Atch 9 FWS San Luis-Alamosa Welcome Center-HQ - Geotechnical Report_0002.pdf

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December 2, 2019

Prepared for:

Wilbert Ortega

USFWS

Region 6 Division of Engineering 134 Union Blvd.

Lakewood, CO 80228

GEOTECHNICAL REPORT

U.S. FISH & WILDLIFE SERVICE

SAN LUIS VALLEY COMPLEX

HQ VISITOR CENTER

TBD EL RANCHO LANE

ALAMOSA, COLORADO

USFWS SLVC HQ Visitor Center Alamosa FINAL geotech report.docx Project #7251.41626.01.01 Page i

TABLE OF CONTENTS

Introduction

Construction Plans

Site Conditions & Approach

Geology

Geologic Hazards

5.1 Shallow Groundwater

5.2 Flooding

5.3 Compressible & Corrosive Soil

5.4 Seismicity

5.5 Radon Gas

Soil Characteristics

6.1 Field Evaluation

6.2 Laboratory testing

6.3 Soils Summary

Engineering Analysis

7.1 Pavement Design

7.1.1 Traffic Analysis

7.1.2 Equivalent Single Axle Loading (ESAL) Calculation

7.1.3 Soil Sample Classification

7.1.4 Subgrade Support Characteristics

7.1.5 Road Section Selection

7.2 Engineering Analysis

Recommendations

8.1 General Design Criteria

8.2 Seismic Design Criteria

8.3 Foundation

8.4 Floor Systems

8.5 Stemwalls

8.6 Drainage and Ventilation

8.7 Compaction and Backfill

8.8 Concrete

8.9 Exterior Concrete Flatwork

8.10 Excavation and Shoring

8.11 Pavement Section Design

Project #7251.41626.01.01 Page ii

8.12 On-Site Wastewater Treatment System Design and Construction

Closing Considerations

9.1 Standard of Care and Interpretation of Subsurface Data

9.2 Use of This Report

9.3 Retention of Samples

Additional Services

APPENDICES

APPENDIX A - Project Maps ................................................................................................ A

APPENDIX B - Borehole & Test Pit Logs ................................................................................ B

APPENDIX C - Laboratory Results ........................................................................................ C

APPENDIX D – Engineering Analysis .................................................................................... D

Project #7251.41626.01

INTRODUCTION

DOWL conducted an evaluation of subsurface and site conditions on October 17, 2019 at the U.S. Fish & Wildlife Service (USFWS) National Wildlife Refuge (NWR) property southeast of Alamosa, Colorado. Our services were performed at the request of the USFWS, to evaluate the property for construction of a new San Luis Valley Complex (SLVC) Headquarters (HQ) Visitor Center. The geotechnical evaluation consisted of a site reconnaissance, drilling seven (7) boreholes, excavating two (2) test pits, logging and testing of representative materials found, and analysis of available data. In this report we present the findings of our evaluation and our geotechnical engineering recommendations for site preparation, foundation design, and management of drainage.

CONSTRUCTION PLANS

Based on the concept plans provided to us (dated 5/7/2019), the HQ Visitor Center planned for this site will be a single-story building that is roughly 34 ft by 120 ft (4,080 square feet) with an 8 ft by 41 ft porch at the entrance. The building will be about 18-inches above existing grade and a concrete walk will be on the west and north sides of the building with a 2-ft wide slab around the remaining perimeter of the building. There will be offices, meeting rooms, public and office bathrooms, a break room/kitchen, and public reception and exhibit/display areas. It is anticipated that 8 to 14 staff will use the offices and facilities at this site and that approximately 10,000 visitors per year are anticipated. The structure will be conventionally framed with wood or steel studs with anticipated loads on the order of 1,000 to 2,000 pounds per linear foot transferred to the foundation level with concentrated loads of approximately 5,000 lbs at the shed roof column. We understand that a floating concrete slab-on-grade with perimeter stemwalls on spread footing is desired, but a framed floor with crawlspace could be recommended depending on potential differential movement of the soil. Frost depth is 4 feet in Alamosa County.

In addition to the HQ Visitor Center to be constructed, other site improvements include an On-site Wastewater Treatment System (OWTS), enclosed water well, entrance road, parking lot (with 31 parking spaces, including ADA parking, and two RV slots), and concrete pads for a trash dumpster and propone tank. In addition to typical cars and trucks, the entrance road and parking lot will need to be designed to accommodate the turning radius of fire trucks, RV’s and school buses.

SITE CONDITIONS & APPROACH

As seen on the Vicinity Map (Map 1, Appendix A), the SLVC site is located on the east side of El Rancho Lane, which is about 3 miles east to southeast of Alamosa. In the attached Site Plan (Map 2, Appendix A) we illustrate the topography of the building site and the locations of our seven (7) boreholes and two (2) test pits with respect to the proposed building footprint and other site improvements. All project features were moved 120 feet east from the original plan to allow for sight distance to existing wireless communications and the new feature locations are shown on Map 2. Based on the Site Plan, the building site is located at an average elevation of 7,530 feet and the terrain is nearly flat with a regional slope of 1-2% down to the southwest.

The SLVC property had been irrigated in the past, as is evidenced by the circular shape of the vegetative pattern from central pivot irrigation that is seen on aerial photographs (Site Plan, Map 1). The building site is vegetated with a sparse cover of seeded grasses and some weeds.

The following photograph (Photo 1) was taken of the building site at the time of our field evaluation.

