HomeMy WebLinkAboutBLD2000-00335 Geotechnical Report To: Kevtn Russell Fax: +1(360)3794451 Page 2 of 3 Tuesday,September05,20008:39 PM
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arbord 13150 S.Macksburg Road
Canby,OR.37013
eptember 5, 2000
Jefferson County Community Development
621 Sheridan Street
Port Townsend, WA. 98368
Attn: Mr. Kevin Russell
Re: CAR 249
Dear Kevin:
As you and I have discussed the original study of this property done by, Mr. Steve
Luxton in April of 1998 was a worst case scenario. I had asked him to prepare a
report that would guarantee we would never have a problem with erosion on the
subject property. This building site has been in existence for approximately 9 years
with no significant changes to it. With the exception of a small slide directly in front of
the building site, that occurred during a period of extremely heavy rainfall a couple of
years before we purchased the property, there has been very little damage done to
the unimproved building site.
We have completed many improvements to the property in the last couple of years to
control water runoff based on the subject report and Steve's recommendations of
what would make the most impact to the property. Those improvements include
draining ground water from the property above the building site and changing the
driveway slope. I intend to continue the improvements until I am certain there will be
no problems with erosion on this site but find it difficult to work on the property from
my house in Oregon. It will be much easier after we can stay at the property.
In the past few days, I have had several conversations with Mr. Bill Payton of
Northwestern Territories, Inc. regarding the subject property and their study of this
17 site. They agree that items 4.1 and 4.4 of the 1998 report do not influence the
building site. The following is an itinerary of the remaining critical improvements that I
n�\ am agreeing to accomplish before we get an occupancy permit.
�\ r 1. Construct a culvert to collect water from the south side of the driveway and divert
it to a ditch on the south side of the property. This will be done with a 6"drainpipe.
G2. Build a retaining wall with trench drain at the back of the building site.
3. All roof runoff will be diverted to the south side of the property. This will be done
with two 6" drainpipes.
You may notice I have changed some of the diversions from the north to the south
side of the property. There is a natural stream bed at the south end of our property.
VISION MACHINERY
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;nougtss Harbord
Site Improvement
• • sept rnber 5, 2000
page 2ofZ
• Based on the above discussion_ we therefore deem-it acceptable to occur y the • :
proposed dwelling-without completing the modifications mentioned in sections 4.1 0 and
4.4 of the original report unless future erosion of these areas warrants such ..
improvement*.
• Limltatierts;
This letter is based on a visual ins
pection of the tixistirtu site conditkras, barring major
catastrophic events such as earthquakes or abnormal weather conditions_No scientific
.measurements, tests or calculations were performed. The findings and letter are limited
to the normal standard of care of the industry for an invcstigation without benefit of
scientific data and calculations. Northwestern Territories, Inc_ and/or its representatives
assume no responsibility for loss or damage.
The sole objective of this investigation has been to diagnose the geotechnical
wr idiliwis at the site, The Idle► l vas f eel 1 wi eparect cut the exclusive use ut Duuglas 1 Ior-srd (..wncr) and/or T.,Uaj-,aric Oanotructic n (L-wilder). It nior lr:. .rac.! Loy.7th.:ro vnl J.
with expressed written permission of the Engineer. The report has not been prepared
for use by others or other uses. The observations, interpretations, and conclusions
herein are based on generally accepted professional engineering and geologic
principles and practice. This warranty is in lieu of all others, either expressed or • •
implied.
Sincerely,
ARRYNORTHWESTERN TERRITORIES,.INC. 4. r vN�k� ;`
72-":"--i-'1-1.- . .. !.., rt•
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J. R. Jerry Newlin, P.E. 'ors TV. _
Principal Engineer tt �`rf - ~
! 73 sy a'l• 1 . .
l' 1,,e_j t ,‘-(y2 :;) ' . . .
Bill Payton, Geologist
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remains at the base of the slope. In addition, the redistribution of weight and mass
that occurred during the slide has probably reduced the potential for further large-
scale sliding.
