14-12-RRSegD2-FinalGeotechStructuralReport.pdf

GEOTECHNICAL ENGINEERING STUDY

FOR

BRIDGE AND RETAINING WALL STRUCTURES

RONALD REAGAN BLVD ‐ SEGMENT D2

FROM EAST OF CR 245 TO SUN CITY BLVD

WILLIAMSON COUNTY, TEXAS

GEOTECHNICAL ENGINEERING STUDY

For

BRIDGE AND RETAINING WALL STRUCTURES

RONALD REAGAN BLVD ‐ SEGMENT D2

FROM EAST OF CR 245 TO SUN CITY BLVD

WILLIAMSON COUNTY, TEXAS

Prepared for

AMERICAN STRUCTUREPOINT, INC.

Austin, Texas

Prepared by

RABA KISTNER CONSULTANTS, INC.

Austin, Texas

PROJECT NO. AAA24‐036‐00

March 27, 2025

Project No. AAA24‐036‐00

TABLE OF CONTENTS

i

INTRODUCTION

PAVEMENT REPORT

PROJECT DESCRIPTION

LIMITATIONS

BORINGS AND LABORATORY TESTS

BORING EXPLORATIONS

LABORATORY TESTING

Sulfate Testing Hydrometer Testing (Scour Analysis) Moisture‐Density Relationship of Subgrade Soil pH‐Lime Series Triaxial Compression for Disturbed Soils

GENERAL SITE CONDITIONS

GEOLOGY

STRATIGRAPHY

GROUNDWATER

BRIDGE FOUNDATION RECOMMENDATIONS

AXIAL CAPACITY

Axial Group Effects

LATERAL RESISTANCE

ESTIMATED SHAFT SETTLEMENT

DRILLED SHAFT CONSTRUCTION CONSIDERATIONS

RETAINING WALL RECOMMENDATIONS

MINIMUM EMBEDMENT

FOUNDATION SOIL

RETAINED SOIL

SELECT BACKFILL (REINFORCED ZONE)

DRAINAGE

MINIMUM REINFORCEMENT LENGTH

External Stability Global Stability

EMBANKMENT DESIGN CONSIDERATIONS

BRIDGE APPROACHES

FILL‐INDUCED SETTLEMENT

EMBANKMENT SLOPES

NEW FILL BENCHING

TABLE OF CONTENTS

ii

EMBANKMENT FILL

SELECT FILL

EXCAVATION

Temporary Slopes Excavation Equipment

EROSION PROTECTION

CULVERT RECOMMENDATIONS

CULVERT FOOTINGS

LATERAL EARTH PRESSURES

BACKFILL COMPACTION

DRAINAGE

CULVERT CONSTRUCTION CONSIDERATIONS

SITE PREPARATION

New Fill Benching Fill Materials Excavations Erosion Protection

CONSTRUCTION RELATED SERVICES

CONSTRUCTION MATERIALS TESTING AND OBSERVATION SERVICES

BUDGETING FOR CONSTRUCTION TESTING

FIGURES

Site Location Map .............................................................................................................................. Figure 1 Geology Map ...................................................................................................................................... Figure 2 Boring Location Map ...................................................................................................... Figure 3A through 3I

APPENDICES

WINCORE Boring Logs ................................................................................................................... Appendix A Results of Soil Sample Analyses .................................................................................................... Appendix B Drilled Shaft Axial Capacity Charts ................................................................................................ Appendix C MSE Wall Stability Analysis Summary ...........................................................................................Appendix D Stability Analysis Calculation Package .......................................................................................... Appendix E Important Information About Your Geotechnical Engineering Report

INTRODUCTION

RABA KISTNER Consultants Inc. (RKCI) has completed the authorized subsurface exploration and foundation bridge foundation and retaining wall analysis for the proposed improvements to Ronald Reagan Boulevard

– Segment D2 in Williamson County, Texas. This report briefly describes the procedures utilized during this study and presents our findings along with our bridge foundation and culvert recommendations and construction considerations for the project.

An overview of the project site extents and surrounding site features is shown on the Site Location Map, Figure 1.

PAVEMENT REPORT

RKCI’s scope of services includes a separate report providing recommendations for the pavement of Ronald Reagan Boulevard – Segment D2.

PROJECT DESCRIPTION

Under consideration in this study are the bridge at Jennings Branch with an estimated 100 ft length, the bridge at Berry Creek with an estimated 1,150 ft length, and three Mechanically Stabilized Earth (MSE) retaining walls proposed along Ronald Reagan Boulevard – Segment D2 in Williamson County, Texas. The bridge foundation and retaining wall design recommendations presented in this report were prepared based on the schematics and profiles provided to us by American Structurepoint on December 5, 2024.

However, we understand that the wall from WBFR STA 2091+00 to STA 2092+00 has been converted to a rip‐rap covered embankment that will have its toe embedded into limestone.

LIMITATIONS

This engineering report has been prepared in accordance with accepted Geotechnical Engineering practices in the region of central Texas and for the use of American Structurepoint, Inc. and its representatives for design purposes. This report may not contain sufficient information for the purposes of other parties or other uses. This report is not intended for use in determining construction means and methods. The attachments and report text should not be used separately.

The recommendations submitted in this report are based on the data obtained from 19 geotechnical borings drilled at this site, our understanding of the project information provided to us, and the assumption that site grading will result in the grade elevations described herein. If the project information described in this report is incorrect, is altered, or if new information is available, we should be retained to review and modify our recommendations.

