24-Attachment AA - Geotechnical Report.pdf

Attachment AA

“Reference Documents” HCFCD Project ID: Q534-01-00-E002

The following documents are included for reference

• Geotechnical Report

• USACE Nationwide Permit

• Draft SWPPP

FINAL REPORT

GEOTECHNICAL INVESTIGATION

PROPOSED HCFCD Q134-00-00-E001

DRAINAGE IMPROVEMENTS

PRECINCT 2, HARRIS COUNTY, TEXAS

All-Terra Project No.: AE20-972

PREPARED FOR:

Harris County Flood Control District c/o Gunda Corporation

11750 Katy Freeway, Suite 300 Houston, Texas 77079

PREPARED BY:

All-Terra Engineering, Inc.

6200 Rothway, Suite 140 Houston, Texas 77041

(713) 574-2371 (office)

(713) 574-2372 (fax)

November 29, 2022

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6200 Rothway, Suite 140, Houston, TX 77041 Tel: 713-574-2371 Fax: 713-574-2372

TBPE Firm No. 9770 All-Terra Engineering, Inc.

Geotechnical Engineering * Construction Materials Testing

November 29, 2022

Harris County Flood Control District c/o Gunda Corporation 11750 Katy Freeway, Suite 300 Houston, Texas 77079

Attn: Mr. Hunter Brennan, P.E.

Re: Final Report Geotechnical Investigation Proposed HCFCD Q134-00-00-E001 Drainage Improvements Precinct 2, Harris County, Texas

Project No.: AE20-972

Dear Mr. Brennan:

All-Terra Engineering, Inc. is pleased to transmit the final report of the geotechnical investigation for the above-referenced project. The report provides data, analyses results, and recommendations that may be used for the design and construction of the proposed drainage improvements. We are providing you the electronic file of the report for your review and use.

We appreciate the opportunity to perform this geotechnical investigation for the above-referenced project. If you have any questions pertaining to this report or if we may be of further service, please contact me at your convenience.

Respectfully submitted, All-Terra Engineering, Inc. (F-9770)

Bonifacio F. Musngi, Jr., P.E.

Senior Geotechnical Engineer

File: c://2020/AE20-972_finalreport_rev.docx

11/29/2022/ /2022

TABLE OF CONTENTS

Page No.

Executive Summary .............................................................................................. i

1.0 Introduction and Summary

1.1 Introduction

1.2 Summary of Findings

1.2.1 Subsurface Soil Conditions

1.2.2 Groundwater Levels in the Borings

1.2.3 Subsurface Variations

1.2.4 Geologic Faults

1.2.5 Slope Stability Analyses Assumptions and Safety Factor Design

Requirements

1.2.6 Soil Parameters Used for Slope Stability Analyses

1.2.7 Slope Stability Analyses Results

1.3 Summary of Recommendations

1.3.1 Recommended Detention Basin and Drainage Channel

Configurations

1.3.2 Recommended Site and Subgrade Preparation – Drainage Ditch ... 18

1.3.3 Recommended Site and Subgrade Preparation – Upper Detention

Portion

1.3.4 Recommended Site and Subgrade Preparation – Lower Wet

Pond Portion

1.3.5 Storm Sewer Design and Bedding/Backfill Requirements

1.3.6 Culverts and Headwalls

1.3.7 Design Considerations for Weir Structures

1.3.8 Embankment Slope Protection

1.3.9 Slope Paving Considerations

1.3.10 Earth Pressure Design of Underground Structures

1.3.11 Recommended Earth Fill Material/Compaction Requirements

1.3.12 Excavation Dewatering Recommendations

1.3.13 Temporary Shoring and Bracing Requirements for Excavations

1.3.14 Recommended Bulking and Shrinking Factors for Excavated Soils .. 29

1.3.15 Driveway Replacement Recommendations

1.3.16 Access Roadway Recommendations

2.0 Field Investigation

3.0 Laboratory Testing

4.0 Subsurface Conditions

4.1 Subsoils

4.2 Ground Water

5.0 Construction Considerations

Project No. AE20-972

TABLE OF CONTENTS

6.0 Closing Remarks

APPENDICES

Appendix A – Plate No. A.1 Vicinity Map Plate No. A.2 Locations of Borings Plate No. A.3 Soil Profile Section A-A Plate No. A.4 Soil Profile Section B-B Plate No. A.5 Soil Profile Section C-C Plate No. A.6 Soil Profile Section D-D Plate No. A.7 Soil Profile Section E-E Plate No. A.8 Typical Pond Section for Slope Stability

Analyses Plate No. A.9 Typical Ditch Section for Slope Stability

Analyses Plate Nos. A.10 – A.25 Boring Logs (Boring Nos. B-1 through B-16) Plate Nos. A.26 – A.41 Summary of Laboratory Test Results Plate Nos. A.42 – A.45 Site Photographs

Appendix B – Laboratory Test Results

Appendix C – Results of Slope Stability Analyses

Appendix D – Boring Logs and Laboratory Tests from AEC Report

Appendix E – Pertinent Data and Calculations

Appendix F – Drawings and Pertinent Data Provided by the Client

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EXECUTIVE SUMMARY

PROPOSED HCFCD Q134-00-00-E001

DRAINAGE IMPROVEMENTS

PRECINCT 2, HARRIS COUNTY, TEXAS

The final report presented herein presents the primary findings, conclusions, and recommendations of the geotechnical investigation pertaining to the design and construction of the proposed Harris County Flood Control District (HCFCD) Q134-00-00- E001 drainage improvement project in Precinct 2, Harris County, Texas (see Key Map Page 339, Squares K, L, P, and Q). The site location for the proposed drainage improvement project is shown on Plate No. 1.

