Geotechnical_Report-WP36.pdf

GEOTECHNICAL EVALUATION

CONROE WATER PLANT NO. 36

6470 FM 1484

CONROE, TEXAS

City of Conroe 700 Metcalf Street Conroe, Texas 77301

September 14, 2026 Project No. 701984001

SOCOTEC

2313 W. Sam Houston Pkwy North, Ste. 119, Houston, Texas 77043 | Tel.: 713 973 8400 www.socotec.us

September 14, 2026 Project No. 701984001

Ms. Jennifer Gilmore Engineering Projects Coordinator City of Conroe 700 Metcalf Street Conroe, Texas 77301

Subject: Geotechnical Evaluation Conroe Water Plant No. 36

6470 FM 1484

Conroe, Texas

Dear Ms. Gilmore:

Ninyo & Moore Geotechnical & Environmental Sciences Consultants (SOCOTEC) has performed a geotechnical evaluation for the above referenced project. The attached report presents our methodology, findings, geotechnical considerations, and recommendations for design and construction of the proposed water plant.

We appreciate the opportunity to be of service to you during this phase of the project

Sincerely, NINYO & MOORE

GEOTECHNICAL & ENVIRONMENTAL SCIENCES CONSULTANTS

TBPELS Firm No. F-9782

Richard J. Whitt, PE Senior Engineer

Jay T. Sunderwala, PE Managing Principal Engineer

IPC/RJW/JTS/vja 9/14/26

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 i

CONTENTS

1. INTRODUCTION 1

2. SCOPE OF SERVICES 1

3. SITE DESCRIPTION 1

4. PROPOSED CONSTRUCTION 2

5. FIELD EXPLORATION 3

6. LABORATORY TESTING 4

7. GEOLOGY AND SUBSURFACE CONDITIONS 4

7.1. Geologic Setting 4

7.2. Subsurface Conditions 4

7.3. Groundwater 5

8. GEOLOGIC HAZARDS 6

8.1. Surface Faulting 6

8.2. Seismic Design Considerations 6

8.3. Expansive Soils 7

9. POTENTIAL SEASONAL MOVEMENTS 7

10. GEOTECHNICAL CONSIDERATIONS 8

11. RECOMMENDATIONS 8

11.1. Earthwork 9

11.1.1. Subgrade Improvement 9

11.1.2. Site Preparation 10

11.1.3. Constructability 11

11.1.4. Excavations 11

11.1.5. Stability of Temporary Excavation Slopes 12

11.1.6. Bottom Stability and Dewatering 13

11.1.7. Pipe Bedding and Trench Backfill 13

11.1.8. Fill Materials 14

11.1.9. Re-Use of Excavated Materials 14

11.1.10. Fill Placement and Compaction 15

11.1.11. Site Drainage 15

11.2. Ground-Supported Water Storage Tank 15

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11.2.1. Tank Settlement 16

11.2.2. Flexible Connections 17

11.2.3. Tank Hydro-Testing 17

11.3. Hydrotank, MCC Building, and Generator Pad Foundations 18

11.3.1. Foundation Construction Considerations 19

11.4. Culvert Design 19

11.5. Drainage Channel 20

11.6. Pavements 21

11.6.1. Pavement Subgrade Preparation 22

11.7. Concrete 23

11.8. Pre-Construction Conference 23

11.9. Construction Observation and Testing 23

12. LIMITATIONS 24

13. REFERENCES 26

TABLES

1 – Boring Purposes and Depths 2 – Depth-to-Water in Boreholes 3 – 2015 International Building Code Seismic Design Criteria 4 – Recommended Depth of Removal and Replacement 5 – Summary of Compaction Recommendations Criteria 6 – Recommended Asphaltic Pavement Sections

FIGURES

1 – Site Location

2 – Boring Locations

APPENDICES

A – Boring Logs

B – Laboratory Testing

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 1

1. INTRODUCTION

In accordance with our proposal dated April 14, 2026, and your authorization on June 2, 2026, we have performed a geotechnical evaluation for a new water plant in Conroe, Texas (Figure 1). The purpose of our evaluation was to assess the subsurface conditions at the site and provide geotechnical recommendations for the design and construction of the project. This report presents the results of our evaluation, geotechnical considerations, and geotechnical recommendations for design and construction of the planned water plant.

2. SCOPE OF SERVICES

Our scope of services included the following:

• Reviewing aerial photographs and published geologic literature in our library, including maps and reports pertaining to the project site and vicinity.

• Conducting a visual reconnaissance of the project site, marking out boring locations, and notifying Texas811 of the boring locations prior to drilling.

• Drilling, logging, and sampling 16 exploratory soil borings at the site to depths of approximately 10 to 65 feet below the ground surface (bgs).

• Performing laboratory tests on selected samples obtained from the borings to evaluate the in-situ moisture content, percent of particles finer than the No. 200 sieve, Atterberg limits, shear strength, and consolidation characteristics.

• Compiling the collected data and performing engineering analyses.

• Preparing an engineering report presenting our findings, geotechnical considerations, and geotechnical recommendations for design and construction of the project.

Our scope of services did not include environmental consulting services, such as hazardous waste sampling or analytical testing, at the site. In addition, a fault study was beyond the scope of this study. If needed, a scope and fee for these services can be provided.

