Amendment_0002_Attachment_Lakeview Site 1 ERI Official Report Final.pdf

United States Department of Agriculture

Page | 1 An Equal Opportunity Provider and Employer

Natural Resources Conservation Services Geological Service Unit 1402 Band Road, Suite 200 Rosenberg, Texas 77471

November 26, 2013

Subject: ERI Report, Lakeview Site 1, Donley County, Texas

To: John Mueller, State Conservation Engineer, NRCS, Temple, Texas

CC: Brain Wenberg, Assistant State Conservation Engineer, NRCS, Temple, Texas

Todd Marek, Dam Safety Engineer, NRCS, Temple, Texas

The purpose of this study was to use earth resistivity imaging on the Lakeview Site 1earthen dam to delineate subsurface conditions and probable flow paths trending to the multiple seeps within the principal spillway basin that could pose subsidence/collapse hazards or undermine the structural integrity of the earthen dam. This report describes the site conditions, specifies the project’s scope, discusses the techniques used to collect and reduce the data, and presents my findings.

Site Description

The area investigated for this study is in Donley County, approximately 13 miles south of Clarendon, Texas west of Highway 70 on County Road 10. The site consists of an earthen dam approximately 1,945 feet long with a 50 foot wide excavated auxiliary spillway. Prior to this study, December 19th-21st 2013, multiple seeps have manifested at the surface within the plunge basin. Recent field reports indicated multiple orange/reddish silt deposits have developed at the seep’s surface location and recently an increase in seep flow. The water is currently flowing clear indicating any continuous removal of material at the time of the study is minimal. There are multiple surface rock outcrops of gypsum beds both upstream and downstream of the dam. The upstream side of the dam, within the pool area, contains evidence of multiple sinkhole developments and collapses. The spillway and dam are underlain by the Permian age Cloud Chief Gypsum and Whitehorse Sandstone undivided formation.

The Cloud Chief Gypsum and Whitehorse Sandstone undivided formation is composed of sandstone, sand, shale, gypsum, and dolomite, interbedded. The Lakeview site is composed of :

some sandstone and sand that is fine grained quartz, silty, massive, locally thin bedded, friable, orange, orange-brown, various shades of mottled grayish-green; shale, sandy in part indistinctly bedded to massive, red and orange-red; gypsum, white, gray, pink locally banded.

Scope

A total of three earth resistivity imaging lines were conducted in this investigation. Line one was conducted along the middle of the downstream berm with an electrode spacing of 2.7 feet (see sheet

1) to provide a fine resolution to locate the preferential flow paths of the seeps. Line two was conducted on the upstream berm with an electrode spacing of 3 feet (see sheet 1) to provide a reference point on the upstream side of the dam. Line three was conducted on the downstream side of the centerline of the dam with an electrode spacing of 5 feet (see sheet 1). The spacing of line three was determined by the depth of the cut off trench in an effort to assess the internal structure

Page | 2 of the dam. The three line configurations used were considered to provide the best possibility for the determination of preferential flow paths and to determine if a potential hazard exists under or within the dam due to internal erosion or preferential flow paths.

A Trimble R8 GNSS was used along each array to obtain surface RTK (Real Time Kinematic) mapping data that is incorporated into the modeling process. Each topographic profile was tied into the permanent elevation control point that is referenced on the as-built drawings. Refer to the attached site plan (see sheet SP) to see the general location of the resistivity lines.

Instrumentation and Field /Analysis Techniques

The earth resistivity imaging method is used for measuring the conductive and resistive properties of the subsurface. It is often used to understand subsurface geology by mapping strata, fracture zones, voids, etc. or to define subsurface anomalies produced by cultural influences such as landfills, buried tanks, and pipelines or industrial facilities. This method utilizes electrical current introduced into the earth by two electrodes. The resistivity of the subsurface is obtained by measuring the potential (drop) between two other electrodes. The depth of investigation is proportional to the current electrode separation and ultimately the length of the array on the surface. By varying the electrode separations, different depths and locations can be sampled allowing a profile of subsurface resistivity to be developed.