The proposed development project is located south of FM 1960 and east of Crosby Huffman Road in Huffman, Harris County, Texas. Information from the design engineer indicated that the proposed project will consist of improving/extending a drainage channel portion (about 2,400 feet in length) of HCFCD Unit Q134-02-00 which is just east of Forest Manor residential subdivision and constructing a detention basin (area of about 45 acres) north of HCFCD Unit Q134-00-00 and west of HCFCD Unit Q134-01-00 in Precinct 2, Harris County, Texas. The proposed detention basin will consist of an upper detention and a lower wet pond that will be excavated down to an elevation of about +52.00 ft-msl with a top of bank elevation of about +65.50 ft-msl (requiring fill material to be placed to achieve the desired grade). The area for the proposed detention basin is owned by the HCFCD and is relatively flat with ground surface elevations ranging from +63.00 ft-msl to +64.40 ft-msl (based on elevations of Boring Nos. B-1A through B-12A of the current study and Boring Nos. B-4, B-6, B-7, B-8, and B-14 of a previous study by another firm) and covered with vegetation consisting of underbrush and grasses. The proposed drainage channel extension for HCFCD Unit Q134-02-00 will be excavated down to a low elevation of about +58.88 ft-msl on the north end to a slightly higher elevation of about +60.69 ft-msl on the south end near Beckham Drive with a top of bank of about +65.80 ft-msl. The area for the proposed drainage channel extension is also relatively flat with ground surface elevations ranging from +64.36 ft-msl to +65.54 ft-msl (based on elevations of Boring Nos. B-13A through B-16A of the current study and Boring No. B-6 of a previous study by another firm) and covered with scattered trees, underbrush, and grasses.

The primary findings, conclusions, and recommendations of this geotechnical investigation are summarized below.

Subsurface Soil Conditions: Based on the 16 borings (Boring Nos. B-1A through B-16A) drilled for the study and the 5 borings (Boring Nos. B-04, B-06, B-07, B-08, and B-14) from a previous geotechnical study, the subsurface soils within the project area generally consisted 2 soil layers consisting of soft to hard clay layers (fat clays classified as CH soils and lean clays classified as CL soils) encountered from 0 to 25

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November 29, 2022 feet beneath the existing ground surface and loose to medium dense sandy and silty layers (sands classified as SC, SM, SP-SM soils and silts classified as ML soils) encountered from 10 to 25 feet beneath the surface in 12 of the 21 borings. Details of the soils encountered during drilling are presented in the boring logs.

Ground Water Conditions: In general, ground water was encountered during the drilling of 16 out of 21 borings with depths ranging from 12.0 to 23.0 feet beneath the surface. Ground water was measured at depth ranging from 5.1 to 15.5 feet beneath the surface in all 21 borings at least 24 hours after the completion of drilling.

A summary of the ground water levels are presented on Section 1.2.2 of the report.

Slope Stability Analyses: We performed slope stability analyses of the planned section of the proposed detention pond (consisting of an 8.05-foot deep lower wet pond portion with a lower slope inclined at 3H:1V, a middle slope inclined at 10H:1V, and upper slope inclined at 4H:1V, a relatively flat (2% slope) maintenance berm portion, and a 5.25-foot deep upper detention portion with slope inclined at 4H:1V) as shown on Plate No. A.8 of the main report and planned section of the proposed drainage ditch extension (consisting of a 7.0-foot deep channel with 4H:1V side slope inclination) as shown on Plate No. A.9 of the main report. Our slope stability analyses of the planned sections of the proposed detention pond and drainage ditch extension indicated safe and stable slopes under short term, long term, and rapid drawdown conditions. The results of our slope stability analyses are presented in Section 1.2.7 of the main report.

Recommended Site and Subgrade Preparation for the Drainage Improvements The excavation for the drainage channel extension will generally cut through the site lean clays and fat clays. Within the proposed detention pond area, excavations will cut through the site lean clays and fat clays as well as the silty sand layer within the locations of Boring Nos. 3A, 5A, and 8. Where these sandy soils are exposed within the slopes and bottom portion at least 20 feet from the toe of slope, it will be necessary that these soils be over-excavated for a depth of at least 2 feet and replaced with properly compacted clay soils (site excavated lean clays and fat clays, as approved by the engineer). Subgrade preparation for the excavations of the detention pond and drainage ditch is typically limited to excavating to the desired base elevation, verifying that a firm and stable base is present, and verifying that no evidence of seepage or raveling is present within the side slopes or bottom before erosion control is implemented.

Culverts and Headwalls With an invert depth within elevation +58 ft-msl, the proposed precast reinforced concrete box culvert will be founded on the lean clay soils or the fat clay soils. A maximum allowable net bearing pressure of 1,600 psf may be used for axial compression dead loads plus sustained live loads for design purposes. A maximum allowable net bearing pressure of 2,400 psf may be used for axial compression dead loads plus sustained and transient live loads for design purposes. These values consider a safety factor against a bearing capacity failure of at least 3 for axial

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November 29, 2022 compression dead loads plus sustained live loads and 2 for axial compression dead loads plus sustained and transient live loads.

Access Roadway Recommendations A 30-foot wide access roadway is part of the proposed drainage improvement and will serve as access road during construction and the maintenance activities for the detention basin and drainage channel extension. Provided below are the recommended aggregate-surfaced roadway thicknesses that may be considered for the project.

- Crushed aggregate base course (unreinforced) …………18.0 inches (minimum)

- Crushed aggregate base course (w/ geogrid, BX1100 or equiv.)…10.0 inches

(minimum)

- Crushed aggregate base course (w/ geotextile)………….12.0 inches (minimum)

This executive summary should not be used alone as a basis for design. The geotechnical report, which contains details of the findings, conclusions, and recommendations of the geotechnical study, should be read in its entirety and serve as the basis for design.

November 29, 2022 Page: 1

GEOTECHNICAL INVESTIGATION

PROPOSED HCFCD Q134-00-00-E001

DRAINAGE IMPROVEMENTS

PRECINCT 2, HARRIS COUNTY, TEXAS

1.0 INTRODUCTION AND SUMMARY

1.1 Introduction

This final report presents the results of the geotechnical investigation in support of the design and construction of the proposed Harris County Flood Control District (HCFCD) Q134-00-00-E001 drainage improvement project in Precinct 2, Harris County, Texas (see Key Map Page 339, Squares K, L, P, and Q). The site location for the proposed drainage improvement project is shown on Plate No. A.1 in Appendix A.

