HomeMy WebLinkAboutBLD2007-00593 Geotechnical Report ! •
GEOLOGICAL SITE EVALUATION
Prepared for:
Mr. & Mrs. Scott Dahlgren
Prepared by:
Craig K. White, Inc.
Geosciences Consulting
P.O. Box 3398
Silverdale, WA 98383
August 31, 2006
ra White, c
Geos CtTi+ZSUI1NG
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Table of Contents
Property Location and Description 1
Field Methods
Reconnaissance Work 2
Property Access 2
Topography and Drainage 3
Vegetation 4
Geology and Ground Materials 5
Soil Characteristics 6
Slope Stability 7
Potential Seismic Hazards g
Conclusions and Recommendations 9
Summary and Limitations 11
Appendices:
Location Map Appendix 1
Slope Stability Map Appendix 2
Potential Seismic Hazards(Fault)Map Appendix 3
Black and White Aerial Photo Appendix 4
LiDAR Based Slope Shaded Map(Slopes>30%) Appendix 5
Detail:Area of Study Appendix 6
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Craig , Inc. ORELINE,BLUFF&SLOPE SPECIALISTS
. \�/hite. I I GEOLOGICAL SITE EVALUATIONS
GEOSCIENCES CONSULTING GEOTECHNICAL STUDIES.REPORTS 13c ASSESSMENTS
LICENSED PROFESSIONAL GEOLOGIST,WASHINGTON&ALASKA
August 31, 2006
GEOLOGICAL SITE EVALUATION
Tax Prc.#.021184005
67 Reef Rd., Marrowstone Island, WA.
SE1/4 Section 18, T30N, ROLE, W.M.
Jefferson County, Washington
Property Location and Description:
The subject property is located at 67 Reef Rd, on Marrowstone Island near Ft.
Flagler State Park, where it occupies a portion of the west-facing coastal bluffs
overlooking Kilisut Harbor. The property is bounded on the north, south and east sides by
other private lands and on the west by coastal tidelands. Property size is approximately
260,000 s.f or about 6.0 acres. Existing structures on the property include a two-story,
wood-frame vacation cabin with a large front deck. The main objective of this study was
to evaluate the stability of the coastal bluff where the construction of a wooden stairway
to the beach is planned; also, to determine ways to minimize the effects of erosion on the
bluff face where uncontrolled storm water runoff from areas along the top of the bluff and
undercutting by storm wave action along the base of the bluff has already produced
several landslides.
Appendices to this report include the following: 1)Location Map, 2) Slope
P.O. box 3398,.SiLverdaLe, \\/ashing±on 98383 (360)830-0718
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
Stability Map, 3) Seismic Hazards Map, 4)Black and White Aerial Photo, 5)LiDAR-
Derived Slope-Shaded Map, 6)Detailed Property Plat showing drainage and slope
features within the Area of Study.
Field Methods:
Reconnaissance Work:
Reconnaissance studies included a review of existing topographic, geologic and
slope stability data along with an analysis of LiDAR(Light Distancing and Ranging) data
using GlobalMapper software. Raw LiDAR imagery was examined to determine the
presence of lineaments possibly related to faults or slide escarpments, and other unstable
landforms. Topographic contours were generated on a 20-foot interval across the property
with slopes steeper than 30 percent(17 degrees) shaded in red (see Appendix 5). Ground-
truth checking of features identified by the imagery was performed with the aid of a
Garmin hand-held 12-channel GPS (satellite positioning receiver).
Property Access:
Access to the property is directly off Reef Rd. approximately 0.1 miles from its
intersection with Fort Gate Rd. onto a gravel driveway that leads to the structure near the
top of the bluff(see Appendix 1). Access to the top of the bluff was achieved on foot from
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
the upland portion of the property. Access to the beach at the base of the bluff was made
by walking down the beach from the parking area at Ft. Flagler State Park.
Topography and Drainage:
Drainage on the subject property in the vicinity of the proposed building site is
generally to the west, from Reef Rd. toward the top of the bluff. A component of drainage
also occurs to the south where the bluff face bends inland to the southeast in the direction
of a natural estuary south of the subject property.
Topography on the bluff face can be subdivided into three parts: a steep, exposed,
upper bluff face, a low-relief middle-bluff area produced by a buildup of slide debris from
the upper bluff face, and a steeper, lower bluff face.
Slope angles were measured at several locations on the subject property using a
Brunton hand clinometer and laser-sighting device(see Appendix 6). In the upland portion
of the property slope angles ranged from five degrees to a maximum of 7 degrees.
