Lauer-2008_USBR_GeolInvRpt.pdf

Prepared for:

U.S. Department of the Interior Bureau of Indian Affairs Safety of Dams Program Pacific Region

Prepared by:

U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado January 2008

Geologic Investigation Report

Lauer Dam

XL Ranch Indian Reservation, California

Bureau of Indian Affairs Safety of Dams Program Pacific Region

Prepared by:

U.S. Department of the Interior Bureau of Reclamation

Denver, Colorado January 2008

Bureau of Indian Affairs Safety of Dams Program Pacific Region

Prepared by:

U.S. Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado January 2008

XL Ranch Indian Reservation, California

Bureau of Indian Affairs Safety of Dams Program

Pacific Region

Prepared by:

Bureau of Reclamation

Denver, Colorado

Gary Russell Curtis Cain

Engineering Geology Group

January 2008

TABLE OF CONTENTS

Page No.

EXECUTIVE SUMMARY …………..…………………………………………………. i

I. Introduction …………………………………………………………...……….…… 1

II. Location and Description ………………………………………...………………… 1

III. Dam Safety Issues ……………………………...……………………..…………… 3

IV. Geologic Investigations …………...……………………..………………………... 4

A. Drill Holes ………………………………………………………………….……. 5 B. Dam Crest SPT Holes ……………...……………………………………………. 8 C. Dike Spillway Test Pits …………………………………...………………..……. 8 D. Borrow Area Test Pits …………………………..……………….………………. 9

V. Regional Geology …………………………………..................………………….. 10

VI. Site Geology ……………………………………………….…………………….. 12

A. Lauer Dam Site …………………………………..…………………………….. 12 Quaternary Colluvium (Qc) ………..…………………………………………... 12 Flow-top Breccia (Tv) ……………...…………………………………………... 12 Basalt (Tv) ……………………..…..…………………………………………... 13 Interflow Sediments (Tv) ………..……………………………………………... 13 B. Existing Lauer Dam Embankment ………..……………………………………. 14 C. Dike Spillway Site …………………………………….……………………...… 14 D. Potential Borrow Area …….………….………………............................……... 15

VII. Results of Explorations, and Evaluation of Data ……………………………….. 17 A. Depth to Bedrock ………………………………..…….……………………….. 17 Lauer Dam Site ………………….....…………………………………………... 17 Dike Spillway Site ……………...…………………………….………………... 17 Potential Borrow Area …………………..……………………………………... 18 B. Bedrock Permeability Test Results ………..…………...………………………. 18 Constant-head Tests ……………......…………………………………………... 19 Falling-head Tests ………….…...…………………………….………………... 19 C. Standard Penetration Test Results …………………….……………………...… 20 D. Dispersibility Test Procedures and Results ……...………………............……... 20 E. Borrow Materials ……...…………………………………………............……... 21

TABLE OF CONTENTS (continued)

Page No.

VIII. Geotechnical Considerations …………………….…………………………….. 22 A. Groundwater Conditions ……………..…………..…………………………….. 22 Lauer Dam Site ………………….....…………………………………………... 22 Dike Spillway Site ……………...…………………………….………………... 23 Potential Borrow Area …………………..……………………………………... 23 B. Foundation Treatment ………..………………………...………………………. 23 C. Excavation Considerations ……………..…….…..…………………………….. 23 Lauer Dam Site ………………….....…………………………………………... 23 Dike Spillway Site ……………...…………………………….………………... 23 Potential Borrow Area …………………..……………………………………... 24

REFERENCES …………….….……………………………………………………….. 25

TABLES

1. Summary of Current Investigations …………………………………..……….……. 5

2. Summary of Depths to Hard Basalt ……………………………..………………… 18

3. Summary of Gravity Permeability Test Results …………………………………... 19

4. Summary of Dispersibility Test Results from Borrow Area Samples …...………... 21

FIGURES

1. General Location Map ………………..…………………………………………….. 2

2. Photograph of Crest of Lauer Dam …………………….…………………………… 3

3. Photograph of CME 750X All- terrain Drill Rig ………………...………………….. 6

4. Photograph of Constant-head Permeability Test ……...……………………………. 7

5. Photograph of Case 580K Backhoe …………………...……………………………. 9

6. Map of California Showing Modoc Plateau Physiographic Province ……….……. 10

7. Geologic Map Showing the Devils Garden Lava Field …………………..….……. 11

8. Photograph of Ground Surface Downstream of the Existing Dike ……..........……. 16

9. Photograph of the Potential Borrow Area ……….…………………...............……. 17

Executive Summary Lauer Dam Geologic Investigation Report i

EXECUTIVE SUMMARY

This report was prepared to document the results of geologic field investigations at Lauer Dam. The investigations include core holes immediately downstream of the existing dam, standard penetration test (SPT) holes on the dam crest, test pits at the dike spillway location, and test pits with inplace densities at a potential borrow area.

Lauer Dam is a rock-faced, homogeneous earthfill structure located in northeastern California, approximately 11 miles north of the city of Alturas. Constructed in 1895, the dam is operated by personnel from the XL Ranch Indian Reservation. The dam has a structural height of approximately 13 feet, a crest length of approximately 830 feet, and a crest width of approximately 12 feet.

There are currently two spillways for the reservoir, one located on the right abutment of the dam, and the other located on the left side of the reservoir approximately one-half mile from the dam. The right abutment spillway is an unlined open channel approximately 50 feet wide. The other spillway (referred to as the “dike spillway”) consists of a 5-foot-wide breach in a dike along the left side of the reservoir rim. The dike spillway is about one foot lower in elevation than the right abutment spillway, and serves as the primary (service) spillway.

