grubbing, temporary access road development, and reduced site disturbance. The rockfill sources are composed of basalt and are anticipated to be developed using ripping tools rather than blasting.

Embankment construction will include excavating and removing soils within the footprint of the new dam, and removing loose, compressible soils to provide a foundation for the upstream and downstream shells on in-situ weathered basalt with the intent of reducing the potential for piping, differential settlement, and strength loss during a seismic event. At the concrete cutoff wall, excavation will extend below the weathered basalt to place the concrete plinth on fresh, unweathered basalt and provide a positive seepage cutoff. If discontinuities, fissures, or shear zones are observed in the concrete wall foundation they will be cleaned and treated with slush grout to address seepage concerns.

3.7 Outlet Works

The outlet works is designed to provide controlled releases for downstream irrigation users and to lower the reservoir level in an emergency drawdown situation. Due to the difficulty in controlling large diameter conduits for low-volume irrigation flows, the outlet works intake

GEI Consultants, Inc. 9 Lauer Dam Rehabilitation is configured with two separate slide gates, with a smaller 18-inch slide gate intended for irrigation purposes and a larger 42-inch slide gate for emergency drawdown situations. The invert elevation of the irrigation conduit is 4975.5 feet and is 4974.5 for the emergency release conduit. Flows through both conduits begin at the concrete headwall through the separate intake gates, but merge into a single conduit a short distance into the embankment.

The intake structure includes a trash rack and stoplogs which allow for the intake to be dewatered in the event maintenance is needed at the gates.

The outlet works are located at the left abutment, approximately 150 feet from the center of the valley to provide a stable rock foundation and reduce the potential of differential settlement along its length.

3.7.1 Intake Structure

The intake is vertically oriented with self-contained slide gates and manual gate operators.

The intake structure includes a retaining wall extending from the intake headwall towards the embankment, and access to the intake is via an earthfill walkway leading from the dam crest to the gate controls. Gate stems will be enclosed in oil-filled sleeves with hand wheels for manual opening and closing of the gates. A vent pipe will extend from behind each gate to the top of the concrete headwall. The outlet works conduits and gates will be guarded from debris at the intake structure by a trash rack panel.

3.7.2 Outlet Works Conduit

The outlet works conduit will be steel pipe encased in concrete for its entire length through the embankment and will be lined for corrosion protection. The purpose of reinforcing the concrete encasement is to provide strength since the pipe will be subjected to sizeable loadings from the embankment and vehicular traffic. In addition, the concrete encasement alleviates concerns of flotation of the conduits and provides protection for the steel pipe during construction.

3.7.3 Energy Dissipation Structure

The energy dissipation structure installed at the downstream end of the outlet works is intended to reduce the hydraulic energy and scour potential in the discharge channel, and is a Reclamation Type VI stilling basin with an internal hanging baffle. The sizing and hydraulic design of the stilling basin is in conformance with Reclamation’s Design of Small Dams (USBR, 1987) and Design of Small Canal Structures (USBR, 1978).

3.8 Spillway

In accordance with the BIA SOD Guidelines, the spillway is designed to pass the routed Inflow Design Flood (IDF). The IDF for Lauer Dam is the flood with a return period of 10,000 years. Additional information on the IDF is contained in Section 6.

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The existing dike spillway will be replaced with a concrete structure to provide grade control at the approximate elevation of the existing dike crest (El. 4985.5). The auxiliary spillway at the right abutment of the existing Lauer Dam will not be incorporated into the new embankment, and all flood flows will be routed to the dike spillway for passage.

Modifications to the dike spillway include removing the existing structure and replacing it with a concrete control sill founded on bedrock. The spillway channel will remain unlined.

3.9 Seismic Design

The 2003 Preliminary Seismic Hazard Assessment provided results of a Probabilistic Seismic Hazard Analysis and produced mean hazard curves based on the seismicity of four identified faults and random background source. Based on results of that analysis, the mean hazard curve estimated the 10,000 year return period to have a peak horizontal ground acceleration of 0.74g. These recommendations were primarily based on a fault activity map of California produced by C.W. Jennings in 1994, which indicated that the dominant source of seismic hazards at Lauer Dam is the East Modoc-North Fork fault. The 2010 Comprehensive Dam Report prepared for Lauer recommended updating the seismic hazard assessment with next generation attenuation relationships which would likely result in a reduced peak horizontal ground acceleration. No updated study has been performed to date, however, hazard curves generated for the National Seismic Hazard Mapping Project and provided by the USGS Hazard Curve Application show a peak horizontal ground acceleration of 0.56g for the Lauer site associated with the 10,000 year recurrence intervals.

