HomeMy WebLinkAbout101324010 Geotech Assessment
GEOLOGICAL SITE EVALUATION
Prepared for:
Prepared by:
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Ms. Carol Akers
Craig K. White, Ine.
Geosciences Consulting
P.O. Box 3398
Silverdale, W A 98383
June 17, 2005
Revised: October 14, 2006
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Table of Contents
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Property Location and Description 1
Topography and Drainage 2
Vegetation 3
Geology and Ground Materials 4
Soil Characteristics 5
Slope Stability 6
Potential Seismic Hazards 7
Conclusions and Recommendations 8
Summary and Limitations 10
Appendices:
Location Map
Appendix 1
Slope Stability Map
Appendix 2
Potential Seismic Hazards (Fault) Map
Appendix 3
Detail: Area of Study
Appendix 4
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Craig ~. \Vhite, Inc.
GEOSCIENCES CONSUL liNG
SHORELINE. BLUFF & SLOPE SPECIALISTS
GEOLOGICAL SITE EVALUATlONS
GEOTECHNICAL STUDIES. REPORTS & ASSESSMENTS , ,
LiCENSED PROFESSIONAL GEOLOGIST, WASHINGTON & ALASKA'
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June 17,2005
Revised: October 16, 2006
GEOLOGICAL SITE EVALUATION
Tax Prc. # 101324010
223 Rainshadow Dr., Port Townsend,
SEY.., Section 32, T31N, ROIW W.M.
Jefferson County, Washington
Property Location and Description:
The subject property is located at 223 Rainshadow Dr., near the city of Port
Townsend, Washington, where it occupies a portion of the north-facing coastal bluffs and
uplands along the Straits of Juan de Fuca. The property is bounded on the south, east and
west by other private lands and on the north by coastal tidelands. Property size is
approximately 4.5 acres. Existing structures on the property include a single family
residence with an attached carport. Initially, the owner had planned to expand this
structure with a room addition on the bluff side, however, these plans have changed and
the owners now intend to demolish the existing structure and construct a proposed new
residence along with a new driveway, septic and drainfield. Accordingly, a site revisit was
performed on October 7, 2006. A Location Map, Slope Stability Map, Seismic Hazards
Map and Detail Plat showing drainage and slope features within the Area of Study are
included with this report as Appendices 1,2,3 and 4, respectively.
P.O. e,ox ??98. 6iLverdale. \Vashington 98?8? (?60) 8?O-O718
Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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Access to the property is from Rainshadow Dr., approximately 0.3 miles east of its
intersection with Jacob Miller Rd. A new gravel driveway curves to the northwest onto
the subject property leading to the residence structure along the top of the bluff Several
soil log test pits were also open in the vicinity of the proposed new drainfield. Access to
the bluff face from the beach was not practical, given its height and steepness. Therefore,
all observations of the bluff were made from points along the top of the bluff using a
safety line and harness.
Topography and Drainage:
Drainage on the upland portion of the subject property in the vicinity of the
residence structure is generally to the south, away from the top of the bluff Near the
northeast property boundary, a minor component of drainage to the west also occurs (see
Appendix 4).
Slope angles were measured at several locations on the property using a Brunton
hand clinometer and laser sighting device. The land surface along the top of the bluff is
nearly level with gentle undulations that create slopes varying from 1 to 3 degrees.
Along the steep upper bluff face, recorded slope angles ranged from 68 degrees to nearly
vertical. In some places, the upper bluff edge is undercut, creating a potentially-hazardous
condition that mandates caution when walking near the b~ufftop. Along the lower bluf(
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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where slides and erosion have produced a wedge of debris, slope angles were generctli)p lJ I
lower and ranged from approximately 25 degrees to as much as 40 degrees. The height of
the upper bluff face was estimated at 160 feet (to the top of the debris wedge) while
overall relief from the top of the bluff to the beach is approximately 220 feet.
