Attachment 2 - Lakeview 1 - Drawings.pdf

TABLE OF CONTENTS

DESIGN REPORT

LAKEVIEW WATERSHED, SITE 1

CONTENTS PAGE NO.

Project Description

General Design of Existing Structure

Survey

Geologic Features, Soil Mechanics, and Observations

Repair Work Design

Cultural Resources

Environmental Considerations

Construction Review

Construction Specifications and Bid Schedule

Construction Schedule

Operation and Maintenance

Appendices:

A. Attachment to the Operation and Maintenance Plan

DESIGN REPORT

LAKEVIEW WATERSHED

SITE 1

PROJECT DESCRIPTION

National Inventory of Dams (NID) Site 1, NID TX04690, is a single-purpose earthen dam designed in 1974 for flood prevention with the construction completed in 1976. The structure is an integral part of the Lakeview Watershed. Site No. 1 is located approximately 12 miles south of Clarendon, Donley County, Texas. The reservoir does not normally hold water and when it has, approximately 6 months is the longest period observed with water. Reservoir sink holes were discovered in 2008 and after the reservoir filled to its highest observable level in 2013, seepage was documented along the downstream toe and in the plunge basin area. An Earth Resistivity Imaging (ERI) investigation was conducted in September 2013 to determine the extent and location of seepage through the dam, foundation, and sub-foundation locations. The Natural Resources Conservation Service (NRCS) dam safety engineer determined that leaving the inlet riser gate valve permanently open would be the best course of action to take until a team could be assembled to investigate the seepage deficiency.

This project is funded through the Remedial Repair Program of the NRCS. According to the USDA-NRCS Abstract of Engineering Report dated August 28, 2014:

“Sinkholes resulted from hydraulic head pressure and the collapse of the permanent pool strata appears to have provided a significant increase in hydraulic conductivity to a verified sandy-silty layer beneath the cut-off trench. The resulting increase in hydraulic flow and pressure resulting from hydraulic head has caused the seepage to express at the surface through the less restrictive material in the plunge basin.”

On October 28, 2013, a multi-state NRCS committee visited the site to investigate the cause and potential solution for repair of the upstream sink holes and the downstream seepage.

According to the USDA-NRCS Engineering Report dated August 28, 2014:

“Based on the evidence, seep water is flowing through the saturated sands, gravels, and silts under the dam. The soil that was being transported from the seeps is the same silty soil observed in the drill holes at the dam foundation. Vegetation observed in wet areas downstream of the dam indicates that the wet areas may have developed historically and infers there has been some seepage in the past, but the seeps transporting soil into the plunge basin have only been observed recently. The presence of gypsum and the formation of the sinkholes in the reservoir infer that dissolved and collapsed gypsum beds have resulted in an increased connectivity from the reservoir to the plunge basin. This explains why the amount of seepage has increased in recent times.”

The remedial repair will consist of constructing a soil-bentonite slurry wall along the upstream embankment of the dam. The slurry wall will attenuate and redirect water trying to flow beneath the dam. The inlet riser slide gate should remain open at all times until the success of the remedial measures to prevent high head seepage gradients has been established.

GENERAL DESIGN OF EXISTING STRUCTURE

Site 1 is a potential significant class structure with an effective height of 31 feet. The drainage area is approximately 1,120 acres with a total storage capacity estimated to be approximately 642 acre-feet during the original design. The dam’s embankment side slopes were designed with a 2.5:1 front slope and a 2.5:1 back slope. Ten anti-seep collars are located along the principal spillway (PS) with center to center distance at 20 feet apart. The PS consists of 270-feet of 24-inch I.D.