4,0 Site Engineering Recommendations
Sitework may include backfilling the•mudslide scarp and/or completing work at the
site that would reduce the possibility of additional smaller scale mudslides or shallow
block slides that might endanger the proposed building site.
•
4.1 Backfilling the Mudslide Scarp
The soil •
above and behind the mudslide is unlikely to be the subject of further sliding
but the scarp is unsightly at present. The Mudslide scarp may be filled by excavating
an 8-foot wide flat base area and loading the toe of the scarp with large blast rock as
illustrated on Drawing #2. The blast rock toe revetment should extend to the top of
the road below. Above the rock, a free-draining ballast of sandy gravel may be
placed in layers of about 2 feet. Each layer should be compacted and flattened.
Place a road fabric over each 2-foot thick layer of gravel and fill another two feet. If
the scarp is backfilled in this manner it will be substantially stronger than the
adjacent fill materials. This work will require about 100 tons of revetment stone and
about 100 yards of imported gravel backfill and about 2 'A days of heavy equipment
time and some hand labor for an approximate cost of about S4 500_
4.2 Drainage Improvements at the Road Leading to the House Site
The chances of additional sliding at the west side of the proposed building site may
be reduced by completing certain improvements to the drainage at the site. Runoff
that follows the•road must be diverted away from the building site. Runoff could be
better controlled by the following actions:
• Pitch the road cross-slope to the north so that water flows off of the road to the
• ditch on the north side
• Improve the ditch on the north side of the road so that it may carry all of the runoff
collected in that area,
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• Construct of a crossing culvert that would collect water captured on the south side
of the road and transfer it across to the main ditch on the north,
• Place a rock revetment in the ditch so that the velocity of runoff is slowed and
further erosion is prevented,
•
Pitching the road cross-slope to the north side ditch can be accomplished by
importing some sandy gravel ballast, grading and compacting it. The ditch itself
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should be,excavated to a depth of 2.5 feet and an overall width of 7 feet. The,ditch
should be completely lined with 6 to 8-inch hand-placed rock blast spells. A 12-inch
concrete crossing culvert about 25 feet in length should be adequate to divert the
water that is captured on the south side of the road.
This work should cost about $000 if it is performed with attention to detail so,that''the
completed job is both functional and pleasing to the eye.
. 4.3 Drainage improvements at the House Site _ •• • .% •
Trench Drain
Additional protection against slope instability may be Obtained by construction-of:en
interceptor trench drain along the axis of the toe of the slope at the east side of the
prospective house site. The trench drain
will
the subsurface water that leaving
would tend to
above the house site and capture some of
destabilize the slopes west of the house site. The trench drain is illustrated on the
attached Drawing #2. .
The completed ,trench drain would be about 100 feet in length and it..can be
constructed for about $28QQ if included with other work that requires the use of a
backhoe at the site and provided that the road is repaired so that trucks may access
the site.
Slope Toe Revetment and or Retaining Wall
,. A 6 foot high retaining wall or rock revetment'will be needed at the toe of the slope
6flerocy-. __above the house site. The retaining wall will provide additional width to the pad so
that the house may be built with a prudent setback. The most width would be gained
by construction of an engineered concrete retaining wall about 6 feet in height leaving
about 2 feet of freeboard behind the wall so that any small slips of earth are arrested.
Such a retaining wall would allow a house width of not more than 30 feet with a
• prudent setback of about 35 to 40 feet.
A concrete retaining wail of this type would cost about;$5500 to build.., Alternately a
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large rock revetment could be placed against the toe of the slope as.illustratedl on
Drawing #2. Such 'a revetment could be adequately constructed_for.$3 500 -bu'.ahe
rock will. require more 'width, forcing: the house footprint' a little closer:;ao..;the'
escarpment. Alternatively, a retaining wailidaylight basement may be engineered to
fit the site and to have sufficient strength to resist small slides from the slope'above
the house pad. .
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4.4 Drainage Improvements Below the House Site
Water must be transported to the beach without causing erosion or further instability.
