GEORESOURCES
earth science & geotechnical engineering 5007 Pacific Hwy E., Suite 16 I
Fife, WA 98424 I 253.896.1011 www.georesources.rocks
May 30, 2018
Oakbrook Townhouses,
Inc. c/o
NW Property Agency, Inc. 9527
Bridgeport Way SW Lakewood, WA 98499
Attn: Mr. Kenneth Karch
Geotechnical
Engineering Report Oakbrook Townhouses
Stability/Settlement Evaluation
7813 Zircon Drive SW
Lakewood, Washington
Doc ID:
OakbrookTH.ZirconDrSW.RG
INTRODUCTION
This geotechnical engineering report summarizes our site observations,
data review and provides
geotechnical recommendations for the existing residential structures located
between Zircon Drive SW and Chambers Creek
in Lakewood, Washington. We have conducted several other Geotechnical Engineering Evaluations in this area, the most recent for a PDD at
7701 Ruby Drive SW. We revisited the site on several occasions between March
and May of 2018. The general location of the site is shown on the attached Site Location
Map, Figure 1.
Based on our discussions, the subject
structures consist of 12 buildings
located between 7407 and 7813 Zircon Drive
SW, on the north side of the street. The structures were constructed between 1968 and 1972 using conventional construction
methods; slab on grade, daylight basement and crawl space/stemwall. We understand that several of units/buildings
experienced localized settlement
following the 2001 Nisqually seismic event, primarily along the north portions
of the structures, generally near the Chambers Creek slope area. We also
understand that you retained several
geotechnical companies following that event, and since that time, to evaluate
that settlement and provide repair
options. Those recommendations were completed; including needle piling behind several structures and drainage
improvements. A list of the previous reports is included as Appendix "A".
The purpose
of our services was to review the existing data and conduct a geologic reconnaissance of the
structures and north slope area to compare the current conditions with the historic data, and
identify any visible changes in slope stability or settlement, as well as
drainage or erosion issues observed. Specifically, the scope of services for this
project included the following:
1. Reviewing the available geologic, hydrogeologic and geotechnical
data for the site area;
2. Reviewing the previous geotechnical documents
and relevant data provided;
3. Describing surface conditions; including soil
type, geologic conditions, drainage issues and;
4. Providing
recommendations for any identified areas of concern related to settlement,
slope stability,
erosion hazards or drainage issues;
5.
Meet
with you to discuss our findings and summarize them in a Summary Report.
SITE CONDITIONS
Surface Conditions
The site is located between 7407 and 7813 Zircon Drive SW, on the north
side of the street in Lakewood, Washington. The site is generally irregular in
shape and is situated on the north side of Zircon Drive, generally at or near the top
of the Chambers Creek steep slope area. The site is bounded by existing
residential development to the east, by Zircon Drive SW to the south, and by
open space to the west.
The site is located south of the Chambers Creek Canyon on a glacial
upland area. According to the site Pierce County GIS, the ground surface of the
upland portion of the site area slopes down to the north at inclinations of less than 8
percent. To the north, the slopes of Chambers Creek range from approximately 35
percent to 120 percent. The majority of the steep slope area is inclined between 65 and 80
percent. Total topographic relief of the Chambers Creek slope is on the order
of 150
feet. The approximate site topography is shown on the Site Vicinity Map,
included as Figure 2b.
Site Soils
The USDA Natural Resource Conservation Service (NRCS) Web Soil Survey
indicates that the upland flatter
portion site is underlain by Spanaway gravelly sandy loam (41A) soils while the
Chambers Creek slope area is underlain by
Xerochrepts Soils. The Spanaway soils are derived from gravelly glacial outwash
and form on slopes of 0 to 8 percent. These soils have "little"
erosion hazard when exposed and are
included in hydrologic soils group A/B. The Xerochrepts soils occur on steep slopes and are typically derived from sand and
gravel outwash or glacial till parent material. The erosion hazard of these
soils is "very severe" and they are classified as a hydrologic Class
A soil. A copy of the soils map for
this area is provided in Figure 3.