This site evaluation was conducted in late spring during periods oflight to
moderate rainfall, and several springs and seeps were noted along the bluff face where
ground water daylights through fractures and a few porous zones in the otherwise
impermeable bluff materials. In the upland portion of the property, near the residence
structure, the ground materials also appear to have low permeability, emphasizing the need
for good drainage provisions and effective water management.
Vegetation:
Large-growth vegetation on the subject property consists of conifers (mostly
Douglas fir and Western red cedar) as well as several varieties of broad leaf trees (Red
alder, P~c madrone and willow) and ornamentals. Lower-growth plants include
Himalayan blackberry, Vme maple, Salal, Swordfern, Nootka rose, wild rhododendron
and native blackberry along with ornamental shrubs and assorted weeds and grasses in
open areas of the property.
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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Native vegetation in the vicinity of the residence structure has been mostly cleared,
along with much of the thin surface soil, where it has been replaced by lawns and
ornamental plants. Vegetation is also absent long the upper bluff where the steepness of
the face and high rates of erosion prevent it from becoming established. Along the lower
bluff: growths of relatively young, same-age Red alder attest to the frequency of slides and
sloughing of materials from above.
Geology and Ground Materials:
A review of existing geologic maps of the area (Jefferson County Water Supply
Bulletin 54) indicates that the subsoil 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.
Reconnaissance mapping indicates that most of the study area is underlain by
Vosbon 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
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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gravels. Along the base of the bluff, the wedge of recent slide debris is identifiJfHf&iJN l:UlJNn m;l1
maps as "Quaternary Undifferentiated (Qu)"
Direct examination of the bluff face at the subject property generally supports the
reconnaissance mapping of the area. A thick sequence of gray to buff-colored, Vashon
"lodgement till" is exposed across the entire upper bluff face. Due to poor sorting, the
high percentage of interstitial clays, and compaction by overlying ice sheets, this unit is
mostly impermeable, however fractures and minor zones of porosity do occur that often
result in seeps and springs that "daylight" along blufffaces where this unit is exposed.
Soil Characteristics:
The principal soil type in the upland area of the subject property as identified on
Jefferson County's Soil Survey maps is ClaUam gravelly sandy loam 0 to 15 percent
slopes. The Clallam series consists of well-drained soils that have a very slowly permeable
cemented layer at a depth of 20 to 40 inches. These soils generally occur on uplands with
slopes ranging from 0 to 30 percent. These soils formed from glacial till under a forest of
mixed conifer and deciduous trees. Clallam soils are well-drained. Permeability is
moderate above the cemented layer and roots penetrate to a depth of 20 to 40 inches.
Runoff is slow to medium and the hazard of water erosion is slight to moderate. The soil is
saturated part of the time during the rainy season and, at times, water moves laterally
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Geological Site Evaluation
Ms. Carol Akers
Revised: October J 4, 2006
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above the cemented layer. "Severe" classifications for this soil are limited to drain fields
due to the soil's low permeability.
Slope Stability:
On Washington State Department of Ecology Coastal Zone Management maps,
the bluff face at the subject property has been classified as "Unstable (S)" with several
areas along the base of the bluff identified as having experienced both old ("Uos") and
recent ("Urs") slides (see Appendix 2).
Direct examination of the bluff face at the subject property reveals a number of
areas where landslides and sloughing of bluff materials have occurred. These areas are
readily identified along the base of the bluff by the accumulations of debris from the upper
bluff face which, in many cases, has been re-vegetated with young, same-age alder that is
quickly established after a slide occurs. While undercutting by storm wave action along the
base of the bluff probably occurs in many areas, the presence of this "debris wedge" acts
as a buffer against wave action during all but the most severe storms when the wedge may
be eroded away, entirely.
The compacted nature ofVashon till makes this geologic unit somewhat more
resistant to erosion than the more-poorly indurated, interglacial sediments that underlie it.
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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However, fractures and weaknesses within the unit still produce slides and sloughing that
often involve large "chunks" of this material which may break off and slide or fall down
onto the debris wedge along the base of the bluff. For this reason, an appropriate setback
for structures from the upper edge of the bluff is an important consideration in the
development of properties in these areas.
