1.9.2_Geotech_Report.pdf

REPORT COVER PAGE

Geotechnical Engineering Report Aransas National Wildlife Refuge

Austwell, Texas

December 21, 2018

Terracon Project No. 90185289

Prepared for:

Hanson Professional Services, Inc.

Corpus Christi, Texas

Prepared by:

Terracon Consultants, Inc.

San Antonio, Texas

Terracon Consultants, Inc. 6911 Blanco Road, San Antonio, Texas 78216

P [210] 641-2112 F [210] 641-2124 t er racon.com Texas Professional Engineers No. 3272

REPORT COVER LETTER TO SIGN

Mr. Terald E. Smith, P.G

Hanson Professional Services, Inc.

4501 Gollihar Road

Corpus Christi, Texas 78061

Re: Geotechnical Engineering Report

Aransas National Wildlife Refuge

FM 2040 and Wildlife Circle

Austwell, Texas

Dear Mr. Smith:

We have completed the Geotechnical Engineering services for the above referenced project. This report presents the findings of the subsurface exploration and provides geotechnical recommendations for the proposed project. We appreciate the opportunity to work with you on this project and look forward to contributing to the ongoing success of this project by providing

Materials Testing services during construction. If you have any questions concerning this report, or if we may be of further service, please contact us.

Sincerely, Terracon Consultants, Inc.

(Firm Registration: TX F3272)

Tariqul Anwar, P.E. Gregory P. Stieben, P.E., D.GE

Project Engineer Senior Consultant

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REPORT TOPICS

REPORT TOPICS

INTRODUCTION

SITE CONDITIONS

GEOTECHNICAL CHARACTERIZATION

PROJECT DESCRIPTION

GEOTECHNICAL OVERVIEW

EARTHWORK

SHALLOW FOUNDATIONS

DEEP FOUNDATIONS

SEISMIC CONSIDERATIONS

PAVEMENTS

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 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 (Boring Logs)

SUPPORTING INFORMATION (General Notes and Unified Soil Classification System) http://client.terracon.com/

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INTRODUCTION

Geotechnical Engineering Report

Aransas National Wildlife Refuge

FM 2040 and Wildlife Circle

Austwell, Texas

INTRODUCTION

This report presents the results of our subsurface exploration and geotechnical engineering services performed for a Maintenance Campus at the Aransas National Wildlife Refuge to be located near the intersection of FM 2040 and Wildlife Circle in Austwell, Texas. This study was performed in general accordance with Terracon Proposal No. P90185289 dated September 19, 2018.The purposes of these services are 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

■ Pavement design and construction

■ Seismic site classification per IBC 2015

The geotechnical engineering scope of services for this project included the advancement of two test borings to depth of about 30 feet below existing site grades. The field work was delayed due to bad weather and soft ground condition.

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 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 publicly available geologic and topographic maps.

Item Description

Parcel Information The project is located near the intersection of FM 2040 and Wildlife Circle in

Austwell, Texas.

Existing Improvements Undeveloped. Majority of the site is heavily wooded.

Current Ground Cover Bare soil, trees and grass.

Aransas National Wildlife Refuge ■ Austwell, Texas

December 21, 2018 ■ Terracon Project No. 90185289

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Existing Topography Based on our field visit, the site is relatively level.

GEOTECHNICAL CHARACTERIZATION

Subsurface Profile

We have developed a general characterization of the subsurface soil and groundwater conditions based upon our review of the data and our understanding of the geologic setting and planned construction. The following table provides our geotechnical characterization.

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.

Based on the results of the borings, subsurface conditions at the boring locations can be generalized as follows:

Approximate

Depth (feet) Material Description Consistency/ Density

0 to 30

POORLY GRADED SAND WITH SILTY (SP-SM) 1;

yellowish brown Loose to Medium Dense

1 The POORLY GRADED SAND WITH SILTY (SP-SM) soils are primarily granular in nature and are expected to possess a negligible potential for volumetric changes as a result of moisture fluctuations. This stratum may become water bearing and prone to sloughing.

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 native soil types; in situ, the transition between materials may be gradual.

Groundwater Conditions

The boreholes were observed while drilling and after completion for the presence and level of groundwater. Groundwater was observed at about 25 feet and 2 feet below existing grade in borings

B-1 and B-2, respectively while drilling, or for the short duration that the borings could remain open.

Note that the ground elevation is about 10 feet higher at the B-1 location. It may be that inadequate time elapsed for an accurate determination of water level at that location. Also, ponded water was observed at the surface near the B-2 location. The borings were backfilled with soil cuttings after the drilling operations and groundwater observations were completed. The groundwater levels are rounded to nearest ½ feet. Specific information concerning groundwater is noted on the boring logs presented in Exploration Results.

