19-Geotechnical Report.pdf

ADDENDUM GEOTECHNICAL RECOMMENDATIONS (MEMORANDUM-2)

CONCRETE-LINED CHANNEL

STATION 11+50 to STATION 40+00

HCFCD PROJECT NO. P118-09-00-E001

PRECINCT 1 – HARRIS COUNTY, TEXAS

Report to:

Woolpert

HOUSTON, TEXAS

Report No. 121-041-03 September 15, 2025

Woolpert 11750 Katy Freeway, Suite 1260 Houston, Texas 77079

Attention: Nathan Wilson, P.E., PMP, CFM, ENV SP Senior Project Manager, South Region-Water

Addendum Geotechnical Recommendations (Memorandum-2) Concrete-lined Channel Along Station 11+50 to Station 40+00

HCFCD Project No. P118-09-00-E001 Precinct 1 - Harris County, Texas

Introduction

Cibor Geoconsultants (Cibor) is pleased to submit this addendum report (Memorandum-2) which provides the lateral earth pressure parameters and slope stability analysis to support Woolpert in their design of the approximately 12.5-foot-deep, concrete-lined channel from Station 11+50 to Station 40+00 (HCFCD Project No. P118-09-00-E001). Cibor submitted the original geotechnical investigation report for this project on August 29, 2023. We subsequently provided draft supplemented recommendations for the design of concrete-lined channel in Memorandum dated December 06, 2023 (Memorandum-1). This report presented revised recommendations for the concrete-lined channel provided in Memorandum-1.

This work has been conducted in general accordance with our Addendum Proposal (Proposal No. 121-041-03 dated June 26, 2025) as authorized by Mr. Sam Talje, Vice President, with Woolpert.

The channel was originally proposed to be concrete lined along the lower banks having 1:1(H:V) side slopes and along channel bottom as indicated in our Memorandum dated December 06, 2023. Following our meeting with Woolpert dated June 9, 2025, it was agreed to eliminate the concrete lining along the bottom of the channel. Instead, it was agreed to embed the 1:1(H:V) concrete-lined side slopes at the toe and place riprap as erosion protection below the flowline of the channel.

Lateral Earth Pressures

The concrete-lined portion of the 1(H):1(V) channel slope will experience lateral earth pressures resulting from a combination of soil pressure and hydrostatic water pressure. Soil pressures behind the concrete lining will impose either a triangular or trapezoidal stress distribution depending on the method of construction and the type of backfill selected. We initially provided the lateral earth pressure parameters to Woolpert for the structural redesign of the concrete lining on July 06, 2025. The schematic for the distribution of lateral pressures on the 1:1(H:V) concrete-lined channel section are illustrated on Plate 1.

Based on our recommendations, Woolpert modified their design of the concrete-lined channel on August 27, 2025, as included in Appendix A in this report.

Channel Cross-section – Station 11+50 to Station 40+00

Woolpert provided channel cross-sections at Stations 18+50 and 34+50 on August 27, 2025, which are included in Appendix A. The channel cross-section consists of a concrete-lined slope that is approximately 7.5 feet deep, with an inclination of 1:1 (H:V). Above this concrete-lined slope is a grass-lined embankment, which has slopes varying from 1.8:1(H:V) to 3.9:1 (H:V). The concrete lined slope will be embedded to a depth of 3 feet below the flowline. Additionally, 2-foot-thick riprap will be placed below the flowline of the channel. The riprap replaces the original concrete lining.

Slope Stability Analysis

Slope stability analyses of the provided concrete-lined channel at Stations 18+50 and 34+50 were performed under short-term, rapid drawdown, and long-term loading conditions as required by the latest HCFCD Geotechnical Guidelines. The Slide2 computer software (Version 7.038) with the Bishop Method was used for slope stability analysis. The soil parameters used in the updated stability analyses remain consistent with those used in our original geotechnical investigation report for this project on August 29, 2023 and presented in Table 1 below.

Table 1. Soil Properties Used for Concrete-Lined Channel – Boring B-7

Soil layers

Design Parameters

Short-term Condition

Long-term Condition/Rapid

Drawdown Effective Stress

Rapid Drawdown Total Stress

Description Depth

Interval (ft)

Total Unit Weight ϒ

(pcf) cu (psf) φu (deg) c’ (psf) φ’ (deg) ccu

(psf) φcu

(deg)

Clayey Sand 0-6 118 35 31 35 31 - -

Sandy Lean Clay 6-10 125 1200 0 160 24 185 19

Fat Clay 10-14 125 1300 0 145 24 165 20

Lean Clay with Sand 14-20 125 1500 0 162 24 190 19

The HCFCD Geotechnical Guidelines require minimum allowable factors of safety of 1.3 for the short-term condition, 1.5 for the long-term condition, and 1.25 for the rapid drawdown condition. The predicted factor of safety values of all analyzed slopes is presented in Table 2 below. The results of the global stability analyses for all conditions are shown in Plates 2 through 7. The slope stability results summarized below indicate that the factor of safety for the proposed concrete-lined channel with side slope configurations 1:1(H:V) meet the minimum requirements only when the strength of the structurally designed concrete lining is incorporated into the analysis.

Table 2. Results of Slope Stability Analysis

Station Boring Factor of Safety

Short-term Long-term Rapid

Drawdown

18+50 B-7

1.75 (Shallow)

10.38 (Deep)

2.01 (Shallow) 2.95(Deep)

1.96 (Shallow)

3.01 (Deep)

34+50 B-7

1.47 (Shallow)

6.67 (Deep)

2.71 (Shallow)

1.93(Deep)

1.47 (Shallow)

3.44 (Deep)

Control of Groundwater and Surface Water Control During Reconstruction of Channel

Water will, of course, be encountered in, and immediately adjacent, to the channel.

Construction activities related to the channel reconstruction will require diversion of channel water by pumping/piping the retained water from upstream to downstream of the excavation.

