7-B08_Final_Geotech_Report_HEHO_160186_12.10.18.pdf

REPORT COVER PAGE

Geotechnical Engineering Report Restore and Stabilize Hoover Creek for Flood Mitigation

West Branch, Iowa December 10, 2018

Terracon Project No. 06185043.01

Prepared for:

HDR Engineering, Inc.

Omaha, Nebraska

Prepared by:

Terracon Consultants, Inc.

Cedar Rapids, Iowa

Terracon Consul tants, Inc. 2640 12 t h S treet SW Cedar Rapids, Iowa P (319) 366 8321 F (319) 366 0032 terracon.com

REPORT COVER LET TER T O SIGN

December 10, 2018

HDR Engineering, Inc.

8404 Indian Hills Drive Omaha, Nebraska 68114-4098

Attn: Mr. L. Randall Stahmer, P.E. – Senior Project Manager P: 402.399.4916 E: Randy.Stahmer@hdrinc.com

Re: Geotechnical Engineering Report Restore and Stabilize Hoover Creek for Flood Mitigation West Branch, Iowa Terracon Project No. 06185043.01

Dear Mr. Stahmer:

We have completed the Geotechnical Engineering services for the above referenced project. This study was performed in general accordance with Terracon Proposal No. P06185043 dated March 5, 2018. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork for the detention basin, Hoover Creek channel modifications, and the sanitary sewer, as well as the design and construction of foundations for new bridge, retaining wall, and detention basin overflow structures.

We appreciate the opportunity to be of service to you on this project. If you have any questions concerning this report, or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

Justin D. Widdel, P.E. Gregory J. Klein, P.E.

Iowa No. 20495 Principal

Copies: PDF – Client PDF – Brandon Luster, P.E., CFM (Brandon.Luster@hdrinc.com) mailto:Randy.Stahmer@hdrinc.com mailto:Brandon.Luster@hdrinc.com

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REPORT TOPICS

REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

GEOTECHNICAL OVERVIEW

SITE PREPARATION

EARTHWORK

RETAINING WALL FOUNDATIONS

LATERAL EARTH PRESSURES

BRIDGE FOUNDATIONS

DETENTION POND RETURN CULVERT

SANITARY SEWER AND WATERLINE

CORROSIVITY

FROST CONSIDERATIONS

SLOPE STABILITY

GENERAL COMMENTS

SIGNATURE PAGE

Note: This report was originally delivered in a web-based format. Orange Bold text in the report indicates a referenced section heading. The PDF version also includes hyperlinks which direct the reader to that section and clicking on the logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.

ATTACHMENTS

EXPLORATION AND TESTING PROCEDURES

SITE LOCATION AND EXPLORATION PLANS

EXPLORATION RESULTS (Geotechnical Models, Boring Logs, and Laboratory Data) SUPPORTING INFORMATION (General Notes, Unified Soil Classification System and Description of Rock Properties) http://client.terracon.com/

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INTRODUCTION

Geotechnical Engineering Report Restore and Stabilize Hoover Creek for Flood Mitigation

West Branch, Iowa Terracon Project No. 06185043.01

December 10, 2018

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed modifications to the Hoover Creek channel, detention basin, bridge over Hoover Creek, and the proposed sanitary sewer for the Herbert Hoover Presidential Library grounds in West Branch, Iowa. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil and rock conditions ■ Foundation design and construction

■ Groundwater conditions ■ Site preparation and earthwork

■ Site preparation and earthwork ■ Lateral earth pressures

■ Excavation considerations

The geotechnical engineering scope of services for this project included the advancement of twenty-three test borings to depths ranging from approximately 15.5 to 78.5 feet below existing site grades.

Maps showing the site and boring locations are shown in the Site Location and Exploration Plan sections, respectively. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included on the boring logs and/or as separate graphs in the Exploration Results section of this report.

SITE CONDITIONS

The following description of site conditions is derived from our site visit in association with the field exploration and our review of topographic maps aerial imagery.

Item Description

Parcel Information The project is located along Hoover Creek, from west of the Herbert Hoover Presidential Library to S 2nd Street in West Branch, Iowa See Site Location

Restore and Stabilize Hoover Creek for Flood Mitigation ■ West Branch, Iowa December 10, 2018 ■ Terracon Project No. 06185043.01

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Item Description

Existing Improvements

■ The detention basin is planned to be constructed in an undeveloped field.

■ The existing Hoover Creek channel is primarily eroded banks lined with trees and shrubs.

■ Existing singe-span bridge on N Downey Street crossing Hoover Creek

■ Failing limestone retaining wall on the south bank of Hoover Creek west of the N Downey Street bridge

■ Pavements, parking lots, and sidewalks in the area of the new Sanitary

Sewer

Current Ground Cover

■ Grass and bare ground in undeveloped areas and Hoover Creek channel embankments

■ Gravel and paved surfacing in developed areas

Existing Topography

■ The existing grades in the area of the detention basin decrease from south to north (towards Hoover Creek) with ground surface elevations ranging from about 730 feet to 715 feet.

