Report (3) 11 xp ro/ ao 9-/`3
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Delson Engineering, Inc. 1204 NE 146 T"AVE
VANCOUVER,WA.98684
PHONE:(360)944-7094
John@Delsonengineering.com
February 13, 2015 RELEI v EP
Re.: Structural Observation of Excavation for the house retaining walls.
BENCHVIEW—ESTATES-RESIDENCE
1 7 2015
13376 SW BENCHVIEW TERRACE CITY Of 1'IGARll
TIGARD, OR 97233 BUILDING DIVISION
(JOB# : A14-6-51)
To Whom It May Concern:
Please find this letter as a confirmation that I have performed a limited Structural Observation
Of the excavation for the above project retaining walls system on February 13, 2015,
Per the contractor request.
Please see the revised retaining walls schedules dated February 13, 2015 for the retaining walls
at grid line "D" (the right side of the house).
The Retaining Walls Schedules and the recommendations are based on the revised
Soil Report, and the Field Soil Observation by RSS dated February 13, 2015.
Due to the existing excavation and no embedment into the native soil the height of the wall shall
be calculated from the base of retaining wall foundation- see R-RW-2.
In addition we propose to provide additional retaining wall perpendicular to grid line"D".
Any other alterations to the project foundation system design or change shall be provided
AND APPROVED by the EOR.
Please see R-CP-3 for LOACTIONS and R-RW-2 FOR NEW SCHEDULE details.
The existing retaining walls are ready for pour after the above recommendations are reviewed
And approved by the local Building Jurisdiction.
The engineering judgments rendered for this report meets current professional standards;no other
warranties,either expressed or implied,are made.
I hope this information meets your needs at this time. Please call our office if you have any questions.
�EpPRO ,
Respectfully, �.
John J. Delson P.E. E.O.R.
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THE OWNER/BUILDER MUST CONFIRM THAT THE SITE SPECIFIC CONDITIONS 4
REQUIREMENTS MEET THE DESIGN PARAMETERS OF T1-415 RETAINING WALL. NOTIFY
ENGINEER OF RECORD OF ANY NON-CONFORMING CONDITIONS PRIOR TO
CONSTRUCTION OF THE RETAINING WALL.
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RECEIVED
FEB 1 7 2015
CITY OF TIGARD
BUILDING DIVISION
GEOTECHNICAL REPORT
13376 SW Benchview Terrace
Tigard, Oregon
For
VP Custom Construction
13 February 2015
19244 '
OREGOW
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EXPIRES
Rapid I SOI1SO1tIOflSLLC
3915 SW Plum Street
Portland, OR 97219
503-285-1636
mia @rapidsoilsolutions.com
Introduction
Rapid Soil Solutions (RSS)has prepared this geotechnical report for the proposed new
construction on the property located at the street address 13376 SW Benchview Terrace in
Tigard, Oregon. The property site is located in the residential neighborhood of Bull Mountain
within the city of Tigard and is zoned R-4.5, single family residential. The subject site is located
on the south side of SW Benchview Terrace, approximately 0.05 miles east of its intersection
with SW Benchview Place/SW Clearview Way and approximately 0.13 miles west of where SW
Benchview Terrace terminates at SW Greenfield Drive. It is situated roughly a half mile south of
SW Walnut Street and a half mile north of SW Bull Mountain Road. It is 0.93 miles west of SW
115th Ave and 1.4 miles west of SW Pacific Highway (99W). At the time of the writing of this
report,the subject site consists of a single taxlot, situated in the southwest quarter of the
southeast quarter of section 04, township 2S, range 1W, W.M. and is identified by Taxlot
Identification Number 2S104DC00400 and R-number R2000323. The location of this 0.24 acre
lot is indicated in Appendix A, Figure 1.
