1_-_Geotechnical_Recommendations_2022__7-6-2022_FINAL_0003.pdf

U.S. Department of the Interior July 2022

Technical Memorandum No. LMDT-86-68313-5

Water Treatment Plant Final Geotechnical Recommendations Leadville Mine Drainage Tunnel Water Treatment Plant Missouri Basin Region

Mission Statements

The U.S. Department of the Interior protects and manages the Nation’s natural resources and cultural heritage; provides scientific and other information about those resources; honors its trust responsibilities or special commitments to American Indians, Alaska Natives, and affiliated Island Communities.

The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public.

Cover Photo – Test pit operations at the project site in 2018 (Bureau of Reclamation/Seth Joramo).

U.S. Department of the Interior July 2022

Water Treatment Plant Final Geotechnical Recommendations

Leadville Mine Drainage Tunnel Water Treatment Plant Arkansas – Rio Grande – Texas Gulf Region

Prepared by:

Bureau of Reclamation Technical Service Center Denver, Colorado

Water Treatment Plant Final Geotechnical Recommendations Leadville Mine Drainage Tunnel Water Treatment Plan Arkansas – Rio Grande – Texas Gulf Region

Prepared by: Michael Henrick Civil Engineer, Geotechnical Group 3, 86-68313

Checked by: Michael J. Gobla, PE Supervisory Civil Engineer, Geotechnical Group 3, 86-68313

Technical Approval: Michael J. Gobla, PE Supervisory Civil Engineer, Geotechnical Group 3, 86-68313

Peer reviewed by: Dennis Hanneman, PE Technical Specialist Geotechnical Group 5, 86-68315

Acronyms and Abbreviations

1D one-dimensional 2D two-dimensional ASTM ASTM International (formerly American Society for Testing and Materials) bgs below ground surface FHWA Federal Highway Administration ft foot/feet ft/s feet per second H:V horizontal:vertical ksf kips per square foot lb/ft3 pounds per cubic foot lb/ft2 pounds per square foot lb/ft2/ft pounds per square foot per foot lb/in3 pounds per cubic inch lb/in2 pounds per square inch LMDT Leadville Mine Drainage Tunnel NAD83 North American Datum of 1983 NAVD29 National Geodetic Vertical Datum of 1929 No. number PHA peak horizontal acceleration Qpg Quaternary terrace gravels Qal Quaternary alluvium Reclamation Bureau of Reclamation TA Technical Approval TM technical memorandum TSC Technical Service Center WTP water treatment plant

Symbols

> greater than < less than # number i

Contents Page

I. Project Introduction A. Site Information B. Proposed Development

II. Subsurface Information A. Regional Geology B. Seismicity

1. Seismic Site Classification C. Available Subsurface Explorations D. Expected Subsurface Conditions E. Groundwater

III. Fill and Excavation Recommendations A. Site Preparation B. Excavation and Temporary Grades

1. Unwatering C. Subgrade Preparation D. Fill Placement and Compaction

IV. Foundation Design A. Cold Weather Design B. Allowable Bearing Capacity C. Settlement D. Lateral Earth Pressures E. Slab-on-Grade Design F. Foundation Drainage Design G. Pipe Trench Requirements H. Pavement Subgrade Parameters

V. Slope Stability and Retaining Walls VI. References

Tables

Table Page

1 Table 1.—Seismic loading conditions for the LMDT, selected from nearby Sugar Loaf Dam [3]

2 2020 Monitoring well installation details [13] ii

Figures

Figure Page

1 Location of Leadville, Colorado 2 LMDT alignment and portal location in Leadville, Colorado [5] 3 Proposed grading plan for the treatment plant project [3] 4 2019 test pit location plan with existing topography [6] 5 2022 Groundwater monitoring well locations with existing topography;

future excavation area shown as hash marked area 6 Geophysical survey line locations overlayed onto satellite and topographic map of the existing site [11] 7 Measured groundwater levels between 2014 and 2017 at the detention pond 8 Detention pond monitoring well locations (circled in blue) from drawing 1335-D-15 (note elevations are in NVGD29) 9 Portion of drawing 1335-D-60601 showing portions of the foundation where unwatering is anticipated

10 Plan view of the WTP wall and leach field wall [16] 11 Profile of the WTP wall (note 2:1 exaggerated vertical scale) [16] 12 Profile view of the leach field wall alignment (note 2:1 exaggerated vertical scale) [16] 13 Plan view of the utility conflicts at the WTP wall, circled in red [16]

I. Project Introduction This report presents the final geotechnical recommendations for the replacement water treatment plant (WTP). Technical Memorandum (TM) No. LMDT-86-68313-2019-3 [1] presented preliminary recommendations based on feasibility designs. This report follows completion of the Technical Approval (TA) Spec package in March 2022 to summarize and document the additional recommendations produced by the Geotechnical Services Division (8300) throughout the design process since feasibility design. This TM supersedes TM No. LMDT-86-68313-2019-3.