This report may not reflect the actual variations of the subsurface conditions across the site. The nature and extent of variations across the site may not become evident until construction commences. The construction process itself may also alter subsurface conditions. If variations appear evident at the time of construction, it may be necessary to reevaluate our recommendations after performing on‐site observations and tests to establish the engineering impact of the variations. The scope of our

Geotechnical Engineering Study does not include an environmental assessment of the air, soil, rock, or water conditions either on or adjacent to the site. No environmental opinions are presented in this report.

BORINGS AND LABORATORY TESTS

BORING EXPLORATIONS

Subsurface conditions at the site were explored by drilling 19 borings using a truck‐mounted drilling rig at the approximate locations shown on the Boring Location Map, Figure 3. The boring locations were documented in the field utilizing a consumer‐grade handheld GPS device. A summary of our field exploration, including the location, drill depth, and ground surface elevation of each of the borings is presented in the table below.

Borings RW‐3 through RW‐5 were drilled to approximate depths of 20 ft but are associated with retaining walls that have since been converted into an embankments.

Structure Boring No.

Coordinates Depth**

(ft)

Surface Elevation*

(ft)

Station

(WBFR STA)

Offset

(WBFR OFF) Latitude* Longitude*

Jennings Branch Bridge/Retaining Wall

(WBFR STA 1987+00

to STA 1992+00)

B‐1 30.736709 ‐97.767886 50 893 1989+12.07 27.95 RT

B‐2 30.736748 ‐97.767809 50 894 1990+64.39 25.94 RT

Retaining Wall

(WBFR STA 2015+00

to STA 2020+00)

RW‐1 30.738555 ‐97.759631 20 883 2016+59.58 16.57' RT

RW‐2 30.738989 ‐97.758514 20 888 2020+44.26 17.73' RT

Berry Creek Bridge

(WBFR STA 2093+00

to 2104+00)

B‐3 30.742999 ‐97.736588 50 839 2092+63.89 18.37' RT

B‐4 30.743066 ‐97.735831 Boring not drilled due to drill rig inaccessibility (steep slopes associated with creek)

B‐5 30.743218 ‐97.735654 50 829 2095+66.14 17.47' RT

B‐6 30.743429 ‐97.735044 50 832 2097+71.62 10.99' RT

B‐7 30.743710 ‐97.734351 50 836 2100+11.43 7.93' RT

B‐8 30.743862 ‐97.733960 50 840 2101+45.25 16.94' RT

B‐9 30.744257 ‐97.733262 50 844 2104+06.00 11.23' RT

Retaining Wall

(WBFR STA 2104+00

to STA 2118+00)

RW‐6 30.744616 ‐97.732793 20 844 2106+02.21 6.20' LT

RW‐7 30.744964 ‐97.732313 20 843 2107+99.44 5.10' LT

RW‐8 30.745227 ‐97.731860 20 845 2109+67.78 24.77' RT

RW‐9 30.745760 ‐97.731426 20 846 2112+02.06 4.05' LT

RW‐10 30.746186 ‐97.731056 20 851 2113+96.24 7.41' LT

RW‐11 30.746643 ‐97.730716 20 860 2115+94.06 13.87' LT

* Boring locations and surface elevations estimated from Google Earth ** Below existing grades at the time of this study

During drilling operations, the following samples were collected:

Type of Sample Number Collected

Split‐Spoon (modified ASTM D1586) 74

Shelby Tube 1

NX Rock Core (ASTM D2113) 437 ft

Texas Cone Penetration (Tex‐132‐E) (no sample) 123

Texas Cone Penetration (TCP) tests were conducted at approximately 5‐ft intervals in each boring in general accordance with TxDOT Tex‐132‐E ‐ Texas Cone Penetration. The TCP test was used to determine the relative density or consistency of the subsurface material, and to develop bearing and skin friction resistance of the subsurface to use in foundation design. The TCP test results are noted as the two consecutive 6‐in. increments following seating. The test consists of driving a 3‐in. diameter cone with a 170‐lb hammer falling 24 in. All drill rigs utilized an automatic hammer to minimize operator error while running the test. The TCP test results are shown on the WINCORE boring logs presented in Appendix A.

Non‐cohesive and relatively very stiff to hard cohesive soils were sampled using a standardized 1.5‐in inner diameter split‐spoon driven with a 170‐lb hammer falling 24 inches. Conversely, the standard penetration test in accordance with ASTM D 1586 ‐ Standard Test Method for Standard Penetration Test (SPT) and Split‐Barrel Sampling of Soils specifies a 140‐lb hammer falling 30 inches. Therefore, hammer drops were not directly used to evaluate relative density or consistency of the subsurface material and results are not presented on the boring logs.

Rock coring was conducted to obtain samples of the in‐situ rock, in general accordance with ASTM D2113 – Standard Practice for Rock Core Drilling and Sampling of Rock for Site Exploration. The core recovery (REC) presented on the logs are the total length of the recovered material divided by the attempted run length during coring activities, presented as a percentage. The Rock Quality Designation (RQD) is the sum of the length of all recovered rock segments measuring 4 inches or more divided by the attempted run length during coring activities, presented as a percentage.

Classification and field test results were recorded onto field logs, which included a visual description in general accordance with ASTM D 2488 – Visual Description and Identification of Soils. After field logging, the individual samples were sealed in plastic bags or wrapped in plastic to prevent loss of moisture and were transported to our laboratory for further evaluation and testing.

Upon completion of the sampling activities, the boreholes were backfilled with auger cuttings and bentonite chips. Samples will be retained in our laboratory for 30 days after submittal of this report. Other arrangements may be provided at the request of the Client.

LABORATORY TESTING

Each sample was visually classified in the laboratory by a member of our geotechnical engineering staff.

Classification testing, which included moisture contents, Atterberg Limits, and grain size analyses, were completed on select samples for classification in general accordance with ASTM D 2487 ‐ Standard Classification of Soils for Engineering Purposes (Unified Soil Classification System).