SITE CONDITIONS
Surface Conditions
The subject site is located on a generally southeast facing slope in a residential neighborhood
along the southern side of SW Benchview Terrace in Tigard, Oregon. It is situated on the side of
a nearly NE-SW ridge along which SW Benchivew Terrace runs. The slope terminates
approximately 240 feet to the southeast of the subject site, at a northeast flowing tributary to
Summer Creek,which in turn is a tributary of Fanno Creek. Metromap' indicates that the
elevation of the subject site starts at 482 feet in the westernmost boundary and goes down to 442
in the eastern corner. This abnormally shaped lot is nearly rectangular with the long axis oriented
NNW-SSE with an elongated access extending towards SW Bench Terrace from the northwest
corner of the subject site. The exact demotions are available in the Benchview Estates Plat map
included in Appendix A, Figure 2. Prior to the new construction undertaken on the site in late
2014 and early 2015,the site was undeveloped and vacant, containing some forest and ground
cover vegetation.
Geology
Current geologic literature2'3'4's classifies the slopes below the project site as part of the
Columbia River Basalt group. This group is a thick accumulation of flood basalts was produced
by dozens of fissure eruptions in eastern Oregon and Washington in the middle Miocene. These
dark grey to black basalts can be divided into 8-10 distinct Columbia River Basalt flow types,
comprised of as many as two dozen individual flows. They present in the region both as
' http://gis.oregonmetro.gov/metromap/
2 Ma,L.,Madin, I.P.,Duplantis,S.,and Williams,K.J.,(2012),Lidar-based surfcial geologic map and database of
the greater Portland, Oregon, area, Clackamas, Columbia, Marion, Multnomah, Washington, and Yamhill
Counties, Oregon, and Clark County, Washington: DOGAMI,Open-File Report 0-2012-02,scale 1:8,000.
3 Gannett,M.W.,and Caldwell,R.R.,(1998).Geologic framework of the Willamette lowland aquifer system, Oregon
and Washington: USGS,Professional Paper PP-1424-A,scale 1:250,000
'Schlicker,H.G.and Deacon,R.J.,(1967),Engineering geology of the Tualatin Valley region: DOGAMI,Bulletin
60,scale 1:48,000.
5 Burns,Scott,Growney,Larry,Brodersen,B.,Yeats,R.S.,and Popowski,T.A.,(1997),Map showing faults,
bedrock geology, and sediment thickness of the western half of the Oregon City 1:100,000 quadrangle, Washington,
Multnomah, and Marion Counties:DOGAMI,IMS-4,scale 1:100,000.
weathered and unweatered flows and contain interflow zones of breccia, ash and baked soil.
Unweathered materials are typically blue-black, dense and finely crystalline basalt with massive
columnar to close cubic jointing. The weathered flows are reddish-brown to gray-brown,
crumbly to medium dense basalt. The slopes below the subject site are further classified as
Grande Ronde Basalts of the Columbia River Basalt Group. This unit is typically a light-to dark-
grey color on fresh exposures,with flows typically displaying blocky t columnar jointing. The
regional climate typically produces a thick layer of colluvium, composed of windblown silts
(loess) and sand, clay and rock fragments produced from the breakdown of the bedrock units,
that overlies the Columbia River Basalts. The slopes in the region are not mapped as including
this surficial layer, but likely have at least a thin accumulation of loess and colluvium. In areas
with steep slopes and thicker sediment accumulations, heavy rainfalls, small landslides and
gravity can move these surficial materials downslope.
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OM Creek Alluvium
Ofte Fine Grained Missoula Flood Sediments
Bedrock(Columbia River Basalt Group)with a thick colluvium
IIIII Bedrock(Columbia River Basalt Group)
Soils observations
Rapid Soil Solutions (RSS)was on site on and excavated two (2) test pits to determine the onsite
soils. The soil was classified using the Unified Soil Classification and Visual Manual Procedure
(ASTM-D 2488) and is classified as a silty CLAY.
Excavations
Excavations can be accomplished with conventional excavating equipment. All excavations for
footings and subgrades in the fine-grained silt should be performed by an excavator or backhoe
equipped with a smooth-faced bucket(no teeth).