A. Site Information The Leadville Mine Drainage Tunnel (LMDT) Treatment Plant is located near the LMDT portal, about 1 mile northeast of the town of Leadville, Colorado, at an elevation of about 9970 feet (ft) above mean sea level.1 The site location is shown on Figure 1 and Figure 2. The LMDT is an underground excavation constructed by the Bureau of Mines during World War II and the Korean War to drain groundwater from metal mines located at Leadville [2].

The existing treatment plant was constructed in the early 1990s to meet discharge criteria of the tunnel effluent imposed by the Environmental Protection Agency under the Clean Water Act and the Federal Water Pollution Act [2].

Figure 1.—Location of Leadville, Colorado.

1 Elevations in this report are in NAD83; previous Leadville Mine Drainage Tunnel reports have used NAVD29.

Preliminary Treatment Plant Geotechnical Recommendations

Figure 2.—LMDT alignment and portal location in Leadville, Colorado [5].

The existing WTP configuration consists of an approximately 180- by 70-ft steel frame treatment plant with attached office, a steel-framed storage building located uphill of the main facility, a clear well, a detention pond, a septic tank, a septic field, and associated site features such as pipelines, parking lots, gates, and security fencing. An existing chemical storage building onsite stores acid and caustic chemicals for use in the treatment plant. Prior to development of the existing facility, the site was used for structures used in construction of the LMDT [2]. Portions of the existing site were developed utilizing waste rock from the tunnel excavation.

Inflows from the LMDT are conveyed to the existing plant through multiple sources, including two separate pipelines from pumphouses located at LMDT Stations 10+25 and 46+96, to the southeast of the WTP site. A gravity-fed influent line also connects the tunnel portal to the existing plant.

B. Proposed Development The existing plant is nearing the end of its service life and will be replaced by a larger plant located approximately 30 ft northeast of and parallel to the existing structure. This report references TA Spec level designs prepared in April 2022 [3]. If significant changes to the civil site layout, excavations, building foundations, or foundation loads are made for SpecB, the Geotechnical Service Division (8300) should be contacted for review.

The new structure requires a large excavation into the existing slope to the east and south of the existing building and installation of a retaining wall approximately 385 linear ft long and a maximum of 22 ft high to support grade changes for the new WTP. A second, shorter wall is required to install a new septic system uphill of the WTP wall. The new treatment plant will contain equipment similar in size and loading to the existing structure and will include large tanks and chemical treatment equipment. The finished floor elevation of the new treatment plant will be 9970 ft. The existing structure will be demolished after completion of the new facility.

The septic system located near the top of the new slope may be reused, and the detention pond will be relined.

Figure 3 shows the proposed layout for the new treatment plant, including the retaining wall and new cut slopes. A large cut slope will create the space required for the new treatment plant. No significant areas of fill are planned, and the planned slope geometry appears to be limited by the existing septic field and chemical storage building.

Figure 3.—Proposed grading plan for the treatment plant project [3].

II. Subsurface Information A. Regional Geology The LMDT lies in the center of the Southern Rocky Mountain physiographic province.

Generally, this province consists of greatly elevated, north-south strips of granite flanked by, and sometimes capped by, sedimentary rocks. Intermountain basins, such as South Park, are common. The Sawatch Range, lying to the west of the tunnel, has the highest peaks of the Rocky Mountains.

The tunnel portal lies near the headwaters of the Arkansas River between the Sawatch and Mosquito Mountain Ranges. The tunnel itself is driven into the Mosquito Range. The portal and first 635 ft of the tunnel lie in a terminal glacial moraine and terrace gravel. Surficial materials (glacial moraine and terrace deposits) consisting of gravel, cobbles, and boulders in a silt and sand matrix overlie the tunnel [2].

B. Seismicity Estimated seismic loadings (Table 1) were derived from peak horizontal acceleration (PHA) hazard curves for Sugar Loaf Dam that were presented in the TM entitled “Screening/Scoping Level Probabilistic Ground Motion Evaluation for Mount Elbert Forebay, Sugar Loaf, and Twin Lakes Dams, Fryingpan-Arkansas Project, Colorado” [3]. PHA hazard curves for Sugar Loaf Dam provide reasonable estimates of seismic loading at the LMDT, located less than 5 miles from dam. Table 1 shows the seismic loading at Sugar Loaf Dam [3].

Table 1.—Seismic loading conditions for the LMDT, selected from nearby Sugar Loaf Dam [3]

Return period (years) PHA

500 0.05g

2,500 0.12g

5000 0.18g

10,000 0.28g

1. Seismic Site Classification

Bureau of Reclamation (Reclamation) Technical Service Center (TSC) staff performed two-dimensional (2D) seismic S-wave refraction surveys at the new WTP site in 2020 [4]. The seismic surveys measured an average shear wave velocity of the top 30 meters (Vs30) between 1,235 and 1,306 feet per second (ft/s). These values allow a seismic site classification of C per the 2018 International Building Code [4].