prestressed, concrete lined, steel cylinder pipe (A.W.W.A. C-301) with a centerline at a 65-degree angle from the dam centerline, right side, looking downstream. The PS is set on shaped concrete bedding with a twenty-foot pipe outlet section resting on an eight-foot reinforced concrete pipe support. The inlet riser is a nineteen-foot high, 24-inch by 48-inch open topped riser with a 12-inch by 12-inch slide gate, four-inch air vent, 11 7/8-inch diameter orifice (constriction) plate, a metal trash guard, and two, 12-inch by 18-inch open ports, one each into the left and right sides of the riser. The riser crest elevation is 2376.6 and the lowest ungated outlet elevation is 2374.6. The conduit outlet discharge is into a rock-lined plunge basin with a bottom width of 12-feet (2.5:1 side slopes). A 13-inch layer of rock cobbles covers the 20-foot wave berm beginning at El 2372.1 on a 5:1 slope and extending approximately 8-feet above the berm along the upstream embankment slope with a 2.5:1 toe slope. There is a 20-foot berm at El. 2365.0 on the downstream slope. The top of dam (TOD) elevation is 2392.0 after allowance for settlement. The auxiliary spillway (AS) ranges in width from 38 to 50 feet and is a vegetated spillway constructed on a 10.4% grade from the crest down to Sta. 4+75 to where it outlets into the natural channel. The AS crest elevation is

2386.0 between Sta. 3+00 and Sta. 3+50. The AS is contained on the left, looking downstream, by a 10-foot wide dike with 3:1 slopes: from Sta. 2+70 to Sta. 2+80 the AS diversion is transitional to the dike; Sta. 2+80 to Sta. 3+50 the dike elevation is 2389.8; Sta. 3+50 to Sta. 4+75 the dike is transitional; and Sta. 4+75 to approximately Sta. 7+80, the height is 2.5-feet. The AS is contained on the right, looking downstream, by a dike: from Sta. 2+70 to embankment at El. 2389.8, a 14-foot wide crest, 3:1 inner slope and 2.5:1 outer slope; from embankment to Sta. 4+75, the dike is a transitional section to a height of 2.5-feet, a crest width 12-feet and both slopes are 3:1.

SURVEY

A repair survey was performed by the NRCS Watershed Construction Office during August 2020. A global positioning system (GPS) survey instrument calibrated to the State Plane Coordinate System (SPCS) was employed for the survey. Donley County is in the Nor th Zone of the SPCS. The GPS data was then processed using Online Positioning User Service (OPUS).

OPUS processes GPS data files with the same models and tools which help manage the nation's Continuously Operating Reference Station (CORS) network, resulting in highly accurate and consistent coordinates that can be utilized among varying project stakeholders.

The existing elevation on the lowest ungated outlet of the PS riser is 2374.6 and was OPUS corrected to elevation 2378.1, a difference of 3.5-feet.

All elevations in the repair construction drawings and in the “Repair Work Design” section of this report reflect the OPUS adjusted elevations. The level of survey performed exceeds the minimum required by NRCS for the type of construction to be performed. It should be noted that no change in water surface elevation is planned as part of this repair work design.

GEOLOGIC FEATURES, SOIL MECHANICS, AND OBSERVATIONS

From the original “Detailed Geologic Investigation of Dam Sites” (July 17, 1973) it is reported that Site 1 is located on the Double Mountain Group of the Permian Bedrock with hardness 2 siltstone and claystone containing disseminated gypsum. Hardness 3-5 gypsum outcrops are on the right abutment and was also encountered at depth in some centerline dam borings as well as in the borrow area. In the higher elevations, remnants of the Ogallala Formation occur. Alluvial soils have a maximum thickness of 44 feet and classify as CL, SC, SM, and GC types. Gravelly horizons are common in the clay and sand materials. No water was found at the time of the original site investigation.

The November 1973 Soil Mechanic Report (SMR) documented:

Based on past experience in this watershed, low in-place densities, and low natural moisture contents of the surface foundation soils, collapse potential was suspected.

Consolidation testing caused settlement as a result of wetting and this was assumed to be collapse potential.

Along dam CL, Sta(s) 14+00 and 15+00 collapse is estimated at 0.5 ft and 1.10 ft respectively equating to 1/3 to ½ of total settlement would result from collapse upon initial wetting.

Foundation removal – because of the relatively high collapse potential and low shear strength, removal of the surface materials was recommended.