We believe installation of temporary barriers/cofferdams will be required to preclude the channel water from entering into the excavation both upstream and downstream of the excavation. The configuration should be sufficient to withstand sudden rises in stage within the channel due to storm events. The soils within the channel are expected to consist of both permeable granular and relatively permeable cohesive material. Consequently, the Contractor should be prepared to handle significant quantities of discharges. Installation of a cut-off extending below the channel bed should be considered to reduce both upward seepage forces and the quantity of inflow.

Some softened and/or sandy conditions are expected at the bottom of the excavation. Drawing the water level down by one or two feet below the base of excavation will be required where these conditions are encountered. Lowering the water levels is required so that the earthwork operations will be performed in reasonably “dry” and free of water conditions, and the strength of the subgrade soils will be preserved against the adverse consequences of upward seepage forces (i.e., bottom heave and/or liquefaction). Control of groundwater and surface water should be in accordance with Section No. 02241 of the HCFCD Standard Specifications. It is recommended that site drainage should be well developed. Surface water should be directed away from the top bank of the slope.

Due to the relatively impervious nature of the slope soils, most of the seepage could likely be handled with sumps and pumps. However, should significant seepage from the near-surface sandy/silty soils be encountered, use of wellpoints could be required. The well point screen openings should be sized so as to avoid the pumping of fines. The discharge from the dewatering operation should be of such quality that contamination of the waters of the channel will not occur. The Contractor should be prepared to handle any water encountered during earthwork operations particularly where permeable sandy/silty soils are encountered.

The following illustrations are attached and complete this report.

ILLUSTRATIONS

PLATE

Lateral Earth Pressure Diagrams

Slope Stability Analysis – Sta. 18+50 .............................................................. 2 to 4

Slope Stability Analysis – Sta. 34+50 .............................................................. 5 to 7

Appendix Cross-sections Provided by Woolpert ............................................ A

Project No. 121-041-03

CONCRETE CHANNEL LINING ALONG P118-09-00

LATERAL PRESSURE PARAMETERS

Lateral Active Earth Pressure Behind Lining

Hydrostatic Pressure(psf)

35H 62.4(H+H1)

H

H1

Top of Bank

Concrete Lining

Slope Varies

1 + +

Surcharge, q=250 psf

14H1

Lateral Pressure due to 3:1 Slope above Concrete Lining

20H1 62.4H1

3' (Min) Embedment Below

Flowline

Maximum Stage in Channel

PLATE 1

1.745

10.376

1.745 W W

250.00 lbs/ft2 1.745

10.376

1.745

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 1200 0

Weathered Fat Clay 110 Mohr-Coulomb 500 0

Fat Clay 125 Mohr-Coulomb 1300 0

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 1500 0

3.9H:1V 2.8H:1V

1H:1V 1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-20 -10 0 10 20 30 40 50 60 70 80

Analysis Description

CompanyScale 1:122Drawn By

File Name V1_ST_Sta. 18+50_Concrete Lining.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

HCFCD P118-09 Channel - Trapezoidal Section

Short-term Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta.18+50

PLATE 2

2.010

2.953

2.010

W W

250.00 lbs/ft2 2.010

2.953

2.010

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 160 24

Weathered Fat Clay 110 Mohr-Coulomb 50 20

Fat Clay 125 Mohr-Coulomb 120 22

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 162 24

3.9H:1V 2.8H:1V

1H:1V 1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-20 -10 0 10 20 30 40 50 60 70 80

Analysis Description

CompanyScale 1:122Drawn By

File Name V1_LT_Sta. 18+50_Concrete Lining.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

HCFCD P118-09 Channel - Trapezoidal Section

Long-term Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta.18+50

PLATE 3

1.956

3.013

1.956

W (Initial)W (Final)

1.956

3.013

1.956

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg) RD Cr (psf)

RD PhiR (deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 160 24 185 19

Weathered Fat Clay 110 Mohr-Coulomb 50 20 50 20

Fat Clay 125 Mohr-Coulomb 120 22 145 18

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 162 24 190 19

3.9H:1V

1H:1V 1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

-10 0 10 20 30 40 50 60 70 80

Analysis Description

CompanyScale 1:120Drawn By

File Name V1_RDD_Sta. 18+50_Concrete Lining.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

2.8H:1V

HCFCD P118-09 Channel - Trapezoidal Section

Rapid Drawdown Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta. 18+50

PLATE 4

1.469

6.672

1.469

W W

250.00 lbs/ft2 1.469

6.672

1.469

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 1200 0

Weathered Fat Clay 110 Mohr-Coulomb 500 0

Fat Clay 125 Mohr-Coulomb 1300 0

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 1500 0

1.8H:1V 2.6H:1V

1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

0 10 20 30 40 50 60 70 80 90

Analysis Description

CompanyScale 1:120Drawn By

File Name V2_ST_Concrete Lining_STA 34+50.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

HCFCD P118-09 Channel - Trapezoidal Section

Short-term Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta.34+50

1H:1V

PLATE 5

1.927

2.718

1.927

W W

250.00 lbs/ft2

1.927

2.718

1.927

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 160 24

Weathered Fat Clay 110 Mohr-Coulomb 50 20

Fat Clay 125 Mohr-Coulomb 120 22

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 162 24

1.8H:1V 2.6H:1V

1H:1V 1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

0 10 20 30 40 50 60 70 80 90

Analysis Description

CompanyScale 1:126Drawn By

File Name V2_LT_Concrete Lining_STA 34+50.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

Long-term Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta.34+50

HCFCD P118-09 Channel - Trapezoidal Section

PLATE 6

1.470

3.449

1.470

W (Initial) W (Final)

1.470

3.449

1.470

Material Name Color Unit Weight (lbs/Ō3) Strength Type Cohesion

(psf) Phi

(deg) RD Cr (psf)

RD PhiR (deg)