C. Available Subsurface Explorations Historic explorations for the LMDT did not adequately cover the current WTP and tunnel portal site, leading to cost overruns during tunnel excavation due to deeper than expected bedrock at the portal area. Bedrock was first encountered in the tunnel approximately 400 ft from the portal entrance. Available subsurface explorations at the WTP site occurred after tunnel abandonment and consisted of four phases:

• 1988 preconstruction explorations for the existing WTP [5]

• 2018 test pit and laboratory testing program [6] [7]

• 2020 two-phased geophysical exploration program [8]

• The 2022 groundwater monitoring well installation program (travel report forthcoming)

In 1988, Reclamation advanced six test pits near the existing water treatment plant site and produced the Leadville Mine Drainage Tunnel Preconstruction Geologic Design Data report.

These test pits encountered either glacial moraine or fill derived from the moraine during tunnel portal area construction. Groundwater was encountered 8 ft below grade. Glacial material was generally described as gravel and cobbles with varying amounts of sand, silt, and clay. Two test pits encountered sandy or silty lean clay from 1.2 and 5 ft below ground surface (bgs) to the maximum depth explored. Several test pits encountered concrete and other debris left over from tunnel construction. Groundwater levels in the 1988 report ranged from 9953.8 to 9961 ft.

Higher groundwater levels were encountered in coarse-grained materials, and the lower levels were observed in test pits that encountered lean clay.

In 2015, Reclamation TSC staff performed pumping tests on the monitoring wells placed around the existing detention pond to the south of the plant. The results of the tests and unwatering modeling are presented in Report No. MERL-2015-083 [9]. Slug tests were performed on the same wells in 2015, documented in Report No. 8550-2019-10 [10].

Reclamation Missouri Basin Region geologists performed a test pit exploration program at the treatment plant site from November 5 to November 8, 2018 [6]. The geologists logged three test pits: two east of the existing perimeter fence to determine conditions for the excavation and retaining wall and one test pit just east of the existing treatment plant to determine foundation conditions for the new plant. The test pits showed most of the site is underlain by Outwash Gravel of Pinedale Glaciation (Qpg) with one test pit encountering clayey Quaternary alluvium

(Qal). The Qpg encountered onsite was generally poorly graded sand with silt and gravel with up to 25 percent cobbles from 3 to 12inches in diameter. The approximate test pit locations are shown on Figure 4.

Groundwater was not encountered to the maximum depth explored or 10 ft below the existing grade east of the existing treatment plant (about elevation 9960). No groundwater was encountered at the top of the cut slope near the existing chemical storage to the maximum depth explored of about 10 ft.

In 2022, a Reclamation GP region drill crew conducted a limited exploration program to install two groundwater monitoring wells on the north side of the existing plant. This program was conducted to more clearly define the potential for unwatering during construction of the new WTP, specifically in deep clear well excavations located around the site. Testing included Standard Penetration Tests and slug tests. Measured groundwater levels are discussed in the “Groundwater” section of this report. Figure 5 presents the approximate location of the piezometers. Test results will be documented in a forthcoming travel report.

Laboratory testing on soils collected during the November 2018 field investigation was performed by the TSC’s Concrete and Geostructural Laboratory [7]. The test program included gradation, Atterberg limits, water content, compaction, minimum/maximum unit weight, direct shear, and corrosion suite testing. Two tests performed using vibratory hammer equipment according to ASTM D7382 Method A on silty sand with gravel – (SM)g – showed maximum dry unit weights of 129.7 and 135.4 pounds per cubic foot (lb/ft3). Two direct shear tests measured friction angles of 38.2 and 44.4 degrees. Corrosivity testing showed the soils are not corrosive, but you should refer to the report for complete results.

A two-phased geophysical investigation program was conducted by Reclamation in 2020. During the first phase [7], three 2D compressional-wave (p-wave) seismic refraction tomography profiling surveys, three one-dimensional (1D) multi-channel analyses of surface waves , shear-wave (s-wave) soundings, and one 2D electrical resistivity tomography survey were performed.

Data showed a possible continuous clay layer in the area of the new plant site; however, the data quality was poor due to snow and frozen ground present during the survey. A second survey was conducted after the snow at the site melted and the ground thawed. The second survey showed no evidence of a continuous clay layer and provided a Vs30 of > 1,200 ft/s. Figure 6 presents the location of the survey lines from the second phase [11]. The survey results indicate dense soils that will not liquify during seismic loading.

Figure 4.—2019 test pit location plan with existing topography [6].

Figure 5.—2022 Groundwater monitoring well locations with existing topography; future excavation area shown as hash marked area.

Figure 6.—Geophysical survey line locations overlayed onto satellite and topographic map of the existing site [11].