The proposed development project is located south of FM 1960 and east of Crosby Huffman Road in Huffman, Harris County, Texas. Information from the design engineer indicated that the proposed project will consist of improving/extending a drainage channel portion (about 2,400 feet in length) of HCFCD Unit Q134-02-00 which is just east of Forest Manor residential subdivision and constructing a detention basin (area of about 45 acres) north of HCFCD Unit Q134-00-00 and west of HCFCD Unit Q134-01-00 in Precinct 2, Harris County, Texas. The proposed detention basin will consist of an upper detention and a lower wet pond that will be excavated down to an elevation of about +52.00 ft-msl with a top of bank elevation of about +65.50 ft-msl (requiring fill material to be placed to achieve the desired grade). The area for the proposed detention basin is owned by the HCFCD, is relatively flat with ground surface elevations ranging from +63.00 ft-msl to +64.40 ft-msl (based on elevations of Boring Nos. B-1A through B-12A of the current study and Boring Nos. B-4, B-6, B-7, B-8, and B-14 of a previous study by another firm), and is covered with vegetation consisting of underbrush and grasses. The proposed drainage channel extension for HCFCD Unit Q134-02-00 will be excavated down to a low elevation of about +58.88 ft-msl on the north end to a slightly higher elevation of about +60.69 ft-msl on the south end near Beckham Drive with a top of bank of about +65.80 ft-msl. The area for the proposed drainage channel extension is also relatively flat with ground surface elevations ranging from +64.36 ft-msl to +65.54 ft-msl (based on elevations of Boring Nos. B-13A through B-16A of the current study and Boring No. B-6 of a previous study by another firm) and covered with scattered trees, underbrush, and grasses.

Additional right-of-way may need to be acquired by the HCFCD for the construction of the proposed drainage channel extension. An outfall structure with headwall will be constructed near the east corner of the proposed detention basin. Side weirs will be constructed at 2 locations where one will be located on the south bank of the pond just on the west side of Boring No. B-10A and the other one will be located on the east bank of the pond just north of Boring No. B-3A. As part of the proposed development project, a 30-foot wide access roadway will be constructed from FM 2100 on the west end to the west end of the proposed detention pond. This access roadway will be

November 29, 2022 Page: 2 used during construction and maintenance of the detention pond and drainage ditch.

An existing 6-inch diameter steel pipe by Sunoco Pipeline will be removed and disposed off-site. It should be noted that the excavated soils within the pipeline may need to be tested for any potential contamination before hauling off the site or use in the site.

The objectives of the geotechnical investigation were to define the subsurface soils and ground water conditions within the project area and provide data/parameters/recommendations that may be used in support of the design and construction of the proposed drainage improvements.

This geotechnical investigation was performed by All-Terra Engineering, Inc. (All- Terra) for the Harris County Flood Control District (HCFCD) and Gunda Corporation in accordance with All-Terra Proposal No. APE20-724 dated July 17, 2020 and authorization through a “Subconsultant Services Agreement” between Gunda Corporation and All-Terra on or about December 10, 2020.

The scope of work for this geotechnical investigation consisted of the following activities:

Drill a total of sixteen (16) geotechnical borings where 12 of the borings (Boring Nos. B-1A through B-12A) were drilled to a depth of 25 feet beneath the surface within the proposed detention basin area and 4 of the borings (Boring Nos. B-13A through B-16A) were drilled to a depth of 20 feet beneath the surface within the accessible portion of the proposed drainage channel improvement area as shown on the attached Plate No. A.2. The boring depths, grid coordinates, and elevations are provided in the table below.

Table 1. Boring Depths and Survey Data

BORING NO.

DEPTH

(FT)

NORTHINGS EASTINGS

ELEVATION

(FT-MSL)

1A 25 13940874.78 3210143.82 64.40

2A 25 13940349.70 3210124.30 63.90

3A 25 13940266.79 3210659.87 63.71

4A 25 13939725.82 3210755.86 63.73

5A 25 13939639.82 3211240.31 63.68

6A 25 13939163.54 3211621.40 63.41

7A 25 13938889.91 3211011.24 63.54

8A 25 13939138.98 3210641.72 63.37

9A 25 13939101.02 3209939.36 63.00

November 29, 2022 Page: 3

Table 1. Boring Depths and Survey Data

BORING NO.

DEPTH

(FT)

NORTHINGS EASTINGS

ELEVATION

(FT-MSL)

10A 25 13938878.65 3209606.77 63.36

11A 25 13939057.52 3209201.64 63.52

12A 25 13938797.36 3208900.73 63.82

13A 20 13937947.43 3208501.44 64.36

14A 20 13937253.85 3208555.12 64.37

15A 20 13936780.39 3208600.24 65.44

16A 20 13936566.91 3208625.66 65.54

Continuously sample each boring to a depth of 20 feet and intermittently at 5-foot interval for the 25-foot deep borings, with both disturbed (cohesionless soils) and relatively undisturbed (cohesive soils) samples being obtained, as applicable.

Measure the depth to ground water during drilling, approximately 15 minutes after the water is initially encountered, as applicable, and within at least 24 hours after the completion of drilling.

Backfill the borings with soil cuttings after obtaining 24-hour ground water level measurements. Due to the relatively tall vegetation within the proposed detention pond area, the borings were marked with stakes that extend at least 6 feet above the ground surface, tie survey flagging near the top of the stakes, and label the stakes with the corresponding boring numbers. The borings within the drainage channel extension area were marked with stakes that extend at least 2 feet above the ground surface, tie survey flagging near the top of the stakes, and label the stakes with the corresponding boring numbers. The layout of the boring locations was provided to the client for surveying of the boring locations. Due to weather related issues, the surveyor obtained grid locations and elevations of 4 borings (Boring Nos. B-9A through B-12A) within the detention pond area based on GPS coordinates that were supplied by the design engineer.

Gather and review existing soils data from a previous geotechnical investigation recently performed by Aviles Engineering Corporation near/within the project area titled “Geotechnical Investigation, Q134-00-00 and Q134-01-00 Advanced Feasibility Study, HCFCD Project ID# Q134-00-00-P001, Harris County Precinct 2, Harris County, Texas, AEC Report No. G130-19” dated February 2020 and referred to in this report as “AEC Report”. Results of geotechnical borings that are near/within the project area and laboratory tests were incorporated into the soils data in our report and were used to supplement our data for our engineering analyses and evaluations.

