MSE_Wall_Design_Letter_FWP1.pdf

amorea Typewritten Text Document#: FWP1

August 15, 2012 HWA Project No. 2012-019-21

Wall Design Report 2 HWA GeoSciences Inc.

The proposed warehouse building will be constructed on strip footings supported by structural fill. The southeastern edge of the building will be positioned behind the top block of the proposed wall. The wall will commence about 10-15 feet east of the northeast corner of the existing south warehouse building and extend a distance of approximately 30 feet before turning due north for a distance in excess of 15 feet where it terminates by stepping into the existing ramp fill. The wall will vary between about 3 to 8 feet high, and will support fill placed which support the proposed warehouse. No survey exists along the proposed wall alignment;

assumptions made herein are based on measurements using a tape in the field.

Based on the recommendations we made previously in our geotechnical report (HWA, 2012) the wall supporting the structural fill will consist of a mechanically stabilized earth (MSE) wall, constructed with Ultra-Blocks™. The block wall is designed to accommodate loads arising from the fill and a design aerial surcharge load of 250 pounds per square foot above the wall to account for building loads.

Soil Conditions

As part of the investigation conducted by HWA for the warehouse building foundation report;

one deep test pit (TP-1) was excavated through the existing structural fill at the location shown on Figure 2. The log of test pit TP-1 is shown in Appendix A. The subsurface information obtained from that test pit was used to design the new retaining walls.

Our test pit excavation encountered a thin blanket of crushed rock which has been placed as the local surface cover. Below the crushed rock, the test pit excavation encountered two distinct layers of fill; both generally consisting of silty sand with gravel. The upper fill appears to consist of imported select borrow material, composed of grayish brown, silty, sand with gravel, is dense to medium dense, and is approximately 2 feet thick. The lower fill layer appears to consist of fill derived from native soil material, and is composed of dark brown, silty, sand with gravel containing organic material and minor amounts of construction debris. The lower fill layer was medium dense and approximately 5.5 feet thick.

Below the crushed rock and fill, at a depth of approximately 8 feet below the existing ground surface, our test pit excavation encountered stiff to very hard, sandy silt, which we interpret to be weathered glacial till. Weathered till has similar composition to un-weathered till, but is generally less dense, and often comprises the upper few feet at the interface between overlying soils and un-weathered glacial till. It also often contains ground water perched at the top of the denser till. Minor seepage was, in fact, observed in TP-1 at a depth of 7.5 to 8.0 feet, just above the inferred contact between the fill and the weathered glacial till.

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Conclusions and Recommendations General

The retaining walls for this project will be required to extend the existing fill pad to support the southeast corner of the new building. We recommend that the fill pad be extended at least 5 feet beyond the exterior wall of the proposed building to accommodate wall reinforcement and reduce the load imposed on the wall by the building. Currently, concrete block walls are in evidence throughout the site and would likely be the most economical to use for this application.

In order to accommodate loads imposed by both the fill and the building, we recommend that this wall be designed as a mechanically stabilized earth wall (MSE). Principal advantages of MSE walls include relatively low unit cost and tolerance of relatively large differential settlements. An MSE wall system is proposed that will be a maximum of approximately 8 feet in height, supporting structural fill placed to support the southeastern corner of the proposed new warehouse.

MSE walls consist of alternating layers of backfill soil and reinforcing material mechanically tied into facing panel or segmental block facing elements. Commonly used reinforcing elements include steel strips and various geosynthetic products such as geogrid and geotextile sheets. The vertical spacing of the reinforcing elements is typically on the order of 1 to 3 feet, depending on the reinforcing material specified and other parameters.

Earthquake Loading

Earthquake loading for the subject wall was developed in accordance with Section 3.10 of the AASHTO LRFD Bridge Design Specifications, 5th Edition, 2010. For seismic analysis, the Site Class is required to be established and is determined based on the average soil properties in the upper 100 feet below the ground surface. Based on our explorations and understanding of site geology, it is our opinion that Site Class C should be used with AASHTO seismic evaluations for this project. A Peak Ground Acceleration (PGA) of 0.42 was used for design of the subject wall.

This PGA is based upon a design event with a 7 percent probability of exceedance in 75 years (equal to a return period of 1,033 years).

Wall Design

The proposed new wall below the southeastern corner of the building should be constructed of

2.5 x 2.5 x 5 foot solid precast concrete blocks capable of interlocking (such as Ultra-Blocks™).

