I s

627.83 N7mdr 1990

MIDDLE CREEK DAM REHABILITATION DESIGN REPORT

OCTOBER 1990

HKM Associates, Engineers/Planners Billings & Bozeman, MT/Sheridan, WY

Executive Plaza, Airport Industrial Park P.O. Box 31318 Billings, MT 59107 (406) 245-6354

'^^TF DOCUMENTS COLLECTfON

3 0864 1002 0343

MIDDLE (HYALITE) CREEK DAM REHABILITATION

DESIGN REPORT

Prepared For ;

Montana Department of Natural Resources and Conservation 1520 East Sixth Avenue Lee Metcalf Building Helena, Montana 59 620

Prepared By:

HKM Associates P.O. Box 31318 Billings, Montana 59107

October 12, 1990 8M0 87 . 14 0/WPWATRES9 61

Digitized by the Internet Archive

in 2015

I

https://archive.org/details/iTiiddlehyalitecre1990mont

TABLE OF CONTENTS

Page

A. INTRODUCTION 1

PURPOSE OF DESIGN REPORT -i 1

PROJECT LOCATION 2

EXISTING PROJECT ! : 2

PROBLEM STATEMENT 4

SUMMARY OF EARLIER INVESTIGATIONS 4

REHABILITATION PLAN 5

PROJECT DATA 7

B. HYDROLOGY / : 10

DESIGN FLOOD 10

FREEBOARD 10

C. GEOTECHNICAL 12

FIELD INVESTIGATION ' 13

LABORATORY ANALYSES 19

FINDINGS 21

D. EMBANKMENT 27

DRAINAGE CONTROL .27 EMBANKMENT CONFIGURATION -/.f^ 31

SEEPAGE THROUGH THE EMBANKMENT 31

BENTON I TE LINER AND SINK HOLES 33 STABILITY ' 34

MONITORING 35

RIPRAP 35

E. EARTH REINFORCED STRUCTURE 38

REINFORCED EARTH SYSTEM 39

GATE TOWER EXTENSION & WALKWAY 4 0

- i -

TABLE OF CONTENTS (Cont.)

Page

F. PRINCIPAL SPILLWAY 4 2

HYDRAULICS 4 2

STRUCTURAL 4 6

GEOTECHNICAL 4 7

TRASH BOOM .::{/y-:- '^^V r^^m:- 50

G. AUXILIARY SPILLWAY 51

HYDRAULICS 52

STRUCTURAL 52

GEOTECHNICAL 52

H. ROADS 54

ALIGNMENT 54

SPEED 54

SURFACE 54

TOLERANCES 54

GUARDRAILS 5 5

GEOTECHNICAL 5 5

I. BRIDGES 57

STRUCTURAL 5 7

GEOTECHNICAL 58

J. RECREATION FACILITIES 6 0

K. GRAYLING SPAWNING MITIGATION STRUCTURE 61

REFERENCES 62

APPENDICES 64

- i i

LIST OF FIGURES

Page

Figure 1 Location Map 3

LIST OF TABLES

Table 1 Informational Goal of Exploration Holes 14

LIST OF ABBREVIATIONS

@ - At

AASHTO - American Association of State Highway and

Transportation Officials

ACI - American Concrete Institute

GBR - California Bearing Ratio

cfs - Cubic feet per second

COE - U.S. Army Corps of Engineers

DNRC - Department of Natural Resources and Conservation

EAL - Equivalent axle load

EQ - Earthquake

FS - Factor of Safety

ft. - Feet

g - Acceleration of gravity

H - Horizontal

HKM - HKM Associates, Engineering Consultant,

Billings, MT

HEC - Hydrologic Engineering Center

K - One Thousand

MCWUA - Middle Creek Water Users Association

MDOH - Montana Department of Highways

NGVD - National Geodetic Vertical Datum

NWS - National Weather Service

PCF - Pounds per cubic foot

PMF - Probable maximum flood

PMP - Probable maximum precipitation

PSF - Pounds per square foot

PSI - Pounds per square inch

SCS - U.S. Soil Conservation Service

SDF - Spillway Design Flood

USBR - U.S. Bureau of Reclamation

USES - U.S. Forest Service

V - Vertical

* - Degrees

- Minutes

- i i i -

A. INTRODUCTION

Plans for the Rehabilitation of the Middle Creek Dam and Reservoir have been progressing since 1980. A number of engineering investigations have been accomplished to increase storage and bring the project into conformance with modern dam safety criteria.

Final design, preparation of drawings and specifications, and construction management services are currently being provided to the Department of Natural Resources and Conservation (DNRC) by HKM Associates (HKM) . These are being provided under Contract No. WE-HKM-239.