3. SITE DESCRIPTION

The approximately 2½-acre project site is located at 6470 Farm-to-Market Road (FM) 1484, on the south side of FM 1484, about 2,000 feet northeast of Technology Parkway in Conroe, Texas.

The site is bounded to the east, south, and west by undeveloped, wooded land.

At the time of our fieldwork, the site generally consisted of heavily wooded land. An unnamed tributary to East Fork Crysal Creek traversed the site from southwest to northeast. An overgrown

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 2 path traversed the site from northwest to southeast and another path traversed along the southern boundary.

Aerial photographs dated 1930 through 2023 were reviewed for this project. A photograph dated

1930 depicts the site as undeveloped, heavily wooded land with an unimproved road traversing the site from northwest to southeast. In photographs dated 1957 through 1995, the road is depicted as overgrowth. The road again is depicted as cleared in photographs dated 2004 through

2012. The 2012 photograph depicts another unimproved road along the southern boundary of the site. Both roads are depicted as overgrown in photographs dated 2014 through 2023. FM 1484 is first depicted under construction in a photograph dated 2009 and as completed in a photograph dated 2010. Photographs dated 2014 through 2023 generally depict the site in its present-day condition.

Based on the United States Geological Survey (USGS) Conroe, Texas, 7.5 Minute Topographic

Quadrangle Map (2022), the elevation of the site ranges from approximately 237 to 247 feet relative to mean sea level (MSL). Based on this topographic map, the topography of the site generally slopes down to the northeast toward tributaries to East Fork Crystal Creek.

4. PROPOSED CONSTRUCTION

Proposed construction will include:

• an approximately 2,000-gallon-per-minute (GPM) water well;

• two 750,000-gallon, 65-foot-diameter, prestressed-concrete ground storage tanks (GST);

• a hydro-pneumatic tank on an approximately 26-foot-by-8-foot concrete pad;

• an approximately 45-foot-by-30-foot precast-concrete motor control center (MCC) building on a concrete slab;

• a standby generator on an approximately 22-foot-by-12-foot concrete pad;

• 12- and 16-inch-diameter waterline yard piping

• an asphalt access driveway, with reinforced concrete pipe (RCP) culverts, and gravel yard pavements;

• two 6-foot-by-4-foot reinforced concrete box (RCB) culverts with headwalls where the tributary to East Fork Crystal Creek traverses the site; and

• reshaping, grading and bank stabilization of the stream.

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Structural loads were not available at the time this report was prepared. We have assumed the loading will be typical for structures of these types. Based on a grading plan dated

June 25, 2026, site grade in the proposed building and tank footprints will be within ±2 feet of existing grade. If finished elevations will vary from existing grades by more than 2 feet, SOCOTEC should be contacted to review the grading plan and evaluate the need for additional recommendations.

5. FIELD EXPLORATION

On June 19 through June 24, 2026, SOCOTEC performed a subsurface exploration at the site to evaluate subsurface soil and groundwater conditions and collect soil samples for laboratory testing. Our evaluation consisted of drilling, logging, and sampling 16 exploratory soil borings, designated as B-1 through B-16 (Figure 2). The borings were drilled using a drilling rig mounted on an all-terrain vehicle (ATV) and equipped with straight-flight augers and rotary wash drilling equipment. The depth and purpose of the borings is presented below in Table 1.

Table 1 – Boring Purposes and Depths

Borings Purpose Depth (feet)

B-1 Entrance Drive and Culvert 10

B-2 Hydropneumatic Tank 20

B-3 Generator 15

B-4 MCC Building 15

B-5 Asphalt Pavement 10

B-6 Gravel Drive and Tributary Culvert 15

B-7 Ground Storage Tank 1

Center 65

B-8 through B-11 Perimeter 40

B-12 Ground Storage Tank 2

Center 65

B-13 through B-16 Perimeter 40

Soil samples were collected at selected intervals and were logged in general accordance with

American Society of Testing Materials (ASTM) Standard D2488. Disturbed soil samples were collected as bag samples from the augers and using a split spoon sampler during Standard

Penetration testing. Relatively undisturbed soil samples were collected by hydraulically pushing

Shelby tube samplers. A pocket penetrometer was used to approximate the unconfined compressive strength as an indicator of soil consistency for intact cohesive samples. The borings were backfilled with soil cuttings on conclusion of the drilling operations.

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Brief descriptions of field sampling procedures used are presented on Figure A-1, Explanation of

Field Sampling Procedures, in Appendix A. Descriptions of the soils encountered in our borings are presented on boring logs in Appendix A.

6. LABORATORY TESTING

The soil samples collected from our drilling activities were transported to our laboratory for geotechnical laboratory testing. Selected samples were visually classified and tested to evaluate their engineering properties as a basis for providing geotechnical design recommendations and construction considerations. Laboratory testing included natural moisture contents (ASTM

D2216), percent of particles finer than No. 200 sieve (ASTM D1140), Atterberg limits (ASTM

D4318 Method B), unconfined compression tests (ASTM D2166), unconsolidated-undrained (UU) triaxial compression tests (ASTM D2850), and two one-dimensional consolidation tests (ASTM

D4186).

Brief descriptions of laboratory test procedures used are presented on Figure B-1, Methods of

Laboratory Testing, in Appendix B. Individual test results are presented on the boring log and/or on summaries of test results on Figures B-2 through B-6 in Appendix B.