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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. Additionally, the sand zone may become water bearing after a precipitation event and can readily transmit water. 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. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.

PROJECT DESCRIPTION

Information Provided Client has selected boring locations for the project.

Project Description

The project includes a Maintenance Campus comprising of several pre-engineered metal buildings to house a vehicle maintenance shop, shade structures, fuel storage, and a bunk house.

Proposed Structure The buildings may be supported either on slab-on-grade foundation or drilled pier foundation.

Pavements Both asphalt and concrete pavements are anticipated.

GEOTECHNICAL OVERVIEW

The following recommendations are based upon the data obtained from our field and laboratory programs, project information provided to us and on our experience with similar subsurface and site conditions.

The foundation being considered to provide support for the planned structures must satisfy two independent engineering criteria with respect to the subsurface conditions encountered at this site. One criterion is the foundation system must be designed with an appropriate factor of safety to reduce the possibility of a bearing capacity failure of the soils underlying the foundation when subjected to axial and lateral load conditions. The other criterion is movement of the foundation system due to compression (consolidation or shrinkage) or expansion (swell) of the underlying soils must be within tolerable limits for the structure.

Potential Vertical Rise (PVR) Considerations

Based on our findings, the subsurface soils at this site generally exhibit a low expansion potential.

Based on the information developed from our field and laboratory programs and on method TEX-

124-E in the Texas Department of Transportation (TxDOT) Manual of Testing Procedures, we estimate that the subgrade soils in the building area exhibit a Potential Vertical Rise (PVR) of about ½ inch in its present condition. The actual movements could be greater than the values presented in this report if inadequate drainage, ponded water, and/or other sources of moisture

Responsive ■ Resourceful ■ Reliable 4 are allowed to infiltrate beneath the structure after construction. Based on the PVR results, building subgrade modifications to provide a uniform soil support for foundation will be required and are discussed in this report.

EARTHWORK

Earthwork will include clearing and grubbing, excavations and fill placement. The following sections provide recommendations for use in the preparation of specifications for the work. These recommendations include critical quality criteria as necessary to render the site in the state considered in our geotechnical engineering evaluation for foundations, floor slabs, and pavements.

Site Accessibility

The loose sand in the upper 2 to 4 feet may be a hindrance to movement of construction equipment, especially in wet climatic condition. These types of soils also have a tendency to become “spongy” and are prone to pumping if the moisture content is elevated due to precipitation. Therefore, it is recommended to provide firm working surface, thus reducing construction delays especially during wet climatic weather. We recommend one of the following methods, to be considered in order to stabilize the soft ground conditions and help create construction access should it become necessary. These are recommended method, it is the responsibility of the contractor to create a stable working surface and access for construction equipment. Construction access is means and method and it’s the sole responsibility of the contractor, and it should be addressed by the contractor.

■ As discussed in Site Preparation section, and/or;

■ A minimum of 6 inches of crushed rock may be pushed over a Tensar TX-140 Triaxial

Geogrid to create a more stable working base. The geogrid and rock are intended as subgrade stabilization only and is not part of the pavement sections.

Site Preparation

Construction operations may encounter difficulties due to the wet or soft surface soils becoming a general hindrance to equipment due to rutting and pumping of the soil surface, especially during and soon after periods of wet weather. If the subgrade cannot be adequately compacted to minimum densities as described in the Fill Compaction Requirements section of this report, one of the following measures may be required:

■ removal and replacement with select fill;

■ chemical treatment of the soil to dry and increase the stability of the subgrade; or

■ drying by natural means if the schedule allows.

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In our experience with similar soils in this area, chemical treatment (cement) may be an effective method to increase the supporting value of wet and weak subgrade. Terracon should be contacted for additional recommendations if chemical treatment of the soils is needed.

Prior to construction, all vegetation, loose topsoil and any otherwise unsuitable materials should be removed from the construction area. The stripped materials consisting of vegetation and organic materials should be wasted from the site, or used to revegetate landscaped areas or exposed slopes after completion of grading operations. Wet or dry material should either be removed or moisture conditioned and recompacted. After stripping and grubbing, the subgrade should be proof-rolled where possible to aid in locating loose or soft areas. Proof-rolling can be performed with a 15-ton roller or fully loaded dump truck. Soils that are observed to rut or deflect excessively (typically greater than 1-inch) under the moving load should be undercut and replaced with properly compacted on-site soils. The proof-rolling and undercutting activities should be witnessed by a representative of the geotechnical engineer and should be performed during a period of dry weather.

Building Pad Preparation

As previously mentioned, the existing PVR at this site is about ½ inch. Moreover, the upper soil at site appears to be relatively loose in condition. The following soil modification methods may be considered to maintain the soil movement to less than 1 inch and provide uniform support to the grade supported slabs and flatwork for this project site. The recommendations provided in this section are based on the Finished Floor Elevation (FFE) near existing grades.

