06 - Amended GEE -Revised Micropile Recommendations.pdf

GEOLABS, INC.

Geotechnical Engineering and Drilling Services

2006 Kalihi Street • Honolulu, Hawaii 96819 Phone: (808) 841-5064 • Facsimile: (808) 847-1749 • E-mail: hawaii@geolabs.net

Hawaii • California

February 3, 2012 W.O. 6246-00(Addendum)

Mr. Stephen Moller

ATKINS

6504 Bridge Point Parkway, Suite 200 Austin, TX 78730

Subject: Revised Micropile Foundation Recommendations F-22 Upgrade Munitions Complex Hickam Air Force Base, Oahu, Hawaii

Reference: Report by Geolabs, Inc. dated December 1, 2010 entitled "Geotechnical Engineering Exploration F-22 Upgrade Munitions Complex Hickam Air Force Base, Oahu, Hawaii"

Dear Mr. Moller:

Due to cost considerations, we understand that higher capacity micropiles are being considered to reduce the number of micropiles that would support Buildings 9000 through 9003, 9007, 9008, 9013, and 9019. Our revised micropile foundation recommendations are provided in this addendum letter.

The revised micropile foundation recommendations provided in this addendum letter are intended to update and supersede the micropile foundation recommendations contained in the referenced geotechnical engineering report for the project dated December 1, 2010. The other recommendations (not revised in this addendum letter) provided in the geotechnical engineering report remain valid for use in the design of the project.

Based on additional engineering analyses, we recommend using a cased micropile system with a grout bulb diameter of at least 7 inches to support the buildings. The permanent casing for the cased micropile system should have an outside diameter (OD) of about 7.625 inches. The permanent casing (unbonded zone) should extend to the top of the soft to medium hard coral formation or medium hard to hard coralline sandstone.

The coral and coralline sandstone formations were encountered at elevations of about -12 to -21 feet Mean Sea Level (MSL).

Based on the subsurface conditions anticipated at the project site, we recommend utilizing a Type I micropile with an allowable compressive load capacity of up to 60 tons to

ATKINS Page 2 W.O. 6246-00 (Addendum) support the building structures. The allowable compressive load capacity of the micropiles are for supporting dead-plus-live loads and may be increased by up to one-third (1/3) for transient loads, such as wind or seismic forces.

The micropiles would derive its vertical support primarily in the bonded zone from skin friction between the grout and the surrounding soft to medium hard coral formation and medium hard to hard coralline sandstone formation. To provide an allowable compressive load capacity of 60 tons, the bonded zone of the micropile should be embedded a minimum of 29 feet below the bottom of the permanent casing. Our recommendations pertaining to the micropile allowable compressive load capacity and lengths are presented in the following table.

COMPRESSION LOAD CAPACITY OF MICROPILE FOUNDATION

Type

Allowable Compressive

Load Capacity (tons)

Bonded Zone

Length (feet)

Average Micropile

Length (feet)

I 60 29 50

Notes:

1. Average Micropile lengths are based on depths below the existing ground surface at elevations of about +3 to +6 feet MSL.

2. Bonded Zone is the length below the bottom of the permanent casing.

A factor of safety of 2.0 was used in arriving at the allowable compressive load capacity provided in the above table.

Downdrag Loads

The coralline detritus and lagoonal deposits at the project site may be susceptible to liquefaction during a seismic event. In the event of seismically induced liquefaction, settlement of the coralline detritus and lagoonal deposits will occur at the project site.

When the coralline detritus and lagoonal deposits move downward relative to the stationary micropile, the downward movement will result in negative skin friction forces, generally known as downdrag forces, acting on the micropile shaft. The magnitude of the downdrag force depends on the thickness of the liquefiable layer, relative density of the potentially liquefiable coralline detritus and lagoonal deposits, and the micropile perimeter.

Due to the potential for liquefaction of the very loose/very soft coralline detritus and lagoonal deposits at the project site, settlement of the coralline detritus and lagoonal deposits will result in downdrag loading of the micropile foundations extending through the potentially liquefiable materials.

Based on the available plans, we understand that compacted earth fill will be placed around and above the igloo structures. The weight of the compacted earth fill will induce settlement of the loose and soft coralline detritus and lagoonal deposits. The

ATKINS Page 3 settlement would result in downdrag forces acting on the pile shaft supporting the igloo structures.

Therefore, the allowable compressive load capacities of the micropiles have been reduced by the expected downdrag loads in the pile foundation analyses. Based on our foundation analyses, we estimate that the downdrag load imposed on the micropiles due to settlement of the lagoonal deposits may be on the order of about 10 kips.

Uplift Load Resistance

Uplift loads imposed on the micropiles may be resisted by shear along the grout bulb (bonded zone) and the adjacent soils. Our recommendations for the micropile uplift load capacity are presented in the following table.