Measured angles along the upper bluff face ranged from 70 degrees to as much as 85
degrees. In the mid-bluff area, slope angles were in the range of 38 to 45 degrees,
steepening to approximately 65 degrees on the lower portion of the bluff. Elevation at the
top of the bluff is approximately 80 feet.
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
Geology and Ground Materials:
A review of existing geologic maps of the area indicates that the geologic units
present in the vicinity of the subject property are glacial and interglacial deposits of
Quaternary age, most of which are less than 20,000 years old. The oldest geologic unit
within the area of study consists of Advance Outwash materials, a series of poorly-
consolidated, fluvial (stream-deposited) sands and gravels laid down during the advance of
the Vashon glacier from the north.
The upland portion of the subject property is capped by the Vashon till, a layer of
compact cobbles and coarse sand in a binder of clay and silt that commonly ranges from a
few feet to as much as 100 feet in thickness. This unit is comprised of materials deposited
directly beneath a massive layer of glacial ice that once overrode the Puget Sound region.
In many places, the weight of the ice lobe compacted these sediments into a concrete-like
mixture that is almost always impermeable, but considerably more resistant to erosion than
the underlying, unconsolidated sands and gravels.
Along the beach and in the mid-bluff area, slide debris is identified on
reconnaissance maps of the area as Quaternary age alluvium.
Examination of the ground materials where exposed along the upper and lower
bluff face at the subject property generally supports the above geologic interpretation. A
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
series of weakly-bedded, buff to light gray, poorly-compacted sand, clay and pebble-to
cobble-conglomerate occurs in patchy exposures along the shoreline to an elevation of
approximately 20 feet above the beach. Above this elevation, the unit is covered by slide
debris from the upper bluff and vegetation. Along the upper bluff face, the sand and
conglomerate unit is exposed again where it is finally overlain by more compact soils
derived from glacially-overridden tills.
Soil Characteristics:
The dominant soil along the lower bluff face at the subject property is Townsend
fine sandy loam, 0 to 15 percent slopes. This nearly level to strongly-sloping soil occurs
in narrow strips along marine bluffs. In these areas, strong prevailing winds blow fine sand
from beaches and bluffs and deposit it on the surface. This soil is generally fine sandy loam
to a depth of 14 to 22 inches. The hazard of bluff slippage or slough off is moderate to
severe. Permeability is moderate above the cemented layer and the soil holds 2 to 5 inches
of water available for plants. This soil can support rural homesites and is often used for
growing a variety of fruits, berries and garden vegetables.
The dominant soil in the upland portion of the property is Hoypus gravelly loamy
sand, 0 to 15 percent slopes. This nearly level soil is located on rolling glacial moraine
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
terraces. In most places, slopes range from 6 to 12 percent. Included with this soil in
mapping are small areas of somewhat poorly-drained silt loam. This soil is somewhat
excessively drained Permeability is rapid and roots may penetrate to a depth of more than
60 inches. The soil holds 2 to 4 inches of water available for plants. Runoff is slow to
medium and the hazard of water erosion is slight to moderate. This soil is used mainly for
production of trees, recreation areas and for homesites.
Slope Stability:
On maps showing Geologically-Critical Areas, prepared by the Jefferson County
Department of Community Development, the northern portion of the coastal bluffs at the
subject property has been identified as an "Unstable Recent Slide" due to the presence of
steep slopes, highly-erodable ground materials and the occurrence of recent slides. The
southern portion of the bluffs has been identified as "Unstable". On Washington State
Department of Ecology Coastal Zone Management maps, the hillside slopes and bluff face
within the Area of Study have been classified as Unstable with recent slides (Urs)
identified in the area where the subject property is located (see Appendix 2).
Direct examination of the lower bluff face, immediately above the beach,
reveals a number of places where slides, erosion and sloughing has occurred, removing
soils and vegetation and exposing the underlying ground materials. This condition is
probably the result active springs within the bluff materials as well as water runoff across
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
Conclusions and Recommendations:
The observations and interpretations outlined in this report support the
classification of the bluff face at the subject property as "Unstable". Although evidence of
erosion, sloughing of ground materials and recent slides is present in some areas along the
lower bluff face, these activities appear to have been mostly shallow-founded and
relatively limited in size. Accordingly, we do not feel that the construction of the proposed
stairway from the upper bluff edge to the beach will significantly increase the risk of
erosion or slides along the bluff face nor should it adversely affect the shoreline
environment. However, the risk that ground movements will still occur that could damage
the structure is high. In summary, it is our opinion that with good water management,
preservation of lower-growth vegetation, and implementation of the slope stability
measures, outlined below, we see no reason why the construction of the proposed
stairway should not occur.