■ After completing stripping operations discussed in the Site Preparation section, excavate about 2 feet of the on-site soil below the Finish Building Pad Elevations (FBPE) within the building pad area. The excavated on-site soils may be stockpiled for later use as fill. The building pad area is defined as the area that extends at least 3 feet (horizontal) beyond the perimeter of the proposed building and any adjacent flatwork. The limits of the building pad should be indicated on the drawings for the project.

■ After removing 2 feet of on-site soil, the exposed subgrade in the pad should be proof rolled with at least a 15-ton roller to evidence any weak yielding zones. A Terracon geotechnical engineer or their representative should be present to observe proof rolling operations.

■ Over-excavate any confirmed weak yielding zones, both vertically and horizontally, to expose competent soil. The upper 6 inches of the exposed subgrade should be moisture conditioned between 0 and +4 percentage points of the optimum moisture content and then compact to at least 95 percent of the maximum dry density determined in accordance with ASTM D 698.

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■ After proof-rolling and the replacement of weak yielding zones, place back the stockpiled onsite soil at the bottom of the excavation in moisture conditioned and compacted lifts to achieve FBPE. The onsite soil should be placed in loose lifts of about 8 inches and compacted thickness not exceeding 6 inches. The onsite soil should be moisture conditioned between -2 and +3 percentage points of the optimum moisture content and with ASTM D 698.

■ The sandy soils may prove to be difficult to compact and have a low cohesion. As a result, these soils may slough excessively when trenched and may affect site access. If this is a concern, these soils can be removed and replaced with cohesive select fill or treated with

2 to 3 percent cement. The means and method to achieve an adequate building pad are the responsibilities of the contractor.

■ If grade needs to be raised, select fill should be used to achieve the Finished Building Pad

Elevation (FBPE). The select fill should be placed in loose lifts of about 8 inches and compacted thickness not exceeding 6 inches. The select fill should be moisture conditioned between -2 and +3 percentage points of the optimum moisture content and with ASTM D 698

■ To provide a more uniform slab support and create a more all-weather working surface, we suggest constructing the final 6 inches of the pad with granular select fill. Details regarding select fill materials, placement and compaction are presented in the following sections Fill Material Types and Fill Compaction Requirements.

This method should result in 2 feet of onsite reworked soils beneath the grade supported floor slab. If grades are to be raised, select fill should then be used to achieve the Finished Building

Pad Elevation (FBPE).

Fill Material Types

Select fill and granular select fill should meet the following material property requirements:

Fill Type 1 USCS Classification Acceptable Location for Placement

Granular select fill 2 Varies Upper 6 inches of the building pad.

Select fill

SC, CL

(LL≤40) and (7≤PI≤20) All locations and elevations.

On-Site Soil SM Onsite soil can be used as moisture conditioned fill.

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1 Prior to any filling operations, samples of the proposed borrow and on-site materials should be obtained for laboratory moisture-density testing. The tests will provide a basis for evaluation of fill compaction by in-place density testing. A qualified soil technician should perform sufficient in-place density tests during the filling operations to evaluate that proper levels of compaction, including dry unit weight and moisture content, are being attained.

2 Granular select fill should consist of 2014 TxDOT Item 247, Type A, Grade 1-2 crushed limestone base material. Plasticity Index (PI) should range from 5 to 15.

Fill Compaction Requirements

Select fill and granular select fill should meet the following compaction requirements.

Item Structural Fill

Fill Lift Thickness All fill should be placed in thin, loose lifts of about 8 inches, with compacted thickness not exceeding 6 inches.

Compaction of On-Site Soil/ Select Fill Soil/ Granular Select Fill

95 percent of materials standard Proctor maximum dry density

(ASTM D 698).

Moisture Content of On-Site Soil, Imported Select Fill and Imported Granular Select Fill

The materials should be moisture conditioned between -2 and +3 percentage points of the optimum moisture content.

Grading and Drainage

All grades must provide effective drainage away from the building during and after construction and should be maintained throughout the life of the structure. Water retained next to the building can result in soil movements greater than those discussed in this report. Greater movements can result in unacceptable differential floor slab and/or foundation movements, cracked slabs and walls, and roof leaks. The roof should have gutters/drains with downspouts that discharge onto splash blocks at a distance of at least 10 feet from the building.

Exposed ground should be sloped and maintained at a minimum 3 percent away from the building for at least 10 feet beyond the perimeter of the building. Locally, flatter grades may be necessary to transition ADA access requirements for flatwork. After building construction and landscaping, final grades should be verified to document effective drainage has been achieved. Grades around the structure should also be periodically inspected and adjusted as necessary as part of the structure’s maintenance program. Where paving or flatwork abuts the structure a maintenance program should be established to effectively seal and maintain joints and prevent surface water infiltration.