PURPOSE OF DESIGN REPORT ,1 L-;

The purpose of this Design Report is to present the criteria used and the engineering decisions made in preparing the construction drawings (APPENDIX A-1, full and half size drawings available under separate cover) and specifications (APPENDIX A-2, under separate cover) for the project.

The design process included a series of coordination meetings with DNRC and the federal reviewing agencies. A series of design working papers were prepared by HKM to facilitate coordination and review. The working papers are included in the text and appendices of this design report.

This report is intended to present all significant data generated/ criteria used, and engineering decisions made. Since the Loan Application Report was published (HKM, 1987) new plan formulation alternatives were developed and are presented herein. This report, however, is not intended to include a complete summary of all available information pertinent to the final design. Reference will be made to earlier documents to supplement data presented herein.

- 1 -

8M087 . 14 0/WPWATRES9 61 08/14/90

PROJECT LOCATION

Middle Creek Dam and Reservoir (also known as Hyalite Reservoir) are located approximately 15 miles south of Bozeman, Montana in Gallatin County, within the Gallatin National Forest. (See Figure 1). The dam and reservoir occupy portions of Sections 15, 22 and 22, T4S, R6E. The drainage basin area above the dam is 27 square miles including primarily the East and West Forks of Hyalite Creek. The facilities are state owned and are operated and maintained by the Middle Creek Water Users Association (MCWUA) . The DNRC constructed the dam under the provisions of a Special Use Permit from the U.S. Forest Service (USES). Operation of the dam began in the fall of 1951.

EXISTING PROJECT

The existing dam is a zoned earthfill structure approximately 117 feet in height (measured from the top of dam elevation 6722 feet NGVD to streambed elevation 6605 feet NGVD at downstream toe) . At centerline it has a structural height of 111 feet based on findings of the 1983 geotechnical investigation. The crest length is approximately 1300 feet long. A 60-inch diameter low level outlet conduit is located near the center of the main embankment. A 40-foot wide concrete spillway with an ogee crest is located on the right abutment.

The existing principal spillway crest is at 6710.8 feet NGVD with a reservoir surface area of 204 acres and a total storage capacity of 7,976 AF. With 2 feet of flashboards these values are 6712.8 feet NGVD, 213 acres, and 8,393 AF. These are total storage amounts including 126 AF of dead storage.

8M08 7 . 14 0/WPWATRES9 61 08/14/90

- 2 -

—TffALnE-i

MIDDLE CREEK DAM REHABILITATION

< >

FIGURE 1 UKW iUWOI li\Ti:S

GENERAL LOCATION MAP

- 3 -

PROBLEM STATEMENT

Based on a U.S. Army Corps of Engineers (COE) Phase I study, (COE, 1980) Middle Creek Dam is rated a large dam with a high downstream hazard potential. Presently the dam can safely pass only 29 percent of the probable maximum flood (PMF) while the recommended spillway design flood (SDF) is the full PMF. The dam has a seriously inadequate spillway and is considered unsafe .

A breach of the dam embankment could occur as a result of overtopping during a large flood event. Damages from the resulting flood could exceed $30 million plus the cost of repairing the dam. The purpose of this rehabilitation is to bring the dam up to present dam safety standards and provide additional storage capacity.

The Phase I report made recommendations for project improvement to correct existing conditions which did not conform to COE recommended guidelines. Key recommendations called for (1) restoration of the principal spillway structure, (2) hydrologic and hydraulic investigations to determine required spillway capacity and downstream hazards, (3) determination of embankment stability, and (4) locating the source of seepage in the left abutment.

SUMMARY OF EARLIER INVESTIGATIONS

DNRC responded to the COE recommendations by initiating a series of investigations that culminated in a rehabilitation plan. HKM, under contract with DNRC, provided the investigations and prepared the feasibility study (HKM, 1985) and loan application report (HKM, April 1987) that defined the rehabilitation plan for this project.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 4 -

HKM has prepared the following documents directly related to the work proposed herein: x

HKM Associates. November 1983. Hydrolooic Potential for Hyalite Creek Watershed. BillingS/ Montana.

HKM Associates. April 1984a. Flood Hydrology, Middle Creek Dam. BillingS/ Montana.

HKM Associates. April 1984b. Geotechnical Investigation, Middle Creek Dam. BillingS/ Montana.

HKM Associates. February 1985. Middle (Hyalite) Creek Dam Rehabilitation Feasibility Study. Billings, Montana .

HKM Associates. December 1986. Middle Creek Dam Rehabilitation Environmental Assessment. Billings, Montana . : "

HKM Associates. April 1, 1987. Middle Creek Dam Rehabilitation Loan Application Report. Billings, Montana .

