Attachment 7 - Appendix A - Geotechnical Engineering Report.pdf

VA Central Iowa, Des Moines, IA VA Project 636-130 Laboratory Remodel and Addition 10-02-2024 3600 30th Street Bid Documents Des Moines, IA 50310

Appendix A

Appendix A:

-Geotechnical Engineering Report

REPORT COVER PAGE

Geotechnical Engineering Report

VA Laboratory Remodel and Addition Des Moines, Iowa

August 17, 2020 Terracon Project No. 08205172-01

Prepared for:

Anderson Engineering of Minnesota, LLC

Plymouth, Minnesota

Prepared by:

Terracon Consultants, Inc.

Des Moines, Iowa

Terracon Consul tants, Inc. 600 SW 7 t h Street, Sui te M Des Moines, Iowa 50309 P (515) 244 3184 F (515) 244 5249 terracon.com

REPORT COVER LETTER TO SIGN

August 17, 2020

Anderson Engineering of Minnesota, LLC 13605 1st Avenue North, Suite 100 Plymouth, Minnesota 55441

Attn: Mr. Jay Koester, P.E. – Senior Engineer P: (763) 412 – 4032 E: JKoester@ae-mn.com

Re: Geotechnical Engineering Report VA Laboratory Remodel and Addition 3600 30th Street Des Moines, Iowa Terracon Project No. 08205172-01

Dear Mr. Koester:

We have performed geotechnical engineering services for the referenced project in general accordance with Terracon Proposal No. P08205172 dated July 9, 2020. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of foundations and floor slabs for the proposed project.

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.

Grace J. Mercado, E.I. Kole C. Berg, P.E.

Staff Geotechnical Engineer Senior Associate/Senior Engineer mailto:JKoester@ae-mn.com

REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

PROJECT DESCRIPTION

GEOTECHNICAL CHARACTERIZATION

GEOTECHNICAL OVERVIEW

EARTHWORK

DEEP FOUNDATIONS

SEISMIC CONSIDERATIONS

FLOOR SLABS

LATERAL EARTH PRESSURES

GENERAL COMMENTS

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 GeoReport 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 PLAN

EXPLORATION RESULTS

SUPPORTING INFORMATION

Note: Refer to each individual Attachment for a listing of contents.

http://client.terracon.com/

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INTRODUC TION

Geotechnical Engineering Report VA Laboratory Remodel and Addition

3600 30th Street Des Moines, Iowa

Terracon Project No. 08205172-01 August 17, 2020

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for the proposed VA building addition to be located at 3600 30th Street in Des Moines. The purpose of these services is to provide information and geotechnical engineering recommendations relative to:

■ Subsurface soil conditions ■ Foundation design and construction

■ Groundwater conditions ■ Floor slab design and construction

■ Site preparation and earthwork ■ Estimated seismic site classification

■ Demolition considerations

The geotechnical engineering Scope of Services for this project included the advancement of three test borings to depths of approximately 55½ feet below existing site grades.

We previously performed geotechnical services near the proposed addition in 2012 and 2016 (Terracon Project Nos. 08125050-01, 08125059-01 and 08165148-01). Borings from these prior exploration programs were reviewed to supplement the borings for the current exploration.

Maps showing the site location and boring locations are shown in the Site Location and Exploration Plan section. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included on the boring logs in Exploration Results.

SITE CONDITIONS

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

VA Laboratory Remodel and Addition ■ Des Moines, Iowa August 17, 2020 ■ Terracon Project No. 08205172-01

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Item Description

Parcel Information

The project is located at existing VA Healthcare Campus located at 3600 30th

Street in Des Moines, Iowa.

Latitude/Longitude: 41.6282°, -93.6610° (approximate) See Site Location

Existing Improvements Existing VA Healthcare Campus buildings, drives, and parking areas

Current Ground Cover

Asphalt pavement driveways and parking areas, manicured grass, and existing buildings

Existing Topography Relatively flat (elevations 915 to 918 feet)

PROJECT DESCRIPTION

Our final understanding of the project conditions is as follows:

Item Description

Project Description

We understand laboratory remodel/demolition and building addition is planned at Building 1 (original building) at the west side of the Central Iowa Campus. The planned building addition will be five to six stories above grade in addition to a sub-basement and basement level. The exact location of the addition was not available at the time of this project.

Building Construction n Pre-cast or cast-in-place concrete framing with brick walls n Based on the structural loads, we anticipate the building will be supported on a deep foundation system consisting of either drilled shafts or augered, cast-in-place piles

Finished Floor Elevation A grading plan was not available at the time of this report. We anticipate final grades will be within about 1 to 2 feet of existing finished floor slab (estimated floor elevation of 921 feet).