Earthwork Construction Considerations

It is anticipated that excavations for the proposed construction can be accomplished with conventional earthmoving equipment. Based upon the subsurface conditions determined from the geotechnical exploration, subgrade soils exposed during construction are anticipated to be relatively stable. However, the stability of the subgrade may be affected by precipitation, repetitive

Responsive ■ Resourceful ■ Reliable 8 construction traffic or other factors. If unstable conditions develop, workability may be improved by scarifying and drying. Over excavation of wet zones and replacement with granular materials may be necessary. Lightweight excavation equipment may be required to reduce subgrade pumping. The use of remotely operated equipment, such as a backhoe, would be beneficial to perform cuts and reduce subgrade disturbance.

All temporary excavations should be sloped or braced as required by Occupational Health and

Safety Administration (OSHA) regulations to provide stability and safe working conditions.

Temporary excavations will probably be required during grading operations. The grading contractor, by his contract, is 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.

Construction Observation and Testing

The earthwork efforts should be monitored under the observation of the Geotechnical Engineer or their representative. Monitoring should include documentation of adequate removal of vegetation and top soil, proof-rolling and mitigation of areas delineated by the proof-roll to require mitigation.

Each lift of compacted fill should be tested, evaluated, and reworked as necessary until approved by the Geotechnical Engineer prior to placement of additional lifts. Each lift of fill should be tested for density and water content at a frequency of at least one test for every 2,500 square feet of compacted fill in the building areas and 5,000 square feet in pavement areas. One density and water content test for every 50 linear feet of compacted utility trench backfill.

In areas of foundation excavations, the bearing subgrade should be evaluated under the observation of the Geotechnical Engineer or their representative. In the event that unanticipated conditions are encountered, the Geotechnical Engineer should prescribe mitigation options.

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.

SHALLOW FOUNDATIONS

Slab-on-Grade Foundation Recommendations

The proposed buildings may be supported on a monolithic slab and grade beam foundation. As stated previously, remedial earthwork measures will be required to reduce potential post-construction movements to a tolerable level as recommended in Building Pad Preparation

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Parameters commonly used to design this type of foundation are provided on the table below.

The slab foundation design parameters presented on the table below are based on the criteria published by the Wire Reinforcing Institute (WRI). This is essentially empirical design method and the recommended design parameters are based on our understanding of the proposed project, our interpretation of the information and data collected as a part of this study, our area experience, and the criteria published in the WRI design manual.

Conventional Method Prepared Subgrade 1

Net Allowable Bearing Pressures 2 2,000 psf

Subgrade Modulus (k) 90 pci

Potential Vertical Rise (PVR) 1 About ½ inch

WRI Method

Effective Plasticity Index (PI) 3 < 10

Soil / Climate Rating Factor (1- C) 0.05

Coefficient of Slab-Subgrade Friction (): 0.75 to 1.00

1 Based on preparing the building pad as discussed in this report.

2 The net allowable bearing pressure provided above includes a Factor of Safety (FS) of at least 3.

3 The WRI effective PI is equal to the near surface PI if that PI is greater than all of the PI values in the upper 10 feet.

We recommend that grade beams be at least 24 inches below the finished exterior grade. Interior grade beams (if any) should bear at least 18 inches below the FFE. These recommendations are for proper development of bearing capacity for the continuous beam sections of the foundation system and to reduce the potential for water to migrate beneath the slab foundation. These recommendations are not based on structural considerations. Grade beam depths may need to be greater than recommended herein for structural considerations and should be properly evaluated and designed by the Structural Engineer. The grade beams or slab portions may be thickened and widened to serve as spread footings at concentrated load areas.

For a slab foundation system designed and constructed as recommended in this report, post construction settlements should be less than 1 inch. Settlement response of a select fill supported slab is influenced more by the quality of construction than by soil-structure interaction. Therefore, it is essential that the recommendations for foundation construction be strictly followed during the construction phases of the building pad and foundation.

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The use of a vapor retarder should be considered beneath concrete slabs-on-grade that will be covered with wood, tile, carpet or other moisture sensitive or impervious coverings, or when the slabs 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 for procedures and cautions about the use and placement of a vapor retarder.

Spread Footings

Isolated spread footings may be used to support the structures. Design recommendations for shallow foundations for the proposed structure are presented in the table below.

Description Parameter

Net allowable bearing pressure 1

- Select Fill / On-Site Soil 2,000 psf

Minimum dimensions 30 inches

Minimum embedment below finished grade for bearing 30 inches

Approximate total settlement from foundation loads 2 Less than 1 inch

Estimated differential settlement from foundation loads < ¾ inch between columns

Ultimate passive pressure 3 250 pcf, equivalent fluid density

Ultimate coefficient of sliding friction 4 0.35

1. The recommended net allowable bearing pressure is the pressure in excess of the minimum surrounding overburden pressure at the footing base elevation. Assumes any soft soils, if encountered, will be undercut and replaced with compacted structural fill. Based upon a minimum Factor of Safety of 3.

