27-Attachment AA - Geotechnical Report.pdf

GEOTECHNICAL INVESTIGATION

LAUDER STORMWATER DETENTION BASIN - PHASE 3

HCFCD PROJECT ID: P500-06-00-E007

KEY MAP NO. 413D AND 414A

Harris County, Texas

SUBMITTED TO

Lockwood, Andrews & Newman, Inc.

2925 Briarpark Drive, Suite 400

Houston, Texas 77042

BY

HVJ ASSOCIATES, INC.

Houston, Texas

August 29, 2025

August 29, 2025

Dr. Yu-Chun Su, Ph.D., P.E., CFM, CPESC, CPSWQ EWRE Technical Director Lockwood, Andrews & Newnam, Inc. (LAN) 3700 West Sam Houston Parkway South, Suite 400 Houston, Texas 77042

Re: Geotechnical Investigation Lauder Stormwater Detention Basin – Phase 2 Harris County, Texas Owner: Harris County Flood Control District (HCFCD) HCFCD Project ID: P500-06-00-E007 Key Map No. 413D and 414A HVJ Report No. HG2210226

Dear Dr. Su:

Submitted herein is the revised draft report of our geotechnical investigation for the above referenced project. The study was conducted in general accordance with our proposal number HG2210226.1 dated January 23, 2025 (Revised January 28, 2025) and is subject to the limitations presented in this report. It has been a pleasure working with you on this project and we appreciate the opportunity to be of service. Please notify us if there are questions or if we may be of further assistance.

Sincerely, HVJ ASSOCIATES, INC.

Texas Firm Registration No. F-000646

DRAFT

Fernando L. Aponte-Rivera, P.E. Babatope James Bada, P.E.

Project Manager Staff Engineer

FA/BJB/AR

This document was released for the purpose of interim review under the authority of Fernando L. Aponte-Rivera, P.E.

130661 on August 29, 2025. It is not to be used for construction, bidding or permit purposes.

TABLE OF CONTENTS

Page

1 EXECUTIVE SUMMARY ........................................................................................................................ I

2 INTRODUCTION

2.1 Project Description

2.2 Scope of Work

3 FIELD INVESTIGATION

3.1 General Site Characterization

3.2 Geotechnical Borings

3.3 Survey Data

3.4 Sampling Methods

3.5 Groundwater Level Measurements

4 LABORATORY TESTING

4.1 General

4.2 Dispersive Tests for Cohesive Soils

4.3 Consolidated Undrained Triaxial Test Results

5 SITE CHARACTERIZATION

5.1 General Geology

5.2 Geologic Faulting

5.3 Soil Stratigraphy

5.4 Groundwater Conditions

6 DETENTION BASIN DESIGN RECOMMENDATIONS

6.1 General

6.2 Method of Analysis and Required Factor of Safety

6.3 Soil Parameters and Water Level

6.4 Results of Slope Stability Analyses

6.5 Recommended Slope Configuration

6.6 Groundwater Control

6.7 Spoil Disposal, Select Fill & Fill Material Excavation from On-Site

7 DISPERSIVE SOILS & EROSION PROTECTION MEASURES FOR NEW BASIN

8 DESIGN CRITERIA AND RECOMMENDATIONS FOR REINFORCED CONCRETE

PIPE (RCP)

8.1 General

8.2 Geotechnical Parameters

8.3 Recommendations for Trenchless Techniques

9 DESIGN REVIEW

10 LIMITATIONS

11 REFERENCES

PLATES

Plate

SITE VICINITY MAP

PLAN OF BORINGS ......................................................................................................................... 2A – 2B

GEOLOGIC MAP

FAULT MAP

LATERAL EARTH PRESSURE DIAGRAMS ............................................................................. 5A – 5B

LINER LOADS

APPENDICES

Appendix

BORING LOGS, PROFILES, AND KEY TO TERMS & SYMBOLS ................................................ A

SUMMARY OF LABORATORY TEST RESULTS ................................................................................. B

PIEZOMETER INSTALLATION RECORDS ........................................................................................ C

CRUMB TEST RESULTS .............................................................................................................................. D

DOUBLE HYDROMETER TEST RESULTS .......................................................................................... E

CONSOLIDATED UNDRAINED TRIAXIAL TEST RESULTS .......................................................F

PROPOSED BASIN PLAN AND PROFILES ......................................................................................... G

EVALUATION OF MOBILIZED SHEAR STRENGTH PARAMETERS ....................................... H

SLOPE STABILITY ANALYSES – PROPOSED SLOPES .................................................................... I

SLOPE STABILITY ANALYSES – OUTPUT REPORTS....................................................................... J

TUNNELMAN’S CLASSIFICATION ........................................................................................................ K

PICTURES FROM HVJ’S SITE VISIT ....................................................................................................... L

REFERENCES................................................................................................................................................ M i

1 EXECUTIVE SUMMARY

HVJ Associates, Inc. (HVJ) was retained by Lockwood, Andrews & Newman, Inc. (LAN) to perform a geotechnical study for the proposed Lauder Stormwater Detention Basin Phase 2 Project in Harris County, Texas (Key Map No. 413D and 414A). The site is bordered by undeveloped land and Greens Bayou to the north, Channel P500-06-00-E006 to the south, and residential development to the west. A site vicinity map is shown in Plate 1.

The overall detention basin area is divided into four (4) compartments. This phase of the project involves the expansion of Compartment 2 towards north and west. The depth of the detention basin is expected to be about 16 feet. Based on the information provided by LAN, we understand that the area of Compartment 2 for the new detention basin is planned to be approximately 44 acres. It is anticipated that Compartment 2 will consist of a dry detention basin with a maximum depth of 16 feet and 3H:1V side slopes.

The project also includes constructing a 72-inch diameter and 270 feet long Reinforced Concrete Pipe (RCP) structure to connect Compartments 1 and 2. We understand that the pipe will be installed below the detention basin banks and under an existing Sunoco gas line using trenchless methods such as tunneling or pipe jacking. Based on the drawings provided by LAN, the pipe is planned to be placed from approximately station 3+30 to 6+10, with an invert elevation of El. 45’.