Geogrid reinforcement is sandwiched between each layer of blocks and extended into the backfill a specified length dependent upon wall height. For the proposed retaining wall at this site, we anticipate maximum height configurations of 2, 3, 4 and possibly 5 blocks. The subject wall was designed for internal and external stability utilizing proprietary design software provided by Ultrablock, Inc. (UltraWall Version 3.1.12193). This program utilizes AASHTO

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2012 design methodologies. A surcharge load of 250 pounds per square foot (psf) was applied at a location about 5 feet behind the top of the wall to account for load of the proposed building.

For the purposes of wall design, we have assumed an allowable bearing pressure of 3,000 psf for walls founded on native soils and 2,000 psf for walls founded on structural fill. We utilized friction angles of 32 degrees for the retained fill soils, 36 degrees for wall backfill, and 28 degrees for native soils. Design calculation summary sheets are compiled and presented in Appendix B.

Based on these analyses, it is our recommendation that the subject wall extension be constructed without batter. The bottom block of the wall should be embedded a minimum of 1.5 feet into the native glacial soil. The wall reinforcement should consist of Miragrid 5XT geogrid or equivalent, installed between each block (2.5 feet vertical spacing), and extending a minimum of

9.5 feet from the face of the wall for walls with an exposed face heights of 8 feet or less. Should the wall be required to have an exposed face height of 8-10 feet then the lowermost layer of geogrid reinforcement should consist of Miragrid 10XT or equivalent and reinforcement should extend 11.0 feet from the face of the wall. All grids should be pulled tight and staked in place prior to covering with wall backfill.

Wall Construction

The wall backfill within the reinforced zone should consist of granular backfill, compacted to 95 percent of modified proctor (ASTM D1557), that meets the requirements and gradation specifications for Gravel Borrow for Geosynthetic Retaining Walls, as specified in the 2012, WSDOT Standard Specification Section 9-03.14(4). Except for the existing, clean imported crushed rock placed as test pit backfill (assuming it is not contaminated with native or existing silty fill soils), on-site material may not be used as structural backfill during wall installation.

Granular fill material shall be placed, spread, and compacted in such a manner that minimizes the development of wrinkles in the geogrid and/or movement of the geogrid.

Excavations for the new wall should be made nearly vertical along the baseline of the wall to reach dense glacial till, as determined by a geotechnical engineer. Excavations for the cut portion of the wall should extend to dense glacial soils and remove all topsoil, organic material, and any loose soil, as determined by a geotechnical engineer. The foundation should be evaluated by a geotechnical engineer prior to wall construction. All loose soils should be over-excavated to medium dense to dense soils and replaced with properly compacted structural fill as described below.

The wall should be constructed on a minimum 6-inch thick structural fill leveling pad consisting of clean, ¾-inch minus, crushed rock with a minimum wall embedment of 1.5 feet. In addition, a 1-foot thick layer of clean, ¾-inch minus, crushed rock backfill should be placed immediately behind the wall. All structural fill should be compacted to at least 95% of the maximum dry density determined by ASTM D1557 Modified Proctor.

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The wall should be keyed into the exiting fill slope to the southwest and northeast by cutting approximately 4-foot wide benches into the existing slopes and placing a 6-inch thick layer of compacted foundation aggregate consisting of crushed surfacing top course meeting the requirements of 2012 WSDOT Standard Specification Section 9-03.9(3) prior to block placement. “Keying” the wall into the slope in this manner reduces the likelihood of a slip surface forming between the contact of the new fill and the existing fill slopes.

A minimum loose fill thickness of 6 inches is required prior to operation of tracked vehicles over the geogrid. Turning of tracked vehicles should be kept to a minimum to prevent tracks from displacing the fill and damaging the geogrid. Rubber-tired equipment may pass over the geogrid reinforcement at slow speeds (less than 10 mph) when integrally-formed geogrids are used.

When woven, multi-layer or welded-strip geogrids are used, rubber-tired equipment shall not be allowed directly on the geogrid. Sudden braking and sharp turning movements shall be avoided.

A four-inch diameter, Schedule 40, perforated, plastic drain pipe should be embedded in the crushed rock below and behind the wall as a subdrain. This pipe should be tight lined to the storm drain system or suitable outlet, with cleanouts provided.

All backfill material placed within 5 feet of retaining walls should consist of structural fill, with the additional requirement that it be free-draining and contain a maximum of 5 percent fines (material passing the U.S. No. 200 standard size sieve). On-site soils will generally be unsuitable for use as wall backfill and the contract documents should include a wall backfill import provision, accordingly. Backfill placed below or behind the wall should be compacted to 95 percent of its maximum dry density (MDD) as determined by ASTM test method D1557.

