HomeMy WebLinkAboutGeoTech Assessment for BLD2006-01529 - BLD Engineering / Geo-tech Reports - 11/9/2006 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton, Washington 98584
DATE: November 9, 2006
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Rebecca Hersha PARCEL# 32127-53-00157
FROM: Patricia Carroll, Public Works BUILDING PERMIT NUMBER: BLD2006-01529
SUBJECT: Geotechnical Assessment NAME: Brett Vanslyke
Hi Rebecca;
The Geotechnical Assessment dated October 3, 2006 has been received and reviewed by Public Works. The
Engineering firm of Bradley-Noble Geotechnical Services stated that based on their assessment of the area
the site soils are conducive to construction of the new residence at 50 East Angus Ct, Shelton.
Recommendations for backfill being placed in conformance with the International Building Code (IBC) to
ensure positive drainage away from the structure are necessary. This will prevent ponding against the
foundations. The collected water is to be conveyed below the drain field and infiltrated into the slope soils.
Installation of roof rain leaders, catch basins, and trench drains to be connected to a tightline collection
system conveyed to the toe of the slope for disposal, which will prevent discharge of storm water on the
surface adjacent to the structure. The footing drain which is also required may be connected to the
common discharge line at an elevation such that backup into the footing zone cannot occur. Use of rigid
plastic pipe for both systems is recommended. The tightline storm water collection system should
incorporate glued watertight joints. Use of flexible ADS pipe used for the infiltration system must be
staked to the slope and maybe connected to this discharge line at an elevation such that backup into the
footing zone cannot occur. Good site grading practices and water management including minimum
vertical separation between bottom of crawl space and top of landscape soils of three inches, which will
be required to ensure a dry crawl space.
Adequate erosion and sediment control features need to be implemented during and after land disturbing
activities to protect neighboring properties and State waters from adverse stormwater runoff impacts. The
migration or release of silty water or mud from the applicant's property will be considered a violation of
County and State water quality protection regulations.
Please feel free to contact me at 619 or John Sliva at 724 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
Sincerely,
1�J
Patricia Carroll
Mason County Public Works
1.0/P2/2006 14: 26 3607053066 B AND G PROPERTIES PAGE 02
Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
PO Box 1 Z267, Olympia WA 98508-2267
Phone 360-357-7883 • FAX 360-867-9307
28 September 2006
Mr. Brett Vanslyke
B & G Properties, Inc.
P.O. Box 2856
Olympia, Washington 98S07-2856
Subject: Geotechnical assessment of the slopes on Lot 157 Tract 4 of the Lake
Limerick Development, Mason County Tax Parcel Number
321275300157..
Dear Mr. Vanslyke:
As requested, we visited the above property in order to observe site geology and slope
conditions on this and adjacent lots. Our work consisted of a site visit, observation of
soils exposed in the septic system design test pits, review of proposed plans for the lot
development, research into the area geology and slope stability history, and preparation
of this letter.
The project site is located in the Lake Limerick Development. This area is a gently
undulating surface of the Vashon recessional outwash plain. At this site, we find the
area adjacent to Angus Court to be of low relief. To the south of the area of low relief
where the new residence will be constructed, we find moderate slopes down to the
northeast and south. Slope angles as measured with a Leitz Pocket Transit varied
between I I to 13 degrees (19- to 23% of slope), Slope angles are significantly less than
the angle of internal friction of the soils mantling the slope and the underlying Vashon
subglacial till. With slope angles less than the angle of internal friction of the material
forming the slopes, slope stability is inferred.
On examination of the soils exposed in road cuts and building pad cuts in this area, we
find a thin layer of Vashon recessional outwash sands and gravels (Qvr). These soils are
thin, typically less than four feet in thickness. Underlying the outwash soils is the
Vashon subglacial till (Qvt). This is consistent with the soil exposures in the septic
system design test pits excavated onto the south-facing slope at this site. The outwash
sands and gravels were fluvially deposited onto the Vashon outwash plain as the Puget
Lobe of the Cordilleran glacier retreated to the north during the Fraser glaciation in late
Wisconsinan time. These soils were deposited onto the subglacial till. Subglacial till is a
mixture of silt, sand, and small gravel. It was deposited at the base of the advancing ice
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sheet and overridden by the ice which is thought to have been between one-half to one
mile thick in this area. The grain size distribution and loading by the ice sheet have
created a soil unit that has many engineering properties similar to those of lean
concrete. It can stand steeply with only slow erosion occurring.