D. Expected Subsurface Conditions Our knowledge of the subsurface conditions at the site is limited to the information gathered from previous site investigations, known prior uses of the site, and our understanding of the geologic setting. Therefore, conditions encountered during construction may vary from the conditions used to develop the recommendations in this report. Based on test pit results and past site experience, the contractor selected for the earthwork can expect generally favorable excavation conditions with some cobbles and boulders up to 2 ft in maximum dimension at the treatment plant site. However, there are localized areas beneath the existing treatment plant where alluvial silts and clays were deposited in glacial kettles and other localized depressions. If encountered at the new plant site, these areas are expected to be less than 5 ft thick and of limited lateral extents. If these soils are encountered under foundations or retaining walls, they must be removed and replaced with clean fill as described in the following sections. It is possible these areas are related to excavation of the tunnel because the existing site is situated at least partially on waste rock.

The predominant soils encountered in the previously undeveloped areas of the site are expected to be gravelly sands or sandy gravels with low (< 20 percent) fines contents. Due to the planned depths of site excavation, these soils are expected to be dense and capable of supporting the building and structures. TSC geotechnical engineers should be contacted immediately if the subsurface conditions encountered during construction differ significantly from this section. In particular, clayey layers encountered during excavation work could result in excessive differential settlement or to slope or foundation instability. These layers would need to be removed if encountered, where possible, or an engineered solution implemented if removal is impractical. An engineer from the TSC should make a site visit during early site work excavation operations to observe the encountered soils near the new site grades.

E. Groundwater Groundwater levels at the site are not expected to be above 9962 ft, which corresponds to 2 ft above the maximum depth of the existing temporary storage pond adjacent to the existing treatment plant. Groundwater depth appears to be higher closer to the Arkansas River wetlands, consistent with observations in a prior report [10].

Piezometer installation details and water levels measured in the newly installed 2022 monitoring wells are shown in Table 2. Locations were shown previously on Figure 5. Additional levels will be measured and included in the SpecB specifications, due for completion in August 2022. The presented levels show the groundwater table in March 2022 at approximate elevation 9957 [13].

Levels typically rise in spring when snowmelt from higher elevations occurs.

Table 2.—2020 Monitoring well installation details [13]

Location Total depth

(ft-bgs) Sand pack depth

(ft-bgs)

Screened interval (ft-bgs)

Water level on completion

(ft-bgs)

DH-22-01 29.0 15.5 – 27.1 17.7 – 25.7 13.0

DH-22-02 29.1 15.5 – 27.1 17.5 – 25.5 13.0

Figure 7 presents groundwater levels measured at the detention pond monitoring wells between 2014 and 2017. Figure 8 presents the locations of the detention pond monitoring wells. MW-2 is closest to the Arkansas River and measures the highest and most stable water level.

Additional measurements are available; however, the treatment plant typically operates two small dewatering wells around the plant to prevent the geosynthetic pond liner from floating during periods of higher water levels between May and November. Plant staff report the wells operate intermittently and have a minimal effect on the water levels of the pond. Several measurements available when the pumps are turned off show peak levels below elevation 9960 in all wells.

Figure 7.—Measured groundwater levels between 2014 and 2017 at the detention pond.

Figure 8.—Detention pond monitoring well locations (circled in blue) from drawing 1335-D-15 (note elevations are in NVGD29).

III. Fill and Excavation Recommendations This section provides recommendations specific to the treatment plant site and proposed construction relating to fill and excavation activities. These recommendations are based on the results of field and laboratory testing and good performance of the existing treatment plant facility. They should be used to develop contract specifications and drawings as the project progresses. Specifications and drawings should be reviewed by the geotechnical team to ensure they adequately satisfy these recommendations.

A. Site Preparation The site should be properly prepared prior to the start of new treatment plant construction.

Buried utilities in conflict with the planned construction should be identified and rerouted to maintain service to the existing treatment plant prior to any excavation. Existing structures, pavement, and vegetation in the new building footprint should be demolished and disposed of offsite. Topsoil should be stockpiled separately to avoid contamination of fill.

B. Excavation and Temporary Grades According to Occupational Safety and Health Administration Technical Manual Section 5, Chapter 2, noncohesive soils should be categorized as Type C for temporary excavation slopes.

At a minimum, excavation slopes should be no steeper than 1.5H:1V (horizontal: vertical) and flattened as site conditions and excavation depths require.

The new site grades will require excavation of up to 30 ft from existing grades. This excavation will be partially supported by a retaining wall up to 22 ft high with a maximum 2:1 (horizontal to vertical) backslope. A steeper temporary excavation may be required to construct portions of the wall and should be limited to a maximum slope of 1.5:1. This temporary slope may be prone to erosion and surficial sloughing from snowmelt and rainfall events and should be in place less than one construction season. Similarly, other temporary excavations for site features, such as foundations or sumps, should be limited to a maximum slope of 1.5:1 or supported through the use of trench boxes or other means of engineered excavation support. Narrow vertical excavations may be stable for short periods of time, but personnel should not be allowed to enter them or be in a position where slope instability could cause injury or damage to property. Soil nail wall construction at the site can be used for temporary exposed slopes up to 6 ft high, but initial facing should be applied immediately after exposure.