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4.0 Geologic Setting and Geotechnical Conditions

4.1 Regional Setting

Lauer Dam is located on the southeastern edge of a large lava field near the edge of the Modoc Plateau known as the Devils Garden. The region consists of a series of north- to northwest-trending block faulted ranges, and the intervening basins are filled with broad-spreading “plateau” basalt flows, or with small shield volcanoes, steeper sided lava or composite cones, cinder cones, and lake deposits. The Modoc Plateau is characterized by a high plain of irregular volcanic rocks of basaltic origin resulting in an undulating topography averaging 4,000 to 5,000 feet in elevation.

The plateau basalt that is widely distributed between the fault-block ranges of the Modoc region is commonly referred to as the Warner Basalt. The largest continuous exposure of the Warner Basalt stretches from Alturas westward more than 20 miles and northward more than 25 miles, and Lauer Dam is located on the eastern edge of this lava field (Figure 4-1). The thickness of the Warner Basalt varies considerably over short distances. In the southeastern edge of the plateau near Alturas, the Warner Basalt ranges in thickness from about 15 to more than 360 feet. The average individual flow unit is approximately 4 to 5 feet thick and ranges from 2 to more than 50 feet thick.

Figure 4-1. Geologic map showing the extent of the Devils Garden area and approximate extent of the Warner Basalt. Figure taken from Geologic Investigation Report, Lauer Dam, XL Ranch Indian Reservation, California

(USBR, 2008).

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4.2 Site Geology

The following units are the dominant geologic materials at the Lauer Dam site, as determined from on-site subsurface investigations.

Quaternary Colluvium (Qc): This colluvial material consists of medium to dark brown, firm to hard, clayey soil derived from the mechanical weathering of volcanic rock. The soil is classified as a sandy lean clay with gravel and gobbles (CL), with about 60 percent medium to high plasticity fines about 20 percent sand and 20 percent angular fragments of basalt. The unit is thin (generally less than 5-feet thick in most locations) and contains some organics. There are some zones of silt and highly dispersive clay. It is present at the ground surface and is generally limited to drainages and topographic low points.

Flow-top Breccia (Tv): This unit is found at the ground surface and represents volcanic breccia that formed contemporaneously with the movement of the lava as the cooling crust became fragmented while still in motion. The resulting unit consists of angular fragments of gray, vesicular basalt in a matrix of brown clay. The clayey matrix contains evidence of volcanic origin, such as small vesicles, weathered glass, and other minerals. The clay is indurated but in some locations but it is generally a soft (soil-like) material. The basalt fragments are hard, moderately weathered, and typical of the underlying basalt flow. The fragments range in size from less than 1/8 inch to about 6 inches.

Basalt (Tv): Basalt at Lauer Dam is medium to dark gray, fine grained, and contains vesicles and vugs, primarily in the upper and lower sections of the basalt flows.

Vesicles average about 1/8-inch to ¼-inch diameter and have a maximum size of about 1 to 2 inches.

Interflow Sediments (Tv): This unit contains generally thin deposits of baked fine grained sediments separating individual basalt flows. It contains subangular clasts of basalt in a reddish-brown very soft clayey matrix. Basalt fragments are ½-inch to 1-inch in diameter.

Figure 4-2 shows examples of these bedrock units as observed in rock cores advanced at the dam alignment, and Figure 4-3 shows these units as exposed at the ground surface.

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Figure 4-2. Core photos from geotechnical boring DH-07-04, advanced by USBR as part of the 2007 investigation at Lauer Dam. This boring was advanced at the right abutment and encountered Flow-top Breccia, basalt, and in the interflow sediments. Black numbers indicate the depth below ground surface.

Figure 4-3. Geologic units at Lauer Dam as observed at the ground surface, a) colluvium downstream of existing Lauer Dam in valley section, b) flow-top breccia on right abutment, c) basalt on left abutment, d) interflow sediments as observed in a roadcut near Alturas.

Flow-top Breccia

Basalt

Interflow Sediments

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4.3 Geotechnical Data

Three subsurface investigations have been performed at Lauer Dam, including a geotechnical site investigation performed by Reclamation in 2007, a clay borrow source investigation performed by GEI in September of 2015, and a rockfill borrow source investigation performed by GEI in October of 2016. Information from the 2015 clay borrow source investigation is contained in the Lauer Dam Rehabilitation Borrow Source Investigation Report (GEI, 2016b). Borelogs and lab data from the 2016 rockfill borrow source investigation are contained in Appendix D.