Maximum Loads n Walls: 12 to 15 kips per linear foot (estimated by Terracon) n Columns: 750 to 850 kips service loads (provided by Anderson

Engineering) n Slabs: 200 pounds per square foot (estimated by Terracon)

Below-Grade Areas

The building will have a sub-basement level (approximately 20 feet below first floor finished elevation, approximate elevation 901 feet) and a basement level (approximately 10 feet below first floor finished elevation, approximate elevation 908 feet)

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GEOTECHNICAL CHARACTERIZATION

Geology

Based on our review of published soils and geology information (including Soil Survey of Polk County, Iowa by the United States Department of Agriculture and Landforms of Iowa by Jean C.

Prior) the site is located within the Des Moines Lobe. The native soils at this site were deposited during the most recent period of glaciation in this area. Bedrock was not encountered within the depth explored.

The risk of ground subsidence has been linked to previous coal mining activities in portions of central Iowa. Based on our review of Abandoned Coal Mines of Des Moines, Iowa and Vicinity (Iowa Department of Natural Resources Technical Paper No. 8), coal mining activities occurred near the project site. According to the information we reviewed, the Blount and Evans Coal Co.

mine and Keystone Coal Co. Mine No. 1 were located less than ½ mile east of the site, and the American Coal Mining Co. Mine was located less than ½ mile south of the site. According to the published information, these mines apparently did not extend below the VA site. Evidence of coal mining was not encountered in our limited subsurface exploration; however, coal mining operations usually occur at depths greater than those explored. If the client is concerned about this potential risk, further studies should be undertaken.

GeoModel

We have developed a general characterization of the subsurface soil and groundwater conditions based upon our review of the data, geologic setting and our understanding of the project. This characterization, termed GeoModel, forms the basis of our geotechnical recommendations.

Conditions encountered at each exploration point are indicated on the individual logs. The individual logs can be found in Exploration Results and the GeoModel can be found in Figures.

Stratification boundaries on the GeoModel and boring logs represent the approximate location of changes in soil types; in situ, the transition between materials may be gradual. As noted in General Comments, the characterizations are based on widely spaced exploration points across the site, and variations are likely.

Model Layer Layer Name General Description

1 Existing Fill n Sandy lean clay, trace gravel and rubble, brown n Extended to depths of about 6 to 8 ½ feet at the boring locations

2 Lean Clay With Sand n Lean clay, with sand, trace gravel, gray brown, medium stiff consistency.

n Encountered to depths of about 13 to 17½ feet bgs n Fine to medium grained sand, loose relative density was encountered to depths of about 14 feet bgs in Boring 3

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Model Layer Layer Name General Description

3 Lower-Strength Lean Clay n Lean clay, soft consistency, brown to gray n Encountered to depths of about 37 to 40 feet bgs

4 Glacial Till n Sandy lean clay, trace gravel, generally stiff consistency n The composition of glacial till soils is often erratic due to the random manner in which these soils were deposited; glacial till frequently contain pockets of sand and gravel, as well as zones of cobbles and random boulders materials n Encountered to boring termination depths of about 55½ feet bgs

The boreholes were observed while drilling and shortly after completion for the presence and level of groundwater. The water levels observed in the boreholes can be found on the boring logs in Exploration Results.

Groundwater was observed in all the borings at depths of about 24 to 36 feet bgs while drilling and 24 to 54 feet shortly after completion. A relatively long period may be necessary for a groundwater level to develop and stabilize in a borehole. Long term observations in piezometers or observation wells sealed from the influence of surface water are often required to define groundwater levels in materials of this type.

Groundwater level fluctuations occur due to seasonal variations in the amount of rainfall, runoff, 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 structures may be different than the levels indicated on the boring logs. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.

GEOTECHNICAL OVERVIEW

Existing fill was encountered in the borings at depths to about 6 to 8½ feet below ground surface (bgs). Existing fill materials consist of sandy lean clay, trace gravel and rubble. Due to the variability in depths of the existing fill, test pits and near surface test probes could be performed in exposed soils to assist in evaluating the depth, extend, and characteristics of any existing fill materials.

Due to the lower strength and high moisture content lean clay soils encountered below GeoModel Layer 2 to depths of about 37 to 40 feet bgs in the borings, we recommend the five to six story building addition be supported on a deep foundation system (drilled shafts or augered, cast-in-place (ACIP) piles) that extend to the glacial till soils. The floor slab of the addition could be supported on structural fill placed as backfill within the excavations for the pile caps. These methods would reduce the potential for differential settlement between the addition and existing buildings, and reduce the transfer of new vertical and lateral loads on the existing foundations and below grade walls.