Photo 1. View looking northeast across the SLVC site, shows the vegetative cover, the local topography, the location of borehole BH#2, and the conditions at the time of our site visit.

DOWL evaluated the subsurface site conditions with a combination of augered boreholes and excavated test pits. The borehole and test pit locations were surveyed by DOWL prior to the drilling so that our testing locations were accurately placed relative to the proposed project features such as the building, parking lot, entrance road, and leachfield site (Site Plan, Map 2).

Two boreholes were drilled within the designated building envelope (BH#1-BH#2), two in the parking lot (BH#3-BH#4), two in the entrance road (BH#5-BH#6), one in the future shade structure area north of the building site (BH#7), and two test pits were excavated in the leachfield area (TP#1-TP#2). The results of our field and laboratory testing are discussed in the Soil Characteristics Section of this report.

GEOLOGY

According to the General Geology of the Northern San Luis Valley, Colorado (J. McCalpin, GEO- HAZ Consulting, 1994), the San Luis Valley is located in the Central Rift portion of the Rio Grande Rift, a major physiographic and structural feature in south-central Colorado. The

Alamosa Basin is the main landform in the San Luis Valley and it is bounded on the west by the San Juan Mountains and on the east by the Sangre de Cristo and Culebra Ranges. Structurally, it is a north-south striking, asymmetric graben with a hinge zone containing multiple faults west of Monte Vista and the Sangre de Cristo Fault along the base of the ranges to the east. The Rio Grande Rift is considered to be an active region of extension and thinning of the crust along these and other Quaternary faults. The Alamosa horst, believed to be a Laramide structure, has dropped down along the flanking faults on the east and west margins of the valley and filled with sediment shed from the adjacent rising mountains and lake bed sediment when it was an internally draining intermontane basin. Although modern measurements of seismic activity in the Rift indicate frequent, but small magnitude earthquakes, geomorphic and paleo-seismic evidence studied by McCalpin suggests Magnitude 6.8 to 7.4 events have occurred on the Sangre de Cristo fault zone.

The Preliminary Geologic Map of the Northwestern Part of the Alamosa 30 x 60 Minute Quadrangle, Alamosa and Conejos Counties, Colorado (USGS OF Report 2005-1392 by M.

Machette and R. Thompson) indicates the SLVC site is located on Quaternary (upper Holocene) floodplain alluvium (Qfp) adjacent to the active channel of the Rio Grande River. These alluvial deposits consist of fine-grained silt and clay to medium-grained sand and pebble gravel. Gravel clasts are generally less than 2 cm in diameter but can be coarser (5-8 cm) along the Rio Grande River, according to the description of the mapping unit on the geologic map. Some infilled oxbow lake loops and subtle terrace margins can be found in this floodplain zone. This alluvial floodplain deposit is underlain by lacustrine (lake bed) deposits from the middle Pleistocene to Pliocene Lake Alamosa. This blue clay lacustrine deposit forms an aquitard layer that separates the floodplain deposits from underlying coarser alluvium consisting of sandy gravels and cobbles. Review of water well logs on file with the Colorado Division of Water Resources (CDWR) indicates these alluvial and lacustrine deposits are hundreds of feet thick in the San Luis Valley. McCalpin (1994) suggests there is more than 10,000 feet of alluvial, lacustrine and aeolian deposits in the San Luis Valley.

GEOLOGIC HAZARDS

This section of the report is included so that the design team is aware that construction in the area comes with certain risks. Some buildings and roadways throughout the region have experienced negative impacts due to slope movement, sensitive soils, and shallow groundwater.

Typical accepted structural engineering practice for design and construction can be used to reduce the potential for undesirable performance related to troublesome weather and soil conditions. However, because of the overall dynamic characteristics of the area, almost every structure is subject to at least some degree of potential risk. The primary geologic hazards relevant to the SLVC property and their associated risks are discussed below.

5.1 Shallow Groundwater

We found water at depths of 8 to 9 feet in the boreholes and test pits. Since we performed our evaluation in October, it is possible that there will be shallower groundwater during the spring snowmelt and early summer irrigation seasons. During that time, mountain soils are usually saturated as melting snow percolates through the soil and runoff from adjacent mountains recharges the groundwater system. Only BH#5 showed signs of rusty mottling, usually indicative of prolonged exposure to groundwater. This could have been a low area before the field was leveled for agriculture. The other boreholes and test pits did not show signs of iron staining or mottling. Therefore, there was only localized evidence in one of the pavement areas that was observed to indicate at least previous shallow groundwater. Since no basement is planned, groundwater is not anticipated to be shallow enough to impact the building foundation. The design of the leachfield will need to consider that there is a separation of at least 4-feet between the bottom of the leachfield and the shallowest level of anticipated groundwater.

5.2 Flooding

Although the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map Panel 80009-40A indicates the SLVC property is in Zone D (areas in which flood hazards are undetermined), the County commissioned a floodplain study and produced the following Figure 1.

Figure 1. Image showing SLVC property relative to the 100 year floodplain boundary in blue. Map courtesy of the San Luis Valley GIS Authority.

We are aware that when there is heavy runoff and regional flooding in the Alamosa area, there can be groundwater flooding, as well, due to the permeable soils underlain by a clay aquitard.

Consequently, it is our opinion there is potential for groundwater flooding at this site.