REHABILITATION PLAN

The principal project features of the rehabilitation plan are:

Enlargement of the dam embankment by 8 feet using an earth reinforced structure in the center of the dam and conventional earthfill on the ends. The top of the impervious section will be 6729.9 feet NGVD.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 5 -

Increase the normal water surface by 8.2 feet (from the top of existing f lashboards) , enlarging the total reservoir capacity from 8,393 AF to 10,310 AF providing an additional 1,917 AF of storage. All storage not presently contracted (2,334 AF) is allocated to municipal use.

An earthen auxiliary spillway excavated in the left abutment which has a base width of 530 feet wide at the crest transitioning to 70 feet wide at the end. A concrete crest control structure will be constructed. An excavated channel and dike will be constructed below the spillway leading flood flows back to Hyalite Creek.

An improved principal spillway will include an inlet weir, a baffled apron drop constructed in the existing spillway, and a channel and dike leading to a new baffled apron drop structure to Hyalite Creek.

; Riprap of the outlet conduit discharge area.

Relocation and construction of replacement campground facilities and boat ramps.

Reconstruct the road across the dam with a new bridge across the principal spillway.

Provide a pedestrian bridge across the principal spillway.

Construction of grayling mitigation structures.

8iyi087 . 14 0/WPWATRES9 61 08/14/90

- 6 -

PROJECT DATA

General

a . Type

b. Height (feet above D/S channel toe)

c. Crest length

d. Crest width

e. Upstream slope

f. Downstream slope

g. Maximum surcharge (Elev. 6729.9)

h. Total storage (Elev. 6721.0)

i. Active storage (Elev. 6721.0)

j . Inactive storage

(Elev. 6637.0) k. Dam crest 1. County-State m. Location

n. Watershed location

o. Year completed p. Drainage area q. Probable Maximum Flood (PMF)

8M087 . 14 0/WPWATRES9 61 08/14/90

Earthfill with Mechanically Stabilized Earth Crest

125

1900 feet 37.5 feet

2H:1V, 3H:1V and 4H:1V, variable, refer to plans 2H:1V and 4H:1V, variable, refer to plans 12,790 acre-feet

10,310 acre-feet

10,184 acre-feet

126 acre-feet

6730.0

Gallatin County, Montana Section 15, Township 4S, Range 6E

Middle Creek, tributary to East Gallatin River,

tributary to Gallatin River, tributary to Missouri River 1991-92

27 square miles 30,500 cfs

Upper Principal Spillway (elevations in feet NGVD)

a . Type

b. Length along flowline

c. Crest elevation (normal pool)

d. Maximum observed flow

e. Capacity at auxiliary Spillway-crest (6723.0)

f. Capacity at maximum surcharge (6729.9)

Baffled apron drop with

labyrinth crest 182 feet 6721.0

Unknown , new 124 0 cf s

3250 cfs

Lower Principal Spillway (elevations in feet NGVD)

a. Type

b. Length along flowline

c. Crest elevation

d. Maximum observed flow

e. Capacity at maximum reservoir

surcharge (6729.9) if dike does not fail

Baffled apron drop 212 feet 6683.3

Unknown, new 2000 cfs

Auxiliary Spillway (elevations in feet NGVD)

a . Type

b. Length

c. Crest elevation

d. Maximum observed flow

e. Capacity at maximum surcharge (6729.9)

Earthen spillway 1150 feet 6723 .0

Unknown, new 27,250 cfs

8M0 87 . 14 0/WPWATRES9 61 08/14/90

- 8 -

Outlet Works (elevations in feet NGVD)

a .

b.

c . d. e . f .

g.

h.

Size

Control

Approximate capacity at normal pool (Elev. 6721.0) Design inlet invert elev.

Design outlet invert elev.

Drain system . ; \ Trashrack , v. ; ? -

Length . ,

5-foot diameter, steel

lined/ cast in place concrete ^ , One 54-inch diameter

butterfly valve

(operating) and one

54-inch diameter gate valve (emergency) 800 cfs

6 637.0

6630 . 5

Jet pump in control tower

Yes r r- .

600 feet

An elevation-area-capacity table and rating tables for the

principal spillway/ auxiliary spillway/ and outlet works are

provided serparately in the "Manual for Operation and Maintenance" .

8M087 . 14 0/WPWATRES9 61 08/14/90

- 9 -

R . HYDROLOGY

A feasibility level design for the rehabilitation of Middle Creek Dam was reported in the loan application report (HKM, April 1987). In preparing the final design, the PMF and associated spillways design have been re-evaluated to incorporate information that has become available since the feasibility study which was completed in 1984. This section presents the design criteria that was used in final design for the design flood and freeboard requirements.