Alternative methods for addressing safety of dams (SOD) deficiencies are documented in the 2006 Conceptual Design Report [10]. The preferred concept identified by the design team includes the construction of a new embankment immediately downstream of the existing dam, and modification of the existing dike spillway. Geologic and geotechnical investigations at the new embankment location, the dike spillway site, and a potential borrow area were completed in September 2007.

Six core holes were completed downstream of the existing embankment to provide foundation data for a proposed new embankment. All six drill holes were completed to a depth of 25 feet by Spectrum Exploration, Inc. using a truck-mounted CME 750X all-terrain drill rig. Gravity permeability tests were conducted in five of the drill holes. Two SPT holes were performed on the crest of the existing embankment to determine embankment soil composition and densities. However, only one of these holes was able to penetrate the embankment and reach the predetermined depth. Four test pits were excavated using a Case 580K backhoe with a 24- inch wide bucket at the dike spillway site for foundation investigations. Six test pits were completed at a potential borrow area using the Case 580K backhoe. Inplace density tests were conducted at depths of 4.0 feet in four of these test pits.

All but one of the drill holes downstream of the existing embankment encountered bedrock essentially at the ground surface. A five-foot-thick surficial deposit of colluvium overlying bedrock was recovered in drill hole DH-07-02 located in the channel area downstream of the existing dam. Bedrock at Lauer Dam consists primarily of Tertiary basalt flows separated by thin, interflow zones of red-colored, baked sediments. Three drill holes located toward the right side of the new dam alignment encountered about four ii to six feet of volcanic breccia overlying hard basalt. The gravity permeability tests indicate low overall bedrock permeability, ranging from about 1 foot per year to about 27 feet per year.

Lauer Dam embankment materials, sampled in drill hole SPT-07-07, consist of about two feet of sandy silt overlying clay to the base of the dam. Laboratory testing of the clayey soils resulted in a classification of Lean Clay, CL, with liquid limits ranging from 45 to 48 and plasticity index ranging from 22 to 27. Standard penetration test blow counts ranged from 6 to 16 blows per foot (bpf), indicating fairly low density materials. The higher blow counts (11 to 16 bpf) occurred in the upper section of the embankment (to a depth of 6.5 feet), and the lower blow counts (6 and 8 bpf) were obtained in the lower part of the embankment

Geology at the dike spillway site consists of a couple inches to a foot of soil or loose sand overlying basalt bedrock. The underlying basalt was decomposed to moderately weathered and difficult to excavate. The backhoe met refusal on hard basalt at depths ranging from 0.8 to 3.0 feet.

The geology of the borrow area is interpreted as volcaniclastic sediments overlying basalt bedrock. The volcaniclastic sediments consist of about half fat clay and half elastic silt.

The fat clay generally consists of 100 percent fines with high plasticity, slow dilatancy, high dry strength, high toughness. The elastic silt consists mostly of 70 percent to 90 percent fines with low to medium plasticity, medium dry strength, low to medium toughness, and 10 to 30 percent fine sand. The underlying basalt bedrock is mostly decomposed and was encountered from depths ranging from 4.0 to 10.0 feet below the surface. The volcaniclastic sediments in the borrow area may be difficult to process and have poor workability as a construction material because of the plastic nature of the clayey soils.

Control of water during construction of a new downstream embankment and modification to the dike spillway is expected to be straightforward and comprise a very small component of the overall work. Dewatering wells or wellpoints are not anticipated to be required, particularly since construction excavations for these features are not anticipated to be particularly deep. Control of water in the construction areas can likely be accomplished by lowering the reservoir, and/or installing unwatering trenches with sump pumps. Groundwater was not encountered in any of the test pit excavations at the proposed borrow area. However, this area is shown as an intermittent lake/pond and may have higher groundwater levels or surface water during certain times of the year. If this borrow area is used as a source of construction materials, unwatering methods may be required depending upon the depths of excavation, the time of year and the amount of precipitation.

Stripping of the surficial soils at the embankment and dike spillway sites can be accomplished by common methods (scraper, dozer, excavator). The near-surface layer of colluvium, the layer of volcanic breccia and the underlying decomposed bedrock should iii also be able to be excavated using common methods. Once the top of the hard basalt is reached, any additional excavation will likely require blasting.

In the borrow area, test pit excavations with the backhoe were generally fast and easy through the volcaniclastic materials and the decomposed basalt. Excavations became more difficult as the bedrock became harder and less weathered. The backhoe met refusal in all but one test pit.

I. Introduction

This report presents the geologic and geotechnical data obtained at Lauer Dam from investigations conducted in September 2007 in support of the Safety of Dams (SOD) program. These investigations were completed under the Revised Project Plan for Final Design, dated July, 2007 [11] and the Technical Service Center (TSC) draft Field Exploration Request (FER), dated August 2007, Appendix A. A primary objective of the investigations was to characterize the geologic conditions downstream of the existing dam, at the dike spillway site, and at a potential borrow area located about three miles south of the dam. Drilling was also performed on the crest of the dam to determine material properties of the existing embankment.

II. Location and Description

No design or construction documentation or design drawings are available for Lauer Dam. Most of the historical information about Lauer Dam has been taken from later reports, primarily the 2006 Conceptual Design Report [10].

Lauer Dam is a rock-faced, homogeneous earthfill structure located in northeastern California, in the high desert of the Modoc Plateau, approximately 11 miles north of the city of Alturas (Figure 1). Constructed in 1895, the dam is operated by personnel from the XL Ranch Indian Reservation. The embankment is located partially on State of California (State) land and partially on Bureau of Land Management (BLM) land. The majority of the reservoir is located on United States Forest Service (USFS) land, but is also located on BLM and State lands. The reservoir is not fed directly by a river or stream, but by runoff and groundwater from a 6.2 square mile drainage basin. The reservoir has an estimated capacity of 1,950 acre-feet at normal reservoir water surface elevation (4980.0).