■ Gradual grade changes north and south of Hoover Creek banks. Steep banks at portions of Hoover Creek due to erosion.

Geology Based on the soil map prepared by the USDA-NRCS, the upper soils near Hoover Creek consist of clay and silt. The landforms consist of drainageways, hillslopes, flood plains, interfluves, and alluvial fans.

PROJECT DESCRIPTION

The Schematic Design Preferred Alternative Final Design documents dated January 10, 2018 were provided for review along with the Civil Detention Area Grading and Layout plan (sheet C13), and the Civil Detention Area Sections (sheet C14) dated August 2018. Our current understanding of the project is summarized in the table below. Should any of the information or assumptions be inconsistent with the planned construction, please let us know so that we may make supplemental recommendations for the project, as necessary.

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Item Description

Planned Improvements

■ Constructing a 6.47-acre detention basin at the west end of the project. An intake weir structure and 24-inch RCP return culvert will connect the basin to Hoover Creek.

■ Widening the Hoover Creek channel and flattening the existing slopes to 4H:1V;

■ Placing bedding and erosion stone/boulders at the base of the channel;

■ Constructing riffle structures along the channel;

■ Replacing the existing bridge along N. Downey Street crossing Hoover Creek. The new bridge will be supported on two abutments;

■ Reconstructing the limestone wall west of the proposed bridge;

■ Rerouting the sanitary sewer from Downey Street to 2nd

Street (approximately 700 linear feet).

Grading (per sheet C13)

■ The crest of the detention basin slopes is expected to range from elevations of about 732 feet on the south side and 717 feet on the north side. The crests will generally match the existing grade elevation, and the detention basin bottom and embankments will be developed from site cuts. We understand the base elevation of the detention basin will be at 710 feet resulting in cuts of about 7 to 17 feet. The base of the detention basin will be sloped 0.5% towards the intake weir.

■ We have assumed that the bridge will match the existing pavement elevations.

Slopes We understand the slopes for the detention basin and Hoover Creek modified channel are currently planned to have inclinations ranging from 4H:1V to 11H:1V.

Sanitary Sewer

■ We understand the invert elevation of the sanitary sewer will be about 5 to 9 feet below existing grade.

■ Construction is planned to be open cut and/or trenchless construction

Waterline

■ 6” fusible PVC waterline (no casing) to be installed under

Hoover Creek at the Downey Street Bridge to be placed using trenchless construction

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Item Description

Limestone Wall

The failing portions of the historic limestone wall located west of N. Downey Street Bridge will be reconstructed with a cast-in-place wall and limestone veneer. A portion of the intact wall, adjacent to N. Downey Street, is planned to be left in place with a protection wall built in front of the historic wall. The base of the retaining wall is planned to be at an elevation of about 702 feet and top of wall at about 710 feet. The finished grade at the base of the wall is planned to be at an elevation of about 706 feet with a 10H:1V slope away from the wall towards Hoover Creek modified channel.

Downey Street Bridge

The existing bridge will be demolished and replaced with a new bridge with a width of 30 feet. The loading conditions were not provided. This report provides recommendations for supporting the new bridge on drilled shaft and micropile foundations.

GEOTECHNICAL CHARACTERIZATION

Subsurface Profile

We have developed a general characterization of the subsurface soil and groundwater conditions based upon our review of the data. Graphical representations of the characterization are provided as Geotechnical Models in the Exploration Results section. Conditions encountered at each boring location are indicated on the individual boring logs shown in the Exploration Results section and are attached to this report. Stratification boundaries on the boring logs represent the approximate location of changes in soil and rock types; in situ, the transition between native materials may be gradual or abrupt in existing fill materials.

The geotechnical characterization forms the basis of our geotechnical calculations and evaluation of site preparation, foundation options and pavement options. As noted in General Comments, the characterization is based upon widely spaced exploration points across the site, and variations are likely.

Groundwater Conditions

The boreholes were observed while drilling and after completion for the presence and level of groundwater. In addition, delayed water levels were also obtained in the piezometers installed in the detention basin borings. The water levels observed in the boreholes can be found on the Geotechnical Models, and the boring logs in the Exploration Results section.

These water level observations provide an approximate indication of the groundwater conditions existing on the site at the time the observations were made. Longer-term observations using

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Groundwater level fluctuations occur due to seasonal variations in the amount of rainfall, runoff, Hoover Creek flow, and other factors not evident at the time the borings were performed.

Therefore, groundwater levels during construction or at other times in the life of the structure may be higher or lower than the levels indicated on the boring logs. Also, trapped or “perched” water could be present within the sand or silt seams within native clay soils and/or in cohesionless soils (fill and native) above lower permeability clay soil layers. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.

GEOTECHNICAL OVERVIEW

General Considerations

The Soil Survey of Cedar County, Iowa was reviewed to identify soil types in the area of the subject site. The document was published in by the U.S. Department of Agriculture (USDA) Soil Conservation Service, now known as the Natural Resource Conservation Service (NRCS).

