14-Report File.pdf

“Reference Documents” HCFCD Project ID: K140-00-00-X025

The following documents are included for reference:

• GEOTECHNICAL STUDY WORK AUTHORIZATION NO. 2

EROSION REPAIRS: VINTAGE RESERVE PARKWAY TO

LOUETTA ROAD CHANNEL K140-00-00

PROJECT ID K140-00-00-X025, May 2023

GEOTECHNICAL STUDY

WORK AUTHORIZATION NO. 2

EROSION REPAIRS:

VINTAGE RESERVE PARKWAY TO LOUETTA ROAD

CHANNEL K140-00-00

PROJECT ID K140-00-00-X025

HCFCD CONTRACT NO. Z100-00-00-X280

HARRIS COUNTY, TEXAS

Prepared for

Gunda Corporation Houston, Texas

May 2023

Prepared by

TEG-TOLUNAY ENGINEERING GROUP, INC.

19407 Park Row Drive, Suite 100

Houston, Texas 77084 281-815-4900

Project No. 19-R034

TEG

Rev. 6, 5/12/2023 Report No. 19-R034 i

TABLE OF CONTENTS

TABLE OF CONTENTS i TABLES, FIGURES AND APPENDICES iii SUMMARY v

1. INTRODUCTION 1-1

1.1 General 1-1

1.2 Project Information 1-1

1.3 Site Visit 1-1

2. SCOPE OF STUDY 2-1

3. FIELD EXPLORATION 3-1

3.1 Test Borings 3-1

3.2 Drilling Methods 3-1

3.3 Soil Sampling 3-1

3.4 Boring Logs 3-2

3.5 Water-Level Measurements 3-2

4. LABORATORY TESTING 4-1

5. SUBSURFACE CONDITIONS 5-1

5.1 General 5-1

5.2 Subsurface Stratigraphy 5-1

5.3 Soil Properties 5-1

5.4 Groundwater Observations 5-3

6. DRAINAGE CHANNEL SLOPE STABILTY 6-1

6.1 Causes of Slope Failure 6-1

6.2 Stability Analysis of Channel Bank at Sta. 6+19.28 6-1

6.3 Slope Stability Analysis -Sta. 1+50.00 6-8

6.4 Slope Stability Analysis -Sta. 13+00.00 6-8

6.5 Summary of Slope Stability Recommendations 6-9

6.6 Erosion Control 6-9

6.7 Flow Line Restoration 6-10

6.8 Soil Dispersion 6-10

6.9 Back Slope Drainage 6-10

6.10 Maintenance 6-10

7. CONSTRUCTION OF CHANNEL BANK 7-1

8. CLOSURE 8-1

8.1 Limitations 8-1

TABLE OF CONTENTS

Report No. 19-R034 ii

8.2 Design Review 8-1

8.3 Construction Surveillance 8-1

9. REFERENCES 9-1

Report No. 19-R034 iii

TABLES, FIGURES AND APPENDICES

Tables

Table 3-1 Groundwater Level Measurements ......................................................................... 3-2 Table 5-1 Summary of Laboratory Test Results (Borings B-1 through B-3) .......................... 5-2

Table 5-2 Summary of CU Triaxial Compression Test Results .............................................. 5-2 Table 5-3 Summary of Crumb Test Results ............................................................................ 5-3 Table 5-4 Summary of Pinhole Dispersion Test Results ......................................................... 5-3 Table 6-1 Soil Parameters at Boring B-2 ................................................................................. 6-2 Table 6-2 Computed Factors of Safety of Channel Bank ........................................................ 6-4

Table 6-3 Recommended Minimum Factors of Safety ............................................................ 6-4

Table 6-4 Soil Parameters at Boring B-2 ................................................................................. 6-5

Table 6-5 Computed Minimum Factors of Safety ................................................................... 6-5 Table 6-6 Soil Parameters at Boring B-2 ................................................................................. 6-6 Table 6-7 Computed Minimum Factors of Safety ................................................................... 6-7 Table 6-6 Summary of Slope Stability Recommendations ...................................................... 6-9

Figures

Figure 1 Site Vicinity Map Figure 2 Repair Locations Figure 3 Plan of Borings Figure 4 Boring Log Profile

Figure 5 Results of Short-Term Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining near Toe Area, and 1.5-ft Thick Imported Select Fill along Slope Face, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Boring B-2 Soil Profile

Figure 6 Results of Rapid Drawdown Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining near Toe Area, and 1.5-ft Thick Imported Select Fill along Slope Face, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Effective Stress, Boring B-2 Soil Profile

Figure 7 Results of Rapid Drawdown Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining near Toe Area, and 1.5-ft Thick Imported Select Fill along Slope Face, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Total Stress, Boring B-2 Soil Profile

Figure 8 Results of Long-Term Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining near Toe Area, and 1.5-ft Thick Imported Select Fill along Slope Face, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Boring B-2 Soil Profile

Report No. 19-R034 iv

Figure 9 Results of Short-Term Condition, Slope Stability Analysis, Shallow Slope Erosion Repair, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Boring B-2 Soil Profile

Figure 10 Results of Rapid Drawdown Condition, Slope Stability Analysis, Shallow Slope Erosion Repair, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Effective Stress, Boring B-2 Soil Profile

Figure 11 Results of Rapid Drawdown Condition, Slope Stability Analysis, Shallow Slope Erosion Repair, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Total Stress, Boring B-2 Soil Profile

Figure 12 Results of Long-Term Condition, Slope Stability Analysis, Shallow Slope Erosion Repair, 3H:1V Slope, West Bank, Cross Section Sta. 6+19.28, Boring B-2 Soil Profile

Figure 13 Results of Short-Term Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining along Slope Face and Toe Area, 3H:1V Slope, West Bank, Cross Section Sta.

6+19.28, Boring B-2 Soil Profile

Figure 14 Results of Rapid Drawdown Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining along Slope Face and Toe Area, 3H:1V Slope, West Bank, Cross Section Sta.

6+19.28, Effective Stress, Boring B-2 Soil Profile

Figure 15 Results of Rapid Drawdown Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining along Slope Face and Toe Area, 3H:1V Slope, West Bank, Cross Section Sta.

