BIBE Panther Junction WWTP Upgrades - Geotechnical Report_0004.pdf

GEOTECHNICAL REPORT

Big Bend National Park

Panther Junction Wastewater Treatment Plant

Upgrade Project, BIBE-218341

Brewster County, Texas

Yeh Project No.: 217-450

January 15, 2019

Prepared for:

Anderson Hallas Architects 715 14th Street

Golden, Colorado 80401 Attn: Mr. Wells Squier

Prepared by:

Yeh and Associates, Inc.

391 Front Street, Suite D

Grover Beach, California 93433 Phone: 805-481-9590

391 Front Street, Suite D Grover Beach, CA 93433

(805) 481-9590 www.yeh-eng.com

Colorado California Denver | Colorado Springs | Durango | Glenwood Springs | Grand Junction | Greeley Grover Beach | Ventura

January 15, 2019 Project No. 217-450

Anderson Hallas Architects 715 14th Street Golden, Colorado 80401

Attn: Mr. Wells Squier

Subject: Geotechnical Report, Panther Junction Wastewater Treatment Plant Upgrade Project, Big Bend National Park, BIBE-218341, Brewster County, Texas

Dear Mr. Squier:

Yeh and Associates, Inc. is pleased to submit this draft geotechnical report for the design of wastewater infrastructure improvements for the National Park Service at the Panther Junction Wastewater Treatment Plant in Big Bend National Park, Texas. This report was prepared in accordance with our professional services agreement with Anderson Hallas Architects dated August 15, 2018. This report provides grading and foundation design recommendations for the design of the treatment plant improvements.

The geotechnical evaluation consisted of a program of field exploration, laboratory testing, and analysis. Field and laboratory data collected for this study are attached to this report. Graphics showing the locations of the field explorations, an interpreted subsurface profile for the percolation basin, and recommendations for the design of earthwork, seismic data, subexcavation, foundations, and pipe trenches and backfill are provided. A summary of the geotechnical considerations for the design of the improvements are as follows:

• The soil encountered within the anticipated depths of excavation are generally sand and gravel associated with existing alluvium. The excavated soil, cleansed of organic material, oversized rock and other deleterious material, should be suitable for reuse as compacted fill.

• Perched water associated with the infiltration pond was encountered in boring 18E-03 near the existing percolation basin at approximately 10 feet below the ground surface during drilling. The infiltration performance of the pond is likely affected by the clay content, cementation and dense nature of subsurface soil. The pond has lush vegetation growing year-round and reportedly has about 1-foot of water standing at all times as it is being supplied with treated water from the existing plant. Improving the infiltration characteristics of the pond would likely include removal of the vegetation, replacing the http://www.yeh-eng.com/

391 Front Street, Suite D Grover Beach, CA 93433

(805) 481-9590 www.yeh-eng.com

Colorado California Denver | Colorado Springs | Durango | Glenwood Springs | Grand Junction | Greeley Grover Beach | Ventura existing gravel layer and possibly installing gravel filled drilled infiltration pits within the pond to increase the infiltration potential.

• Perched water should be anticipated for excavations extending below about 10 feet near the location of the percolation basin. Groundwater was not encountered in borings 18E-01 and 18E-02, and is not anticipated for excavations near the proposed treatment structure.

• The design of temporary slopes and shoring systems should consider and provide a means to properly support adjacent structures, pipelines or other existing facilities that will need to remain in operation while the improvements are being constructed.

• Proposed structures can be supported on shallow foundations supported on compacted native materials as recommended in the report. Recommended bearing capacity, lateral load resistance and earth pressures are presented in this report.

• The design earthquake for the site is a M6.1 earthquake, with a resulting peak ground acceleration of approximately 0.17g. The foundation support soil for the new structure is predominantly dense alluvium that is not considered vulnerable to liquefaction or settlement in response to the design earthquake. No special geotechnical recommendation to mitigate liquefaction are considered necessary for design.

We appreciate the opportunity to be of service. Please contact Judd King at 805-481-9590 x 285 or jking@yeh-eng.com if you have questions or require additional information.

Sincerely, YEH AND ASSOCIATES, INC.

Jamie L. Cravens Judd J. King, P.E., G.E. (CA) Staff Engineer Senior Geotechnical Engineer

Reviewed by:

Hai Ming Lim, P.E. (TX) Project Manager

1-15-19 http://www.yeh-eng.com/

Geotechnical Report Yeh Project No. 217-450 BIBE Panther Junction WWTP Improvements January 15, 2019 i