It is recommended that vehicles and soil stockpiles be kept a minimum lateral distance from the crest of the slope equal to no less than the slope height. The exposed slope face should be protected against the elements, and stability of the slopes should be reevaluated after weather events.

Significant amounts of utility installations up to 10 ft below grade onsite are expected in the new WTP yard. Support for trenching will likely be required given the tight site constraints. Support can be provided by temporary trench boxes. The gravel and cobble content expected in the soils onsite will make sheet pile or other driven pile support, such as a soldier pile and lagging, difficult or impossible. Drilled soil nail walls could be used for temporary support, but casing maybe required to prevent caving of the drillholes.

Specifications should require that excavation slopes do not undermine foundation soils of the existing structures onsite. Excavations should not enter the area below the outside corners of existing foundations defined by a 1:1 line extending down and outward. Further, excavations and the storage of materials should not impact the LMDT portal structure. Equipment and material should not be placed on the hillside area that is directly above the tunnel alignment.

1. Unwatering

Unwatering of excavations extending below the groundwater level may be required for cuts deeper than about 10 ft below the existing grade around the treatment plant. This may be accomplished with the use of sumps and submersible pumps. A layer of gravel or cobbles across the base of the excavation, in combination with a sump and submersible pump, may be used to maintain foundation surfaces free of standing water prior to concrete placements. Sloughing of excavation sidewalls can be expected, as surrounding soils drain into the excavation. Slopes should be flattened as needed to minimize sloughing.

Drawing 1335-D-60601, reproduced on Figure 9, shows the concrete outline for elevation

9953.0. Elevations shown are top of slab, and additional excavation will be required for the slab thicknesses. Unwatering should be anticipated for excavation and construction of these concrete structures, with typical groundwater levels varying from 9957 to 9960 depending on seasonality.

The remainder of the WTP slabs and foundations are planned for elevations above 9964 and are unlikely to require unwatering.

Figure 9.—Portion of drawing 1335-D-60601 showing portions of the foundation where unwatering is anticipated.

C. Subgrade Preparation The base of the excavation for footings and slabs below the existing ground surface should be maintained dry, and subgrade disturbance should be minimized. The subgrade under foundations and slabs should be free from silty or clayey material and should be inspected by the Contracting Officers Representative prior to placement of concrete. The soils should be moistened and compacted to form firm foundations. Where unsuitable material is encountered, the COR should direct additional excavation and replacement with the overlying fill. Areas noted to rut or deflect excessively under compaction, and those that do not tighten up under repeated passes, should be removed to the level of firmer materials.

D. Fill Placement and Compaction Soil for backfilling utilities, adjacent to or under cast foundation structures or slabs and under pavements, should generally consist of all granular soils except those with the classifications of Peat (PT), Silt (ML), Lean Clay (CL), Fat Clay (CH), Silty Sand (SM), Clayey Sand (SC), Organic Silts and Organic Clays (OL and OH), or Elastic Silt (MH) per ASTM D2487. New fill under slabs or foundations should be placed in maximum of 8-inch lifts and compacted to a minimum of 95 percent of the maximum dry unit weight as determined by the Vibrating Hammer method (ASTM D7382-08) due to the high gravel contents in onsite soils and specified materials. Moisture contents should be within 2 percent of optimum moisture during placement and compaction. Field density tests should be conducted on a regular basis to confirm the level of compaction.

The excavation onsite is expected to produce a large quantity of fill that may be processed for use as granular backfill for foundation preparation described in the following section and backfill behind retaining walls. Processing should consist of screening to remove 1.5-inch and larger particles and washing to remove fines passing the #200 sieve. The result should be clean gravelly sand and sandy gravel fill. Soils containing 6 percent or more of minus 200 sieve fines in fill placed under slabs and foundations will increase the risk of frost heave of soils in the winter and spring months. Gravel and cobbles > 1.5 inches may be used on the ground surface as erosion protection for slopes or in landscaped areas.

IV. Foundation Design Foundation design at the site should adhere to the recommendations in this section. The site geology is generally favorable to shallow foundation support of the planned structures and equipment, but special measures will be required to avoid frost heave due to the high elevation subalpine site and high groundwater table. Structures founded deeper than 8 ft from the proposed final grades will not require additional treatment, as they are expected to extend below the depth of frost penetration.