Additional laboratory testing was completed on select samples to evaluate the geotechnical engineering properties of the subsurface soils existing at the site. Additionally, testing was performed on bulk samples, Samples BS‐1 through BS‐5.

Type of Sample Type of Test Number Conducted

Borings (B‐1 through B‐9 and RW‐1 through RW‐11)

Natural Moisture Content (Tex‐103‐E) 60

Atterberg Limits (Tex‐ 104‐E, Tex‐105‐E, Tex‐106‐E) 8

Sieve Analysis (Tex‐110‐E) 5

Percent Passing a No. 200 Sieve (Tex‐111‐E) 3

Unconfined Compression (rock) with Unit Weight

(ASTM D4543) 16

Sulfate Concentration (Tex‐145‐E) 3

Bulk Samples (BS‐1 through BS‐5)

Atterberg Limits (Tex‐ 104‐E, Tex‐105‐E, Tex‐106‐E) 4

Sieve Analysis, with Hydrometer (Tex‐110‐E) 3

Sieve Analysis (Tex‐110‐E) 2

Soil pH‐Lime Relationship (Tex‐121‐E, Pt. III) 1

Moisture‐Density Relationship (Tex‐114‐E) 2

Triaxial Compression (Tex‐117‐E) 2

The results of natural moisture content, Atterberg Limits, and percent passing No. 4, 40 and 200 sieves are shown on the WINCORE boring logs presented in Appendix A. A summary of laboratory testing results including grain size distribution plots is also included with the Results of Soil Sample Analyses presented in Appendix B. Bulk sample testing results are listed below and included in Appendix B.

Sulfate Testing

Sulfate testing was performed on selected samples from our borings. The purpose of the sulfate testing was to determine the concentration of soluble sulfates in the subgrade soils, in order to investigate the potential for an adverse reaction to lime in sulfate‐containing soils. The adverse reaction, referred to as sulfate‐ induced heave, has been known to cause cohesive subgrade soils to swell in short periods of time, resulting in pavement heaving and possible failure.

Boring No.

Depth Below Existing Ground Surface (ft)

Soil Type Sulfate

Concentration (ppm)

RW‐3 0 to 1.5 Brown to Tan Gravelly Lean Clay with sand 120

RW‐6 2 to 3.5 Tan Sand 100

RW‐9 0 to 1.5 Dark Brown Fat Clay 120

Based on the laboratory test results, the reported sulfate concentration values were generally determined to be on the order of 120 parts per million (ppm) or less. Reported sulfate concentrations above 3,000 ppm are known to cause sulfate induced heaving when the soils are mixed with lime. It should be understood that the identification of sulfates based on discrete soil samples cannot totally identify sulfates in all areas.

If the option for lime is considered, a quality assurance program should be implemented to assist in reducing the risk of sulfate induced heaving.

Hydrometer Testing (Scour Analysis)

Hydrometer testing was performed on three bulk samples as listed below, for use in scour analyses. The D50 grain sizes and the percent fines (percentage passing the No. 200 sieve) are shown in the table below, and the complete results will be presented on the Grain Size Curve in Appendix B.

Sample No. Depth (ft) Percent Fines (Passing

No. 200 Sieve) D50

(mm) Depth to

Limestone* (ft)

BS‐2 0 to 2 78 0.005 9

BS‐4 0 to 2 86 0.005 1

BS‐5 0 to 2 87 0.005 12 *Based on ground surface existing at the time of exploration and nearby borings.

Moisture‐Density Relationship of Subgrade

Moisture‐density relationships were completed for the bulk samples listed below in general accordance with TxDOT, Tex‐114‐E laboratory procedures. In addition, Atterberg Limits and sieve analysis testing was performed to characterize the bulk sample. A summary of the test results is shown in the table below and the detailed results are presented in Appendix B.

Sample Maximum Dry Density (pcf)

Optimum Moisture Content (%)

Subgrade Classification

BS‐1 82.5 30.2 Fat Clay (CH)

BS‐3 93.9 21.3 Fat Clay (CH)

Soil pH‐Lime Series

Soil pH‐Lime series testing was conducted on a selected sample collected from Sample BS‐1. Testing was performed in general accordance with TxDOT Tex‐121‐E Part III laboratory procedures. The results of the soil pH‐lime testing are presented in the table below and the detailed results are presented in Appendix B.

pH

Sample Baseline 2% Lime 4% Lime 6% Lime 8% Lime 10% Lime

BS‐1 8.38 11.81 12.29 12.36 12.37 12.39

Triaxial Compression for Disturbed Soils

Triaxial compression testing was conducted on the bulk samples listed below in general accordance with TxDOT, Tex‐117‐E laboratory procedures. The results of the triaxial compression testing are presented in the table below and the detailed results are presented in Appendix B.

Sample Friction Angle (degrees)

Cohesion (psi) Texas Triaxial Classification

BS‐1 25.8 7.1 4.1

BS‐3 23.9 12.3 3.7

GENERAL SITE CONDITIONS

GEOLOGY

A review of the Geologic Atlas of Texas, Austin Sheet, indicates that this site is naturally underlain with the soils/rock (limestone) of the Edwards Group. Edwards limestone is generally considered hard in induration and typically contains harder zones/seams of chert and dolomite. Edwards limestone also typically contains karstic features in the form of open and/or clay‐filled vugs, voids, and/or solution cavities that form as a result of solution movement through fractures in the rock mass. A general overview of the project alignment geology is presented in the Geology Map in Figure 2.