Water from the ditch on the north side of the approach road may be routed to the
: ' beach in two ways described below
Restrained Drainage Tube
Drainage from the ditch north .of the,road may be collected .into a fabric hose or
'drainage tube that resembles a fire hose. A 12-inch diameter hose of this type
anchored by a concrete wall embedded in the ground would•allow drainage to be
directed over the slope along the northerly line of the parcel. Such a device should
be designed by an engineer and construction costs would be about $3800 for a
properly detailed job that will give enduring service.
Rock Revetment of Existing Ditch Along the Road to the Beach
Drainage could continue to flow along the road to the beach although it is desirable to
divert water away from the slopes that lie below the house site. If drainage is to
remain in its current alignment, the ditch should be deepened and rock rip raps
should be placed along the ditch all the way to the beach to reduce future erosion. It
would be best to avoid any widening of the road to the beach. Thus, if it is absolutely
necessary to maintain the road for narrow rough terrain vehicles, then, drainage must
be routed through a separate tube system.
CompletIon of hand placed rip rap and improved ditching to the beach along the
existing road should cost about $2200 for a carefully completed job that is attractive
and functional_
5.0 Summary and Conclusions
Gross Slope Stability
Generally the property appears to have good prospects for continued gross stability.
The buttressing effect of the existing landslide debris below the proposed..house site
is helpful in stabilizing the deeper siltstone against deep-seated slides,• The historic
slide has also redistributed the mass and weight of the slope so• that large scale
slides are less likely. •
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Local Stability of the Proposed Building Site
The proposed building site has been eroded by mudsliding due to the presence of
subsurface water and flooding with surface water during storms. The existing
conditions, geometry of the building site and the presence of fills on the slope beyond
the building site suggests that similar slides in adjacent areas could occur in the
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future. Site planning should be carried out to provide at least 35 feet and preferably
40 feet of setback from the line where the building site descends rapidly to the beach
below. This can be achieved by an appropriate earth retaining structure behind the
prospective house and by selecting a house plan that is relatively narrow. The
house may also be built with an integral daylight basement that is appropriately
engineered.
Site improvements Can Reduce the Risk of Sliding
Several site improvements may be considered to reduce'the risk of future:sliding
controlling and directing'drainage. These. improvements include better.. i rainage
along the 'approach .road, ditch improvements and rip rap, drainage tube
construction, a trench drain along the east side of the building site and the addition
of a culvert to direct water away from the house site. These improvements are
expected to cost a total of about $12,000.
Retaining Structure Behind the Prospective House Suggested
in order to achieve the desirable setback it will be necessary to situate the house well
back on the lot. This is achievable at the site if some retaining of earth is done on the
east side of the building site_ Costs of this may vary from $3500 to $5500.
Mudslide Scarp Can be Repaired
Although the existing mudslide scarp is unlikely to be the site of additional sliding, the
scarp is unsightly at present. The mudslide scarp may be filled beginning with a large
rock toe revetment and finishing with free-draining sandy gravel fills with fabric
reinforcements. Access to the slide is difficult and the work is expected to cost about
$4500. This work enhances the appearance of the site but it does nothing to
enhance the future stability of the home site.
6.0 Limitations
This report and the conclusions herein were based upon a reconnaissance.inspection
of the subject parcel. Although much can be inferred from inspection of surface
conditions, the subsurface cannot be completely understood without borings at
frequent intervals, more detailed mapping of the surface and laboratory testing of soil
samples. Users who require a higher degree of reliance on the investigations and
conclusions of the report are encouraged to obtain such studies.
Unless construction plans are reviewed and the construction work is inspected by the
Engineer, no warranty is made by the Engineer concerning conformity of the
construction to the conclusions and recommendations of this report.
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• This report and its conclusions were prepared in "accordance wit the principles of
,geologic science and 'standards. of professional erigineerin .Practice. •No other
• wprranty, either-express or implied.•is provided herewith. • ' '
Y Sincerely yours, ' ' ' ('.
•
•; 'tEeve S•Cu don; MSc. P.E. •- • :.-.,irA',.'.'t'''','.rV.-,t..',.. - •,• ..,= •
. Professidnal Engineer - ?`7.
IR: Newlin, P.E.' I epees. ff I • '
Principal Engineer .
' SSUJRN_eas •
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