Therefore, it is recommended to determine flood elevations and possible protection recommendations and ground floor elevation to be above the 100-year flood event. The

SLVC site

Rio Grande River 100 yr floodplain planned 18-inches of elevation of the ground floor above surrounding grade and using a mounded septic system will help to minimize the hazard from potential surface and groundwater flooding.

5.3 Compressible & Corrosive Soil

Compressible soils are those that have generally been deposited rapidly, such as floodplain deposits or windblown loess, have a loose particle structure, are not sorted or reworked, have a weak matrix containing voids and possibly organic debris, and/or are not naturally in a dense or compacted state. Compressible soils typically have a large proportion of fines (i.e., silt and clay). The near surface soil at this site was found to be moderately to highly compressible under the loads tested in the laboratory. Standard Penetration Tests (SPT) also indicate variable densities throughout the soil column. The potential hazard from compressible soil is excessive settlement or differential settlement of foundation soils under loads applied through the foundation. Reduction of the hazard depends on the nature and extent of the compressible soil.

However, settlement can be minimized by treatment of foundation soils, management of on-site drainage, foundation systems that extend to more competent soil or rock, design of foundation systems that have sufficient strength to resist differential movements, or removal and recompaction of native soil or replacement with compacted structural fill. These methods are discussed in further detail below in the Recommendations Section.

Corrosive soils, which are known to be common in the area, are those which contain chemical compounds that, when water is added, form and acid or base that can be corrosive to concrete, steel or other building materials. Typical salts that create acidic or alkaline conditions in soils are water soluble sulfates such as gypsum (calcium sulfate) or chlorides like magnesium chloride.

Testing for these salts is helpful to identify corrosive soils, but other indicators are pH and electro-conductivity. Laboratory tests of two soil samples indicates moderate to very high values for water soluble sulfates, chlorides and electro-conductivity. Using sulfate-resistant cement in concrete, providing cathodic protection, using corrosion-resistant coatings on pipes and other utility lines, and not having native fine-grained soil in direct contact with foundation components or flatwork will all reduce corrosion from on-site soil.

5.4 Seismicity

According to the Colorado Geological Survey (CGS) report Earthquake Potential in Colorado (CGS Bulletin 43), the San Luis Valley is in Southern Rio Grande Rift Seismotectonic Subprovince in Colorado, where maximum credible earthquakes are estimated to be on the order of magnitude 6.5 to 7.5 (CGS Bulletin #43). Although the Rio Grande Rift, as discussed in the Geology section of this report, contains numerous Quaternary faults and the region is considered to be seismically active, the magnitudes of recorded earthquakes have been low.

Refer to the Seismic Design Criteria Section of the Recommendations section for site-specific seismic design recommendations based on the Applied Technology Council (ATC) website (www.hazards.atccouncil.org) that relies on the ASCE7-16 guidelines and U.S. Geological Survey Seismic values determined from the latitude and longitude coordinates for the site.

http://www.hazards.atccouncil.org/

5.5 Radon Gas

Radon gas is produced by decay of radioactive minerals contained in subsurface rock and soil.

The U.S. Environmental Protection Agency (EPA) has determined that radon is the second leading cause of lung cancer and that radon can accumulate in buildings and homes if the gas is not properly ventilated. The EPA map of Radon Zones indicates that Alamosa County is in Zone 2 (www.epa.gov/radon/zonemap.html). Although there is no known safe level of radon, Zone 2 is the zone of second highest risk for exposure to radon gas [i.e., 2 to 4 picoCuries per Liter (pCi/L)]. The EPA recommends that occupied structures should be tested regardless of zone designation.

The EPA does not specifically address testing and management of radon in commercial structures. However, the design team should be aware of the potential health implications of constructing upon radon emitting soils, assess risks, and take appropriate action. Additional information about radon gas can be found on the EPA radon website www.epa.gov/radon/ or the CDPHE radon website www.cdphe.state.co.us/hm/rad/radon.

SOIL CHARACTERISTICS

6.1 Field Evaluation

A total of seven (7) boreholes were drilled and two (2) test pits were excavated at the SLVC site on October 17, 2019. Two boreholes were drilled to 31.5 feet within the designated building envelope (BH#1-BH#2), two were drilled to 10.5 feet in the parking lot (BH#3-BH#4), two were drilled to 10.5 feet in the entrance road (BH#5-BH#6), and one was drilled to 10.5 feet in the future shade structure area north of the building site (BH#7). The two test pits (TP#1- TP#2) were excavated in the leachfield area using a backhoe to depths of 8 to 9 feet. A CME 75 truck drill rig was used for the borings at the locations noted on the attached Site Plan (Map 2, Appendix A). The locations of the borings were selected and surveyed prior to the field evaluation based on the locations of the proposed improvements. The boreholes were drilled with an 8-inch hollow-stem continuous-flight auger. Soil samples were obtained at discrete depths using either a standard 1.375-inch inside diameter (I.D.) split-spoon sampler without liners or a 2-inch I.D. split-spoon “California” sampler to perform in-situ Standard Penetration Tests (SPTs) in general accordance with ASTM Standard D-1586. The number of blows required to drive the sampler 12 inches in 6-inch increments were recorded (SPT “N” penetration resistance values) and indicate the relative density or consistency of the soils. The soil, bedrock, and groundwater conditions were logged and the borehole and test pit logs are presented in Appendix B. Representative samples of soil and rock materials were tested in our laboratory and the results are presented in Appendix C. Laboratory results are also summarized on the borehole and test pit logs.