The purpose of this study is to evaluate the stability of the proposed side slopes within Compartment 2 and provide design and construction recommendations for the proposed RCP structure. The information and recommendations provided in this report will supplement HVJ’s Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020, prepared for the previous phase of the project.

The subsurface stratigraphy at the project site was investigated by drilling seven (7) soil borings, labeled as B-11, B-12, B-14, B-15 (PZ-5), B-16, T-1 (PZ-4) and T-2 to a depth of 30 feet below the existing grade. Boring B-13, originally part of the proposed borings, was not drilled due to access constraints. The borings will be supplemented by three (3) other previously drilled borings extending to a depth of 40 feet labeled as B-1(PZ-1), B-2, and B-10 and included in HVJ’s Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020.

Based on the subsurface conditions revealed by the soil borings performed for the project site, the findings and recommendations of this report are summarized below:

1. Predominantly soft to hard cohesive soils (CL-ML, CL, and CH) with varying amounts of sand and medium dense cohesionless soils (ML) generally encountered from the surface to the termination depth across borings. Undocumented fill soils were encountered to a depth of about 1.5 feet below the existing grade in boring T-1 (PZ-4). The generalized soil stratigraphy observed in our borings is presented in the boring logs in Appendix A of this report.

2. According to the USGS, the project site is located within the Beaumont and Lissie geologic formations. The Beaumont formation generally consists of clay, silt and sand. Includes mainly stream channel, point-bar, natural levee, backswamp and to a lesser extent coastal marsh and mud flat deposition. The Lissie formation is heterogeneous, containing ii interbedded layers of clay, sand, and silt. It was deposited in mid-Pleistocene times in shallow coastal river channels and flood plains.

3. A literature review was conducted to identify the documented faults in the project area. The primary objective of this review was to evaluate available information from published and open file reports. Based on Sachin D. Shah and Jennifer Lanning-Rush, 2005, the project site is located about 4 miles southeast of Hardy fault and approximately 3.5 miles southwest of Lee fault. Based on literature, the project site is not anticipated to be impacted by an existing fault. Therefore, a detailed fault study is not recommended for this project. A fault map is presented in Plate 4.

4. Groundwater was observed in borings B-14, B-15 (PZ-5), T-1 (PZ-4), and T-2 at depths that ranged from 10 feet (El. 53.90’) to 30 feet (El. 35.42’) below the existing grade during drilling operations. The 24-hour water level readings varied from 3.58 feet (El. 62.89’) to 18.08 feet (El. 46.53’). Two piezometers were installed in borings T-1 (PZ-4) and B-15 (PZ-5) to record water level readings at 24 hours and 30 days after the piezometer installation.

Piezometer water level readings in boring T-1 (PZ-4) were measured at 9 feet (El. 54.90’) and at 8.42 feet (El. 55.48’) for 24 hours and 30 days after installation, respectively.

Piezometer water level readings in boring B-15 (PZ-5) were measured at 8.5 feet (El. 56.92’) and at 6.33 feet (El. 59.09’) for 24 hours and 30 days after installation, respectively.

It should also be noted that groundwater levels determined during drilling may not accurately reflect the true groundwater conditions and therefore should only be considered as approximate. Groundwater levels measured in open standpipe piezometer are more accurate. However, these readings will fluctuate seasonally and in response to rainfall.

5. Based on the information provided by LAN and the latest cross section drawings provided to us, we understand that the detention basin within Compartment 2 is planned to be constructed as a dry basin with side slopes of 3H:1V. HVJ evaluated the stability of the side slopes for cross sections A-A, C-C, D-D and E-E. The slope stability analysis was performed considering short term, long term, and rapid drawdown loading conditions for both global and local failures. The results of our analyses indicate that the proposed detention basin configuration meets the HCFCD required factor of safety for slope stability for the analyzed cross sections. The slope stability outputs and reports are presented in Appendix I and J respectively. The global stability analysis results are summarized in Table 1-1.

Table 1-1 – Global Slope Stability Analysis Results (Proposed Configuration)

Location (Cross

Section)

Compartment Bank

Boring Used

Factor of Safety

Short-Term (Required FOS

= 1.30)

Long-Term (Required FOS

= 1.50)

Rapid Drawdown (Required FOS = 1.25)

Effective Stress

Parameters

Total Stress Parameters

A-A

West B-12 2.23 2.00 1.61 1.38

East B-14 1.34 1.96 1.45 1.27 iii

(Cross

Section)

Compartment Bank

Boring Used

Factor of Safety

Short-Term (Required FOS

= 1.30)

Long-Term (Required FOS

= 1.50)

Rapid Drawdown (Required FOS = 1.25)

Effective Stress

Parameters

Total Stress Parameters

C-C

South B-16 3.05 2.06 1.39 1.51

North B-11 1.85 1.40 1.50 1.61

D-D

South

T-1 (PZ-

4)

2.49 1.73 1.48 1.45

North B-15

(PZ-5)

4.39 1.72 1.54 1.41

E-E

South

T-2

2.18 1.93 1.41 1.29

North 2.60 2.18 1.63 1.39

6. Dispersive soils are clays that disintegrate in the presence of relatively pure water as a result of their chemical composition. The results of crumb tests revealed the presence of highly dispersive cohesive soils in borings B-16 and T-1 (PZ-4). Double hydrometer tests performed on samples obtained from borings B-16 and T-1 (PZ-4) showed no dispersion and intermediate dispersion, respectively.

Due to the presence of dispersive soils, water should not be allowed to accumulate at the top of the detention basin slopes. We recommend installing backslope interceptors/swale systems at close intervals to quickly dissipate water from the ground surface. The interceptor spacing should not be greater than 400 feet at locations where dispersive clays are encountered as stated in Section 11.1 of HCFCD Policy, Criteria and Procedure Manual.

7. Trenchless construction recommendations for the proposed RCP structure are provided in Section 8.

Please note that this executive summary does not fully narrate our findings and opinions. Detailed findings and opinions are presented in the subsequent sections of the report.