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Care should be taken to avoid undermining existing foundations, walls, and floor slabs during foundation construction, utility installation, and other below-grade construction activities.

EARTHWORK

Site Preparation

Site preparation within planned new construction areas should include:

n Demolition of existing structures within the proposed construction area n Removal of all debris materials from past and current demolitions (e.g., sidewalks, pavements, slabs, footings, below-grade features) n Stripping of all vegetation, organic soils, root systems, and any unsuitable materials (e.g., debris, desiccated soil, frozen soil, impacted soils scheduled for removal, etc.). Root systems should be satisfactorily grubbed and removed from the proposed construction areas n Completing undercuts of existing fill soils as needed to develop recommended thicknesses of new structural fill below floor slabs

Existing utilities should be rerouted around areas of new construction. The trench backfill and bedding materials of any rerouted utilities should also be removed and replaced with newly placed structural fill. Sloping or benching of the sides of these excavations may be needed to accommodate proper placement of new structural fill.

Demolition of existing structures and utilities should be performed with close construction observation and testing. Any unsuitable fill or demolition debris should be removed. Demolition contractors should be aware of project requirements for earthwork (e.g., structural fill placement and compaction) so that reworking fill materials placed by demolition contractors is not necessary prior to construction of new structures.

The demolition of the existing structures and removal of utilities should be performed with close observation and testing. Any unsuitable fill, demolition debris (e.g., foundations, slabs, etc.), and lower strength native materials should be removed at this time. Portions of the new construction will likely be supported on the new fill placed in the demolition excavations. The demolition contractor should be aware of project requirements for backfilling so that removal of these fill materials and replacement under controlled conditions is not necessary upon construction of the new building addition. It is imperative that demolition be observed by Terracon and new structural fill be placed in accordance with the recommendations in this report.

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Prior to the placement of new structural fill in areas below design grade and for development of the subgrades for the slab and other areas cut below current grade, the exposed soils should be subjected to scarification to a depth of at least 9 inches, adjusted to the recommended range of moisture contents, and compacted to the density ranges recommended in this report. The stability of the subgrade should also be evaluated for the presence of soft or rutting subgrades during or just after the compaction phase. If unsuitable areas are observed, subgrade improvement will then be necessary to establish a suitable subgrade support condition.

Subgrade stabilization may be required to complete earthwork operations. The use of low contact pressure tracked equipment or remote excavation equipment may be necessary to assist in earthwork operations.

Potential methods of subgrade improvement are described below. The appropriate method of improvement, if required, would be dependent on factors such as schedule, weather, the size of area to be stabilized, and the nature of the instability. More detailed recommendations can be provided during construction as the need for subgrade stabilization occurs. Performing site grading operations during warm seasons and dry periods would help reduce the amount of subgrade stabilization required.

n Scarification and Compaction – Soils can be scarified, moisture condition (i.e., dried or wetted), and compacted. The success of this procedure depends primarily on favorable weather and sufficient time to manipulate the soils. Even with adequate time and favorable weather, stable subgrades may not be achieved if the thickness of the unstable material is greater than about 1 to 1½ feet.

n Undercut and Replacement with Crushed Stone/Aggregate – The use of crushed stone, crushed concrete, and/or gravel could be considered to improve subgrade stability.

To limit depths of undercuts, the use of a geogrid could be considered after underground work, such as utility construction, is completed. Equipment should not be operated above the fabric or geogrid until one full lift of crushed stoned/aggregate is placed above it. The maximum particle size of material placed over geotextile fabric or geogrid generally should not exceed 1½ inches. Gradation requirements for backfill materials provided by the geosynthetic product manufacturer should be verified prior to material purchase and delivery to the site.

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Structural Fill Material Types

Structural fill should meet the following material property requirements.

Soil Type 1 USCS Classification Acceptable Location for Placement

Low plasticity fine-grained 2, 3 CL

■ Site mass grading fill

■ Below aggregate base for grade-supported slabs Moderate to high plasticity fine-grained 4 CH, MH ■ General site grading fill limited to

■ Not to be used in areas of future buildings.

Coarse-grained

(granular) 5

GW, GP, GM, GC,

SW, SP, SM, SC

■ Below grade-supported slabs

■ Backfill adjacent to below grade walls

1. Structural fill should consist of approved materials that are free of organic matter and debris. 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 the Geotechnical Engineer for evaluation prior to use on this site.

2. Fine grained material (e.g., clays) can be difficult to compact in relatively small areas (e.g., excavations for foundations). Moisture conditioning (e.g., wetting or drying) would be necessary to achieve compaction requirements if fine grained material is used as structural fill for this project.

