HomeMy WebLinkAboutBLD2008-00262 Geotechnical Report •
Based on the predicted behavior of Indianola soils, this plan assumes a
maximum design infiltration rate of 10 inches per hour.
K. Geotechnical Report
As requested, NTI Engineering & Surveying (NTI) completed a geotechnical
assessment of the above referenced property consisting of research of
available literature and geologic maps of the area, and a site visit on
September 17, 2008, for visual observations. This assessment was in
response to the property being listed as a landslide hazard area by the
Jefferson County Department of Community Development.
NOV 1 !` 2008
1. Site Description
dLtrctiJUIV 1;U N(Y OCD
The subject property is located on Spring Hill Road off of S.R. 19, south of
Chimacum in Jefferson County (Map 2 - East Chimacum Creek Valley on
Page 9). The hilly wooded property is bounded on the north, east and
south by wooded residential property, and on the west by SR 19. Existing
improvements to the property include a driveway and benched home site.
The property is situated on a west facing slope overlooking Chimacum
Valley. The eastern two thirds of the property has an overall average
natural slope angle of approximately 18°, and the western one third has a
slope of 13°. These slope angles are below the "angle of repose" which is
defined as the maximum slope or angle at which loose, cohesionless
material remains stable, and commonly ranges between 33 and 37
degrees on natural slopes. Three west and southwest trending ravines cut
through portions of the property (Map 6 - Drainage Basins on Page 13).
The benched home site is nearly flat and encompasses an area about 200
feet by 120 feet (Map 5 - Site Sketch on Page 12). The cut slope has a
maximum height of approximately 29 feet with an average slope of 40°
and the fill slope is about 23 feet high with a slope of 30°. The material
exposed in the cut slope is dense silty sand and gravel.
Except for the benched home site, the property is well vegetated with
young to mature trees and brush. Predominant species include fir, cedar,
maple, alder, and madrona, with an understory of salal and sword fern.
The property has been logged probably twice in the past as evidenced by
old growth stumps with spring board notches as well as second growth
stumps. Some of the trees exhibit curved trunks, suggesting that
downslope creep of the surface soil is occurring in places. No groundwater
seeps were observed on the slopes, however, seeps were visible at the
west side of the property near SR 19. No evidence of past landslides was
noticed on the slopes.
39
• 4
2. Site Geology
The Department of Ecology's "Geology and Ground-Water Resources of
Eastern Jefferson County, Washington" maps the soil in the area of the
property as Lodgement till (Qvt) and as Undifferentiated glacial, fluvial,
glaciofluvial, lacustrine and glaciolacustrine deposits (Qu). The till is
described as boulders, cobbles and pebbles in a matrix of sand, silt and
clay; a compact and unsorted mixture. The undifferentiated deposits are
older than the Fraser glaciation and include sediments from pre Fraser
glaciations. This unit contains a variety of soil types and is generally
mapped when more detailed data is not available.
According to the Soil Survey of Jefferson County Area, Washington
(United States Department of Agriculture, 1975), the subject site is in an
area mapped as Cassolary sandy loam (CfD), and Indianola loamy sand
(InC & InD) (Map 8 - Soils on Page 36). The Cassolary soil formed in
glacial drift and/or marine deposits on terraces and is composed of sand,
silty sand and silt/clay with gravel. Runoff is listed as medium and the
hazard of water erosion as moderate. The Indianola soil formed on eskers,
kames, or moraines in sandy glacial outwash and is composed of silty
sand with gravel. Runoff is listed as slow to medium and the hazard of
water erosion as slight to moderate.
Visual observations of exposures and cuts on the property are consistent
with the above descriptions.
3. Conclusions and Recommendations
The subject property appears grossly stable and the project seems
feasible from a geotechnical perspective. Based upon our assessment of
the property, it is our opinion that the hillside does not represent a
landslide hazard. However, the property is subject to erosion due to the
sloping terrain. Specific sediment and erosion control measures will be
addressed in the engineered drainage, sediment and erosion control plan
being completed for this property by NTI.