Table of Contents

1. PURPOSE AND SCOPE OF STUDY

2. PROJECT UNDERSTANDING

2.1 EXISTING FACILITY AND SITE DESCRIPTION

2.2 AS-BUILT DATA

2.3 PROPOSED DESIGN

3. FIELD EXPLORATION AND TESTING

3.1 HOLLOW STEM AUGER DRILLING

3.2 HAND EXCAVATION

3.3 LABORATORY TESTING

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

4.2 SUBSURFACE CONDITIONS

4.2.1 PROPOSED IFAS TREATMENT STRUCTURE LOCATION

4.2.2 EXISTING PERCOLATION BASIN LOCATION

4.3 GROUNDWATER CONDITIONS

5. CONCLUSIONS AND RECOMMENDATIONS

5.1 SEISMIC DATA

5.1.1 LIQUEFACTION

5.2 EXPANSIVE SOIL

5.3 ADJACENT STRUCTURES

5.4 EARTHWORK - GENERAL

5.4.1 SUGGESTED MATERIAL SPECIFICATIONS

5.4.2 CLEARING AND GRUBBING

5.4.3 COMPACTION AND GRADING

5.4.4 FILL PLACEMENT

5.4.5 DESIGN OF GRADED SLOPES

5.4.6 FILL PLACEMENT ON SLOPES

5.4.7 EROSION AND DRAINAGE CONSIDERATIONS

5.4.8 REUSE OF EXCAVATED ONSITE MATERIAL

5.5 BURIED STRUCTURE DESIGN

5.5.1 FOUNDATION EARTHWORK

ii

5.5.2 BEARING RESISTANCE

5.5.3 FOUNDATION SETTLEMENT

5.5.4 RESISTANCE TO LATERAL LOADS

5.5.5 STATIC LATERAL EARTH PRESSURES

5.5.6 DYNAMIC LATERAL EARTH PRESSURES

5.6 TRENCH DESIGN

5.6.1 BEDDING

5.6.2 PIPE ZONE MATERIAL

5.6.3 TRENCH BACKFILL

5.7 SOIL CORROSIVITY DATA

5.8 PERCOLATION BASIN

6. CONSTRUCTION CONSIDERATIONS

6.1 TEMPORARY EXCAVATIONS

6.2 SUBGRADE EVALUATION

6.3 GRADING OBSERVATION

7. LIMITATIONS

8. REFERENCES

List of Figures

FIGURE 1: PROJECT LOCATION MAP

FIGURE 2: BORING LOCATION MAP

FIGURE 3: GEOLOGIC MAP

FIGURE 4: GRADING ON SLOPES TYPICAL DETAIL

FIGURE 5: TRENCH DETAIL

FIGURE 6: PERCOLATION BASIN

List of Tables

TABLE 1: GEOTECHNICAL PROPERTIES TEST SUMMARY FOR THE PROPOSED IFAS STRUCTURE LOCATION

TABLE 2: GEOTECHNICAL PROPERTIES TEST SUMMARY FOR THE EXISTING PERCOLATION BASIN LOCATION

TABLE 3: SEISMIC DATA

TABLE 4: RECOMMENDED RELATIVE COMPACTION

List of Plates

Plate 1: Subsurface Profile A-A’ file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327449 file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327450 file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327451 file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327452 file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327453 file://synologygbca/GB_Server/Projects%20-%20Grover/217-450%20ES%20NPS%20BIBE%20Panther%20Junction%20WWTP%20-%20DEN%20copy%208-15-18/1.0%20Geotechnical%20Report/217-450%20BIBE%20Panther%20Junction%20WWTP%20Upgrades%20-%20Draft%20Geotechnical%20Report%20V1.docx#_Toc535327454 iii

List of Appendices

Page/No.

Appendix A – Boring Log Legend for Symbols used on Boring Logs ..................................................................................... A-1 Boring Logs for 18E-01 through 18E-04 ................................................................................ A-2 to 8

Appendix B – Results of Laboratory Testing Summary of Laboratory Test Results ............................................................................................ B-1 Mechanical Sieve Analysis .................................................................................................... B-2 to 7 Percent Passing No. 200 Sieve ...................................................................................................... B-8 Atterberg Limits ........................................................................................................................... B-9 pH and Resistivity ........................................................................................................................ B-10 Soluble Sulfates and Chlorides .................................................................................................... B-11 Organic Content .......................................................................................................................... B-12 Consolidated Undrained Triaxial Shear Strength .............................................................. B-13 to 14 Hydraulic Conductivity ...................................................................................................... B-15 to 16

1. PURPOSE AND SCOPE OF STUDY

Yeh and Associates was retained by Anderson Hallas Architects (AHA) to provide geotechnical engineering services for the design of wastewater infrastructure improvements for the National Park Service at the Panther Junction Wastewater Treatment Plant in Big Bend National Park, Texas. The approximate location of the site is shown on Figure 1.

The geotechnical evaluation consisted of a program of project coordination, data review, field exploration, laboratory testing, and engineering analyses as a basis for providing the recommendations in this report. This report provides recommendations for grading, trench design and foundation design for the project. The report also presents our observations of the infiltration characteristics of the existing pond and presents considerations for improving infiltration of effluent.

2. PROJECT UNDERSTANDING

2.1 EXISTING FACILITY AND SITE DESCRIPTION

The Panther Junction Wastewater Treatment Plant (WWTP) is located near the intersection of Main Park Road and Park Route 12, adjacent to the Panther Junction Visitor Center for Big Bend National Park. The Rio Grande River borders Big Bend National Park to the south, and runs northeast approximately 16 miles southeast of the treatment plant. The river flows from the northwest around Big Bend National Park before running into the Gulf of Mexico at Port Isabel, about 438 miles southeast of the site. The site is bordered by the Chisos Mountains to the south and open desert in the other directions. The existing plant services the Big Bend National

Figure 1: Project Location Map

Park area and operates at about a 42,000 gallon-per-day flow according to the Basis of Design Report by AHA (2018).

The existing ground surface at the treatment plant ranges from about elevation 3,670 feet to 3,693 feet. The ground elevation of the proposed Integrated Fixed Film/Activated Sludge (IFAS) structure is approximately at elevation 3,682 feet. The terrain in the plant vicinity consists of flatlands with interspersed meandering canyons that transition from gently to moderately sloping hillsides and mountainous terrain closer to the center of Big Bend National Park. The surrounding undeveloped areas are covered with brush, grasses, and other vegetation. The average annual rainfall in the project area is approximately 13.8 inches with the average daily high temperatures ranging from 53.3°F to 79.1°F (according to www.usclimatedata.com, for Big Bend National Park).