6+19.28, Total Stress, Boring B-2 Soil Profile

Figure 16 Results of Long-Term Condition, Slope Stability Analysis, 1.5-ft Thick Riprap Lining along Slope Face and Toe Area, 3H:1V Slope, West Bank, Cross Section Sta.

6+19.28, Boring B-2 Soil Profile

Appendices

Appendix A Project Pictures Appendix B Profiles of Slope Cross Sections for Channel K140-00-00 Provided by

Gunda Corporation Appendix C gINT Logs (Borings B-1 through B-3) Appendix D Pinhole Dispersion, Crumb and Consolidated Undrained (CU) Triaxial

Compression with Pore Pressure Measurements Test Results Appendix E Output for Slope Stability Analysis, Riprap Lining at Toe and Imported

Select Fill along Slope (Sta. 6+19.28) Appendix F Output for Slope Stability Analysis, Shallow Slope Erosion Repair

(Sta. 6+19.28) Appendix G Output for Slope Stability Analysis, Riprap Lining along Slope (Sta.

6+19.28)

Report No. 19-R034 v

SUMMARY

The principal findings and conclusions of the geotechnical study for the proposed repair of HCFCD channel K140-00-00 are summarized as follows.

It is our understanding that channel K140-00-00 has experienced erosion at the slope bank and along the toe area. The proposed channel repair section is approximately 1820-ft long and 12-ft to 17-ft deep. The side slope declivities of the existing channel range from 2.1H:1V to 5.8H:1V.

During our initial site visit on May 7, 2019, we observed damaged storm sewer pipes at multiple locations, south of Louetta Road and at mid of the channel. The back slope interceptors also appeared to be old/damaged and need to be replaced. On October 1, 2020, we visited the site again to observe the erosion at the toe areas and channel banks to revise our slope stability recommendations. The representatives of HCFCD, Gunda Corporation, TEG and Gradient Group were present at that time. Based on the subsurface soil conditions, we recommend that a 1.5-ft thick riprap lining at the toe area and 1.5-ft thick imported select fill be placed along the slope face to stabilize the channel bank from Sta. 6+00 to 11+50 with side slope declivity of 3H:1V.

In addition to that, we analyzed the bank of channel with side slope declivity of 3H:1V using shallow slope erosion repair. It should be noted that this slope repair option may experience sloughing and erosion due to presence of sandy soils at the project site.

The slope bank may be stabilized using a 1.5-ft thick riprap lining along the slope face and at the toe area with side slope declivity of 3H:1V.

Project pictures were taken during our site visit to describe the slope conditions of the channel, and are presented in Appendix A.

The subsurface soil stratigraphy along channel K140-00-00, based on borings B-1 through B-3, generally consists of very loose to medium dense cohesionless/semi-cohesive silty sands (SM), silty clayey sands (SC-SM), clayey sands (SC) and to lesser extent sandy lean clays (CL) to 19.5-ft to 20-ft depth, followed by firm to hard, but mostly stiff to very stiff consistency, medium to high plasticity cohesive sandy lean clays (CL), lean clays (CL), lean clays with sand (CL), fat clays (CH) and to lesser extent semi-cohesive clayey sands (SC) to the 35-ft explored depth. A surficial 2-ft thick layer of semi-cohesive fill (silty clayey sand) was encountered in boring B-1 during our field exploration.

Based on the groundwater level measurements in the open boreholes, it appears that the groundwater level was at about 5.5-ft to 8.9-ft below existing grade at the time of the field exploration.

A slope stability analysis was performed to evaluate the channel (K140-00-00) bank at Sta.

6+19.28 with side slope declivity of 3H:1V using 1.5-ft thick riprap lining at the toe area and a 1.5-ft thick imported select fill along slope face and the high bank. Results of the slope stability analysis for proposed repair of channel bank for short term, rapid drawdown and long-term conditions are presented in the text of this report and on Figures 5 through 8. The outputs of slope

Report No. 19-R034 vi stability analysis are presented in Appendix E. This slope repair method can be used from Sta.

6+00 to Sta. 11+50 on both sides of the channel.

We analyzed the channel bank with side slope declivity of 3H:1V using shallow slope erosion repair. Results of the slope stability analysis for proposed repair of channel bank for short term, rapid drawdown and long-term conditions are presented in the text of this report and on Figures 9 through 12. The outputs of slope stability analysis are presented in Appendix F. It should be noted that this slope repair option may experience sloughing and erosion due to presence of sandy soils at the project site.

The channel bank was also analyzed using a 1.5-ft thick riprap lining along the slope face and at the toe area with side slope declivity of 3H:1V. Results of the slope stability analysis for proposed repair of channel bank for short term, rapid drawdown and long-term conditions are presented in the text of this report and on Figures 13 through 16. The outputs of slope stability analysis are presented in Appendix G.

From Sta. 0+00 to 6+00, the side slope declivities ranged from 3.2H:1V to 4.2H:1V along the east bank of the channel. The side slope declivities along the west bank ranged from 2.6H:1V to 2.9H;1V. The west bank may be flattened to a 3H:1V slope with shallow slope erosion repair option or, may be repaired with riprap lining at toe area and along slope face from Sta. 0+00 to 6+00. The east bank may be kept as is.

The side slope declivities between Sta. 11+50 and Sta. 13+50 ranged from 3.4H:1V to 4.1H:1V along the west side of channel. The slope bank at the west side of channel may be kept as is. The side slope declivities along the east bank ranged from 2.8H:1V to 2.9H:1V. The east bank may be flattened to 3H:1V slope with shallow slope erosion repair option or, repaired with riprap lining at toe area and along slope face from Sta. 11+50 to 13+50.

The side slope declivities between Sta. 13+50 and Sta. 18+00 ranged from 3.4H:1V to 5.8H:1V at the west bank and 3.2H:1V to 4.8H:1V at the east bank of the channel. The slope banks at both sides of the channel may be kept as is.