Our findings and recommendations are based on materials found within the above-referenced profile depths. Soil conditions may change between boreholes and below these depths. If deeper excavations are contemplated or if soil/rock materials different than those found in the two boreholes are encountered during construction, DOWL should be called to verify soil continuity and validity of our recommendations. This may require drilling boreholes or http://www.epa.gov/radon/ http://www.cdphe.state.co.us/hm/rad/radon excavation of additional test pits into the subgrade material below foundation grade, or other typical excavation verification.

In the building site boreholes, we found 3 to 4.5 feet of light brown/tan, dry, stiff, silt to silty lean clay underlain by sand, silty sand, and sand with fine gravel to 26 feet. From 26 to 31.5 feet is silty to fine sandy clay. Groundwater was measured during drilling at 8 feet and no bedrock was encountered. SPT N-values of 7-13 blows per foot (bpf) were recorded in the upper 10 feet of BH#1 and 6-12 bpf in the upper 10 feet of BH#2. From 15 to 31.5 feet in BH#1 N-values ranged from 17-35, while from the same depths in BH#2 N-values ranged from 12-17 bpf except for a value of 5 bpf at 25 feet. The following photographs show the site and typical soils that were taken during our field investigation.

Photo 2 (left) is a view northeast at BH#7 being drilled and Photo 3 (right) shows the granular nature of the native sandy soils below 3-5 feet.

For the remainder of the boreholes (BH#3-BH#7), similar soil and groundwater conditions were encountered as were found at the building site. The texture of the alluvial deposits found at this site appear to be variable laterally and vertically, but they are dominated by sand ranging from fine to coarse. We did not observe large gravels or cobbles, mostly fine gravels.

The soils at the leachfield site contain 3-3.5 feet of fine-grained silty clay loam (Type 3A for LTAR – Long-Term Acceptance Rates) underlain by sand to loamy sand (Type 1) with variable amounts of small gravel to 7.5 feet. Below 7.5 feet was medium to coarse sand (Type 0) with gravel. Groundwater was recorded at 8.5 feet in TP#2 and the soil was wet at 8 feet in TP#1.

6.2 Laboratory testing

Laboratory tests were performed on the predominant native soil types to evaluate the range of plasticity and particle size characteristics (Appendix C). Atterberg limits tests of the fine-grained soil at 2-3.5 feet in BH#2 was found to have a liquid limit (LL) of 46, plastic limit (PL) of 24, and plasticity index (PI) of 22. A soil with a PI of between 15 and 30 is considered to have moderate potential for swelling or shrinking. Gradation analysis performed on this soil indicates that it is composed of 38% clay, 51% silt, 11% sand, and 0% gravel (sample DS9). Based on these laboratory test results, these soils classify as sandy lean clay (CL) according to the Unified Soil Classification System (USCS). A swell/consolidation test performed on this sample indicates that, under a seating pressure of 100 pounds per square foot (psf) and left at its in-situ moisture content of 19.2%, the sample compressed 1.4%. When inundated with water at constant stress, the sample swelled 0.4%. Upon the addition of progressively increasing pressures to 4,000 psf, the sample consolidated a total of 17.1%. The initial dry density of this sample was 67 pounds per cubic foot (pcf). The estimated free swelling pressure generated within the sample is 160 psf. Ordinarily, a lean clay with a PI of 24 would have more swell potential, but the low density and high silt content indicate it is not in a naturally dense state and likely has the potential to collapse under loads when the moisture content increases.

Gradation analyses of the sand, silty sand and sand with gravel found throughout the majority of the soil column indicated 6-30% fines (silt/clay), 49-95% sand, and 0-21% gravel. These soils classify as SC, SM and SP soils.

Soil strength and moisture-density (Standard Proctor) tests were performed on a bulk sample obtained from a depth of 2 to 4 feet in BH#6. The Standard Proctor was 95.6 pcf at 20.9% moisture content. The California Bearing Capacity (CBR) test result was 3.6.

A series of geochemical tests were conducted on soil samples from 2-3.5 feet in BH#7 and 1.5- 2 feet in TP#1. The soils had water-soluble sulfate concentrations of 0-0.29%, chlorides contents of 0.042-0.075%, electro-conductivity of 582-3,482 µS/cm, and pH values of 7.0-7.1.

The water-soluble sulfates, chlorides, and electro-conductivity are indicative of moderately to highly corrosive soil. Recommendations for addressing the corrosive nature of the soil are presented in the Recommendations Section of this report.

6.3 Soils Summary

In summary, the field observations and laboratory testing indicate that the soils to the depths explored are non-plastic to moderate plasticity, loose to moderate density, low to moderate swell potential, and moderate to high consolidation potential. To reduce the potential for differential settlement, an over-excavation and replacement with compacted structural fill will be necessary to create a uniformly dense foundation surface. The soils are also moderately to highly corrosive, so providing protection to concrete and steel as well as not using the native fine-grained soil as backfill or foundation soils will be important.

ENGINEERING ANALYSIS

7.1 Pavement Design

7.1.1 Traffic Analysis

The Institute of Transportation Engineers (ITE) Handbook does not have a land use code specific to the visitor’s center and the National Monument land use code (418) uses the acreage of the monument as a means of calculating average daily trips (ADT) for the facility. The Alamosa visitor’s center is proposed to have approximately 31 parking spaces for cars and 2 spaces for RV, so we estimated that a peak day would turn those spaces over three times for a total of 99 ADT.