For this study, an Advanced Geosciences, Inc. SuperSting R8/IP resistivity system was employed. The SuperSting is a resistivity meter, data logger, and a switching box that configures the various electrodes used during the data acquisition process. Fifty-six electrodes were used with electrode spacings of 2.7, 3, and 5 feet. In order to increase overall line length without sacrificing resolution for the long arrays along the dam, a roll-along technique was employed which allowed a leapfrog-like progression of the array to continuously record data with minimal interruption between array spreads. All data was collected using both the Dipole-Dipole and Inverse Schlumberger resistivity method and processed using EarthImager 2D. This software uses a forward modeling subroutine for calculating apparent resistivity values and a least-squares optimization inversion process to create the profiles.

Results

The data collected from the earth resistivity imaging arrays were processed to create two-dimensional profiles showing the subsurface characteristics below each array. The profiles show the ground surface along the top of the sections with the contouring below indicating resistive/conductive changes in the subsurface. Colors in the red and yellow end of the ohm-meter scale indicate areas of higher resistivity such as rock, dry soil or air filled voids. The shades of green found in the profile are typically an indication of an increase in soil moisture from dry to moist; however, green along with some shades of yellow may also be an indication of a well-drained, possibly coarse grained soil when observed at the surface. Colors towards the blue end of the scale are representative of a more conductive condition such as wet soils, water-filled voids, mud-filled voids, or water- filled fractures.

The three resistivity lines generated a data set on the upstream berm, crest of the dam and downstream berm of the dam. A moisture verification hole (see sheet 1) was bored (12 feet 11

Page | 3 inches) using a hand auger due to initial data in the field producing evidence of an unusually high possible water zone. The verification hole indicates the red highly resistive anomalies along the berm are compacted materials that contain slight moisture and lack the ability to transport ground water within the berm. The yellow to green banding is indicative of slight moisture variations and trace minerals. This material did contain some sand at approximately 2.5 feet. The light blue to green contact is the transition point from slightly moist to moist material.

The darkest blues are most likely a preferential seep flow path or a water laden void.

The verification hole produced a dry to very slightly moist material to a depth of 8.5 feet where the material became moist. Saturated material was encountered at a depth of 11.5 feet. The 8.5 feet contact correlated to the green to light blue resistive banding contact and the 11.5 feet contact point correlated to the second darkest blue of 4.198 Ohms. A 24 hour stabilization period produced 8 inches of water in the bottom of the hole

Row one (see sheet 1) has three prominent conductive anomalies from 13-35 feet, 362-364 feet, and 391 – 405 feet on the profile. There appears to be three electrical stratum layers that are fairly uniform and layered horizontally. The first anomaly from 13-35 feet is approximately station 12+84. The same anomaly appears in row three approximately the same stationing. This feature trends in line with the sinkholes that have developed on the front toe of the structure. The second anomaly from 362-364 feet is approximately station 16+26. This feature continues to manifest into row three and there is some indication that it is present in row 2. The presentation of the feature would indicate the possibility of a vertical fracture or the contact of a shelf type structure. The third anomaly from 391-405 feet is approximately station 16+62. This feature continues to manifest into row three and there is some indication that it is also present in row 2.

There is a possibility that anomalies located at stationing 16+26 and stationing 16+62 are hydraulically connected due to their close proximity and the increased moisture band that joins the two together. The highly resistive anomaly located at 389 feet is questionable if its and artifact or actual data. If the anomaly does exist, it falls within the range for a transitional gypsum formation and could possibly be gypsum or a very dry clay stone.

Row two (see sheet 4) has two conductive anomalies from 78-93 feet and 441-459 feet on the profile. There appears to be three electrical stratum layers that are fairly uniform and layered horizontally. The significant moisture indicated in blue is a result of the row location on the upstream side of the dam. The first anomaly from 78-93 feet is approximately station 12+69.

The same anomaly appears in row three approximately 15 feet to the right as viewed facing downstream. The elevation of the anomaly is 10 feet deeper than the anomaly found in row 1;

however, the anomaly in row 3 appears that it is large enough to provide a possible hydraulic connection. The second anomaly from 441-459 feet is approximately station 16+26. This feature is also deeper than the anomalies found in rows one and three however it does align with the anomalies found in row one. There is indication of extensive saturation from 255 feet to the end of the row.