Because of safety considerations and the nature of temporary excavations, the Contractor should
be made responsible for maintaining safe temporary cut slopes and supports for utility trenches,
etc. We recommend that the Contractor incorporate all pertinent safety codes during
construction, including the latest OSHA revised excavation requirements, and based on soil
conditions and groundwater evidenced in cuts made during construction.
Structural Fills
Depending upon finished building pad elevations, structural fills may be required to raise the site
grades. Additionally, fill may be required for the backfilling of the proposed new foundation
walls. Native or imported material may be used for fill, provided the soil is free of organics,
cobbles larger than 6 inches in maximum diameter, or other deleterious matter; is of low
plasticity; and, is at the proper water content.
Fills should be placed on level benches in thin lifts and compacted to a dry density of at least
92% of its Maximum Dry Density (MDD) as determined by the Modified Proctor Test (ASTM
D-1557).
For any over-excavation completed in the area of footings or slabs, the backfill material shall
consist of free-draining, well-graded, crushed aggregate base with a maximum particle size of 3/4
inch. The rock shall not contain more than 5% fines (material passing the No. 200 sieve, as tested
by ASTM D-1140). The rock shall be compacted to a dry density of at least 92%of its MDD.
Foundation Design
Based on the field exploration and our experience with this soil formation it is our opinion that
the foundation should consist of conventional spread footings. RSS found no stability issues with
the site.
Footing excavations should be evaluated by the Engineer to confirm suitable bearing conditions.
Observations should also confirm that all loose or soft material, organics, unsuitable fill, prior
topsoil zones, and softened subgrades, if present, have been removed. Localized deepening of
footing excavations may be required to penetrate through the upper, softer site soils.
In order to reduce disturbance to the silty soil found at the site, we recommend all excavations
for footings be accomplished with an excavator or backhoe equipped with a smooth-faced bucket
(e.g., no teeth). If the bases of the footing excavations are disturbed by man or equipment, the
bases should be compacted to a smooth, unyielding surface with a plate compactor.
All concrete footings should be founded at least 2.0 feet below the lowest exterior grade, and 16
inches below the finished floor elevation, whichever is deeper. Interior footings may also be
founded at a depth of 16 inches below the finished floor elevation.
The new footings should be designed for a maximum allowable bearing pressure of 1,500
pounds per square foot (psf) as per scribed in 2012 IBC code book under section 1804.2 Table 2
Allowable Foundation and Lateral Pressures. When sizing footings for seismic considerations,
the allowable bearing pressure may be increased by 1/3 to 2,000 psf. Lateral pressures may be
resisted by friction between the bases of the footings and the underlying ground surface.
Settlement
Based on our knowledge of the project scope, and for footings designed as described in the
preceding paragraphs, maximum settlement should not exceed 1 inch. Differential settlement
should be on the order of 50 to 75% of the maximum settlement over 50 feet. Our settlement
estimate assumes that no disturbance to the foundation soils would be permitted during
excavation and construction, and that footings are prepared as described in the preceding
paragraphs.
Retaining walls and embedded basement walls
These design parameters have been provided assuming that back-of-wall drains will be installed
to prevent buildup of hydrostatic pressures behind all walls.
The backfill material placed behind the walls and extending a horizontal distance equal to at least
half of the height of the retaining wall should consist of granular retaining wall backfill as
specified in the "Structural Fill" section of this report. The wall backfill should be compacted to
a minimum of 95 percent of the maximum dry density, as determined by ASTM D698. However,
backfill located within a horizontal distance of 3 feet from the retaining walls should only be
compacted to approximately 92 percent of the maximum dry density, as determined by ASTM
D698. Backfill placed within 3 feet of the wall should be compacted in lifts less than 6 inches
thick using hand-operated tamping equipment(e.g.,jumping jack or vibratory plate compactors).
If flat work (e.g., sidewalks or pavements) will be placed atop the wall backfill, we recommend
that the upper 2 feet of material be compacted to 95 percent of the maximum dry density, as
determined by ASTM D698.
