February 28, 2009

Tetra Tech | Infrastructure Group

230 South Franklin

Suite 212

Juneau, Alaska 99801

! Attention:!Don Beard, P.E.

! Subject:!CBJ Garage and Transit Center

! !Foundation Recommendations

! !DMA Job No.: 4256.001

The CBJ Garage and Transit Center will be founded on reinforced concrete

foundations. Based on additional site geotechnical information, refinements to

the allowable bearing capacities for each structure are summarized with this

letter.

CBJ Garage

The CBJ Garage will be founded on either bedrock or till. Geotechnical

drilling and rock coring was conducted by R&M Engineering, Inc. (R&M) along

Main Street in July 2008 for this structure. Based on the R&M soils logs, an

allowable bearing capacity of 15,000-pounds per square foot (psf) can be used for

reinforced concrete foundation cast directly on a clean bedrock surface.

Settlements are expected to be negligible for concrete foundations cast directly on

clean bedrock. The CBJ Garage foundations not cast directly on a clean bedrock

surface will need to be founded on a till as identified on the R&M soils logs,

primarily along Main Street. Assuming that a “medium dense to dense till” as

identified on the R&M soils logs is present under the continuous strip

foundations, an allowable bearing capacity of 4,000-psf can be used, provided a

DMA representative is present to verify the condition of the till prior to

placement of foundations. Settlements in the range of 0.5-inch total and 0.25-

differential should be expected for continuous strip reinforced concrete

foundations on these tills. The majority of the settlement is expected to occur

concurrent with construction.

Transit Center

The Transit Center foundations are expected to be founded on bedrock

along the northern portion of the structure and over “AJ” fill along the southern

portion of the structure, Plate 1. The current design for the Transit Center

includes glass panels that will tolerate more than 0.5-inch settlement.

Foundation geotechnical preparation recommendations were requested for the

bedrock and “AJ” fill areas in maintain settlements to the glass panel

manufacturer’s tolerance.

“AJ” fill is the spoil material derived from the former hard rock mine and is

considered an unclassified material, generally granular soil, but may contain

cobbles and boulders as well as fines (silts and clays). Organic matter may also

be present.

A concern with the “AJ” fill is its behavior under cyclic (seismic) loading. If

saturated finer grained sands are present, shear strength loss and potential

liquefaction may occur. These conditions may result in excessive total or

differential settlements. The liquefaction potential of the “AJ” fill at the Transit

Center is undetermined at this time. However, the former structure at this site

was reportedly a CMU block structure without reinforcing. This structure and its

concrete floor slab reportedly did not exhibit any noticeable distress prior to

demolition.

For the Transit Center foundations over bedrock, we recommend removing

rock to foundation grade. Site inspection prior to foundation installation should

verify that no weathered, weak, or heavily fractured rock is present in the

foundation footprint. The bedrock surface should be clean and the continuous

reinforced concrete strip foundations cast directly on the bedrock surface. If so,

an allowable bearing capacity of 15,000-psf can be used. Settlement is expected

to be negligible for foundations cast directly over bedrock

For the Transit Center foundations over “AJ” fill, we recommend the

foundation footprint be excavated into the “AJ” fill at least 8-inches below the

foundation grade. Once exposed, “AJ” fill (subbase) will require verification for

material consistency. Boulders, organics, silts or clays, debris, or other

deleterious matter must not be present at the foundation grade. The exposed

“AJ” fill should be proof compacted with a vibratory plate of roller compactor.

CBJ Garage and Transit Center Foundation Refinements! Duane Miller Associates LLC

February 28, 2009Page 2

a b

A performance-based compaction specification may be necessary in lieu of field

density testing since maximum dry density test may be difficult to measure with

the “AJ” fill material. We recommend a performance-based compaction

specification include at least three passes with vibratory plate or roller

compaction that is rated for at least 6,000-lbs centrifugal force with at least 5,000

vibrations per minute (VPM). A visual inspection for weak, soft or yielding areas

in the “AJ” fill should be conducted by an experienced engineer or material

technician. Any soft or yielding area should be removed to firm material and

replaced with structural fill.

Once the subbase is properly prepared, a non-woven geotextile, such as

Geotex 401, or similar, should be placed over the prepared subbase. A crushed

sand and gravel conforming to ADOT/PF D-1 specification should be placed

over the geotextile and compacted with a vibratory compactor to a minimum

95% of modified Proctor, ASTM D-1557.

We understand the Transit Center foundations over “AJ” fill will be

continuous strip reinforced concrete sufficiently wide to reduce bearing loads to

1,000-psf, or lower. If, so we would expect settlement to be less than 0.5-inch

total with less than 0.25-inch differential for foundations installed over the “AJ”

fill prepared as discussed above. However, the potential for poor quality

material in the “AJ” fill under Transit Center footprint may exist. If so, a greater

total and differential settlement may occur, particularly during cyclic (seismic)

loading conditions.

Please feel free to call me or Duane Miller if you have any questions.

Respectfully submitted

Duane Miller Associates LLC

Richard Mitchells, P.E.

RAM/DLM/bs

Attachments!Plate 1, Transit Center

! ! R&M Test Hole Location Plan and Soils Logs

CBJ Garage and Transit Center Foundation Refinements! Duane Miller Associates LLC

February 28, 2009Page 3

a b

Plate

1Inferred Bedrock Contact

Juneau, Alaska

CBJ DOWNTOWN TRANSIT CENTER

Job No.: Date:

4169.007February 2009

Duane Miller Associates LLC

Base Plan: Tetra Tech, Preliminary Sheet C1.01 Downtown Transportation Center, Contract No. E09-079, Feb 05, 2009

N

Approximate Scale In Feet

50 0 50 100

1in = 50 ft, approx

Inferred BedrockContact

ProposedTransit Center

Bedrock

“AJ” Fill

SCALE:

DATE:

CKD:

DWN:

SURVEYORSENGINEERS GEOLOGISTSSOILS LOG 1 OF 1

081134

SOILS LOGS

DWG No:

PROJ No:

GRID:

1" = 80'

M.L.L.

E.B.C.

AUG. 2008

CBJ PARKING GARAGEGEOTECHNICAL INVESTIGATION

DOWNTOWN, JUNEAU, ALASKA

DRILL TEST HOLE LOCATION MAP

SYMBOL

TEST HOLE