A. Cold Weather Design The expected depth to frost for an unheated structure at the treatment plant site is 8 ft. Heated structures reduce this depth to 4 ft based on the 2012 International Building Code [4], but local experience has shown this to be insufficient in especially bad winters. The original structures onsite used insulated foundations designed in accordance with Reclamation publication “Frost Action in Soil Foundations and Control of Surface Structure Heaving” [12]. The original design appears to have performed well and could be repeated for the new treatment plant. The design consisted of removing the top 5 ft of soil from the treatment plant footprint and replacing it with compacted clean gravel fill and a 6-inch perforated polyvinyl chloride drainpipe around the building perimeter and 3-inch and 2-inch thick rigid foam insulation along and extending out from exterior footings, respectively. Drains should be designed to flow by gravity to allow groundwater to daylight downslope from the treatment plant.

B. Allowable Bearing Capacity The allowable bearing capacity for strip footings up to 3 ft wide and isolated footings up to 8 ft in maximum dimension should be limited to 4 kips per square foot (ksf). Larger foundations for equipment, such as tanks, used for chemical treatment larger than 8 ft in diameter should be limited to 3 ksf to avoid excessive settlement. Strip footings should be a minimum of 18 inches wide to ensure lateral stability. All foundations should be supported by gravel backfill and insulation in accordance with the cold weather design as described previously.

The existing plant used a continuous slab thickened under load bearing walls and structures to contain chemical spills, reduce foundation construction costs, and reduce potential for differential settlement. This method appears to have performed well at the plant and is being replicated for the future plant.

C. Settlement Consolidation or settlement of the foundation soils is expected from placement of the fill to raise the building sites and construction of the new buildings. Based on the subsurface profile, groundwater levels, and previous experience, about 0.5 to 1 inch of total settlement is expected beneath the buildings. Most settlement is expected to occur as the new construction loads are being placed. Based on the relatively uniform subsurface conditions across the treatment plant site, settlement is expected to be relatively uniform, and differential settlement should be minor (on the order of ¼ to ½ inch), assuming recommendations for foundation preparation are followed.

D. Lateral Earth Pressures Earth retaining walls onsite should be designed for lateral earth pressures based on the slope of the soil at the top of the wall, the density of the soil, and the direction of wall movement.

Glacial soils at this site are expected to be very dense; therefore, we recommend a unit weight of 135 lb/ft3. Backfill thickness placed between the retaining wall and cut slope soils should be well drained and meet the backfill requirements provided previously.

Walls with a level backfill can be designed to an active earth pressure coefficient of 0.24 or 32 pounds per square foot per foot (lb/ft2/ft). Walls supporting a 2:1 slope should be designed for an active earth pressure coefficient of 0.36 or 50 lb/ft2/ft. Pressures at the toe of the wall that resist movement should be designed with an at-rest lateral earth pressure coefficient of 0.90 or

122 lb/ft3; however, passive earth pressure contribution toward wall stability from the top 2 ft of soil should be ignored in case there are future changes to the site grade. We recommend the at-rest coefficient for use at the wall toe because passive earth pressures require excessive deformations to develop.

Compacted backfill placed behind structures or retaining wall can assume a friction angle of 32 degrees, a unit weight of 135 lb/ft3, an active earth pressure of 0.31, and an at-rest earth pressure of 0.47. Similar to above, passive earth pressures should not be used.

Allowances for drains behind the wall should be made, but it is prudent at this site to assume some buildup of water pressure behind the wall due to freeze/thaw conditions in winter and spring and the potential effects of the nearby septic leach field. We recommend assuming saturated soil to the full height of the wall in case of drainage system failure. Water inflows from the leach field can be limited by regrading the surface to drain toward the tunnel portal, and away from the 2:1 slope below it, and covering the leach field with a waterproof geomembrane liner.

This will limit inflow from rainwater and snowmelt and help prevent the buildup of water pressures behind the wall and saturation of the 2:1 slope above the wall that could lead to localized sloughing and instability.

E. Slab-on-Grade Design Provided the site is prepared as previously described in this report, uniform slab-on-grade support can be used to support light loads on 5 ft of compacted engineered fill using a Modulus of Subgrade Reaction of 300 pounds per cubic inch (lb/in3) for uniform slab pressures up to 500 lb/ft2 when supported by 5 ft of compacted clean gravel fill and 100 lb/in3 otherwise.

The 1991 Design Summary Report [13] states that the final design combined the floor slab and exterior strip footing into a thickened slab section at the exterior wall to reduce the potential for differential settlement between the perimeter footing and slab and to provide secure containment areas for acid and caustic storage. Provided the original design has performed well, the same design could be used for the replacement building provided the slab is designed with extra reinforcement to bridge some of the perimeter footing load.

F. Foundation Drainage Design Groundwater levels are not expected to reach the foundation drainage system of the new plant;

however, some inflows from snowmelt and stormwater will likely intercept the system. General guidance for sump pumps for sandy sites above the water table provided by [12] recommends a system capacity of 14 gallons per minute per 1,000 square feet of building footprint. The drainage system around the new WTP should be designed following these general guidelines.