Key geotechnical engineering considerations for development supported on this formation will be the depth to rock, the expansive nature of the overlying clays, the condition of the rock, and the presence/absence of karstic features.

STRATIGRAPHY

The subsurface conditions encountered at the boring locations are shown on the WINCORE logs presented in Appendix A. These boring logs represent our interpretation of the observed subsurface conditions based on the field logs, visual examination of field samples by our personnel, and test results of selected field samples. Each stratum has been designated by grouping soil/rock layers that possess similar physical and engineering characteristics. The lines designating the interfaces between strata on the boring logs represent approximate boundaries. Transitions between strata may be gradual.

Stratum I consists of stiff to hard, dark brown fat clay (CH), and brown to tan and gray lean clay (CL) with some clayey sand (SC). These soils are classified as moderate to highly plastic based on measured plasticity indices ranging from 13 to 56. Measured moisture contents range from 1 to 35 percent. Based on grain size analyses, the percentage of fines (percent passing a No. 200 sieve) ranges from 36 to 94 percent. This stratum extends to depths ranging from 1 to 12 ft below the existing ground surface in our borings.

Stratum II consists of hard, light gray and light tan, decomposed to highly fractured and weathered vuggy limestone. TCP test N‐values range from 73 to 100 blows for refusal. Core recovery values range from 17 to 100 percent, and RQD values range from 0 to 100 percent. All borings terminate in this stratum except Boring B‐7.

Stratum III consists of hard, dark gray, highly fractured and weathered clayshale and was only encountered in Boring B‐7 at a depth of 46 ft below the existing ground surface. TCP test N‐values range from 100 blows for 1‐1/2 inch of penetration to 100 blows for 1 inches of penetration. Boring B‐7 terminates in this stratum.

GROUNDWATER

Based on the results of our field exploration, free water was not observed within the open boreholes during drilling. However, it is possible for groundwater to exist beneath this site at shallow depths on a transient basis, particularly at the clay/limestone interface, within weathered limestone seams, and following periods of precipitation. Fluctuations in groundwater levels typically occur due to variation in rainfall and surface water run‐off. The construction process itself may also cause variations in the groundwater level.

BRIDGE FOUNDATION RECOMMENDATIONS

The proposed bridge at Jennings Branch is anticipated to be approximately 100 ft in length and the proposed bridge at Berry Creek is anticipated to be approximately 1,150 ft in length. The approximate locations of the bridges are shown in the Boring Location Map in Figures 3A and 3I. It is our understanding that the proposed bridge at Jennings Branch will replace an existing culvert. It should be noted the specific stationing of the bridge foundations and the diameters of the drilled shafts were not finalized at the time of this report.

AXIAL CAPACITY

The TxDOT TCP values from the bridge borings (B‐series borings) and Figures 5‐1 through 5‐4 of the 2020 TxDOT Geotechnical Manual were used to develop the allowable axial capacities for straight‐sided drilled shafts to support the proposed bridge. These figures utilize factors of safety of at least 2 for skin friction and end bearing in determining allowable capacities (see individual figures from the Geotechnical Manual for specific factors of safety). The capacities were developed based solely on the TCP blow count data obtained from the borings. A soil reduction factor of 0.7 was used to compute the skin friction values for drilled piers.

Disregard depths were applied to the axial capacity curves. Specifically, the embankment fills for abutment shafts and 10 ft below existing ground surface were disregarded.

Axial capacity charts for each boring are presented in Appendix C. The maximum allowable drilled shaft service loads should be determined in accordance with Table 5‐1 of the 2020 TxDOT Geotechnical Manual.

If higher loads are planned, a detailed structural analysis of the drilled shaft design is required. Our recommendations for axial capacity of deep foundations assume drilled shafts will be constructed following the procedures and requirements in the TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, November 2014 Edition.

Axial Group Effects

In some cases, the total allowable axial capacity for foundation groups may be less than the sum of the individual foundation capacities within the groups. This is common when the center‐to‐center spacing of foundations are less than three (3) pier diameters. Based on our experience with similar projects, we assume that the foundations for each abutment and bent will have a center‐to‐center spacing greater than three pier diameter widths. Therefore, we do not anticipate reduction in the individual foundation capacity to affect the axial capacity for the foundation groups. We should re‐evaluate the axial group effects if the foundations within the groups are spaced less than three center‐to‐center shaft diameters apart.

LATERAL RESISTANCE

Soil parameters for evaluating the lateral resistance characteristics of the drilled shafts are shown in the tables below. The provided soil parameters are based on the LPile User’s Manual version 2022 (Released February 2022), laboratory testing results and TCP blow count correlations. We understand that detailed lateral capacity analysis will be performed by others. We recommend that appropriate measures be taken during the LPile analysis to account for sloping ground surface and the use of temporary casing during drilled shaft installation.

Assumed Behavior for Analysis Depth* (ft) c (psf)

Ɛ50

(pcf)

UCS

(psi)

Soft Clay (Matlock) 0 to 5 500 0.02 100 ‐

Stiff Clay without Free Water (Reese) 5 to 10 2,500 0.007 115 ‐

Strong Rock (Vuggy Limestone) 10 to 50 ‐ ‐ 140 2,000 *Depth below existing ground surface at the time of this study

Where: c = undrained cohesion

Ɛ 50 = strain factor = total unit weight UCS = unconfined compressive strength

ESTIMATED SHAFT SETTLEMENT

Based on the subsurface conditions encountered at the site and provided the drilled shafts are designed in accordance with current TxDOT standards, we estimate total vertical settlements on the order of less than 1 inch for the foundations. Post‐construction settlement is estimated to be on the order of less than 1/2 inch between adjacent substructures.