Construction of the slope banks should comply with the HCFCD 2020 Standard Specifications Book, Section 02316 entitled "Structural Excavating and Backfilling". Care should be taken to assure that the surface of the existing ground surface is benched or stepped prior to fill placement so that the channel bank fill is keyed into the existing ground surface. Fill for grading purposes, if needed, should preferably be earth fill. Earth fill should have plasticity index similar to the on-site soils or select fill. If select fill is used, it should have a plasticity index (PI) between 12 and 30, and a liquid limit of less than 45%. The fill should be placed on 8-in. thick loose lifts and compacted to an in-place dry density equal to at least 95 percent of the maximum standard dry density (ASTM D 698). The fill moisture content during compaction should be at the optimum moisture content or within plus or minus 3 percent of the optimum moisture content.

Fill soil should have plasticity index (PI) of 12 to 30, and liquid limit of less than 45%. The material passing No. 200 Sieve should be 60% to 85%. The dispersion potential should be ND1-

ND2.

Report No. 19-R034 1-1

1. INTRODUCTION

1.1 General

Gunda Corporation (GC) retained TEG-Tolunay Engineering Group, Inc. to perform a geotechnical study for Channel K140-00-00 in Harris County, Texas (Key Map 329 U, Y). The study was performed in general accordance with TEG Proposal No. 19-P025, dated June 13, 2019, that was part of the Subcontract for Consultant Services Between GC and TEG. The agreement was authorized by Mr. Rajesh Tanwani, P. E. of GC on July 9, 2019.

Work Authorization No. 2 for the channel K140-00-00 repair work was sent by Mr. Hunter Brennan, P.E. of GC via email transmittal dated April 7, 2020. The authorization letter was issued by Mr. Daniel O’Donnelly, P.E. of HCFCD on April 1, 2020.

1.2 Project Information

The project consists of repair works for channel K140-00-00 in Harris County, Texas. The project alignment is about 1820-ft long and 12-ft to 17-ft deep. The project site has experienced erosion along the toe areas. Back slope interceptors and storm sewers out fall pipes were observed to be damaged and need to be repaired/replaced. The work included slope erosion repairs, regrading channel banks, repair or upgrade of backslope swales. We received the cross-sections of the channel from Mr. Brennan of GC via e-mail transmittal dated June 25, 2020. The additional cross-sections prepared by Gradient Group were emailed on May 28, and June 2, 2021.

The profiles of cross-sections for channel K140-00-00 are presented in Appendix B. The site vicinity map is shown on Figure 1. The repair locations are presented in Figure 2. The boring locations are shown in Figure 3.

1.3 Site Visit

We visited the HCFCD channel K140-00-00 on May 7, 2019, and October 1, 2020. Our site visits indicated that the channel slope to the south of Louetta Road has experienced some failure, and erosion has occurred at the toe area. The back-swale drainage system appeared to be damaged.

The storm sewer pipes were old and need to be repaired/replaced. We recommend that the slope of the channel banks be repaired between Louetta Road and Vintage Preserve Parkway.

During our site visit, we observed corrugated metal pipes (CMP) at Sta. 5+73.80 (Location # 6), Sta. 13+11.64, Sta. 13+87.63 (Location # 4), Sta. 16+82.11, Sta. 16+99.52 and Sta. 17+77.53 (Location # 1). These storm sewer outfall pipes are 18-in. to 72-in. diameter. These pipes appear to be old and damaged and need to be repaired/replaced. We also observed numerous back slope interceptors of 18-in. diameter near Sta. 0+12.09 (Location # 8), Sta. 0+21.59 (Location # 7), Sta.

10+79.97, Sta. 14+68.49. These pipes appear to be damaged and need to be replaced. An existing

Report No. 19-R034 1-2 concrete pilot ditch towards the channel was observed near Sta. 17+25 (Location # 2) at the east bank. The concrete paving of the pilot channel should be repaired/replaced.

We observed a significant amount of soil has been eroded at the toe areas along the channel within the middle portion of the channel section i.e., between Sta. 6+00 and Sta. 11+00. We recommend that slope banks be stabilized at those areas using riprap at the toe and clay liner along the slope face.

Project pictures were taken during our site visit and are presented in Appendix A.

Report No. 19-R034 2-1

2. SCOPE OF STUDY

The study included field exploration to obtain subsurface information and to secure representative soil samples, laboratory testing to measure selected soil engineering properties, and geotechnical analyses to develop recommendations for stable side slopes for the drainage channel. Information developed as part of the study includes:

• Subsurface stratigraphy

• Groundwater condition

• Site preparation

• Drainage channel slope stability

• Dispersive soil remediation

• Backslope drainage

An environmental site assessment (ESA), areas not covered by the boring layout and a detailed site-specific fault study were outside the scope of work for this study.

Report No. 19-R034 3-1

3. FIELD EXPLORATION

3.1 Test Borings

The fieldwork was performed on April 16, 2020. The field exploration consisted of drilling, logging and sampling three (3) 35-ft deep soil borings at channel K140-00-00. Drilling and sampling were performed by Discovery Acquisition Services (DAS). A TEG representative coordinated the field activities, logged the boreholes, and obtained water level readings during drilling, at the end of the day of drilling and the next day after drilling completion. The boring locations are shown on Figure 2.

3.2 Drilling Methods

The field operations were performed in general accordance with Standard Practice for Soil Investigation and Sampling by Auger Borings [American Society for Testing and Materials (ASTM) D 1452]. Soil borings were drilled with a track rig and were dry augered to depths where wall instability or free water was encountered and completed using rotary wash drilling techniques in an attempt to measure the groundwater level. Soil samples were obtained at continual 1.5-ft/2-ft intervals to 20-ft depth, and at 5-ft intervals thereafter to boring completion depths of 35-ft. The soil bores were backfilled with bentonite chips/grout one day after completion of sampling and after groundwater level readings.

3.3 Soil Sampling

Cohesive soil and soil interpreted to be cohesive during drilling were sampled by hydraulically pushing a 3-inch diameter, thin-walled tube a distance of about 24 inches. The field sampling procedure was conducted in accordance with the Standard Practice for Thin-Walled Tube Sampling of Soils (ASTM D 1587). The field technician extruded some of the soil samples in the field, visually classified the recovered soils, and obtained penetration resistance measurement of the cohesive soils using a calibrated pocket penetrometer. The pocket penetrometer readings are presented on the boring logs in Appendix C. Based on experience with local soils, a factor of 0.67 was applied to penetrometer reading to estimate consistency of the cohesive over-consolidated soils.