Terracon utilized the NRCS on-line Web Soil Survey (WSS)1 to identify soil types. The soil map provided by the NRCS is provided below.

Figure 1: NRCS Soil Map

1 Posted at: http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm, accessed June 27, 2018.

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The primary (more than 4% of site surface area) soil types mapped on the site by the USDA NRCS, and the degree of limitations for embankments, dikes, and levees, and surface water management systems associated with these soil types are tabulated below along with the susceptibility of the soil to sheet and rill erosion by water (K Factor, whole soil). The classifications provided are for the USDA textural soil classification system for approximately the upper 80 inches of the soil profile.

Mapped Soil Types (Unit Symbol)

Type and Degree of Limitations Soil Properties

Embankments, Dikes, and Levees

Surface Water Management

System

K Factor, Whole Soil 1

Lindley loam, 9 to 14 percent slopes, moderately eroded

(65D2)

Somewhat limited:

piping, dusty Not rated 0.37

Tama silty clay loam, 5 to 9 percent slopes, eroded

(120C2)

Somewhat limited:

dusty

Very limited: slope, water erosion 0.32

Colo silt loam, 0 to 2 percent slopes, occasionally flooded, overwash (133+)

Somewhat limited:

dusty Not limited 0.32

Ely silty clay loam, 2 to 5 percent slopes

(428B)

Very limited: depth to saturated zone, dusty

Somewhat limited:

slope, water erosion 0.32

Colo-Ely complex, 0 to 5 percent slopes

(911B)

Very limited: depth to saturated zone, dusty

Somewhat limited:

slope, water erosion 0.28

Downs silt loam, till plain, 9 to 14 percent slopes, eroded

(M162D2)

Somewhat limited:

dusty

Very limited: slope, water erosion 0.37

1. Per the NRCS, the Erosion factor K indicates the susceptibility of a soil to sheet and rill erosion by water. Factor K is one of six factors used in the Universal Soil Loss Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) to predict the average annual rate of soil loss by sheet and rill erosion in tons per acre per year. The estimates are based primarily on percentage of silt, sand, and organic matter and on soil structure and saturated hydraulic conductivity (Ksat).

Values of K range from 0.02 to 0.69. Other factors being equal, the higher the value, the more susceptible the soil is to sheet and rill erosion by water. Erosion factor for whole soil indicates the erodibility of the whole soil. The estimates are modified by the presence of rock fragments.

Detention Basin, Overflow Structure, and Culverts

We understand the base of the detention basin is planned to be at an elevation of about 710 feet.

Based on the water level observations in borings in the detention basin area, groundwater is

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Topsoil was encountered in the borings to depths ranging from about 1.5 to 4.5 feet below existing grade. The topsoil should be removed from below the overflow and culvert structures. The topsoil layer has benefits for vegetation growth, but due to variations in topsoil thickness, strength, and density, the permeability of the topsoil is anticipated to vary. Therefore, unfavorable drainage conditions (erosion, seepage, piping, etc.) could be associated with the topsoil if left in-place below the detention basin.

Creek Channel Modifications

The near-surface soils encountered in the borings performed near the existing creek channel (Borings C-1 to C-6) encountered about 1.5 to 3 feet of topsoil. The topsoil layer may be susceptible to erosion and other unfavorable drainage conditions, as noted above. We understand the channel modifications will consist of shifting the creek channel, and cutting/flattening the existing creek banks. To reduce erosion, the topsoil layer should be removed and replaced with less-erodible soils as discussed in the Site Preparation and Earthwork sections.

Existing fill and possible fill was encountered in Borings C-4 and C-5 to depths of about 6 to 9 feet. Concrete was encountered in Boring C-4 which may have been placed for previous bank stabilization. Existing fill encountered at time of construction should be removed due to the potential for voids and/or low density material within the fill stratum which could result in lower slope stability and higher potential for erosion and piping.

Limestone Retaining Wall

Existing fill was encountered in Boring RW-1 to a depth of about 5 feet below the existing grade (elevation of about 705.5 feet). We understand the retaining wall foundation is anticipated to be at an elevation of about 702 feet. Lower strength native soils, groundwater, and/or existing fill could be encountered below the new retaining wall foundations. Temporary dewatering and overexcavation and crushed stone backfill should be anticipated to improve the bearing conditions of the retaining wall as discussed in the Retaining Wall Foundations section.

We understand a portion of the limestone retaining wall is planned to remain in place near N.

