0 City box number • _ :.r) -:- y,.t.c... ~ 0 Title/cover page withe following info:

Ea' Company (author) name

CY'Report Date ~ 0 Project name ~ D ·Company's job number

CJ j:ity DCLU project number (7-digit number)

El City Permit number (6-digit number)

CJ Kroll map index number (3-digit number, w?/E,W,N,S) 0 Green label

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0 Executive Summary and associated figures

D Table of Contents

D Project Location Plan/Map or Vicinity Map

~ite Plans, Boring Location Pl~s, or Exploration Plans

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D Geologic Maps

~ross Sections/Subsurface Profiles

D Fill or Peat Thickness Maps and Contour Maps

it Boring LogsCV) X S d-~ 0 Geology Text (if no logs)

0 Soil Classification Key/Boring Log Key D ProbeLogs

D Test Pit Logs

D Monitoring Well Logs

0 Cone Penetrometer Logs

0 Shear Wave Velocity Measurements

D Groundwater Maps

~ GW Elevation Tables/Data

D Soils Lab Testing (Geotechnical) Summary Tables

0 Grain Size Analyses/Hydrometer Analyses

CJ Atterberg Limits

0 Strength tests: Triaxial, Unconfined, Direct Shear

0 Organic Content

0 14C or Radiocarbon Testing 0 Other ______________ _

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NEIL H. TWELKER & ASSOCIATES AUG l Ci 1976

CONSULTING SOILS ENGINEERS CHESTER L. LINDSEY ARCHli£ClS

Alaska Trade Building Mail To: P.O. Box 21328 Seattle, Washington 98111

August 6, 1976

Chester L. Lindsey, Architects 1711· - 12th Ave Seattle, Washington 98122

Attention: Mr. Curtis L.; Beattie

Gentlemen:

Re: Soils and Foundation Investigation for Proposed Office Building, 4th and Battery, Seattle, Washington

At your request we have conducted a soils and foundation investiga­tion for a proposed office building at 4th and Battery in Seattle, Washington. We submit herewith a report of our Findings, conclusions and recommendations.

Site Description The property extends from Wall St to Battery St between 4th Ave and t~e alley to the west. It has been-recently razed and cleared of debris, leaving a relatively level surface, except for areas in the southeast and northwest corners formerly occupied by basement and crawl space excavations. The Battery St margin of the property is immediately adjacent to the wall of the Battery St tunnel, which is supported by piling extending 30 ft below surface grade. The street level in the tunnel is 17 ft below grade one. Soil exposures on the site are of a brown silty sand and gravelly sand.

Surface Explorations In crder· to determine the sub~urface soils characteristics at the site, four test borings were made using a truck mounted hollow stem power auge~ Samples were taken at 5-Ft intervals with the Standard Penetration Test, in which a 2-inch OD split spoon sampler is driven into the undisturbed soil formation at the bottom of the boring by repeated blows of a 140-lb pin­guided hammer falling 30 inches. The number of blows required to advance the sampler a given distance is a measure of soil consist­ency. The logs of these borings were combined with topographic information to construct the geologic section shown in Fig. l, attached.

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Chester Lindsey, Arch. Augusts, 1976 Page 2

Subsurface Conditions Three principal soil units are present at the site; these are described briefly as Follows:

1. In the northerly-portion or the site, a gray, loose, sandy, gravelly, silty fill extends to a maximum depth of 7 feet.

2. In the southerly portion or the site (adjacent to th~ Battery Street tunnel) a unit of hard gray silt and clay with dense fine to coarse sand lenses extends from ground surfac~ to a depth of 22 feet, apparently thinning towsrcl the north.

3. Underlying the fill in the r.orth and clay in the south, and exposed Qt the surFace throughout the central portion or the site, is a unit of hard sandy gravelly silt and ail~y gravelly sand, extending to an unknown depth.

Groundwater was encountered in all the borings in thin intermit:ont pervious layers. The highest water-bearing layer was round in tho center of the site, at a depth of 3 ft.