DESIGN FLOOD ^

The PMF was re-evaluated using several different approaches (see Appendix B-2) . During a meeting of the USER, DNRC and HKM on March 24, 1988 at the Engineering and Research Center in Denver, Colorado, it was decided that a PMF having a peak flow of 30, 500 cfs is most appropriate for use at Middle Creek Dam. The USER and USES again approved in concept the PMF at the agency review meeting held on March 31, 1988 at the HKM offices in Eillings, Montana. The COE's HEC-1 computer output showing the revised PMF and reservoir routing is located in Appendix E-1. Additional information pertaining to the revised PMF is located in Appendix B-2.

FREEBOARD

The amount of freeboard during the PMF was also discussed with the USER at the March 24, 1988 meeting. The USER stated that it would be acceptable to allow the maximum water surface to rise to the top of the impervious section of the dam during the 30,500 cfs PMF. During a PMP general storm, SCS and NWS meteorologists predict that there would not be any significant wind associated with the flood peak. Any wind associated with the storm would be from the east which would blow waves away

8M0 87 . 14 0/WPWATRES9 61 08/14/90

- 10 -

from the dam. It is also believed that any winds associated with a thunderstorm event would precede the rain and runoff. This confirms that allowing water to rise to the top of the impervious surface is an appropriate criteria to utilize at Middle Creek Dam. The freeboard methodology was again approved by the USER and USFS at the March 31, 1988 agency review meeting. Additional freeboard information is found in Appendix B-3 .

After review of this criteria, DNRC and HKM have selected the following elevations for final design:

Principal Spillway Crest = 6721.0

Auxiliary Spillway Crest = 6723.0

Reservoir Water Surface 30, 500 cfs = 6729.9

Top of Impervious Section = 6729.9

8M087 . 14 0/WPWATRES9 61 08/14/90

- 11 -

C. GEOTECHNICAL

A geotechnical investigation was performed as part of the final design for the Middle Creek Dam Rehabilitation project. The geotechnical investigation included a field drilling program, a ground water monitoring program, soil laboratory testing and material source identification for borrow selection. Each of these portions of the investigation will be described in the design report. Geotechnical considerations not included here are found in other sections of the report.

The geotechnical investigation was performed in two parts. The first part was an investigation for the Middle Creek Dam Rehabilitation Feasibility Study. This study was done in 1983 and 1984. That study consisted of a field drilling program, the geological investigation, laboratory testing, engineering analysis and seepage monitoring.

The 1983-84 geotechnical investigation included fifteen exploration drill holes and fourteen test pits to determine soil conditions and obtain samples for laboratory testing. Instrumentation, to monitor embankment movement and changes in ground water levels, was installed. A geologic investigation was made to assist in evaluating the soils and the impact of a seismic event on Middle Creek Dam. The engineering analyses included seepage and stability studies. Both static and dynamic stability analyses were performed.

Rehabilitation alternatives were conceptualized at that time, however, design details had not yet been developed. Additional geotechnical work was required to develop the final design.

The results were reported (HKM, 1984b). The detailed information will not be repeated herein except for general conclusions and findings relating to design criteria as appropriate .

8M087 . 14 0/WPWATRES9 61 08/14/90

- 12 -

The second investigation began in 1987 and is concluded with this report. It included a field drilling program, laboratory- testing, borrow source investigation, and engineering analyses. The results are presented in detail herein.

FIELD INVESTIGATION

The geotechnical investigation was performed to accomplish the following :

Determine the sub-surface conditions at the proposed upper and lower principal spillway structures.

Expand knowledge of the sub-surface conditions in the auxiliary spillway area.

To monitor the phreatic surface throughout the existing embankment and in the left abutment.

To perform borrow and materials investigation specific for final design.

A field exploration program was performed during November and December of 1987. The investigation consisted of exploration borings using a truck mounted drill rig and a borrow investigation using a backhoe.

The field exploration program included 8 drill holes and 14 exploration test pits. The locations of these holes are shown on Sheet Nos. 41 and 44 in Appendix A-1 of this report. The drilling was performed by Rollins, Brown & Gunnell, Inc., Provo, Utah, under the direction of a HKM professional engineer and a HKM professional geologist.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 13 -

The borings were extended to feet. Ground water levels were taken for field classification, were made and are detailed on (Appendix A-1, Sheet 57 and 60).

depths from 41.5 feet to 121 measured and soil samples were Logs of the exploration holes the logs of exploration holes

The informational goal of each drill hole (DH) and test pit (TP) including those from the previous investigation (HKM, 1984b) is summarized in Table 1.