The dam has a structural height of approximately 13 feet, a crest length of approximately 830 feet, and a crest width of approximately 12 feet (Figure 2). A survey of the two drill holes on the dam crest, DH-07-07 and DH-07-08, resulted in crest elevations of 4987.5 and 4987.7 (NAVD88 vertical datum), respectively.

Figure 1. General map of the area north of Alturas, California showing the location of Lauer Dam and the potential borrow area.

There are currently two spillways for the reservoir, one located on the right abutment of the embankment, and the other located on the left side of the reservoir approximately one-half mile upstream of the dam. The right abutment spillway is an unlined open channel approximately 50 feet wide. The other spillway (referred to as the “dike spillway”) consists of a 5-foot-wide breach in a dike along the left side of the reservoir basin. Because this spillway is about one foot lower than the right abutment spillway, it serves as the primary, or service spillway.

The original outlet works at Lauer Dam consisted of two 8- inch-diameter steel pipes with two 8- inch-diameter valves at the downstream end. Records indicate that the outlet works pipes were modified in 1931 by installing a hydraulically controlled slide gate at the intake. Presently, the outlet works are in unsatisfactory condition and inoperable.

The slide gate structure and equipment at the crest no longer exist as a complete unit.

Figure 2. View of the crest of Lauer Dam, looking toward the left abutment.

Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by M.

McCulla, September 2007.

III. Dam Safety Issues

The existing embankment of Lauer Dam was identified as being in poor condition with over-steepened upstream and downstream slopes, as well as rodent burrows and extensive seepage. The dam has also been recognized as having dam safety deficiencies related to slope stability and potential piping of embankment materials (primarily along or into the outlet works conduit). Verified and assumed deficiencies are presented in detail in the Deficiency Verification Analysis (DVA) report [9]. Alternative methods for addressing the deficiencies and other safety issues are documented in the 2006 Conceptual Design Report [10]. Subsequent to the issuance of the Conceptual Design Report, a Value Engineering (VE) study was performed. The combination of the two design alternatives that comprise the preferred concept identified by the design team after the VE study that will address the verified and assumed Safety of Dams (SOD) deficiencies are as follows:

• VE Proposal No. 1 – Replace the existing dam with a new dam immediately downstream of the existing dam.

• Alternative S4 (modified) – Enlarge the existing dike spillway.

An alignment for the proposed new embankment was selected in a location that ranges from approximately 50 to 150 feet downstream of the existing embankment. The present design for the proposed replacement dam is a homogeneous earthfill structure with an internal filter blanket and toe drain system near the downstream toe. All overburden materials would be stripped so that the entire embankment footprint would be founded on bedrock. The new embankment would be approximately 1,500 feet in length with a maximum height of about 17 feet. The current design for the proposed dike spillway includes a 400-foot wide, riprap- lined main channel with a 10-foot-wide pilot channel.

Additional engineering analysis will be required during final design to further refine the alternatives. Borrow materials to construct the new embankment and dike spillway need to be identified, as well as a source of riprap for both features. Data obtained during these geotechnical field investigations will help with the analyses and selection of possible borrow areas.

IV. Geologic Investigations

No prior geologic investigations in the immediate vicinity of Lauer Dam have been conducted, other than collection of limited samples of embankment soils by hand-sampling methods. A study of five potential construction access roads to the dam site and potential sources of road-base borrow was completed in 2003 [8]. Field work for this study consisted of surface examination of geology along about 49 miles of existing road and about 5 miles of potential new road alignment. A cursory examination of a local cinder borrow pit was also completed. No soil samples were collected and no subsurface investigations were conducted for this phase of the study.

The current investigations at Lauer Dam, dike spillway and borrow area were conducted in September 2007. Geologic and geotechnical data requirements for the program are outlined in the Revised Project Plan for Final Design [11], the August, 2007 draft FER (Appendix A), and the 2007 Statement of Work [12]. The investigation program was intended to provide data for the design alternatives that will address the verified and assumed Safety of Dams (SOD) deficiencies described above. Core drilling was done along the proposed new dam alignment to determine the depth to bedrock, rock quality, discontinuity characteristics, groundwater conditions, and foundation permeability. Two Standard Penetration Test (SPT) holes were drilled from the crest of the existing embankment to collect samples of the embankment soils and obtain penetration blow counts. Test pits were completed at the dike spillway site to provide foundation data which will be used during final design to further refine the new spillway design. Test pits were also completed at a potential borrow area located about three miles south of Lauer Dam.

Mobilization of the drilling rig, water truck, backhoe and other equipment from the town of Alturas to the dam site took about five hours. Both the drill rig and backhoe had rubber tires to aid with access to the site and to minimize cultural, biological and environmental disturbance. Presently, the unimproved access roads to the dam and borrow sites are very rough, particularly where the road traverses the surface of a basalt flow. In these sections, cobble- to boulder-size blocks of hard basalt often line the road surface. Some stretches of road cross deposits of fine-grained clayey airfall tuff. When these materials become wet, they can be very soft and slick.

The following table (Table 1) and the paragraphs below provide a summary of the current investigations. The locations of these explorations are shown on Figures Dwg-2 through Dwg-5 (Appendix E).

Table 1. Summary of Current Investigations.