Downey Street. Construction activities that could cause disturbance to the portion of the wall left in place should be closely monitored and reduced to the extent practical. These activities include, December 10, 2018 ■ Terracon Project No. 06185043.01

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N. Downey Street Bridge

Fill and possible fill materials were encountered in Borings BR-1 and BR-2 to depths of about 9 to 14 feet. Existing fill should not be relied on for axial support of the bridge foundations. The construction of the new bridge, and demolition of the existing bridge, may cause disturbance to nearby structures such as the historic retaining wall. Some damage or distress could result from vibrations from construction activities. In our experience, limiting peak particle velocities as low as 0.1 to 0.2 inches per second may be considered as tolerable vibration levels adjacent to historic structures. The contractor should perform the construction activities so as to limit the damage to adjacent structures and utilities. However, the owner should understand that conventional construction equipment and demolition/construction methods may not be capable of producing tolerable vibration levels especially for the historic wall in close proximity to the bridge. To reduce the risk of excessive vibrations, consideration should be given to performing vibration monitoring during demolition and construction. In addition, the project specifications could require that the contractor develop a Vibration Control Plan. To reduce the level of vibration associated with foundation construction for the new bridge, recommendations for micropile and drilled shaft/pile foundations are provided in the Bridge Foundations section. Difficult drilling conditions could be encountered if rubble material is encountered within the existing fill stratum as well as within gravel or cobble layers within the native soil strata.

The General Comments section provides an understanding of the report limitations.

SITE PREPARATION

Site Stripping

Complete stripping of the vegetation and topsoil should be performed below the structures and below embankment fills. To reduce the potential of deep topsoil erosion, the topsoil should be removed to a depth of at least 2 feet below planned finished grades in the creek channel, channel slopes, and the base of the detention basin. The exposed subgrade at the base of the stripping depth should be scarified, moisture conditioned, and recompacted in place to the moisture content and percent compaction levels provided in the Fill Compaction Requirements section. Topsoil placement for re-vegetation of slopes or the channel bottom should be consistent with needs to support intended plant growth. Turf reinforcement/geosynthetics can reduce erosion from rainfall events before vegetation is reestablished.

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Shallow Groundwater

Shallow groundwater is anticipated on this site and, if present, will likely contribute to subgrade instability. If similar groundwater levels are present at the time of construction, it may be necessary to pre-drain portions of the site by establishing an effective means of controlling groundwater to reduce disturbance of subgrade soils and allow for operation of construction equipment. This may be accomplished with a series of ditches, “French drains”, and/or subdrain lines in order to lower the groundwater level to at least 2 feet below finished site grades. It is important to note that stable soil or compacted fill can lose strength or soften if groundwater is allowed to rise towards the working construction grade and may require removal and replacement of the disturbed materials.

Excavation of the detention basin is anticipated to encounter soft, saturated soils. We are unaware of any structural features intended to be supported in the basin or the earthwork contractor’s methods or equipment support needs. This report assumes that limited subgrade preparation (densification, etc.) is necessary at the pond bottom to achieve long-term needs, and excavation techniques using equipment operating above the level of the basin bottom can be conducted. Contractor use of heavy equipment directly on the pond bottom (such as scrapers or haul roads) is expected to significantly rut the surface, and improvement will be necessary to provide adequate equipment support or repair the subgrade surface following removals.

Improvement would commonly include temporary dewatering to a depth of several feet below the level that equipment is operating. Because temporary dewatering could create an unintended outflow path from the pond, the contractor should submit dewatering plans to the design team for approval. Any dewatering trenches (or French drains) that extend through an intended berm area or well points that have increased potential to intercept sand seams should be avoided or have positive plans for abandonment.

Temporary dewatering of sewer trenches and other localized deeper excavations such as the culvert and retaining wall foundations should be anticipated, even with site pre-draining implemented. We expect that sump pits and pumps would generally be adequate for dewatering smaller trench excavations in clay soils. More extensive dewatering measures such as well points and sheeting should be anticipated for excavations extending into water bearing sand soils.

Diversion of the existing Hoover Creek will be required for the channel modifications and other improvements planned to occur within the existing creek channel.

Construction dewatering system design and implementation should be conducted by contractors with demonstrable experience in similar soil-water conditions to limit impacts to surrounding infrastructure. We recommended that a dewatering performance specification be included as part of the bidding documents.

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Subgrade Preparation

After rough grade has been established, the exposed subgrade should be proofrolled by the contractor and test probed by Terracon. However, obviously unstable subgrades should not be proofrolled to reduce disturbance of the subgrade soils, until after these soils have been stabilized.

Proofrolling could be accomplished by using heavy, rubber-tired construction equipment or sheepsfoot roller. This surficial proofroll would help to provide a stable base for the compaction of new structural fill, and delineates low density, soft, or disturbed areas that may exist below subgrade level. Soft or loose areas should be undercut, moisture conditioned, and recompacted or replaced with approved fill.

Soil Stabilization

Due to the lower strength (soft to medium stiff) clays encountered in the borings, corrective measures will probably be required to increase subgrade stability during subgrade preparation.

Additional costs should be budgeted to provide the required corrective measures. The unstable subgrade soils could be undercut, scarified on-site, and compacted with moisture and density control in maximum 9-inch loose lifts up to final subgrade elevation to provide a uniform thickness of well-compacted material.