A minimum 12-inch-wide zone of drain rock, extending from the base of the wall to within 6
inches of finished grade, should be placed against the back of all retaining walls. Perforated
collector pipes should be embedded at the base of the drain rock. The drain rock should meet the
requirements provided in the "Structural Fill" section of this report. The perforated collector
pipes should discharge at an appropriate location away from the base of the wall. The discharge
pipe(s) should not be tied directly into storm water drain systems, unless measures are taken to
prevent backflow into the wall's drainage system.
Settlements of up to 1 percent of the wall height commonly occur immediately adjacent to the
wall as the wall rotates and develops active lateral earth pressures.
Engineering values summary
Bearing capacity 1,500psf
Coefficient of friction 0.35
Active pressure 35pcf
Passive pressure 350pcf
Drainage
The Contractor should be made responsible for temporary drainage of surface water and
groundwater as necessary to prevent standing water and/or erosion at the working surface.
Water should not be allowed to "pond" or collect anywhere on the site. The ground surface
around the structure should be sloped to create a minimum gradient of 2% away from the
building foundations for a distance of at least 5 feet. Surface water should be directed away from
all buildings into drainage swales or into a storm drainage system. "Trapped" planting areas
should not be created next to any buildings without providing means for drainage.
The roof downspouts shall discharge out to pipes that are connect to curb cut outs .Footing
drains should be installed around the building perimeter to help intercept any water migrating
towards the building subgrade. The footing drain and any slab drains shall remain independent
of surface water drain systems(e.g. downspouts, etc.)and pumped up to street.
We recommend that typical footing drains be placed on the exterior of the foundations to
intercept any water"chasing"the utility lines, or that an impermeable trench plug (e.g. concrete,
etc.) be installed to stop water before it reaches the building envelope.
Geohazard review
RSS site reconnaissance on 29 January 2015 did not find any seeps, cracks, hummocks or any
signs of slope instability.
The Oregon HazVu: Statewide Geohazard Viewer6 was reviewed on February 9th, 2015 to
investigated mapped geological hazards see figure 3. This review indicates that the project site is
situated outside the 100-year floodplain. The expected earthquake-shaking hazard is classified as
`very strong' with a no classified earthquake liquefaction hazard. The nearest mapped active
fault is a NW-SE oriented fault situated over 1.5 miles to the east of the subject site. IMS-157
rates the site vicinity as having a peak horizontal acceleration of 0.4 to 0.5 g for a magnitude 6.8
Portland Hills Fault earthquake. This movement class is described by the Modified Mercalli
Intensity as `severe shaking', capable of producing slight damage in specially designed
structures, considerable damage in ordinary substantial buildings with partial collapse, and can
result in great damage in poorly built structures. Slopes in this drainage and adjacent drainages
contain small earth flows,but none of these are located on or directly adjacent to the subject site.
Limitations
This report has been prepared for the exclusive use of the addressee, and their architects and
engineers for aiding in the design and construction of the proposed development. It is the
addressee's responsibility to provide this report to the appropriate design professionals, building
officials and contractors to ensure correct implementation of the recommendations.
6 http://www.oregongeology.org/hazvu/
7 Wong,I., Silva,W.,Bott,J.,Wright,D.,Thomas,P.,Gregor,N.,Li,S.,Mabey,M.,Sojouner,A.,and Wang,Y.,
(2000),Earthquake scenario ground shaking map for the Portland Oregon, metropolitan area: Portland Hills Fault
M 6.8 earthquake, Peak horizontal acceleration(g) at the ground surface: DOGAMI,IMS-15.Scale 1:62,500
The opinions,comments and conclusions presented in this report were based upon information
derived from our literature review, field investigation and laboratory testing. Conditions
between,or beyond,my exploratory test pits may vary from those encountered. Unanticipated
soil conditions and seasonal soil moisture variations are commonly encountered and cannot be
fully determined by merely taking soil samples. Such variations may result in changes to our
recommendations and may require that additional expenditures be made to attain a properly
constructed project. Therefore, some contingency fund is recommended to accommodate such
potential extra costs.
.
.
.
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