7. GEOLOGY AND SUBSURFACE CONDITIONS

The geology and subsurface conditions at the site are described in the following sections.

7.1. Geologic Setting

The site is located in the West Gulf Coastal Plain Province of the Atlantic Plain physiographic region. This province extends from the southern tip of Texas along the Gulf Coast to the

Mississippi Alluvial Plain to the east. This physiographic region is characterized as a gently sloping plain with gentle rolling hills.

The Geologic Atlas of Texas, Beaumont Sheet (1992) describes the geology of the site as the

Willis Formation. This geologic formation generally consists of fluvial deposits of clay, silt, sand, clay, and gravel.

7.2. Subsurface Conditions

Our understanding of the subsurface conditions at the project site is based on the results of our field exploration, the laboratory testing, and our experience. More detailed stratigraphic

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 5 information is presented on the boring logs in Appendix A. The boring logs contain our field and laboratory test results, as well as our interpretation of conditions believed to exist between actual samples retrieved. Therefore, these boring logs contain both factual and interpretive information.

Lines delineating subsurface strata on the boring logs are intended to group soils having similar engineering properties and characteristics. They should be considered approximate, as the actual transition between soil types (strata) may be gradual. A key to the soil symbols and terms used on the boring logs is provided in Appendix A.

Naturally-deposited soils from the Willis Formation were encountered in our borings from the ground surface to the total explored depths of about 10 to 65 feet bgs. Predominantly cohesive soils, consisting of fat clays (CH), lean clays (CL), and silty clays (CL-ML) with varying sand fractions, were generally encountered in our borings from the ground surface to depths of about

17 to 27 feet bgs. The underlying soils generally consisted of clayey sands (SC), silty sands (SM), poorly graded sands with silt (SC-SM), and poorly graded sands (SP) in our borings. In addition, a 2-foot stratum of silty sand (SM) was encountered at the ground surface in Borings B-1 through

B-3.

7.3. Groundwater

The borings were initially drilled using dry-auger techniques in an attempt to measure depth-to-water in the open boreholes. Free water was not observed in Borings B-1 through

B-6 (drilled to depths of 10 to 20 feet) during or on conclusion of drilling. In Borings B-7 through

B-12, free water was encountered at depths of about 18 to 25 feet and rose to depths of about

16 to 21½ feet after about 15 minutes. About 24 hours after drilling, water levels were measured five of the borings at about 18½ to 19½ feet bgs. The depths where free water was observed in the borings are presented below in Table 2.

Table 2 – Depth-to-Water in Boreholes

Boring No.

Boring Depth

(feet) Depth to Water

During Drilling (feet) Water Level after 15 Minutes (feet)

Water Level after 24 Hours (feet)

B-1 10 Not Encountered N/A N/A

B-2 20 Not Encountered N/A N/A

B-3 15 Not Encountered N/A N/A

B-4 15 Not Encountered N/A N/A

B-5 10 Not Encountered N/A N/A

B-6 15 Not Encountered N/A N/A

B-7 65 25 21½ N/A

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Table 2 – Depth-to-Water in Boreholes

Boring No.

Boring Depth

(feet) Depth to Water

During Drilling (feet) Water Level after 15 Minutes (feet)

Water Level after 24 Hours (feet)

B-8 40 23 16½ N/A

B-9 40 25 17½ 18½

B-10 40 25 Caved at 20 19½

B-11 40 25 17 19½

B-12 65 23 17½ 19½

B-13 40 18 16 N/A

B-14 40 20 17 N/A

B-15 40 23 17 18½

B-16 40 25 18½ N/A

Fluctuations in groundwater may occur as a function of seasonal moisture variation, precipitation, temperature, and groundwater withdrawal. Future construction activities may alter the surface and subsurface drainage characteristics of this site. In addition, perched groundwater could be encountered at the site, particularly after periods of precipitation.

8. GEOLOGIC HAZARDS

The following sections describe potential geologic hazards at the site, including faulting, seismicity, and expansive soils.

8.1. Surface Faulting

A geologic fault study was not part of our scope of work for this project. Based on a review of published geologic data in our library, the closest documented surface expression of a non-seismic growth fault to the project area is the Conroe Fault, mapped about 4 miles southwest of the site (Campbell et al, 2018). This fault does not trend in the direction of the site.

8.2. Seismic Design Considerations

Design of the proposed structures should be performed in accordance with the requirements of the governing jurisdictions and applicable building codes. Table 3 presents the seismic design parameters for the site in accordance with the 2015 International Building Code (IBC) guidelines and adjusted maximum-considered earthquake spectral response acceleration parameters evaluated using the web-based Structural Engineers Association of California (SEAOC) Seismic

Design Map tool.

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Table 3 – 2015 International Building Code Seismic Design Criteria

Site Coefficients and Spectral Response Acceleration Parameters Values

Site Class D

Site Coefficient, Fa 1.6

Site Coefficient, Fv 2.4

Mapped Spectral Acceleration at 0.2-second Period, Ss 0.076g

Mapped Spectral Acceleration at 1.0-second Period, S1 0.042g

Spectral Acceleration at 0.2-second Period Adjusted for Site Class, SMS 0.122g

Spectral Acceleration at 1.0-second Period Adjusted for Site Class, SM1 0.101g

Design Spectral Response Acceleration at 0.2-second Period, SDS 0.082g

Design Spectral Response Acceleration at 1.0-second Period, SD1 0.067g

8.3. Expansive Soils

Moisture variation within expansive soils can lead to vertical rise in these soils. Expansive soils typically contain significant clay fractions, and the expansive characteristics of these clay fractions depend upon a number of factors, which include clay mineralogy, degree of compaction (density) for reworked soils, moisture content, etc. Expansive soils tend to have a more adverse effect on lightly loaded construction elements (e.g. exterior flatwork, floor slabs, etc.) than on more heavily loaded foundations.