2. The above settlement estimates from foundation loads have assumed that the maximum footing size is 6 feet for column footings and 1.5 feet for continuous footings and that the area has been thoroughly proof-rolled as recommended in this report.

3. The spread footing foundation excavation sides must be nearly vertical and the concrete should be placed neat against these vertical faces for the passive earth pressure values to be valid. If the loaded side is sloped or benched, and then backfilled, the allowable passive pressure will be significantly reduced. Passive resistance in the upper 12 inches of the soil profile should be neglected. If passive resistance is used to resist lateral loads, the base friction should be neglected.

4. A Factor of Safety of 2 should be applied to the ultimate value.

The spread footings can provide some uplift resistance for those structures subjected to wind or other induced structural loading. The uplift resistance of a spread footing may be computed using the effective weight of the soil above the spread footing along with the weight of the spread footing and structure. A soil unit weight of 120 pcf may be assumed for the on-site soils placed above the footing, provided the fill is properly compacted.

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Foundation Construction Considerations

As noted in Earthwork, the footing excavations should be observed by the Geotechnical Engineer or their represented. Grade beams for the slab foundation and footings should preferably be neat excavated. Excavation should be accomplished with a smooth-mouthed bucket. If a toothed bucket is used, excavation with this bucket should be stopped 6 inches above final grade and the grade beam excavation completed with a smooth-mouthed bucket or by hand labor. Debris in the bottom of the excavation should be removed prior to steel placement. Due to the presence of sandy clay soils, grade beams and footings excavation may experience caving. Therefore, the foundation contractor should be prepared to use formwork.

The foundation excavations should be sloped sufficiently to create internal sumps for runoff collection and removal of water. If surface runoff water or subsurface water seepage in excess of

1 inch accumulates at the bottom of the foundation excavation, it should be collected and removed and not allowed to adversely affect the quality of the bearing surface. Special care should be taken to protect the exposed soils from being disturbed or drying out prior to placement of the concrete.

If unsuitable bearing soils are encountered at the base of the planned footing excavation, the excavation should be extended deeper to suitable soils, and the footings could bear directly on these soils at the lower level or on lean concrete backfill placed in the excavations. This is illustrated on the sketch below.

Over-excavation for structural fill placement below footings should be conducted as shown below.

The over-excavation should be backfilled up to the footing base elevation, with describe soil type placed, as recommended in the Earthwork section.

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DEEP FOUNDATIONS

Proposed structures may be supported on drilled piers. Piers should bear no shallower than 15 feet below existing grade. Due to the presence of water bearing sand, Auger Cast in Place (ACIP) piles are suggested, rather than conventional drilled piers. Therefore, this report only provides straight sided pier recommendations.

Deep Foundation Design Parameters

We recommend both ACIP piles and Drilled Piers have a minimum shaft diameter of 18 inches and be installed to a minimum depth of 15 feet below existing grade. Soil design parameters are provided below in the Design Summary table for the design deep foundations. The values presented for allowable side friction and end bearing include a factor of safety.

Design Summary 3

Approximate Depth

(feet)

Allowable Skin Friction

(psf)

Allowable End Bearing Pressure

(psf)

0 – 5 --- ---

5 – 10 150 ---

10 – 15 250 ---

15 – 30 400 6,000

1. The allowable skin friction values include a factor of safety of 2.

2. The bearing pressure includes a factor of safety of 3.

3. Minimum center to center pier spacing should be at least three shaft diameters. If design or construction considerations require that the center to center pier spacing is less than three shaft diameters, pier capacity should be decreased by 25 percent. However, no piers should be less than 2 shaft diameter center to center.

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The cross-sectional area of the reinforcing steel should not be less than 1 percent of the gross cross-sectional area of the drilled pier shaft. The reinforcing steel should extend from the top to the bottom of the shaft to resist this potential uplift force.

For the straight-sided drilled piers, the uplift force due to axial tension forces due to structural loading conditions can be resisted by the allowable side-shear of the drilled pier. The allowable uplift resistance of the straight sided drilled piers can be evaluated using the following equation:

Qar = 1.4· d · Dp + 0.9Wp + PDL

Qar = Allowable uplift resistance of pier in kips (k)

Where: d = Diameter of pier shaft in feet (ft)

Dp = Founding depth of pier in natural soils minus the upper 5 feet of shaft in contact with the soil in feet (ft)

Wp = Weight of the drilled pier in kips (k)

PDL = Permanent sustained dead Load acting on the drilled pier in kips (k)

The structural engineer may want to factor the dead load value based on their degree of certainty.