Compaction of backfill within 3 feet of the wall should be accomplished using equipment of limited size such as walk-behind rollers or jumping jacks in order to limit the surcharge on the wall. Fill compacted with such equipment should be placed in lifts no more than 6 inches thick.

Utility Relocation

We anticipate that excavation made for wall foundations will require care through the southeast portion of the existing fill area containing the existing CCTV conduit (oriented N-S) and HDPE storm drain pipe (oriented E-W). It is likely that the presence of these utilities will interfere with wall construction as they are located at or slightly below the base of the existing fill. Provision should be made during construction to relocate or reposition these utilities deeper below ground and provide some form of structural cushion or encapsulation (CIP or pre-cast concrete) such that the over lying wall loads do not damage the conduit or pipe.

Temporary Excavations and Dewatering

Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the contractor. Temporary excavations should be performed in accordance with

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the current requirements of federal, state and/or local agencies. Temporary excavations exceeding 4 feet in depth must be sloped, benched or shored, according to Part N of Section 196- 155 of the Washington Administrative Code (WAC). The existing medium dense fill soils at this site classify as Type C Soil; temporary excavations in Type C Soil may be inclined no steeper than 1.5: H: 1V (horizontal: vertical).

Specific design information for temporary excavation shoring is not included herein, since the contractor has control over factors during construction that are critical to the stability of the temporary slopes. Such factors include the amount of slope opened at one time; the length of time the slope is left open, and weather conditions. Thus, maintaining safe and stable temporary excavations is the responsibility of the contractor.

Earthwork-related construction within excavation areas should proceed as quickly as possible, to limit the time temporary excavations are open. During wet weather, runoff water should be prevented from entering excavations, and should be collected and disposed of outside the construction limits. Heavy construction equipment, building materials, and surcharge loads such as excavated soil should not be allowed within ½ the slope height behind the top of any excavation.

Time and the presence of seepage and/or precipitation can significantly reduce the stability of temporary unsupported cut slopes. Therefore, all temporary slopes should be protected from erosion by installing a surface water diversion ditch or berm at the top of the slope and by covering the cut face with well-anchored plastic sheets. In addition, the contractor should monitor the stability of the temporary cut slopes and adjust the construction schedule and slope inclination accordingly.

Conditions and Limitations

We have prepared this report for FEMA for use in design of a portion of this project. This report should be provided in its entirety to prospective contractors for bidding and estimating purposes.

However, the conclusions and interpretations presented in this report should not be construed as a warranty of the subsurface conditions. Experience has shown that soil and ground water conditions can vary significantly over small distances. Inconsistent conditions can occur between explorations and may not be detected by a geotechnical study. If, during future construction or site operations, subsurface conditions are encountered which vary appreciably from those anticipated, HWA should be notified for review of the recommendations of this report, and revision of such if necessary.

Sufficient geotechnical monitoring, testing, and consultation should be provided during construction to confirm that the conditions encountered are consistent with those indicated by explorations, to provide recommendations for design changes should conditions revealed during

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References:

AASHTO, 2010, LRFD Bridge Design Specifications, Fifth Edition.

Race, R., (2012), ULtraBlock Retaining Wall Design Software, (version 3.1.12193.1459), [Computer Program], available at http://www.ultrablock.com/ultrablock_008.htm

WSDOT, 2012, Standard Specifications for Road, Bridge and Municipal Construction, Washington State Department of Transportation.

EXPLORATION PLAN MAP

MSE WALL DESIGN REPORT

FEMA WAREHOUSE

130 228TH STREET SW

BOTHELL, WASHINGTON

2012-019

FIGURE NO.

PROJECT NO.

NORTHApproximate Test Pit Location

Approximate Hand Boring Location

TP-1

HB-1

Approximate limits of existing fill pad and ramp.

Existing Bldg

Existing Bldg

Approximate Location of New Warehouse Building

Concrete Slab

Conduit under crossing

Approximate extent of area requiring fill and retaining structure.

Approximate area of potential Wall/Utility conflict.

SITE DEVELOPMENT LAYOUT SCHEMATIC

MSE WALL DESIGN REPORT

FEMA WAREHOUSE

130 228TH STREET SW

BOTHELL, WASHINGTON

2012-019

FIGURE NO.

PROJECT NO.

NORTH

Note: Approximate scale; 1-inch = 20 feet. This schematic is for illustrative purposes only, and not suitable for construction

APPENDIX A

HWA EXPLORATION LOG

APPENDIX B

ULTRAWALL COMPUTER WALL DESIGN OUTPUT

SUMMARIES