2 INTRODUCTION

2.1 Project Description

HVJ Associates, Inc. (HVJ) was retained by Lockwood, Andrews & Newman, Inc. (LAN) to perform a geotechnical study for the proposed Lauder Stormwater Detention Basin Phase 2 Project in Harris County, Texas (Key Map No. 413D and 414A). The project site is surrounded by undeveloped land and Greens Bayou to the north, Channel P500-06-00-E006 to the south, and residential housing development to the west of the planned detention basin. A site vicinity map showing the project location is presented in Plate 1 of the report.

The overall detention basin area is divided into four (4) compartments: Compartment 1, Compartment 2, Compartment 3 and Compartment 4. This phase of the project involves the expansion of Compartment 2 towards north. We understand that additional right-of-way of about 3 acres was acquired recently to expand the basin within Compartment 2. The depth of the detention basin is expected to be about 16 feet. Based on the information provided by LAN, we understand that the area of Compartment 2 for the new detention basin is planned to be approximately 44 acres.

It is anticipated that Compartment 2 will consist of a dry detention basin with a maximum depth of 16 feet and 3H:1V side slopes.

The project also involves the construction of a Reinforced Concrete Pipe (RCP) structure to connect Compartment 1 and Compartment 2 within the Lauder Storm Water Detention Basin. We understand that the proposed RCP structure is planned to be installed underneath the detention basin banks and below an existing Sunoco gas line using trenchless construction methods. We also understand that the trenchless construction methods may consist of either tunneling or pipe jacking techniques. Based on the drawings provided to us, one 72-inch RCP structure is planned to be placed from approximately station 3+30 to 6+10, with an approximate length of 270 linear feet.

Based on the drawings provided by LAN, the invert level of the proposed RCP structure is at El.

45’.

The purpose of this study is to evaluate the stability of the proposed side slopes within Compartment 2’s detention basin area and provide design and construction recommendations for the proposed RCP structure. The information and recommendations provided in this report will supplement HVJ’s Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020, prepared for the previous phase of the project.

The subsurface stratigraphy at the project site was investigated by drilling seven (7) soil borings, labeled as B-11, B-12, B-14, B-15 (PZ-5), B-16, T-1 (PZ-4) and T-2 to a depth of 30 feet below the existing grade. Boring B-13, originally part of the proposed borings, was not drilled due to access constraints. The borings will be supplemented by three (3) other previously drilled borings extending to a depth of 40 feet labeled as B-1(PZ-1), B-2, and B-10 and included in HVJ’s Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020.

This geotechnical study was performed in accordance with the Harris County Flood Control District (HCFCD) Policy Criteria, and Procedure Manual dated July 2019 and HCFCD Geotechnical Investigation Guidelines dated December 2021.

2.2 Scope of Work

The objectives of this study were to gather information on subsurface conditions at the project site to provide design and construction recommendations for the proposed detention basin and RCP structure. The objectives were accomplished by:

Drilling seven (7) soil borings, labeled as B-11, B-12, B-14, B-15 (PZ-5), B-16, T-1 (PZ-4) and T-2 to a depth of 30 feet below the existing ground surface. Borings B-11, B-12, B-14, B-15 (PZ-5) and B-16 were performed within the proposed detention basin area. Borings T- 1 (PZ-4) and T-2 were drilled within the proposed RCP structure location. The soil borings were performed to determine soil stratigraphy and to obtain samples for laboratory tests;

Installing two piezometers to gain an understanding of the groundwater conditions at the site;

Performing laboratory tests to determine physical and engineering characteristics of the soils;

Performing engineering analyses to develop design recommendations for the proposed detention basin.

Performing engineering analyses to develop design guidelines and recommendations for the proposed RCP structure.

Subsequent sections of this report contain descriptions of the field exploration, laboratory testing program, general subsurface conditions, design recommendations and construction considerations.

3 FIELD INVESTIGATION

3.1 General Site Characterization

A site visit was conducted by HVJ to observe the existing features at the project site. The project site is located south of the intersection of Miranda Street and Maximilian Street in Harris County, Texas.

The site is bordered by undeveloped land and Greens Bayou to the north, Channel P500-06-00- E006 to the south, and residential development to the west. The project area is highly vegetated with unpaved roads allowing traffic from north to south and east to west. In addition, a pipeline easement can be seen in the southeast portion of the site near borings T-1 and T-2. An existing detention basin is located south of the project site. Existing features like outfall structures and pilot channels can be observed within the existing detention basin area. Pictures obtained from the site visit can be found in Appendix L.

3.2 Geotechnical Borings

Subsurface conditions were investigated by drilling seven (7) soil borings, labeled as B-11, B-12, B- 14, B-15 (PZ-5), B-16, T-1 (PZ-4) and T-2 to a depth of 30 feet below the existing ground surface.

Borings B-11, B-12, B-14, B-15 (PZ-5) and B-16 were performed within the proposed detention basin area. Borings T-1 (PZ-4) and T-2 were drilled within the proposed RCP structure location.

Borings were drilled with ATV mounted drilling equipment in accordance with HCFCD “Geotechnical Investigation Guidelines”. Drilling was performed using dry auger drilling techniques to the termination depth of borings B-11, B-12, B-14, B-15 (PZ-5), B-16 and T-2. Wet rotary was conducted below a depth of 10 feet to the termination depth of boring T-1 (PZ-4). Borings were backfilled with cement grout by the tremie method. Two piezometers were installed in borings T-1 (PZ-4) and B-15 (PZ-5) to monitor long term groundwater levels. The approximate boring and piezometer locations are presented in the Plan of Borings, Plate 2.

During our investigation we did not observe any odor or contamination to suggest any environmental concerns.

3.3 Survey Data

Survey information for the drilled borings has been provided by LAN. The table below includes Northing, Easting, and surface elevation values for each boring location.