In order to limit differential settlement between wingwalls and abutments, drilled shafts for these structures should be of similar length. Post‐construction settlement will be dependent on the final structural loading, foundation diameter, foundation depth, foundation spacing, and group size. We recommend that RKCI be retained to review the final loads, foundation group layouts, and to check estimated foundation settlements.

It should be noted that groups of drilled shafts may settle more than individual shafts subject to the same load per shaft. The settlement of shafts groups is dependent on several variables, including the dimension of the shafts group, the shafts length, the sustained structural loads, and the compressibility characteristics of the foundation soils. For the shaft groups planned for this project, we do not anticipate groups greater than 5‐by‐5, or shafts spacing less than three shaft diameters.

DRILLED SHAFT CONSTRUCTION CONSIDERATIONS

Drilled pier construction should follow TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, November 2014 Edition, Item 416. Presented below are a few specific considerations.

Drilled shaft excavations should be inspected for verticality and side sloughing. Verticality is specified at one inch in ten feet of the shaft length and should be checked to the full depth of dry augering prior to introducing drilling mud. The shaft should be within 1 inch of horizontal plan when the shaft is under a column, and 3 inches when under a footing.

Drilling slurries should meet the requirements of TxDOT Standard Specification Item 416, Table 3. Surface casing should be installed a minimum of 10 feet below the surface before introducing slurry.

Before placing concrete, the shaft bottoms should be cleaned with a "clean‐out" bucket until rotation on the bottom without crowd (i.e. penetration under force) produces little spoil. An air lift can also be used to remove any lose material in the bottom of the shaft.

Probing after clean out is essential to verify the condition of the base of the shaft.

Concrete placement should be accomplished as directed in TxDOT Standard Specification Item 416.3.6. A water‐reducing, retarding admixture in accordance with DMS‐4640, “Chemical Admixtures for Concrete” should be used in all concrete when using casing that will be pulled or when placing shafts under water or under slurry.

Concrete should be placed using a closed tremie or be pumped to the bottom of the excavation when using slurry displacement or underwater concrete placement methods.

The tremie pipe diameter should be at least eight times as large as the largest concrete aggregate size. Concrete should be placed continuously from beginning of placement until the shaft is complete.

A computation of the final concrete volume for each shaft should be made. Shafts taking an unreasonably high or low volume of concrete should be cored to check their integrity.

If casing is used, it should be extracted slowly and smoothly with a vibratory hammer. The casing should always remain at least one foot below the level of the concrete during placement.

Shaft excavations should not be made within three shaft diameters (edge to edge) of shafts that have been concreted within the last 24 hours.

RETAINING WALL RECOMMENDATIONS

We understand mechanically stabilized earth (MSE) retaining walls are proposed at several locations along the project alignment, including at the locations of bridge approach embankments. The approximate locations of the proposed retaining walls are shown in Figures 3A through 3D, and 3F through 3I.

The maximum exposed heights of the retaining walls are anticipated to be on the order of 10 to 25 ft. As such, we have assumed a maximum wall height of 25 ft for our analysis. Moreover, we assumed that the wall base will be supported on undisturbed native clay or select fill, meeting the requirements described in the Embankment Fill section of this report. Recommendations for design of the retaining walls are presented in the following sections.

MINIMUM EMBEDMENT

Minimum wall embedment, taken as the distance from the top of the leveling pad to the ground surface in front of the wall, should be at least 1 ft where there is level ground in front of the wall and there is no potential of erosion or future excavation. The minimum embedment should be 2 ft when there are slopes steeper than 4H:1V but less than 3H:1V in front of the wall. Adjacent improvements such as pavement and sidewalk sections or landscaping topsoil should not be considered in the wall embedment.

Additionally, the wall embedment should increase in areas of potential excavation so that the base of the wall is at least 1 ft below the bottom of the excavation.

FOUNDATION SOIL

Leveling pads should be established on competent, undisturbed native soil or new embankment fill overlying native soil. Based on results of our borings, we assume native subgrade at the retaining wall locations consists of Stratum I very stiff to hard clays and/or Stratum II limestone.

RETAINED SOIL

Retained soil should consist of embankment fill material meeting the specifications of TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, Item 132 Type A or B.

The retained soil zone should extend from behind the base of the select backfill (reinforced zone) and project upwards at a 1V:1H or flatter. Additional recommendations for use of embankment fills are discussed in the Embankment Fill section of this report. Recommendations for fill benching are presented in the New Fill Benching section of this report.

SELECT BACKFILL (REINFORCED ZONE)

Select backfill is required within the reinforced zone of the MSE retaining wall. The reinforced zone extends horizontally from the back of the wall to at least 2 ft beyond the end of the reinforcement. The reinforced zone extends vertically from the top of the leveling pad to the top of the wall. Select backfill material should meet the specifications of TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, Item 423 Type BS.

Select backfill should be placed in loose lifts not exceeding 8 in. and be compacted to a minimum of 95 percent of the maximum dry density, as determined by Tex‐114‐E. It should be noted that over compaction of select backfill can result in additional lateral forces on the wall facing units. Therefore, we recommend hand‐operated or walk‐behind equipment be used for backfill placed within 3 ft of the back of wall face or as specified by the wall manufacturer.

DRAINAGE

The retaining wall recommendations presented herein have been provided assuming that drains will be installed to prevent buildup of hydrostatic pressures behind all walls. A minimum 6‐inch‐diameter, perforated collector pipe should be placed at the base of the walls directly below the reinforced zone. The pipe should be embedded in a minimum 2 ft wide and 2 ft deep zone of angular drain rock that is wrapped in a drainage geotextile fabric. The perforated collector pipes should discharge at an appropriate location away from the base of the wall. The discharge pipe(s) should not be tied directly into stormwater drain systems, unless measures are taken to prevent backflow into the drainage system of the wall.