November 29, 2022 Page: 4

Perform geotechnical laboratory tests on soil samples recovered during drilling in order to properly classify the site soils, identify dispersive soils, develop engineering design parameters for use in our engineering analyses, and develop engineering design parameters that will support our conclusions and recommendations. Laboratory testing will include Atterberg limits tests, tests to determine percent soil finer than the No. 200 sieve, sieve analyses including hydrometer analyses, specific gravity tests, double hydrometer tests, crumb tests, unconfined compression tests, unconsolidated undrained triaxial tests, and consolidated undrained triaxial tests with pore pressure measurements. Soil classifications were performed in strict compliance with ASTM D 2487.

Characterize the site subsoil and ground water conditions and provide the results on “gINT” boring logs and soil profiles.

Perform engineering analyses in order to develop data, parameters, and recommendations that may be used for the design and construction of the proposed detention basin and channel improvement. Engineering analyses and recommendations included the following items.

Stability (global and local) of slopes of the proposed detention basin and drainage channel extension.

OSHA requirements for trenching and bracing.

Earth pressure coefficients and equivalent fluid unit weights.

Allowable bearing capacities for structures, as applicable.

Ground water control and dewatering requirements during construction

All-weather access road alternatives and recommendations.

Stormwater runoff and surface water control measures.

Theoretical shrinking and bulking factors for use on excavated materials.

Stockpiling recommendations for excavated materials.

Pavement recommendations for driveway relocation.

Perform slope stability analyses in order to determine the stability of the side walls of the proposed detention basin and channel improvement in the short-term, rapid drawdown, and long term conditions using subsoil parameters, peak and residual shear strength parameters, derived from field and laboratory tests. All-Terra coordinated with the design engineers, Gunda Corporation, for the updated basin and channel designs in order to assure proper applicability of our engineering analyses.

Prepare a draft report in the format that contains the information as required by Part 6 of the December 2010 HCFCD “Geotechnical Investigation Guidelines”.

The report presents project description, existing site condition based on site visit and pictures taken during site visit, field investigation, laboratory test results, engineering analyses, the results of the analyses, and design recommendations as required by the work scope items described above. All-Terra will provide/send a digital copy (in pdf format) of the report for HCFCD’s and Gunda Corporation’s review. The final report will incorporate responses to the HCFCD’s and Gunda

November 29, 2022 Page: 5

Corporation’s review comments on the draft report and will be submitted to the HCFCD within 2 weeks upon receipt of the review comments. Both the draft and final reports will be sealed by a professional engineer licensed in the State of Texas. Electronic files of the project will be submitted to the HCFCD after the completion of the project.

1.2 Summary of Findings

The pertinent findings of this geotechnical investigation pertaining to the design and construction of the proposed drainage improvements are provided below.

1.2.1 Subsurface Soil Conditions

The subsurface soil conditions at the boring locations within the proposed drainage improvement project are described by the laboratory test results and the boring logs for Boring Nos. B-1A through B-16A performed for this study as well as Boring Nos. B-04, B-06, B-07, B-08, and B-14 from AEC Report.

Data from the 21 geotechnical borings drilled suggest that the upper 25 feet of the overburden soils are generally composed of two (2) separate soil layers as described below.

Table 2. Soil Layers and Descriptions

Layer Depth *

(ft) Soil Description

I 0 - 25

Clay layers consisting of black, dark gray, gray, light gray, tan, tannish gray, dark brown, reddish brown, light brown, and brown FAT CLAY (CH), FAT CLAY WITH SAND (CH), SANDY FAT CLAY (CH), LEAN CLAY (CL), LEAN CLAY

WITH SAND (CL), and SANDY LEAN CLAY (CL), medium to very high plasticity, soft to hard, moist to wet w/ ferrous nodules, ferrous stains, calcareous nodules, sand seams, sand pockets, sand partings, silt seams, silt partings, slickensides, organics, and roots.

II 10 – 25

Sandy and silty layers consisting of gray, brown, light brown, and tannish gray CLAYEY SAND (SC), SILTY SAND (SM), POORLY GRADED SAND WITH SILT (SP-SM), SILT (ML), SILT WITH SAND (ML), and SANDY SILT (ML), none to medium plasticity, moist to wet w/ clay seams and clay pockets (not encountered in Boring Nos. B-1A, B-4A, B-7A, B- 10A, B-11A, B-13A, B-15A, B-16A, and B-14).

* Measured below the ground surface at the time of our field exploration activities.

November 29, 2022 Page: 6

Laboratory testing was performed on selected samples of the subsurface materials obtained to classify the soils in accordance with ASTM D 2487 and to define the engineering properties of the soils. Portions of the test results indicating the high and low values of specific testing are provided in the table below.

Table 3. Highs and Lows of Laboratory Test Results

LAYER

DEPTH

(FT)

LIQUID

LIMIT

(ASTM D-4318)

PLASTICITY

INDEX

(ASTM D-4318)

MOISTURE

CONTENT

(ASTM D-2216)

PASSING

NO. 200

SIEVE

(ASTM D-1140)

SHEAR

STRENGTH

(TSF)

(ASTM D-2166,

(ASTM D-2850)

HIGH LOW HIGH LOW HIGH LOW HIGH LOW HIGH LOW

I 0 – 25 86 27 61 12 47.6 13.9 99.7 52.3 1.58 0.28

II 10 - 25 27 NP 12 NP 29.2 15.1 87.6 9.7 --

-- No sample was tested NP - - - Non-Plastic

For our slope stability analyses, 2 consolidated undrained (CU) triaxial compression tests were performed on selected clay soils. The results of these tests are summarized below and in Appendix B.

Table 4A. Triaxial CU Test Results

(ASTM D-4767)

Type of Material (Boring No., Depth)

LL PI

Pass #200 Sieve

TOTAL

STRESS

EFFECTIVE

STRESS

Lean Clay (CL) (Boring No. B-1A, 10’-14’)

41 23 92.1 Ctot = 370.6 psf tot = 16.8

Ceff = 276.1 psf eff = 25.0

Fat Clay (CH) (Boring No. B-6A, 6’-12’)

78 54 97.8 Ctot = 249.8 psf tot = 6.4

Ceff = 168.8 psf eff = 12.5

To supplement our data for our slope stability analyses, 2 consolidated undrained (CU) triaxial compression tests from AEC Report were obtained and the results are shown below and attached in Appendix D.