Settlement – For piers, total settlements, based on the indicated bearing pressures, should be less than 1 inch for properly designed and constructed drilled piers. Settlement beneath individual piers will be primarily elastic with most of the settlement occurring during construction. Differential settlement may also occur between adjacent piers. The amount of differential settlement could approach 50 to 75 percent of the total pier settlement. For properly designed and constructed piers, differential settlement between adjacent piers is estimated to be less than ¾ of an inch.

Settlement response of drilled piers is impacted more by the quality of construction than by soil-structure interaction.

Auger Cast in Place Construction Considerations

The performance of ACIP piles is dependent on installation procedures. We recommend the following measures be taken for ACIP pile installation at this site:

Contractor Experience and Equipment – The selected ACIP contractor should have relevant project experience with augering and pumping equipment, installation of similar sized ACIP piles in similar subsurface soil conditions, placement of reinforcing steel, as well as experience handling the special grout mixes and admixtures. Our experience indicates that not all contractors have the specialized equipment and experience required to successfully install ACIP piles with diameters of 18 inches and larger. We strongly recommend that the contractor’s qualifications and experience records be reviewed as part of the selection process, especially if pile diameters equal to or greater than 24-inches are planned for this site.

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Grout Mix – The ACIP pile contractor should be able to demonstrate that a grout mix can be furnished to meet this project's requirements. The results of test mix design or test results from a previous project are generally required to be submitted prior to approval of the grout mix for construction. During construction, the fluidity of the grout mix should be frequently tested using a ¾-in. diameter flow cone. Flow rates of 10 to 20 seconds are typically specified. Compressive strength of the grout mix should be checked by making at least six, 2-inch square cubes for each day of pile installation. Test cubes should be cured and tested in accordance with ASTM C109 and be restrained from expansion as described in ASTM C942. The Deep Foundation Institute recommends testing two grout cubes at 7 days, two at 28 days, and keeping two on hold.

ACIP Pile Installation and Monitoring – The installation process makes it inherently difficult to verify the integrity of the installed ACIP piles, yet the integrity is essential to the load carrying capacity of the piles. A comprehensive construction monitoring and record keeping program is recommended to help reduce the risks associated with improper construction techniques.

Construction monitoring should be performed in accordance with the Inspector’s Guide to

Augered Cast-in-Place Piles, prepared by the Deep Foundations Institute. We recommend that construction monitoring be performed by qualified personnel independent of the contractor.

There are several aspects of the installation procedure that can be monitored to aid in assessing whether the ACIP pile is being installed properly. These aspects are: 1) viscosity of the grout mixture to be pumped, 2) initial grout placement prior to raising the augers and resulting grout head observed upon completion of pile installation, 3) incremental grout factor (volume of grout pumped relative to the theoretical volume) computed over intervals of 5 ft. or less as the auger is withdrawn, 4) uniformity of grout placement and computed grout factor along the length of the completed pile, 5) continuous grout placement and auger withdrawal without delays or grout pressure fluctuations, and 6) unobstructed and successful reinforcing steel placement in a timely manner following completion of grout placement. Records kept during construction monitoring should include these aspects of the pile installation.

The incremental volume of grout pumped as the auger is withdrawn is one of the more important installation controls. Common practice is to have the contractor calibrate the grout pump at the start of the work. The number of pump strokes are then computed to provide an initial grout head of 5 to 10 ft. and at least 15 percent greater than the theoretical volume for each 5 ft. increment of auger withdrawal. Typically, the actual volume of grout pumped is in the range of 15 to 60 percent greater than the theoretical volume. The required grout volume to obtain a uniform pile will vary depending on the soil type and consistency. Typically, the grout take will range from about 1.1 times the theoretical volume in clays to 1.35 or more in sands. An independent observer should continually monitor the number of pump strokes pumped over each 5-ft interval. We recommend the grout volume and pressure be monitored with the aid of a Pile Installation

Recorder (PIR).

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Construction Sequence – The Contractor’s installation plan should be reviewed prior to the start of construction. The installation plan should address the allowable spacing between piles to be grouted the same day. In general, previously cast piles should achieve their initial set (at least 6 hours and preferably more) prior to augering adjacent piles. Based on our experience, we strongly recommend installing ACIP piles spaced closer than about 6 ft (clear spacing) on alternate days to reduce the risk of interference between adjacent piles. A drop in the grout level of a completed pile (commonly called “blowups”) when installing an adjacent pile is an indication that the spacing between piles installed on the same day should be increased.