Proposed Construption We understand that an,eleven-story oFrice building with a basement parking garage will-be constructad on the site approximately 11 feet below existing grade.

Conclusions and Recommendations On the basis of our invcstigatior.o at this site we draw the following principal-conclusions:

l. The natural _soils on the site are in a dense state and ·will provide excellent support for the proposed struc~urc. Spread footings of conventional configuration may be used on undisturbed soil and should be. designed for bearing pres­sures of 10,000 psf.

2. Groundwater will be encountered sporadically throughou~ the excavation. We anticipate that inflow of groundwater can be controlled satisfactorily by a modest pumping eFFort, using open sumps.

3. Excavation slopes in the native soil units may be cut near-vertical for heights up to 12 feet, and 4 vertical to 1 horizontal to 20 feet. Temporary excavations made in this manner are expected to be stable; they are, however, subject to minor sloughing without warning.

4. The portion of the building to be constructed adjacent to the existing Battery Street tunnel would result in increased horizontal loads on the tunnel section i~ constructod at tho same grade as foundation elements in the remainder of the building. Rather than attempt to evaluate the increase in

Chester Lindsey, Arch. Augus"C 6 ,1 l 576

pressure against the existing tunnal wail and then determ:na the ability oF the wall to withs~and tho increased pruss~re~ we believe the more straight-forward solution to ccr.sis;:: oF Footings in open excavations a~ tMa level of t~e lower roac­way. We have been advised that ~ha existing tun~el wQs constructed by placing soldier pi:es (12 SP 53] at a S?aci~£ oF 3 ft 9 inches in prebcred holes to the roadway alevGtion, followed by driving to a depth oF at least 5 ~t below ths subgrade elevation of the lower roadway. We Further underst~~d that neither air nor w~ter jets were used to Facilit8te thG driving. Because oF the sequence used in construction oF t~e tunnel1 negligible disturbance would have been created in the in-situ soil beyond the soldier pile lins; conseGuently, lateral earth pressure delivered by the extremely competant, over-consolidated and cohesive soil units against the tun~8l wall would be or a very low order. We estimate ~he probable maximum lateral pressure on the tunnel to be oF the orcer c~ 1,000 lbs· per lineal Foot. The open excavation adjac~r.t ~o the tunnel will oF course remove lateral Force Fro~ th~ t~~~sl, thereby requiring the tunnel structure [walls, roadway, flea~~ etc) to distribute the lateral earth leads oF tna cpposi~e tunnel wall to the undisturbed portions cf tho s-..bs~ruct:wr~. Although we have not reviewed tha ab:lity oF ~M~ ~~nna~ structure to so distribute these Forces, it appezrs tha~ little problem should be occasioned on this account.

S. Backfill oF the deeper excavations may be m~de either ~•w•,

compacted on-site material (provided that Favorabl~ woa~h~r and adequate moisture control can be maintained) or with pcia gravel, which may be placed without compsctiva effort.

6. An increase in allowable bearing prossure oF as much as 40 per cent may be permitted for transient loads (wind or seismic).

7. Soils on which spread footings will··be constructad ar~ susceptible to disturbance in the presence of excess mois~~~c. Should rainy weather threaten during the Founda~ion prcpar3ti~n phase oF the project, appropriate steps should be taken to p~o­tect prepared bearing surfaces. These could include plzci~g oF a 3-inch blanket oF washed sand over-the Freshly p~epsra~ bearing surfaces.

We point out that interpretation of soil conditionG at the projac~ site has been made on the basis oF a limited number oF explora~cry borings. We should be advised when the excavation is complete i~ order that we may determine that soil condit:i·ons at the site c;ll"'S

consistent with our previous intarpretaticns. ~.1-..~'-*'-~w,

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Very truly yours,

NEIL H. TWELKEA ANO ASSOCIATES

. ~ by mdzt:t~d~

Neil H. Twelker

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