The existing Wastewater Treatment Plant consists of the headworks, aeration tank, septic tank, sludge receiving facility, sludge drying beds, blower room, disinfection system, and an effluent percolation basin (AHA 2018). Currently, treated wastewater from the existing plant is infiltrated to the groundwater in the percolation basin.

2.2 AS-BUILT DATA

As-built data provided by AHA for Panther Junction Wastewater Treatment Plant, Percolation Basin and Meter Box (Sheet 27), October, (1976) was reviewed for this project.

2.3 PROPOSED DESIGN

The planned improvements include an Integrated Fixed Film/Activated Sludge (IFAS) treatment train that will consist of an anoxic tank, an aeration tank, clarifier, chlorine contact tank, and an aerobic digester. The treatment train will be a concrete structure that will incorporate a 16-foot wide and 52-foot long mat foundation to support the various components. The foundation will support concrete walls that will separate the various treatment systems. The structure will be buried approximately 10 feet below the existing grade and will extend approximately 3.5 feet above grade. Grading for the project is anticipated to include excavation to construct the structure and installation of new piping. Other improvements will include rerouting of piping, repairs to the existing headworks, improving the infiltration rate of the percolation basin, and other maintenance items identified in the Basis of Design Report (AHA 2018).

3. FIELD EXPLORATION AND TESTING

3.1 HOLLOW STEM AUGER DRILLING

Drilling services were provided by Speedrilling of El Paso, Texas. Speedrilling used a truck-mounted CME-55 drill rig equipped with 8-inch outside diameter hollow stem augers to advance three borings (18E-01 through 18E-03) to depths of approximately 31 to 41 feet below existing ground surface (bgs) on September 13, 2018. The approximate boring locations are presented in Figure 2. Elevations of the ground surface at the boring locations were estimated using preliminary plans by AHA. Logs of the borings are presented in Appendix A.

Yeh and Associates personnel classified, labeled and secured samples for subsequent laboratory testing, recorded blow counts (N-Values) for drive samples, and prepared logs of subsurface conditions encountered. Sampling within the borings was performed by driving either a modified California or standard penetration test (SPT) split spoon sampler at specified intervals.

The SPT sampler has a 2-inch outside diameter, 1-3/8-inch inside diameter and is equipped for

Figure 2: Boring Location Map but was used without liners. The modified California sampler has a 3-inch outside diameter, 2- 3/8-inch inside diameter and was used with 1-inch high brass liners. Drive samples were collected using a 140-pound automatic hammer dropping a distance of 30 inches in accordance with ASTM D-1586 (the Standard Penetration Test) procedures. Samples collected using these methods are generally considered disturbed, but are suitable for the testing performed. Bulk samples of the subgrade soil were collected from the augers at the depth intervals noted on the logs.

3.2 HAND EXCAVATION

A grab sample and a bulk sample were collected from the existing percolation basin by Yeh personnel on September 14, 2018. The samples were hand-excavated and subsequently classified, labeled, and secured for subsequent testing. A log of the subsurface conditions encountered in the percolation basin is presented in boring log 18E-04 in Appendix A. The location of the excavation is presented in Figure 2.

3.3 LABORATORY TESTING

Laboratory testing was performed on selected samples recovered from the field exploration program. Tests for moisture content, unit weight, percent passing the 200 sieve, particle size analyses, Atterberg limits, pH, and resistivity were performed at the Yeh office and laboratory in Grover Beach, California. Tests for triaxial compressive strength using consolidated undrained (CU) loading and test for hydraulic conductivity were performed at the GEO-E lab at the Cal Poly Civil Engineering Department in San Luis Obispo, California. Tests for soluble sulfates and soluble chlorides, as well as organic content were performed by Cooper Testing Labs in Palo Alto, California. Testing was performed in accordance with applicable ASTM standards.

Laboratory test results are presented in Appendix B. After the completion of the laboratory testing, the field descriptions on the borings logs were confirmed or modified as necessary.

4. SITE CONDITIONS

4.1 GEOLOGIC SETTING

The project site is located within Big Bend National Park near the border of the Great Plains Province to the northeast and the Sonoran section of the Basin-and-Range structural province to the southwest (Bohannon 2011b). The Great Plains Province is characterized by flat-lying or gently dipping sedimentary strata, low topographic relief, shallow stream valleys, and a general lack of faulting. The Basin-and Range province is characterized by linear parallel mountain ranges, deep sediment-filled valleys, high structural and topographic relief, and is known for its complex patterns of Cenozoic faulting. The area of Big Bend National Park is understood to have been subject to episodes of deformation, sediment deposition, large-scale erosion, and volcanic activity that led to the formation of the Chisos Mountains and surrounding topography. Basement rocks within the Park are predominantly Cretaceous age-rock with Tertiary-to Quaternary-age rocks commonly overlying the older rock along the flanks and foothills of the Chisos Mountains.

The predominant geologic structure within the Chisos Mountains are a series of nonuniform trending faults of various styles, activity and orientation. The closest fault to the site is an unamed fault, mapped by Bahannon (2011a) approximately about 1.6 miles northeast of the site as shown on Figure 3.

The site is within the gently sloping alluvial plain northeast of the Chisos Mountains. Surface geology in the site vicinity as mapped by Bohannon (2011a) is shown on Figure 3. Bohannon maps the site as Quaternary- to intermediate-age undivided alluvial deposits (Qia). The alluvial deposits are described as weakly to moderately well-consolidated, poorly to moderately well-

Figure 3: Geologic Map sorted, poorly to moderately well-bedded gravel, sand, and silt with cobble and small boulder-sized clasts common and abundant.

