HomeMy WebLinkAboutGeoTechnical Report Comments - COM Engineering / Geo-Tech Reports - 8/29/2002 08z'-9/2002 09: 41 4258817731 AMERICAN ENGINEERING PAGE 02
American Engineering Corporation
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Engineers . Planners • Surveyors
VIA FAX AND U.S. MAIL
August 29, 2002 AEC Job#9953
Via Fax: (360) 427-8425
Mr. Alan.A. Tahja
Pierce Co. Hydr. Engineer
Mason County Public Works Department
P.O. Box 1 S50
Shelton, WA 98584
Subject: Johnson @ BelltowneBelfair
Comments to Geotechnical Report, Seismicity Evaluation
Dear Mr. Tahja:
Thank you for responding to my phone call regarding your letter and comments of August 16, 2002,
regarding the Johnson site at BelltowneBelfair. I have received your table regarding the
probabilistic ground motion values and have input.them into a second analysis.
Based on our site investigation and regional studies relating to the infiltration studies,we found that
the site materials consisted of very dense silty gravelly sand and sand. The regional water table was
determined to be at a depth of 60 feet or greater(from the base of the slope). It is our opinion that
the very dense nature of the soils extends to a depth of 100 feet or more and there is no chance of
liquefaction on this site.
Perched water may be possible on this site; however,no features were identified on or near the site.
,Adjacent upslope development has modified infiltration characteristic and on site filling and surface
drainage has reduced the amount of infiltration to a large degree. Our analyses used the undrained
site conditions.
The average bulk density of the site soils was considered to be 120 pounds per cubic foot. The
compacted soil placed on the site during.grading yielded a maximum dry density of 130 to 140 pcf
within the 95 percent compaction requirement. This density significantly reduces the porosity and
permeability of the reworked soils. It is our opinion that the 120 pcf is a reasonable value.
The cohesion used in the analysis was considered because of the fact that there were low, vertical
slopes on the site that inferred cohesion in the soil mass. A lower cohesion value of 10 psf would
reduce the original slope factors of safety to a level below numerical stability. The 100 psf value was
used to bring the SF (static) above 1.0 to 1.5 depending on the combination of other soil parameters.
This was part of our parametric evaluation of the soil parameters.
rd9.1;1 W-7d3n Fax (425) 881-7731
08;'29/2002 09: 41 4258817"31 AMERICAN ENGiNEEHiN6 PAGE
American Engineering Corporation
Mr. Alan Tahja
Re: Comments to Geotechnical Report, Seismicity Evaluation Engineers • Planners • Surveyors
Johnson EelltowneBClfair
AEC Job#9953
Aug.ust 29, 2002
Page 2 of 2
The recalculation of the slope stability using the higher horizontal acceleration of 0.27 and 0.3 are as
shown in the following table:
Trial Acceleration Pseudostatic Site Condition
Factor of Safety(SF)
1 0.2 1.21 Undrained, graded
2 0,27 1.20 Top of slope excavated
3 0.3 1.15 Top excavated
4 0.2 1.37 Top excavated
5 0_27 1.07 Natural state
This data shows that the original analysis (1) exhibited a SF of 1.21 in the undrained, but final graded
state prior to construction. That slope analysis (2)using the 0.27 yielded 1.20 with 8 feet of the top
of the slope removed to represent the construction of structures and detention/treatment systems_ The
(3) analysis input 0.3 to demonstrate the sensitivity of the analysis to changes of the horizontal
acceleration_ The (4) trial shows the SF for the site using 0.2g with the top of slope graded for
construction. The difference between 1.37 and 1.21 is the change caused by the site preparation for
structures at the top of the slope. The trial (5) shows the SF for the slope in the natural state using the
higher acceleration—0.27g.
It is our opinion that the site fulfills the general requirements of most municipalities of at least 1.1 SF
for the pseudostatic state.
If you have any questions regarding this review,please call Richard Bielefeld at(425) 828-3162.
Sincerely, GUNDF
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AMERICAN ENGINEERING CORPORATION
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RICH D J. BIE FEI.D, CEG PE
Engineering Geologist P foNAL�'
Attachments ExrIREs 04-05-di
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08/29/2002 09:41 4258817731 AMERICAN ENGINEERING PAGE 04
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