Table 3-1 – Survey Data

Boring Coordinates (Grid), Feet1 Elevation2

(Feet) Northing Easting

Recently Drilled Borings3

B-11 13901545.91 3124345.69 66.47

B-12 13901206.28 3124254.40 65.85

B-14 13900742.57 3126413.57 64.61

B-15 (PZ-5) 13901072.21 3124796.63 65.42

B-16 13900612.63 3124389.81 65.62

T-1 (PZ-4) 13900512.72 3125453.46 63.90

T-2 13900686.74 3126268.34 63.50

Previously Drilled Borings4

B-1 (PZ-1) 13900927.66 3124848.20 65.41

B-2 13900278.62 3124025.37 67.38

B-10 13901203.75 3126139.46 65.73

1. Coordinates shown are referenced to State Plane Coordinate System.

2. Elevations are referenced to North American Vertical Datum (NAVD) 88, 2001 Adjustment.

3. Borings drilled during the current investigation.

4. Previous borings from HVJ Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020.

3.4 Sampling Methods

Soil samples were obtained continuously to a depth of 20 feet and at 5-foot intervals thereafter to the termination depth of borings. Cohesive soil samples were generally obtained with a three-inch thin-walled (Shelby) tube sampler in general accordance with ASTM D-1587 standard. Each sample was extruded in the field, visually classified, and its strength estimated with a pocket penetrometer.

Cohesionless soil samples were obtained with the split spoon sampler in accordance with ASTM D- 1586 standard. Suitable portions of each sample were sealed and packaged for transportation to our laboratory.

3.5 Groundwater Level Measurements

Groundwater levels were measured during drilling—initially encountered, after 15 minutes, and approximately 24 hours later. Additional readings at 24 hours and 30 days were taken from piezometers PZ-4 and PZ-5, installed in borings T-1 and B-15, respectively. Measurements are presented in Section 5.4.

4 LABORATORY TESTING

4.1 General

Selected soil samples were tested in the laboratory to determine physical and engineering properties applicable to the site. Tests were performed according to the relevant ASTM Standards. The tests performed consisted of moisture content measurement, percent finer than No. 200 sieve, Atterberg limits, Unconfined Compression (UC), Unconsolidated Undrained (UU) triaxial, Consolidated Undrained (CU) triaxial, Crumb, and Double Hydrometer tests.

The Atterberg Limits and percent passing number 200 sieve tests were utilized to verify field classification by the Unified Soils Classification System (ASTM D2487). Crumb and double hydrometer tests were conducted to identify the presence of dispersive soils. Compression tests were performed to obtain the undrained shear strength of the clay soils. The type and number of tests performed for this investigation are summarized below:

Table 4-1 – Type and Number of Laboratory Tests

Laboratory Test Name Number of Tests

Moisture Content (ASTM D2216) 68

Atterberg Limits (ASTM D4318) 37

Percent Finer than No. 200 Sieve (ASTM D1140) 37

Unconfined Compression (ASTM D2166) 9

Unconsolidated Undrained (ASTM D2850) 20

Consolidated Undrained (ASTM D4767) 1

Crumb Test (ASTM D6572) 7

Double Hydrometer Test (ASTM D4221) 2

The laboratory test results are presented on the boring logs in Appendix A. A summary of laboratory test results, except for the crumb and double hydrometer tests is provided in Appendix B.

4.2 Dispersive Tests for Cohesive Soils

Crumb and double hydrometer tests were conducted to identify the presence of dispersive soils at the project location. Crumb tests were initially assigned on selected soil samples. Double hydrometer tests were then assigned on soil samples that exhibited dispersion potential based on the results from the crumb tests. The results of crumb and double hydrometer tests indicate that the cohesive soils at the proposed detention basin location varies from non-dispersive to highly dispersive. The test results are summarized in Table 4-2. Laboratory test results of crumb and double hydrometer tests are presented in Appendix D and Appendix E, respectively.

Table 4-2 – Crumb and Double Hydrometer Test Results

Sample

Depth, Feet Soil Description Crumb Test

Double Hydrometer Test (Percent Dispersion)

B-11 6-8 Sandy Lean Clay Non-dispersive Not Performed

B-12 6-8 Lean Clay Non-dispersive Not Performed

B-14 4-6 Lean Clay w/ Sand Non-dispersive Not Performed

B-15 (PZ-5) 4-6 Lean Clay w/ Sand Non-dispersive Not Performed

B-16 4-6 Lean Clay w/ Sand Highly Dispersive Non-dispersive (0%)

T-1 (PZ-4) 2-4 Lean Clay w/ Sand Highly Dispersive Intermediate Dispersion

(49.5%)

T-2 4-6 Lean Clay Non-dispersive Not Performed

According to ASTM D4221, dispersiveness from double hydrometer is categorized as non-dispersive, intermediate, and highly dispersive based on the percent dispersion of less than 30 percent, 30 to 50 percent and more than 50 percent, respectively.

4.3 Consolidated Undrained Triaxial Test Results

The results of the two consolidated undrained (CU) triaxial compression tests previously performed during the geotechnical investigation documented in HVJ’s Final Geotechnical Report No.

HG101766.1.4 dated August 28, 2020, were supplemented with one CU test performed for this project. The results of the CU tests are provided in Table 4-3.

Table 4-3 – Consolidated Undrained Triaxial Test Results

Boring (Depth) Soil

Description Plasticity

Index

Total Stress Parameters

Effective Stress Parameters Failure

Criteria ccu, psf cu, degrees c’, psf ’, degrees

B-15 (PZ-5) (4’-6’) Lean Clay w/Sand

21 125 16.8 151 25.7 Effective Stress Obliquity

B-3 (8’-10’) Fat Clay 311 1491 21.11 941 29.01 Effective Stress Obliquity

B-19 (8’-14’) Lean Clay w/ Sand

211 1601 24.71 3131 27.01 Effective Stress Obliquity

1. Consolidated undrained (CU) triaxial compression tests previously performed during the geotechnical investigation documented in HVJ’s Final Geotechnical Report No. HG101766.1.4 dated August 28, 2020

Where:

ccu: Consolidated Undrained Cohesion ’cu: Consolidated Undrained Friction Angle c’: Consolidated Drained Cohesion ’: Consolidated Drained Friction Angle

5 SITE CHARACTERIZATION

5.1 General Geology

There are two major surface geological formations that exist in the Houston area: the Beaumont formation and the Lissie formation. The Beaumont formation is a relatively younger formation generally found to the southeast of the Lissie formation. The Beaumont formation dips southeastward and extends beneath beach sand and waters of the Gulf of Mexico as far as the continental shelf. The project site is primarily located within the Beaumont formation, with a small portion on the west side in the Lissie formation. A geologic map is presented on Plate 3.