4.2 SUBSURFACE CONDITIONS

The subsurface conditions encountered in the borings at the proposed IFAS structure location and at the existing percolation basin location consisted of alluvium (Qia). The approximate locations of the borings are shown on Figure 2 and an interpreted subsurface profile of the conditions encountered near the existing percolation basin are shown on Plate 1. A description of the unit encountered is summarized below for each location below.

4.2.1 PROPOSED IFAS TREATMENT STRUCTURE LOCATION

Alluvium (Qia). Alluvium was encountered from the ground surface in borings 18E-01 and 18E- 02 to the maximum depth explored, approximately 41 feet below the ground surface. The alluvium generally consisted of medium dense to very dense poorly and well-graded gravel with varying amounts of silt and sand (GP, GW, GW-GM, GM) as well as medium dense to very dense well-graded sand with varying amounts of silt, clay, and gravel (SW, SW-SC, SM). Cobbles were encountered within the sand and gravel layers between depths of approximately 5 to 25 feet below the ground surface in borings 18E-01 and 18E-02.

A summary of the laboratory test results for the alluvium at the proposed IFAS treatment structure location is presented in Table 1.

Table 1: Geotechnical Properties Test Summary for the Proposed IFAS Structure Location1

Geologic Unit

Locations Encountered

Dry Unit Wt.

(pcf)

Moisture Content

Particle Size

Analyses (%G, %S, %F)

Atterberg Limits

LL, PI

Corrosion pH, ρ (Ω-cm)

Strength Parameters Other

Alluvium (Qia)

18E-01 18E-02 79 - 125 5 - 112

43 - 74 G 17 - 42 S 10 - 15 F

36 LL

15 PI

pH = 7.61

- 7.84 ρ = 600 – 1,063 ϕ’CU = 50° c’CU= 0 ksf

Sulfates = 113 - 349 mg/kg, Chlorides =

13 - 25 mg/kg

1 Geotechnical properties are noted for dry unit weight (ɣd) and moisture content (wo); particle size as percent gravel (G), sand size (S) and fines content (F); electrical resistivity (ρ) in ohm-centimeters (Ω-cm), soluble sulfates (SO4

2-) Atterberg liquid limit (LL) and plasticity index (PI); average permeability at 20°C (k) in centimeters per second; shear strength (S) in kips per square foot (ksf) measured by pocket penetrometer (pp), torvane (tv) or unconsolidated undrained (uu) tests; friction angle (φ) or cohesion (c) in kips per square foot measured from direct shear (ds) or consolidated undrained (cu) tests, uniaxial compressive strength (σa) in kip per square inch (ksi).

4.2.2 EXISTING PERCOLATION BASIN LOCATION

Alluvium (Qia). Alluvium was encountered from the ground surface in boring 18E-03 to the maximum depth explored, approximately 31 feet below the ground surface (or approximately

24.5 feet below the bottom of the percolation basin). The alluvium generally consisted of medium dense to dense well-graded gravel with varying amounts of silt, and clay, and sand (GW-GM, GW-GC, GC) as well as medium dense to very dense well-graded sand with varying amounts of clay and gravel (SW, SW-SC). An approximately 6-inch thick layer of organic silty sand with gravel (SM) was encountered at the ground surface in the bottom of the percolation basin in the hand excavation boring, 18E-04.

A summary of the laboratory test results for the alluvium at the existing percolation basin location is presented in Table 2.

Table 2: Geotechnical Properties Test Summary for the Existing Percolation Basin Location

Geologic Unit

Locations Encountered

Dry Unit Wt.

(pcf)

Moisture Content

Particle Size

Analyses (%G, %S, %F)

Atterberg Limits

LL, PI

Corrosion pH, ρ (Ω-cm)

Strength Parameters Other

Alluvium (Qia)

18E-03 18E-04 119 - 141 2 - 15

24 - 68 G 21 - 43 S 10 - 37 F

130 LL

21 PI

(organic sample) k = 8.8x10-5 cm/sec to 2.6x10-6 cm/sec Organic Content =

31% (organic sample)

4.3 GROUNDWATER CONDITIONS

Groundwater was initially encountered during drilling at approximately 10 feet below the ground surface (at approximately elevation 3,662 feet) in boring 18E-03 adjacent to the percolation basin, and was not encountered in borings 18E-01 and 18E-02 at the proposed IFAS structure. The groundwater depth in boring 18E-03 was measured at 13 feet below the ground surface (elevation 3,659 feet) approximately 24 hours after drilling. The groundwater level in the site vicinity is likely dependent on the water level and infiltration in the percolation basin, and will vary in association with changes in precipitation, runoff, and other factors. The encountered water in 18E-03 is likely perched within the formation and the level is influenced by the water from the adjacent percolation basin.

5. CONCLUSIONS AND RECOMMENDATIONS

5.1 SEISMIC DATA

Structures should be designed to resist the lateral forces generated by earthquake shaking in accordance with the building code and standard design practice. This section presents seismic design parameters for use with the American Society of Civil Engineers (ASCE) 7-10 and the International Building Code (IBC). The site coordinates and the United States Geological Survey (USGS) Seismic Design Maps were used to obtain the seismic design criteria. An estimated average shear wave velocity within the upper 100 feet of the site of approximately 268 meters per second, corresponding to a Site Class D, stiff soil site was used along with the assumption of ASCE Risk Category “I, II, or III.” A summary of the seismic data for the tank location is presented in Table 3. The USGS Unified Hazard Deaggregation Tool was also used to estimate the peak ground acceleration for the site corresponding to the design earthquake having a 2 percent probability of occurrence in 50 years.