TABLE 1

INFORMATION GOAL OF EXPLORATION HOLES

EXPLORATION

HOLE NO. PURPOSE

DH-1 To investigate the foundation of the existing

spillway and determine excavation needs

DH-2 To investigate the foundation of spillway and the

character of the abutment

DH-3 To investigate the right abutment

DH-4 To determine the depth to bedrock and obtain

undisturbed samples of the foundation; critical for the dynamic analysis

DH-5 To investigate the pervious section of the dam

and the foundation; critical for dynamic analysis

DH-6 To investigate the semi-pervious section of the

embankment and identify the phreatic surface; critical for the dynamic analysis

8M087 . 14 0/WPWATRES9 61 08/14/90

- 14 -

TABLE 1

INFORMATION GOAL OF EXPLORATION HOLES (Continued)

EXPLORATION HOLE NO.

PURPOSE

DH-7

To investigate the impervious section of the embankment; critical for the static and dynamic stability analyses

DH-8

To investigate the left abutment

DH-9

To investigate the seepage in the left abutment

DH-10

To investigate the left abutment

DH-11

To investigate the potential auxiliary spillway area

DH-12

To investigate soils in area of the potential auxiliary spillway

DH-13

To investigate the old landslide in the right abutment

DH-13A

To redrill DH-13 as it was lost due to difficult dr i 1 ling

DH-14

To investigate the right abutment at the contact with the downstream slope

DH-201

To investigate foundation conditions at the toe of the proposed lower baffled apron drop

DH-202

To investigate the foundation at the upper end of the proposed lower baffled apron drop

15 -

8M087 . 14 0/WPWATRES9 61 08/14/90

INFORMATION GOAL OF

TABLE 1 EXPLORATION

HOLES

(Continued)

EXPLORATION

HOLE NO. PURPOSE

DH-203 To investigate the position of the phreatic

surface in the left abutment and to help evaluate the pervious section of the existing embankment

DH-204 To better define the phreatic surface through the

embankment and left abutment

DH-205 To investigate the location of the phreatic

surface through the embankment and left abutment

DH-206 .:, To investigate the location of the phreatic

surface between drill holes DH-5 and DH-6 on the maximum section of the embankment

DH-207 To investigate the subsoil conditions at the toe

of the upper baffled apron drop in the principal spillway

DH-208 To investigate the subsoil conditions in a

seepage area at the toe of the existing small landslide

TP-101 To investigate the ease of excavation of the

principal spillway

TP-102 To obtain a bulk sample of the pervious section

of the embankment and to observe seepage

TP-103 To obtain a bulk sample of the impervious section

Qf the embankment and to investigate the old landslide area

8iyi087 . 14 0/WPWATRES9 61 08/14/90

- 16 -

TABLE 1

INFORMATION GOAL OF EXPLORATION HOLES (Continued)

EXPLORATION HOLE NO.

PURPOSE

TP-104

To investigate the upstream impervious blanket on the left abutment

TP-105

To investigate the ease of excavation of the potential auxiliary spillway area

TP-106, 107 & 111

To obtain a bulk sample of the borrow from the potential auxiliary spillway excavation and observe seepage

TP-108-110

To obtain samples of the borrow areas on the east side of the reservoir

TP-112-113

To investigate the potential borrow area on the mountain side above the right abutment

TP-114

To investigate the area of the trench drain on the left abutment downstream from the embankment

TP-301

To investigate foundation conditions at the toe of the lower baffled apron drop

TP-302

To investigate foundation conditions at the lower baffled apron drop

TP-303

To investigate foundation conditions at the site of the proposed labyrinth weir

- 17

8M0 87 . 14 0/WPWATRES9 61 08/14/90

TABLE 1

INFORMATION GOAL OF EXPLORATION HOLES (Continued)

EXPLORATION

HOLE NO. PURPOSE

TP-304 To investigate foundation conditions at a

potential site for a bridge which would provide access to the Edsall cabin

TP-305 & 306 To investigate foundation conditions in the

proposed campground areas

TP-309 To determine the condition of the existing drain

system in the left abutment

TP-310 To evaluate the foundation conditions at the

; . training dike which is part of the auxiliary

spillway system

TP-307, 308 To investigate the characteristics of materials TP-311 thru to be excavated from the auxiliary spillway TP-314

Source: HKM Associates

Field permeability tests were taken in each exploration drill hole and laboratory samples were obtained for testing. Monitoring tubes were installed in each of the exploration drill holes. A description of the completion of each of these holes is provided on Sheet Nos. 41-43 in Appendix A-1.

Slope stability monitoring tubes were also installed in Drill Holes No. 5 and 208. The purpose of these tubes is to monitor potential horizontal movements in the downstream embankment slope and the right abutment, respectively.