Hole Designation Ground Elevation Hole Location (Coordinates) Total Depth

Drill Holes Along the Proposed Dam Alignment

DH-07-01 4984.3 N2491042.4 E6980856.8 25.0

DH-07-02 4972.6 N2490838.6 E6980815.4 25.0

DH-07-03 4980.6 N2490625.8 E6980632.1 25.0

DH-07-04 4983.1 N2490430.4 E6980481.2 25.0

DH-07-05 4983.4 N2490250.6 E6980326.1 25.0

DH-07-06 4986.6 N2490055.3 E6980183.9 25.0

Dam Crest SPT Holes

DH-07-07 4987.5 N2490975.3 E6980664.2 13.0

DH-07-08 4987.7 N2490887.5 E6980618.4 2.0

Dike Spillway Test Pits

TP-07-01 4982.5 N2493231.9 E6979848.2 3.0

TP-07-02 4982.4 N2493139.2 E6979862.8 0.8

TP-07-03 4982.2 N2493180.7 E6979816.9 2.0

TP-07-04 4982.4 N2493202.9 E6979785.2 2.0

Potential Borrow Area Test Pits

TP-07-05 5017.5 N2475044.4 E6982042.6 10.0

TP-07-06 5016.7 N2474901.3 E6983293.7 11.0

TP-07-07 5016.9 N2474720.8 E6983929.2 8.0

TP-07-08 5016.8 N2474329.6 E6984704.5 4.1

TP-07-09 5017.1 N2475090.5 E6984759.9 12.0

TP-07-10 5017.2 N2474242.1 E6982500.4 11.5

Notes:

All depths and elevations are in feet.

Coordinates and ground elevations are reported in 83/88 datum.

A. Drill Holes

Six core holes, DH-07-01 through -06, were completed downstream of the existing embankment to provide foundation data for a proposed new embankment. All six drill holes were completed by Spectrum Exploration, Inc.

using a truck-mounted CME 750X all-terrain drill rig with all-wheel drive and 31-inch tires (Figure 3). Coring was accomplished using a wireline NQ core system with a 3.79-inch O.D. diamond bit and 5-foot- long split inner barrel. All core holes were drilled to a depth of 25 feet using reservoir water for drilling fluid.

The reservoir water contained a considerable amount of floating algae and other suspended substances.

Figure 3. CME 750X all-terrain drill rig. Coring was accomplished using a wireline NQ core system with a 3.79-inch O.D. diamond bit and 5-foot- long split inner barrel.

Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by G.

Russell, September 2007.

Gravity permeability tests were conducted in drill holes DH-07-02 through DH- 07-06. A permeability test was not attempted in drill hole DH-07-01 because the basalt core samples were only slightly fractured and the water level in this hole dropped extremely slowly. In the other holes, constant-head and falling-head tests were conducted in a completely open, uncased and unscreened hole. Prior to starting the gravity permeability tests, water was added to each hole to saturate the bedrock interval above the natural water table. The volume of water needed to maintain a constant head was generally so small that inflow could not be sufficiently limited using the conventional water-valve and flow-meter arrangement on the drill rig. In order to accurately control the flow of water into the holes and maintain a constant head, a water delivery system was constructed using a five-gallon bucket with a small rubber tube from the base of the bucket (Figure 4).

Figure 4. Constant-head permeability test being conducted in drill hole DH-07-05.

Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by G.

Russell, September 2007.

The flow of water from the tube was adjusted until a uniform inflow resulted in stabilization of water level at a predetermined level. After stabilization was attained, the actual flow rate was determined by the time required to fill a container of known volume. A constant-head test was continued for at least one hour in each of the holes. The water delivery system was then removed from the hole in order to allow the water level to drop during the falling-head test period.

Instrumentation Northwest, Inc. data logger equipment and a laptop computer were used to record water pressure and temperature data during each test session, which included both constant-head and falling-head test periods. A PT2X sensor (transducer) was lowered down the hole inside an open PVC pipe to prevent the sensor from getting stuck in the hole. Results of the gravity permeability tests are summarized in section VII (Results of Explorations and Evaluation of Data), and the calculated K-values are also included on the individual drill hole logs. Water Depth vs. Time graphs and Displacement vs. Time graphs for individual gravity tests are included on Figures Perm-1 through Perm-7 in Appendix D. After the permeability tests were completed, each hole was backfilled with bentonite chips, which were hydrated by the remaining water in the holes and pouring additional water down the holes.

B. Dam Crest SPT Holes

Two SPT holes (DH-07-07 and DH-07-08) were performed on the crest of the existing embankment to determine embankment soil composition and densities.

Both holes were drilled by Spectrum Exploration, Inc. using the CME 750X. The holes were drilled dry, without the use of any drilling fluid. Only drill hole DH- 07-07 reached the required depth to the base of the embankment (13.0 feet). Drill hole DH-07-08 met refusal on a hard cobble at a depth of only 2.0 feet. Another crest hole was attempted in an area located about 5 to 10 feet toward the right abutment. However, this hole also met refusal near the surface of the embankment.

The CME 750X was equipped with a 140-pound automatic hammer capable of consistent and accurate SPT results, and meeting all ASTM requirements. A constant 1-3/8- inch I.D., 2.3-foot- long split-barrel type penetration sampler was used for the standard penetration tests. A 4.25- inch I.D. by 8.25-inch O.D. flight auger with pilot bit (FAPB) was used to advance the hole to the next SPT interval (one foot below the previous SPT interval). No samples, other than auger cuttings, were retrieved with the FAPB. For each SPT test, the sampler was driven 1.5 feet, recording blow counts in the six- inch seating interval and within the one-foot test interval. The sum of the blow counts recorded in this one-foot test interval is referred to as “N” (or the “N-value”). The drill-hole log for DH- 07-07 (Appendix B) displays the field blows for each six- inch interval, as well as uncorrected N-values for the test intervals.