Clayey Sand 118 Mohr-Coulomb 35 31

Sandy Lean Clay 125 Mohr-Coulomb 160 24 185 19

Weathered Fat Clay 110 Mohr-Coulomb 50 20 50 20

Fat Clay 125 Mohr-Coulomb 120 22 145 18

Concrete Lining (Designed Structurally) 150 Mohr-Coulomb 50000 35

Riprap 150 Mohr-Coulomb 0 40

Lean Clay with Sand 125 Mohr-Coulomb 162 24 190 19

1.8H:1V 2.6H:1V

1H:1V 1H:1V

Safety Factor 0.000 0.250 0.500 0.750 1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000 3.250 3.500 3.750 4.000 4.250 4.500 4.750 5.000 5.250 5.500 5.750 6.000+

0 10 20 30 40 50 60 70 80 90 100 110

Analysis Description

CompanyScale 1:140Drawn By

File Name V1_RDD_Concrete Lining_STA 34+50.slimDate 9/5/2025

Project

SLIDEINTERPRET 7.038

HCFCD P118-09 Channel - Trapezoidal Section

Rapid Drawdown Condition - 1(H):1(V) Concrete Retaining Wall (Structurally Designed) - Sta.34+50

PLATE 7

APPENDIX A:

CROSS-SECTION PROVIDED BY WOOLPERT

CROSS SECTION (LOOKING UPSTREAM)

0+35 0+40 0+50 0+60 0+70 0+80 0+90 1+00 1+10 1+20 1+30 1+40 1+50 1+60 1+65

EXIST.40' DRAINAGE EASEMENT EXIST. 60' TEXAS PIPELINE COMPANY

R.O.W.

8" THICK

CHANNEL LINING

EXIST GROUND

TOE WALL

ELEV= 51.55

1.0 :1

1.0:1

ELEV= 47.75

10'

UTILITY EASEMENT

10'

UTILITY EASEMENT

8" THICK

CHANNEL LINING

1' TOE WALL

3'

1' 2'

2.8:1 3.9:1

ELEV= 49.40

4'

RIPRAP GRADATION NO. 2

(24" THICK)

BETWEEN BIGWOOD ST PARKER RD

(STATION 18+50)

CROSS SECTION (LOOKING UPSTREAM)

0+35 0+40 0+50 0+60 0+70 0+80 0+90 1+00 1+10 1+20 1+30 1+40 1+50 1+60 1+65

ELEV= 55.24

1.0 :11.0:1

EXIST.40' DRAINAGE EASEMENT

10'

UTILITY EASEMENT

EXIST. 60' TEXAS PIPELINE COMPANY

R.O.W.

10'

UTILITY EASEMENT

8" THICK

CHANNEL LINING

EXIST GROUND

TOE WALL

8" THICK

CHANNEL LINING

1'

TOE WALL

3'

1'

2'

ELEV= 51.72

ELEV= 55.78

1.8:1 2.6:1

RIPRAP GRADATION NO. 2

(24" THICK)

GRASS-LINE GRASS-LINE

4'

BETWEEN PARKER RD TO GLEASON RD

(STATION 34+50)

Report No. 121-041-05 July 06, 2025

AƩenƟon: Nathan Wilson, P.E., Woolpert From: Subhas Kandel, Graduate Engineer Subject: Response to Woolpert’s AddiƟonal Geotech QuesƟons

We reviewed Woolpert quesƟons following the Structural Engineer’s review of our Memorandum dated June 20, 2025, and appreciate his comments regarding the lateral earth pressures behind the concrete lining.

Since no consolidated undrained (CU) triaxial tests were performed on samples recovered from these borings, the selected soil design parameters for each individual soil layer in our calculaƟons were based on our prior experience with similar soil types, focusing on the most criƟcal (i.e.

weakest) relaƟvely loose and/or soŌ soil condiƟons.

We iniƟally used the Coulomb Method to calculate the acƟve earth pressure, taking into account both the interface fricƟon angle (Φ) and the slope angle (β). Although the Coulomb formula indicates a "zero" lateral earth pressure coefficient because the fricƟon angle is less than the slope angle of 45 degrees, we sƟll believe the acƟve case for lateral earth pressures should be considered in addiƟon to the lateral pressures due to the 3:1(H:V) slope above the lining, as well as the hydrostaƟc pressures under a rapid drawdown condiƟon. This recommendaƟon is based on our evaluaƟon and consideraƟon of other factors, including potenƟal pressures associated with shrink-swell. We did not, however take into account the impact of the 250 psf surcharge on the concrete lining due to its significant distance from the top of the lining.

We have revised Plate 7 of our Memorandum dated December 7, 2023, by updaƟng our lateral earth and hydrostaƟc pressure diagrams as discussed above and removing the concrete lining from the boƩom of the channel. The updated lateral pressure diagrams are now aƩached as AƩachment 1.

The lateral pressures acƟng behind the concrete lining and those due to the 3:1 slope are horizontal, while the hydrostaƟc pressures are acƟng perpendicular to the face of face of the 1:1(H:V) concrete-lined slope. For calculaƟng lateral earth pressures due to a 3:1(H:V) slope above the concrete lining, we referenced the publicaƟon Perloff W. H., Baladi G. Y., and Harr M.

E., 1967, Stress DistribuƟon Within and Under Long ElasƟc Embankments.