Site Geology
The draft
the Geologic Map of the Steilacoom 7.5-minute
Quadrangle, Pierce County, Washington by Kathy Goetz Troost et al. (in review)
maps the west portion of the site as being underlain by Steilacoom Gravel
(Qvsp4), a subunit of the Steilacoom gravel recessional outwash deposit. The central and east
portions of the site are mapped at Steilacoom Gravel South Tacoma Channel deposit (Qvsst4),
also a subunit of the Steilacoom Gravel, deposited in an outwash channel. The lower portion of the
Chambers Creek slopes are mapped as Advance outwash deposits (Qva).
These soils were generally deposited during the most recent Vashon Stade
of the Fraser Glaciation, some 11,000 to 12,000 years ago. The recessional
outwash/Steilacoom soils consist of a poorly sorted, lightly stratified mixture
of sand and gravel that may contain localized deposits of clay and silt that were
deposited by meltwater streams emanating from the retreating ice mass. The
recessional outwash deposits are considered normally consolidated and offer
moderate strength characteristics. The underlying Advance outwash deposits were
deposited in front of the advancing glacial ice and subsequently overridden, and
therefore are in a very dense condition where undisturbed. An excerpt of the above
reference geologic map is attached as Figure 4.
No evidence of significant erosion or other active deep seated landslide
activity was observed at the time of our site visits. No areas of landslide deposits
or mass wasting are noted on the referenced map within the immediate vicinity of the
site. We did observe evidence of surficial or shallow slope movement behind several of the
units, which will be discussed in the "Areas of Required
Maintenance/Mitigation" section of this report. In general, the slope
movement included soil creep, ground settlement and a localized ground rupture
(crack). The soil creep and associated settlement area common in steep slope areas.
The previously installed needle piling at select locations of the buildings mitigated
previous similar settlement at the site.
Subsurface
Conditions
GeoResources, LLC (GeoResources) has conducted previous geotechnical
investigations in the site area
which included subsurface explorations, test pits and borings. These exploraitons/investigations have confirmed the
mapped soil conditions in the area.
In general, out
previous explorations encountered several feet of sod and gravelly topsoil over
medium
dense dark sandy gravel with silt and trace roots. We interpreted these
surficial soils to be recessional outwash gravels mixed with volcanic ash
forming what is commonly referred to as Black Steilacoom Gravel. Underlying these
surficial soils, we observed medium dense to dense tan sandy gravel. We
interpreted these soils to be recessional glacial outwash. Borings in the
general area encountered similar gravel deposits to several 10s of feet where either
glacial till or advance outwash material was encountered, based on our interpretation.
Groundwater Conditions
Groundwater was not observed in any of our previous explorations at the
time of completion. Based on the nature of the near surface soils, we
anticipate fluctuations in the local groundwater levels may occur in response
to precipitation patters, off-site construction activities, and site
utilization. We did observe one area of water tolerant vegetation about mid slope below
units 7631 and 7629. Above this area of seepage we observed a small crack or ground
rupture about 3 to 5 feet below the top of the slope. The seepage zone may be related
to a localized perching layer of soils (silty material) or a leaking drain pipe.
CONCLUSIONS AND RECOMMENDATIONS
Based on the results of our data review, site reconnaissance and our
experience in the area, it is our opinion that the site is generally stable
relative to deep seated slope failures. The greatest risk of slope
movement is likely related to a significant seismic event, or failure of a
drainage pipe in the slope area.
As you
are aware, the City of Lakewood Municipal Code (LMC), Title 14A defines a Landslide Hazard Area as an area potentially
subject to mass movement because of a combination of geologic, seismic,
topographic, hydrologic, or manmade factors. These areas may be identified by the presence of any
of the following indicators:
1. Areas of historic
failures including areas of unstable, old and recent landslides.
2.
Areas with both all
three of the following characteristics:
a.
Slopes
steeper than 15%; and
b. Hillsides
intersecting geologic contacts with a relatively permeable sediment overlying a
relatively impermeable sediment or bedrock; and
c. Springs or groundwater seepage.