Specific recommendations are as follows:
1. Because of the unconsolidated nature of the ground materials and the presence
of undercutting along the upper bluff edge, it is recommended that the foundation for the
upper landing and supports for the portion of the stairway across the upper bluff face, be
installed as far inland - away from the upper bluff edge- as possible. This may require
cantilevering the upper supports out over the upper bluff face. Any excavation that may be
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
required of the ground materials on the face of the upper bluff should be performed in a
manner that will cause as little disturbance to these materials as possible.
2. The platform landing and stairway supports across the middle bluff area will be
founded in slide debris from the upper bluff and the uppermost layers of these materials
will probably be poorly consolidated. For this reason, we recommend that any pilings or
supports required for the stairway or platform be founded well below the surface layer
where more stable ground materials should be present.
3. The lower landing and stair supports should be installed above the OHWM
(Ordinary High Water Mark) on the beach, so as not to be exposed to regular wave action
during high tides. The OHWM is easily identifiable in this area and occurs below the lower
limit of the lower bluff face slide debris.
4. An effective water collection system(gutters and downspouts) should be
installed on the existing structure and on any new structures to be built with collected
water tightlined away from the area of the upper bluff edge. In no case should water from
any source be allowed to discharge from the upland portion of the subject property onto
the face of the bluff and areas where water collects along the upper bluff edge should be
identified and remediated by filling or draining.
5. Efforts should be made to promote and maintain a growth of healthy
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
vegetation, wherever possible, along the bluff to aid in retaining surface soils and reduce
the effects of erosion. The accumulation of construction materials, stumps, branches,
cuttings or other yard debris on the mid-slope or bluff face should be discouraged as these
materials may inhibit the growth of such vegetation.
6. Accumulations of slide debris as well as drift logs or other large flotsam that
washes up onto the beach and collects along the upper shoreface should be preserved in
place for as long as possible, as these materials may serve to absorb storm wave action and
reduce the rate of undercutting along the base of the bluff.
7. Periodic examinations of the bluff face should be conducted during and after
periods of heavy rainfall, to determine the locations of any new springs or seeps, check for
areas of excessive water runoff or erosion, and identify any unstable areas. Recording the
appearance of the bluff in photographs, taken each year, and comparing them with current
conditions may assist in this evaluation.
Summary and Limitations:
Although the development of properties along hillside slopes and coastal bluffs in
the Puget Sound region is common, it should be acknowledged by property owners that
such areas may be inherently unstable and involve higher risks than other areas due to the
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
steepness of slope faces and the unconsolidated nature of the ground materials. Frequent
and severe winter storms commonly produce large volumes of water runoff that may cause
extensive surface erosion, saturate ground materials and destabilize slopes, resulting in
ground movements that often occur without warning. Given these conditions, it should be
expected that erosion and occasional sloughing of ground materials on the subject
property may be a continuing problem. While the potential for larger-scale deep-seated
movements, such as may be precipitated by a seismic event, are not well-understood in
this area, the hazards that may be posed by such an occurrence should also not be ignored.
This report has been prepared for the exclusive use of Mr. Scott Dahlgren for
specific application to the referenced site, and the conclusions and recommendations
presented, herein, should be applied in their entirety. Within the limits of scope and
budget, this study was conducted in accordance with generally-accepted practices
employed at the time this work was done.No other warranty of conditions is expressed or
implied. You should also be aware that these conclusions and recommendations are based
on a general knowledge of this area and the interpretations of surface and subsurface
conditions as they are believed to exist. These conditions may, in fact, be different than
interpreted and events may inevitably occur that were not predicted.
Clients should also be aware that there is a practical limit to the usefulness of this
report and recommendations without a site re-evaluation and critical review. Although this
time limit is somewhat arbitrary, it is suggested that 2 years be considered a reasonable
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Geological Site Evaluation
Mr. &Mrs. Scott Dahlgren
August 31, 2006
limit for the usefulness of this report.
q;APIA
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°�c� 5%s o�� raig Vhite, Engineering Geologist
S�'d G2°� Aup. t 31, 2006
CRAIG K. WHITE
13
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APPENDICES
Appendix 1: Location Map
Appendix 2: Slope Stability Map
Appendix 3: Potential Seismic Hazards (Fault)Map)
Appendix 4: Black and White Aerial Photo
Appendix 5: Lidar-based Slope-Shaded Map(slopes>30%)
Appendix 6: Detail,Area of Study
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Attachment 2
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