- 18 -

8M0 87 . 14 0/WPWATRES9 61 08/14/90

Drill pads were constructed to provide access to many of the exploration holes. These access roads and drilling pads were constructed using a dozer and were reclaimed upon completion of the work. Complete reclamation included recontouring of the ground surface, construction of runoff control appurtenances using stop logs, reseeding the area and removal of downed timber. This reclamation work has been reviewed and approved by the USFS .

Protective caps were constructed for each of the exploration drill holes. The method of the construction was the same as for the previous work (HKM, 1984) with the exception that a loose fitting slip-on cap was placed over the casing instead of a screw-on cap to provide future access to the monitoring tubes.

LABORATORY ANALYSES

Representative field samples were selected for laboratory testing after careful visual examination of the soil and consideration of the design criteria. Most of the physical and engineering soils property tests were performed in the HKM laboratory in Billings, Montana. Some engineering property tests and chemical tests were performed by Lyndes Laboratory in Billings, Montana.

Test

Purpose of Test

Natural Moisture Content

To determine the natural (in situ)

(ASTM D-2216)

water content and to correlate the

moisture

contents

with

the

phreatic surface.

Atterberg Limits

(ASTM D-423 & D-424)

To provide an indication of the shear strength and compressibility of the soil.

- 19 -

8M087 . 14 0/WPWATRES9 61 08/14/90

Test

Purpose of Test

Particle-Size Distribution (ASTM D-4 22) . , .

To determine the grain sizes of the soils for classification and identification of physical

characteristics.

Natural Unit Density (ASTM D-2937)

To determine the (in situ) dry unit weight of the soil.

Moisture - Density Curve (ASTM D-698)

To determine the relationship of water to the density of soil during a compaction or remolding process .

Laboratory Bearing Ratio (ASTM D-1883)

To measure the shearing resistance of soil under controlled moisture and density conditions. Used to evaluate the relative quality of subgrade soils for pavement support .

Laboratory Permeability

To determine the permeability of soils remolded to simulate embankment construction.

Corrosivity SO4 / pH, Resistivity

To determine the corrosion potential of soils on buried metal and concrete

Direct shear

(ASTM D-3080)

To determine the shear strength of the soils

L.A. Abrasion

To determine the quality of rock relative to hardness.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 20 -

The laboratory tests were performed in strict accordance with applicable ASTM procedures. A Summary of the Laboratory Test Results is presented on the Geotechnical Information sheets of Appendix A-1. Additional test data for individual tests is presented on Figures 1-13 of Appendix C-1.

Laboratory test results from the earlier geotechnical report. Middle Creek Dam Geotechnical Investigation, April 1984, are presented on Plate Nos. 1-27 in Appendix C-1. These results are included to provide a complete compilation of soils grain-size distribution tests.

FINDINGS

This section presents general engineering considerations based on the results from the geotechnical investigation. Some items are only briefly discussed in this section but are detailed in other pertinent sections of this report.^

Foundation Conditions at Principal Spillway - ' '

Three exploration holes were made along the alignment of the principal spillway, DH-201, DH-202 and DH-207. One of the primary purposes of these holes was to determine the depth to bedrock. The findings indicate that bedrock is well below these structures. Therefore, soil anchors are designed for structural stability.

The natural foundation soils are tills and are primarily gravel with some clay and sand. Because of the high quantity of clay in the till soil, the clay typically governs the shear strength. Therefore, anchor capacities are limited.

Results of the exploration borings also indicate that ground water at the lower baffled apron drop structure is near the

8M08 7. 140/WPWATRES9 61 08/14/90

- 21 -

stream elevation. It is anticipated that during periods of high precipitation and when the pool is near full the ground water may raise significantly.

A detailed explanation of the design criteria and design of the soil anchors for the baffled apron drop structures is discussed in Appendix F-3.

Monitoring . h ;

The phreatic surface through the embankment and ground water in the abutments at the site has been monitored extensively since 1984. The monitoring has been performed in a joint effort between personnel from HKM and the DNRC.

Exploration holes DH-204 through DH-206 were located for the purpose of identifying the phreatic surface through the existing embankment. Although the location of the phreatic surface was previously identified and reported in the April 1984 geotechnical report, additional exploration was needed to confirm its location along another cross section through the embankment. Results of the monitoring have confirmed that the phreatic surface previously identified appears to be correct. With at least two locations indicating the location of the phreatic surface, confidence is gained in the seepage model. Seepage analysis through the embankment is described under the Embankment section.