Project No. 121-041 7/6/2025

CONCRETE CHANNEL LINING ALONG P118-09-00

LATERAL PRESSURE PARAMETERS

Lateral Active Earth Pressure Behind Lining

Hydrostatic Pressure(psf)

35H 62.4(H+H1)

H

H1

Top of Bank

Concrete Lining

1 + +

Surcharge, q=250 psf

14H1

Lateral Pressure due to 3:1 Slope above Concrete Lining

20H1 62.4H1

3' (Min) Embedment Below

Flowline

Maximum Stage in Channel

ATTACHMENT 1

GEOTECHNICAL INVESTIGATION

P118-09-00 DETENTION BASIN AND CHANNEL IMPROVEMENTS

HCFCD PROJECT ID NO. P118-09-00-E001 BOND ID C-24

HARRIS COUNTY, TEXAS

Precinct 1

Report to:

CivilTech Engineering, Inc., a Woolpert Company

HOUSTON, TEXAS

i

TABLE OF CONTENTS

1. INTRODUCTION

2. PROJECT AND SITE DESCRIPTION

3. PURPOSE AND SCOPE OF STUDY

3.1 APPLICABILITY OF REPORT

4. SITE EXPLORATION

4.1 FIELD RECONNAINSSANCE

5. SITE GEOLOGY

5.1 FAULTING

6. FIELD INVESTIGATION

6.1 DRILLING AND SAMPLING METHODS

6.2 DEPTH-TO-WATER OBSERVATIONS

6.3 ENVIRONMENTAL ISSUES

7. LABORATORY TESTING

7.1 CLASSIFICATION TESTS

7.2 STRENGTH TESTS

7.3 DISPERSIVE SOILS CHARACTERIZATION TESTS

8. GENERALIZED SUBSURFACE AND GROUNDWATER CONDITIONS

8.1 SOIL STRATIGRAPHY AND PROPERTIES

8.2 VARIATIONS

9. ENGINEERING ANALYSIS AND RECOMMENDATIONS

9.1 SLOPE STABILITY ANALYSES

9.2 SEEPAGE AND RECHARGE CONSIDERATIONS

10. CONSTRUCTION CONSIDERATIONS

10.1 GROUNDWATER CONTROL DURING EXCAVATION AND OPERATION

10.2 CONSTRUCTION ACCESS GRADING AND TEMPORARY EXCAVATIONS

10.3 TEMPORARY DEWATERING AND DIVERSION

10.4 OTHER TEMPORARY SHALLOW EXCAVATIONS

10.5 SEEDING AND SODDING

ii

LIST OF TABLES

Table 1. Boring Coordinates, Ground Surface Elevation, and Depth Table 2. Laboratory Test Summary Table 3. Dispersive Testing Characterization Test Summary Table 4. Soil Properties Used for Upstream Basin & Channel - Borings B-4 & B-6 Table 5. Soil Properties Used for Downstream Channel – Boring B-9 Table 6. Soil Properties Used for Concrete Channel – Boring B-7 Table 7. Results of Slope Stability Analysis

LIST OF ILLUSTRATIONS

Vicinity Map Plan of Boring 2D Stratigraphy Log of Borings ................................................................................................. 4 to 10 Boring Log Terms and Symbols ...................................................................... 11 & 12 Summary of Laboratory Test Results ............................................................. 13 to 15 Consolidated-Undrained Triaxial Compression Results ................................. 16 to 21 Field Reconnaissance Locations and Pictures……………………………………………. 22 to 38 Slope Stability Test Results ............................................................................ 39 to 53

APPENDICES

Selected Plan Set Provided by CivilTech UC/UU Test Results Input-Output Files

A B C iii

EXECUTIVE SUMMARY

Cibor Geoconsultants (Cibor) performed a geotechnical investigation for the P118-09-00 Detention Basin and Channel Improvement Project (HCFCD Project ID No. P118-09-00-E001).

The study was performed to assist CivilTech Engineering, Inc. (CivilTech) in their design of improvements along Channel P118-09-00 and the construction of a detention basin along the west bank of the channel.

The project is located approximately 2.4 miles east of Interstate I-69 and 3.6 miles north of Interstate I-610 in Harris County, Precinct 1. Channel P118-09-00 is located between Little York Road to the north, Wayside Drive to the east, Homestead Road to the west, and Bigwood Street to the south. The current design layout of detention basin consists of two options, namely:

Section AA’, “wet-bottom” design with a bottom elevation of 51.1 feet (depth of about 7 feet);

and Section BB’, “dry-bottom” design with a bottom elevation of 53 feet (depth of about 5 feet).

A detailed field reconnaissance was completed to evaluate basin slopes and existing conditions along the channel from Little York Road in the north to Bigwood Street in the south of the existing HCFCD Unit No. P118-09-00. Based on our field observations, the existing channel side slopes between Little York Road and Cheeves Drive were grass-lined and generally stable. A majority of the channel side slopes between Langley Road and Gleason Road were also lined with grass. The channel slopes were lined with riprap where the channel changed from flowing eastward to flowing south. The riprap used to line the channel side slopes consists of random concrete pieces and appears not to be within HCFCD specifications. The reach of the channel between Little York Road and Cheeves Drive should be de-silted and the flowline should be regraded/re-established. The channel had rotational failures scattered between Lera Street and Bigwood Street.

Our cursory Geologic Fault (Desktop) Review did not provide evidence for surface faulting intersecting the proximity of the project site. The most proximal previously mapped fault exists approximately 3.6 miles to the southeast of the project location.

Seven borings were drilled along the proposed P118-09-00 channel modifications reach. All seven borings extended to a depth of 20 feet. Three borings were not drilled due to lack of access (R.O.E was not granted by HCC at the time of this study) to the initially proposed boring locations within the basin site.

The laboratory testing program was conducted to classify the subsurface materials at the site and to assess their undrained/drained shear strength properties. The presence of dispersive soils was evaluated by on-site observations of the surficial soils and performing crumb tests on near-surface samples.

In general, the soil stratigraphy consisted primarily of stiff to hard sandy lean clay and medium dense to very dense clayey sand, underlain by stiff to hard fat clay, overlying very stiff to hard sandy lean clay to lean clay with sand.

iv

As we understand from the cross-sections provided by CivilTech, the basin bottom elevation is approximately at an elevation of El. 51 feet. The groundwater elevation in proximity to the basin along the channel ranges from elevations of El. 49.7 feet to El. 48.7 feet. The groundwater will likely be encountered where sandy soils are present during construction, particularly as the stage in adjacent Channel P118-09-00 rises.