9. POTENTIAL SEASONAL MOVEMENTS

Potential seasonal ground movements due to shrink/swell of the underlying soils were estimated at the site using the Texas Department of Transportation (TxDOT) Method for determining

Potential Vertical Rise (PVR) (TEX-124-E), and engineering judgment and experience. The PVR varies across the site, calculated to be about 2½ inches at the MCC building site and about

4 inches in the area of the GSTs. These values were estimated using a surcharge load of

1.0 pound per square inch (psi) and dry moisture conditions within the zone of seasonal moisture variation (about 12 feet).

The TxDOT Method for PVR is an empirical method, and it should be noted that actual soil movements may exceed the estimated PVR, depending on moisture fluctuation, water seepage, etc. For example, movements exceeding those predicted above could occur if positive drainage of surface water is not maintained away from foundation elements or if soils are subject to an outside water source, such as leakage from a utility line or subsurface moisture migration from offsite locations.

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10. GEOTECHNICAL CONSIDERATIONS

Based on the results of our subsurface evaluation, laboratory testing, and data analyses, the proposed construction is feasible from a geotechnical standpoint, provided the recommendations in this report are incorporated into the design and construction of the project. Geotechnical considerations include the following:

• Due to the widely spaced nature of our soil borings, soil conditions that differ from what were encountered in our borings may be encountered during construction.

• A PVR of about 4 inches is estimated for the GSTs and about 2½ inches at the MCC site.

Select fil pads will be needed beneath mat/slab foundations and other lightly loaded structures. The PVR will also need to be reduced beneath the GSTs to account for an empty tank condition.

• The onsite soils should generally be excavatable to the anticipated depths with conventional heavy-duty equipment.

• Earthwork contractors should be made aware of the moisture sensitivity of near surface soils and potential compaction difficulties in wet weather conditions.

• Groundwater was generally observed in our borings at depths of about 18 to 25 feet during drilling and subsequently rose to as shallow as about 16 feet bgs. In addition, relatively shallow perched groundwater may be encountered during construction, particularly after periods of precipitation.

• Due to the presence of cohesionless/granular soils in our borings, and the potential for groundwater, care should be taken prior to excavation. The Contractor is responsible for evaluating the depth of groundwater at the time of construction, the need for shoring, and the need for dewatering prior to commencing any excavation.

• Imported soils and soils generated from onsite excavation activities that exhibit a very low to low swell potential, have a plasticity index (PI) between 8 and 20, liquid limit of less than 40, and a percent passing the No. 200 sieve of 60 or more can generally be used as select, engineered fill, provided they are free of deleterious materials.

• Many of the onsite soils will not be suitable for reuse as select, engineered fill. However, these soils may generally be reused as general fill for site grading and trench backfill, provided they are free of deleterious materials.

11. RECOMMENDATIONS

The following sections present our geotechnical recommendations and were developed based on our understanding of the proposed construction, the observed subsurface conditions, and our experience. If the proposed construction is different from that discussed in Section 4, or if subsurface conditions other than those shown on the boring logs are observed at the time of

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 9 construction, SOCOTEC should be contacted to review the new information and evaluate the need for additional recommendations.

11.1. Earthwork

The following sections present our earthwork recommendations for this project. In general, City of Conroe (COC) construction standards and specifications are expected to apply, unless otherwise noted.

11.1.1. Subgrade Improvement

As discussed above, a PVR of about 2½ inches was calculated for the subsurface conditions observed at the MCC building and about 4 inches at the GSTs. Structural engineers in the

Houston area generally consider potential seasonal soil-related movements of more than about 1-inch excessive for structures with a slab-on-grade. In addition, tank designers generally consider a PVR of more than about 1-inch excessive for a GST considering a dry-tank condition. As such, soil improvement to reduce the magnitude of potential swell is needed.

We recommend subgrade improvement within the footprint of the proposed GSTs, MCC

Building, and generator pad consist of overexcavating a portion of the existing expansive soils in the footprints of the structures and replacing it with non-expansive select, engineered fill. The recommended thickness of select, engineered fill needed beneath the structures is presented in Table 4.

Table 4 – Recommended Depth of Removal and Replacement

Structure PVR, inches Thickness of Select, Engineered

Fill Pad, feet

Ground Storage Tanks 4 7

MCC Building and Generator Pad 2½ 5

Select fill should meet the requirements described in Section 11.1.8 and should be placed and compacted as described in Section 11.1.10. The select fill should extend 5 feet or more beyond the structure footprint and under adjoining flatwork.

As an alternative to overexcavation and replacement of the expansive soils, chemical injection may be considered. Chemical injection involves use of propriety products which undergo a chemical reaction with the in-situ clay to reduce the clay’s affinity for water.

Chemical injection is generally performed by a specialty contractor.