Row three (see sheet 5) has multiple large conductive anomalies from 197 - 210 feet, 570-580 and 595 – 620 feet on the profile. The dam appears to have variations in moisture content throughout. The first anomaly from 197-210 feet is approximately station 12+87. The proximity to the principal spillway pipe does cause some interference; however, it does appear that there is

Page | 4 possibly water flowing within approximately 20 feet from the left side of the principal spillway pipe within the profile. The anomaly aligns with the sinkholes on the front toe and the anomaly in row 1. The data suggests that the preferential flow path is presumably through the dam. The second anomaly from 570-580 feet is approximately station 16+62. The presentation of the feature would suggest the possibility of a preferential flow path existing though the dam or possibly at the base of the cut off trench. The third anomaly from 595-620 feet is approximately station 16+95. The feature is appears as a highly saturated zone. It could possibly be wet soil, water-filled void, mud-filled void, or water- filled fractures.

Conclusion

The earth resistivity imaging data of the structure suggest the seeps to the left of the principal spillway pipe appear to be from water flowing through the dam based on row three data from the top of the dam overlain by the cut off trench data from the as-built (see sheet 12). All of the anomalies appear occur below the pool elevation at the time of the study except for the anomaly in row three located at stationing 16+95.

The water appears to be migrating through the dam to the left of the pipe quasi parallel from station12+69 upstream 81 feet to station 12+84 downstream 77 feet. The initial cause of the seeps at this location is undeterminable; however, there is evidence on site that indicates some amount of gypsum rock was incorporated in to the dam. There are multiple large pieces of gypsum partially exposed on the dam.

The seeps beneath the principal spillway pipe and within the plunge basin are probably caused by interstitial flow from the surrounding subterranean seeps; however, its existence is only known due to its surface expression and cannot be seen in the data.

There is indication in the data of a possible seep to the right of the principal spillway pipe approximately 373 feet. Data from rows one and two indicate seep water most likely is flowing just under or at the cut off trench. The subterranean seep is at depth and hasn’t exposed at the surface near the dam. The only indications of a possible subterranean seep at the surface are the many trees and healthy grasses in a localized area.

The anomaly located at 600 feet (see sheet 12) does cause some concern due to indications of a possible saturated or mud filled void within the dam. The anomaly appears to be above a possible rock outcrop in the hillside. Ground truthing will be required to make a definite determination. Currently the only explanation for the anomaly’s presence is hillside seep. There is a possibility that the anomaly located at station 16+62 and this anomaly is hydraulically connected based on the resistivity readings in the data.

It should be noted that horizontally banded gypsum seams on site are both upstream and downstream of the dam. I believe it is reasonable to assume that the gypsum seams were notched into for placement of the cut off trench and the dam core on the right side facing downstream. This appears to be what is probably presenting the highly resistive anomaly on row three (see sheet 10) 605 feet.

Page | 5

Ground truthing the earth resistivity imaging data is recommended for final determinations to be made prior to any repair design or construction.

Daniel Wilkinson State Geologist

USDA-NRCS

Geological Services Unit Rosenberg, TX

DATEREVISIONS

SHEET

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Upper San Marcos Site 2 Resistivity Scatter Plot Model

Lakeview Site 1 Site Plan

Legend Row 1 Row 2 Row 3

! Seeps Gypsum Sinkhole Permanent Marker

USDA Permanent Marker N: 3559252.519 E: 832776.602 Z: 2390.20

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3Electrical Resistivity Imaging Survey Layout

Legend Row 1 Row 2 Row 3

! Seeps Gypsum Sinkhole Permanent Marker

USDA Permanent Marker N: 3559252.519 E: 832776.602 Z: 2390.20

Row 1 Start

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Row 2

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3Electrical Resistivity Imaging Survey Layout

Row 3 Start

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Ele va tio n ( ft)

Ohm-m

Lakeview Site 1 Row 1 Inverted Resistivity Section

Sta. 12+62 Sta. 17+12

Approximate cut off trench limits from asbuilts

Principal Spillway Pipe Verification Hole

Viewed facing downstream

Approximate ground line from asbuilts

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3Electrical Resistivity Imaging Survey Layout

Row 3 Start

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Lakeview Site 1 Row 1 Inverted Contrasted Resistivity Section

Sta. 12+62 Sta. 17+12

Approximate cut off trench limits from asbuilts

Principal Spillway Pipe Verification Hole

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Lakeview Site 1 Row 1 Inverted Resistivity Section Viewed facing downstream

Ohm-m

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460 92 138 184 229 275 321 367 413 0.0

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Measured Apparent Resistivity Pseudosection

Calculated Apparent Resistivity Pseudosection

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2, 13Electrical Resistivity Imaging Survey Layout

Row 3

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Lakeview Site 1 Row 2 Inverted Resistivity Section