Table 4B. Triaxial CU Test Results from AEC Report

Type of Material (Boring No., Depth)

LL PI

Pass #200 Sieve

TOTAL

STRESS

EFFECTIVE

STRESS

Fat Clay (CH) (Boring No. B-7, 7’-10’)

74 51 97.0 Ctot = 240.0 psf tot = 17.2

Ceff = 270.0 psf eff = 23.6

November 29, 2022 Page: 7

Table 4B. Triaxial CU Test Results from AEC Report

Type of Material (Boring No., Depth)

LL PI

Pass #200 Sieve

TOTAL

STRESS

EFFECTIVE

STRESS

Fat Clay (CH) (Boring No. B-16, 12’-14’)

50 31 92.0 Ctot = 400.0 psf tot = 21.7

Ceff = 330.0 psf eff = 28.6

Results of Other Pertinent Tests Performed – Other pertinent laboratory tests were also performed for the proposed project.

Crumb testing, a test to determine dispersibility of clayey soils, was also performed on selected soil samples obtained from the field investigation.

The results of the tests are summarized below and are shown on the boring logs provided in Appendix A as well as in Appendix B.

Table 5. Crumb Testing Results

Boring No.

Depth (ft.)

Soil Layer No.

Type of Material Dispersion

Characteristic

B-1A 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

B-1A 8 – 10 I Fat Clay (CH) Grade 2 (Intermediate)

B-2A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-2A 12 – 14 I Lean Clay (CL) Grade 2 (Intermediate)

B-3A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-3A 10 – 12 I Fat Clay (CH) Grade 2 (Intermediate)

B-4A 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

B-4A 10 – 12 I Fat Clay (CH) Grade 2 (Intermediate)

B-5A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-5A 8 – 10 I Fat Clay (CH) Grade 2 (Intermediate)

B-6A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-7A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

November 29, 2022 Page: 8

No.

Depth (ft.)

Soil Layer No.

Type of Material Dispersion

Characteristic

B-7A 10 – 12 I Fat Clay (CH) Grade 2 (Intermediate)

B-8A 6 – 8 I Fat Clay (CH) Grade 1 (Nondispersive)

B-8A 12 – 14 I Lean Clay (CL) Grade 2 (Intermediate)

B-9A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-9A 10 – 12 I Lean Clay (CL) Grade 2 (Intermediate)

B-10A 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

B-10A 8 – 10 I Fat Clay (CH) Grade 1 (Nondispersive)

B-11A 8 – 10 I Fat Clay (CH) Grade 1 (Nondispersive)

B-12A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-12A 10 – 12 I Lean Clay (CL) Grade 2 (Intermediate)

B-13A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-14A 2 – 4 I Fat Clay With Sand

(CH)

Grade 2 (Intermediate)

B-15A 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-16A 2 – 4 I Fat Clay With Sand

(CH)

Grade 1 (Nondispersive)

B-04 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

B-04 8 – 10 I Lean Clay (CL) Grade 2 (Intermediate)

B-06 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-06 8 – 10 I Fat Clay (CH) Grade 1 (Nondispersive)

B-07 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

B-07 6 – 7 I Fat Clay (CH) Grade 1 (Nondispersive)

B-08 4 – 6 I Fat Clay (CH) Grade 1 (Nondispersive)

B-08 8 – 10 I Fat Clay (CH) Grade 1 (Nondispersive)

B-14 2 – 4 I Fat Clay (CH) Grade 1 (Nondispersive)

November 29, 2022 Page: 9

No.

Depth (ft.)

Soil Layer No.

Type of Material Dispersion

Characteristic

B-14 8 – 10 I Lean Clay With Sand

(CL)

Grade 1 (Nondispersive)

The results of the crumb testing on selected undisturbed soil samples obtained from the geotechnical borings revealed that the site lean clay soils and fat clay soils exhibited nondispersive to intermediately dispersive characteristics (Grades 1 and 2).

Double hydrometer, another test to determine dispersibility of clayey soils, was also performed on selected soil samples obtained from the field investigation. The results of the tests are summarized in the table below and are shown on the boring logs provided in Appendix A as well as in the Summary of Laboratory Tests in Appendix A.

Table 6. Double Hydrometer Testing Results

No.

Depth (ft.)

Soil Layer No.

Soil Type

% Pass

5-m

(DIS)

% Pass

5-m

(NODIS)

Dispersion*

B-1A 4 – 6 I Fat Clay (CH) 63.5 3.9 6.1

B-2A 6 – 8 I Fat Clay (CH) 60.0 0.5 0.8

B-3A 6 – 8 I Fat Clay (CH) 67.5 2.5 3.7

B-4A 10 – 12 I Fat Clay (CH) 58.5 0.3 0.5

B-5A 4 – 6 I Fat Clay (CH) 70.5 1.0 1.4

B-6A 14 – 16 I Lean Clay (CL) 34.5 9.6 27.8

B-7A 6 – 8 I Fat Clay (CH) 71.0 2.5 3.5

B-8A 8 – 10 I Fat Clay (CH) 68.5 0.2 0.3

B-9A 8 – 10 I Fat Clay (CH) 54.8 2.5 4.6

B-11A 6 – 8 I Fat Clay (CH) 59.2 0.1 0.2

B-14A 4 – 6 I Fat Clay (CH) 38.5 0.1 0.3

November 29, 2022 Page: 10

Table 6. Double Hydrometer Testing Results

No.

Depth (ft.)

Soil Layer No.

Soil Type

% Pass

5-m

(DIS)

% Pass

5-m

(NODIS)

Dispersion*

B-07 10 – 12 I Fat Clay (CH) 67.5 16.5 24.4

B-14 6 – 8 I Fat Clay (CH) 49.5 8.5 17.2

* When the percent dispersion is less than 30, it indicates that the soil is non-dispersive.

When the percent dispersion equals 30 but is less than 50, it indicates that the soil is intermediately dispersive. When the percent dispersion is greater than 50, it indicates that the soil is dispersive.