In addition to geotechnical drill holes, GEI performed four seismic refraction survey lines at the dam and used site using Refraction Microtremor (ReMi) methods. Survey lines included a profile along the dam centerline, along the outlet works alignment, and at each of the potential rockfill borrow areas.

4.3.1 Clay Borrow Investigations

2007 Clay Borrow Investigation

The 2007 investigation focused on obtaining foundation data along the proposed dam alignment and at the dike spillway, and also included evaluating the suitability of a potential clay borrow source. Six test pits were also excavated at a 165 acre clearing on the Modoc Plateau, about 3 miles south of the reservoir (USBR, 2008). The test pits ranged from approximately 4 to 12 feet deep and either encountered refusal on bedrock or reached the limit of the excavation equipment. Soils were classified based on samples collected from the backhoe bucket or the spoil pile, and were submitted for laboratory testing to determine the index properties including grain size distribution, Atterberg limits, dispersivity, moisture content, and compaction requirements.

All of the test pits encountered high plasticity (fat) clay overlying an elastic silt layer above basalt bedrock except for one test pit which encountered elastic silt at the surface. Beneath the upper soils, test pits encountered weathered and decomposed basalt bedrock above hard, fresh to moderately weathered, basalt. Laboratory analysis of the materials in this borrow source indicated they were not suitable for embankment construction due to the high plasticity and high dispersivity of the clays.

2015 Clay Borrow Investigation

The 2015 clay borrow source investigation included advancing test pits at two potential borrow sources; one in an area at the northwest end of Lauer Reservoir and another in a clearing west of the dam site. The borrow sources are shown in Figure 4-4, and are in clearings filled with clay, silt, and sand, and surrounded by dense timber and shallow basalt

GEI Consultants, Inc. 15 Lauer Dam Rehabilitation bedrock. The sediment deposits are thin (generally less than 8 feet thick) and become thinner towards the edge.

Figure 4-4. Clay borrow source locations investigation in 2015.

A total of 32 test pits were excavated between 2 and 9 feet deep before hitting refusal on basalt bedrock, except for one test that extended to 14 feet before the limits of the backhoe were reached. The borrow sources contained interbedded clay, silt, and sand which could be distinguished from each other in the field by composition, plasticity, and color. Laboratory analyses were performed on these samples and indicated that the soils were generally more suitable than those encountered at the off-site borrow location identified by Reclamation.

However, the locations presented difficult access issues, and borrow development would have been challenging due to the thin interbedded nature of the deposits and difficulties during processing.

4.3.2 Dam Alignment Borings

Six vertical rock core borings were advanced along the proposed alignment to a depth of 25 feet by Reclamation in 2007. The boring on the left abutment is labeled DH-07-01, the boring in the center of the valley is DH-07-02, and borings at the right abutment are labeled DH-07-03 through DH-07-06. Each boring encountered relatively shallow bedrock, with overburden soils less than one foot thick except in the center of the valley (DH-07-02), where 5 feet of colluvium was observed above weathered basalt bedrock. Each of the borings

GEI Consultants, Inc. 16 Lauer Dam Rehabilitation included constant or falling head tests. Tests were performed in uncased borings over the entire drilled length, and were performed using water from Lauer Reservoir which was dirty, containing algae, which likely influenced results by showing lower permeability values than if a clean water source was used.

In addition to the borings advanced by Reclamation, GEI advanced one boring, labeled DH- 16-01, at a location slightly downstream of the new embankment alignment. This drill hole was inclined 30 degrees from vertical and included water pressure (packer) testing to characterize seepage conditions and permeability of steeply dipping discontinuities. The locations of dam alignment borings are shown in Figure 4-5.

Figure 4-5. Borings advanced for foundation investigations along the new dam alignment.

Bedrock conditions were logged in general accordance with descriptors provided in the USBR Engineering Geology Field Manual. Field descriptors are provided in Appendix C (USBR, 2001). A generalized subsurface profile is shown below in Figure 4-6.

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Figure 4-6. Generalized subsurface profile along the dam alignment, looking downstream.