Representative portions of the soil samples extruded in the field were wrapped in foil, placed into plastic bags, and transported to our laboratory.

Cohesionless to semi-cohesive soils were sampled by driving a 2-inch-diameter, split barrel sampler.

The sampler was driven 18 inches by a 140-pound hammer falling about 30 inches in general accordance with the Standard Method for Penetration Test and Split-Barrel Sampling of Soils (ASTM D 1586). The field technician recorded the number of blows required to drive the sampler through three consecutive 6-inch sampling intervals. The sum of the blows required to penetrate the final 12 inches is the Standard Penetration Test (SPT) “N” value. Each sample obtained from the split barrel was visually classified, placed in an air-tight plastic bag, and transported to our laboratory for testing. The compactness of the cohesionless/semi-cohesive soils were inferred from N60 (i.e., Report No. 19-R034 3-2

SPT “N” blow count value corrected for field procedure to an average energy ratio of 60%). The SPT data are presented on the log of borings in Appendix C.

3.4 Boring Logs

Our interpretations of general soil and groundwater conditions at the boring locations are included on the boring logs. The interpretations of the soil types throughout the boring depth and the locations of strata change were based on visual classifications during field sampling and laboratory testing, following ASTM D 2487 and 2488. The boring logs include the type and interval depth for each sample along with the corresponding penetration resistance of soils. The boring log profile is included in Figure 3. The project boring logs are presented in Appendix C.

3.5 Water-Level Measurements

Groundwater level measurements taken during drilling and one day after completion of drilling are summarized in Table 3-1.

Table 3-1 Groundwater Level Measurements

Boring No. Channel ID Boring

Depth

Depth of Dry

Augering

Depth to Free Water During Drilling

Groundwater Level Observations

Depth Caved Depth

Time Lapse

B-1

K140-00-00

35 ft 14 ft 14 ft 5.5 ft* 15.7 ft 18.7 hours

B-2 35 ft 18 ft 18 ft 8.9 ft 13.5 ft 16.9 hours

B-3 35 ft 18 ft 18 ft 7.4 ft 19.7 ft 21.9 hours *May be influenced by drill water.

Water levels measured in open boreholes may not accurately reflect the true (static) groundwater condition since it may be influenced by drill water, surface water seepage, wall instability, and may be time dependent. A more accurate determination of the static groundwater level is made in open standpipe piezometers. Piezometer installation was not included in the work scope of this project.

Report No. 19-R034 4-1

4. LABORATORY TESTING

Laboratory tests were performed on selected soil samples to measure physical and engineering properties. A brief description of the tests is presented below.

• Laboratory Determination of Water (Moisture) Content of Soil and Rock -ASTM D 2216. The water content of the soil/rock, expressed as a percentage, is defined at the ratio of the mass of fluid to the mass of soil solids. The moisture content may provide an indication of cohesive soil shear strength and compressibility when compared to Atterberg Limits.

• Liquid Limit, Plastic Limit and Plasticity Index of Soils -ASTM D 4318. These tests, also known as Atterberg limits, are used for soil classification and provide an indication of volume change potential when considered in conjunction with the natural moisture content.

The liquid limit and plastic limit establish the boundaries of the consistency states of plastic soils. The plasticity index (PI) is the difference between the liquid limit and the plastic limit.

• Amount of Material in Soils Finer Than the No. 200 (75-m) Sieve – ASTM D 1140.

This test measures the total amount of material in soils finer than the No. 200 sieve. The test result is presented as the percentage of silt and clay sizes by weight in the sample and may provide an indication of the soil hydraulic conductivity (permeability).

• Unconsolidated-Undrained Triaxial Compression – ASTM D 2850. This compression test consists of placing a relatively undisturbed cylindrical specimen in a pressurized triaxial chamber and then loading it to failure, without allowing drainage, using strain-controlled application of the axial load. The compressive strength of a cohesive soil sample is twice its undrained shear strength.

• Unconfined Compressive Strength of Cohesive Soil – ASTM D 2166. This test measures the unconfined compressive strength of cohesive soils in undisturbed or remolded condition, using strain-controlled deformation under load application. The undrained shear strength of a cohesive soil sample is one-half of the unconfined compressive strength.

• Density of Soil - ASTM D 7263, Method B. Total unit weight of the soil aggregate is defined as the weight of the aggregate (soil plus water) per unit volume. Knowing the total unit weight and moisture content, dry unit weight can be computed.

• Multi-stage Consolidated-Undrained (CU) Triaxial Compression With Pore Pressure Measurements –ASTM D 4767. In this test, the samples are allowed to consolidate and loaded to predetermined effective confining pressures (estimated from the sample depth).

The applied loads for all three stages (under three different confining pressures) are loaded to failure and the tests are extended to reach at least 15% strain to reach “fully softened”

Report No. 19-R034 4-2 condition. The pore water pressures are continuously monitored during loading. The test provides measured total (undrained) and effective (drained) soil parameters (cohesion and soil friction).

• Determining Dispersive Characteristics of Clayey Soils by Crumb Test - ASTM D 6572.

This test method provides a qualitative indication of the natural dispersive characteristics of clayey soils. A cube of remolded soil approximately 15 mm (5/8 in.) on each side is placed on the bottom of a white porcelain dish containing 250 mL of distilled water during the test. Temperature of water is recorded during the test. The visual determinations of dispersion grade are made and recorded at 2 minutes, 1 hour and 6 hours.

• Identification and Classification of Dispersive Clays by Pinhole Test- ASTM D 4647. In the pinhole test method, distilled water flows horizontally initially under a hydraulic head of 50 mm through a 1.0 mm diameter hole punched in the soil specimen. Test results are evaluated from the appearance of the flowing solution emerging from the specimen, the rate of flow, and the final size of the hole through the specimen.

The results of the laboratory tests are shown on the boring logs in Appendix C. Consolidated Undrained Triaxial Compression with Pore Pressure Measurements, Pinhole Dispersion and Crumb test results are presented in Appendix D.