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11.1.2. Site Preparation

Prior to placing any fill soils, structures, or pavement, the site should be prepared by clearing existing vegetation, trees, and root balls, grubbing, and root raking. Root balls that remain after tree removal operations should be removed and the resulting voids filled with engineered fill placed and compacted as outlined in Section 11.1.10. Excavations to remove root balls (or other obstructions) should have side slopes of 3:1 (horizontal: vertical) or flatter so as not to create abrupt changes in fill thickness. The root balls should be removed as early as possible prior to construction to allow the ground to reach a new equilibrium for the moisture content.

Soft and/or saturated soils may be encountered near the ground surface following wet weather. As such, good clearing techniques should be used to reduce the potential for trees, branches, roots, and other organic materials to be pushed deeper into the subgrade soils by the clearing equipment. Poor clearing techniques may increase the potential for soil settlement, reduced bearing capacity, etc. We recommend the clearing contractor retained have experience with clearing in the types of soil conditions encountered at this site.

Once the trees and root balls have been removed, the Contractor should root rake the area to remove the remaining organic material. We recommend that SOCOTEC be retained to perform a root rake inspection and observe the excavations for the root ball removal prior to placement of fill or other construction.

After stripping, clearing, grubbing, and root raking is performed and prior to placement of any fill soils, the exposed subgrade should be evaluated by proofrolling. Proofrolling should be accomplished using a pneumatic-tired roller, loaded dump truck, or similar equipment weighing approximately 20 tons and observed by the Geotechnical Engineer-of-Record or the Engineer’s designated representative. Any soft or weak areas observed during the proofrolling process should be removed and replaced with compacted general fill as outlined in Section 11.1.10.

Due to the nature of the surficial soils, traffic of heavy equipment (including heavy compaction equipment) may create pumping and general deterioration of shallow soils. Therefore, some construction difficulties should be anticipated, especially during periods when these soils are saturated.

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11.1.3. Constructability

Approximately 2 feet of silty soils (CL-ML and SM) were encountered at the surface in several of our borings and may be encountered elsewhere at the site. Soils with a high silt content are commonly slow to drain and can hold perched water. Earthwork contractors should be made aware of the moisture sensitivity of these soils and potential compaction difficulties when these soils are wet. If construction is undertaken during wet weather conditions, the surficial soils may become saturated, soft, and unworkable.

Although having a thin layer of silty soils overlying cohesive soils is typical of this geologic formation, our experience suggests that earthwork contractors can find these materials troublesome and this can often be a source of change orders, construction delays, and budget overruns. Earthwork contractors have employed several techniques intended to mitigate these conditions such as the use of sumps and drainage trenches, reworking the problematic soils using conventional moisture-conditioning and compaction methods, chemical treatment (e.g. lime-fly ash or cement treatment), and/or removal and replacement of the problematic soils. Mitigation techniques should be evaluated on a case-by-case basis.

We recommend that consideration be given to construction during drier months. Alternatively, the Contractor should protect exposed areas once topsoil has been stripped, as well as provide positive drainage during earthwork operations.

11.1.4. Excavations

Based on the results of our exploratory borings, site observations, and experience with similar materials, excavations for this project may be performed using conventional heavy-duty excavation equipment. Due to the heterogeneous nature of the project area soils, and the relatively wide spacing between our borings, soils different than those encountered in our borings should be anticipated during construction.

Equipment and procedures should be used that do not cause significant disturbance to excavation bottoms. The bottoms of excavations should expose competent soils and should be dry and free of loose, soft, or disturbed soil. Any soft, wet, weak, or deleterious materials should be overexcavated to expose competent soils.

As discussed above, free water was generally observed in our borings at depths of about

18 to 25 feet during drilling and subsequently rose to as shallow as about 16 feet bgs. In addition, the Contractor should be prepared to encounter relatively shallow perched water

Conroe Water Plant No. 36, Conroe, Texas | 701984001 | September 14, 2026 12 during construction, particularly after periods of precipitation. This may result in difficulty achieving compaction of soils, as well as subgrade pumping, etc., during earthwork activities.

Wet or saturated soils at the excavation bases may soften under the action of light equipment and foot traffic. Drying or overexcavation of these materials may be appropriate prior to filling.

If the subgrade becomes disturbed, it should be compacted before placing the backfill material.

The sides of excavations should be sloped back and/or shored, as discussed in Section

11.1.5. Dewatering is discussed in Section 11.1.6.

11.1.5. Stability of Temporary Excavation Slopes

Excavations that are 20 feet deep or less can be constructed using sloped sides designed based on the soil types encountered in accordance with current applicable state and federal trenching guidelines, including the Occupational Safety and Health Administration (OSHA) requirements for excavations. Excavations over 20 feet deep should be designed by the

Contractor’s engineer based on site-specific geotechnical analyses and evaluation of settlement-sensitive features.

Upon excavation, soil classifications should be evaluated in the field by the Contractor’s geotechnical consultant in accordance with OSHA regulations. For estimating purposes, an

OSHA “Type B” soil classification may be used for dry excavations in clays at this site. This corresponds to temporary slopes of 1:1 (horizontal: vertical). However, where cohesionless/granular soils and/or groundwater seepage are encountered, an OSHA

Soil “Type C” classification should be used. This corresponds to temporary slopes of

1.5:1 (horizontal: vertical).