DIS - - - - With the use of dispersant NODIS - - - Without the use of dispersant

The results of the dispersion testing using double hydrometer testing on selected undisturbed soil samples obtained from the geotechnical borings revealed that the site lean clay soils and fat clay soils exhibited nondispersive characteristics (% dispersion less than 30).

Selected samples of the site clays tested for triaxial CU testing were also tested for specific gravity and the results are as follows:

Table 7. Specific Gravity Test Results

Boring No.

Depth

(ft) Type of Material Specific Gravity

B-1A 12 - 14 Lean Clay (CL) 2.642

B-6A 6 - 8 Fat Clay (CH) 2.726

1.2.2 Groundwater Levels in the Borings

Groundwater was encountered during the drilling of 16 of the 21 borings (including the 5 borings from AEC Report). The following table presents the results of the ground water level measurements at each of the geotechnical borings. The borings were backfilled with soil cuttings after the completion of the drilling activities.

Table 8. Ground Water Levels

BORING

NO.

DEPTH

OF

BORING

(FT.)

DEPTH TO

WATER

DURING

DRILLING

(FT.)

DEPTH TO

WATER

15 MINUTES

AFTER

INITIAL

ENCOUNTER

(FT.)

DEPTH TO

WATER

AT LEAST 24

HOURS AFTER

DRILLING

COMPLETION

(FT.)

DEPTH TO

BOTTOM OF

BORING AT

LEAST 24 HOURS

AFTER DRILLING

COMPLETION

(FT.)

1A 25 14.0 10.5 6.5 9.4

November 29, 2022 Page: 11

Table 8. Ground Water Levels

BORING

NO.

DEPTH

OF

BORING

(FT.)

DEPTH TO

WATER

DURING

DRILLING

(FT.)

DEPTH TO

WATER

15 MINUTES

AFTER

INITIAL

ENCOUNTER

(FT.)

DEPTH TO

WATER

AT LEAST 24

HOURS AFTER

DRILLING

COMPLETION

(FT.)

DEPTH TO

BOTTOM OF

BORING AT

LEAST 24 HOURS

AFTER DRILLING

COMPLETION

(FT.)

2A 25 12.0 9.5 5.5 9.0

3A 25 12.0 9.0 7.5 10.5

4A 25 12.0 10.5 8.5 9.8

5A 25 12.0 9.0 6.5 10.1

6A 25 16.0 13.5 7.5 11.5

7A 25 18.0 17.5 12.5 15.8

8A 25 16.0 15.0 13.5 14.9

9A 25 18.0 16.5 12.5 14.1

10A 25 dry dry 13.5 16.1

11A 25 18.0 17.0 11.5 14.0

12A 25 18.0 17.2 13.8 15.2

13A 20 dry dry 15.5 16.2

14A 20 12.0 11.5 10.0 10.8

15A 20 dry dry 12.5 16.2

16A 20 14.0 13.0 11.0 13.4

04 * 25 14.0 11.6 7.4 12.7

06 * 20 dry dry 7.1 11.7

07 * 25 23.0 8.8 5.1 5.7

08 * 25 14.0 8.9 8.4 5.9

14 * 25 dry dry 6.0 12.6

Note: Depths are referenced from the existing ground surface elevation at the time the borings were drilled.

1.2.3 Subsurface Variations

The information in this report summarizes conditions found on the dates the geotechnical borings were drilled and the dates the groundwater levels were monitored. It should be noted that the groundwater data and soil moisture contents will vary with environmental variations/fluctuations

November 29, 2022 Page: 12 such as the frequency, duration, and magnitude of rainfall as well as the time of year when construction is in progress.

Clay soils in the Harris County area typically exhibit secondary features such as slickensides and sand or silt fissures/lenses/pockets/seams.

Data gathered and presented in our boring logs are based on a 3-inch diameter soil samples obtained during our field investigation. Due to the sampling size, a detailed description of the secondary features may not have been observed/encountered and not indicated on the boring logs.

Therefore, while some of our boring logs show soil secondary features, it should not be assumed that these secondary features are absent where not indicated on the boring logs.

1.2.4 Geologic Faults

Based on the limited scope of the preliminary fault study performed by Aviles Engineering Corporation (AEC) and shown on Page 11 of AEC Report, there are no faults within the project area. However, their fault study is limited to the inaccessibility to the project area; the scale of maps available; the presence of obscuring vegetation, environmental features, and manmade structures; modification of the land surface by human activities; and lack of documentation in the available literature.

Their report indicated that faults may be present below ground, but which do not currently have surface expressions. No further study was done.

1.2.5 Slope Stability Analyses Assumptions and Safety Factor Design Requirements Slope stability analyses for this project were performed using the typical pond and ditch section configurations provided by the client and the subsoil profiles defined by the geotechnical borings. The typical slope section configurations (detention pond and drainage ditch) used in our slope stability analyses are as shown in Plate Nos. A.8 and A.9, respectively.

As required by the HCFCD, the minimum acceptable factors of safety against slope stability failures used for channel side slope design purposes are provided in the following table:

Table 9. HCFCD Factor of Safety Requirements

Condition Factor of Safety

Short Term Condition (End of Construction) 1.30

Intermediate Condition (Rapid Drawdown) 1.25

Long Term Condition 1.50

November 29, 2022 Page: 13

The results of the slope stability analyses for the typical section configurations, as described above, are shown in Appendix C and are discussed in detail in the following section.

1.2.6 Soil Parameters Used for Slope Stability Analyses

Results of our laboratory tests on selected soil samples from the geotechnical borings as well as applicable laboratory test data from AEC Report were used as soil parameters for our slope stability analyses. The soil parameters include moist unit weights, unconsolidated-undrained (UU) shear strengths, and shear strengths (effective stress and total stress) obtained from consolidated-undrained (CU) triaxial tests.

Based on the borings obtained within the project area, the subsurface soils generally consisted of upper fat clay soils and lower lean clay soils with sandy/silty soils in majority of the borings drilled. Laboratory tests revealed that the upper fat clay soils exhibited very high plasticity and shrink/swell potential. Exposing these fat clay soils to the atmosphere and cycles of wetting-drying from seasonal moisture changes will result in dessication, cracking, and reduction in soil shear strengths, which in turn could cause progressive slope movement and slope failure. The dessication zone or “weathered zone” of the fat clays is estimated to be about 8 feet below the ground/slope surface. For fat clays and lean clays (PI ≥ 20) within the weathered zone, it is appropriate to use effective stress residual shear strengths (cres and res) to evaluate slope stability for both the long-term and rapid-drawdown conditions.