The Beaumont formation was deposited on land near sea level in flat river deltas and in inter-delta regions. Soil deposition occurred in freshwater streams and in flood plains (as backwater marsh and natural levees). The courses of major streams and deltaic tributaries changed frequently during the period of deposition, generating within the Beaumont clay a complex stratification of sand, silt and clay deposits. Frequently, stream courses were diverted significant distances from a given point in a backwater marsh, and the water overlying the soil would evaporate since it was cut off from a drainage path. Such water, which would be highly alkaline, would precipitate large nodules of calcium carbonate (calcareous nodules) throughout the surface of evaporation. With the coming of the Second Wisconsin Ice Age, the nearby sea withdrew, leaving the formation several hundred feet above sea level and permitting the soil to desiccate. The process of desiccation compressed the clays in the formation such that they became significantly overconsolidated to a large depth. In addition to preconsolidating the soil, the process of desiccation, together with the later rewetting, produced a network of fissures and slickensides that are now closed but which represent potential planes of weakness in the soil.

The upper Lissie formation is sometimes denoted as the Montgomery formation. The upper Lissie formation is heterogeneous, containing interbedded layers of clay, sand and silt. It was deposited in mid-Pleistocene times in shallow coastal river channels and flood plains. The clay present in the formation has been preconsolidated by a process of desiccation. Numerous wetting and drying cycles have produced a network of randomly oriented and closely spaced joints, which are sometimes slickensided, that is, have a shiny appearance when exposed. The joint pattern strongly influences the engineering behavior of the soil. The sand layers vary in compactness from loose to very dense, and in thickness from a fraction of an inch to many feet due to an irregular depositional environment. Sands are generally subrounded to subangular and vary from coarse to very fine, are poorly graded, and often contain significant amounts of silt-sized particles in the sand matrix.

5.2 Geologic Faulting

The tectonic history of the Texas Gulf Coast includes a relatively stable depositional cycle since the Cretaceous Period (about 65 million years). During this period the area was subjected to deposition of clays, silts, and sands resulting in over 30 thousand feet of sedimentary rocks. Underlying this clastic sequence are salt formations, which have migrated upwards to produce the typical salt dome features associated with the Texas Gulf Coast. In conjunction with salt movement, dewatering and compaction of some of the deeper sediments in the basin have resulted in the development of growth faults.

A literature review was conducted to identify the documented faults in the project area (Sachin D.

Shah and Jennifer Lanning-Rush, 2005). The primary objective of this review was to evaluate available information from published and open file reports. Based on our review, the project site is located about 4 miles southeast of Hardy fault and approximately 3.5 miles southwest of Lee fault.

We anticipate that faulting will not affect the project site. Therefore, a detailed fault study is not recommended for this project. A fault map is presented on Plate 4.

5.3 Soil Stratigraphy

Our interpretation of soil and groundwater conditions at the project site is based on information obtained at the boring locations only. This information has been used as the basis for our conclusions and recommendations. Significant variations at areas not explored by the project borings may require re-evaluation of our findings and conclusions.

A generalized summary of the soil profile encountered at the project site indicates predominantly soft to hard cohesive soils (CL-ML, CL, and CH) with varying amounts of sand. Medium dense cohesionless soils (ML) were encountered in boring T-2 at 23 feet below the existing ground surface.

Undocumented fill soils were encountered to a depth of about 1.5 feet below the existing grade in boring T-1. A summary of cohesionless soil layers encountered along the RCP structures alignment is presented in Table 5-1 below. The detailed subsurface conditions observed in our borings are presented in the boring logs in Appendix A of this report. In addition, a profile showing the subsurface conditions obtained from the borings is also included in Appendix A.

Table 5-1 – Location of Cohesionless Soils

Approx. Depth below

Surface, Feet Material

T-2 23-30 Medium Dense Sandy Silt (ML)

5.4 Groundwater Conditions

Groundwater level was measured in the borings during drilling operations. We installed two piezometers at locations suggested by HCFCD to monitor long term groundwater readings. The groundwater levels were measured after 24 hours and 30 days of piezometer installation. The table below summarizes the water readings.

Table 5-2 – Groundwater Measurements

Groundwater Depth (Elevation), Feet

During Drilling After 15 minutes After 24 hours After 30 days

B-11 --1 --1 3.58 (EL. 62.89) --2

B-12 --1 --1 --1 --1

B-14 25.0 (EL. 39.61) 19.75 (El. 44.86) 18.08 (EL. 46.53) --2

B-15 (PZ-5) 30.0 (El. 35.42) 28.67 (El. 36.75) 8.5 (El. 56.92) 6.33 (EL. 59.09)

B-16 --1 --1 --1 --1

T-1 (PZ-4) 10.0 (EL. 53.9) 5.5 (El. 62.52) 9.0 (EL. 54.90) 8.42 (EL. 55.48)

T-2 25.0 (El. 38.5) 15.42 (El. 48.33) --2 --2

1. Groundwater was not encountered.

2. Not measured.

Two piezometers were installed in borings T-1 (PZ-4) and B-15 (PZ-5) to record water level readings at 24 hours and 30 days after the piezometer installation. Piezometer water level readings in boring T-1 (PZ-4) were measured at 9 feet (El. 54.90’) and at 8.42 feet (El. 55.48’) for 24 hours and 30 days after installation, respectively. Piezometer water level readings in boring B-15 (PZ-5) were measured at 8.5 feet (El. 56.92’) and at 6.33 feet (El. 59.09’) for 24 hours and 30 days after installation, respectively.

We believe the 24-hour and 30-day water level readings are considered more representative of the groundwater levels than the “during drilling” readings. The reason for this is that it is expected that the groundwater level stabilizes with time.