Based on the observed soil and free water conditions, it may be preferable to shore or brace trenches in lieu of sloping back the sides. Temporary earth retention systems may include braced systems, such as trench boxes or shields with internal supports, or cantilever systems

(e.g., soldier piles and lagging). The Contractor should retain a qualified and experienced structural engineer to design any shoring system.

Flatter slopes or bracing may be needed if excessive sloughing or raveling is observed. If material is stored or equipment is operated near an excavation, flatter slopes or shoring should be used to resist the extra pressure due to superimposed loads.

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Temporary excavations that encounter groundwater seepage or surface runoff may need shoring or dewatering. Excavations encountering seepage should be evaluated on a case-by-case basis. Dewatering is discussed in Section 11.1.6.

11.1.6. Bottom Stability and Dewatering

The stability of the bottom of excavations is dependent on the excavation geometry, soil strength, and the presence of groundwater. The bottom of an excavation in cohesionless/granular soils is susceptible to “blow out” conditions when groundwater is present. Instability and quick conditions can occur when excavations approach saturated cohesionless/granular layers under a hydraulic head without dewatering. In addition, if a saturated sand layer is situated beneath a clay layer, seepage pressures within the clay layer at the bottom of an excavation will result in unstable ground conditions. These conditions may endanger workers and equipment.

As discussed above, free water was generally observed in our borings at depths of about

18 to 25 feet during drilling and subsequently rose to as shallow as about 16 feet bgs. In addition, the Contractor should be aware of the potential for encountering relatively shallow perched water at the site and that a dewatering operation may need to be implemented. The depth of groundwater and the need for dewatering should be evaluated by the Contractor prior to beginning any excavation. Typically, the Contractor is responsible for designing, installing and maintaining the dewatering system. Dewatering systems should be designed, installed, and monitored by qualified personnel familiar with dewatering soils in the project area.

We recommend groundwater levels be kept 5 feet or more below the bottom of excavations.

In addition, we recommend the use of an independent back-up system whenever workers are in or nearby excavations needing dewatering. For utility line construction, dewatering operations should be continued until backfilling operations are concluded to reduce the potential for hydrostatic uplift (“floating”) of the pipe.

11.1.7. Pipe Bedding and Trench Backfill

Pipe bedding and trench backfill materials should meet any requirements specified by the

City of Conroe. Trench backfill as discussed herein refers to the material placed above the pipe zone/bedding backfill material. Based on the results of our study, the onsite soils may generally be used as trench backfill.

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Trench backfill material should be compacted as discussed in Section 11.1.10. Lift thickness for backfill will be dependent upon the type of compaction equipment used, but should generally be placed in lifts not exceeding 8 inches in loose thickness. Care should be exercised to avoid damaging the pipe or other structures during the compaction of the backfill.

Properly compacted trench backfill may still be subject to settlement over time of about 1 to

2 percent of the total fill thickness. To reduce the potential for settlement of fills in relatively deep trenches, we recommend compacting soils below a depth of 10 feet to 98 percent or more of the maximum dry density per ASTM D698. Alternatively, trenches deeper than

10 feet below grade may be backfilled using cement stabilized sand (CSS). The CSS should be placed in accordance with local specifications and compacted to 95 percent or more of the maximum density per ASTM D558, at a moisture content within ±2 percent of optimum.

11.1.8. Fill Materials

Select, engineered fill should consist of imported soils that exhibit relatively low plasticity indices and very low to low expansive potential. Relatively low plasticity indices are defined as a PI of 20 or less per ASTM D4318. We recommend select engineered fill should have a liquid limit (LL) less than 40, a PI between 10 and 20, and a percent passing the

No. 200 sieve of 60 or more.

Suitable fill soils (select fill or general fill) should not include organic material, construction debris, or other non-soil fill materials. Clay lumps and rock particles should not be larger than

6 inches in dimension. Fill materials in contact with ferrous metals should also have low corrosion potential (minimum resistivity more than 2,000 ohm-cm, chloride content less than

25 parts per million [ppm]). Fill material in contact with concrete should have a soluble sulfate content of less than 0.1 percent. The Geotechnical Engineer-of-Record should evaluate such materials and details of their placement prior to importation.

11.1.9. Re-Use of Excavated Materials

Based on laboratory test results and our general observations, we anticipate the onsite soils will generally not meet the specifications provided in Section 11.1.8 above and will not be suitable for re-use as select, engineered fill. Soils that do not meet the criteria for select engineered fill, may be used as general fill for site grading and utility trench backfill, provided they are free of deleterious materials.

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11.1.10. Fill Placement and Compaction

Fill soils should be moisture conditioned within the moisture range shown in Table 5 below and mechanically compacted to the percent compaction shown. Fill should generally be placed in 8-inch-thick loose lifts and compacted such that each lift is firm and non-yielding under the weight of construction equipment.

Table 5 – Summary of Compaction Recommendations Criteria

Description Percent Compaction1 Moisture Content2

Select, Engineered Fill3, 4 95 or more -2% to +2%

General Fill – Sand 95 or more5 -2% to +2%

General Fill – Clay4 95 or more5 -1% to +3%

Lime and Lime-Fly Ash Treated Subgrade 95 or more -1% to +3%

Notes:

1Percent compaction is the ratio expressed as a percentage of compacted dry density to the maximum dry density per ASTM D698.

2The range shown is relative to the optimum moisture content per ASTM D 698.