3. Slopes that are
parallel or sub-parallel to planes of weakness, such as bedding planes, joint
systems and fault planes, in subsurface materials;
4. Slopes having
gradients steeper than 80% subject to rockfall during seismic shaking;
5. Areas potentially
unstable as a result of rapid stream incision, streambank erosion, and undercutting by wave
action;
6. Areas located in a
canyon or on an active alluvial fan, presently or potentially subject to inundation by debris
flows or catastrophic flooding;
7. Any area with a slope
of 30 percent or steeper and with a vertical relief of ten or more feet. A slope is delineated by establishing the
toe and top and measured by averaging the inclination
over at least ten feet of vertical relief;
8. Areas which have a
"severe" limitation for building site development because of slope conditions, according
to the Soil Conservation Service.
We also reviewed the Washington State DNR Landslide inventory
Map-Shallow Susceptibility which identified most of the steep slope area as
"High Susceptibility". The areas at the top and toe of the slope are
mapped as having a "Moderate Susceptibility". A excerpt from that map
is included as Figure 5. Relative to deep-seated susceptibility, the extreme west
end of the property has a "High Susceptibility" while the remaining
portions of the property have a "Moderate Susceptibility". A copy of the map are included as Figure 6.
Typically deep-seated failures are associated with glacial outwash sediments in areas of over-steepened
slopes where significant groundwater seepage or surface water runoff occurs. We
did not observe any areas of significant uncontrolled surface water or groundwater seepage in the slope areas below
the site.
Based on the above,
our observations of the site and review of published and previous geotechnical information provided, as well
as our review of other information provided, the site meets several of the technical criteria of a landslide and/or erosion
hazard area. In addition, the site has
a history of soil creep, minor surficial settlement and localized erosion. The
Condominium Association has been
generally responsible in mitigating a number of these issues. As you are aware,
steep slopes throughout the Puget Sound area are at risk of movement if
appropriate mitigation measures are not
utilized, in particular the management or control of drainage. Based on our data review and site observations, it appears
that drainage is being generally managed at the site. It should be verified
that the drainage from the roof areas is being directed towards the street. If not, we recommend that the system be evaluated
and its function verified. We did observe several drainage issues above the steep slope area; drainage from an
out building, and from below deck
areas. These will be discussed in the recommendations section of this report.
The LMC Chapter 14A.146.020 defines an Erosion Hazard
Area as one
identified by the presence of vegetative
cover, soil texture, slope, and rainfall patterns, or human-induced changes to such characteristics, which create site
conditions which Ure vulnerable to erosion. Erosion hazard areas are those areas
that are classified as having moderate to severe, severe or very severe erosion
potential
by the Soil Conservation Service, United States Department of Agriculture
(USDA). The geologic units considered as potential erosion hazards within areas of
slopes greater than 15% may consist
of the following: m (modified land, Qal (alluvium), Qw (wetland deposits), Qb
(beach deposits), Qtf (tide flat deposits),
Qls (landslide deposits), Qf (fan deposits), the Qvr and Qvs series
(Vashon
recessional outwash, and Steilacoom Gravel), and Qvi (Ice contact deposits).
These units are identified because of density and composition.
As previously stated,
the site is mapped as being underlain by Spanaway (41A) soils. These soils have
"little" erosion hazard. The site is also mapped as being underlain
by glacial recessional outwash deposits: Steilacoom Gravels (Qvsp4), however, no
slopes greater than 15 percent were observed or are mapped on the site. It is
our opinion that an erosion hazard does not exist on the site in its current
condition.
Based on the above, it appears that the site does have several of the
above listed landslide hazards
indicators. No evidence of deep-seated landslide activity was observed at the
site at the time of our site visit. During
our recent site reconnaissance, we observed evidence of soil creep and localized
foundation settlement that had previously impacted several of the building units
or structures. Some of this has been
mitigated with placement of fill material, needle piling and/or concrete
footings or grade beams. It appears that the needle piling and other mitigation
measures have prevented the ongoing
ground settlement from adversely impacting the associated structures.
It does appear that there has been
continued soils creep in these and several additional areas. We observed areas of soil creep on most of the steep slope area,
extending to near the top of the steep slope. This is common in sandy or
granular soils on steep slopes in the Puget Sound area. And as you have done in
the past, can be managed with relatively simple measures.