Report No. 19-R034 5-1

5. SUBSURFACE CONDITIONS

5.1 General

Our interpretations of soil and groundwater conditions at the site are based on information obtained at the soil boring locations only. The project boring logs are presented in Appendix C. This information has been used as the basis for our conclusions and recommendations. Significant variations at areas not explored by the project borings will require re-evaluation of our recommendations.

5.2 Subsurface Stratigraphy

The subsurface soil stratigraphy along channel K140-00-00, based on borings B-1 through B-3, generally consists of very loose to medium dense cohesionless/semi-cohesive silty sands (SM), silty clayey sands (SC-SM), clayey sands (SC) and to lesser extent sandy lean clays (CL) to 19.5-ft to 20-ft depth, followed by firm to hard, but mostly stiff to very stiff consistency, medium to high plasticity cohesive sandy lean clays (CL), lean clays (CL), lean clays with sand (CL), fat clays (CH) and to lesser extent semi-cohesive clayey sands (SC) to the 35-ft explored depth. A surficial 2-ft thick layer of semi-cohesive fill (silty clayey sand) was encountered in boring B-1 during our field exploration.

It should be stressed that it is relatively difficult in practice to accurately delineate fill from similar natural soils. Fill classifications are made based upon visual observations and require considerable judgment. The interpreted fill depth may vary somewhat from the actual condition.

Ferrous and calcareous nodules, sand lenses, organics and roots were observed within the cohesive soil matrix. Detailed descriptions of the soils encountered in the project borings are presented on the logs of borings in Appendix C.

5.3 Soil Properties

We determined moisture content, plasticity (Atterberg limits), fines content (soil finer than the No.

200 sieve) and performed unconfined compression (UC), unconsolidated-undrained (UU) triaxial compression and consolidated undrained (CU) triaxial compression with pore pressure measurements tests on selected samples recovered from borings B-1 through B-3. The results of the tests are summarized below and are included in Appendices C and D of this report.

Report No. 19-R034 5-2

Table 5-1 Summary of Laboratory Test Results (Borings B-1 through B-3)

Stra-tum No.

Soil Classification

Depth Range

(ft)

Boring

Moist ure

LL

PI -Sieve #200

Pocket Pen.

(tsf)

Qc / Qu (tsf)

Dry Unit Weight

(pcf)

SPT

“N” (bpf)

I Silty Clayey Sand

“Fill” 0-2 B-1 8 20 6 37 - - 109 -

II

Silty Clayey Sand

(SC-SM)

0-2 B-2 9 24 6 37 - - - -

8-20 B-1, B-3 12-18 21-22 4-5 17-25 - - - 8-27

Silty Sand (SM)

0-8 B-1, B-2, B-3 2-9 16 1 30-42 - - - 7-26

14-23 B-2 16-22 21 3 47 - - - 15-26

Clayey Sand (SC) 6-16 B-2, B-3 12-19 24 10-11 28-40 - - - 3-13

Sandy Lean Clay

(CL) 12-14 B-2 15 31 18 60 3.00 - 122 -

III

Sandy Lean Clay, Lean Clay, Lean

Clay w/ Sand

(CL)

19.5-35 B-1, B-2, B-3 13-26 30-42 16-28 52-96 2.00-4.50 0.66-3.42 100-123 -

Fat Clay (CH) 23-28 B-1 23 54 37 91 3.25 - 102 -

Clayey Sand (SC) 28-33 B-1 20 24 10 33 - - - 17

LL = Liquid Limit PI = Plasticity Index Qc = Unconfined Compressive Strength Qu = Unconsolidated-Undrained Triaxial Compressive Strength bpf = blows per foot

5.3.1 Additional Test Results

We performed two (2) consolidated-undrained (CU) triaxial compression tests with pore water pressure measurements on the cohesive samples recovered from respective depths of 12-ft to 14-ft and 23-ft to 25-ft in bores B-2 and B-1. The results of the tests are summarized in Table 5-3 and are presented in Appendix D.

Table 5-2 Summary of CU Triaxial Compression Test Results

Boring No./ Sample Interval

Soil Sample Classification

Total Stress Parameters

Effective Stress Parameters c (psf) (deg.) c’ (psf) ’ (deg.)

B-1/23’-25’ Fat Clay (CH) 425 14 374 20

B-2/12’-14’ Sandy Lean Clay

(CL)

461 32 173 34

Report No. 19-R034 5-3

We selected two (2) soil samples recovered from the upper 2-ft depth in borings B-1 and B-2 for crumb, and one (1) sample recovered from the upper 2-ft depth in boring B-1 for pinhole dispersion test. The results are presented in Tables 5-3 and 5-4, and in Appendix D.

Table 5-3 Summary of Crumb Test Results

Boring Sample Depth Interval (ft) Soil Sample Description Dispersive

Classification B-1 0-2 Tan & brown Silty Clayey Sand

(SC-SM)

Grade 3 - Dispersive

B-2 0-2 Light gray & tan Silty Clayey Sand

(SC-SM)

Grade 2 - Intermediate

Table 5-4 Summary of Pinhole Dispersion Test Results

Boring Sample Depth Interval (ft) Soil Sample Description Dispersive Classification

B-1 0-2 Tan & brown Silty Clayey Sand

(SC-SM)

D1 - Dispersive clay with colloidal erosion under 50 mm (2-in.) head. Flow rate is 1.0 to

1.4 mL/s.

5.4 Groundwater Observations

Based on the groundwater level measurements presented in Table 3-1, it appears that the groundwater level was at about 5.5-ft to 8.9-ft below existing grade at the time of the field exploration for channel K140-00-00. Fluctuations in groundwater levels may occur with changes in seasonal and climatic conditions. Depending on when the project is developed, the groundwater condition observed may not be representative during construction. We recommend that the water level be verified prior to construction.

Report No. 19-R034 6-1

6. DRAINAGE CHANNEL SLOPE STABILTY

It is planned to repair drainage channel K140-00-00 in northwest Harris County, Texas. Furnished information indicates that the channel is about 1820-ft long and 12-ft to 17-ft deep. The depth of the channel near critical section at Sta. 6+19.28 is about 17-ft. The channel bank may be stabilized with 1.5-ft thick riprap lining near toe area with side slope declivity 3H:1V at this location. During our analysis, we assumed a 1.5-ft thick imported select fill along the slope face and high bank due to presence of sandy soils at this site. In addition to that, we also analyzed the slope bank at this location with side slope declivity of 3H:1V using 1.5-ft riprap lining along slope face from toe to high bank.