Each soil sample recovered from the SPT sampler was classified visually in the field. Each sample was also submitted to the TSC materials laboratory for testing of standard soils index properties including gradation, consistency (Atterberg limits) and moisture content. Laboratory data summary sheets are included in Appendix C.

Drill hole DH-07-07 was backfilled with hydrated bentonite chips, and drill hole DH-07-08 was filled with embankment material auger cuttings.

C. Dike Spillway Test Pits

Four foundation investigation test pits were excavated using a Case 580K backhoe with a 24- inch-wide bucket. Total depths of the pits ranged from 0.8 to

3.0 feet, with all test pits meeting refusal on bedrock. Descriptions and classifications of the soil and rock materials were based on samples taken from the spoil piles. The test pits did not encountered any seepage water.

Selected samples recovered from the test pits were bagged and sent to the Mid Pacific Construction Office (MPCO) materials laboratory in Willows, California for testing of index properties including gradation, consistency (Atterberg limits) and moisture content. Laboratory data summary sheets are in Appendix C.

D. Borrow Area Test Pits

Six test pits for borrow investigations were excavated using a Case 580K backhoe (Figure 5) with a 24- inch-wide bucket. Total depths of the six pits ranged from

4.1 to 12.0 feet, depending on meeting refusal on bedrock or reaching the maximum reach of the backhoe. For safety purposes, no test pit over four feet in depth was entered by field personnel. Soil and rock classifications were performed on samples taken from the spoil pile and the backhoe bucket.

Figure 5. Case 580K backhoe. Excavation was accomplished using 24- inch-wide bucket with teeth. Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by C. Cain, September 2007.

Groundwater was not encountered in any of the test pits. The volcaniclastic materials and the decomposed basalt were generally easy to excavate. The moderately weathered to fresh basalt was difficult to excavate, often resulting in refusal of the backhoe. Inplace density tests were conducted at depths of 4.0 feet in four of the test pits. Selected soil samples recovered from the pits were bagged in 50-pound sacks and sent to the Mid Pacific Construction Office (MPCO) materials laboratory in Willows, California for testing of standard soils index properties including gradation, consistency (Atterberg limits) and moisture content. Additional soil materials were collected from test pits TP-07-06, -07 and -09 to get a broader representative (overlapping) sample of the deposits. These samples were tested in the TSC materia ls laboratory in Denver, Colorado.

V. Regional Geology

The Modoc Plateau lies in the northeast corner of California (Figure 6) as well as parts of Oregon and Nevada. Nearly one million acres of the Modoc National Forest lie on the plateau between the Medicine Lake Highlands in the west and the Warner Mountains in the east. The Modoc region consists of a series of north- to northwest-trending block-faulted ranges. 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 geology in the area of Modoc County, California is all Cenozoic, mainly Miocene to Holocene volcanics, volcaniclastic sediments, and lake deposits, including diatomite. The oldest rocks in the Modoc region are exposed only in the uplifted and tilted fault blocks.

Figure 6. Map of California showing the location of the Modoc Plateau Physiographic Province.

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 thickness of the Warner Basalt varies considerably, even over short distances. At the edge of the plateau near Alturas, California, the Warner Basalt ranges in thickness from about 15 feet to more than 360 feet [4]. Individual flow units range from less than 2 feet to more than 50 feet, and probably average 4 to 5 feet. Thin basalt flow units are vesicular throughout, but thicker flows may be very dense in their middle and lower parts. The surface of the plateau is generally gently undulating. In some areas tumuli are common, which are domes or small mounds on the crust of the lava flow, caused by pressure due to the difference in rate of flow between cooler crust and the more fluid lava below.

The largest continuous exposure of the Warner Basalt is that of the Gardens Basalt on the high plateau that stretches from Alturas, California westward more than 20 miles and northward more than 25 miles, with extensions reaching north of Clear Lake Reservoir (Figure 7). This lava field, commonly called “The Gardens” or “The Devils Garden”, lies between the Basin and Range and Cascade Range physiographic provinces. The basaltic lavas of the entire region are similar in petrology and age and thus constitute a coherent lava field. The volume of the basalt of Devils Garden is estimated to be at least 850 km3 [4]. Radiometric dating indicates that the basalt of Devils Garden was emplaced during late Miocene and earliest Pliocene time, resulting in an age range mostly between 5 and 10 million years [5]. The basalts originated in the upper mantle and were erupted through crust thinned by tectonic extension behind the Cascade Range volcanic arc.

Figure 7. Geologic map showing the Devils Garden lava field (in yellow) with relationship to Lauer Dam and the city of Alturas, California.

From Gay T. E., California Division of Mines and Geology.

VI. Site Geology

A. Lauer Dam Site

Lauer Dam is located about six miles south of Goose Lake on the southeastern edge of the Devils Garden lava field (Figure 7). Bedrock was very shallow in five of the six drill holes located downstream of the existing dam along the alignment for the proposed new structure. Based on the drill hole data, the existing dam embankment and outlet works appear to have been constructed directly on Tertiary volcanic bedrock (Tv). Overburden materials were less than one foot thick in all drill holes except DH-07-02. Drill hole DH-07-02, located in the downstream channel area, encountered about five feet of Quaternary colluvium (Qc) overlying decomposed bedrock.

Tertiary volcanic bedrock (Tv) in the drill holes consisted primarily of basalt flows separated by thin, interflow zones of red-colored, baked sediments. The three drill holes located toward the right side of the new dam alignment (DH-07- 04, -05, and -06) encountered about four to six feet of volcanic flow-top breccia in the upper section of the holes. The following paragraphs describe the composition and properties of the Quaternary colluvium (Qc) and volcanic bedrock (Tv) units.