3Select, engineered fill should have a liquid limit of 40 or less, a PI between 10 and 20, and a percent passing the No. 200 sieve of 60 or more.

4Clayey soils used as fill should be processed so that particles or clods are no more than 6 inches in diameter prior to compaction.

5Soils more than 5 feet below finished floor elevation to 98 percent or more of the maximum density per ASTM D698.

11.1.11. Site Drainage

Adequate drainage should be provided to reduce seasonal variations in the moisture content of foundation soils. Pavement and flatwork within 5 feet of structures should be sloped away from the structure to reduce the potential for water ponding near the foundation. Finished grade within 5 feet of structures should be adjusted to slope away from the structure at a slope of 2 percent or more. The long-term performance of the foundation system depends, in part, on maintaining positive surface drainage over the life of the structure.

11.2. Ground-Supported Water Storage Tank

We understand the two GSTs are planned to be prestressed concrete tanks with diameters of approximately 65 feet. Based on a capacity of 750,000 gallons, the tanks will be on the order of

32 feet in height. We understand the high water level will be 30 feet above the floor.

Concrete water storage tanks should be designed in general accordance with American Water

Works Association (AWWA) Standard D110-13(R18), Standard for Wire- and Strand-Wound, Circular, Prestressed Concrete Water Tanks. The following recommendations are based on the data obtained from our subsurface exploration and laboratory testing, our experience with geotechnical conditions similar to those at this site, and our engineering design analyses.

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As discussed in Section 11.1.1 above, the concrete tanks should be constructed on 7-foot-thick pads of select, engineered fill. Site grading should provide for positive drainage away from the tanks.

Foundation systems for GSTs should be designed considering three basic and independent criteria. First, the bearing pressure transmitted to the foundation soils should not exceed the net allowable soil bearing capacity, which includes an appropriate safety factor. Second, settlements during the operating life of the tank should not be of a magnitude that will cause structural damage to the tank or its piping connections. Third, settlements should not impair the normal operations or serviceability of the tank.

Concrete water tanks are typically designed using a footing at the perimeter to support the weight of the shell. The footing transitions to a relatively thin concrete membrane floor beneath the tank.

Footing and floor are generally placed in a monolithic concrete pour. If the proposed construction will differ from that discussed herein, tank plans should be provided to SOCOTEC for review to evaluate the need for additional recommendations.

A perimeter footing bearing on select, engineered fill at about 2 feet or more below finished grade may be designed using an allowable bearing pressure of 2,400 pounds per square foot (psf) for static conditions. This value may be increased by a factor of ½ for total loads, including wind and seismic. We recommend the perimeter footing have a width of 30 inches or more.

Lateral loads transmitted to the foundation will be resisted by soil-concrete friction on the base of the footing. Frictional resistance may be estimated using an allowable coefficient of friction of

0.25 for a footing bearing on compacted, engineered fill.

11.2.1. Tank Settlement

We performed a generalized settlement analysis for the proposed water storage tank using the computer software program Settle3D by RocScience for the calculation of stress distribution and total and differential settlements. The program computes the net stress changes at selected locations and depths beneath loaded areas using Westergaard’s theories of stress distribution and overlap from adjacent structures. The program then uses soil compressibility parameters to evaluate the change in thickness of individual layers caused by the loaded areas and then computes the overall movement of the foundation at user defined points of interest.

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Our soil model was based on conditions observed in the borings performed in the tank footprints, the results of our laboratory testing, an assumed depth to groundwater of about

16 feet, and our professional judgment. Based on the proposed height of about 30 feet, the total static pressure beneath the interior membrane floor of the storage tank will be on the order of 1,900 pounds per square foot (psf). The pressure beneath the perimeter footing will be about 2,400 psf (based on the allowable bearing pressure provided above). The compressibility of the bearing soils underlying the proposed ground-supported water storage tank were characterized by a comparison of the consolidation test results (and the results of previous nearby consolidation tests in our library) and published correlations with laboratory index tests.

Based on the existing soils in the footprints of the tanks, the results of our analyses indicate that about 3 inches of total settlement is anticipated at the center of the proposed GSTs and about 2¼ inches at the perimeter, indicating differential settlements up to about ¾-inch across the floor. If 7 feet of select, engineered fill is placed in the tank footprints, as recommended in Section 11.1.1, about 2½ inches of total settlement is anticipated at the center of the proposed GSTs and about 1¾ inches at the perimeter, indicating differential settlements up to about ¾-inch across the floor.

11.2.2. Flexible Connections

Based on the magnitude of the settlements estimated along the tank perimeter and the calculated PVR, we recommend a vertical movement of 6 inches or more be accounted for in the design of utility connections and surface drainage features. Ball joints and sleeve-type or other flexible couplings, as appropriate, should be considered between external piping and the tank wall connections.

11.2.3. Tank Hydro-Testing

Before being placed into service, we recommend that the storage tank structure be tested using a controlled and monitored, stage-loaded hydro-test in accordance with standard industrial practices such as American Petroleum Institute (API) standards and guidelines.

Hydro-testing can be accomplished by filling the tank with water in successive stages under controlled conditions to check the competency of the welding, the ability of the foundation material carrying the imposed load by the tank structure, and to reduce the amount of settlement that the tank will experience after it is placed in service.