Engineering analyses were performed using typical cross-sections, provided by CivilTech; they were used to perform stability analyses for the detention basin, the improved unlined channel banks, and the concrete-lined channel slopes. The conceptual-level design drawings for the detention basin included two cross-sections proposed by CivilTech. Cross-section AA’ consists of a “wet-bottom” design with side slopes at 4(H):1(V) from an elevation of EI. 58 feet (top-of-berm) to El. 52 feet at the toe, two-percent side slopes from El. 52 feet to EI. 51.1 feet, and 4(H):1(V) side slopes from El. 51.1 feet to a basin bottom elevation of El. 43 feet. Cross-section BB’ consists of a “dry-bottom” design with a top of basin elevation of EI. 58 feet and bottom elevation of EI. 53 feet.

The global stability analyses of the selected cross-sections were performed under short-term, rapid drawdown, and long-term loading conditions as required by the latest HCFCD Geotechnical Guidelines. The Slide2 computer software (Version 7.038) with the Bishop method was used for slope stability analysis. Undrained shear strength parameters (undrained cohesion cu and friction angle ɸu) were used to analyze the short-term condition. Effective stress shear strength parameters (drained cohesion c' and friction angle ɸ') were used to analyze the long-term condition. Both effective (c' and ɸ'), and total shear strength parameters (consolidated-undrained cohesion ccu and consolidated-undrained friction angle ɸcu) were used to analyze the rapid drawdown condition.

The results of the global stability analyses for all conditions indicate that the factors of safety for the basin and channel side slope configuration (i.e., 4(H):1(V)) meet the minimum requirements of the HCFCD Geotechnical Guidelines. Also, the factor of safety for the proposed concrete-lined channel with side slope configurations 1(H):1(V) meets the minimum requirements.

Due to the presence of sands/silts encountered at borings B-4 and B-6, specifically at the “wet bottom” design excavation depth, the wet sandy/silty slopes may become unstable during excavation and/or operation of the basin. During the lifetime of the basin, the “wet pool” water level may lower following extended periods of dry weather, which may cause slope instabilities due to seepage flow through the wet sandy/silty side slope soils. In addition, excess pore pressures developed during excavation are likely to result in “flow” of the wet, saturated sand/silt layers.

The control of groundwater during construction will require the use of gravel-lined ditches and sump pumps over much of the excavation in clayey soils. However, use of well points during construction of the basin may be necessary where silty/sandy soils are encountered in, or immediately below, the design bottom of the excavation.

1. INTRODUCTION

CivilTech Engineering, Inc. (CivilTech) was retained by Harris County Flood Control District (HCFCD) for the P118-09-00 Detention Basin and Channel Improvement Project (HCFCD Project ID No. P118-09-00-E001). Our geotechnical investigation was performed in general accordance with our original proposal (Proposal No. 121-041 dated September 17, 2021) and addendum proposal (Proposal No. 121-041-02 dated June 1, 2022). The addendum proposal was prepared to re-locate borings B-7 and B-8, and to add borings B-9 and B-10 as needed by a change in CivilTech’s scope of work. The study was performed under a Professional Service Agreement authorized by Mr. Darrell L. Kaderka, P.E., with CivilTech, dated April 11, 2022, and October 6, 2022, for original proposal and addendum proposal, respectively. Background information was initially provided by Mr. Barry Vanderwalt, P.E., and additional information was provided throughout the course of this study.

This report contains the results of our field exploration and laboratory testing, a description of the observed subsurface conditions encountered at the site, the results of our engineering analyses, and our recommendations for the development of PER-level plans and construction specifications for the proposed P118-09-00 Detention Basin and Channel Improvements.

2. PROJECT AND SITE DESCRIPTION

The project consists of improvements along Channel P118-09-00 and the construction of a detention basin along the west bank of the channel between Little York Road and Langley Road.

The project area is located approximately 2.4 miles east of Interstate I-69 and 3.6 miles north of Interstate I-610 in Harris County, Precinct 1. More specifically, Channel P118-09-00 is located between Little York Road to the north, Wayside Drive to the east, Homestead Road to the west, and Bigwood Street to the south. The proposed channel improvements, which will be approximately 10 feet deep, will run along an approximately 2,700-foot-long reach of Channel P118-09-00, south of Gleason Road and north of Bigwood Street. This channel runs downstream from Little York Road to Bigwood Street. We understand that the current design layout of detention basin under consideration, as provided by CivilTech, consists of two options, namely: Section AA’, “wet-bottom” design with a bottom elevation of 51.1 feet (depth of about 7 feet); and Section BB’, “dry-bottom” design with a bottom elevation of 53 feet (depth of about 5 feet). The proposed cross-sections are included in Appendix A.

3. PURPOSE AND SCOPE OF STUDY

A geotechnical study was performed for the proposed P118-09-00 detention basin and channel improvements to provide information related to the soil conditions, slope stability, and geotechnical design parameters along the project’s alignment. To accomplish these purposes, the following tasks were performed and are discussed in this report:

• Field Investigation o Performed field reconnaissance to assess site conditions and accessibility for geotechnical borings o Drilled, sampled, and logged 7, 20-foot-deep geotechnical borings o Measured short-term and long-term depth-to-water levels in the 7 borings

• Laboratory Testing of Soils including:

o Visual Classification and Water Content o Unit Dry Density o Liquid and Plastic Limits o Percent Passing No. 200 Sieve o Specific Gravity o Crumb Tests o Unconsolidated Compression Triaxial Compression Tests o Unconsolidated-Undrained Triaxial Compression Tests o Consolidated-Undrained Triaxial Compression Tests

• Engineering Analyses and Recommendations o Generalizing the soil conditions, stratigraphy, and groundwater levels o Slope stability analysis for basin slopes under short-term, long-term, and rapid drawdown conditions o Assessing the dispersive nature of soils and recommending methods to mitigate the presence of dispersive soils where encountered during our field reconnaissance o Characterizing the possible cause(s) of any observed/potential channel bank failures observed during the detailed field reconnaissance o Performing engineering analyses pertaining to the channel global stability of selected approved channel configurations o Providing construction considerations to assist in undertaking channel improvements and construction of the detention basin o Preparing this report presenting our geotechnical findings, conclusions, and recommendations

Environmental or biological assessment, wetlands assessment, compliance with state and federal regulatory requirements, and/or environmental analyses were beyond the scope of this study.