Quaternary Colluvium (Qc). A five-foot-thick surficial deposit of colluvium overlying basalt was recovered in drill hole DH-07-02 located in the channel area downstream of the existing dam. The colluvial material consists of medium to dark brown, firm to hard, clayey soil derived from the decomposition of volcanic rock. Occasional organics (small roots, etc.) were observed in the soil. Based on results of visual and manual tests, the soil classifies as a Sandy Lean Clay with Gravel and Cobbles, s(CL)gc, containing about 60 percent clayey fines with medium to high plasticity and dry strength, about 20 percent sand and about 20 percent angular fragments of basalt (which are more abundant toward the bottom of the interval). A 0.4-foot fragment of hard basalt was recovered from the very bottom of the interval. Underlying the colluvium (to a depth of about 10 to 11 feet) is orange-brown basalt which is decomposed to the extent that the bedrock is discolored and weakened throughout, soft, friable, and has properties of a clayey soil. Below this decomposed interval the basalt becomes less weathered, showing rock properties similar to the basalt in the other drill holes.

Flow-top Breccia (Tv). Volcanic breccia was encountered in the upper four to six feet of bedrock core in drill holes DH-07-04, -05, and -06. The breccia was likely formed contemporaneously with the movement of the upper lava flow as the cooling crust became fragmented while the flow was still in motion. The resulting rock unit consists of angular fragments of gray, vesicular basalt in a matrix of brown, indurated clay. The clayey matrix contains evidence of volcanic origin, such as small vesicles, weathered glass and other minerals (green olivine).

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. The core breaks through the clayey matrix (around the fragments), resulting in an overall core hardness of moderately soft (H5). The core is intensely fractured (FD7), but the joints are poorly defined due to the nature of the rock (i.e. the combination of hard fragments in a softer matrix).

Some of the fractures may have been mechanically induced by the drilling action.

Basalt (Tv). The primary rock type in all of the drill holes is medium to dark gray, fine grained basalt. Vesicles and vugs occur primarily in the upper and lower sections of the basalt flows. They average about 1/8-inch to ¼-inch diameter and have a maximum size of about 1 to 2 inches. Vesicles and vugs are generally randomly oriented, elliptically shaped, open (not filled) and not interconnected. Occasionally, the vesicles are oriented in a band along the flow structure.

For the most part, the basalt is slightly weathered (W3) and hard (H3). However, in drill hole DH-07-02 the basalt was moderately weathered (W5) to decomposed (W9), discolored and weakened throughout due to mineral alteration to clay.

Fracture density generally ranges from moderately to slightly fractured (FD3 and FD4); however, short intervals of intensely to moderately fractured (FD5, FD6 and FD7) basalt were also encountered. The joints are mostly subhorizontal with occasional subvertical fractures. Joint surfaces are usually thinly coated with clay and have minor discoloration or oxidation. Rarely the joints (primarily the higher-angle joints) are filled with a thicker (up to ¼-inch) infilling of moderately hard to soft, tan to whitish-brown, clayey material.

At the base of the upper basalt flow, directly overlying the unit of interflow sediments is a zone of basal breccia material. This breccia zone, encountered in three of the drill holes (DH-07-03, -04, and -06), is less than one foot thick and consists of very intensely fractured (FD9) rock containing angular fragments and chips of hard, vesicular basalt and interflow sediments in a matrix of soft, brown to tan, clayey material.

Interflow Sediments (Tv). These zones are thin (generally less than three feet thick) sediment deposits separating individual basalt flows. Baking of the fine-grained sediments by the overlying molten basalt flows resulted in orange-red, moderately hard (H4) rock. Occasional angular fragments of gray basalt are included within in the sediments, especially toward the bottom of the interflow units. The core samples were moderately to slightly weathered (W3 to W5), and moderately fractured (FD5) to intensely fractured (FD7) with core lengths ranging from fragments to 0.9 feet and averaging about 0.4 feet. The joints are generally subhorizontal with very little staining or infilling on the fracture surfaces. Some of the fractures may have been mechanically induced by the drilling action.

B. Existing Lauer Dam Embankment

Based on the samples recovered from drill hole DH-07-07, the Lauer Dam embankment (Emb) consists of about two feet of sandy silt overlying clay to the base of the dam. The soils from each SPT interval were visually classified in the field, and laboratory tested for physical properties, gradations and Atterberg limits. Test results are included on the drill hole log (Appendix B) and on the laboratory summary sheets (Appendix C).

The surficial layer of silt is medium brown, dry and had no reaction with dilute hydrochloric acid (HCl). Visual classification of this material resulted in a Sandy Silt, s(ML), containing about 60 percent non-plastic fines with no dry strength, rapid dilatancy, and low toughness, about 40 percent fine to coarse (predominantly fine) sand, and a trace of fine gravel. Laboratory classification of this material resulted in a Sandy Lean Clay, CL, with 50.9 percent fines, 35.5 percent sand and 13.6 percent gravel.

The underlying clay is medium to dark brown and slightly moist (becoming moist with depth). Visual classification of the clay ranged from Fat Clay, CH; Fat Clay with Sand, (CH)s; to Sandy Lean Clay, s(CL). The consistency of the clayey soils varied from hard (thumb would not indent the soil) toward the upper part of the embankment, to soft to firm (thumb would readily penetrate the soil) toward the lower part of the embankment. Laboratory testing of the clayey soils resulted in a classification of Lean Clay, CL, with liquid limits ranging from 45 to 48 and plasticity index ranging from 22 to 27. The upper two samples (above 6.5 feet depth) contained about 8 percent sand and 2 percent gravel, whereas the lower three samples (below 7.5 feet depth) contained between about 15 and 20 percent sand and zero gravel.