7.1.2 Equivalent Single Ax le Loading (ESAL) Calculation

Of the 99 average daily vehicles, we estimated that there would be:

95% Passenger vans and pickup trucks x 99 = 94 5% Single Unit Trucks, Buses, and RV’s x 99 = 5

Each of these classification totals was multiplied by the appropriate Colorado Equivalency Factor for flexible pavement from CDOT Table C-1. Those results are:

94 passenger car/pickup truck ADT x 0.003 = 0.282 5 single unit trucks x 0.249 = 1.245

The total of these three classifications is 0.282 + 1.245 = 1.527, which was then multiplied by 365 days per year and by 20 years to get the total number of 18-kip ESAL’s for design.

That total is: 1.527 x 365 x 20 = 11,147 ESAL’s.

7.1.3 Soil Sample Classification

Individual laboratory testing results on representative soil samples obtained from the boreholes are attached (Appendices B and C). As discussed previously in this report, the dominant silt and lean clay (ML & CL) found in the upper 3 to 5 feet, have a California Bearing Ratio (CBR) of 3.6.

This value was used to calculate the Resilient Modulus (MR) of the site soils. Equation 3-1 from the CAPA Guideline for the Design and Use of Asphalt Pavements for Colorado Roadways states:

MR (psi) = 1500 x CBR, so the average MR = 1500 x 3.6 = 5,400 psi

Table 1 shows the Structural Number (SN) calculations given for the MR above.

TABLE 1. PAVEMENT THICKNESS DESIGN FACTORS

Parameter Subgrade Resilient Modulus (psi) 5,400 Drainage coefficient 1.0 Reliability (%) 75 Standard Normal Deviate (ZR) -0.674 Standard Deviation 0.44 Serviceability Loss 2.0 Strength coefficients:

HMA

ABC

SUBBASE

0.44 0.12 0.10

7.1.4 Subgrade Support Characteristics

In Table 1 (above) DOWL summarizes the typical subgrade support characteristics for the soils encountered. Assumptions for choosing the subgrade characteristics listed in Table 1 are:

(a) The pavement structures on site will be exposed to moisture levels approaching saturation more than 25% of the time and that these areas of potential saturation will be have a “good” quality of drainage. No typical drainage information was available for the soils encountered on the site at the time of this report, therefore a CDOT recommended drainage coefficient of m1 = 1.0 was used.

(b) From CDOT’s Table 5.6 Drainage Quality (2014 Edition), it was assumed that water would be removed from all pavement structures within one day and so an overall drainage quality of “good” was used.

(c) A reliability factor of 75% was assumed due to the conservative ESAL count in Section (1) above. CDOT’s Table 1.3 – Reliability (Risk) recommends a range of reliability of 50-85 % for roads. However, 75% reliability was used to represent an acceptable long-term service life.

(d) A standard normal deviate (ZR) of -0.674 and a standard deviation of 0.44, as required by CDOT for all designs, was used.

(e) Per CDOT recommendations, initial and terminal Serviceability Indexes were assumed to be

4.5 and 2.5 respectively, thus a Design Serviceability Loss (ΔPSI) of 2.0 was calculated by subtracting the terminal serviceability index from the initial serviceability index.

CDOT Equation 3.2 was used to calculate the required pavement section for flexible (asphalt) pavement. Equation 3.2 states:

SN = a1D1 + a2D2m2 + a3D3m3

Where: a1, a2, a3 = structural layer coefficients D1 = thickness of bituminous surface course (inches) D2 = thickness of base course (inches) D3 = thickness of sub base (inches) m2 = drainage coefficient of base course m3 = drainage coefficient of sub base

The minimum recommended structural number (SN) was calculated to be = 2.221.

7.1.5 Road Section Selection

Based on the design criteria and calculations presented above, we included concrete and asphalt structural sections (Table 2). Alternative #1 is for the concrete apron pavement section and consists of 6 inches of CDOT Class D concrete on 6 inches of compacted Class 6 aggregate base course (A.B.C.) Alternatives #2 and #3 are two options for the asphalt parking lot and entrance road areas. This section consists of either a 3-inch depth asphalt section on 6-inches of CDOT Class 6 A.B.C. on top of 6 inches of Class 1 sub-base for a total thickness of 15 inches or a 4-inch asphalt section on 8-inches of Class 6 for a total thickness of 12 inches.

Table 2. Pavement Section Alternatives

Pavement Section

Alternatives

SN

Class D Concrete Pavement

(in)

HMA

Thickness

(in.)

Class 6 Base Course

Thickness (in.)

Class 1 Sub-base Course

(ABC) Thickness (in.)

Total Section

Thickness (in.)

Alternative #1 -- 6 -- 6 0 12

Alternative #2 2.76 -- 3 6 6 15

Alternative #3 2.76 4 8 12

Pavement design recommendations are presented in the Recommendations section below.

7.2 Engineering Analysis

We used the data obtained from our field exploration and laboratory testing to evaluate bearing capacity and settlement. We utilized both Shmertmann’s and Janbu’s methods for estimating settlement. We used both Terzaghi’s and Vesic’s method to evaluate bearing capacity. Based on our analysis, settlement controls the design. However, we observed about 2.5 to 4 feet of low density silt near the surface. Based on our experience in the area, we believe this material is a wind-blown loess that is potentially collapsible. We performed a consolidation test on the soil taken from Boring BH#2 and measured about 13% settlement under a load of 2,000 pounds per square foot (psf), when the sample was inundated. In addition, the same sample had a low dry density (67 pcf) also indicative of potentially collapsible soil. We recommend this soil be removed and recompacted or replaced below structural loads such as foundations. In addition, we recommend recompacting 24 inches of subgrade soil beneath the pavement section to reduce the potential for pavement distress. We present the recommended bearing pressure and other foundation recommendations in the following sections.