Sta. 11+79 Sta. 16+81 approximate cut off trench limits from asbuilts

Principal Spillway Pipe

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Row 3

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Lakeview Site 1 Row 2 Inverted Contrasted Resistivity Section

Sta. 11+79 Sta. 16+81 approximate cut off trench limits from asbuilts

Principal Spillway Pipe

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Lakeview Site 1 Row 2 Inverted Resistivity Section Viewed facing downstream

Ohm-m

Ohm-m510 102 153 204 255 306 357 408 459

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2, 13Electrical Resistivity Imaging Survey Layout

Row 3 Start

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2392.5

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2375.8

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Ele va tio n ( ft)

Ohm-m

Lakeview Site 1 Row 3 Inverted Resistivity Section

Sta. 10+ 87 Sta. 17+ 84 approximate cut off trench limits from asbuilts

Principal Spillway Pipe

Viewed facing downstream

Hole 5 Hole 4Hole 7

Hole 6 Hole 3

Hole 2

Boring Data is from Asbuilt Geology Diagram

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Ohm-m

Lakeview Site 1 Row 3 Inverted Contrasted Resistivity Section

Sta. 10+ 87 Sta. 17+ 84 approximate cut off trench limits from asbuilts

Principal spillway Pipe

Viewed facing downstream

Hole 5 Hole 4Hole 7

Hole 6 Hole 3

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Lakeview Site 1 Row 3 Inverted Resistivity Section Viewed facing downstream

Ohm-m

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700 140 210 280 350 420 490 560 630 0.0

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14.7

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2.6

46.0

17.7

42.1

8.7

5.1

24.9

14.7

Inverted Resistivity Section Iteration= 6 RMS=3.68% L2=1.42 Electrode spacing=5 ft

DRAWING NO.

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2, 13Electrical Resistivity Imaging Survey Layout

Row 3 Start

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2392.5

2359.2

2325.9 2342.5

2375.8

700 140 210 280 350 420 490 560 630 5040

17.3 2.6

Iteration= 6 RMS=3.68% L2=1.42 Electrode spacing=5 ft

Ele va tio n ( ft)

Ohm-m

Lakeview Site 1 Row 3 Natural Scale Resistivity Section

Sta. 10+87 Sta. 17+84 approximate cut off trench limits from asbuilts

Principal spillway Pipe

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2, 13Electrical Resistivity Imaging Survey Layout

Row 3 Start

End

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2392.5

2359.2

2325.9 2342.5

2375.8

700 140 210 280 350 420 490 560 630 5040

17.3 2.6

Iteration= 6 RMS=3.68% L2=1.42 Electrode spacing=5 ft

Ele va tio n ( ft)

Ohm-m

Lakeview Site 1 Centerline Profile vs Bore Locations

Sta. 10+ 87 Sta. 17+ 84 approximate cut off trench limits from asbuilts

Principal spillway Pipe

Viewed facing downstream

Hole 5 Hole 4Hole 7

Hole 6 Hole 3

DRAWING NO.

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2, 13Electrical Resistivity Imaging Survey Layout

Row 3 Start

End

End

2392.5

2359.2

2325.9 2342.5

2375.8

700 140 210 280 350 420 490 560 630 120

6.8 2.6

46.0 17.7

Iteration= 6 RMS=3.68% L2=1.42 Electrode spacing=5 ft

Ele va tio n ( ft)

Ohm-m

Lakeview Site 1 Centerline Profile Anomaly Comparisson

Sta. 10+ 87 Sta. 17+ 84 approximate cut off trench limits from asbuilts

Principal spillway Pipe

Viewed facing downstream

Hole 5 Hole 4Hole 7

Hole 6 Hole 3

Hole 2

Boring Data is from Asbuilt Geology Diagram

X XX

Sta. 16+62Sta. 16+26Sta. 12+84

Indicates location of Row 1 anomaliesX

E E

Indicates location of Row 2 anomaliesE

Sta. 12+69

Upstream toe sinkholes Sta. 12+87

Water Level 2362.4 feet

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5, 13Electrical Resistivity Imaging Survey Layout

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Lakeview Site 1 Model

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5, 13Electrical Resistivity Imaging Survey Layout

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5, 13Electrical Resistivity Imaging Survey Layout

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5, 13Electrical Resistivity Imaging Survey Layout

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DRAWING NO.

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5, 13Electrical Resistivity Imaging Survey Layout

Row 3 Start

End

End