3.1. APPLICABILITY OF REPORT

We have prepared this geotechnical engineering report exclusively for CivilTech to assist in their development of the necessary PER-level plans, specifications, and bid documents relating to the P118-09-00 detention basin and channel improvements along the study alignment as described earlier in the section entitled Project and Site Description, and as shown in the Vicinity Map on Plate 1. The explorations and analyses for this study, as well as the conclusions and recommendations in this report, were selected or developed based on our understanding of the scope of the proposed project as described herein. If there are differences in site conditions as we understand them, we should be authorized to review the changes and, if necessary, to modify our conclusions and recommendations.

We have conducted this geotechnical investigation using the standard level of care and diligence normally practiced by recognized engineering firms now performing similar services under similar circumstances. We intend for this report, including all illustrations and appendices, to be used in its entirety. Furthermore, this report should not be construed to represent a warranty of subsurface conditions, nor should this report be used, whether in whole or in part, as a stand-alone construction specification document. Cibor makes no claim or representation concerning any activity or condition falling outside the specified purposes to which this report is directed.

4. SITE EXPLORATION

The project site is located downstream of Little York Road and upstream of Gleason Road in Harris County, Texas, as illustrated in the Vicinity Map on Plate 1. The project includes channel improvements south of Gleason Road and north of Bigwood Street consisting primarily of desilting and removing and replacing the concrete-lined reaches of the channel. The aerial extent of the proposed P118-09-00 U/S Detention Basin is approximately 9.4 acres and will have a storage capacity of 35 acre-feet. The study site currently has an existing channel that is approximately 10 feet deep, with well-vegetated and generally stable side slopes of approximately 3(H):1(V).

4.1. FIELD RECONNAINSSANCE

On February 15, 2023, two Cibor Field Geologists performed a site reconnaissance along the approximately 6000-foot-long channel reach from Little York Road in the north to Bigwood Street in the south of the existing HCFCD Unit No. P118-09-00. The Cibor Field Geologists walked along the entire channel reach. They documented the site conditions by taking numerous photographs and making notes of the site conditions. Twenty-four photographs, representing typical channel conditions, were selected for inclusion in the report. The approximate locations and orientations of the 24 photographs with captions are provided on Plates 22 through 38. The site conditions at the time of the reconnaissance are described by the photograph captions.

The following comments/notes are provided below based on our field observations:

• The depth of Channel P118-09-00 was approximately 10 feet;

• The unlined side slope inclinations of Channel P118-09-00 appeared to be approximately

3(H):1(V), whereas the downstream concrete-lined channel appeared to have 1(H):1(V) slope;

• The existing channel side slopes between Little York Road and Cheeves Drive were grass-lined and generally stable;

• A majority of the channel side slopes between Langley Road and Gleason Road were also lined with grass. The channel slopes were lined with riprap where the channel changed from flowing eastward to flowing south;

• The channel side slopes between Gleason Road and Bigwood Street were intermittently concrete-lined and lined with riprap;

• The riprap used to line the channel side slopes consists of random concrete pieces and appears not to be within HCFCD specifications;

• The reach of the channel between Little York Road and Cheeves Drive should be de-silted and the flow line should be regraded/re-established; and

• The channel had rotational failures scattered between Lera Street and Bigwood Street.

These failures are identified on Plates 23 through 26.

5. SITE GEOLOGY

The site is mapped within the Beaumont Formation, an approximately 100-foot-thick layer of modern sediment composed of predominantly plastic clay and mud, interbedded with silt and sand lentils. The Beaumont Formation has been dated as late Pleistocene to Quaternary Age.

The sediments are interpreted to have been oxidized and desiccated during the Wisconsin Glacial Stage when the sea level was more than 400 feet lower than at present. The Beaumont Formation in the studied area consists of predominantly clay and mud characterized by low permeability, high compressibility, low shear strength, and high plasticity (Geologic Atlas of Texas, 2020).

5.1. FAULTING

Our cursory Geologic Fault (Desktop) Review did not provide evidence for surface faulting intersecting the proximity of the project site. The most proximal previously mapped fault exists approximately 3.6 miles to the southeast of the project location.

6. FIELD INVESTIGATION

Field exploration included advancing seven borings utilizing an ATV-mounted drilling rig to explore the subsurface soil conditions and installing seven temporary standpipes to measure longer-term groundwater levels.

6.1. DRILLING AND SAMPLING METHODS

The purpose of our field exploration was to obtain information on the subsurface stratigraphy and groundwater conditions. We also gathered samples to be tested in the laboratory to evaluate the engineering properties of on-site soils within the study area. Our field exploration activities were planned based on information provided to us by CivilTech and our observations of the existing conditions during the field reconnaissance.

Seven borings (i.e., borings B-4 through B-10) were drilled along the proposed P118-09-00 channel modifications reach. All seven borings extended to a depth of 20 feet. Three borings (i.e., borings B-1 through B-3) were not drilled due to lack of access (R.O.E was not granted by HCC at the time of this study) to the initially proposed boring locations within the basin site.

The locations of all borings are shown in the Plan of Borings on Plate 2. Drilling of the borings was observed, and the recovered soil samples were logged, by a Cibor Field Engineer. Soil samples were obtained continuously to 20-foot-depths, the depth of completion. Temporary standpipes were installed in all seven borings for longer-term groundwater measurements.