RECOMMENDATIONS

Based upon our site evaluation and results of our subsurface testing, the proposed building site is suitable for the construction of the intended structure with special attention to foundation subgrade preparation, foundation design, general site preparation and drainage design. The following recommendations are offered to enhance the long-term performance of the foundation soils, foundations and site improvements. It should be noted that the measures offered address only the construction at the proposed building site. They cannot and will not arrest or prevent large-scale geologic processes that may be on-going elsewhere on the property and within the Alamosa area.

8.1 General Design Criteria

1. Shallow components of the foundation system should be extended into the soil a minimum depth of 4 feet below finished grade as prescribed by the local building official to reduce the negative effects of frost heave.

2. In the context of climate change, geologic hazards such as flooding, debris flows, prolonged drought (causing plant stress, wildfires, disease and insect infestations), super rain and snow storms (i.e. 1,000 year events), rockfall, avalanche, etc. in the region may become more common, intense, and destructive than previously observed and/or documented in recent history. Therefore, we encourage the owner to consider design measures that are more conservative and protective than have been historically employed and incorporated into jurisdictional “design standards.” Prediction of such events is beyond the scope of this report and as such, available information and studies which DOWL relies upon, may only meet what would be considered the current minimum standard of care of the local geotechnical engineering industry.

8.2 Seismic Design Criteria

Based on the 2018 IBC we used ASCE7-16 guidelines and our knowledge of the site, we conservatively recommend that this site be designated as Site Class D (stiff soil with 15<N<50).

This classification is based on limited shallow exploratory data, is based on the predominant soil condition near the surface and assumes similar conditions for a depth of 100 feet. For Site Class D, the maximum spectral response acceleration at short periods (0.2 second, SMS) is 0.539 and at one second (SM1) is 0.278g. These values are taken from the Applied Technology Council (ATC) website (www.hazards.atccouncil.org) that relies on the U.S. Geological Survey Seismic Design Web Services and is based on the latitude and longitude coordinates for the site. See ATC seismic parameter output in Appendix D.

8.3 Foundation

As we discussed above, the silt near the surface has the potential to settle when the moisture content increases. Due to that potential, we recommend excavating that material and replacing http://www.hazards.atccouncil.org/ it with structural fill (defined below in the Site Preparation and Grading Section). Based on the boring logs there is 3 to 4.5 feet of silt within the building footprint. The following recommendations are provided to guide foundation design and construction. Based on our analysis, and if the following recommendations are followed during construction, we estimate total settlement will be less than one inch and differential settlement will be less than ½-inch in 25 feet.

1. Design the footings, bearing pads, and retaining walls placed on the prepared structural fill using a net allowable bearing capacity (qa) of 2,500 psf.

2. Excavate and remove the silt soil from below footings and slabs-on-grade. Contact DOWL to identify the depth and extent of the silt soil during construction. Replace the over-excavated area replaced with compacted structural fill. The footing trench should extend a minimum distance equal to one-half the depth of over-excavation horizontally from the edge of the footings. Compact subgrade soil disturbed by excavation operations prior to placing the structural fill. If large (larger than 4-inches in any dimension) rocks are present in the bottom of the excavation, remove and replace the rocks with compacted structural fill. If soft or yielding soils are encountered, contact DOWL to assess the soil conditions and recommend remedial measures. Typical procedures involve removing soft/yielding subgrade soils to firm material and replacing them with compacted structural fill. A geotextile fabric should be placed on the subgrade prior to placing the structural fill to minimize the migration of fines into the structural fill.

The structural fill, as defined in Table 2, should be compacted in 6-inch lifts until the desired footing elevation is achieved.

3. Once the excavation is exposed, but prior to placement of any fill or footing forms, a representative of DOWL must be called out to verify the nature and density of the foundation excavations and cut slope materials, to ensure that relatively uniform soil conditions are present and to confirm that our recommendations are consistent with actual conditions. If we do not verify the soil conditions, DOWL cannot be held responsible for recommendations that may be inconsistent with actual conditions.

4. Observation and testing during construction are essential to ensure that the geotechnical recommendations are consistent with conditions and that the project is constructed in general conformance with project design and specifications. Any geotechnical observations or testing will be provided at additional charge and we should be contacted at least 2 days in advance for scheduling site visits. In addition to excavation observations, we can provide observation and testing of soil density, concrete and grout, foundation forms and rebar, pile installation, steel, welds, grading features, and drain systems.

5. Foundation stemwalls should be designed with sufficient strength to resist lateral earth pressures and to bridge an unsupported span of at least 10 feet. The components of the foundations should be sufficiently interconnected to ensure that they act as a unit. This will provide some degree of resistance to the forces associated with some unanticipated minor soil movement and will provide some degree of unity to the foundation systems.

6. All concrete used in foundation components at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.

8.4 Floor Systems

1. A floating slab-on-grade may be used at the site. However, as we discussed above, the upper native soil has the potential to settle when the moisture content increases. We recommend excavating the silt soil below slabs-on-grade and replacing the soil as compacted structural fill.

2. To provide an adequate bearing surface remove any topsoil and organic material. The subgrade material should be proof-compacted and soft spots removed and replaced with structural fill. If additional fill is needed to elevate the slab area to the desired foundation grade, this can be accomplished using structural fill.