An initial grout head should be developed at the bottom of excavation, prior to withdrawal of the auger. A positive pressure head of at least 5-feet of grout should be maintained at all times. We recommend that the initial 5-feet of grout be pumped prior to initial withdrawal of the auger, rather than over-pumping the initial five-foot increment. The number of pump strokes required to maintain the 5-feet of head should be determined during installation of the test reaction piles and can be visually monitored by observing the “grout return depth” upon removal of the auger. A grout return depth of 5 to 10 feet is generally considered acceptable. Grouting of the piles should continue until grout contaminated with soil cuttings has stopped flowing from the hole and uncontaminated grout has completely filled the hole. The grout pressure should be monitored during the field grouting and maintained at a pressure between 350 to 500 psi to reduce the potential for grout necking and soil caving.

Reinforcement – Because the piles will be subjected to uplift and downdrag movements as the subgrade heaves and shrinks, the ACIP piles will have to be reinforced throughout their length.

The pile reinforcement is inserted immediately after the pile excavation is grouted while the grout is still in a fluid / plastic state. Centering devices, such as steel footballs or plastic wheels, should be used to maintain the vertical alignment of the reinforcement during the installation. Where

ACIP piles penetrate sandy soils, the moisture content of the plastic grout will immediately decrease after pumping due to the weight of the overhead grout column and permeability of the sand. It has been our experience that the installation of long reinforcing steel cages for ACIPs could be difficult due to the moisture migration from the grout into the sands and the grout stiffening. Consequently, consideration should be given to using a combination of a single rebar and a steel rebar cage in each ACIP pile and limiting the length of the steel rebar cage to about

20 to 25 feet. However, the length of the cage will also need to be sufficient under lateral pile loading conditions. The use of fine-grained sand and fly ash and the absence of pea- or larger-sized gravel in the grout mix will decrease the difficulties in inserting the reinforcing steel and reinforcing cages. To compensate for the loss of larger aggregate, the cement content in the grout mix needs to be increased.

Sonic Integrity Logging and Pile Integrity Testing – A non-destructive technique currently available to observe completed ACIP piles is the Sonic Integrity Logger. We believe it is essential that a

Responsive ■ Resourceful ■ Reliable 16 qualified geotechnical engineering firm experienced with ACIP piles observe the installation using sonic integrity logging to provide a higher degree of confidence that the piles are properly installed.

Defects such as caving, necking or soil inclusions in the pile can be detected using sonic integrity logging. Use of the Sonic Integrity Logger at the start of construction to evaluate alternate installation procedures and to help establish an acceptable production pile installation procedure is particularly beneficial. We can provide literature and/or a demonstration of the sonic integrity logging equipment if requested. Performing sonic integrity testing on about 10 percent of the production piles should be sufficient to ascertain that the ACIP piles have been properly installed.

Sonic integrity testing is typically performed at the beginning of pile installation and at about 1/3 and 2/3 completion points of pile installation.

A second type of non-destructive testing applicable to ACIP piles is Pile Integrity Tester (PIT) testing. PIT testing may be employed for additional testing if an anomaly is suspected in the upper portion of the pile. Due to various limitations of PIT testing, this technique is not recommended as a replacement of the sonic logging, but rather as a supplement.

Observation – Production ACIP pile installation should be closely observed by a qualified technician experienced in ACIP pile installation. The installation sequence and technique should be observed to evaluate that ACIP pile installation is being performed in general accordance with project specifications.

Drilled Pier Construction Considerations

The pier excavations should be augered and constructed in a continuous manner. Steel and concrete should be placed in the pier excavation immediately following drilling and evaluation for proper bearing stratum, embedment, and cleanliness. In no circumstances should the pier excavation remain open overnight.

During the time of our drilling operations, subsurface water was encountered at 2 feet in the borings. Subsurface water levels are influenced by seasonal and climatic conditions which result in fluctuations in subsurface water elevations. Sand encountered in some of the borings is prone to sloughing. Therefore, the contractor should be prepared to use temporary casing or use slurry drilling methods should water be encountered and/or sloughing of the excavation sidewalls occur.

Slurry method may be considered since casing may not seal groundwater due to the presence of sand and a watertight seal maybe not achieved. The casing and slurry methods are discussed in the following paragraphs.

Casing Method- Casing will provide stability of the excavation walls but may not completely eliminate subsurface water influx potential or stability of the pier excavation bottom unless the casing penetrates below any pervious soils. Casing that terminates in pervious soils may generate “boils” due to the head differential between the inside and outside of the casing and require that the casing be extended until the excess

Responsive ■ Resourceful ■ Reliable 17 seepage or boils are eliminated. The drilling subcontractor should determine casing depths and casing procedures. Water that accumulates in excess of six (6) inches in the bottom of the pier excavation should be pumped out prior to steel and concrete placement. If the water is not pumped out, a long closed-end tremie should be used to place the concrete completely to the bottom of the pier excavation in a controlled manner to effectively displace the water during concrete placement. If this operation is not successful or to the satisfaction of the foundation contractor and engineer, the pier excavation should be flooded with fresh water to offset the differential water pressure caused by the unbalanced water levels inside and outside of the casing. If water is not a factor, concrete should be placed with a short tremie so that the concrete is directed to the bottom of the pier excavation. The concrete should not be allowed to ricochet off the walls of the pier excavation nor off reinforcing steel. When the pier excavation depth is achieved and the bearing area has been cleaned, steel and concrete should then be placed immediately in the excavation.