We reviewed the slope banks of the drainage channel at Sta. 1+50 and Sta. 13+00 near the respective borings B-3 and B-1. The depth of the channel at Sta. 1+50 is about 13-ft and at Sta.

13+00 about 15.5-ft. The slope repair recommendations provided in this section are based on the results of borings B-1 through B-3, and laboratory test results from soil samples obtained during this study.

6.1 Causes of Slope Failure

The main causes of the slope failure appear to be presence of cohesionless soils along the slope banks, and the slope banks are steeper. The tendency of this type of soil is to experience erosion in the sheet flow or during heavy rain. Furthermore, we observed a significant amount of soil erosion at the toe area due to flow/current of water in the channel during our site visits.

6.2 Stability Analysis of Channel Bank at Sta. 6+19.28

6.2.1 Riprap Lining at Toe and Benched Slope Erosion Repair along Channel Bank

A slope stability analysis was performed to evaluate the channel K140-00-00 cross section at Sta.

6+19.28 with a 1.5-ft thick riprap lining near the toe area and 1.5-ft thick imported select fill along the slope face due to presence of sandy soils at the site (Please refer to Picture Nos. 6, 7 and 8).

The side slope declivity at this location ranged from 2.3H:1V (west bank) to 2.8H:1V (east bank).

During analysis, we used a 3H:1V side slope declivity for this section.

The stability analysis was performed using the computer program Slide 2 (Version: 9.024 by Rocscience Inc.) and the Simplified Bishop slope stability analysis procedures. Soil parameters used in the stability analysis were based on experience with local soils and laboratory test results of the soil samples obtained during this study. The Slide 2 computer software searches for the critical slope failure plane and computes the minimum safety factor for a given slope geometry and subsurface soil and groundwater conditions.

Total stress cohesion and total stress angle of internal friction for clay soils for the short-term condition were based upon unconfined compression test results and TEG’s experience with similar soils. Total and effective stress cohesion and total and effective stress angle of internal friction for the clay soils for the intermediate term (rapid drawdown) condition were based upon consolidated-

Report No. 19-R034 6-2 undrained triaxial compression with pore pressure measurements test results and TEG’s experience with similar soils. Effective stress cohesion and effective stress angle of internal friction for clay soils for the long-term condition were based upon consolidated-undrained triaxial compression with pore pressure measurements test results and TEG’s experience with similar soils.

The slope stability analysis for the channel bank was performed for the soil profile observed in boring B-2, which consisted of a layered soil stratigraphy, for the following three design conditions:

• Short Term (End of Construction) Condition - The end of construction condition models the initial undrained condition of the soil. Total stress shear strength parameters were used in our analysis. A partially saturated slope was assumed for the short-term condition. For the channel slope analysis, the piezometric level was assumed at 8.9-ft from the high bank (El. 109.1-ft).

• Intermediate Term (Rapid Drawdown) Condition - Total and effective stress shear strength parameters from CU triaxial compression with pore pressure measurements tests and TEG’s experience with similar soils were used in the analysis. The rapid drawdown case models the condition where high flood waters saturate and piezometrically “load” a slope, and then quickly recede, leaving a large unbalanced piezometric head in the slope. The unbalanced head significantly increases shear stresses in the slope. Stresses relax as the unbalanced piezometric head drains out of the slope with time. For our analysis, the high water was placed near the high bank at El. 118-ft, along the slope face, and down to near the toe area (El. 105-ft).

• Long Term Condition - Effective stress shear strength parameters, a partially saturated ditch side-slope, and a saturated bottom were used in the analysis. The water level for the long-term condition was assumed at the same elevation as the short-term condition. The long-term case represents steady state piezometric and stress conditions. The long-term model is a case in which the slope soils rely on their available drained shear strength for long-term stability.

Soil parameters used in the channel slope stability analysis for 3V:1H slope at boring B-2 is summarized in the following table.

Table 6-1 Soil Parameters at Boring B-2

Strat um

Depth

(ft) Description

Unit Weight ST RDD LT t

(pcf) sat

(pcf) cuu

(psf) uu

(deg) ccu

(psf) cu

(deg) c’

(psf)

(deg)

1 - Riprap Lining at Toe 130 130 0 38 0 38 0 38

2 -

1.5-ft thick Imported Select Fill along Slope face and high Bank (Lean Clay “Fill”)

123 125 1000 0 250* 20 200* 25

Report No. 19-R034 6-3

Strat um

Depth

(ft) Description

Unit Weight ST RDD LT t

(pcf) sat

(pcf) cuu

(psf) uu

(deg) ccu

(psf) cu

(deg) c’

(psf)

(deg)

3 0-2 Silty Clayey Sand (SC-

SM) 118 130 0 32 0 32 0 32

4 2-6 Silty Sand (SM) 125 125 0 32 0 32 0 32

5 6-12 Clayey Sand (SC) 127 130 0 30 0 30 0 30

6 12-14 Sandy Lean Clay (CL) 140 140 2000 0 250 22 173 30

7 14-23 Silty Sand (SM) 125 125 0 32 0 32 0 32

8 22-35 Lean Clay (CL) 131 132 1400 0 250 20 200 25 *We used soil parameters of similar soils from another project located in the general area of current project (See CU Test results in Appendix D).

Where:

ST: Short Term Condition RDD: Rapid Drawdown Condition LT: Long Term Condition (Drained Condition) t: Moist Unit Weight of Soil sat: Saturated Unit Weight of Soil cuu: Unconsolidated-Undrained Cohesion uu: Unconsolidated-Undrained Friction Angle ccu: Consolidated-Undrained Cohesion cu: Consolidated-Undrained Friction Angle c’: Consolidated-Drained Cohesion ’ Consolidated-Drained Friction Angle

During our slope stability analysis, we used a 1.5-ft thick riprap lining near toe area and 1.5-ft thick imported select fill along the slope face and high bank due to presence of cohesionless/semi-cohesive soils at the channel bank. This slope repair option may be applicable from Sta. 6+00 through Sta. 11+50.