3. By our definition, low plasticity materials should have a liquid limit of 45 or less and a plasticity index of 23 or less (ASTM D4318).

4. Recommendations for moderate to high plasticity fine-grained soil apply to on-site materials only. Import of moderate to high plasticity fine-grained soil is not recommended.

5. Specific material requirements will need to be satisfied based on intended use.

Structural Fill Compaction Requirements

Structural fill should meet the following compaction requirements.

Item Description

Maximum individual lift thickness

■ 9 inches or less in loose thickness when heavy, self-propelled compaction equipment is used

■ 4 inches or less in loose thickness when hand-guided equipment (i.e.

jumping jack or plate compactor) is used

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Item Description

Minimum compaction requirements 1

Fine-grained soils:

■ 95% of maximum

Coarse-grained soils:

■ 98% of maximum

■ If the material is a coarse sand or gravel, or of a uniform size, or has a low fines content, compaction comparison to relative density may be more appropriate. In this case, coarse-grained soils should be compacted to at least 70% relative density (ASTM D4253 and D4254).

Water content range 1

Fine-grained soils:

■ 0 to +4% of optimum

Coarse-grained soils:

■ Water content should be maintained at levels satisfactory for compaction to be achieved without the coarse-grained material bulking during placement or pumping when proofrolled.

1. Maximum density and optimum water content as determined using standard effort (ASTM D698).

Utility Trench Backfill

All trench excavations should be made with sufficient working space to permit construction, including backfill placement and compaction. If utility trenches are backfilled with relatively clean coarse-grained material, they should be capped with at least 18 inches of fine-grained structural fill to reduce the infiltration and conveyance of surface water through the trench backfill.

Utility trenches are a common source of water infiltration and migration. Utility trenches constructed in fine-grained soils that penetrate beneath the building should be effectively sealed to restrict water intrusion and flow through the trenches, which could migrate below the building.

We recommend constructing an effective clay “trench plug” that consists of low permeability clay soils or flowable fill that extends at least 5 feet out from the face of the building exterior. The trench plug material should be placed to completely surround the utility line and any granular envelope, and be placed and compacted as recommended in this report. Care should be taken to not damage the in-place utilities.

Grading and Drainage

During earthwork, the site should be graded to prevent ponding of surface water on the prepared subgrade or in excavations. Surface water should be promptly removed.

Final surrounding grades should be sloped away from the planned building on all sides to prevent ponding of water next to the structure. Gutters and downspouts that drain water a minimum of 10 feet beyond the footprint of the proposed building are recommended. This can be accomplished

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Planting trees, large shrubs or other vegetation adjacent to structures is not recommended. Trees and large shrubs can develop extensive root systems that can draw moisture from the subgrade soils, causing them to shrink during dry periods of the year. Although the new building will be supported on deep foundations that will not be affected by moisture content changes in the shallow soils, drying or desiccation of clay soils below grade-supported floor slabs can result in settlement of the slabs. Irrigation should also be avoided adjacent to the building.

Earthwork Construction Considerations

Unstable subgrade conditions could develop during general construction operations, particularly if the soils are allowed to become saturated and/or subjected to repetitive construction traffic. In order to improve subgrade stability and help expedite construction, consideration should be given to performing earthwork in the late summer and fall when groundwater levels are generally lower and weather is conducive to drying. Where soft and wet subgrades are encountered, stabilization measures will be required to help provide a stable working base for construction. The use of low contact pressure, track equipment, or remote excavation equipment may be necessary to assist in earthwork operations.

As a minimum, all temporary excavations should be sloped or braced as required by Occupational Safety and Health Administration (OSHA) regulations to provide stability and safe working conditions. Temporary excavations will be required during grading operations and/or installation of utilities. Contractors, by their contract, are usually responsible for designing and constructing stable, temporary excavations and should shore, slope or bench the sides of the excavations as required, to maintain stability of both the excavation sides and bottom. All excavations should comply with applicable local, state and federal safety regulations, including the current OSHA Excavation and Trench Safety Standards.

Upon completion of filling and grading, care should be taken to maintain the subgrade moisture content prior to construction of foundation elements and grade-supported floor slabs.

Construction traffic over the completed subgrade should be avoided to the extent practical. If the subgrade should become frozen, desiccated, saturated, or disturbed, the affected material should be removed or these materials should be scarified, moisture conditioned, and compacted prior to floor slab construction.

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Construction Observation and Testing

Terracon’s involvement during 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.