Removal of casing should be performed with extreme care and under proper supervision to reduce mixing of the surrounding soil and water with the fresh concrete.

Rapid withdrawal of casing or the auger may develop suction that could cause the soil to intrude into the excavation. An insufficient head of concrete in the casing during its withdrawal could also allow the soils to intrude into the wet concrete. Both of these conditions may induce “necking”, a section of reduced diameter, in the pier.

Slurry Method- As an alternate to the use of casing to install the pier foundations, water or a weighted drilling fluid may be considered. Slurry displacement drilling can only prevent sloughing and water influx but cannot control sloughing once it has occurred.

Therefore, slurry displacement drilling techniques must begin at the ground surface, not after sloughing materials are encountered.

Typical drilling fluids include those which contain polymers or bentonite. If a polymer is used with “hard” mixing water, a water softening agent may be required to achieve intimate mixing and the appropriate viscosity. The polymer manufacturer should be consulted concerning proper use of the polymer. If bentonite slurry is used, the bentonite should be mixed with water several hours before placing in the pier excavation. Prior mixing gives the bentonite sufficient time to hydrate properly. The drilling fluid should only be of sufficient viscosity to control sloughing of the excavation walls and subsurface water flow into the excavation. Care should be exercised while extracting the auger so that suction does not develop and cause disturbance or create

“necking” in the excavation walls as described above. Casing should not be employed in conjunction with the slurry drilling technique due to possible trapping of loose soils and slurry between the concrete and natural soil.

The use of weighted drilling fluid when installing drilled pier foundations requires extra effort to ensure an adequate bearing surface is obtained. A clean-out bucket should be

Responsive ■ Resourceful ■ Reliable 18 used just prior to pier completion in order to remove any cuttings and loose soils which may have accumulated in the bottom of the excavation. Steel and concrete should be placed in the excavation immediately after pier completion. A closed-end tremie should be used to place the concrete completely to the bottom of the excavation in a controlled manner to effectively displace the slurry during concrete placement. The concrete should be placed completely to the bottom of the excavation with a closed-end tremie in the pier excavation if more than six (6) inches of water is ponded on the bearing surface or the water should be pumped from the excavation. A short tremie may be used if the excavation has less than 6 inches of ponded water. The fluid concrete should not be allowed to strike the pier reinforcement, temporary casing (if required) or excavation sidewalls during concrete placement.

All aspects of concrete design and placement should comply with the American Concrete Institute

(ACI) 318 Code Building Code Requirements for Structural Concrete, ACI 336.1 Standard

Specification for the Construction of Drilled Piers, and ACI 336.3R entitled Suggested Design and

Construction Procedures for Pier Foundations. Concrete should be designed to achieve the specified minimum 28-day compressive strength when placed at a 7 inch slump with a 1 inch tolerance. Adding water to a mix designed for a lower slump does not meet the intent of this recommendation. If a high range water reducer is used to achieve this slump, the span of slump retention for the specific admixture under consideration should be thoroughly investigated.

Compatibility with other concrete admixtures should also be considered. A technical representative of the admixture supplier should be consulted on these matters.

Successful installation of drilled piers is a coordinated effort involving the general contractor, design consultants, subcontractors and suppliers. Each must be properly equipped and prepared to provide their services in a timely fashion. Several key items of major concern are:

■ Proper drilling rig with proper equipment (including casing, augers, high torque, high powered equipment).

■ Reinforcing steel cages tied to meet project specifications;

■ Proper scheduling and ordering of concrete for the piers; and

■ Monitoring of installation by design professionals.

Pier construction should be carefully monitored to assure compliance of construction activities with the appropriate specifications. A number of items of concern for pier installation include those listed below.

Pier locations Concrete properties and placement

Vertical alignment Casing removal (if required)

Competent bearing Proper casing seal for subsurface water control

Steel placement

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If the contractor has to deviate from the recommended foundations, Terracon should be notified immediately so additional engineering recommendations can be provided for an appropriate foundation type.

Foundation Construction Monitoring

The performance of the foundation system for the proposed structure will be highly dependent upon the quality of construction. Thus, we recommend that fill pad compaction and foundation installation be monitored full time by an experienced Terracon soil technician under the direction of our Geotechnical Engineer.