For the construction, we used 250 psf live load for short term condition during our stability analysis. For long term analysis, we assumed the same load for the operation of maintenance vehicles.

The computed minimum factors of safety for the channel bank with design slope of 3H:1V (west bank) are presented on Figures 5 through 8, and are tabulated as follows:

Report No. 19-R034 6-4

Table 6-2 Computed Factors of Safety of Channel Bank

Condition Analyzed

Factor of Safety B-2

Sta: 6+19.28 Riprap Lining at Toe Area &

Imported Select Fill along Slope Face 3H:1V Slope (West Bank)

Short Term 2.07

Rapid Drawdown Effective Stress 1.35

Total Stress 1.37

Long Term 1.76

Typical minimum factors of safety against slope failures, which are used for design purposes, are shown in Table 6-3.

Table 6-3 Recommended Minimum Factors of Safety

Condition Minimum Recommended Factor of Safety

Short Term 1.3

Rapid Drawdown 1.25

Long Term 1.5

Results of the stability analysis indicate that drainage channel at Sta. 6+19.28 with a 1.5 thick riprap near toe, and 1.5-ft thick imported select fill along the slope face and high bank with side slope declivity of 3H:1V will have adequate factors of safety against a failure for the short term (end of construction), intermediate term (rapid drawdown), and long-term conditions. The output of the slope stability analysis is presented in Appendix E.

6.2.2 Shallow Slope Erosion Repair

Slope stability analysis for the channel cross-section at Sta. 6+19.28 was performed to evaluate the shallow slope erosion repair with slope inclination of 3H:1V (Please refer to Picture Nos. 6, 7 and 8).

Soil parameters used in the stability analyses were as discussed above in Section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28, - 6.2.1 Riprap Lining at Toe and Benched Slope Erosion

Report No. 19-R034 6-5

Repair along Channel Bank.” The SLIDE computer software was used as discussed above to compute the minimum safety factors.

The slope stability analysis for the flattened slope of shallow slope erosion repair at Sta. 6+19.28 was performed using the soil profile from boring B-2 for the three (3) design conditions as described in Section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28, - 6.2.1 Riprap Lining at Toe and Benched Slope Erosion Repair along Channel Bank.”

Soil parameters from boring B-2 used in the slope stability analysis for channel bank at Sta.

6+19.28 with side slope declivity of 3H:1V are summarized in Table 6-4.

Table 6-4 Soil Parameters at Boring B-2

Strat um

Depth

(ft) Description

Unit Weight ST RDD LT t

(pcf) sat

(pcf) cuu

(psf) uu

(deg) ccu

(psf) cu

(deg) c’

(psf)

(deg)

1 0-2 Silty Clayey Sand (SC-

SM) 118 130 0 32 0 32 0 32

2 2-6 Silty Sand (SM) 125 125 0 32 0 32 0 32

3 6-12 Clayey Sand (SC) 127 130 0 30 0 30 0 30

4 12-14 Sandy Lean Clay (CL) 140 140 2000 0 250 22 173 30

5 14-23 Silty Sand (SM) 125 125 0 32 0 32 0 32

6 22-35 Lean Clay (CL) 131 132 1400 0 250 20 200 25

The computed minimum factors of safety against slope failure for the section at Sta. 6+19.28 using shallow slope erosion repair are presented in Figures 9 through 12 and are summarized in Table 6- 5:

Table 6-5 Computed Minimum Factors of Safety

Condition

Calculated Factor of Safety

B-2 Sta. 6+19.28 3H:1V Slope w/ Shallow Slope Erosion Repair Short Term 2.08

Rapid Drawdown Effective Stress 1.28

Total Stress 1.28

Long Term 1.63

Report No. 19-R034 6-6

The recommended minimum factors of safety for slope stability analysis are presented in Table 6-3 in the previous section.

Results of the slope stability analyses indicate that the proposed shallow slope erosion repair at Sta. 6+19.28 with side slope declivity of 3H:1V may provide stable slope for the short term, intermediate term (rapid drawdown) and long-term conditions. The results are presented in Figures 9 through 12. The output of slope stability analysis is included in Appendix F. It should be noted that this slope repair option may experience sloughing or erosion due to presence of sandy soils at the project site.

6.2.3 Riprap Lining

Slope stability analysis for the slope cross-section at Sta. 6+19.28 was performed to evaluate the riprap lining slope with side slope declivity of 3H:1V (Please refer to Picture Nos. 6, 7 and 8).

Soil parameters used in the stability analyses were as discussed above in Section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28, - 6.2.1 Riprap Lining at Toe and Benched Slope Erosion Repair along Channel Bank.” The SLIDE computer software was used as discussed above to compute the minimum safety factors.

The slope stability analysis for the riprap lining slope at Sta. 6+19.28 was performed using the soil profile from boring B-2 for the three (3) design conditions as described in Section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28, - 6.2.1 Riprap Lining at Toe and Benched Slope Erosion Repair along Channel Bank.”

Soil parameters from boring B-2 used in the slope stability analysis for channel bank at Sta.

6+19.28 with side slope declivity of 3H:1V are summarized in Table 6-4.

Table 6-6 Soil Parameters at Boring B-2

Strat um

Depth

(ft) Description

Unit Weight ST RDD LT t

(pcf) sat

(pcf) cuu

(psf) uu

(deg) ccu

(psf) cu

(deg) c’

(psf)

(deg)

1 - Riprap Lining 130 130 0 38 0 38 0 38

2 0-2 Silty Clayey Sand

(SC-SM) 118 130 0 32 0 32 0 32

3 2-6 Silty Sand (SM) 125 125 0 32 0 32 0 32

4 6-12 Clayey Sand (SC) 127 130 0 30 0 30 0 30

5 12-14 Sandy Lean Clay (CL) 140 140 2000 0 250 22 173 30

6 14-23 Silty Sand (SM) 125 125 0 32 0 32 0 32

7 22-35 Lean Clay (CL) 131 132 1400 0 250 20 200 25

Report No. 19-R034 6-7

The computed minimum factors of safety against slope failure for the section at Sta. 6+19.28 using riprap lining are presented in Figures 13 through 16 and are summarized in Table 6-5:

Table 6-7 Computed Minimum Factors of Safety

Condition

Calculated Factor of Safety

B-2 Sta. 6+19.28

3H:1V Riprap Lining along Slope Face

Short Term 1.74

Rapid Drawdown Effective Stress 1.32

Total Stress 1.33

Long Term 1.68

The recommended minimum factors of safety for slope stability analysis are presented in Table 6-3 in the previous section.