MINIMUM REINFORCEMENT LENGTH

Based on the results of external and global stability analyses, the minimum reinforcement lengths for MSE walls at the site are presented in the table below. The minimum reinforcement lengths satisfy minimum factors of safety (F.S. = 1.5 to 2) for sliding, overturning, eccentricity, and bearing, as well as for global stability. A summary of our external and global stability analyses is presented in Appendix E.

Retaining Wall (Approximate Station Range)

Associated Borings

Maximum Wall Height (ft)

Minimum Strap Length (ft)*

WBFR STA 1987+00 to STA 1992+00 B‐1, B‐2 15 H ≤ 15 ft 0.9H

WBFR STA 2015+00 to STA 2020+00 RW‐1, RW‐2 15 H ≤ 15 ft 0.9H

WBFR STA 2104+00 to STA 2118+00 RW‐6 to RW‐11 25 20 ft < H ≤ 25 ft 1.0H

H ≤ 20 ft 0.9H

H = MSE Wall Height *Minimum strap length shall be no less than 8 ft.

For our drained analyses, we assumed no hydrostatic pressures are developed behind the walls. Also, we included a surcharge load of 250 psf to account for vehicular loading, and used the properties of the backfill and retained fill material as recommended by TxDOT RW‐MSE‐22.

The following sections provide a discussion of the analyses performed in order to arrive at the minimum strap length ratios above.

External Stability

The computer program MSEW+, version 2024.01, developed by ADAMA Engineering Inc. was used to analyze external stability of the walls. External stability analyses were performed for varying wall heights ranging from 10 to 25 ft tall (as appropriate – see table above) using the soil parameters and loading conditions discussed in this report. The minimum factors of safety and eccentricity considered in our analyses included the following:

Bearing Capacity Failure = 2.0 Base Sliding = 1.5 Overturning = 1.5 Eccentricity within the middle third of the width of the reinforced mass

The soil parameters presented in the table below were assigned for our external stability analyses based on our findings and the TxDOT standard sheet Mechanically Stabilized Earth Retaining Walls, RW (MSE). The friction angle of the foundation soil was specifically assigned based on TCP blow counts.

Soil Parameters for External Analysis

Soil Description

Sliding, Overturning, Eccentricity Bearing c

(deg)

Reinforced Zone 105 0 34 125 0 34

Retained Soil/New Embankment Fill 125 0 30 125 0 30

Foundation Clay Subgrade 115 1,500* 26** 115 1,500* 26** *Only considered for undrained analysis **Only considered for drained analysis

Where: = total unit weight c = cohesion

= friction angle

A summary table of the minimum reinforcement lengths required to satisfy the above factors of safety and eccentricity resulting from our external stability analyses is included in Appendix D. In addition, a comprehensive analysis package is presented in Appendix E.

Global Stability

The global stability analyses were performed using the limit equilibrium computer program Slide2, ver. 9.0345, developed by Rocscience. The analyses were performed on typical MSE wall cross‐sections for each wall at wall heights ranging from 10 to 25 ft. The Spencer method was used to evaluate the short‐term (undrained) and long‐term (drained) conditions of the wall section using the soil parameters shown in the table below. The soil parameters used to represent the foundation soil are based on the results of soil classification laboratory testing and the measured TxDOT TCP values. The reinforced zone was modeled as having infinite strength, in order to force the failure surface to not go through the reinforced zone, since the external stability had been evaluated in the External Stability section of the report.

Soil Parameters for Global Stability Analysis

Soil Description

Undrained Condition Drained Condition

Reinforced Zone 125 Infinite Strength Infinite Strength

Retained Soil/New Embankment Fill 125 0 30 0 30

Foundation clay subgrade 115 1,500 0 100 26

Where: = total unit weight c = cohesion = friction angle

TxDOT specifies a minimum factor of safety of 1.3 for global stability of non‐critical walls, and 1.5 for global stability of walls supporting abutments and structures with low failure tolerance. Based on prior experience and the project information provided to use, RKCI selected a minimum factor of safety for undrained and drained global stability analysis of 1.5 for all walls on this project. The results of our analyses indicate critical slip surfaces maintain a minimum factor of safety on the order of 1.5 and 2.0 for the drained and undrained conditions, respectively. We also evaluated the walls for rapid drawdown conditions. For rapid drawdown condition analysis, a minimum factor of safety of 1.3 was selected. The factors of safety for the critical slip surfaces are presented in a comprehensive analysis calculations package in Appendix E.

Retaining Wall (Approximate Station

Range)

Associated Borings

Maximum Wall

Height (ft)

Minimum Strap Length

(ft)*

Resulting Factor of Safety

Drained Undrained Rapid

Drawdown

WBFR STA 1987+00 to

STA 1992+00 B‐1, B‐2 15 0.9H 1.5 > 2.0 1.3

WBFR STA 2015+00 to

STA 2020+00 RW‐1, RW‐2 15 0.7H 1.5 > 2.0 1.5

WBFR STA 2104+00 to STA 2118+00 RW‐6 to RW‐11 25 1.0H 1.5 1.7 1.3

*H = Wall Height

EMBANKMENT DESIGN CONSIDERATIONS

BRIDGE APPROACHES

Placement and compaction of embankment fill at bridge approaches should be considered during design.

Improper fill placement can result in excessive differential movement between the bridge abutment and adjacent embankment fills. Historical cases have shown that this type of differential settlement has been a problem at some bridge locations and should be considered specifically during bridge design. When a problem is encountered it develops over time after the bridge is opened to traffic.