Incorporation of hydrated lime, portland cement, or Class C fly ash could be considered for chemical stabilization of the unstable soils. Chemical modification should be performed by a pre-qualified contractor having experience with successfully stabilizing subgrades in the project area on similar sized projects with similar soil conditions. Due to the proximity of existing public use buildings and park facilities, and the hazards posed by blowing chemicals, the chemical modifiers should be placed by a wet slurry or other containment method. Additional testing would be needed for us to develop specific recommendations to improve subgrade stability by blending chemicals with the site soils. However, on a preliminary basis, 14 to 16 percent by soil weight of Class C fly ash and 4 to 6 percent by soil weight of hydrated lime or portland cement could be used for budgeting purposes. Additional testing could include, but not be limited to, determining the most suitable stabilizing agent, the optimum amounts required, the potential for sulfate induced heave, and freeze-thaw durability of the subgrade. Chemical stabilization should not be performed on the topsoil layer or partly organic site clays.

Similarly, where fill is placed for berms, unstable soils may be present. It may be possible to improve these soils to support the placement of new structural fill by scarification, drying, and recompaction; however, where this is not feasible, other stabilization methods will be required.

Stabilization with a crushed stone layer is not recommended below berms due to the higher permeability of these materials; chemical modification with lime, portland cement, and/or Class C fly-ash could be required. Alternatively, the unstable materials could be removed and replaced with low plasticity fill provided the requirements in Earthwork are achieved. Typically, stabilization with clay soils requires glacial till-origin clays (i.e. sandy lean clay). These soils were encountered in the borings, but not within anticipated excavation/stripping depths. As such, a

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The soils could become unstable with typical earthwork and construction traffic, especially after a precipitation event; therefore, effective drainage should be implemented early in the construction sequence and maintained during construction. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. If possible, the grading should be performed during the warmer and drier time of the year. If grading is performed during the winter months, an increased risk for possible undercutting and replacement of unstable subgrade will persist.

Stabilization techniques will likely be required for haul roads or areas subject to heavy or repeated loads. In sewer excavations, trench stabilization consisting of overexcavation of unstable lower strength soils and replacement with 1½ to 2½-inch nominal diameter crushed stone with less than 10 percent passing the U.S. No. 16 sieve to create a stable bearing surface

Excavation Considerations

Classification of the soil types presented in this report is based solely on the materials encountered in the borings. The subsurface material and conditions may vary between the borings and/or other unexplored locations across the site. Therefore, it is possible that the subsurface materials at various locations and depths may vary significantly. The contractor should verify that similar conditions exist throughout the excavation areas. Additional recommendations may be required if subsurface conditions are significantly different from those encountered in the borings.

As a minimum, excavations should be performed in accordance with OSHA 29 CFR, Part 1926, Subpart P, “Excavations” and its appendices, and in accordance with any applicable local, state, and federal safety regulations. The contractor should be aware that slope height, slope inclination, and excavation depth should in no instance exceed those specified by these safety regulations. Flatter slopes than those dictated by these regulations may be required depending upon the soil conditions encountered and other external factors. The stability of the excavation slopes should be reviewed continuously by qualified personnel.

All vehicles should be kept at least the distance equal to the depth of the excavation from the crest of excavation slopes to prevent slope failures and ensure safe working conditions. Vehicles adjacent to excavations would significantly influence the stability of the slopes as outlined by the OSHA regulations. Surface water should be diverted away from the excavation areas. Trench excavation, pipe placement, and backfilling should be completed as rapidly as possible. The length of open trench should be held to a minimum. Due to shallow groundwater, the site soils could be susceptible to sloughing or slope failure, even with relatively flat slopes, and this should

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Construction site safety and dewatering are the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety, or the contractor's activities; such responsibility shall neither be implied nor inferred.

EARTHWORK

Fill Material Requirements

The inorganic (i.e. less than 5 percent organic content) lean clay soils encountered in the borings are considered suitable for use as site mass grading fill. The lean to fat and fat clay soils will have a relatively high potential for shrink/swell which could develop surficial turf cracks. As such, they should not be used as fill in the upper 2 feet of finished subgrade elevation. Significant moisture conditioning (e.g. drying of clays) should be anticipated for on-site soils used as fill.

Compacted fill should meet the following material property requirements:

Fill Type 1 USCS Classification Acceptable Location for Placement Low plasticity cohesive 2 CL, CL-ML General site grading fill, berms

High plasticity cohesive CL/CH, CH More than 2 feet below finished subgrade in

“green space” areas

Granular GW, GP, GM, GC

SW, SP, SM, SC

Structural fill placed below culvert, weir, and retaining wall, and utility trench backfill 3, 4

Unsuitable ML, MH, CL-OL, OL, CH-

OH, CH, PT

More than 2 feet below finished subgrade in “green space” areas

On-site soils CL, CH, CL/CH, CL-OL, SP, SP-SC, SC Per the USCS classifications noted in this table

1. Suitable fill should consist of approved materials that are free of organic matter and debris, and should have a minimum maximum standard Proctor density of 95 pcf. Frozen material should not be used, and fill should not be placed on a frozen subgrade. A sample of each material type should be submitted to Terracon for evaluation prior to use on this site.