Table 3: Seismic Data

Seismic Parameter Value

Latitude, Degrees 29.32739

Longitude, Degrees -103.20129

Peak ground acceleration 0.17g

Design Earthquake Magnitude 6.1

Ss, Seismic Factor, Site Class B at 0.2 second 0.247g

S1, Seismic Factor, Site Class B at 1 second 0.071g

Site Class D, Stiff Soil

Fa, Site Coefficient for Site Class 1.600

Fv, Site Coefficient for Site Class 2.400

SMS, Site Specific Response Parameter for Site Class D at 0.2 second 0.395g

SM1, Site Specific Response Parameter for Site Class D at 1 second 0.170g

SDS = 2/3 SMS 0.263g

SD1 = 2/3 SM1 0.113g

5.1.1 LIQUEFACTION

Liquefaction is the loss of soil strength due to an increase in soil porewater pressure resulting from seismic ground shaking. Liquefaction typically occurs in loose to medium dense granular soil that is below the groundwater table. The extent and severity of liquefaction is dependent upon the intensity and duration of the strong ground motion.

The project site is underlain by predominantly dense sand and gravel with varying amounts of silt, clay, and cobbles. The liquefaction potential of the soil at the project site was evaluated using NCEER procedures (Youd and Idriss 2001). Soil at the site is not considered vulnerable to liquefaction due to the dense conditions and lack of consistent groundwater near the surface.

Therefore, liquefaction is not a consideration for this project, and no special recommendations are needed for design to address liquefaction or seismic settlement related hazards.

5.2 EXPANSIVE SOIL

Expansive soil conditions can cause differential movement and damage to foundations, slabs, flatwork, slopes, pavement and other improvements due to shrinking and swelling of the soil in response to moisture fluctuations. These movements are most common in near surface soil, between the interior and exterior of structures, and near the edge of slabs or pavements where seasonal moisture contents in the soil fluctuate the most.

Soil encountered within the improvement areas generally consisted of sand and gravel with less than 15% fines. These materials are not considered susceptible to expansion, and therefore is not a design consideration for this project.

5.3 ADJACENT STRUCTURES

The IFAS treatment structure will be constructed in close proximity (within 12 feet) of the aeration tank. Rerouting of piping, tight shoring and/or underpinning of the adjacent structure should be provided where excavations intersect and/or extend below existing improvements.

Excavations should generally not extend within a 1:1 line projected downward and out from the existing structure, unless special considerations for the design or shoring of excavated slopes are made with specific considerations for maintaining proper foundation support for the structure and piping. The contractor should submit an excavation plan for review by the geotechnical professional prior to beginning the excavation.

5.4 EARTHWORK - GENERAL

5.4.1 SUGGESTED MATERIAL SPECIFICATIONS

Standard Specifications refers to the 2014 edition of the Standard Specifications for Construction and Maintenance of Highways, Street and Bridges (TxDOT 2014) and DMS refers to the Departmental Material Specifications (TxDOT 2018), published by the Texas Department of Transportation. The following specifications are suggested for materials referenced in various sections of this report.

Aggregate Base. Aggregate base shall consist of imported material conforming to Section 247 of the Standard Specifications for Flexible Base.

Compacted Fill. Compacted fill material shall consist of imported or on-site material free of organics, oversize rock (greater than 3 inches), trash, debris, corrosive, and other deleterious materials. Imported fill to be placed below tank or pump station shall be non-expansive material having an Expansion Index of less than 20 when tested according to latest approved edition of ASTM D4829. Fill and borrow sources shall be reviewed by the Engineer before being imported to the site. Fill materials should comply with all specified material requirements for the designated placement location as-placed at the site. Fill materials used in structure locations should not include expansive or collapsible materials.

Pipe Bedding Material. Pipe bedding material shall consist of imported or onsite material conforming to Section 400.3.2 of the Standard Specifications.

Pipe Bedding Material – Gravel. Gravel placed for trench foundation stabilization shall consist of material conforming to ASTM C33 No. 8 Coarse Aggregate (pea gravel).

Geotextile for Separation Filter (Filter Fabric). Geotextiles for filtration shall conform to DMS Section 6200.

Initial/Pipe Zone Material. Pipe zone material shall consist of imported or onsite material having a sand equivalent (SE per ASTM 2419) of at least 30 and conform to Section 400.3.3.3 of the Standard Specifications. Slurry cement used for pipe zone material should conform to Section 401 of the Standard Specifications.

Subsequent/Trench Backfill. Trench backfill shall consist of imported or onsite material that is free of organics, debris, oversized material (greater than 3 inches), and other deleterious materials. Trench backfill material shall have at least 85 percent of the material passing the U.S.

Standard No. 4 sieve, and/or comply with the applicable requirements for the area where trench backfill is being placed (such as the pavement structural section). Trench backfill placed below pavement areas shall conform to the requirements for Pipe Bedding.

5.4.2 CLEARING AND GRUBBING

Clearing and grubbing should be performed to remove existing vegetation and objectionable material from improvement areas that will be graded, receive fill, or serve as borrow sources.