The earthwork efforts should be observed and tested by a representative of the Geotechnical Engineer. Observation and testing should include documentation of adequate removal of vegetation and topsoil, delineation of areas requiring subgrade stabilization and assessment of existing fill materials.

Each lift of structural fill should be tested, evaluated, and reworked as necessary until approved by the Geotechnical Engineer prior to placement of additional lifts. Each lift of structural fill should be tested for density and water content at a frequency:

■ Subgrade Soils – one test for every 2,500 square feet per lift in building areas. Additional tests in backfill of isolated excavations during demolition or foundation overexcavations.

■ Utility Trench Backfill – one test for every 50 linear feet of utility trench length per lift

Foundation construction should be observed and tested by a representative of the Geotechnical Engineer. In the event that unanticipated conditions are encountered, the Geotechnical Engineer should be contacted to discuss mitigation options.

Construction Adjacent to Existing Structures

We understand the existing buildings adjacent to the planned addition includes a basement level and sub-basement. New foundation elements (grade beams and/or pile caps above new deep foundations) constructed adjacent to the existing building will be constructed to match the existing building’s foundation elevation.

Care should be taken to avoid undermining existing foundations, walls, grade-supported slabs, and utilities during any foundation or utility excavations, or any overexcavations necessary to remove unsuitable materials. Excavations adjacent to the existing building should not be deeper than the bottom of the existing foundation elements unless the existing foundations or walls are shored or underpinned. Care should be taken during excavations to protect the existing foundations and bearing soils. The foundation systems and bearing elevations of existing structures should be verified prior to the start of construction.

Some differential settlement between new and existing structures will occur and the settlement may approach the maximum total settlement estimated for the new addition. Structural connections with sufficient flexibility to accommodate independent movement between the new addition and the

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DEEP FOUNDATIONS

Deep foundations (either drilled shafts or augered, cast-in-place (ACIP) piles) are suitable to support the relatively heavily loaded walls and columns of the five to six story building addition.

The deep foundations are anticipated to develop their capacities by a combination of side friction in the various soil strata and in end bearing in the stiff glacial till soils encountered below depths of about 37 to 40 feet below existing grade (i.e., below approximately elevation 878 feet). Based on the subsurface conditions encountered in the borings, we have developed the following design parameters.

Drilled Shaft/ACIP Pile Design Parameters

Elev. (ft) (site datum)

Effective Unit

Weight 1, 2 (pcf)

Estimated Cohesion 2,3 (psf)

Rankine Coefficient (for sands)

Estimated Friction Angle

(degrees)

Allowable Compressive

Side Resistance

(psf)

(FS = 2)

Allowable End

Bearing (psf)

(FS = 3) 4

Static Lateral

Soil Modulu s kh

(pci)

Soil Strain ε50

908 to 904½ 120 Recommend neglecting upper 3½ feet due to freeze-thaw effects

904½ to 900½ 6 130 1,000 -- -- 300 NR 5 500 0.007

904½ to 900½ 7 110 -- 2.77 28 50 NR 5 20 --

900½ to 878 60 300 -- -- 100 NR 5 100 0.020 878 to 872 65 2,500 -- -- 800 7,500 1,000 0.005

872 to 859½ 70 3,500 -- -- 1,000 12,000 1,000 0.005

1. Maximum groundwater level considered at elevation 844 feet (site datum).

2. Effective unit weight and estimated cohesion values have no Factor of Safety applied.

3. Cohesion, internal friction angle, unit weight and Rankine passive coefficient values have no factor of safety applied.

4. Drilled shafts should penetrate at least one shaft diameter into the bearing stratum when designing for allowable end bearing pressures.

5. NR = Not recommended

6. For fine-grained soils

7. For coarse-grained soils

Group action for lateral resistance of shafts/piles should be taken into account when the center-to-center spacing is less than 8 diameters. For a group of shafts/piles oriented parallel to a lateral

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Laterally Loaded Drilled Shafts/ACIP Piles – Group Reduction Factors Shaft/Pile Spacing 1

(Diameters) Leading Row Reduction

Factor Second Row Reduction

Factor Third or Higher Row

Reduction Factor

8D 1.0 1.0 1.0 5D 1.0 0.8 0.7 3D 0.8 0.4 0.3

1. Center-to-center spacing in the direction of loading. If the loading direction for a single row of shafts/piles is perpendicular to the row, a group reduction factor of less than 1.0 should be used if the shaft/pile spacing is less than 5D (i.e., all shafts/piles are located in the leading row).

Lateral deflections of the drilled shaft should be evaluated using an appropriate analysis procedure and would be dependent on the physical characteristics of the drilled shaft, subsurface conditions, applied loading, restraint conditions at the bottom and top of the shaft and ground slope. We can provide analyses of lateral deflections upon request.