Given below is a table of the short term, rapid-drawdown, and long term parameters used for the various layers of the subsurface soils within the development area.

Table 10. Soil Parameters for Slope Stability Analyses

Type of Material

Wet Density

(pcf.)

Sat.

Density

(pcf.)

Short Term Parameters

Intermediate Term Shear

Strength Parameters

Long Term Shear

Strength Parameters

Fat Clays (W_Zone*)

125.0 128.0 C =1,200 psf

= 0

Ctot = 75 psf tot = 13.0

Ceff = 75 psf eff = 13.0

Lean Clays (W_Zone*)

130.0 132.0 C =1,400 psf

= 0

Ctot = 160 psf tot = 24.0

Ceff = 160 psf eff = 24.0

Sands/ Silts

125.0 128.0 C = 0 psf

= 30

Ctot = 0 psf tot = 30

Ceff = 0 psf eff = 30

November 29, 2022 Page: 14

Table 10. Soil Parameters for Slope Stability Analyses

Type of Material

Wet Density

(pcf.)

Sat.

Density

(pcf.)

Short Term Parameters

Intermediate Term Shear

Strength Parameters

Long Term Shear

Strength Parameters

Fat Clays 125.0 128.0 C =1,200 psf

= 0

Ctot = 240 psf tot = 11.8

Ceff = 220 psf eff = 18.0

Lean Clays 130.0 132.0 C =2,000 psf

= 0

Ctot = 370 psf tot = 16.8

Ceff = 275 psf eff = 25.0

Embankment fills to achieve top of bank

May require the use of fill material that complies with Section 02314 titled “Fill Material” from the 2020 HCFCD Standard Specifications Book. For conservative slope stability analysis, the fill material was assumed to have the same property as the Fat Clays within the weathered zone. If the HCFCD allows the use of the on-site fat clay and lean clay soils, their use had been factored in using the soil properties for fat clay soils (CH) on our slope stability analyses which results in slightly lower factor of safety values.

* W_Zone - - - -- weathered zone (upper 8 feet of the ground/slope surface)

In the above table, the effective stress residual shear strength values (cres and res) for the site fat clays within the weathered zone were derived from the average of the values from the AEC Report and the results of our evaluation using the analyses methods presented in a published journal by G. Mesri and M.E.M. Abdel-Ghaffar titled “"Cohesion Intercept in Effective Stress-Stability Analysis". The effective stress residual shear strength values (cres and res) for the site lean clays within the weathered zone were derived from the results of our evaluation using the same Mesri/Abdel- Ghaffar journal. Our evaluation pertaining to the effective stress residual shear strength values provided in the above table are shown in Appendix E.

1.2.7 Slope Stability Analyses Results

Engineering analyses were performed in order to determine the factors of safety of the selected slope section configurations of the proposed detention pond and drainage ditch extension against a slope stability failure. For our slope stability analyses, the section configurations used were based on Plan Sheets dated August 11, 2011 provided by Gunda Corporation as shown on Plate Nos. A.8 and A.9 for the detention pond and drainage ditch extension, respectively, and attached on Appendix F.

The section configurations consisted of the following:

As shown on Plate No. A.8, the detention pond section configuration consisted of a lower wet pond portion that has a flat bottom (at Elev.

+52.00 ft-msl) with a lower wall (sloped at 3H:1V) and rises about 4.16 feet (at Elev. +56.16 ft-msl), a middle wall (sloped at 10H1V) that rises

November 29, 2022 Page: 15 about 2.49 feet (at Elev. +58.65 ft-msl), an upper wall (sloped at 4H1V) that rises about 2.50 feet ((at Elev. +61.15 ft-msl). At this point, a 10-foot wide bottom shelf will be constructed with a slope of 2% into the toe of the upper detention portion. At the toe (at Elev. +61.35 ft-msl) , the sidewall of the upper detention (sloped at 4H:1V) will rise about

4.15 feet to the top of bank (at Elev. +65.50 ft-msl) where a maintenance berm (sloped at 2% downward) will be constructed.

As shown on Plate No. A.9, the drainage channel section configuration consisted of a flat bottom (at Elev. +58.77 ft-msl) with a sidewall (sloped at 4H:1V) and rises about 7.03 feet to reach the top of bank (at Elev. +65.80 ft-msl) where a 20-foot wide maintenance berm (sloped at 2% downward) will be constructed.

Slope stability analyses involve the determination of the most likely sliding surface by comparing the developed shear stresses with the weight forces associated with the sliding soil. Slope stability analyses are performed to check the stability of the soil slope. The method involves the determination of the factor of safety along the surface where the soil mass may slide. The failing or sliding surface is typically assumed to be circular. The analysis also includes the determination of a particular circle (i.e., sliding surface or critical circle) that has a least factor of safety against sliding.

Recommendations concerning side slope inclinations were based on results of slope stability analyses performed using the STABL6H computer program developed by Purdue University. The Simplified Bishop procedure was adopted in the analyses to determine the factor of safety against sliding. The STABL6H computer program searches for the critical slope failure plane and computes the minimum factor of safety for the given slope geometry and subsurface soil conditions.

For the slope stability analyses, 3 conditions were analyzed for the selected section configurations analyzed. The 3 different conditions are:

1) Short Term or Undrained Condition - This condition occurs when the pore pressures within the soil mass are not dissipated. Typically, this condition corresponds to the state of the soils, which exist during and immediately after performing any cut/fill during the construction of any slope. For this condition, slopes were analyzed using undrained soil parameters obtained from our laboratory testing program for the site clays and the corresponding friction values obtained from field standard penetration tests and typical values from various references for the site sandy and silty soils. In the short term condition, a distributed load of 250 psf was applied at the top of the embankment to simulate construction equipment loadings.