It should be noted that groundwater levels determined during drilling may not accurately reflect true groundwater conditions and should therefore be considered approximate. Groundwater levels measured in open standpipe piezometers are generally more accurate; however, these readings can fluctuate seasonally and in response to rainfall. The reason for the rise in groundwater levels recorded after 24 hours is difficult to determine. Changes in water levels are influenced by various factors, such as the permeability of on-site soils, rain events, seasonal variations (e.g., winter or summer), and fissures within the soils that may create preferential flow paths for groundwater. In this case, if possible, we recommend leaving the piezometers in place until just before construction begins to allow for continued monitoring of groundwater levels. Piezometer readings can be taken approximately every 30 days to gain a clearer understanding of the “true” groundwater level. It is expected that water levels will stabilize over time. However, it is also important to closely monitor rain events and correlate them with the measured water levels.

6 DETENTION BASIN DESIGN RECOMMENDATIONS

6.1 General

The overall detention basin area is divided into four (4) compartments: Compartment 1, Compartment 2, Compartment 3 and Compartment 4. This phase of the project involves the expansion of Compartment 2 towards north and west. We understand that additional right-of-way of about 3 acres was acquired recently to expand the basin within Compartment 2. Based on the information provided by LAN, we understand that the area of Compartment 2 for the new detention basin is planned to be approximately 44 acres. It is anticipated that Compartment 2 will consist of a dry detention basin with a maximum depth of 16 feet and 3H:1V side slopes.

Details from the slope stability analysis are provided in the subsequent sections of this report.

6.2 Method of Analysis and Required Factor of Safety

Slope stability analyses were conducted using the 2021 R2 version of slope stability program SLOPE/W using the Morgenstern-Price method. The program calculates the factor of safety against slope failure using a two-dimensional limit equilibrium method.

The factor of safety represents the calculated resisting forces and moments divided by the calculated driving forces and moments of the various potential failure surfaces analyzed. These forces and moments are based on the estimated unit weights and shear strengths of the various soils in the slope profile. Accordingly, a factor of safety of 1.0 indicates impending failure. The larger the factor of safety is above 1.0, the lower the risk is that the slope will fail. Practically, and in due consideration of the variables and unknowns involved, the risk cannot be reduced to zero. The goal is to reduce the risk of slope failure to a reasonable and acceptable level, with consideration of the consequences of failure.

The HCFCD requires that slope stability analyses be performed for the short term (end of construction), long-term and rapid drawdown loading conditions. The proposed slope should have safety factors of 1.3, 1.5 and 1.25 for short term, long-term and rapid drawdown loading conditions, respectively.

6.3 Soil Parameters and Water Level

The soil parameters used in each case were estimated based on the field and laboratory data developed for this study and are presented in Table 6-1. Unconfined Compression (UC) and Unconsolidated Undrained (UU) triaxial tests were utilized to obtain end of construction shear strength parameters. Consolidated Undrained (CU) triaxial test results with pore water pressure measurements were utilized to estimate total and effective stress shear strength parameters of cohesive soils. For cohesionless soils, the friction angles were determined based on correlations with SPT results (Peck, R.B., Hanson, W.E., and Thornburn, T.H. (1974) Foundation Engineering, John Wiley & Sons, Inc., p. 87).

Short-Term: The short-term case models the initial undrained condition of the soil. Undrained shear strength parameters were used for this analysis. The piezometric level was considered at the top of the bank, along the slope and bottom of the basin. A surcharge of 500 psf was assumed along the bank for construction equipment loading.

Long-Term: The long-term design case represents steady state piezometric and stress conditions.

When a slope is excavated, altered stress conditions create pore pressure changes within the slope and the undrained strength of the bank soils is mobilized. With time, the soil pore pressures adjust to the imposed stress and piezometric conditions, and the bank soils rely on their available strength for long term stability. The piezometric level was considered at the bottom of the basin. A surcharge of 250 psf was assumed along the bank to account for maintenance equipment traffic.

Rapid Drawdown: The rapid drawdown case models the condition where high floodwater saturates and piezometrically “loads” the slope, and then quickly recedes leaving a large unbalanced piezometric head in the bank slope. This unbalanced force increases the shear stresses in the bank soils. In this analysis, we have assumed the water level along the bank. We performed two separate analysis considering effective and total stress parameters for this loading condition. Surcharge was not considered for rapid drawdown analyses per HCFCD guidelines.

Heavily overconsolidated high plasticity clays that are exposed at the surface swell when exposed to water and shrink when dried. Alternate cycles of this process cause the strength of these clays to reduce over time after initial construction. Hence, the slopes must be designed considering the reduction in soil strength when high plasticity soils are encountered within the zone of desiccation (typically to depths of 7 to 10 feet). Based on the subsurface conditions observed in the borings within the zone of desiccation, medium to high plasticity clay soils were encountered within this zone. Therefore, the reduction in the soil’s shear strength was considered for our slope stability analysis.

Table 6-1 presents the soil parameters used for slope stability analyses.

Table 6-1 – Soil Parameters for Slope Stability Analyses

Location Boring Depth

(Elevation), Feet Soil

Description t, pcf

ST

Cu, psf

RDD/LT

'peak, deg c’peak, psf peak, deg c peak, psf

East Bank (Cross

Section A-A) B-14

0-6 (64.6-58.6) Lean Clay w/Sand

125 500 25 150 22 125

6-10 (58.6-54.6) Lean Clay I 125 2000 25 150 22 125

10-23 (54.6-41.6) Sandy Lean Clay/Lean

Clay II 125 500 25 150 22 125

23-30 (41.6-34.6) Fat Clay 125 500 22 94 21 149

West Bank (Cross

Section A-A) B-12

0-6 (65.9-59.9) Lean Clay w/Sand I

125 1000 25 150 22 125

6-12 (59.9-53.9) Lean Clay I 125 2500 25 150 22 125

12-23 (53.9-42.9)