Drilled shafts for this project should have a minimum diameter of 30 inches in order to accommodate temporary casing and equipment for removal of water and for concrete placement.

ACIP piles for this project should have a minimum diameter of 14 inches.

Drilled shafts or ACIP piles designed and constructed as recommended in this report would be expected to experience settlement on the order of ½ inch or less (not including elastic compression of the shafts/piles). Proper reinforcing steel should be included in the drilled shaft/ACIP pile designs for the combined axial and lateral loading, as well as internal bending moments.

For drilled shafts that will not be topped with grade beams, the portion of the shaft within 3½ feet of ground surface should be formed (using sonotubes, steel casing, or an alternate forming method) to prevent widened or “mushroomed” tops of the shafts. Shafts with a “mushroomed” top could be subject to uplift loads from freeze-thaw movements of the surrounding soils. If grade beams and/or pile caps are used, the bases of the grade beams/pile caps should extend at least 3½ feet below lowest adjacent finished grade.

Drilled Shaft Construction Considerations

We anticipate that conventional drilling equipment (e.g. soil augers) should be able to penetrate the soils unless cobbles and/or boulders are encountered within the glacial till. If these conditions are encountered. The sides of the drilled shaft excavation should be protected from disturbance during shaft excavation and concrete placement. The shaft bottom should be cleaned of loose or disturbed material prior to concrete placement. Groundwater was encountered within the

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Where temporary casing is used, care should be taken during removal of the casing as concrete is placed. During casing removal, a sufficient concrete level should be maintained to counteract hydrostatic pressure on the annular space outside of the casing. We recommend the concrete mixture be designed to have a slump in the range of 5 to 7 inches to facilitate removal of temporary casing. Arching of the concrete, loss of seal and other problems can occur during casing removal and result in contamination of the drilled shaft. Placement of loose backfill or cave-in material in the perimeter annulus around the casing should not be permitted.

A representative of the Geotechnical Engineer should observe the drilled shaft excavation to evaluate the suitability of the bearing materials and to verify that conditions in the drilled shaft excavation are consistent with those encountered in the boring. If unsuitable materials are encountered at planned depths, it may be necessary to redesign the shaft

ACIP Pile Construction Considerations

Proper ACIP pile installation is highly operator-dependent and requires a greater than average dependence on quality workmanship and quality control monitoring. In addition, the successful ACIP pile completion largely depends on the equipment and installation procedures. The auger should be withdrawn in a controlled manner, and a sufficient head of grout should always be maintained in the augers to prevent necking of fluid grout due to hydrostatic pressures. Excess rotation of augers could result in loss of ground adjacent to previously constructed piles or other structures. We recommend that an automated pile installation recorder (PIR or similar device) be employed for installation of the piles for this project. A representative of the Geotechnical Engineer should observe installation of ACIP on a full-time basis to determine when suitable bearing depths have been reached.

ACIP piles installed within a clear distance spacing of less than ten (10) pile diameters should be delayed until the grout in the nearby piles has set. This is recommended to avoid possible loss of support around relatively weak grout of the previously installed pile, which could jeopardize pile integrity.

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SEISMIC CONSIDERATIONS

The seismic design requirements for buildings and other structures are based on Seismic Design Category. Site Classification is required to determine the Seismic Design Category for a structure.

The Site Classification is based on the upper 100 feet of the site profile defined by a weighted average value of either shear wave velocity, standard penetration resistance, or undrained shear strength in accordance with Section 20.4 of ASCE 7 and the International Building Code (IBC).

Based on the soil properties encountered at the site and as described on the exploration logs and results, it is our professional opinion that the Seismic Site Class is D. Subsurface explorations at this site were extended to a maximum depth of 55½ feet. The site properties below the boring depth to 100 feet were estimated based on our experience and knowledge of geologic conditions of the general area. Additional deeper borings or geophysical testing may be performed to confirm the conditions below the current boring depth.

FLOOR SLABS

Design parameters for floor slabs expect the requirements for Earthwork have been followed.

Specific attention should be given to positive drainage away from the structure and positive drainage of the aggregate base beneath the floor slab.

Floor Slab Design Parameters

Item Description

Floor slab support

■ Prepared according to the Earthwork section.

■ Minimum 6 inches of free-draining granular material 1

Estimated modulus of subgrade reaction 2 125 pounds per square inch per inch (psi/in) for point loads

1. Free-draining granular material should have less than 6 percent fines (material passing the No. 200 sieve), e.g., IaDOT 4121. Other design considerations such as cold temperatures and condensation development could warrant more extensive design provisions.