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Left Abutment

Boring DH-07-01 was advanced on the left abutment in a location with large (1-5 feet in diameter) basalt blocks and very little soil (Photo 4-1). The drill hole encountered medium to dark gray, fine grained basalt, which in this location is slightly weathered (W3) and hard (H3). RQD values were 68% in the upper 5 feet and increased to between 84 and 100% from 5 to 25 feet. The core was slightly fractured (FD3) with core lengths ranging from 0.1 to 2.6 feet. Joint surfaces were clean to coated with a thin layer of clay. Thin zones of interflow sediment (less than 0.7 feet thick) were observed at 1.4 to 2.1 feet, 3.2 to 3.8 feet, and 11.7 to

12.1 feet.

Photo 4-1. Surface conditions at left abutment, showing location of boring DH-07-01.

Valley Section

The valley section at Lauer Dam is approximately 400 feet long and was investigated by borings DH-07-02 and DH-16-01. The ground surface in the valley section contains a sequence of colluvial clay above weathered basalt bedrock, and this area has standing water at the ground surface seasonally and during and following periods of precipitation.

Boring DH-07-02 was advanced along the centerline and encountered 5.0 feet of clayey colluvium overlaying 10.5 feet of decomposed (W9) to intensely weathered (W7), intensely (FD7) to intensely to moderately fractured (FD6) basalt. At 15.5 feet, basalt weathering decreased to moderately weathered (W5), hard (H3) and moderately fractured (FD5). Joint surfaces were generally oxidized with a very thin coating of clay, slightly rough and undulatory. RQD values were 0% in the upper 5 feet of bedrock, and ranged from 24 to 42%

GEI Consultants, Inc. 19 Lauer Dam Rehabilitation for the remainder of the boring. Groundwater was recorded at 10.4 feet below ground surface two days after drilling was complete. Results of the constant head test showed a permeability value of 5.7x10-6 cm/s.

Boring DH-16-01 was drilled approximately 200 feet downstream from station 12+00 on the new dam alignment. The boring was inclined 30 degrees from vertical with an azimuth of approximately 200° and had a drilled length of 40.5 feet. It encountered about seven feet of high plasticity clay grading to silt overlying approximately 1.5 feet of flow-top breccia with basalt fragments in a soil-like clay matrix. Beneath the brecciated basalt was approximately

8.5 feet of decomposed to moderately weathered (W9 to W5), hard to moderately soft (H3 to H5) basalt. This upper, weathered basalt was generally described as being similar to soil, with easy drilling conditions. Beneath the weathered basalt was hard to very hard (H3 to H2), slightly to moderately weathered basalt (W3 to W5) with localized and thin zones less and 1.5 feet of decomposed (W9) material. RQD values were zero in the upper weathered materials, but had increased to between 20 and 66% at a drilled depth of 23 feet. Vesicles were generally throughout the basalt except for several six to twelve inch layers and were occasionally infilled with clay or calcium carbonate crystals. Groundwater was observed at 20 feet below ground surface at the beginning of the second day drilling with the hole advanced to 29 feet.

Packer testing on DH-16-01 was performed in 10 foot intervals and resulted in bedrock permeability values of 1.0 x10-4 to 2.0 x 10-4 cm/s. Packer testing was performed using clean water and the intercepted steeply dipping bedrock fractures, which may have resulted in the increase in permeability when compared to the 2007 constant and falling head tests. Results of the packer testing are shown Lugeon Pattern in Figure 4-7.

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Figure 4-7. Summarized Lugeon Patterns and permeability values from water pressure testing in boring DH-16-01.

Laboratory testing was performed on rock samples from DH-16-01 to characterize the physical and engineering properties of the sample, and results are summarized below in Table 4-1.

Table 4-1. Summarized results from lab testing performed at DH-16-01.

Boring Sample

Depth (ft)

Unit Weight

(pcf)

Moisture

Unconfined Compressive

Strength (psi)

Specific Gravity

Absorption

(ASTM

D2937)

(ASTM D2216)

(ASTM D7012 -

Method C)

(ASTM

D6472)

(ASTM

D6472)

DH-16-01 14.5-14.8 - 16.2 - - -

DH-16-01 25.3-25.9 157.7 0.8 14832 2.52 0.4

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Right Abutment

Borings DH-07-03 through DH-07-06 were advanced on the right abutment in a location that contains an upper layer of flow-top breccia above basalt bedrock. Surface conditions are shown in Photo 4-2. The drilling generally encountered bedrock at the ground surface.