2. Low plasticity cohesive soil has a liquid limit less than 45 and a plasticity index less than 23.

3. Granular soils should not be used as utility trench backfill below berms.

4. SC soils could potentially be used for berm construction, after evaluation by Terracon.

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Appropriate laboratory tests, including Atterberg Limits for cohesive soils, organic content tests for dark colored soils and/or those that exhibit a noticeable odor, grain size analysis, and standard Proctor (ASTM D698) moisture-density relationship tests should be performed on proposed fill materials prior to their use as structural fill. Further evaluation of any on-site soils or off-site fill materials should be performed by Terracon prior to their use in compacted fill sections.

Fill Compaction Requirements

Fill materials should meet the following compaction requirements.

Item Description

Loose fill lift thickness 1 n 9 inches or less in loose thickness when heavy, self-propelled compaction equipment is used n 4 to 8 inches for pipe bedding and trench backfill n 8 to 12 inches for final trench backfill

Minimum compaction requirements 2, 3 n 95% below grade-supported slabs, culvert, and basin berms n 98% below retaining wall foundation n 90% for “green” areas

Moisture content range 2 n Low plasticity cohesive: -2% to +3% o Berms: 0 to +4% n High plasticity cohesive: 0 to +4% n Granular: -3% to +3%

1. Thinner lifts could be required if the required degree of compaction is not able to be obtained.

2. As determined by the standard Proctor test (ASTM D698).

3. If the granular material is a coarse sand or gravel, or of a uniform size, or has a low fines content, compaction comparison to relative density (ASTM D 4253 and D 4254) may be more appropriate.

Earthwork Construction Considerations

The lower strength clay subgrades will likely not support heavy rubber-tire excavation equipment such as scrapers especially in areas working near the groundwater level. Excavation by track-mounted backhoes with spoils removed by equipment operating above the finished subgrade level should be anticipated. Construction traffic over the completed subgrade should be avoided to the extent practical.

Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to construction of grade-supported slabs. Construction traffic over the completed subgrades should be avoided. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over, or adjacent to, construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the

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Construction Observation and Testing

Terracon should be retained during the construction phase of the project to observe earthwork and to perform necessary tests and observations during subgrade preparation, proofrolling, placement and compaction of controlled compacted fills, backfilling of excavations into the completed subgrade, and any other earthwork related construction activities.

In areas of foundation excavations, the bearing subgrade should be evaluated under the direction of Terracon. In the event that unanticipated conditions are encountered, the Geotechnical Engineer should provide recommendations.

In addition to the documentation of the essential parameters necessary for construction, the continuation of the Geotechnical Engineer into the construction phase of the project provides the continuity to maintain the Geotechnical Engineer’s evaluation of subsurface conditions, including assessing variations and associated design changes.

RETAINING WALL FOUNDATIONS

As noted in Project Description, portions of the existing limestone retaining wall are planned to be reconstructed with a cast-in-place concrete wall with limestone veneer. We understand approximately 30 feet of the wall adjacent to N. Downey Street is planned to be left in place, and a protection wall will be constructed in front of the historic wall. To reduce disturbance to the existing bearing soils of the existing wall during construction of the protection wall, consideration could be given to supporting both the existing and proposed wall on a specialty foundation system such as helical piers or push piles. Helical piers are designed to develop their capacity from a combination of side friction and/or end bearing in the native soils. Once the highly weathered limestone is reached, the benefits of multiple helices are largely diminished, and the foundation element is in essence a push pile deriving a substantial portion of its design resistance in end bearing on limestone bedrock. Helical piers and push piles allowable capacities can approach 50 to 60 kips for single elements, but site constraints may limit the equipment required to attain these capacities. Organic clay was encountered to a depth of about 14.5 to 24 feet below the existing grade in the boring performed near the retaining wall (Boring RW-1), and lower strength soils were encountered to the termination depth of about 30.5 feet. Medium dense sands and stiff clays were encountered below these a depth of about 40 feet below the existing grade in the nearby bridge boring (Boring BR-2). To reduce the risk of excessive long-term settlements of helical pier foundations, the helices/shafts should fully extend into (or below if required to develop capacity) the medium dense sand or stiff clay. Organic soils are typically corrosive to buried metals and protection should be provided.

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Due to the relatively slender cross-sections of each of these specialty foundation elements, lateral load resistance is typically limited, and often accommodated by passive resistance on the pile cap or additional elements placed as tie-back anchors. Eccentrically loaded foundations should be avoided or designed using pile groups or shafts in tension or compression. Push piles extended to bedrock will have a marked decrease in uplift resistance, and should not be used in designs requiring tensile resistance.