2. Modulus of subgrade reaction is an estimated value based on our experience with the subgrade condition, the requirements noted in Earthwork, and the floor slab support as noted in this table. It is provided for point loads. For large area loads the modulus of subgrade reaction would be lower.

Where floor slabs are tied to perimeter walls or turn-down slabs to meet structural or other construction objectives, our experience indicates differential movement between the walls and slabs will likely be observed in adjacent slab expansion joints or floor slab cracks beyond the length of the structural dowels. The Structural Engineer should account for potential differential settlement through use of sufficient control joints, appropriate reinforcing or other means.

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The use of a vapor retarder should be considered beneath concrete slabs on grade covered with wood, tile, carpet, or other moisture sensitive or impervious coverings, or when the slab will support equipment sensitive to moisture. When conditions warrant the use of a vapor retarder, the slab designer and slab contractor should refer to ACI 302 and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder.

Exterior Slabs and Frost Considerations

The soils on this site are frost susceptible, and small amounts of water can affect the performance of doorways, sidewalks, and pavements. Exterior slabs should be anticipated to heave during winter months. If frost action needs to be eliminated in critical areas, we recommend the use of low-frost susceptible fill or structural slabs (e.g., structural stoops in front of building doors). Low-frost susceptible materials should consist of a well-graded, clean granular material with less than 6% passing the No. 200 sieve. Placement of low-frost susceptible material in large areas may not be feasible; however, the following recommendations are provided to help reduce potential frost heave:

■ Providing surface drainage away from the buildings and slabs and toward the site storm drainage system

■ Installing drain tiles around the perimeter of the exterior slabs and pavements, and connect them to the storm drainage system

■ Grading clayey subgrades such that groundwater potentially perched in overlying more permeable subgrades, such as sand or aggregate base layers, slope toward the site drainage system

■ Placing low-frost susceptible materials in critical sidewalk areas

As an alternative to extending the low-frost susceptible fill to the full frost depth, consideration can be made to placing extruded polystyrene or cellular concrete under a buffer of at least 2 feet of low-frost susceptible fill.

Floor Slab Construction Considerations

Grading for floor slab subgrades is typically accomplished relatively early in the construction phase. Fills are placed and compacted and the initial surface is prepared in a relatively uniform manner. However, as construction proceeds, utility excavations, rainfall, and heavy construction traffic can disturb the subgrade. Surface irregularities are often filled with loose materials to temporarily improve trafficability. As a result, the floor slab subgrade, prepared earlier during initial site grading operations should be carefully evaluated as the time for slab construction approaches. Particular attention should be given to high traffic areas that become rutted and disturbed and to areas where backfilled trenches are located.

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Areas where unstable conditions exist should be repaired by removing and replacing the materials with low plasticity structural fill. All floor slab subgrade areas should be moisture conditioned and compacted to the recommendations in Earthwork immediately prior to placement of the aggregate base materials and concrete.

Care will be necessary to avoid contaminating the aggregate base layer located directly below the floor slabs with soil prior to floor slab placement. We recommend the aggregate base layer be placed only immediately prior to slab concrete placement.

LATERAL EARTH PRESSURES

Lateral Earth Pressure Design Recommendations

Reinforced concrete walls 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, final grades or sloping of ground adjacent to the walls, surcharges, methods of construction and/or compaction and the strength of the materials being restrained. The recommended design lateral earth pressures provided in this section are for cast-in-place, reinforced concrete walls only, and are not applicable to other wall systems (e.g., segmental block, landscaping walls, etc.).

Two wall restraint conditions are provided in the following table. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes wall movement. The "at-rest" condition is commonly used for design of below grade building walls and other walls restrained from movement and assumes no wall movement. The surcharge component applies where floor loads or other loading will be applied adjacent to the below grade walls. The recommended design lateral earth pressures do not include a factor of safety.

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Lateral Earth Pressure Design Parameters

Earth Pressure Condition 1

Coefficient for Backfill Type 2

Surcharge Pressure 2, 3, 4

(psf)

Effective Fluid Pressures (psf)2, 4, 5, 6

Drained Undrained

Active (Ka) Coarse-grained: 0.33 Fine-grained: 0.42

(0.33)S (0.42)S

(40)H (50)H

(80)H (85)H

At-Rest (Ko) Granular: 0.50 Cohesive: 0.59

(0.50)S (0.59)S

(60)H (70)H

(90)H (95)H

Passive (Kp) Granular: 3.0 Cohesive: 2.4

(360)D (285)D

(235)D (200)D

Sliding Resistance Ultimate coefficient of sliding friction for suitable bearing soils: 0.4

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. Considers a horizontal ground inclination uphill of the wall, and a horizontal ground inclination downhill of the wall.