Lean Clay with Sand

II/Lean Clay

II

125 500 25 150 22 125

23-30 (42.9-35.9) Fat Clay 125 2500 22 94 21 149

North Bank (Cross

Section C-C) B-11

0-10 (66.5-56.5) Sandy Lean

Clay 125 1200 25 150 24.7 125

10-14(56.5-52.5) Sandy Fat

Clay 125 500 22 94 21 149

14-30 (52.5-36.5) Fat Clay 125 500 22 94 21 149

South Bank (Cross

Section C-C) B-16

0-6 (65.6-59.6) Lean Clay w/Sand I

125 500 25 150 22 125

6-10 (59.6-55.6) Lean Clay I 125 2000 25 150 22 125

10-18 (55.6-47.6) Fat Clay 125 1000 22 94 21 149

18-23 (47.6-42.6) Lean Clay II 125 500 25 150 22 125

23-30 (42.6-35.6) Lean Clay w/Sand II

125 1500 25 150 22 125

North Bank (Cross

Section D-D)

B-15 (PZ-

5)

0-6 (65.4-59.4) Lean Clay w/Sand I

125 500 25 150 22 125

6-12 (65.4-53.4) Lean Clay w/Sand II

125 1000 25 150 22 125

Location Boring Depth

(Elevation), Feet Soil

Description t, pcf

ST

Cu, psf

RDD/LT

'peak, deg c’peak, psf peak, deg c peak, psf

12-23 (53.4-42.4) Lean Clay 125 2500 25 150 22 125

23-30 (42.4-35.4) Fat Clay 125 2500 22 94 21 149

South Bank (Cross

Section D-D)

T-1 (PZ-4)

0-10 (63.9-53.9) Lean Clay w/Sand I

125 500 25 150 22 125

10-16 (53.9-47.9) Lean Clay w/Sand II

125 2000 25 150 22 125

16-23 (47.9-40.9) Fat Clay 125 1800 22 94 21 149

23-30 (40.9-33.9) Sandy Silty

Clay 125 2000 25 150 22 125

North and South Bank (Basin Cross Section E-E)

T-2

0-5.5 (63.5-58) Lean Clay 125 1900 25 150 22 125

5.5-8.5 (58-55) Lean Clay w/Sand I

125 2500 25 150 22 125

8.5-13.5 (55-50) Lean Clay w/Sand II

125 500 25 150 22 125

13.5-17.5 (50-46) Sandy Lean Clay

125 1500 25 150 22 125

17.5-22.5 (46-41) Silty Clay w/Sand

125 1000 25 150 22 125

22.5-30 (41-33.5) Sandy Silt 125 0 30 0 22 125

1. Softened shear strength parameters for exposed clay soils with plasticity index (PI) greater than 20 are as follows: For lean clay (CL), c = c’ = 101 psf, = ’ = 25° & for fat clay (CH), c = c’ = 51 psf, = ’ = 27°.

Where:

cu: Unconsolidated Undrained Cohesion ST: Short Term c’peak: Peak Consolidated Drained Cohesion RDD: Rapid Drawdown

’peak: Peak Consolidated Drained Friction Angle LT: Long Term cpeak: Peak Consolidated Undrained Cohesion peak: Peak Consolidated Undrained Friction Angle

6.4 Results of Slope Stability Analyses

Based on the information provided by LAN and the latest cross section drawings provided to us, we understand that the detention basin within Compartment 2 is planned to be constructed as a dry basin with side slopes of 3H:1V. HVJ evaluated the stability of the side slopes for cross sections A- A, C-C, D-D and E-E. The slope stability analysis was performed considering short term, long term, and rapid drawdown loading conditions for both global and local failures. The results of our analyses indicate that the proposed detention basin configuration meets the HCFCD required factor of safety for slope stability for the analyzed cross sections. The slope stability outputs and reports are presented in Appendix I and J respectively. The global and local stability analysis results are summarized in Table 6-2 and Table 6-3, respectively.

Table 6-2 – Global Slope Stability Analysis Results (Proposed Configuration)

(Cross Section)

Compartment Bank

Boring Used

Factor of Safety

Short-Term (Required FOS

= 1.30)

Long-Term (Required FOS

= 1.50)

Rapid Drawdown (Required FOS = 1.25)

Effective Stress

Parameters

Total Stress Parameters

A-A

West B-12 2.23 2.00 1.61 1.38

East B-14 1.34 1.96 1.45 1.27

C-C

South B-16 3.05 2.06 1.39 1.51

North B-11 1.85 1.40 1.50 1.61

D-D

South

T-1 (PZ-

4)

2.49 1.73 1.48 1.45

North B-15

(PZ-5)

4.39 1.72 1.54 1.41

E-E

South

T-2

2.18 1.93 1.41 1.29

North 2.60 2.18 1.63 1.39

Table 6-3 – Local Slope Stability Analysis Results (Proposed Configuration)

(Cross Section)

Compartment Bank

Boring Used

Factor of Safety

Short-Term (Required FOS

= 1.30)

Long-Term (Required FOS

= 1.50)

Rapid Drawdown (Required FOS = 1.25)

Effective Stress

Parameters

Total Stress Parameters

A-A

West B-12 4.58 2.20 1.71 1.46

East B-14 2.11 2.32 1.53 1.30

C-C

South B-16 5.14 2.11 1.47 1.64

North B-11 3.24 1.43 1.53 1.66

D-D

South

T-1 (PZ-

4)

2.43 2.13 1.74 1.52

North B-15

(PZ-5)

4.16 1.96 1.67 1.45

(Cross Section)

Compartment Bank

Boring Used

Factor of Safety

Short-Term (Required FOS

= 1.30)

Long-Term (Required FOS

= 1.50)

Rapid Drawdown (Required FOS = 1.25)

Effective Stress

Parameters

Total Stress Parameters

E-E

South

T-2

3.76 2.29 1.64 1.36

North 4.02 2.32 1.65 1.40

6.5 Recommended Slope Configuration

As discussed in Section 6.4, the results of our slope stability analyses indicate that the proposed detention basin configuration with 3H:1V side slopes meets the minimum required factor of safety for slope stability. HVJ recommends that 3H:1V sides slopes be used for the construction of the new detention basin expansion area within Compartment 2.