G. Pipe Trench Requirements Pipe trench subgrades should be scarified and compacted after final grades are reached.

Inappropriate materials, such as organic and fat clay soils, if encountered, should be removed and replaced with compacted fill. Cementitious low-strength material can then be placed as pipe bedding.

H. Pavement Subgrade Parameters Subgrade around the new WTP will consist of either exposed glacial materials from the existing and proposed excavations to the east of the existing plant, existing gravel surfacing, or previously placed gravel fill from the existing WTP’s foundation subgrade. All of these materials will provide good subgrade support for flexible and rigid pavements, but designs should conservatively assume a more variable and more fines-rich glacial soil. The most fines-rich gradations from the 2018 test pit program correspond to the Colorado Department of Transportation’s pavement recommendations for either an A-2-6 or A-6 soil; therefore, we would recommend a resilient modulus of 15,200 lb/in2 for flexible and 12,900 lb/in2 for rigid pavements at the site. Subgrade drainage classification should be fair, as the subgrade generally has 10 to 30 percent nonplastic fines but will be excellent under the existing WTP foundation.

However, in general, it is generally better to design for a conservative subgrade support for the entire site than to do piecemeal pavement designs. Contractors often ignore the resulting minor changes in pavement design, and areas of site variability are difficult to accurately identify after large amounts of civil site work have taken place.

V. Slope Stability and Retaining Walls TM No. 86-68313-2019-3 [1] presented preliminary stability analyses for the 2019 retaining wall layouts. The retaining wall analyses are no longer relevant to the current retaining walls planned for the WTP site because the WTP building was expanded during the design process, resulting in taller and longer retaining walls. The 2019 TM also included stability analyses for temporary, 1.5H:1V slopes, which showed a factor of safety of 1.3. That analysis is still germane. This section, instead, discusses the final design retaining wall layouts, the assumptions, and the analyses performed for quantity estimations.

Two walls are planned for the site: an upper wall to retain soil for construction of the new septic leach field and a larger lower wall to provide grade separation for the new WTP. Both walls will be cut from existing grades and have slopes up to 3H:1V above them. Site soils contain gravel and cobbles; therefore, a drilled soil nail wall was selected for quantity estimation. A storm sewer line is planned near the toe of the WTP wall; therefore, the critical case considered a wall with an additional 6-ft of excavation at the toe. The WTP wall will also pass over the connection between the existing influent line from the LMDT Station 10+25 pumphouse line and the connection to the new WTP. The location of the existing LMDT Station 10+25 pumphouse influent line was not known at the time of this TM. The March 16, 2022, TA Spec design retaining wall plan view, WTP wall profile view, and leach field wall profile view are shown on Figure 10, Figure 11, and Figure 12, respectively. Figure 13 presents the area where the WTP wall and planned utility connections conflict.

Both walls will be contractor designed as defined by a performance-based specification. A new connection to this influent line is planned for the new WTP behind the proposed wall. The contractor-designed wall must be constructed near the existing influent line and allow for installation of a new manhole on the existing line and a new WTP influent line to the WTP. The new influent line must cross under the WTP wall alignment as shown on the project drawings.

Further, the excavation support systems for storm sewers planned along the base of the WTP wall may impact the wall design, sequencing, and wall type selection. These considerations, along with time constraints of the design schedule, led the project team to have the wall be contractor designed and built.

The TSC explored altering the grading to eliminate the western portion of the wall with utility conflicts, but surface water flow requirements dictate the minimum width between the wall and WTP building in that area. The location of the utilities is limited by the gravity-fed influent line hydraulics and the number of additional utilities in the same area.

Preliminary designs for a soil nail wall were prepared for cost estimation purposes only. The details described below are for informational purposes; the actual wall types and details will be contractor designed. Calculations supporting cost estimates will be located in the project files but not available in the contract documents.

Our analyses assumed a soil nail wall for both the WTP and leach field walls with conservative designs to account for additional costs related to the planned new LMDT Station 10+25 pumphouse influent line. The wall supports are soil nails consisting of No. 9, Grade 75 deformed steel bars centered in a grouted 6-inch-diameter drillhole. A nominal pullout capacity per unit length of bond of 20 lb/in2, or approximately 4.5 kips per foot, was selected for the glacial soils.

The nominal bond strength of 20 lb/in2 was estimated using Table 4.4a in Federal Highway Administration (FHWA) Geotechnical Engineering Circular (GEC) No. 7, Soil Nail Walls Reference Manual [14] for sand and gravel using rotary drilling methods. Uniform 23-foot soil nails were selected for the WTP wall and 11-foot soil nails for the leach field wall (including 1 foot for facing hardware). Cost estimates assumed both walls used a constant maximum vertical and horizontal nail spacing of 4.5 and 5 ft, respectively.