Results of the slope stability analyses indicate that the proposed riprap lining slope at Sta. 6+19.28 with side slope declivity of 3H:1V will provide stable slope for the short term, intermediate term (rapid drawdown) and long-term conditions. The results are presented in Figures 13 through 16.

The output of slope stability analysis is included in Appendix G.

From Sta. 0+00 to 6+00, the side slope declivities ranged from 3.2H:1V to 4.2H:1V along the east bank of the channel. The side slope declivities along the west bank ranged from 2.6H:1V to 2.9H;1V. The west bank may be flattened to a 3H:1V slope or, may be repaired with shallow slope erosion repair option from Sta. 0+00 to 6+00. The east bank may be kept as is.

The side slope declivities between Sta. 11+50 and Sta. 13+50 ranged from 3.4H:1V to 4.1H:1V along the west side of channel. The slope bank at the west side of channel may be kept as is. The side slope declivities along the east bank ranged from 2.8H:1V to 2.9H;1V. The east bank may be flattened to 3H:1V slope or, repaired with riprap lining at toe and shallow slope erosion repair option from Sta. 11+50 to 13+50.

The side slope declivities between Sta. 13+50 and Sta. 18+00 ranged from 3.4H:1V to 5.8H:1V at the west bank and 3.2H:1V to 4.8H:1V at the east bank of the channel. The slope banks at both sides of the channel may be kept as is.

Report No. 19-R034 6-8

6.3 Slope Stability Analysis -Sta. 1+50.00

We reviewed the slope cross-section at Sta. 1+50.00. Our review indicates that the existing side slope declivity is 2.8H:1V at the west bank and 3.3H:1V at the east bank at this location (Please refer to Picture Nos. 11, 12 and 13). Based on boring B-3, the subsurface soils appeared to be sandy.

Slope stability analysis was not performed for profile at Sta. 1+50.00, because the subsurface soils are sandy and west bank slope is similar to the previously analyzed profile which was discussed in report section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28.” Therefore, the recommendations provided in the previous section may be interpolated and applied to profile at Sta. 1+50.00.

The slope bank at the west side of channel may be flattened to 3H:1V slope with shallow slope erosion repair option or, repaired with riprap lining at toe area and along the slope. The east bank may be kept as is.

6.4 Slope Stability Analysis -Sta. 13+00.00

Our review of the cross-section at Sta. 13+00.00 indicates that the existing side slope declivity is 4.1H:1V at the west bank and 2.9H:1V at the east bank at this location (Please refer to Picture Nos.

3, 4 and 5). Based on boring B-1, the subsurface soils are sandy and consistent at the channel alignment.

Slope stability analysis was not performed for profile at Sta. 13+00.00, because the subsurface soils are sandy, and the east bank of channel is similar to the previously analyzed profile which was discussed in report section 6.2 “Stability Analysis of Channel Bank at Sta. 6+19.28.”

Therefore, the recommendations provided in the previous section may be interpolated and applied to profile at Sta. 13+00.00.

The slope bank at the west side of the channel may be kept as is. The east bank may be flattened to a 3H:1V slope with shallow slope erosion repair option or repaired with riprap lining at toe area and along the slope.

Report No. 19-R034 6-9

6.5 Summary of Slope Stability Recommendations

We performed the slope stability analysis for the channel cross-section at Sta. 6+19.28 based on boring B-2. Presented below is the summary of our slope stability recommendations for channel K140-00-00.

Table 6-6 Summary of Slope Stability Recommendations

Station No. Existing Slope Recommended Slope

West Bank East Bank West Bank East Bank

0+00 to 6.00 2.6H:1V to

2.9H:1V

3.2H:1V to

4.2H:1V

Shallow slope erosion repair* or, Riprap lining

Keep as is

6+00 to 11+50 2.1H:1V to

2.9H:1V

2.3H:1V to

2.8H:1V

Riprap lining at toe

Benched slope erosion repair along slope face

11+50 to 13+50 3.4H:1V to

4.1H:1V

2.8H:1V to

2.9H:1V

Shallow slope erosion repair* or, Riprap lining

13+50 to 18+00 3.4H:1V to

5.8H:1V

3.2H:1V to

4.8H:1V

*In general, the channel bank is constructed with the side slope declivity of 4H:1V or flatter within sandy soils for stability purposes. The shallow slope erosion repair option with 3H:1V side slope declivity may experience sloughing/erosion due to the presence of sandy soils at the project site.

6.6 Erosion Control

Some erosional distress and shallow slides should be expected on slopes that are subjected to overbank runoff or are located in the zone of temporary inundation by varying water level or wave action. A grass cover for cohesive slopes is an essential protection for erosion. HCFCD recommends establishing turf grass on all exposed or disturbed soil (HCFCD Policy, Criteria and Procedure Manual, October 2018, Section 6.14.1). A concrete liner or geotextile will provide better protection against erosion and shallow slides. In addition, erosion of the low plasticity cohesive soils may occur due to overbank runoff.

Zones of relatively permeable soils are susceptible to erosion under the action of flowing water, such as overbank flow and sheet flow. These soil zones along the slope, if encountered, may require hard armor such as concrete lining or riprap if grass cover does not produce a good response. Additional erosion control methods include chemical stabilization, erosion control blankets, bonded fiber matrices, or removal and replacement with high plasticity clays.

Report No. 19-R034 6-10

Back slope drainage systems are required where the natural ground slopes toward the channel. In addition, concrete lining or riprap may be necessary around pipe inlets and outlets if erosive velocities and turbulence are expected.

The bank of channel should be protected with vegetation as specified in the HCFCD Policy, Criteria and Procedural Manual, October 2018, Section-10 “Erosion and Sediment Control (10.3 Turf Establishment)”.

6.7 Flow Line Restoration

Our review of channel flow line (Figure 4) indicates that channel bed near boring B-2 has…