Helical piers and push piles are proprietary systems designed by licensed contractors, based on an interpretation of the boring logs. The potential for corrosive environments should be considered by the foundation designer. We recommend that a performance specification be developed regarding allowable capacities and tolerable settlements. In addition, if helical piles are used, we recommend a load test be performed to evaluate the actual capacity of the foundations.

The new retaining wall foundation is planned to be at an elevation of approximately 702 feet. Stiff clay was encountered at this elevation in Boring RW-1. However, the stiff clay was underlain by medium stiff clay, and groundwater (potentially perched) was encountered at an elevation of about

700.5 feet at the time of drilling. We have assumed the retaining wall will have relatively low contact pressure. However, due to the potential for lower strength clay and potential disturbance from groundwater and construction, granular fill is recommended to be placed below the retaining wall foundation. For preliminary planning purposes, we recommend at least 1 foot of relatively clean crushed limestone (IDOT Section No. 4121 or 4123) be placed below the retaining wall foundation. The depth of crushed stone may need to be increased depending on the maximum pressures exerted at the base of the wall foundation. Lateral widening of the overexcavation is discussed in the Shallow Foundation Construction Considerations section below. A separation fabric should be placed between the crushed stone and surrounding soil to prevent the migration of fine particles. Groundwater should be lowered to at least 2 feet below the excavation depth.

The following design parameters are applicable for the retaining wall foundation.

Shallow Foundation Design Parameters

Item Description Maximum Net Allowable Bearing pressure 1, 2 1,500 psf

Required Bearing Stratum 3 Compacted crushed limestone extending to suitable native soil

Ultimate Passive Resistance 4

(equivalent fluid pressures) See Lateral Earth Pressures section

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Item Description

Ultimate Coefficient of Sliding Friction 5 0.40 for cast-in-place foundations placed on a minimum of 1 foot of crushed limestone

Minimum Embedment below

Finished Grade 6 48 inches

Estimated Total Settlement from Structural Loads Less than about 1 inch

Estimated Differential Settlement About 2/3 of total settlement

1. The maximum net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. An appropriate factor of safety has been applied.

2. Values provided are for maximum loads noted in Project Description.

3. Unsuitable or soft soils should be overexcavated and replaced per the recommendations presented in this section.

4. Use of passive earth pressures require the sides of the excavation for the spread footing foundation to be nearly vertical and the concrete placed neat against these vertical faces or that the footing forms be removed and compacted structural fill be placed against the vertical footing face.

5. Can be used to compute sliding resistance where foundations are placed on suitable soil/materials.

6. Embedment necessary to minimize the effects of frost. For sloping ground, maintain depth below the lowest adjacent exterior grade within 5 horizontal feet of the structure.

Shallow Foundation Construction Considerations

As noted in Earthwork, the footing excavations should be evaluated under the direction of Terracon. If unsuitable bearing soils are encountered in footing excavations, the excavations should be extended deeper to suitable soils. Overexcavation for compacted backfill placement below footings should extend laterally beyond all edges of the footings at least 8 inches per foot of overexcavation depth below footing base elevation. The overexcavation should then be backfilled up to the footing base elevation with dense-graded granular material placed in lifts of 6 inches or less in loose thickness and compacted to at least 98 percent of the material's maximum standard Proctor dry density (ASTM D698).

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The base of all foundation excavations should be free of water and loose or soft soils prior to placement of reinforcing steel and concrete. If groundwater is encountered at the time of construction, it should be lowered and controlled to a minimum depth of 2 feet below the excavation elevation. Should the soils at the bearing level become disturbed, the affected soil should be stabilized or removed prior to placement of concrete. Concrete should be placed as soon as possible after excavating to minimize disturbance of bearing soils.

LATERAL EARTH PRESSURES

Design Parameters

Structures with unbalanced backfill levels on opposite sides should be designed for earth pressures at least equal to values indicated in the following table. Earth pressures will be influenced by structural design of the walls, conditions of wall restraint, methods of construction and/or compaction and the strength of the materials being restrained. Two wall restraint conditions are shown. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The "at-rest" condition assumes no wall movement and is commonly used for basement walls, loading dock walls, or other walls restrained at the top.

The recommended design lateral earth pressures are for cast-in-place concrete walls only, and do not include a factor of safety

Lateral Earth Pressure Design Parameters

Earth Pressure Condition 1

Coefficient for Backfill Type2

Surcharge Pressure 3, 4, 5 p1 (psf)

Effective Fluid Pressures (psf) 2, 4, 5

Unsaturated 6 Submerged 6

Active (Ka) Granular - 0.33 Lean Clay - 0.41

(0.33)S (0.41)S

(40)H (50)H

(80)H (85)H

At-Rest (Ko) Granular - 0.50 Lean Clay - 0.58

0.50)S (0.58)S

(60)H (70)H

(90)H (95)H

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Lateral Earth Pressure Design Parameters

Earth Pressure Condition 1

Coefficient for Backfill Type2

Surcharge Pressure 3, 4, 5 p1 (psf)

Effective Fluid Pressures (psf) 2, 4, 5

Unsaturated 6 Submerged 6

Passive (Kp) 7 Granular – 2.93 Lean Clay – 2.39 Fat Clay – 2.13

(350)H (290)H (255)H

(230)H (200)H (185)H

1. For active earth pressure, wall must rotate about base, with top lateral movements 0.002 H to 0.004 H, where H is wall height. For passive earth pressure, wall must move horizontally to mobilize resistance.