Potential Seismic Hazards:
The Puget Lowland has been classified as a Seismic Zone III (moderately high
risk) by the Uniform BUilding Code (UBC). In 2003, the state of Washington replaced the
UBC with the International Building Code (ffiC) which provides guidelines for
construction based on "Seismic Design Categories" rather than seismic zones. Architects
and builders should refer to these guidelines when designing structures in this area.
Crustal earthquakes, which occur along surface faults, have been documented in
several areas of the Puget Sound region (see Appendix 3: Seismic Hazards Map).
However, the largest earthquakes in this region have been subcrustal events,
ranging in depth from 30 to 60 miles or more. These deeper-focus earthquakes are not
related to surface faults but are usually caused by movements along plate boundaries or
within deeper sub-crustal slabs. Although the mapping of faults and study of earthquakes
in the Puget Sound region is a continuing effort, the potential effects of a large seismic
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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event on hillsides and slopes in this area are still not completely understood.
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Conclusions and Recommendations:
The observations and interpretations outlined in this report support the
classification of the coastal bluff within the study area as "Unstable" and the indications
are that slides and sloughing of ground materials along the bluff face at the subject
property will continue to occur. However, given an adequate setback from the top of the
blufffor all structures and proper management of storm water runoff, we see no reason
why the proposed re-development of this property should not occur.
Our specific recommendations are as follows:
1. It is recommended that a minimum setback distance of 100 feet from the upper
edge of the bluffbe observed for all structures. Due to the hazardous nature of
undercutting along portions of the upper bluff, we strongly recommend that residents and
visitors to the property maintain a distance of at least ten feet from the upper bluff edge.
2. An effective water collection system (gutters and downspouts) should be
installed on all new structures with water tightlined away from the bluff edge to inland
areas of the property where this water may be distributed over the forested area or
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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disposed ofin an infiltration pit. Buried portions oftightlines should utilize 4- or gtfffflsun: !,:UlJrurr
smoothwall, sewer/drainpipe firmly cemented at the joints.
3. Because of the low permeability of the ground materials, grading and site
preparation should be conducted in a manner that will allow water from uphill sources to
flow away from the proposed structure. Consideration should also be given to installing a
foundation drain on the uphill side of the structure to intercept surface and near-surface
runoff water from higher elevations. that might, otherwise, collect near the foundation. The
outlet ends of this drain can be connected to the tightline system.
4. Efforts should be made to promote and maintain a healthy growth of deep-
rooting vegetation, wherever possible, along the upper bluff edge to aid in retaining
surface soils and reduce the effects of erosion from storm water runoff.
5. Periodic inspections of the bluff face should be made by qualified individuals
using safety equipment, especially after storms and periods of heavy winter rainfall, to
determine the location of any new slides or areas of undercutting and to identify places
where excessive water runoff or erosion may be occurring. Recording the appearance of
potentially unstable areas in photographs, taken each year, and comparing them with
current conditions may assist in this evaluation.
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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Summary and Limitations:
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Although the development of properties along coastal bluffs in the Puget Sound
region is common, it should be acknowledged by property owners that these areas may be
inherently unstable and involve higher risks than other areas due to the steepness of bluff
faces and the unconsolidated nature of the ground materials. Frequent and severe winter
storms commonly produce high 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
slides, sloughing of ground materials and erosion along the bluff face at the subject
property will continue occur. While the potential for larger-scale 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 Ms. Carol Akers 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
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Geological Site Evaluation
Ms. Carol Akers
Revised: October 14, 2006
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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
limit for the usefulness of this report.
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APPENDICES
Appendix 1: Location Map
Appendix 2: Slope Stability Map
Appendix 3: Potential Seismic Hazards (Fault) Map)
Appendix 4: Detail, Area of Study
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MAP EXPLANATION
o Measured Slope Angle (Degrees)
Appendix 4
Detail - Area of Study
~ Arrows Indicate Approximate Direction of Drainage
NOTE SKETCH NOT TO SCAlE; Dl5TANCES AND LOCAll0NS OF OBJECIS 15 APPROXlMA IE