As settlement continues to occur with time the pavements adjacent to the bridge abutments settle. Since the bridge abutment is supported on deep foundations it settles much less. Resulting differential settlement may be compounded by improper fill compaction, impacting rideability as vehicles encounter the bridge at speed. When the vehicle crosses onto the bridge, the vehicle rises abruptly, and when the vehicle crosses off the bridge, the vehicle falls abruptly. Even relatively small levels of differential settlement create a rideability problem. As previously discussed, we estimate that, if embankment fill is properly constructed, the majority of soil settlement will be completed during the construction period;

therefore, there is a low potential for differential settlement problems.

FILL‐INDUCED SETTLEMENT

Based on the subsurface conditions encountered in our borings, fill‐induced settlements on the order of

1.0 percent of the new fill heights is estimated for the site. Based on preliminary cross section drawings provided by American Structurepoint Inc. on December 5, 2024, we estimate maximum fills are on the order of 10 ft or less. The majority of fill‐induced settlements should be elastic and occur during fill placement. We estimate that post‐construction fill‐induced settlement will be on the order of 1/2 inch or less with differential settlement also on the order of 1/2 inch or less.

EMBANKMENT SLOPES

Embankment slopes that will be mowed should not exceed 3H:1V. Embankment slopes steeper than 3H:1V should be covered with a non‐erodible material such as stone riprap or concrete. Likewise, a non‐ erodible material should cover slopes where portions of the slope is below the high water elevation (i.e.

subject to inundation). Also, if stone riprap is used, we recommend a filter geotextile fabric be placed between the riprap and fill soil. The non‐erodible material should key into the toe of the slope no less than 1.5 feet or the potential scour depth, whichever is greater.

Pavements should be located at least 5 feet from the slope face. The slopes should be planted with appropriate vegetation as soon as possible after grading to provide protection against erosion. Surface water runoff should be collected and directed away from slopes to prevent water from running down the face of the slope.

NEW FILL BENCHING

Fills should not be placed on slopes without benching; slopes either existing or created during the construction process. We anticipate that the new fill material for embankments will be in accordance with TxDOT Item 132. Benching will help reduce the potential for sloughing or creating an artificial failure plane in which the material is being placed on. The benches should be approximately 6 feet wide or 1‐1/2 times the width of the compaction equipment, whichever is wider. Embankment slopes should be constructed by extending the compacted fill beyond the planned profile of the slope and then trimming the slope to the desired configuration.

EMBANKMENT FILL

Fill materials should meet the specifications of TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, Item 132, Type A or B. We recommend the liquid limit and plasticity index of the material be less than 45 and 25, respectively. If gravel is used as fill material, we recommend the gravel be processed to have at least two crushed faces.

The material should be compacted to at least 98 percent of maximum dry density as determined by Tex‐ 114‐E. The moisture content during compaction should be equal to or greater than the optimum moisture content.

SELECT FILL

Select backfill material should meet the specifications of TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, Item 423 Type AS or BS meeting the “rock backfill” designation (85 percent or more retained on the No. 4 Sieve). Select backfill should be compacted to a minimum of 95 percent of the maximum dry density, as determined by Tex‐114‐E. If MSE walls are designed to be inundated during flood events, Type DS backfill should be used instead.

EXCAVATION

All excavations should be made in accordance with applicable OSHA and TxDOT regulations. While RKCI describes certain approaches to excavations, the contractor is responsible for selecting the excavation methods, monitoring the excavations for safety, and providing shoring as required to protect personnel and adjacent areas.

Excavations should not undermine adjacent utilities, foundations, walkways, streets, or other hardscapes unless shoring or underpinned support is provided. Unsupported excavations should not be constructed with a slope steeper than 1H:1V (Horizontal: Vertical) from 2 feet outside the edge of an adjacent structural feature.

Temporary Slopes

The development of a trench safety plan, which could include designs for sloping, benching, various types of temporary shoring or a combination thereof are the responsibility of the Contractor. Any such designs and safety plans shall be developed in accordance with current Occupational Safety and Health Administration (OSHA) guidelines and other applicable industry standards. Based on encountered subsurface conditions, we recommend excavation side slopes up to 20 feet high be no steeper than the following recommendations:

Soil Classification Soil Type Slopes (H:V)

Clay Type A 3/4:1*

Sand/Clayey Gravel Type C 1.5:1

Limestone Stable Rock Vertical *Per OSHA 1926 Subpart P App B, where Type A soils overlie Type C soils, the overlying clays should also be sloped back at a slope of 1.5:1 or flatter. All cut slopes should be protected from erosion. If sloughing or instability is observed, the slope should be flattened, or the cut supported by shoring.

Excavation Equipment

Due to the shallow nature of the surficial soils, excavations at this site will likely require removal of the underlying rock formation. Thus, the need of rock excavation equipment should be anticipated for construction at this site. Our boring logs are not intended for use in determining construction means and methods and may therefore be misleading if used for that purpose.

We recommend that earth‐work and utility contractors interested in bidding on the work perform their own tests in the form of test pits to determine the quantities of the different materials to be excavated, as well as the preferred excavation methods and equipment for this site.

EROSION PROTECTION

The establishment of permanent erosion control measures is beneficial for long‐term aesthetics and reduces soil erosion. We recommend that until permanent erosion control measures can be established, temporary erosion measures should be implemented. These temporary measures may include, but not limited to, straw mulching or matting, and/or straw wattles.

CULVERT RECOMMENDATIONS

CULVERT FOOTINGS

Footings should be founded on competent, undisturbed native soil. The footings should be embedded at least 18 inches below the finished grade. If downslopes are present, the embedment should increase so that a minimum horizontal distance of 5 ft is provided between the bottom outer edge of the footing and the face of the adjacent downslope. Additionally, the footings should be protected from undermining such caused from erosion/scour.