2. Uniform, horizontal backfill (in the retained zone), compacted to at least 95 percent of the ASTM D 698 maximum dry density, rendering a maximum unit weight of 120 pcf.

3. Uniform surcharge, where S is surcharge pressure.

4. Loading from heavy compaction equipment is not included.

5. No safety factor is included in these values.

6. In order to achieve “Unsaturated” conditions, follow guidelines in Subsurface Drainage for Below Grade

Walls below. “Submerged” conditions are recommended when drainage behind walls is not incorporated into the design.

7. Passive pressures are based on a 10H:1V ground inclination downhill of the wall.

Backfill placed against structures should consist of granular soils or low plasticity cohesive soils.

For the granular values to be valid, the granular backfill must extend out and up from the base of the wall at an angle of at least 45 and 60 degrees from vertical for the active and passive cases, respectively.

Subsurface Drainage for Below Grade Walls

A perforated rigid plastic drain line installed behind the base of walls and extends below adjacent grade is recommended to prevent hydrostatic loading on the walls. The invert of the drain line should be placed near foundation bearing level. The drain line should be sloped to provide positive gravity drainage to daylight. The drain line should be surrounded by clean, free-draining granular material having less than 6 percent passing the No. 200 sieve. The free-draining aggregate should be encapsulated in a filter fabric. The granular fill should extend to within 2 feet of final grade, where it should be capped with compacted cohesive fill to reduce infiltration of surface water into the drain system.

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As an alternative to free-draining granular fill, a pre-fabricated drainage structure may be used. A pre-fabricated drainage structure is a plastic drainage core or mesh which is covered with filter fabric to prevent soil intrusion, and is fastened to the wall prior to placing backfill.

BRIDGE FOUNDATIONS

Drilled Shaft Design Parameters

Drilled shafts extending into the limestone could be used for support of the N. Downey Street bridge abutments. Bedrock was encountered at depths of about 58 and 60.5 feet (elevations of about 656 to 653.5 feet) below the existing grades at Borings BR-1 and BR-2, respectively. Drilled shafts extending to limestone bedrock should be designed for end bearing conditions only in compression. Uplift capacities could be developed from frictional resistance between the shaft and soil/bedrock interface. Drilled shaft design methodology is in general accordance with Drilled Shafts: Construction Procedures and LRFD Design Methods, Publication No. FWHA-NHI-10-016 (LRFD Manual).

Geotechnical resistance factors are provided in Table 10-5 of the LRFD Manual for side resistance in uplift and base resistance. We recommend the following geotechnical resistance factors be used for design for calculating uplift and base resistances.

Geotechnical Resistance Factors (ϕ)

Soil Type (IDOT Soil Classifications) Side Resistance Base Resistance

Lean Clay and Sandy Lean Clay (silty clay and sandy clay)

0.35 - - -

Poorly Graded Sand and Gravel (fine sand, clayey sand, and coarse sand)

0.45 - - -

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Geotechnical Resistance Factors (ϕ)

Soil Type (IDOT Soil Classifications) Side Resistance Base Resistance

Limestone (bedrock N>200)

0.45 0.50

The soil design parameters provided below in the DRILLED SHAFT DESIGN SUMMARY tables should be used for the design of drilled shaft foundations for the north and south abutments, Borings BR-1 and BR-2, respectively. The values presented are nominal unit side and base resistances and do not include the resistance factors noted above. Uplift resistance should be ignored in existing and possible fill layers as well as the potential scour zone.

DRILLED SHAFT DESIGN SUMMARY (BORING BR-1)

Approximate

Elevation (feet)

Stratigraphy Nominal Unit Side Resistance

(psf)

Nominal Unit Base Resistance

(psf) 1Material

714 - 700 Topsoil and possible lean clay fill - - - - - -

700 - 694 Lean Clay 200 - - -

694 - 690 Sandy Lean Clay 550 - - -

690 - 679 Poorly Graded Sand with Clay and Gravel 1,500 - - -

679 - 673 Sandy Lean Clay 400 - - -

673 - 667 Sandy Lean Clay 200 - - -

667 – 659.5 Poorly Graded Sand with Clay and Gravel 2,000 - - -

659.5 - 656 Sandy Lean Clay 550 - - -

656 - 640 Limestone with shale and siltstone seams and layers 30,000 60,000

1. Shafts should extend at least one diameter into limestone bedrock.

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DRILLED SHAFT DESIGN SUMMARY (BORING BR-2)

Approximate

Elevation (feet)

Stratigraph…