November 29, 2022 Page: 16

2) Long Term or Drained Condition - This condition occurs when the pore pressures within the soil mass are dissipated. Typically, this condition corresponds to the state of the soils a few months or years after the construction is complete. For this condition, slopes were analyzed using drained and effective stress soil parameters based on results of laboratory testing program for the site clays and the corresponding friction values obtained from field standard penetration tests and typical values from various references for the site sandy and silty soils. In the long term condition, each configuration analyzed consisted of the slope section in a consolidated and drained condition and effective shear strength parameters prevail. The peak shear strength values are normally used for low to medium plasticity lean clays and residual shear strength values are used for highly plastic lean clays and fat clays. In the long term condition, a distributed load of 250 psf was applied at the top of the embankment to simulate construction equipment loadings.

3) Rapid Drawdown Condition – This condition occurs when the water level in the pond rises during a flood, saturating the slope and then drains rapidly after the flood recedes. The state of stress within the soils of the slope after a flood event depends largely on the permeability and drainage characteristics of the slope. When the water levels are lowered rapidly against a clay slope, excess pore pressures may not have enough time to dissipate and an undrained strength analysis is required. For this condition, slopes were analyzed using total stress soil parameters based on results of laboratory testing program for the site clays and corresponding friction values obtained from field standard penetration tests and typical values from various references for the site sandy and silty soils. Each configuration analyzed for the intermediate condition consisted of a fully saturated slope section. The peak shear strength values are normally used for low to medium plasticity lean clays and residual shear strength values are used for highly plastic lean clays and fat clays.

Stability analyses were performed using peak shear strength parameters for the site soils in the short term condition and residual shear strength parameters for the rapid-drawdown and long term conditions. The results of these analyses are shown below and in Appendix C.

November 29, 2022 Page: 17

Table 11. Slope Stability Analyses Results in Detention Basin

SECTION/LOCATION

ANALYZED

FACTOR OF SAFETY AGAINST A SLOPE STABILITY FAILURE

SHORT

TERM

CONDITION

(FS=1.30 MIN)

RAPID

DRAWDOWN

CONDITION

(FS=1.25 MIN)

LONG

TERM

CONDITION

(FS=1.50 MIN)

L G L G L G

Typical Pond Section Configuration, Boring No. B-4A (See Plate No. A.8)

8.02 5.43 1.82 1.54 2.05 1.58

L- Local Slope Stability. G- Global Slope Stability.

Table 12. Slope Stability Analyses Results in Drainage Channel Extension

SECTION/LOCATION

ANALYZED

FACTOR OF SAFETY AGAINST A SLOPE STABILITY FAILURE

SHORT

TERM

CONDITION

(FS=1.30 MIN)

RAPID

DRAWDOWN

CONDITION

(FS=1.25 MIN)

LONG

TERM

CONDITION

(FS=1.50 MIN)

L G L G L G

Typical Ditch Section Configuration, Boring No. B-13A (See Plate No. A.9)

12.24 7.61 1.40 1.30 2.08 1.73

L- Local Slope Stability. G- Global Slope Stability.

The results of the stability analyses indicated that the slopes for the proposed drainage ditch channel and detention pond will be stable in the short term, rapid drawdown, and long term conditions. The detailed data and results of our slope stability analyses are shown in Appendix C.

Additionally, in a situation where the detention pond is totally dry due to prolonged lack of rain and the complete drying of the water in the lower wet pond area, a slope stability analysis was performed in the long term condition in order to check the stability of the slopes. The results of our slope stability analysis revealed the following factors of safety: 1.85 for local stability failure and 1.60 for global stability failure which satisfied the minimum required factor of safety of 1.50 in the long term condition.

The detailed data and results of our slope stability analyses are also shown in Appendix C.

November 29, 2022 Page: 18

1.3 Summary of Recommendations

The recommendations, as summarized below, are provided for use in the design and construction of the proposed drainage improvement project.

1.3.1 Recommended Detention Basin and Drainage Channel

Configurations The results of our slope stability analyses for the detention basin section configuration, shown on Plate No. A.8, indicate that the proposed basin section (including an upper detention portion and lower wet pond portion), with excavations that will extend to a depth of about 12 feet beneath the existing ground surface, with an embankment fill of about

1.5 feet over the existing ground surface, and with side slopes inclined no steeper than 3H:1V and 4H:1V for the upper detention portion and lower wet pond portion, respectively, should have an adequate factor of safety against deep-seated failures. Steeper slopes are not recommended due to the high potential for shallow failures that may be caused by loss of strength in the surficial soils resulting from shrink-swell cycles (i.e., within the active zone or weathered zone) where surface water may infiltrate through the shrinkage cracks and cause downhill creep. Therefore, it is recommended that the side slopes for any portion of the proposed detention basin be constructed with side slopes inclined no steeper than 3H:1V for the wet pond portion and 4H:1V for the upper detention portion, unless as desired by the client, further analysis with steeper slopes are performed.

The results of our slope stability analyses for the drainage ditch extension section configuration, shown on Plate No. A.9, indicate that the proposed ditch section, with excavations that will extend to a depth of about 5.6 feet beneath the existing ground surface, with an embankment fill of about 1.4 feet over the existing ground surface, and with side slopes inclined no steeper than 4H:1V, should have an adequate factor of safety against deep-seated failures. Steeper slopes are not recommended due to the high potential for shallow failures that may be caused by loss of strength in the surficial soils resulting from shrink-swell cycles (i.e., within the active zone or weathered zone) where surface water may infiltrate through the shrinkage cracks and cause downhill creep. Therefore, it is recommended that the side slopes for any portion of the proposed drainage ditch extension be constructed with side slopes inclined no steeper than 4H:1V, unless as desired by the client, further analysis with steeper slopes are performed.

1.3.2 Recommended Site and Subgrade Preparation – Drainage Ditch Extension It is our understanding that the proposed drainage ditch extension will be excavated to an elevation of about +58.88 ft-msl on the north end and to an elevation of about +60.69 ft-msl on the south end. As shown on the soil profile on Plate No. A.7 in Appendix A, the excavation for the

November 29, 2022 Page: 19 proposed drainage ditch extension will cut through the upper fat clays. It is not anticipated that the Layer II sandy/silty soils will be encountered during the excavation activities.

Subgrade preparation is typically limited to…