The USDA Web Soil Survey for the area indicates a predominance of the Obaro-Quinlan association within the immediate vicinity of the dam and to a lesser degree, Clairemont silt loam along the creek channel, and Aspermont silty clay loam along the southeast section of Site 1.The Obaro-Quinlan soils receive a "Very Limited" rating class and limiting features for "Pond reservoir areas” with a 0.70 value for seepage and 0.5 value for piping (rating between 0.1 to 1 with 1 being the most limiting), consists of ML, CL, and CL-ML variations and have PI values ranging from a low of 1 to a high of 25. The Aspermont soils receive a "Somewhat Limited" rating class and limiting features for "Pond reservoir areas” with a 0.70 value for seepage. The engineering properties of the Aspermont soils indicate CL materials at multiple depths with CH, CL located in the deeper depths (45 to 80 inches) and with higher PI values ranging from 8 to 39.

After the discovery of upstream reservoir sinkholes and downstream seepage an Earth Resistivity Imaging (ERI) investigation and report was made (November 26, 2013). Using specialized equipment, three rows across the dam were measured: the top of dam, and the upstream and downstream berms. The tests were conducted to determine if water was moving through the embankment fill, dam foundation, or below the dam foundation and to identify areas to focus future geological investigations (GI) and drilling. The conclusion provided in the report was that water appeared to be flowing both through and under the dam and that a drilling program should be conducted to finalize the conclusions.

The NRCS Dam Safety engineer, after observing 11 downstream seepage points along the upstream face of the plunge basin concluded in his November 2013 report:

Despite falling head, increasing seepage volumes were recorded over 30 days’ time from 10 gallons per minute (gpm) to a max between 100- 200 gpm (200gpm = 36.4 cubic feet per minute = 0.44 cubic feet per second)

Conclusion/recommendation: gypsum below reservoir went into solution causing a flow path or soil collapsed creating a differential settlement crack and thus in either case a hydraulic connection was formed to thin fine to medium grain gravel layers that are present below the foundation of the dam. Water flowing below the foundation of the dam and the water is passing beneath the foundation and surfacing in the plunge pool. Since the flow rate has been observed to be increasing from the seeps, despite the water level dropping, it is concluded that somewhere along the flow path additional flow path area is being created. Whether or not these areas will collapse in time is unknown. But one thing is for certain, now that the paths are established, they will very likely be there from now on. Leave gate valve permanently open.

During 2013 and 2014 ten more bore holes were drilled along the centerline of the dam and along the downstream berm. Some vertical cracking within the embankment zones were discovered in the sample cores. Discussions with the geologists revealed that the high gradient seepage paths observed by the NRCS personnel were likely going from the reservoir seep holes, below the cutoff trench of the original dam construction, and exiting along the downstream toe and the upstream face of the plunge basin.

REPAIR WORK DESIGN

The scope of this repair design is to install an upstream soil-bentonite slurry barrier wall to intercept and redirect seepage flow paths below the dam and originating in reservoir sinkholes. A clay blanket along the upstream embankment slope of the dam was also considered for seepage interception along the upper embankment areas but, after further discussion, it was determined that the clay blanket would be redundant to the Zone 1 core of the dam which, after observations made during the most recent geological drilling operations, appears intact and extends up to the AS crest elevation of the dam. A downstream drainage system and a downstream slurry wall were also considered during the design process but put on hold so that the effectiveness of the upstream soil-bentonite wall in cutting off seepage can be first assessed.

The existing reservoir sinkholes, approximately eight (8), will be filled with Class A compacted CL materials.

Due to the high seepage gradients observed, apparently flowing from the sinkholes and below the dam, the design team concluded the estimated seepage flows of 200 gpm (38,500 cubic feet per day) would be too great a capacity for even a high capacity, downstream foundation trench drain to intercept and drain seepage water using the traditional materials of fine and coarse sand and slotted pipe. Furthermore, a foundation trench drain in this scenario would have to be located further upstream and trenched fairly deeply to intercept the thin gravelly layers below the dam that are suspected of conveying the bulk of the seepage flow.