We recommend that building foundations be placed in undisturbed native
soil and comply with Section 1805.3 (see below) of the International
Building Code (IBC)which deals with footings on or adjacent to slopes.
40
! •
1805.3 Footings on or adjacent to slopes.The placement of (2134 mm)from the top of the slope shall be capable of sup-
buildings and structures on or adjacent to slopes steeper than porting the water in the pool without soil support.
one unit vertical in three units horizontal(33.3-percent slope) 1805.3.4 Foundation elevation.On graded sites,the top of
shall conform to Sections 1805.3.1 through 1805.3.5. any exterior foundation shall extend above the elevation of
1805.3.1 Building clearance from ascending slopes. In the street gutter at point of discharge or the inlet of an
general,buildings below slopes shall be set a sufficient dis- approved drainage device a minimum of 12 inches (305
tance from the slope to provide protection from slope drain- mm)plus 2 percent.Alternate elevations are permitted sub-
age,erosion and shallow failures.Except as provided for in ject to the approval of the building official,provided it can
Section 1805.3.5 and Figure 1805.3.1,the following criteria be demonstrated that required drainage to the point of dis-
will be assumed to provide this protection.Where the exist- charge and away from the structure is provided at all loca-
ing slope is steeper than one unit vertical in one unit hori- [ions on the site.
zontal (100-percent slope), the toe of the slope shall be 1805.3.5 Alternate setback and clearance.Alternate set-
assumed to be at the intersection of a horizontal plane drawn backs and clearances are permitted,subject to the approval of
from the top of the foundation and a plane drawn tangent to
the slope at an angle of 45 degrees(0.79 rad)to the horizon- the building official. The building official is permitted to
tal.Where a retaining wall is constructed at the toe of the require an investigation and recommendation of a registered
design professional to demonstrate that the intent of this sec-
slope,the height of the slope shall be measured from the top
of the wall to the top of the slope. tion has been satisfied. Such an investigation shall include
consideration of material, height of slope, slope gradient,
1805.3.2 Footing setback from descending slope sur- load intensity and erosion characteristics of slope material.
face.Footings on or adjacent to slope surfaces shall be
founded in firm material with an embedment and set back
from the slope surface sufficient to provide vertical and lat-
eral support for the footing without detrimental settlement.
Except as provided for in Section 1805.3.5 and Figure
1805.3.1, the following setback is deemed adequate to .4
meet the criteria.Where the slope is steeper than 1 unit ver-
tical J „
in 1 unit horizontal(100-percent slope),the required
setback shall be measured from an imaginary plane 45 j
degrees (0.79 rad) to the horizontal, projected upward 0 ` 1 2Oe
gT P ��'lr
from the toe of the slope.
1805.3.3 Pools.The setback between pools regulated by
this code and slopes shall be equal to one-half the building ` •!!,` r
footing setback distance required by this section.That por- s)C t r E ti 1>!nn� L lj d N i 1v CE"
Lion of the pool wall within a horizontal distance of 7 feet
FACE OF
FOOTING
TOP OF TThs
SLOPE
FACE OF / IT
E
STRUCTURE
SLOPESLOPE
ry1 BUT NEED NOT H
EXCEED 40 FT
MAX
H/2 BUT NEED NOT EXCEED 15 FT.MAX.
For SI: 1 foot=304.8 mm.
FIGURE 1805.3.1
FOUNDATION CLEARANCES FROM SLOPES
2006 INTERNATIONAL BUILDING CODE®
In this case, the face of the structure will need to be a minimum of 14.5
feet from the toe of the cut slope unless a retaining wall is utilized, and the
face of the footing will need to be a minimum of 8 feet from the edge of the
fill slope.
The following recommendations should also be considered with regards to
development of the subject property:
1. It will be necessary to replant and maintain vegetative ground cover
on the slopes to reduce erosion from surface runoff. Native deep-
rooted vegetation that requires little or no irrigation would be the
most beneficial.
41