Grubbing should include removing stumps, roots and buried vegetation. Care should be taken not to injure trees, plants or existing improvements outside of the clearing limits. Soil containing pavement, debris, organics, unsuitable, loose or disturbed materials should be removed prior to placing fill. Demolition areas should be cleared of old foundations, existing fill, pavement, abandoned utilities, and soil disturbed during clearing and grubbing.

Depressions or disturbed material left from the removal or demolition of materials should be replaced with compacted fill.

5.4.3 COMPACTION AND GRADING

Fill placement and grading operations should be performed according to the recommendations of this report. Table 4 provides a summary of the recommended minimum levels of compaction for locations where fill will be placed. Relative compaction should be assessed according to the latest approved edition of ASTM Standard Test Method D1557.

Table 4: Recommended Relative Compaction

Location of Fill Placement Recommended Minimum

Relative Compaction*

General 90% U.O.N.2

Utility trench bedding, pipe zone or backfill 90% U.O.N.

Fill or backfill placed within 3 feet of finished grade in pavement areas 95%

Aggregate base, or subbase 95%

Foundation areas and within 2 feet horizontal of foundations 95%

Buried structure 90% U.O.N.

*Moisture content of soil should be placed at +/- 2% of optimum moisture content.

5.4.4 FILL PLACEMENT

Fill should be mechanically compacted. Jetting or ponding should not be permitted for placement or compaction of fill materials. Fill materials should be moisture conditioned and spread in lifts that are suitable for compaction with the equipment being used. Control of compaction layer thickness will be necessary to achieve compaction throughout the material being placed. Fill will/should typically need to be spread in loose lifts of approximately 8 inches

2 U.O.N. – unless otherwise noted or less, and within 2 percent of the optimum moisture content, to achieve the recommended compaction.

The moisture content of the material should be such that the specified compaction can be achieved in a firm and stable condition. Each layer should be spread evenly, bladed and mixed to provide relative uniformity of material within each layer, and be moisture conditioned by adding water or drying the material to provide a moisture content suitable for compaction, and be thoroughly mixed during the spreading to provide relative uniformity of material within each layer. Soft or yielding materials should be removed and replaced with properly compacted fill material prior to placing the next layer of fill. Fill and backfill materials may need to be placed in thinner lifts to achieve the recommended compaction with the equipment being used.

Particles greater than half the compacted lift thickness can limit compactive effort. The fill should not contain rocks, gravel or other solid particles larger than 3 inches in the greatest dimension. Deleterious materials, such as soft rock particles, concrete or pavement rubble, metal, glass or sharp objects should not be placed within the fill material being placed. Recycled or reused materials should only be used and placed within the fill when specifically permitted by the project specifications and approved by geotechnical engineer. Rocks should not be nested, and voids should be filled with compacted fill material.

5.4.5 DESIGN OF GRADED SLOPES

Cut slopes less than 20 feet high can be designed to an inclination of 2h:1v or flatter. Fill slopes less than 20 feet high should be graded to an inclination of 2h:1v or flatter. Fill slopes should be constructed by placing compacted fill beyond the finished grade and then cutting the slope back to exposed compacted fill at the finished grade. The slope can then be track-walked or prepared for placement of landscaping or planting. Fill slopes can be steepened by using internal geosynthetic reinforcement, retaining walls, and/or select backfill. If steeper or taller slopes are needed, Yeh should be contacted to provide additional evaluation and recommendations for the slope design.

5.4.6 FILL PLACEMENT ON SLOPES

Fill to be placed on existing ground that is 20 percent grade or steeper should be keyed and benched into the existing slope as shown in Figure 4. The fill should be initiated from a base key that is excavated at the toe of the slope. The base key should be at least 8 feet wide (about the width of 1 piece of equipment), at least 2 feet deep and sloped at 2 percent into the hillside.

The base key should be deepened if needed to excavate the base into firm and stable material, or below the minimum depth of removal recommended for building or pavement areas in this report. When placing fill against an existing natural slope, embankment, or previous phase of the fill that was already constructed, the new fill should be keyed into the slope such that at least the outer 5 feet of the existing slope is removed and the key extends 2 feet into the remaining slope. The excavated material can be incorporated into the fill being placed as the keying and benching progresses up the slope.

5.4.7 EROSION AND DRAINAGE CONSIDERATIONS

Drainage should be provided such that surface water does not run over slopes or pond on pavements, slabs, or adjacent to foundations. Downspouts from roofs should be provided to collect drainage and direct surface water to drainage pipes or areas away from foundation areas. Concentrated flows and runoff should not be permitted to discharge on slopes. Down drains, solid pipes, or lined ditches should be provided to carry water to the base of the slope.

Energy dissipation and erosion control devices should be provided at the outlet of drain pipes and in areas of concentrated runoff to reduce the potential for erosion. Landscaping and

Figure 4: Grading on Slopes Typical Detail maintenance of graded areas and slopes should be provided to assist the establishment of vegetation and reduce the potential for erosion.

5.4.8 REUSE OF EXCAVATED ONSITE MATERIAL

Soil encountered at the site consisted of medium dense to very dense poorly and well-graded gravel with varying amounts of silt, clay, and sand (GP, GW, GW-GM, GW-GW, GM, GC) as well as sand with varying amounts of silt, clay, and gravel (SW, SW-SC, SM). Excavated onsite soil, cleansed of organic material, oversized rock and other deleterious material, should be suitable for reuse as compacted fill. Excavated onsite material should not be considered suitable for reuse as select material such as aggregate base, pipe zone material or bedding.

5.5 BURIED STRUCTURE DESIGN

Design of the buried structures will need to include static and seismic lateral earth pressures.