Cohesive samples were collected by hydraulically advancing a thin walled (i.e., Shelby) tube approximately 24 inches. The field procedure for the collection of cohesive soils was in general accordance with American Society for Testing and Materials (ASTM) D1587. Granular cohesive soil samples were collected by performing Standard Penetration Tests (SPTs) in general accordance with ASTM D1586. N-values (i.e., blow count number per 12-inch penetration) were recorded for SPTs during field exploration. Estimates of the shear strength of any recovered samples were obtained with a hand-held penetrometer. Representative portions of each sample were properly packaged for transport to our laboratory.

Detailed descriptions of the encountered soils are presented in the Logs of Borings on Plates 4 through 10. A key identifying the terms and symbols used on the boring logs is provided on Plates 11 and 12. We obtained the GPS (Latitude-Longitude) coordinates of the boring location by using our hand-held GPS device. The recorded GPS coordinates of each boring locations were then converted to X-Y coordinates which are based on the NAD 83, Texas South Central FIPS 4204. The boring coordinates, elevation, and depth are presented in Table 1 below.

Table 1. Boring Coordinates, Ground Surface Elevation, and Depth Boring No. Northing Easting Elevation [ft] Depth [ft] Date

B-4 13882153.64 3142130.195 60.10 20 11/23/2022 B-5 13881957.169 3142130.077 59.90 20 11/23/2022 B-6 13881608.343 3142144.298 60.02 20 11/23/2022 B-7 13879246.914 3143516.144 56.68 20 1/5/2023 B-8 13878705.353 3143533.319 54.81 20 1/5/2023 B-9 13879926.591 3143494.589 57.00 20 1/5/2023

B-10 13877953.386 3143569.852 52.06 20 1/5/2023

6.2. DEPTH-TO-WATER OBSERVATIONS

Dry drilling was initially adopted to obtain the first measurement of depth-to-groundwater. Wet rotary drilling techniques were used once groundwater was encountered to allow drilling to proceed to the completion depth of the borings. Short-term depth-to-water measurements were made during drilling. The longer-term groundwater levels of the borings were measured by installing and monitoring seven temporary slotted standpipes. Longer-term groundwater level readings were taken after waiting at least 24 hours after the completion of drilling. Both the short-term and long-term groundwater measurements are presented in the Logs of Borings on Plates 4 through 10.

6.3. ENVIRONMENTAL ISSUES

No environmental issues were encountered during our field exploration campaign. The temporary standpipes were removed after long-term readings were taken and the boreholes were backfilled with a combination of soil cuttings and Holeplug.

7. LABORATORY TESTING

Soil classifications and shear strengths were further evaluated by performing laboratory tests on selected representative samples of the major strata. The laboratory tests were performed in general accordance with ASTM Standards. Specifically, ASTM D2487 was used for the classification of the soils for engineering purposes. A summary of the laboratory tests performed for this investigation is presented in Table 2 below. The table includes the test description, quantity of tests performed, and the testing standard generally followed. A summary table of laboratory test results is presented on Plates 13 through 15.

Table 2. Laboratory Test Summary Laboratory Test Quantity Testing Standard

Water Content 43 ASTM D2216 Atterberg Limits 33 ASTM D4318

Percent Passing No. 200 Sieve 34 ASTM D1140 Unconsolidated-Undrained (UU) 11 ASTM D2850 Unconfined-Compression (UC) 5 ASTM D2166 Consolidated-Undrained (CU) 2 ASTM D4767

Crumb Test 10 ASTM D6572 Specific Gravity 2 ASTM D854

7.1. CLASSIFICATION TESTS

As an aid to visual soil classification, physical properties of the soils were evaluated by classification tests. These tests were conducted in general accordance with ASTM Standards.

These tests consisted of natural moisture content tests (ASTM D2216), Percent Passing No. 200 Sieve tests (ASTM D1140), and Atterberg Limits determinations (ASTM D4318, Method A).

Results of these tests performed on samples collected during drilling are provided in the Logs of Borings on Plates 4 through 10 at the respective sample depths, and in the Summary of Laboratory Test Results on Plates 13 to 15.

7.2. STRENGTH TESTS

The undrained shear strengths of selected soil samples were estimated by performing pocket penetrometer tests in the field as well as unconsolidated-undrained (UU) triaxial compression, and unconfined compression (UC) tests (see Plates 4 through 10). The results of the UU and UC tests are also included in Appendix B. Undrained and drained shear strength parameters were evaluated by performing multi-stage consolidated-undrained (CU) triaxial tests in our laboratory (see Plates 16 through 21). Water contents and dry unit weights were determined as part of the UU, UC, and CU tests.

7.3. DISPERSIVE SOILS CHARACTERIZATION TESTS

The presence of dispersive soils at the site was evaluated by on-site observations of the surficial soils and laboratory testing. Crumb tests were performed on all 7 borings. The results are included in the Logs of Borings on Plates 4 through 10 and are presented in Table 3 below.

The crumb tests were performed in accordance with ASTM D6572 procedures. The dispersive nature of the soils is evaluated based on the turbidity of a distilled water solution when a 1.5-cm diameter or square soil sample is introduced into this solution. Dispersive soils will lead to the solution becoming cloudy, while the solution remains clear with non-dispersive soils. The dispersive nature of these soils is evaluated by the degree of sample reaction with water. Grade 1 indicates no reaction or a non-dispersive soil; Grade 2 indicates slight reaction or a slightly dispersive soil; Grade 3 indicates moderate reaction or a moderately dispersive soil; and Grade

4 indicates strong reaction or a highly dispersive soil.

The results are included in the Logs of Borings on Plates 4 through 10 and are presented in Table 3 below.

Table 3. Dispersive Testing Characterization Test Summary

Location Depth (ft.)

Crumb Grade Hydrometer Percentage Dispersion

Temperature (°C) 2 mins 1 hr. 6 hrs.

B-4 5 1 1 1 - 22.0 B-5 2 1 1 1 - 22.0 B-6 2 1 1 1 - 22.0 B-7 4 1 2 2 - 22.0 B-8 2 1 1 1 - 22.0 B-8 4 1 1 1 - 22.0 B-9 3 1 1 1 - 22.0 B-9 6 1 1 1 - 22.0

B-10 3 1 1 1 - 22.0 B-10 5 1 1 1 - 22.0

The results of our dispersive characterization testing indicated that there were no highly dispersive soils present in any of the borings. Double Hydrometer tests were not necessary in any of the borings. There were slightly dispersive soils encountered in boring B-7 at a depth of 4 feet.