Subglacial till is for all practical purposes impermeable. Site excavation for the building
pad that exposes subglacial till will require special drainage considerations to ensure a
dry crawl space. You should expect flows of water to develop on the surface of the till
during periods of heavy or prolonged rainfall. To aid in ensuring a dry crawl space and
to protect the drain field that will be constructed downslope of the building site, we
recommend that all roof rain leaders, catch basins, and trench drains be connected to a
tightline disposal system. The collected water is to be conveyed below the drain field
and infiltrated into the slope soils.
To prevent localized saturation from developing, we recommend that the water be
infiltrated by perforated pipes parallel with the slope contours. A footing drain system
will also be required. Use of rigid plastic pipe for both system is recommended. Rigid
pipe is more resistant to crushing and deflection during backfilling than flexible ADS type
of pipe. The tightline storm water collection system should incorporate glued
watertight joints. Use of flexible ADS pipe is acceptable for the infiltration system. This
pipe must be staked to the slope. The footing drain may be connected to the common
discharge line at an elevation such that backup into the footing zone cannot occur.
Backfill against foundations must be placed in conformance with the International
Building Code (IBC), Section 1803.2 Placement of Backfill. Site grading around the
residence must be in conformance with the requirements of Section 1803.3 of the IBC
to ensure positive drainage away from the structure and to prevent ponding against the
foundation. We also recommend a minimum vertical separation between bottom of
crawl space vents and top of landscape soils of three inches. This is to prevent the
vents from becoming a conduit for water entry into the crawl space and to promote
good ventilation.
Geologic Map of the Shelton 1:100,000 Quadrangle, Washington by Robert L. Logan, 2003,
and published by the Division of Geology and Earth Resources maps this area as Qgt,
Till, late Wisconsinan. Plate I of Water-Supply Bulletin 29 Geology of Southeastern
Mason County, Washing by John B. Noble and Dee Molenaar maps the project and
genera areas as Qvt, Vashon subglacial till. This mapping is consistent with our field
observations.
With the slopes being underlain at shallow depth by subglacial till, deep rotational type
r d i failures is not considered to be a risk at this site. Relative Stability a Sta Slop ty of the Southern
Hood Canal Area, Washington by Mackey Smith and R. J. Carson and published by the
1.0/P2/2006 14:26 3607053066 B AND G PROPERTIES PAGE 04
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Division of Geology and Earth Resources 1977 presents an opini9on on area slope
stability. In this mapping, which is at a scale that cannot be considered more than
general, we find the project area mapped as a Class I. Class I areas are believed to be
stable. This opinion is supported by our field observations. We find the small diameter
Douglas firs growing on the slope to be vertical with no evidence of surface soil
movement rotating the root mass. No geomorphic expression of recent or past mass
wasting events were observed on this and adjacent slopes.
Based on our site observations and research, we have no objections to the placement of
a new single-family residence at the location indicated based on geotechnical
considerations. We do not consider that the project site or surrounding slopes are at
risk of mass wasting events that would adversely affect soil foundation support. Good
site grading practices and water management will be required to ensure a dry crawl
space and to protect the slopes from concentrated flows of storm water. We do not
consider the preparation of a Geotechnical Report is warranted at this site due to the
site being underlain at shallow depth by highly overconsolidated soils.
If you have any questions, or if we may be of additional service to you on this project,
please contact us at our Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL SERVICES o� 1Nashi�
Ilk
David C. Strong, L.E.G.
1129 �tiH
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C_."YID(;. S;k-ING
MASON COUNTY PUBLIC WORKS WORK ORDER
Permit#: ,� —O/5� RECEIVED
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Date: �� a� OCT 0 4 2006
Requested by: /-/ NIASONCOUNTYP11RIIraw
Authorized by: 17anl7,A
Type of Work:
Public Works
employee in charge: ) J/,
CHARGE TO:
Name:
Billing Address:
Phone: 7�
WORK PERFORMED:
Employee:
Date: i t ( o G i' 0(,0
Hours:
Hourly Rate:
Total:
ACCOUNTS RECEIVABLE INFO:
Billed Date:
Invoice #:
Amount Billed:
Receipt #
Date:
Amount Paid: