HomeMy WebLinkAboutBLD1995-00284 Geo Tech ilit _____. - NORTHWESTERN TERRITORIES, INC.
V. Engineers ■ Land Surveyors ■ Planners
Construction Coordination ■ Materials Testing
NTI
GEOLOGIC HAZARD AND GEOTECHNICAL SITE ASSESSMENT
REPORT OF FINDINGS
Lot 4, Coyle Peninsula Properties -
Short Plat II
Located in Section 16, Township 27, Range 1 East, W.M.
Jefferson County, Washington
Prepared for
CHUCK THURMAN
Prepared by
NORTHWESTERN TERRITORIES, INC.
November 1994
717 SOUTH PEABODY, PORT ANGELES,WA 98362 (206)452-8491 1-800-654-5545 FAX 452-8498
REPORT OF FINDINGS
During November 1994, Northwestern Territories, Inc. (NTI) conducted a reconnaissance
geotechnical study of the geologic and foundation conditions at Lot 4 of the Coyle
Peninsula Properties Short Plat. The 1.56 acre parcel lies on a gentle slope above
Hood Canal in Section 16, Township 27 North, Range 1 East, W.M., Jefferson County,
Washington. The lot offers a relatively flat building site with a spectacular view to the east
across Hood Canal. Special considerations are necessary for the site development due
to descending slopes on the east beyond the building site and grading of the site that is
necessary to the proposed development. Drainage aspects of the proposed development
are also reviewed.
I. GENERAL SITE CONDITIONS
An inspection of Lot 4 of the Coyle Peninsula Properties Short Plat indicates that the
entire parcel enjoys a medium sand soil. Study of the terrain in the vicinity of the project
suggests that the sand is extensive in the vertical section beneath the proposed homesite.
The homesite is a tabular bench which was graded to a flat condition during the original
division of the property several years ago. The on-site sandy soils are an excellent
foundation material, if properly handled. The preliminary site plan prepared by the owner,
Mr. Chuck Thurman, indicates that most of the foundation level will be below the
uppermost soils that may have been disturbed during the original grading. (Please note
attached site plan.)
Northeast of the homesite, the bench area gives way to a descending slope which ends
at the ditch line of Edna Way. (Please refer to the attached plat map of the division.) At
the highest point, these slopes measure about 36 feet vertically, and at the most severe
point slopes are about 38 degrees from the horizontal or about 75 percent grade.
A very small stream has been routed along the outside the western boundary of Lot 4.
The presence of the stream creates a constraint on the position of the septic drainfield
that must be placed on the parcel. The parcel is served with water by a public water
system.
The benched building site must receive a home and the septic drainfield. As a result,
space and setbacks on the site are limited. Specific foundation recommendations
applicable to daylight basement houses are indicated and apply to any home of this type
that is selected by the owner. The house footprint shown on the site plan is the
"Herrington" produced by Kappler Plan Service, Inc.
1
II. GEOLOGIC SETTING
The homesite and lot are situated on a formation of outwash sands deposited toward the
close of the Ice Age. During the glacial epoch, Hood Canal on the east was the site of
immense lobes of glacial ice. As glaciation waned in the region, melting ice released
torrents of sand-filled water that braided across the flanks of the ice lobe forming terraces.
These meltwater rivers deposited sand in longitudinal ridges that are nearly parallel with
Hood Canal in the vicinity of the site. The occurrence of the sand in the vicinity resulted
in the siting of a community septic drainfield just east of Lot 4.
In the vicinity of the site, outwash gravels with glacial silt interbeds are believed to form
most of the subsurface. These materials comprise the forest-covered slope west of Lot
. The slope on the west creates no hazard to the proposed development.
For all practical purposes the stability and capacity of the site to undergo development
is determined by the sand soils which appear to be predominant both vertically and
horizontally at the site. At present the site is nearly entirely cleared and covered with a
dense ground cover of grasses and brush.
III. GEOTECHNICAL INDICATORS AT THE SITE
Sand soils at the site are capable of excellent bearing capacity and sand may remain
stable at slopes exceeding about 33 degrees from the horizontal. The soils at the site
are, however, subject to water and wind erosion and precautions should be taken to
ensure that little or no erosion results, especially where setbacks are limited.
Geotechnical sections were constructed from field measurements provided by the owner.
Sections A through C illustrate the approximate profile of the descending slopes at the
points where the sections are noted on the owner's preliminary site plan. The well-
drained sand soils underlying the site are likely to remain stable under seismic loadings
within envelopes that do not exceed 33 degrees from the horizontal. Thirty-three degree
envelopes have been sketched on the approximate profiles of the slope at critical points
along the slope.
The profile analysis indicates that approximately ten feet of lateral and horizontal setback
is required for structures built on the bench. Under the preliminary site development plan,
a daylight basement format would be used for the house. Thus, foundation levels would
be lowered seven or more feet from the existing elevation of the flattened bench.
The geometry of the slopes and the lay of the proposed single-family residence shown
on the preliminary site plan indicates that the proposed lowering will create a flattened
setback zone at the top of the slope. Profile analysis indicates that setbacks will be ten
or more feet in width.
2
The Uniform Building Code (UBC) provides guidance for building foundations of this type.
The UBC indicates that footings should be embedded deep enough so that the minimum
horizontal distance between any part of a footing and the breakout point on the adjacent
slope is not further than the height of the slope divided by three. The lowered foundation
level is never more than 30 feet above Edna Place below, and the lateral breakout
distance may be taken as a minimum ten feet for field applications.
Natural deposits of sand at field densities are capable of good bearing capacities.
Undisturbed sands that are presumed to occur at the proposed foundation level may
readily support bearing pressure of 2,500 pounds per square foot. Earth materials must
be embanked in the area to receive the garage indicated in the preliminary site plan.
Native soils may be used for such embankments provided they are placed and com-
pacted in accordance with the recommendations outlined below.
IV. GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS
The following conclusions and recommendations are based on the occurrence of sand
in the subsurface in the foundation areas of the proposed structure. If silty soils, peats,
or wet and unstable soils are encountered, notify the undersigned engineer before
proceeding with any foundation or embankment work.
4.1 Bearing Capacity of Soils
Sandy soils that underlie the proposed building site are capable of good bearing
strength. UBC prescriptive footings that are embedded two or more feet below the
existing grade will provide bearing capacity of 2,500 pounds per square foot. Footings
must be placed on undisturbed native soil. If soils must be embanked under footings, the
recommendations in part 4.4 should be followed.
4.2 Lateral Forces for Retaining Wall Design
Retaining walls that are provided with drainage blankets and perimeter footing drains
and/or weep holes at frequent intervals may be designed for an equivalent fluid density
of 40 pounds per cubic foot. Total lateral pressures and force diagrams for retaining
walls may be calculated in the conventional manner.
4.3 Slope Setbacks
House foundations must be set back from the edge of all descending slopes. No portion
of the foundation footing should extend closer to the natural descending slope than
ten feet measured horizontally. If possible, more setback should be provided for
additional protection from small surficial sliding or erosion which could occur on the
adjacent slopes if the vegetative cover is disturbed.
3
4.4 Embanking of Native Soils
Native soils may be embanked on the site to receive foundations for portions of the house
or garage. Native soils may be embanked by removing all vegetable material from the
surface to receive the fill, placing the soils in lifts not exceeding 8 inches and compacting
with high-frequency vibratory plate in trenches. Usually three or four passes are required
to achieve the density specified below.
Heavy self-propelled vibratory compactors should be used for mass embankment under
the garage floor and approach road areas. Water flooding should be used to accelerate
the compaction process whenever possible. Each lift of soil should be compacted to at
least 90 percent of the Modified Proctor control density. Such a surface will be notably
firm and tight under foot. Most embankment failures occur because lifts of soil are too
thick for effective compaction and both owner and contractor should monitor the embank-
ment work to ensure good workmanship.
If silty, wet and fluid soils or mucks are encountered on excavation of the site, notify the
engineer immediately.
4.5 Protection of the Slopes
The principal erosional hazards for the sand soils at the site are water and wind erosion.
The existing ground cover of plants on the slope at the easterly margin of the building site
must be permanently protected and remain undisturbed. Plant covers are effective in pre-
venting the drying of the soil which leads to a loss of cohesion and hastens wind erosion.
All drainage from rooftops and driveways must be permanently diverted from the slope
to prevent scouring of the sand.
V. SITE GRADING AND DRAINAGE
5.1 Site Grading
Preliminary calculations indicate that approximately 800 cubic yards of earth will be
excavated from the footprint of the proposed home to be developed. Under the current
plan about 300 to 400 cubic yards of this material is likely to be embanked west of the
proposed house site. At present, the area to the west is an average of about 31 feet
lower than the bench elevation. Although an additional 100 to 200 cubic yards could be
embanked on the property, it is likely that 300 or 400 cubic yards of soil must be hauled
away from the site.
To prevent erosion of the newly embanked soils, work should be planned so that the
disturbed areas can be seeded and germination can occur before heavy seasonal rains.
Silt fencing should be provided on the west side of the lot to protect the small stream
from silts and mud. Standards for silt fencing may be obtained from this office.
4
No soil or debris should be placed on the existing slopes east of the proposed house
without a positive program of restoration. Dozing or dumping of soil over the slope is
more likely to destabilize the slope than to enhance it.
5.2 Site Drainage
Site drainage is regulated by Jefferson County. Under applicable ordinances, runoff from
rooftops and driveways must be detained on the site and slowly released to prevent
downstream flooding and erosion.
Preliminary inspections of the soil and the topography of the site suggest that drainage
water may be effectively detained in the area west of the proposed house. From
detention water may be routed through a controlled release orifice and directed to the
small creek through a filtration swale.
Alternately, the sand soils at the site offer an excellent opportunity to dispose of runoff in
subsurface percolation trenches. These percolation trenches detain the 25-year storm
runoff and allow the runoff to percolate into the subsurface. Percolation trenches and
drywells must be set back from the sloped area as much as possible. Therefore, the area
west of the proposed garage appears to be ideal for such a percolation area. Detailed
design of such facilities can be performed by NTI.
VI. STANDARD DISCLOSURE
This report is preliminary in nature and based on an inspection of the existing site
conditions. The site investigation was made without the benefit of detailed measurements,
geotechnical materials tests, borings or detailed investigation of the subsurface conditions.
The work was done to the standard of care and in accordance with the practice of the
engineering profession but unforeseen and unknown subsurface conditions cannot be
completely understood by rapid surveys or examination of surface conditions. Reduction
in risk and better reliability of conclusions and recommendations can be obtained by more
detailed investigations of the site that include borings, accurate measurements and
laboratory testing of materials. The Engineer encourages the user who wishes a higher
degree of reliability of investigation results to obtain such services.
Unless construction plans are reviewed and construction inspected by the Engineer, no
warranty is made concerning the conformity of the finished construction to the conclu-
sions and recommendations of this report.
The observations, interpretations and conclusions herein are based on the generally
accepted principles of engineering and geology. This warranty is in lieu of all others,
either express or implied.
5
This report is intended for the sole use of Mr. Chuck Thurman and may be used by
others only with his express permission.
tu-se Are co v*A
Steve S. Luxton, Msc., P. . "'
Professional Engineer
-5-rsTr,
FC: Thurman.Geo
DIR: SSL\Rpt
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