C. Dike Spillway Site

The ground surface downstream of the existing dike is covered with scattered basalt cobbles and boulders (Figure 8) resting on soils consisting of fine grained sand and silt mixed with angular pieces of moderately to slightly weathered basalt. The fine grained soil with angular pieces of basalt was encountered from

0.0 to 1.0 feet in test pit TP-07-01. The soil consists of tan to reddish brown, firm clayey soil derived from the decomposition of volcanic rock. Roots were observed in the soil. Based on the results of visual and manual tests the soil consists of about 50 percent clay fines with medium to high plasticity and medium to high dry strength, about 25 percent sand, and about 25 percent angular fragments of basalt. The soil classifies as a Clayey Sand with Gravel, (SC)g. The underlying basalt was decomposed to moderately weathered and difficult to excavate. The backhoe met refusal on hard basalt at 3.0 feet depth in test pit TP- 07-01.

The dike spillway site geology upstream of the existing dike consists of several inches of recent wave deposited sand from Lauer Reservoir overlying basalt bedrock. The sand layer was consistently 0.2 feet thick in the test pits (TP-07-02, TP-07-03, and TP-07-04). Based on the results of visual and manual tests, the sand classifies as poorly graded sand, with about 100 percent fine to medium sand with a trace of non-plastic fines. The basalt was decomposed to moderately weathered and difficult to excavate.

Fifty-pound sack samples of the weathered basalt encountered in the four dike spillway test pits were taken to the Mid Pacific Construction Office materials laboratory in Willows, California for testing. Due to the decomposed nature of the rock, the materials were able to be tested using soils testing methods. The sample recovered from test pit TP-07-01, completed on the downstream side of the dike spillway, classified as a Silty Gravel with Sand (GM)s with 48.6 percent gravel, 36.9 percent sand, 14.5 percent fines, with a liquid limit of non-plastic, a plasticity index of non-plastic, and a field moisture of 15.7 percent.

Upstream of the dike spillway, test pit TP-07-02 has a laboratory classification of Silty Sand with Gravel and Cobbles (SM)gc with 57.8 percent sand, 25.3 percent fines, 16.9 percent gravel, 8.0 percent cobbles, with a liquid limit of non-plastic, a plasticity index of non-plastic, and a field moisture of 6.9 percent. Test pits TP- 07-03 and TP-07-04 have laboratory classifications of Silty Sand with Gravel (SM)g. The laboratory test result for the sample for TP-07-03 is; 49.6 percent sand, 35.3 percent gravel, 15.1 percent fines, liquid limit is non-plastic, plasticity limit non-plastic, and field moisture 11.1 percent. The laboratory test result for the sample for TP-07-04 is; 50.1 percent sand, 33.0 percent gravel, 16.9 percent fines, liquid limit non-plastic, plasticity index non-plastic, and field moisture 9.2 percent.

D. Potential Borrow Area

A potential borrow area is located approximately three miles south of Lauer Dam in a topographically flat area which is interpreted as an intermittent lake/pond.

The area is approximately 4000 feet long by 1800 feet wide. Figure 9 is a general photo of the area, and Figure Dwg-2 (Appendix E) shows its location in relationship to the damsite.

Geologic investigations at the site revealed volcaniclastic sediments overlying basalt bedrock. The volcaniclastic sediments consist of about half fat clay and half elastic silt. The fat clay consists mostly of 100 percent black to chocolate brown fines with high plasticity, slow dilatancy, high dry strength, high toughness, and a trace of fine sand. The elastic silt consists mostly of 70 percent to 90 percent light brown to black fines with low to medium plasticity, medium dry strength, low to medium toughness, and 10 percent to 30 percent fine sand.

Except for test pit TP-07-05 where the elastic silt was encountered at the surface, the elastic silt layer was encountered underneath a fat clay layer. The underlying basalt bedrock is mostly decomposed and was encountered from depths ranging from 4.0 to 10.0 feet below the surface. Hard, fresh to moderately weathered basalt was encountered from depths ranging from 4.1 to 12 feet. All test pits but one (TP-07-09) met refusal on fresh to moderately weathered basalt. Test pit TP- 07-09 was stopped at a depth of 12.0 feet, the reach limit of the backhoe.

Figure 8. View of the cobbles and boulders on the surface immediately downstream of the existing dike. Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by C. Cain, September 2007.

Figure 9. View of the potential borrow area. Lauer Dam, XL Ranch Indian Reservation, California – Bureau of Reclamation photograph by C. Cain, September 2007.

VII. Results of Explorations, and Evaluation of Data

A. Depth to Bedrock

Lauer Dam Site. Five of the six downstream drill holes encountered volcanic bedrock very near the ground surface. However, not all of the near-surface bedrock consisted of hard basalt. An upper layer of moderately soft volcanic breccia was encountered in three of the holes. Drill hole DH-07-02 encountered a layer (about five feet thick) of colluvial soil material overlying decomposed basalt.

Dike Spillway Site. Hard basalt bedrock was encountered very near the ground surface in all four of the test pits. The Case 580K backhoe met refusal on basalt bedrock at depths of less than three feet. The backhoe was only able to penetrate the very thin deposit of loose, poorly graded beach sand and the upper surface of decomposed to moderately weathered basalt.

Potential Borrow Area. Within the potential borrow area, the Case 580K backhoe was able to easily excavate the layer of volcaniclastic tuff (which ranged in depth from 4.0 to 10.0 feet), and the underlying layer of decomposed to moderately weathered basalt. The backhoe met refusal on hard basalt bedrock at depths ranging from 4.1 to 11.5 feet. Test pit TP-07-09 reached a depth of 12.0 feet without encountering hard basalt.