Four additional monitoring wells (DH-301 through DH-304) were installed in August 1990. These wells will serve to monitor the performance of the barrier drain after rehabilitation. These four wells and 14 others that will be disturbed during rehabilitation construction were equipped with vibrating wire piezometers to allow for measurement after rehabilitation.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 22 -

Seepage has been identified near the surface at the contact between the embankment and the right abutment on the downstream face. This seepage was identified in the earlier report and was investigated again during this exploration program. Results of the field monitoring program suggests that there is more seepage in this area than was previously identified. A trench drain has been designed to control the seepage.

The existing drain on the left abutment (1956 Rehabilitation) discharges approximately 200 ft. horizontally and 50 ft. vertically from the outlet channel. Discharge from this drain flows along the abutment contact to the outlet channel at the downstream end of the outlet conduit. New pipe drain will be installed to extend the existing drain to the outlet channel. This will keep the surface flow off the abutment and allow for measurement of flow.

In April 1984 a slope indicator tube was placed in DH-5 on the main embankment. Slope movements have been monitored in this hole and results indicate that surface sloughing is occurring on the face of the embankment. It is felt that this sloughing is minor and typical of earth embankments. Movements since installation have been less than % inch near the ground surface. There is some indication that the movement may be related to disturbance by vehicles crossing the berm of the embankment during the exploration program. In any case, the results suggest that there is not a particular concern with stability on the downstream face of the embankment. *

A slope monitoring tube was also installed in DH-208 during 1987. The purpose of this tube is to monitor slope movements on the right abutment. During this investigation a small landslide has been identified on the right abutment slightly downstream of the embankment. This slide appears to be shallow, approximately three to five feet deep, and 20 to 40 vertical feet in height. The slide is approximately 30 feet in

8M087 . 14 0/WPWATRES9 61 08/14/90

- 23 -

width. The exact dimensions of the slide are not obvious. Surface sloughing appears to be occurring as a result of high ground water and low strength soils. This slide does not appear to impact embankment stability.

Dewater inq .

The following areas may require dewatering during construction:

The downstream toe of the upper baffled apron drop.

The right abutment and wall area of the upper baffled apron drop .

The upstream entrance area for the lower baffled apron drop . ^

The downstream toe of the lower baffled apron drop.

The dike foundation area between the upper and lower baffled apron drops.

, Excavation in the auxiliary spillway area.

There is a spring located in the area of DH-207 adjacent to the upper baffled apron drop. Ground water was encountered near Elevation 6698 feet NGVD. This spring creates hydrostatic pressures behind the east wall of the existing principal spillway. The proposed construction for the new spillway will require that this water be controlled during construction. Additionally, subsurface drainage behind the east wall of this upper spillway structure will be required. A description of the drain design is presented in Section F, Principal Spillway.

The static ground water elevation at the toe of the upper baffled apron drop is near elevation 6697.6 feet NGVD (November 18/ 1987). It is anticipated that the static water elevation may increase after the rehabilitation construction and the pool is filled.

8M0 87 . 14 0/WPWATRES9 61 08/14/90

- 24 -

Presently, there is a swamp like condition in the area of the training dikes between the upper and lower baffled apron drop structures. High ground water in this area is the result of springs from the east and from minor seepage through the embankment and abutment near the existing spillway. Saturated conditions in this area during the early spring and summer months may create construction problems. Dewatering should be anticipated in this area.

At the upper end of the lower baffled apron drop there is an existing drainage channel from the swampy area previously described. During the summer months, surface waters from the swamp flow into the channel creating saturated soils. Dewatering in this area may be reduced if the swampy area previously described is also dewatered. Surface runoff from this area may also create problems during construction and should be controlled.

The static water elevation at the downstream toe of the lower baffled apron drop is near or slightly above the stream elevation. Springs from the east create saturated conditions above the streambed in some areas. It is possible that a spring will develop during foundation construction for the lower baffled drop.

Dewatering is anticipated at the toe of the lower baffled apron drop. The primary source for this water will be from the low level outlet conduit under the existing embankment. Because of the high porosity and permeability of the soils in this area large dewatering volumes should be anticipated. Underdrains are designed to handle seepage after construction. This drainage control is described in the Principal Spillway Section.

Presently there is a spring in the area of the proposed auxiliary spillway. It is expected that this spring will continue to flow during construction. Deep soft sediments are

8M087. 140/WPWATRES9 61 08/14/90

- 25 -

located where spring water ponds in the auxiliary spillway- area. Equipment may find it difficult to work in these saturated soils. De-watering may be accomplished by sub-surface trench drains. Surface drainage may also be used provided it does not have to be crossed by heavy equipment.

8M087 . 14 0/WPWATRES9 61 08/14/90

- 26 -

D . EMBANKMENT

To control the PMF, the top of the impervious embankment must be raised to elevation 6729.9 feet NGVD. At the earth reinforced structure the minimum elevation of the impervious upstream face will be 6730.0 feet NGVD.