3. To provide a capillary break and/or radon under-slab mitigation collection, slabs on-grade should be placed on 4 inches of ¾-inch to 1½-inch washed rock on the prepared subgrade. Where moisture-sensitive interior floor finishes are applied to the slab, we recommend an unpunctured vapor retarder between the gravel and the floor slab.

4. Under-slab plumbing should be avoided to minimize the potential for leakage under the slab. When necessary, under-slab plumbing should be provided with flexible couplings and should be leak-tested prior to concrete floor construction.

5. All concrete slabs used at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.

6. Suspended floors (framed floor over a crawlspace) may be considered for use at this site. Suspended floors can consist of conventionally-framed wood flooring systems, thin concrete slabs supported on steel or wood decking, or prestressed slabs. These systems have historically performed well, but may be susceptible to moisture accumulation in the crawlspaces that can not only cause environmental concerns but also compromise the structural integrity of the flooring system over time. To enhance the long-term performance of these systems, provide site grading to drain water away from the structure and consult with the architect and structural engineering to vent the crawlspace. In addition, we recommend you consider constructing a sump with a sump pump system in the crawlspace.

8.5 Stemwalls

1. Stemwalls acting to restrain soil should be designed using the lateral earth pressures given in Table 1 below. These values assume a level backslope (slope behind the walls) or outboard slope (slope below the toe of wall), no hydraulic pressures behind the wall, the use of “free-draining” structural fill, and no surcharge loads applied within the backslope zone. Native clayey/silty soils are not recommended for backfill due to their low permeability and corrosion potential.

Table 1. Lateral Earth Pressures

Structural Fill

Active Earth Pressure 34 pcf* Passive Earth Pressure 400 pcf* At-Rest Earth Pressure 54 pcf* Unit weight of soil 130 pcf** Coefficient of Friction 0.32 ***

* pounds per cubic foot (fluid equivalent) ** pounds per cubic foot *** concrete on dry soil conditions

2. Design stemwalls that retain soil with drainage features so that hydrostatic pressures do not develop behind the walls. This is usually accomplished by providing free-draining granular backfill between the wall and retained soil and a collection drain provided at the bottom of this granular zone. The drain system should be continuous and have a positive outfall or a sump and pump which releases the collected water well away from the wall in a manner that minimizes the erosive energy of concentrated flow. The design engineer should ensure that drainage design is compatible with design assumptions.

3. Excavations for foundation stemwalls should be laid back a minimum of 33° (1.5H:1V) from the vertical prior to backfilling against retaining structures. For safety, excavations should also be in accordance with OSHA Regulations 29 CFR 1926. Consequently, gentler excavation faces may be required.

4. Fill material placed behind the stemwalls should consist of free-draining granular material (specified below) compacted as shown in Table 3. Native soil should not be used as backfill due to the predominance of fines and their corrosive qualities, or the concrete must be resistant to sulfate attack (see Section 8.8). Be careful not to over-compact the backfill soil immediately adjacent to the wall to avoid excessive pressures against the retaining wall. Unless expressly approved by the design engineer, only use hand-operated light-duty compaction equipment within three feet of the wall. The upper one foot of backfill should consist of clayey (i.e. less permeable) soil to reduce infiltration of surface runoff. Flatwork and other improvements supported on the lightly compacted backfill may likely settle over time. Such improvements should be designed to accommodate such settlement or be founded through the backfill on native undisturbed soils.

5. Concrete used for retaining structures at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.

8.6 Drainage and Ventilation

It is important to minimize moisture penetration into the soil beneath or adjacent to the structure. Moisture can accumulate because of poor surface drainage, drywell and infiltration systems, over-irrigation of landscaped areas, waterline leaks, melting snow, subsurface seepage, or condensation from vapor transport.

1. Per the 2018 IBC, slope the ground surface within 10 feet of the structure downward a minimum of 5% away from the structure. Slope the ground surface beyond 10 feet of structures downward at least 2% away from the structure.

2. Apron slabs and pavement may be used to further reduce infiltration adjacent to structures. Aprons should consist of asphalt or Portland cement concrete pavement that is placed directly adjacent to the foundation stemwalls. An elastomeric sealant should also be considered between aprons and foundation stemwalls to further reduce the potential for moisture to infiltrate the area directly adjacent to foundations. Slope apron slabs and pavement a minimum of 2%, downward, away from the building.

3. Install eve gutters, downspouts, and extensions such that they dispose of water a minimum of 10 feet away from the structure or into solid pipes connected to the storm drain system. Equip roof drainage with gates and/or heat traces to melt snow and ice.

Do not discharge downspouts into the perforated foundation drain system. These points of discharge should be identified in the site drainage plan so that water is readily removed from the site. Do not irrigate within 10 feet of the building. Periodically inspect and flush irrigation systems to detect potential leaks and avoid saturation of foundation backfill.

4. We recommend xeriscape or dryland shrub landscaping be used within 10 feet of foundations to reduce the amount of infiltration. Xeriscape landscaping practices are recommended for this site so that daily applied irrigation waters do not exceed daily evapotranspiration rates applicable for the time of year when irrigating. A NRCS soil scientist or Colorado landscape specialist should be consulted for selection of xeriscaping species and methods.

5. Maintain and check sump systems for proper operation.

6. Seal cracks in sidewalks, driveway and patio slabs, floor slabs, and foundation walls.

Maintain sealant between adjacent slabs and between slabs and adjacent walls.