NRCS past experience with several other sites with similar soil characteristics and high capacity seepage problems demonstrate the effectiveness of the soil-bentonite slurry wall as the remedial component of choice for sub-grade seepage control. The slurry wall location was set with an alignment located parallel to the upstream toe of the dam and crossing the PS centerline at approximately station 1+03. The intent of the slurry wall will be to intercept high seepage gradients and redirect them to either side of the dam where soils capable of attenuating the high gradients or flow paths directing the seepage away from the dam can be actualized.

The NRCS geologist compiled a record of all the boring holes taken at the site and projected a station and depth geological profile along the upstream toe of the dam. The depth to bedrock plus an additional 2-feet depth to key the slurry wall into the bedrock, was identified to establish the trench bottom extents of the trench and the top of the slurry wall was set at elevation 2360. The slurry wall alignment spans between stations 11+00 and 16+74.

It is assumed that all fill areas will be supplied by the soil from the cut and borrow areas. Assuming 25% shrinkage, approximately 6,757 cubic yards of fill is required for the embankment and sinkhole repair areas and approximately 6,123 cubic yards is available from the excavations. This leaves a borrow material requirement of approximately 634 cubic yards. Multiplying this by a factor of 1.5 results in a borrow area requirement of 951 cubic yards. According to Web Soil Survey, borrow material of suitable quantity and quality appears to be available on site. The required borrow materials will be excavated from multiple borrow areas totaling approximately

0.5 acres, as designated in the drawings. Reserving the top foot of this area for topsoil, approximately 1,613 cubic yards of borrow material remains by excavating approximately 2 feet more into the designated borrow area.

Detailed geometry and construction information can be obtained from the drawings. Topsoil will be placed over all the repaired areas and vegetation established.

CULTURAL RESOURCES

A TX-NRCS Cultural Resource Specialist (CRS) gathered information concerning previously recorded historically or culturally significant sites at Lakeview 1 (located at Lat/Long 34.76388889, -100.90944444) by searching the database of the Texas Historic Sites Atlas (https://atlas.thc.texas.gov/ ) for Donley County. In addition, other relevant databases were searched including the National Register of Historic Places (https://www.nps.gov/subjects/nationalregister/index.htm) and the National Association of Tribal Preservation Offices (http://nathpo.org/wp/thpos/find-a-thpo/), to identify historically significant sites within the boundaries of the project and Native American interests in the area. Based on a review of these databases, historical imagery, topographic maps, and the Web Soil Survey (WSS) (https://websoilsurvey.sc.egov.usda.gov), there are no previously recorded cultural resources or historically significant sites that may be impacted by the proposed project. The project engineer and/or inspector will monitor for previously undiscovered cultural resources during construction, including prehistoric and historic artifacts and features that may appear as a result of excavation.

The TX-NRCS CRS will be notified if cultural resources are encountered in the work areas.

Construction activities in the area of the discovery will be suspended until the Cultural Resources Specialist provides clearance to proceed.

ENVIRONMENTAL CONSIDERATIONS

An Environmental Evaluation Worksheet (Form NRCS-CPA-52) has been completed for Lakeview Site 1. Under current regulations of the National Environmental Policy Act (NEPA), it is required to have one completed by an interdisciplinary team to determine the level of NEPA documentation required. Based on the proposed work of backfilling sinkholes in the dry reservoir pool area upstream of the embankment and installing approximately 575 feet of soil-bentonite slurry wall below the upstream embankment it is not anticipated that the repair construction of the structure will have any detrimental effects on the environment. The construction specifications call for control of pollution in any form resulting from construction operations.

With adequate pollution prevention measures in place the project is anticipated to fall within the guidelines of NWP-3 for maintenance. The NWP 3 will not require a Pre-Construction Notification (PCN).

CONSTRUCTION REVIEW

Equipment capable of trenching to depths of 60 to 65 feet will be required at the jobsite.

Equipment capable of ripping through rock materials will be required at the jobsite.

A construction campsite of approximately 2 acres shall be staked out as shown in the drawings for the soil-bentonite operations.

An NRCS geologists is required to be on site during the trenching and soil-bentonite slurry wall installation to ensure sufficient bedrock key-in depths have been reached as shown in the drawings.

The contractor shall provide a written plan as to how the slurry wall trench will be installed around the existing PS conduit and how the existing PS inlet riser tower, PS conduit, and first anti-seep collar (looking downstream) will be protected during construction.