The following table (Table 2) lists the depths to hard basalt in each of the downstream drill holes, dike spillway test pits and borrow area test pits.

Table 2. Summary of Depths to Hard Basalt.

Hole Designation Depth to Hard Basalt

(feet) Overlying Material (s) and Depths

Lauer Dam - Downstream Drill Holes

DH-07-01 0.0 NA

DH-07-02 15.5 Colluvium (0.0 – 5.0) Weathered Basalt (5.0 – 15.5)

DH-07-03 0.0 NA

DH-07-04 3.9 Volcanic Breccia (0.0 – 3.9) DH-07-05 6.0 Volcanic Breccia (0.0 – 6.0) DH-07-06 5.0 Volcanic Breccia (0.0 – 5.0)

Dike Spillway Test Pits

TP-07-01 3.0 Topsoil (0.0 – 1.0) Weathered Basalt (1.0 – 3.0) TP-07-02 0.8 Poorly Graded Sand (0.0 – 0.2) Weathered Basalt (0.2 – 0.8) TP-07-03 2.0 Poorly Graded Sand (0.0 – 0.2) Weathered Basalt (0.2 – 2.0) TP-07-04 2.0 Poorly Graded Sand (0.0 – 0.2) Weathered Basalt (0.2 – 2.0)

Potential Borrow Area Test Pits

TP-07-05 10.0 Volcaniclastic Tuff (0.0 -6.0) Weathered Basalt (6.0 – 10.0) TP-07-06 11.0 Volcaniclastic Tuff (0.0 -10.0) Weathered Basalt (10.0 – 11.0) TP-07-07 8.0 Volcaniclastic Tuff (0.0 -5.0) Weathered Basalt (5.0 – 8.0) TP-07-08 4.0 Volcaniclastic Tuff (0.0 -4.0) TP-07-09 Not Reached Volcaniclastic Tuff (0.0 -9.0) Weathered Basalt (9.0 – 12.0) TP-07-10 11.5 Volcaniclastic Tuff (0.0 -5.0) Weathered Basalt (5.0 – 11.5)

Notes:

All depths are in feet.

B. Bedrock Permeability Test Results

Constant-head and falling-head permeability tests were conducted in five of the six drill holes (DH-07-02 through -06) located downstream of Lauer Dam. The tests were conducted in an open hole with no casing or screened intervals.

Gravity tests were not attempted in drill hole DH-07-01 due to the relatively unfractured basalt core samples and the very slow rate of water- level drop after the hole was filled. In drill hole DH-07-04, only a falling-head test was conducted because of the extremely small amount of water needed to maintain a constant head, and the difficulty of accurately controlling and measuring this flow rate.

A range of calculated permeability values were obtained from each drill hole because of the slightly different input parameters and formulas used for the constant-head versus the falling-head tests. Also, it should be pointed out that the gravity tests were conducted using reservoir water which contained algae and other debris which may have reduced permeability values.

The following table (Table 3) provides a summary of the gravity permeability test results from each of the drill holes.

Table 3. Summary of Gravity Permeability Test Results.

Hole Designation Coefficient of Permeability (K, feet/year) Average K (feet/year)

Constant-Head Test Falling-Head Test

DH-07-02 9.6 2.1 5.9

DH-07-03 6.1 4.5 5.3

DH-07-04 Not Conducted 1.0 1.0

DH-07-05 40.6 13.8 27.2

DH-07-06 17.4 1.4 9.4

Constant-head Tests. The coefficient of permeability results from the constant-head tests were calculated using the Gravity Permeability Test (Method 2) from the USBR Ground Water Manual, page 352, figure 10-13 [7]. Zone 3 of this method can be applied to tests run in consolidated materials using an open hole for the test section. It assumes a uniform flow of water sufficient to maintain a constant water level in the hole. The equation which was used for the constant-head calculations, and the input parameters are show on the test result sheets (Figures Perm-6 and Perm-7) in Appendix D.

Falling-head Tests. The coefficient of permeability results from the falling-head tests were calculated using the software program Aqtesolv, developed by HydroSOLVE, Inc. [3]. The Bouwer-Rice (1976) solution for a slug test in an unconfined aquifer was the solution method used for calculating the K values.

Included on the test result sheets (Figures Perm-1 through Perm-5 in Appendix D) are the falling head time-displacement graphs, and a description of the different parameters and variables contained in the equations. Certain parameters (such as the hydraulic conductivity anisotropy ratio) were assigned assumed values based on knowledge of the bedrock characteristics. The assumed values were varied to see how much influence the variation would have on the final calculated permeability results. A very minimal effect was seen because the permeability results are controlled primarily by the falling-head time and displacement field data.

C. Standard Penetration Test Results

Standard penetration tests were performed within the existing Lauer Dam embankment in drill hole DH-07-07, located on the dam crest. This hole was drilled to a depth of 13.0 feet (refusal on rock) using the CME 750X drill rig. The hole was drilled dry, without the use of any drilling fluid. Uncorrected blow counts, as both blows per foot and blows per 6- inch interval are reported on the drill hole log (Appendix B). Blow counts in this drill hole ranged from 6 to 16 blows per foot (bpf), indicating fairly low density materials. The higher blow counts (11 to 16 bpf) occurred in the upper section of the embankment (to a depth of 6.5 feet), and the lower blow counts (6 and 8 bpf) were obtained in the lower part of the embankment. Visual inspection of the SPT samples confirmed that the clayey soils in the upper part of the embankment are firm to hard, and soft to firm in the lower part of the embankment.

D. Dispersibility Test Procedures and Results

Both the pinhole and the crumb…