We did observe one
area of slope movement near the west portion of Building 5 that should be address
before the next wet weather season. A small ground rupture, 2 to 4 inches of
downset, was observed near the top of the slope below a small out building near
7623 and extending eastward across units 7621 and 7617. The crack is approximately 30 feet
below the units, and they are not likely at risk of damage or distress at the
current time. We also observed that the downspout drains for the out building
discharge directly on the ground surface, generally above the failure area. This
drainage system is likely contributing to this ground movement and should be addressed.
Further, there are a number of deck supports (without piling) that have
settled/tilted, several areas of settlement below footings where there is no observed
impact to the structures currently, and several buildings that have drainage
systems that are in need of repair.
The area of slope
movement, drainage issues and areas of more recent or continued settlement currently
impacting deck supports and slabs behind several additional units are identified below, as well as on Figure 2.
We also observed localized evidence of seepage and uncontrolled drainage with associated erosion behind several of the
units. These are also identified.
Areas of Required Maintenance/Mitigation
Starting in the west portion of the site, at Building 1:
·
Building 2; visqueen below decks needs maintenance,
localized erosion below unit 7717 with ongoing ground settlement, property owner has
indicated there is ponding of water as well as overflow of the drainage system and
settlement of landscape blocks;
·
Building
3; ground settlement on west end with tilting pier pads;
·
Building 4; west end, soil creep and associated
settlement below foundation;
G EO RESOU RCES
·
Building 4/5; ground rupture just below top of slope,
uncontrolled drainage from out building, upslope soil creep and associate settlement of
ground around foundations, deck pads settling/tilting (larger pads are not
necessarily a good thing here);
·
Building 5; downslope seepage may be related to out
building drainage, trees being undermined down slope;
·
Building 6; soil creep and upslope ground settlement,
minor settlement of deck supports/patios, mostly west end;
·
Building 8 (east)/9 (west); soil creep and ground
settlement around foundations, erosion down slope in damp area, drainage system function
should be checked;
We also observed
several trees down in the mid and lower portions of the slope. There are some exposed soils in
these areas, although vegetation has started to regrow. Several trees have also
been cut on portions of the upper slopes behind several units. The logs and
debris remain on the slope. We would recommend that if the trees are to be trimmed, a
minimum of 15 feet of the maple/alder tree stumps be left to try and keep the tree
alive, and fir or cedar trees be limbed up leaving the tops of the trees. The only
trees that should be stumped are identified hazard trees. Where the trees are
cut, the debris should be removed from the slope, typically by hand to minimize
the disturbance of the slope area.
Once you have reviewed
the report, we can meet at the site to review and observe the site conditions. We can also discuss site
specific mitigation measures, similar to what you have completed in the past.
LIMITATIONS
We have prepared this report for use by Oakbrook Condominiums for use
in planning for future site mitigation measures, as appropriate. The data used in
preparing this report and this report may be provided to prospective contractors
for their bidding or estimating purposes if utilized. Our report, conclusions
and interpretations are based on our review of the available public
information, provided data, our previous subsurface evaluations in the general
area, data from others and limited site reconnaissance, and should not be
construed as a warranty of the subsurface conditions.
Variations in
subsurface conditions are possible and even likely laterally and vertically,
and may also occur with time. If subsurface work is expected, we recommend that
appropriate subsurface explorations be completed with supporting geotechnical
analysis and recommendations. Our recommendations are not intended to direct
the contractor's methods, techniques, sequences or procedures, except as
specifically described in our report for consideration in design.
The scope
of our services does not include services related to environmental remediation
and construction
safety precautions.
·
• •
We have appreciated
the opportunity to be of service to you on this project. If you have any questions or
comments, please do not hesitate to call at your earliest convenience.
Respectfully
submitted, GeoResources, LLC
Brad P. Biggerstaff, LEG, LHG, QE Seth Mattos, LEG
Principal Senior Geologist
BPB:SM:bpb
DocID: OakbrookTownhomes.
Attachments: Figure
1: Site Location Map
Figure 2a: Site Plan
Figure 2b: County Topographic Map
Figure 3: NRCS Soils Map
Figure 4: USGS Map
Figure 5: WDNR Shallow Susceptibility Map Figure 6: WDNR
Deep-Seated Susceptibility Map Appendix A - Photographs