RQD’s recorded for DH-07-03 through DH-07-06 ranged from 26% to 100%. DH-07-06 and DH-07-03 recorded a moderately to intensely fractured baked sediment interflow zones at

21.9 to 25.0 feet and 18.0 to 19.5 feet respectively. DH-07-04 encountered moderately soft (H5), intensely fractured (FD7) and intensely to moderately weathered (W6) volcanic breccia from 0.0 to 3.9 feet. Otherwise, the borings generally encountered slightly weathered (W3), hard (H3) moderately to slightly fractured (FD3 to FD4) vesicular Basalt. Vesicles and vugs were generally randomly oriented and unfilled. Joints were generally discolored or oxidized and often coated with clay or a soft, orange-white material. Groundwater was recorded from

6.3 to 13.0 feet below ground surface.

Photo 4-2. Surface conditions at right abutment, near boring DH-07-03.

4.3.3 Rockfill Borrow

The October 2016 investigation by GEI included two geotechnical borings in potential rockfill borrow areas. Each boring was inclined 30 degrees from vertical to target vertically oriented fractures and had a drilled length of 25 feet.

Boring RQ-16-01 was drilled downstream of the right abutment. The boring showed approximately 3.5 to 5 feet of high plasticity clay, above dark gray, fine grained Tertiary volcanic basalt with vesicular texture. Calcium carbonate crystals were observed in some of the shallower vesicles and fractures were closely to moderately spaced the entire depth.

RQD’s ranged from 32 to 82% the length of the boring with most fractures occurring on 5-inch intervals. Groundwater was measured at 23 feet at completion of the drilling.

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Boring RQ-16-02 was drilled downstream of the left abutment. This boring encountered sandy high plasticity clay overburden and intensely weathered basalt to approximately 12 feet, followed by a 1.5 foot thick layer of baked interflow sediments. Freshly weathered basalt was observed below the interflow sediments to the bottom of the boring. Fractures were generally randomly oriented, moderately open and contained clay or calcite crystal infilling. The fresh basalt also contained a vesicular texture from pinhead to 1-inch diameters. RQD values were generally 0% to 15 feet, and increased from 25% to 84% for the rest of the boring. Groundwater was measured in the borehole after completion at 16.2 feet.

Based on boring RQ-16-01 the rockfill borrow location at the right abutment is the preferred source of rockfill. Surficial boulders collected during site clearing and grubbing may be of use for larger diameter rock if they are durable enough to exhibit minor damage during handling and stockpiling. Additional boulders may be sourced from the excavation of the dam foundation. Select material handling procedures may be required to economically provide the required rockfill quantities.

Geophysical seismic survey lines through the borrow areas show near surface shear wave velocity generally less than 2000 fps and up to 5,000 fps (Figure 4-8). Based on the Caterpillar 12th edition handbook of ripping, a D-9 dozer or similar equipment is able to rip igneous rock with seismic velocity values up to 7,000 ft/s. Therefore, it is anticipated that limited to no blasting will be required for rockfill borrow excavation.

In general, flow top breccia and similar baked sediment material should be selectively screened for use as Zone 2 Rockfill. The ripping process is expected to produce a variety of rockfill sizes which must be screened and sorted on site for mixing into the specified mix gradations.

Figure 4-8. Geophysical profiles obtained from ReMi seismic survey. Upper plot shows velocity profile of the borrow area downstream of left abutment, and lower plot shows the velocity profile of the borrow area downstream of the right abutment.

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5.0 Embankment Designs

5.1 Embankment Configuration

5.1.1 Crest Elevation and Width

The crest of the new embankment dam will serve as the primary access road for dam maintenance and operations and will provide access to the spillway and land on the east side of the reservoir. Dam safety modifications include covering the new crest with a minimum of 6 inches of base course aggregate to provide a stable driving surface. The crest will be sloped at 2 percent toward the reservoir to minimize erosion and runoff on the downstream embankment slope. The design crest elevation is El. 4990.5, which allows for 5 feet of freeboard above the spillway crest.

The crest width for the new embankment dam was selected in accordance with criteria specified in Design of Small Dams (USBR, 1987), which indicates the minimum crest width should be equal to the vertical height of the dam in feet divided by 5 plus 10 feet. To allow for constructability using typical earthwork equipment and considering the crest will serve as the primary access for dam maintenance and operations, the 15-foot wide design crest was selected.

The new dam will be constructed with a crest elevation that is approximately 3 feet higher than the existing structure, requiring a longer embankment length to intercept the higher topography on the right abutment. A bend in the alignment at embankment station 3+50 was included to reduce the required dam length due to the relatively shallow topography at the right abutment.