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. H value is used for active and at-rest pressure computations from the top of wall. D value (not shown on figure) is used for passive pressure computations and is depth of foundation embedment below lowest grade at base of wall. Passive pressure resistance distribution should consider frost effects as discussed in the Shallow Foundations section.

6. In order to achieve “Drained” conditions, follow guidelines in the Below Grade Walls Subsurface Drainage section. “Undrained” conditions are recommended when drainage behind walls is not incorporated into the design or where walls will be submerged during flooding events.

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/at-rest and passive cases, respectively.

Below Grade Walls Subsurface Drainage

To prevent hydrostatic loading on the below grade walls, we recommend a drainage system be installed along the walls and extend to the foundation of the below grade walls. The wall drain system should be designed according to the following table.

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Item Description

Below grade wall subdrain pipe

■ Perforated rigid plastic drain line with a minimum 4-inch diameter.

■ Pipe perforations should be appropriately sized to prevent free-draining granular material from entering the subdrain pipe.

■ Pipe invert should be at least 3½ feet below proposed exterior grade or at the foundation of the wall, whichever is deepest.

■ Subdrain lines should be sloped to provide positive gravity drainage to daylight or to a reliable discharge point (e.g., storm sewer, sump pit and pump, etc.).

■ Pipes should be embedded in at least 4 inches of wall drainage backfill material.

Wall drainage backfill 1

■ A minimum 2-ft wide section of coarse-grained (granular) fill located above the drain line and adjacent to the walls, consisting of either:

o IaDOT porous backfill (Section 4131), or o Free-draining coarse-grained material encapsulated with non-woven geotextile filter fabric (Contech C60NW or equivalent).

■ The coarse-grained fill should extend to within 2 feet of final grade, where it should be capped with fine-grained structural fill to reduce infiltration of surface water into the subdrain system.

1. 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.

If walls must resist combined hydrostatic and lateral earth pressures, then combined hydrostatic and lateral earth pressures should be calculated using the “undrained” values in Lateral Earth Pressure Design Recommendations. Water stops and other wall waterproofing measures should also be considered if undrained designs are used.

GENERAL COMMENTS

Our analysis and opinions are based on our understanding of the project, the geotechnical conditions in the area, and the data obtained from our site exploration. Natural variations will occur between exploration point locations or due to the modifying effects of construction or weather.

The nature and extent of such variations may not become evident until during or after construction.

Terracon should be retained to review the final design plans and specifications so comments can be made regarding interpretation and implementation of our geotechnical recommendations in the design and specifications. Terracon should be retained to provide observation and testing services during grading, excavation, foundation construction, and other earth-related construction phases of the project. If variations appear, we can provide further evaluation and supplemental recommendations. If variations are noted in the absence of our observation and testing services

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Our Scope of Services does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken.

Our services and any correspondence or collaboration through this system are intended for the sole benefit and exclusive use of our client for specific application to the project discussed and are accomplished in accordance with generally accepted geotechnical engineering practices with no third party beneficiaries intended. Any third party access to services or correspondence is solely for information purposes to support the services provided by Terracon to our client.

Reliance on the services and any work product is limited to our client, and is not intended for third parties. Any use or reliance of the provided information by third parties is done solely at their own risk. No warranties, either express or implied, are intended or made.

Site characteristics as provided are for design purposes and not to estimate excavation cost. Any use of our report in that regard is done at the sole risk of the excavating cost estimator as there may be variations on the site that are not apparent in the data that could significantly impact excavation cost. Any parties charged with estimating excavation costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing.

Site safety, cost estimating, excavation support, and dewatering requirements/design are the responsibility of others. If changes in the nature, design, or location of the project are planned, our conclusions and recommendations shall not be considered valid unless we review the changes and either verify or modify our conclusions in writing.

SIGNATURE PAGE

I hereby certify that this engineering document was prepared by me or under my direct personal supervision and that I am a duly licensed Professional Engineer under the laws of the State of Iowa.

__________________________________8/17/2020_____ Kole C. Berg, P.E. Date

My license renewal date is December 31, 2021.

ATTACHMENTS

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EXPLORATION AND TESTING PROCEDURES

Field Exploration

Boring Numbers Boring Depths (feet) Location

1 through 3 55½ Building addition

Boring Layout and Elevations: Terracon personnel staked the boring locations using handheld GPS equipment with respect to planned building areas indicated on concept plans provided to us and coordinates determined from a “best fit” overlay of the plans on a GIS system. The boring locations are shown on the Exploration Plan. The coordinates of the borings are indicated on the boring logs.