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To reduce the potential for increased settlement due to stress overlap of the pressures beneath footings, we recommend spread footings be spaced with approximately one footing width apart (edge-to-edge using the larger footing). If spread footings are to be considered for heavily loaded structures, we recommend detailed settlement analyses be performed.

11.3. Hydrotank, MCC Building, and Generator Pad Foundations

The proposed hydropneumatic tank may be supported on spread footings. The footings should bear on native soils at a depth of 4 feet or more below existing grade. Such footings may be designed using an allowable bearing pressure of 3,500 psf. This bearing pressure may be increased by a factor of 1/3 for total loads, including wind and seismic.

Provided the building pad has been prepared as recommended in Section 11.1.1, the MCC building may be supported on shallow spread footings or a stiffened-beam-and-slab foundation.

Footings bearing on select, engineered fill at a depth of 2 feet or more below existing grade may be designed using an allowable bearing pressure of 2,000 psf. This bearing pressure may be increased by a factor of 1/3 for total loads, including wind and seismic. Continuous (strip) footings should have a width of 18 inches or more and isolated spread footings should have a width of

24 inches or more.

If a stiffened-beam-and-slab foundation is selected for the MCC Building, we recommend a stiffening beam width of 12 inches or more, founded 24 inches or more below the adjacent grade.

The beams can be designed using a net allowable bearing pressure of 1,500 psf.

Provided the pad area has been prepared as recommended in Section 11.1.1, the generator pad and water well equipment pads may be designed as mats/slabs-on-grade. We recommend the slabs bear on select, engineered fill at a depth of 18 inches or more below finished grade. The slabs may be designed using an allowable net bearing pressure of 1,500 pounds per square foot

(psf) and a modulus of subgrade reaction of 125 pounds per cubic inch (pci).

The foundations should be reinforced in accordance with the Structural Engineer’s recommendations. Foundation excavations should be protected against any significant change in soil moisture content and disturbance by construction activity.

We estimate foundation movements of 1-inch or less may occur. Differential movements should not exceed half the predicted movement. These settlement estimates are based on the assumption that the foundations act as isolated foundations, that is, the clear spacing between the foundation elements are the width or diameter of the largest adjacent foundation or more.

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The ultimate resistance of shallow foundations to uplift forces is limited to the weight of the foundation plus the weight of any soil above the foundation. We recommend total unit weights of about 120 pcf for soil and 150 pcf for concrete be used in calculations. The ultimate uplift resistance should be reduced by a factor of safety of 1.2 to calculate the allowable uplift capacity.

Lateral loads transmitted to foundations will be resisted by soil-concrete friction on the base of the footings or stiffening beams. Frictional resistance may be estimated using an allowable coefficient of friction of 0.25. The foundations should preferably be proportioned such that the resultant forces from the loads, including lateral loads, fall within the middle one-third of the footing base.

11.3.1. Foundation Construction Considerations

The Geotechnical Engineer-of-Record or his representative should monitor foundation excavations to locate any pockets or seams of unsuitable materials (organic material, wet, soft, or loose soil), which might be encountered in excavations for foundations. Unsuitable materials encountered at the foundation bearing level should be removed and replaced with select fill as defined in Section 11.1.8 or lean concrete (with a compressive strength of about

1,000 psi at 28 days).

Foundation excavations should be protected against any significant change in soil moisture content and disturbance by construction activity. Sides of excavations may slough to some extent with time; sloughed soils and other debris in the bottom of the excavation should be removed prior to steel placement. Water should not be allowed to accumulate at the bottom of footing excavations.

Steel should be placed and concrete poured the day of excavation. If for some reason the foundation slab cannot be poured the day of excavation, a seal slab should be placed to protect the exposed foundation soils.

11.4. Culvert Design

As discussed above, two 6-foot-by-4-foot reinforced concrete box (RCB) culverts are planned where the tributary to East Fork Crystal Creek traverses the site. Prior to construction of the culverts, any areas of soft or saturated soils within the channel should be overexcavated to expose firm underlying soils.

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We recommend headwalls and wingwalls for the planned culvert be supported on shallow spread footings designed using an allowable bearing pressure of 2,000 pounds per square foot (psf).

Such footings should bear on native soils at a depth of 2 feet or more below the scour elevation or lowest adjacent grade, whichever is deeper.

Resistance to lateral loads on the headwall/wingwall footings may be analyzed using a combination of soil-concrete friction on the base of the footings and/or passive pressure on the face of the foundation. We recommend an allowable coefficient of friction along the base of the footings of 0.25. Passive resistance can be analyzed using an equivalent fluid pressure of

150 psf per foot of foundation embedment of the footing and the stem of the wall. The upper

2 feet of soil (or the depth of scour, whichever is deeper) should be neglected for passive resistance of the footings where the soils are not protected with concrete slope paving. If passive and frictional resistances are to be used in combination, we recommend the calculated passive resistance be reduced by one-half.

We recommend measures be taken so that moisture does not build up behind the walls. Drainage measures should include free-draining backfill material and perforated drainpipes or weep holes.

Free draining material should consist of clean, non-plastic, ½- to ¾-inch drain rock with less than

10 percent finer than the No. 200 sieve size. The drain rock and pipe should be wrapped in a separation fabric, such as Mirafi 140N or equivalent.

Headwalls/wingwalls should be designed to withstand anticipated lateral earth pressures and (to account for a rapid-drawdown condition) a full hydrostatic profile.