7. Do not construct landscaping, curbs or other barriers that impair drainage.

8. Do not burry rain gutter discharge pipes because they can leak, which often goes undetected. Seepage problems can also be caused by clogging, crushing, and poor grading of the pipes.

9. Do not construct infiltration basins adjacent to or up gradient of the structures. If detention is required by statute, infiltration basins should be located down gradient and at least 30 feet from foundations.

10. Floor systems and confined areas above concrete floor slabs should be properly ventilated to allow for the release of radon gas. See the Radon Gas Section of this report for more radon information.

11. Based on the results of a flood study, the main floor of the proposed structure should be elevated above the 100-year flood elevation relative to finished grade.

12. Development should utilize "best practices" for design and construction so that on-site erosion is minimized. This may include selective thinning of vegetation, construction of temporary diversion ditches, silt fencing, and/or dust suppression. The local building official will be able to provide specific details regarding these requirements.

8.7 Compaction and Backfill

1. Backfill placed in utility trenches leading to the structure should be densely compacted in accordance with project specifications to inhibit surface water infiltration and migration towards the foundation, as well as minimize post-construction settlement of the trench backfill. We recommend low-permeability check-dams be installed in the trenches at the lot line and the structure to inhibit water flow along any utility trenches.

2. Fill used at this site should meet the gradational and compaction requirements listed in Tables 2 and 3 below. Fill should be placed and compacted in maximum 6-inch lifts, unless otherwise directed by the design engineer. Structural fill should not be placed on frozen or wet existing soil or fill material. It is recommended that the foundation excavation be open a minimal amount of time to avoid degradation of the foundation soils.

Table 2. Gradation Requirements for Fill Material

Type Sieve %Passing, by weight Structural Fill (CDOT Class 6 roadbase) 3/4” (19.0 mm) 100 #4 (4.75 mm) 30-65 #8 (2.36 mm) 25-55 #200 (0.075 mm) 3-12

Structural Fill (CDOT Class 1) 2.5” (63.5 mm) 100 2” (50 mm) 95-100 #4 (4.75 mm) 30-65 #200 (0.075 mm) 3-15

Fill under exterior concrete flatwork 3” (75 mm) 100 #200 (0.075 mm) 0-5

Free-draining fill 3” (75 mm) 100 ¾” (19 mm) 20-90 #4 (4.75 mm) 0-20 #200 (0.075 mm) 0-3

Note: The Plasticity Index for all fill soils should be less than 6.

Table 3. Compaction Requirements for Fill Material

Application Compaction Requirement Proctor Moisture

Under footings and slabs 95% max. dry density Modified ±2% of optimum Under exterior flatwork 92% max. dry density Modified ±2% of optimum Road Subgrade 95% max. dry density Standard 0-4% above optimum Road Subbase 95% max. dry density Modified ±2% of optimum Road base course 95% max. dry density Modified ±2% of optimum Behind retaining walls 95% max. dry density Standard ±2% of optimum Utility Trenches 95% max. dry density Standard ±2% of optimum General landscaping Per project specifications*

*As specified by the design engineer on project documents or in accordance with local municipal requirements.

3. Do not place soils containing more than four percent organics, debris, topsoil, frozen soil, snow, ice, and other deleterious materials. Soil containing organics or topsoil may be used for landscaping.

4. Contact a representative of DOWL to observe placement of structural fill and verify the compacted density. The owner should contact DOWL in advance of the excavations to discuss the specific testing requirements, budget, and scheduling needed for these services.

8.8 Concrete

Water-soluble sulfate tests conducted on soil samples from BH#7 and TP#1, showed sulfate concentrations of 0-0.29. This is a very wide range, but the higher level is indicative of highly corrosive soil. Therefore, we recommend that the cementitious material requirements for Class 2 sulfate exposure in Section 601.04 of CDOT’s Specifications for Road and Bridge Construction publication be consulted and followed.

8.9 Exterior Concrete Flatwork

1. Flatwork should be placed on a minimum of 1-foot of structural fill (see Tables 2 and 3), placed and compacted in accordance with project specifications overlying at least 1 foot of scarified, moisture conditioned and compacted subgrade soil. As previously discussed, flatwork and other improvements supported on the lightly compacted backfill will likely settle over time.

2. Flatwork adjacent to buildings should be placed on properly compacted fill. To minimize future settlement and damage to the flatwork and/or adjacent foundations, the fill should consist of approved material placed and compacted per project specifications.

3. Flatwork adjacent to exterior doorways should be dowelled into the foundation to reduce long-term differential movement between the flatwork and structure.

4. Exterior concrete flatwork should be designed and constructed so that it drains freely away from the structure. Concrete flatwork adjacent to the foundation should slope away at a grade of at least ¼-inch per foot.

5. All concrete used at this site in contact with native soil should comply with the recommendations in the Concrete Section of these recommendations.

8.10 Excavation and Shoring

1. Temporary excavations should be in accordance with Occupational Safety and Health

Administration (OSHA) regulations and with worker safety in mind.

2. Construction equipment, materials, and soil stockpiles should be located a minimum horizontal distance equal to the height of the excavation from the crest of the excavation unless otherwise approved by the design engineer.

3. The contractor’s “competent person” (defined by OSHA as “an individual capable of identifying existing and predictable hazards…and who has the authorization to take prompt corrective measures to eliminate or manage these hazards and conditions) should evaluate the soil materials exposed…