6.6 Groundwater Control

Assessment of the need for groundwater control and installation of appropriate dewatering equipment is the contractor's responsibility at the time of construction. The following comments are intended to represent common solutions to groundwater control problems encountered in similar soil conditions in the Houston area and may not be construed as dewatering system design recommendations.

Groundwater was encountered at depths between 6.33 feet (El. 59.09’) and 8.24 feet (El. 55.66’) below the existing grade at the boring locations. We have considered the groundwater readings recorded in Piezometers only. Based on the water level readings, groundwater may be encountered during excavation for the detention basin. A conventional pump and sump arrangement may be adequate if water bearing cohesive soils are encountered during construction. Well points may be utilized to lower the groundwater level to at least three feet below the excavation level if water bearing cohesionless soils are encountered.

6.7 Spoil Disposal, Select Fill & Fill Material Excavation from On-Site The select fill material should consist of a fine-grained lean clay with sand (CL) or sandy lean clay (CL) soil when classified in accordance with ASTM D2487. The select fill material should follow the specifications presented in item 2.1 of Section 02314 Fill Material from the 2020 HCFCD Standard Construction Specifications and Details as summarized in the table below.

Table 6-4 – Select Fill Criteria (Section 02314, Item 2.1) Test Description ASTM Test Value

Maximum Liquid Limit D4318 45%

Plasticity Index Range D4318 12% to 30%

Passing No. 200 Sieve D1140 60% to 85%

Dispersion Potential D4647 ND1 to ND2

The select fill material should be placed in 8-inch maximum layers, loose measure, and be compacted to not less than 95 percent of maximum standard dry density (ASTM D698) within plus or minus 3 percent of optimum moisture content.

The following on-site soils meet HCFCD’s criteria for select fill.

Table 6-5 – Location of On-Site Soils for Select Fill Material

Where no other fill material is specified or shown, use soils from the on-site excavation that are free from roots, trash, organic matter and other objectionable material in accordance with item 2.2A of Section 02314 Fill Material from the 2020 HCFCD Standard Construction Specifications and Details. The fill material should be classified by their group name and symbol in accordance with ASTM D2487 as follows:

Table 6-6 – Fill Material Criteria (Section 02314, Item 2.2A)

The following on-site soils meet the criteria for item 2.2A Fill Material Excavation from On-Site of Section 02314 Fill Material from HCFCD Standard Construction Specifications and Details.

Table 6-7 – Location of On-Site Soils for Fill Material

Location Elevation of Soil Strata (feet) Boring B-11 El. +66.47 to El. +56.47 Boring B-12 El. +65.85 to El. +59.85

El. +53.85 to El. +49.85 Boring B-14 El. +64.61 to El. +41.61

Boring B-15 (PZ-5) El. +65.42 to El. +53.42 Boring B-16 El. +65.62 to El. +59.62 Boring T-2 El. +57.50 to El. +45.50

Group Name Group Symbol Lean Clay CL

Lean Clay with Sand CL Sandy Lean Clay CL

Location Elevation of Soil Strata (feet) Boring B-11 El. +66.47 to El. +56.47 Boring B-12 El. +65.85 to El. +42.85 Boring B-14 El. +64.61 to El. +41.61

Boring B-15 (PZ-5) El. +65.42 to El. +42.42 Boring B-16 El. +65.62 to El. +55.62

Boring T-1 (PZ-4) El. +63.90 to El. +47.90 Boring T-2 El. +63.50 to El. +45.50

7 DISPERSIVE SOILS & EROSION PROTECTION MEASURES FOR NEW BASIN

Dispersive soils, sands and silts are highly susceptible to erosion and piping and are a major cause of slope distress in the Houston area. Dispersive soils are clays that disintegrate in the presence of relatively pure water as a result of their chemical composition. Crumb and double hydrometer tests were assigned on selected soil samples to determine the potential dispersion of the on-site soils. The results of crumb and double hydrometer tests indicate that the cohesive soils at the proposed detention basin location varies from non-dispersive to highly dispersive.

Rainfall and runoff on exposed slopes containing potential dispersive clays, sands and silts may result in surficial erosion which often results in rilling and gullying on the slopes. In other cases, standing water at top of the slopes may result in a distinctive erosion pattern referred to as “tunnel and jug hole”. The typical slope failure due to dispersive soils is depicted in Figure 7-1.

Figure 7-1 Typical Slope Failures due to Dispersive Soils (Ref., Soil Mechanics Note No. 13, USDA, 1991)

We recommend that water not be allowed to accumulate at the top of the basin slopes. The finish grading adjacent to the cells should be sloped away not less than one vertical in twelve horizontal (1V:12H) for a horizontal distance of at least 10 feet. The surrounding site-finish grading should be established so that surface water runoff may be collected at areas of the site where it may be captured into a drainage structure. We recommend installing backslope interceptors/swale systems installed at close intervals to quickly dissipate water from the ground surface. Please, refer to section

11.1 of HCFCD Policy, Criteria and Procedure Manual for the design of backslope interceptors/swale systems. Stone riprap should be used in areas where high flow velocities are anticipated such as near outfalls and confluences.

Following is a discussion of various erosion protection measures that can be considered for this project.

Turf establishment: Turf establishment following HCFCD Standard Specifications, Section 02921 and HCFCD Policy Criteria & Procedure Manual, Section 10.3 may be used as an erosion protection measure for the basin slopes. Turf establishment must be performed as quickly as practical after slope construction.

Stone Riprap: The stone riprap must be installed following HCFCD Standard Specification Section 02378. We recommend that a geotextile filter conforming to the requirements of HCFCD Standard Specification, Section 02379 should be installed as a filter beneath stone riprap to reduce erosion of soils from beneath the riprap. The gradation of the riprap should be determined based on the flow velocity as stated in Section 10.5.4 of the HCFCD Policy, Criteria and Procedure Manual. We recommend considering maximum velocity in determining the limits and gradation of stone riprap.

8 DESIGN CRITERIA AND RECOMMENDATIONS FOR REINFORCED

CONCRETE PIPE (RCP)

8.1 General

In addition to the…