The rehabilitation of the existing Middle Creek Dam embankment consists of two general items:

Construction of an earth reinforced structure across the main portion of the embankment, approximately 850 feet long .

9 Construction of an enlarged embankment at each end of the earth reinforced structure to raise the crest up to the desired elevation.

The rehabilitation plan addresses the following areas: tux^z^.-'

Internal seepage control

Feasible and economical embankment construction

Crest road design

Erosion control on the upstream face

Materials selection

Embankment stability

DRAINAGE CONTROL Impervious Barrier

The pool elevation will be raised from the existing elevation of 6712.8 to 6721 feet NGVD. This increase in head has the potential to create an entire new seepage pattern through the existing embankment. The result would be to raise the phreatic

- 27 -

8M087 . 14 0/WPWATRES9 61 08/14/90

surface throughout the embankment. Raising of the phreatic surface would create unacceptable stability factors of safety. The combination of an addition to the dam crest elevation and an increase in the phreatic surface in the embankment would be unacceptable unless the phreatic surface could be controlled to maintain stability.

The design feature which will control the phreatic surface through the embankment is an impervious barrier trench located along the main portion of the embankment as shown in Appendix D-3 . This barrier trench would consist of an impervious flexible membrane buried to a depth of approximately 24 feet below the proposed crest elevation. A 12-inch diameter drain pipe and a high capacity drain fill would be placed at the lowest point of the impervious membrane. The purpose of this drain is to prevent an increase in the phreatic surface on the downstream side of the impervious barrier.

This proposed system was evaluated using a finite element computer model to represent flows through the embankment. Results of the computer modeling indicate that the phreatic surface will not raise above the elevation used to evaluate stability of the embankment except at the upper most elevations. At the barrier trench the phreatic surface will be about two feet higher than the phreatic surface used in the original stability analysis. This is because the trench is downstream from the location originally conceptualized. This higher phreatic surface was used in the final stability calculations. Results indicate stability is adequate. Stability is discussed later in this section.

Embankment Section

Rehabilitation in the dike portion (embankment beyond ends of earth reinforced structure) of Middle Creek Dam will consist of

8M0 87 . 14 0/WPWATRES9 61 08/14/90

- 28 -

raising the crest elevation by placing new fill over the downstream face of the existing embankment. Crest alignment for the new embankment will be approximately parallel to the existing embankment with slightly wider radius.

The process of constructing this embankment will consist of excavating the upper three feet of the existing material off the embankment then placing the new fill. The new fill will include an impervious zone which will tie into the existing impervious upstream section of the existing embankment. A more pervious embankment zone will be constructed on the downstream side. Internal seepage will be controlled by a filter drain located at the toe of the existing embankment.

The new filter will be approximately 9 feet wide and 5 feet deep at the toe of the existing embankment below the new fill. There will be a drain pipe in the gravel filter section to collect seepage . .

Embankment Drains and Filter Design ' c ^ ^ .v . ;

New drains will be installed in the abutments and under the Earth Reinforced Structure (ERS) . A barrier trench drain as discussed previously will be utilized to lower the phreatic surface under the upstream portion of the ERS. An "ERS Foundation Drain" will be placed under the downstream edge of the ERS .

The ERS foundation drain will be utilized to pick up any water that enters the ERS either from surface infiltration or through the upstream face. Both drains will drain laterally in both directions from the center of the embankment toward the abutments. The barrier trench drainage will be collected at each abutment by non-perforated collector pipes that will direct flows toward manholes situated on the downstream side of

8M087 . 14 0/WPWATRES9 61 08/14/90

- 29 -

the dam crest in each abutment. The ERS foundation drain will drain directly into the manholes. Configuration of the inlets to the manholes is such that flows for each of the drains can be monitored . , ,

All drains within the embankment will consist of varying diameters of factory slotted AWWA C-900 Class 200 pressure pipe with drain gravel envelopes. The abutment drains that have only small over burden pressures will consist of perforated Polyethelene tubing with drain gravel envelopes.

The majority of the gravel envelopes for the drains will consist of Grade B drain/filter gravel. The gravel for the barrier trench drain will consist of a washed Grade A drain gravel. A filter fabric will be required to prevent potential piping of the existing embankment soils into the washed Grade A drain gravel of the barrier trench.

Standard gravel filter design criteria selecting compatible filter/drain gravel, filter fabric.

Gradation of the Grade B drain/filter gravel was determined using the following criteria;

filter <_ 4 ^85 soil filter >. 5 ^15 soil

As presented by (Cedergren, 1977) and (Jansen, 1988) representative gradations