8. GENERALIZED SUBSURFACE AND GROUNDWATER CONDITIONS

8.1. SOIL STRATIGRAPHY AND PROPERTIES

The subsurface soil and groundwater conditions described in this section are based on our field observations and in-situ testing, and the results of laboratory tests performed in the laboratory.

In general, the soil stratigraphy consisted primarily of medium to high plasticity, stiff to hard sandy lean clay and slight to medium plasticity, medium dense to very dense clayey sand, underlain by high to very high plasticity, stiff to hard fat clay overlying medium plasticity, very stiff to hard sandy lean clay to lean clay with sand. The soil stratigraphy defined by the seven borings is presented in the 2D Stratigraphy on Plate 3.

Detention Basin and Northern Reach of Channel. The site soil stratigraphy, based on the three borings drilled along the channel bordering the east side of the proposed detention basin, is shown on Plate 3. The extrapolated soil stratigraphy of the proposed detention basin is derived from borings B-4, B-5, and B-6. The soils in borings B-4 and B-6 primarily consisted of 9-10 ft of medium dense to dense clayey sand, underlain by approximately 10 ft of stiff to very hard lean clay and fat clay soils. Boring B-5 consisted of upper 5 ft of stiff to very stiff sandy lean clay and sandy silty clay underlain by 4 ft of very stiff to hard sandy lean clay, 5 ft of stiff to very stiff sandy fat clay, and 6 ft of very stiff to hard lean clay with sand.

Southern Reach of Channel. The site soil stratigraphy along the southern reach of the channel, based on the four borings drilled between Gleason Road and Bigwood Street, is shown in the 2D Stratigraphy on Plate 3. The soil stratigraphy of the proposed channel improvements is derived from borings B-7, B-8, B-9, and B-10. The near-surface soils in borings B-7 consisted of 6 ft of medium dense to very dense clayey sand overlying 4 ft of very stiff to hard sandy lean clay.

Underlying the sandy lean clay was a 4-foot-layer of stiff to very stiff fat clay and a 6-foot-layer of stiff to hard lean clay with sand. The soils encountered in boring B-8 consisted of 10 feet of stiff to hard sandy lean clay overlying 4 feet of very stiff to hard fat clay. Underlying the fat clay was a 6-foot-layer of very stiff to hard lean clay with sand. The soils encountered in boring B-9 consisted of the upper 10 feet of dense to very dense silty clayey sand and clayey sand.

Underlying the sandy soil was a 10-foot-layer of stiff to hard lean clay with sand and sandy lean clay. The soils in boring B-10 consisted of 11 feet of stiff to very stiff sandy lean clay overlying 5 feet of stiff to hard fat clay underlain by 4 feet of very stiff to hard sandy lean clay.

DEPTH-TO-WATER

Short-term groundwater measurements, taken during drilling, were encountered in boring B-9 at an approximate depth of 6.7 feet. Groundwater was not encountered during drilling in six borings, B-4, B-5, B-6, B-7, B-8, and B-10. Longer-term groundwater measurements, taken from borings B-4 through B-10, and varied from depths of approximately 4.8 to 10.6 feet. Detailed depth-to-water measurements are provided in the Logs of Borings on Plates 4 through 10. Due to the proximity of the borings to channel flow within Channel P118-09-00, we expect that the groundwater levels will reflect the stage in the channel, although the response will generally be more delayed within the more distant area of the basin due to the presence of primarily relatively impervious clayey soils throughout the site.

8.2. VARIATIONS

Our interpretations of soil and depth-to-water conditions, as described in this report, are based on visual observations of soil samples recovered from our borings, laboratory test results, and observations during field reconnaissance, all performed for this study.

We expect significant variations in water levels within the channel slopes as the stage in the adjacent Channel P118-09-00 fluctuates. The changes will be delayed more within the basin due to the presence of relatively clayey soils. We recommend careful observations during construction to assess both soil and groundwater conditions. Should variations from our interpretations be found, we recommend that we be notified and authorized to evaluate what, if any, revisions should be made to our recommendations.

9. ENGINEERING ANALYSIS AND RECOMMENDATIONS

Channel and Basin Recommendations. Typical cross-sections were sent to Cibor via email on February 9, 2023, from Mr. Barry Vanderwalt of CivilTech. The approximate locations for the channel and basin are shown in Plan of Borings on Plate 2. These cross-sections are also included in Appendix A. After further discussions between Cibor and CivilTech, it was agreed to perform stability analyses using two different cross-sections for the detention basin.

Concrete-lined Channel. Additional slope stability analysis was performed to address the failed concrete-lined channel downstream of Gleason Road to upstream of Bigwood Street as shown in Plan of Borings on Plate 2. This request was made by Mr. Barry Vanderwalt of CivilTech in a meeting on August 18, 2023. A typical cross-section for concrete channel was sent to Cibor via email on August 18, 2023, from Mr. Pedro Gonzalez of Woolpert, as presented in Appendix A-5.

9.1. SLOPE STABILITY ANALYSES

Selected Representative Sections. Global slope stability analyses were performed to assess the stability of the proposed detention basin and channel, as drawn in conceptual-level layout sheets provided by CivilTech in an e-mail dated February 9, 2023. The conceptual-level design drawings for the proposed detention basin included the two cross-sections. Cross-section AA’ consists of a “wet-bottom” design with side slopes at 4(H):1(V) from an elevation of El. 58 feet (top-of-berm) to El. 52 feet, two-percent side slopes from El. 52 feet to El. 51.1 feet, and finally 4(H):1(V) side slopes from El. 51.1 feet to the basin bottom elevation of El. 43 feet.