Footings bearing above the existing base elevation of the channel may be designed using a maximum allowable bearing pressure of 2,500 psf. Footings extending below the base elevation of existing channels and bearing on very stiff to hard clays or limestone, may be designed using a maximum allowable bearing pressure of 4,000 psf.

Culvert footings will likely be subject to inundation and potentially scour. Therefore, protection measures should be implemented to protect these footings from undermining resulting from erosion/scour. Some protection measures include armoring the existing slopes with ballast or concrete riprap and deepening the footing to a depth greater than potential scour. Another available option to reduce the risk of undermining is utilizing controlled low‐strength material (CLSM) below the footings. If this option is utilized, the CLSM should meet City of Austin Standard Specification, Item 402S and have a minimum 28‐ day compressive strength of 100 psi.

An ultimate coefficient of friction of 0.35 may be used to evaluate resistance to sliding for footings founded on native soils. If a key is planned beneath the footing, a passive earth pressure coefficient of 200 psf may be assumed in design. If the passive earth pressure and friction components are combined, the passive component should not exceed 2/3 of the total.

LATERAL EARTH PRESSURES

Equivalent fluid density values for computation of lateral soil pressures acting on retaining walls were evaluated for various types of backfill materials that may be placed behind the retaining walls. These values, as well as estimated unit weights, are presented below in preferential order for use as backfill materials.

Backfill Type

Estimated Total Unit Weight (pcf)

Active Condition At Rest Condition

Earth Pressure Coefficient, ka

Equivalent Fluid Density (pcf)

Earth Pressure Coefficient, ko

Equivalent Fluid Density (pcf)

Select Fill 135 0.28 38 0.44 60

Embankment Fill 125 0.33 41 0.50 63

The values tabulated above under “Active Conditions” pertain to flexible retaining walls free to tilt outward as a result of lateral earth pressures. For rigid, non‐yielding walls, the values under “At‐Rest Conditions” should be used.

The design equivalent fluid densities should be increased for walls that retain sloping soil. We recommend the equivalent fluid densities listed above be increased using the following factors when designing walls that retain sloping soil.

Slope of Retained Soil (degrees)

Lateral Earth Pressure Increase Factor

0 1.00

5 1.06

10 1.12

20 1.33

25 1.52

30 2.27

The use of fat clay soils as backfill against the proposed retaining structures is not recommended. These soils generally provide higher design active earthen pressures, as indicated above, but may also exert additional active pressures associated with swelling. Controlling the moisture and density of these materials during placement will help reduce the likelihood and magnitude of future active pressures due to swelling, but this is no guarantee.

BACKFILL COMPACTION

If settlement sensitive structures are present within the backfill zone of the wall, the backfill materials should be placed and compacted as recommended in the Embankment Fill section of this report, or to the manufacture’s specifications. Furthermore, within at least a 3‐ft zone directly behind the wall, we recommend that compaction be accomplished using hand‐guided compaction equipment capable of achieving the maximum density in a series of 3 to 5 passes.

DRAINAGE

If applicable, drainage may be provided by the use of a drain trench and pipe. The drainpipe should consist of a slotted, heavy‐duty, corrugated polyethylene pipe and should be installed and bedded according to the manufacturer’s recommendations. The drain trench should be filled with drainage aggregate and extend from the base of the structure to within 2 ft of the top of the structure.

The bottom of the drain trench will provide an envelope of gravel around the pipe with minimum dimensions consistent with the pipe manufacturer’s recommendations. The gravel should be wrapped with a suitable geotextile fabric (such as Mirafi 140N or equivalent) to help reduce the intrusion of fine‐ grained soil particles into the drain system. The pipe should be sloped and equipped with clean‐out access fittings consistent with state‐of‐the‐practice plumbing procedures.

As an alternative to a full‐height gravel drain trench behind the proposed retaining structures, consideration may be given to utilizing a manufactured geosynthetic material for wall drainage. A number of products are available to control hydrostatic pressures acting on earth retaining structures, including Amerdrain (manufactured by American Wick Drain Corp.), Miradrain (manufactured by Mirafi, Inc.), Enkadrain (manufactured by American Enka Company), and Geotech Insulated Drainage Panel (manufactured by Geotech Systems Corp.). The geosynthetics are placed directly against the retaining structures and are hydraulically connected to the gravel envelope located at the base of the structures.

Weepholes may be provided along the length of the proposed retaining structures, if desired, in addition to one of the two alternative drainage measures presented above. Based on our experience, weepholes, as the only drainage measure, often become clogged with time and do not provide the required level of drainage from behind retaining structures. We recommend that RKCI review the final retaining structure drainage design before construction.

CULVERT CONSTRUCTION CONSIDERATIONS

SITE PREPARATION

Fill areas should be stripped of all vegetation and organic topsoil. Tree roots greater than 1 inch in diameter should be grubbed and removed. Exposed subgrades should be thoroughly proofrolled in order to locate weak, compressible zones. A minimum of 5 passes of a fully‐loaded dump truck or a similar heavily‐loaded piece of construction equipment should be used for planning purposes. Proofrolling operations should be observed by the Geotechnical Engineer or their representative to document subgrade condition and preparation. Weak or soft areas identified during proofrolling should be removed and replaced with suitable, compacted on‐site soils, free of organics, oversized materials, and degradable or deleterious materials.

New Fill Benching

Fills should not be placed on slopes without benching in accordance with the New Fill Benching section of this report under Embankment Design Considerations.

Fill Materials

Fill materials should meet the specifications of TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges, Item 132, Type A or B.