Buried structure backfill material should be compacted to at least 90 percent relative compaction, unless 95 percent relative compaction is otherwise recommended below pavement or foundation areas. Values for static and seismic lateral earth pressures that can be used for design are provided below. The values presented are for level backslope conditions with either onsite or imported granular material.

5.5.1 FOUNDATION EARTHWORK

A mat of compacted fill should be provided below the IFAS structure to provide a suitable foundation support. The existing soil within foundation area of the IFAS structure should be excavated to the bottom of the foundation elevation. The foundation area is defined as the footprint of the foundation and extending a minimum of 2 feet horizontally beyond those edges. The bottom of the excavation should be reviewed by the geotechnical professional prior to placing fill to evaluate whether or not the subgrade is suitable for fill placement, or to provide additional recommendations if needed. The bottom of the excavation should then be scarified to a depth of at least 9 inches, moisture conditioned, and compacted in-place to at least 95 percent relative compaction as presented in Table 4.

5.5.2 BEARING RESISTANCE

The IFAS treatment train structure can be supported on shallow foundations or a mat foundation bearing on compacted native material prepared according to the recommendations of this report (see Section 5.5.1). The foundation can be designed using a maximum allowable bearing pressure of 3,500 pounds per square foot and have a minimum embedment of 2 feet below lowest adjacent exterior grade. The maximum allowable bearing pressure can be increased by one-third when considering short-term wind or seismic loads.

5.5.3 FOUNDATION SETTLEMENT

Foundation settlements were estimated using the Schmertmann (Schmertmann 1970) elastic strain method and SPT (Standard Penetration Test) blow count data collected during our field exploration program. We estimate that settlements resulting from static foundation loads should generally be on the order of approximately 1-inch total and approximately ½-inch differential across the IFAS foundation designed according to the recommendations of this report.

5.5.4 RESISTANCE TO LATERAL LOADS

Resistance to lateral loading of the foundation can be provided by sliding friction acting on the base of footings combined with passive pressure acting on the sides of the foundations. A nominal coefficient of friction of 0.5 and a nominal passive resistance of 420 pounds per cubic foot (equivalent fluid weight) be used to estimate the sliding resistance along the bottoms of footings bearing in compacted soil. Passive and sliding resistance should not be used for the upper two feet of soil below the adjacent surface. Appropriate factors of safety should be applied by the structural engineer to the provided values. A one-third increase in the passive value can be used when considering short-term or seismic loads.

5.5.5 STATIC LATERAL EARTH PRESSURES

The static lateral earth pressures should be designed considering either at-rest or active conditions. Active earth pressure conditions can be assumed where the top of the wall is free to rotate away from the backfill. If the wall is braced or non-yielding, the wall should be designed to resist at-rest conditions. Drained conditions should be assumed for the design of buried structures provided the backfill is capped with a minimum of 1-foot of clayey soil or flatwork.

Equivalent fluid weights of 35 pounds per cubic foot for active and 55 pounds per cubic foot for at-rest conditions should be used in calculating lateral earth pressures for buried structure design.

The recommended equivalent fluid weights do not account for surcharge loads acting on the backfill. Traffic surcharge can be estimated as an additional 2 feet of soil cover, equal to a uniform pressure of 72 pounds per square foot, applied to the upper 5 feet of the wall. Traffic surcharge can be neglected if the backside of the wall is outside the limits of a 0.5h:1v (horizontal: vertical) line projected downward from the edge of the pavement.

5.5.6 DYNAMIC LATERAL EARTH PRESSURES

The general limit equilibrium method (GLE) was used to estimate dynamic lateral earth pressures on buried structure walls up to a depth of 6 to 10 feet using SLIDE 7 software

(Rocscience 2016). Yeh estimated a horizontal pseudostatic coefficient (kh) of 0.09 from the design peak ground acceleration and methods referenced in CGS (2008). The pseudostatic coefficient corresponds to a driving force causing a seismic wall displacement up to approximately 2 inches. Buried structures designed for active earth pressures should be designed using an additional dynamic lateral earth pressure of 4H (where H is the retained height in feet) applied as a uniform pressure in pounds per square foot on the back of the wall.

Seismic lateral earth pressures may be neglected if the buried structure is designed for at-rest conditions and/or if the structure will retain 6 feet or less of soil.

5.6 TRENCH DESIGN

A typical trench detail for the design of buried pipes is presented in Figure 5. The detail and the following recommendations are intended to be used with Texas Department of Transportation standard plans and specifications as applicable, except where modified by the recommendations of this report. The excavated onsite soil may be used as trench backfill provided the fill is placed and compacted as recommended in this report. Excavated onsite soil is not considered suitable for use as pipe bedding or pipe zone material.

5.6.1 BEDDING

The soil at the bottom of trenches in the upper 10 feet at the site is expected to consist of medium dense to very dense, poorly and well-graded gravel with sand, clay, and silt as well as medium dense to dense silty and well-graded sand with gravel that should provide adequate foundation support for new pipes. Bedding material for pipes can be placed on undisturbed soil at the bottom of the trench. The bedding should have a minimum thickness of 4 inches and be compacted to at least 90 percent relative compaction. The pipe should be placed on the bedding such that the middle third of the pipe (Do/3 on Figure 5) is in contact with the bedding prior to placing initial backfill within the pipe zone. The bedding maybe loosened along the invert of the pipe if necessary to help form the cradle.

Gravel bedding material could be provided in the event that trench bottom is in wet or soft ground. For areas where the trench bottom is soft or yielding, the trench should be deepened to at least 1-foot below the normal bottom of the trench and be refilled with gravel bedding.