We selected the same initial and final facings for both walls. The initial wall facing consisted of 4 inches of 4,000 lb/in2 shotcrete placed over W2.9xW2.9 mesh with two No. 4 grade 60 horizontal and vertical walers. The anchor plates consisted of grade 36 8x8x1-inch plates with 1/2-inch x 4-1/8-inch studs. The final facing consisted of 8 inches of 4,000 lb/in2 shotcrete with No. 4 bars on 1-foot centers placed horizontally and vertically. The tensile force at the nail head was determined using methods described in FHWA GEC No. 7 (2015) [14] and guidelines provided in Clouterre (2002) [15]. The maximum nail loads from slope stability analyses performed in Geostudio during the individual WTP wall excavation phases were used for the initial facing. The maximum nail loads from the finished wall were used for the final facing design.

Figure 10.—Plan view of the WTP wall and leach field wall [16].

Figure 11.—Profile of the WTP wall (note 2:1 exaggerated vertical scale) [16].

Figure 12.—Profile view of the leach field wall alignment (note 2:1 exaggerated vertical scale) [16].

Figure 13.—Plan view of the utility conflicts at the WTP wall, circled in red [16].

VI. References

[1] Bureau of Reclamation, “Leadville Water Treatment Plant, Preliminary Geotechnical

Recommendations,” TM No. 86-68313-2019-3, Leadville Mine Drainage Tunnel Project, Leadville, Colorado, November 2019.

[2] Gobla, M. and Vandeberg, M. (2008). Existing Condition of the Leadville Mine Drainage Tunnel, Leadville Mine and Drainage Tunnel Project, Colorado, Great Plains Region.

U.S. Department of the Interior Bureau of Reclamation, Technical Service Center, Denver Colorado, June 2008, 74 p.

[3] “Screening/Scoping Level Probabilistic Ground Motion Evaluation for Mount Elbert Forebay, Sugar Loaf and Twin Lakes Dams, Fryingpan-Arksansas Project, Colorado,” U.S. Department of the Interior Bureau of Reclamation, Technical Service Center, Denver Colorado, 2002.

[4] International Code Council. “International Building Code.” Falls Church, Virginia, International Code Council, 2018.

[5] Bureau of Reclamation “Preconstruction Design Data” Leadville Mine Drainage Tunnel, Water Treatment Plant, Leadville Mine Drainage Tunnel Project, Colorado, May 1988.

[6] Travel Report “Test Pits at Leadville Mine Drainage Tunnel - Water Treatment Plant Leadville, CO” Joramo, Seth, Bureau of Reclamation Great Plains Region, May 2019.

[7] Technical Memorandum No. 8530-2019-25 “Geotechnical Laboratory Testing Results for Leadville Mine Drainage Tunnel Water Treatment Plant Foundation Material” Bureau of Reclamation, Technical Service Center, Denver, Colorado, May 23, 2019.

[8] Technical Memorandum No. 85-832000-2020-06 – Geophysical Surveys: P-wave Seismic Refraction, MASW, and Electrical Resistivity Tomography, Water Treatment Plant Replacement – Leadville Mine Drainage Tunnel Project, Leadville, Colorado.

[9] Bureau of Reclamation “Leadville Mine Drainage Tunnel Treatment Plant Unwatering Under Retention Pond,” Report No. MERL-2015-083, Leadville Mine and Drainage Tunnel Project, Colorado, Great Plains Region, September 21, 2015.

[10] Bureau of Reclamation “Detention Pond Replacement and New Lining System for the Leadville Water Treatment Plant” Geotechnical Laboratory and Field Support, 86-68550, Report No. 8550-2019-10, Leadville Mine and Drainage Tunnel Project, Colorado, Great Plains Region, August, 2019.

[11] Bureau of Reclamation, Technical Memorandum 85-832000-2020-15 – Geophysical Surveys: Phase II 2D Seismic S-wave Refraction and Electrical Resistivity Tomography, Water Treatment Plant Replacement – Leadville Mine Drainage Tunnel Project, Leadville, Colorado, 2020.

[12] Bureau of Reclamation “Frost Action in Soil Foundations and Control of Surface Structure Heaving” REC-ERC-82-17, Engineering and Research Center, June 1982.

[13] Bureau of Reclamation “Design Summary – Treatment Plant and Tunnel Lining, Leadville Mine and Drainage Project, Colorado” Denver, Colorado, January 1991.

[14] U.S. Department of Transportation, Federal Highway Administration, “Soil Nail Walls Reference Manual,” National Highway Institute, Publication No. FHWA-NHI-14-007, Washington, DC, February 2015.

[15] Clouterre “Additif 2002 aux recommandations Clouterre 1991,” (Trans: Addenda to Recommendations Clouterre 1991), in French, Presses de l’Ecole Nationale des Ponts et Chaussées, Paris, France.

[16] Bureau of Reclamation, Technical Service Center, TA Spec Drawings and Specifications, Microsoft Teams file, downloaded April 14, 2022.