Excavated foundations shall likewise be examined to ensure all low density materials have been removed and that a stable foundation exists on which to lay earthfill. If low density materials are still present, the construction engineer shall adjust the excavation limits as necessary to remove the materials. Open excavations should be backfilled in a timely manner to ensure that density and moisture requirements for the material are maintained at specific levels. All excavated subgrade or earthfill materials that exhibit drying cracks, slaking, and other evidence of being unsuitable shall be removed or reworked prior to placement of fill.

It should be noted that subgrade excavations in the vicinity of the principal spillway conduit shall be carefully monitored to ensure the underlying pipe is not damaged or disturbed. The excavation should be monitored to assure a minimum of 2 feet of cover is maintained over the pipe for a distance of 10 feet left and right of the principal spillway centerline before heavy equipment may be driven over it.

The State Design Engineer will be notified of any significant discontinuities in the slurry wall and/or embankment. A course of action will be decided upon by consultation with other personnel as necessary and by field review if required.

A dewatering plan will be specified in the Construction Specifications.

Policy by the Texas Commission on Environmental Quality (TCEQ) requires new permanent reference markers (PRMs) to be placed on reconstructed dams. The old PRMs shall be located and removed and then replaced with new markers. Horizontal and vertical measurements recorded with a global positioning system (GPS) receiver must be based on the North American Datum of 1983 and the North American Vertical Datum of 1988 horizontal and vertical reference datum. Elevation data must be recorded using a survey instrument or survey grade GPS receiver. Latitude and longitude measurements must be provided in decimal degree to six places. Control points for the layout of the embankment centerline will be established by the NRCS.

For the bentonite material delivery delays an additional 10 days has been added to the overall performance time. An 80-hour curing time post soil-bentonite installation has been allocated in the estimated performance time for this project.

Extra fill shall be left in the location and condition as designated by the Engineer.

APPENDIX A

LAKEVIEW WATERSHED SITE 1

ATTACHMENT TO THE OPERATION AND MAINTENANCE PLAN

Operation and maintenance of this structure will be the responsibility of the SLO and subject to periodic inspection and review by the NRCS. A copy of the O&M agreement is on file in the NRCS State Office and is a Standard agreement for operation and maintenance agreement.

a. Inspection

(1) Upstream embankment – inspect embankment for abnormal (desiccation) cracking, (longitudinal and transverse), rilling, and erosion on crown, berm and slopes; for slope failure; for excessive or irregular settlements; for excessive or undesirable vegetative growth; new deformations along the toe; for excessive accumulations of debris on slopes; for slippage of slopes; and for wave erosion.

(2) Downstream embankment - inspect for abnormal (desiccation) cracking (longitudinal and transverse), rilling, and erosion on crown, berm and slopes; for slope failure; for excessive or irregular settlements; for excessive or undesirable vegetative growth; for excessive accumulations of debris on slopes; for slippage of slopes; for excessive seepage from the toe of the embankment or upstream face of the plunge basin.

(3) When dry, inspect the upstream reservoir for the appearance of new sink holes.

The sink holes should be promptly filled with compacted lean clay materials.

(4) Slide gate - inspect and monitor that the inlet riser slide gate valve is left in the fully open position at all times until the success of the remedial measures to prevent high head seepage gradients has been established.

(5) Fences - inspect for deterioration of coatings and/or materials; check for bent or overturned line, brace, stub out, corner or gate post; check for adequate tension of line wire and brace wire. Inspect swinging gates for proper opening and closing.

b. Maintenance

(1) Downstream seepage – monitor the downstream toe of the dam and upstream face of the plunge basin for seepage. Report findings to the NRCS and continue to monitor seepage.

(2) The location of new reservoir sink holes should be documented and promptly filled with compacted lean clay materials.

(3) Slide gate – keep the inlet riser slide gate valve in the fully open position at all times until the success of the remedial measures to prevent high head seepage gradients has been established.

(4) Fences - replace excessively deteriorated, damaged, bent, or overturned line, brace, corner, stub out, or gate post; replace excessively deteriorated wire and