5.2 Embankment Materials

The new rockfill embankment includes four material types, Zone 1 and Zone 2 Rockfill, slope protection, and a clay blanket in addition to the concrete cutoff wall. Figure 5-1 shows the embankment prism. Note that the slope protection will be incorporated into the Zone 1 Rockfill, and the thickness and extent of the slope protection will be determined based on the size and quantity of large diameter material as a result of rockfill borrow development.

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Figure 5-1. Typical section through Lauer Dam showing distribution of materials.

5.2.1 Zone 1 and Zone 2 Rockfill

Rockfill materials are intended to provide embankment slope stability and will be made of two material types, Zone 1 and Zone 2. Both materials will be obtained from the same borrow source and are anticipated to have similar material properties, but will differ in the maximum grain size. Zone 1 Rockfill will be pit run materials with a maximum particle size of 9-inches, and Zone 2 Rockfill will be screened to have a maximum particle size of 3-inches. Zone 2 Rockfill is intended to function as a cushion layer and will be placed in thin vertical zones approximately 5-feet wide against concrete structures at the embankment, including the concrete cutoff wall, the outlet works conduit, and retaining walls at the outlet works intake. A thin protective layer of Zone 2 Rockfill will be placed above the clay blanket prior to placement of the Zone 1 material. The gradation of the Zone 1 and Zone 2 Rockfill materials will be determined during rockfill borrow development at the start of construction.

A surficial layer of coarse rockfill will be placed on the outer upstream and downstream surfaces to provide slope protection. Design calculations show the optimal thickness of the upstream slope protection is a D50 of 18-inches and a thickness of 30-inches, but the actual thickness and gradation of the slope protection will be determined during rockfill borrow development based on the size of excavated material. The slope protection is intended to provide resistance against wave action on the upstream slope, and erosion protection on the downstream slope in the event accidental leakage or overtopping occurs. Due to the steep slopes, special attention will be paid to placement procedures.

5.2.2 Clay Blanket

Clay materials within the footprint of the new dam will be used to construct a clay blanket upstream of the concrete cutoff wall. The clay blanket is intended to reduce the hydraulic gradient beneath the concrete cutoff wall, and will be placed above a layer of 16 oz/yd2 non-woven needle punched geotextile. The geotextile will also provide additional protection from piping along fractures that may not be observed or treated during foundation cleaning.

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The clay blanket will be compacted to 95% of modified proctor, -2 to +2% of optimum, and will be placed in 9-inch thick lifts to a finished thickness of 3 feet.

5.3 Slope Stability Analysis

Two dimensional seepage and slope stability analyses were performed using the SEEP/W and SLOPE/W computer programs (Version 8.15.5.11777, GEO-SLOPE International, Ltd.).

Results of the analyses are summarized in this section, and complete results are contained in Appendix G.

5.3.1 Hydraulic Conductivity Values

Material properties were selected based on available data and published research results for similar materials. The 2007 site investigation performed by Reclamation measured hydraulic conductivity of core holes along the proposed dam alignment with gravity constant head and falling head tests. However, the tests used water that contained significant amounts of algae and suspended solids, and were performed in 25-foot deep, uncased vertical core holes.

Permeability values ranged from 3.9 x 10-5 cm/s to 1.3 x 10-6 cm/s. These tests did not capture the permeability of steeply dipping bedrock fractures that are visible on the ground surface, and whose orientation matches regional trends observed in air photos. The 2016 investigation performed by GEI included inclined boring and packer tests and were performed using clean water and in isolated 10 foot zones. Measured permeability values ranged between 1.0 x 10-4 cm/s and 2.2 x 10-4 cm/s. For our model, a value of 1x10-4 cm/s was applied to the competent bedrock and a value of 2 x 10-4 cm/s was applied to the weathered bedrock. Soil hydraulic conductivity was based on standard values referenced in USBR Design Standards No. 13 Chapter 8. Hydraulic conductivity values are summarized below in Table 5-1.

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Table 5-1. Summarized hydraulic conductivity values for Lauer Dam.

Placed Materials

Vertical Hydraulic Conductivity, kv

Anisotropy Ratio (kH/kV)

Comment Range

(cm/sec)

Selected for Design

(cm/sec)

Range for Standard

Placement of Embankment

Material

Selected for Design

Zone 1 Rockfill -- 1x10-2

4 to 9

4 Some fines content increase is expected during compaction, resulting in a lower hydraulic conductivity than normal.

Zone 2 Rockfill

1.0 to 1x10-3 1x10-3 4

Zone 3 Clay…