The gravel should be fully encased in geotextile to provide separation between the gravel and the in-situ soils and subsequent backfill and to reduce the potential for migration of soil into the gravel.

Prior to placing bedding material, the foundation support soil exposed at the trench subgrade should be reviewed to allow for proper bedding and stabilization of the subgrade below the trench if needed. We suggest that a quantity be estimated to include gravel bedding below the pipe, and to allow for the quantity to be increased, decreased or deleted based on review of the trench subgrade conditions encountered during construction.

5.6.2 PIPE ZONE MATERIAL

Pipe Zone Material should be placed from the top of the bedding material to at least 6 inches above the crown of the pipe and compacted to at least 90 percent relative compaction. Pipe zone material should be placed such that there is an equal distribution of compaction and

Figure 5: Trench Detail material on both sides of the pipe. Backfill should not be placed above the springline of the pipe until the fill below the springline has been placed and compacted to properly support the haunches. Compaction within the pipe zone should be performed such that the pipe is fully supported during compaction and such that excessive deformation, displacement or damage to the pipe does not occur. Trench backfill should not be placed until compaction and testing of the pipe zone material is complete.

5.6.3 TRENCH BACKFILL

Trench backfill is material placed in the trench from the top of the pipe zone to finished grade, and includes the roadway structural section. Trench backfill placed within 3 feet of the finished grade below the path should be compacted to at least 95 percent relative compaction. Trench backfill should otherwise be compacted to at least 90 percent relative compaction unless a higher degree of compaction is recommended. In non-pavement areas the upper 2 feet of the backfill should consist of on-site native soil or approved landscape soils to help reduce the potential for surface water to infiltrate the trench if granular materials are used as trench backfill above the pipe zone. Slurry cement can also be used to backfill trenches.

5.7 SOIL CORROSIVITY DATA

Yeh performed tests for pH and electrical resistivity on four selected samples from the site.

Testing was performed on the selected samples in accordance with ASTM test methods. Test results are presented in Appendix B. The pH for the four samples tested ranged from 7.61 to

7.84. Minimum soil resistivity of the samples ranged from 600 to 1,063 ohm-centimeters. Three of the samples had resistivities less than 1,000 ohm-centimeters and were subsequently sent for soluble sulfate and soluble chloride testing. Results of the soluble sulfate and chloride testing are also presented in Appendix B. Soluble sulfate content for the tested samples ranged from 113 to 349 ppm, and soluble chloride content for the samples ranged from 13 to 25 ppm.

Design of the project should consider corrosivity test results using appropriate design standards, and designed by a corrosion engineer.

5.8 PERCOLATION BASIN

We found a lush vegetation layer growing atop an approximate 6-inch thick organic silty sand layer within the bottom of the existing basin (see Figure 6). The organic layer was underlain by an approximate 12-inch gravel layer that was placed as a part of the construction according to the as-built plans from 1976. We were not able to excavate any further in the basin, but did drill boring 18E-03 adjacent to the basin where we found a clayey gravel with sand layer between 10 and 15 feet below the ground surface. These depths correspond to the approximate bottom of the percolation basin. The clayey gravel with sand was initially wet, but became moist to dry with depth indicating the subsurface water observed in the boring was likely perched on or within this layer. We were not able to perform a percolation test in this hole due to the inflow of water likely emanating from the adjacent basin. Plate 1 presents an interpreted subsurface profile for the existing percolation basin.

Hydraulic conductivity testing was performed on two selected samples obtained from the boring drilled near the basin (18E-03). Testing was performed on the selected samples in accordance with ASTM test methods. Test results are presented in Appendix B. Average permeability values ranged between 2.6x10-6 centimeters per second and 8.8x10-5 centimeters per second for depths of approximately 10 feet and 12.5 feet below the ground surface (or approximately 3.5 to 6 feet below the bottom of the percolation basin). These samples also had void ratios of approximately 0.4.

We qualitatively evaluated the contributing factors as to why water is not rapidly infiltrating at the percolation pond. Contributing geotechnical factors include:

Figure 6: Percolation Basin

Left: Hand excavation within basin. Right: Depth to bottom of gravel layer.

• The clayey gravel with sand found between 10 and 15 feet below the surface has low permeability compared to other more suitable drainage materials such as well-graded sand and gravel with little to no fines. The low permeability could be attributed to caliche layers and cementation within the subsurface strata

• The vegetation build up within the bottom of the pond is also likely slowing infiltration.

Biomass reduces effective permeability and slows the vertical and horizontal infiltration of the pond.

Improving the infiltration performance of the pond would involve the following considerations:

• Remove the vegetation and organic material from within the pond;

• Remove the previously placed gravel layer and clean the organic matter and fines;

• Install a series of drilled holes that are backfilled with gravel and inflow piping to increase the infiltration area. The holes would need to be large enough and deep enough to provide sufficient head pressure to increase the flow of infiltration. Design of mitigation of the infiltration issues for the pond should be further supported by infiltration tests at the site or the pond or designed using permeability tests performed on the samples obtained from boring 18E-03; and

• Implement a maintenance program to remove vegetation from the pond and cleaning of infiltration gravel to maintain performance.

Yeh’s scope did not include development of details or design recommendations for improving the infiltration characteristics of the pond, but we can assist with the design if requested.

6. CONSTRUCTION CONSIDERATIONS

6.1 TEMPORARY EXCAVATIONS

Temporary slopes should be braced or sloped according to the requirements of the Occupational Safety and Health Administration…