UPLIFT LOAD CAPACITIES

FOR TRANSIENT LOADING CONDITIONS

Type

Uplift Load Capacity (kips)

I 180

The uplift load capacity provided is intended for transient load applications only.

Therefore, the uplift load capacity should not be further increased for transient load applications. For sustained uplift loads, we recommend reducing the uplift load capacity provided above by 50 percent to resist sustained uplift loads. The project structural engineer should check the capacity of the micropile member in tension.

Lateral Load Resistance

It should be noted that the micropile foundation system provides low lateral load resistance due to the relatively small diameter of the micropile. Therefore, the lateral load resistance contributed by the vertical micropiles should be neglected. Lateral loads imposed on the new structure foundations may be resisted primarily by passive earth pressure resistance against the vertical faces of the pile cap and grade beams and by shear acting along the sides of the pile caps and grade beams.

Passive pressure resistance against the vertical faces of the pile cap and grade beams may be estimated using an equivalent fluid pressure of 350 pounds per square foot per foot of depth (pcf) above the groundwater level and 175 pcf below the groundwater level. Additional resistance to lateral loads may be provided by shear resistance between the sides of the pile caps and grade beams and the adjacent soils. A side shear resistance of up to 150 pounds per square foot (psf) may be used for transient loads such as wind or seismic forces only.

ATKINS Page 4

Micropile Foundation Settlements

Settlements of the micropile foundations will result primarily from elastic compression of the pile member and subgrade response. We estimate the total settlement of the micropile-supported foundations to be less than 0.5 inch with differential settlements between columns supported on micropiles not exceeding about one-half of the total settlement. We believe that these settlements are essentially elastic and should occur as the loads are applied.

Micropile Load Test Program

It should be noted that the compressive and uplift load capacities of the micropiles are highly dependent on the drilling procedures and the grouting methods employed by the contractor to install the micropile. Therefore, the compressive and uplift load capacity of the micropile may vary considerably between different contractors and micropile foundation systems. In order to determine whether the contractor’s methods of micropile installation are adequate and to determine the ultimate compressive load capacity, we recommend performing one pre-production compressive load test at the site. In general, the purpose of the pre-production load test on a micropile is to fulfill the following objectives:

To examine the adequacy of the methods and equipment proposed by the contractor to install the micropiles to the depths required

To assess the contractor’s method of drilling and grout injection To evaluate the degree of long-term settlements of the micropile

In general, the pre-production load test should be performed in accordance with

ASTM D 1143. Based on experience, we believe that the load tests should be conducted at least seven days after completion of the micropile installation to allow the grout adequate time to cure. Two to four additional micropiles may be used for reaction during the compressive load testing of the pre-production load test micropile. The reaction micropiles may be installed to depths as deep as the load test micropile to provide adequate reaction in uplift (to be determined by the contractor).

The load test micropile should be loaded gradually to a maximum compressive load of 250 kips. We recommend holding the micropile for a minimum of 4 hours at 200 percent of the allowable design load depending on the recorded movements of the load test micropile. Please note that the pre-production load test is an integral part of the design of the micropile foundation system.

In addition to the pre-production load test, we also recommend performing pullout tests (proof tests) on selected micropiles during construction to confirm the load carrying capacity of the installed micropiles. We recommend testing a minimum 10 percent of the total number of micropiles for pullout. The pullout tests should consist of subjecting the micropile to at least 133 percent of the design load, and the maximum test load should be

ATKINS Page 5 held for at least 10 minutes. Pullout tests on the micropiles also are integral parts of the micropile foundation system design.

Micropile Construction Considerations

A specialty subcontractor experienced in the construction of a micropile foundation system should install the micropiles. Based on the borings, coral and coralline sandstone formations were encountered at about -12 to -21 feet MSL. Therefore, some hard drilling conditions may be anticipated during micropile installation.

It should be noted that the hardness of the coral and coralline sandstone formations is highly variable across the site. In addition, there is potential for encountering cavities and/or voids in the coral and coralline sandstone formations. Due to the possible cavities and/or voids that may be encountered in the coral and coralline formations, we believe that there is a potential for significant grout loss during micropile installation. Where significant grout loss is observed, we recommend removing the permanent casing and allowing the grout in the drilled hole to set for up to one day. After the grout sets, the hole should be redrilled and grouted. This procedure should be repeated until significant grout loss is not observed during micropile installation.

Due to the specialized nature of the micropile foundation construction, observation of the micropile foundation installation system and testing of the micropiles should be designated “Special Inspection” items. We understand that Geolabs may not be retained to provide on-site construction observation during foundation construction. A contractor quality control (CQC) may be used for the project. The qualified geotechnical engineer should be a licensed civil engineer in the State of Hawaii with a minimum of 8 years of geotechnical engineering design and construction experience in similar subsurface conditions. The field observations should be performed by the geotechnical engineer or a representative from his/her firm under the direct supervision of the geotechnical engineer.