HomeMy WebLinkAboutGeological Assessment Report - GEO General - 6/11/2007 MARTIG ENGINEERING
P.O. Box 11850
Olympia, WA 98508-1850
(360) 754-9687
Fax (360) 705-0789
July 11, 2007
Mason County
Department of Community Development
426 W. Cedar
P.O. Box 578
Shelton, WA 98584
Attn: Rebecca Hersha, Land Use Planner
Re: Geotechnical Compliance Report for:
* Contractor: Huston Excavating (Jack Huston, 427-0952)
1320 SE Arcadia Rd
Shelton, WA 98584 (360) 426-0550
* Owner: Jack A. Huston
1320 SE Arcadia Rd.
Shelton , WA 98584
* Property Site Addresses: 351 E Dartmoor Dr;
Shelton, WA 98584
* Tax Parcel No: 32127-53-00109,
* Installed Septic System: SWG2002-00408
* County Reference Number: BLD2007-00280
* Mason County Sec. 27, T 21 N, R 03 W;
* Hersha Letter dated 3/6/07;
* GEOTECHNICAL ASSESSMENT for placing New 3-Br Single Family Residence
Dear Ms. Hersha:
I, Kenneth W. Martig, Jr., PE, of Martig Engineering have completed my engineering
investigations regarding the Geotechnical Assessment Report requirements associated
with the above referenced property, as required by Mason County.
As a result of our investigations, outlined within this Report, we can affirm that all
potential Landslide Hazards can be overcome in such a manner as to prevent harm to
property and public health & safety and that no significant Environmental Impacts will
result if this project is permitted, as outlined within this Report and its attachments.
1
SUMMARY
The above referenced property requires a Geotechnical Assessment Report, since the
existing ground slopes on and near the site exceed 15%, but are less than 40%.
The USGS Union Quad Topography Map, Attachment 2, shows that the average slope
across the property is 20.0%, and even less northerly, easterly, and southerly of the
property.
Measured slopes range from 7.2% to 25.6%, as shown on the Attachment 5 Site Plan
and the Attachment 6 Elevation Views.
The open excavation for the Septic System tanks and the Dartmoor Drive cut-slopes
confirm the presence of Alderwood series soils.
Martig Engineering recommends that this proposal be approved by Mason County, as
discussed within this Geotechnical Assessment Report.
PROPOSED PROJECT
Earlier clearing and grading has prepared this property for a 3-Bedroom Single Family
Residence.
The Driveway was installed as per Permit #9142, with a slope of 7.2%.
The Septic System has been installed as per Permit #SWG2002- 00408.
A public Water System serves the property.
GEOTECHNICAL ASSESSMENT REPORT
1. Geologic Conditions:
a. Topography:
This site was found to be stable. No landslides were found on or near this site.
The Attachment 2 USGS Union Quadrangle Map and the Attachment 6 Elevation
Views show the site sloping from west to east:
(i) The Quad Map shows a west to east slope of 20.0% across the parcel
with lesser slopes northerly, easterly, & southerly of the site;
(ii) Measurements show an average slope of 25.6% across the property as
shown on Section A-A of Attachment 6 (west to east);
(iii) The northeast to southwest measured slope along the Driveway is 7.2%
as shown on Section B-B;
2
(iv) Section C-C shows the graded flat Building Pad at Elevation 257.0,
looking at the southerly wall of the proposed residence.
#1: Looking northerly at Section A-A. #2: Looking northwesterly along Section C-C
-
:+ .
b. Soils Type:
The Mason County Soils Survey shows the site to be Ab, Alderwood gravelly
sandy loam with slopes from 5 to 15% (see Attachment 3 soils map).
Although the Mason County Soils Survey identifies the site soils to be
Alderwood gravelly sandy loam with slopes of 5 to 15%, our investigations
confirm the soils to be as indicated but that the natural slopes are as high as 15
to 30%.
These Alderwood soils have loose, single-grained, pale-brown sandy loam soil
to depths approaching 34-inches. Below these unconsolidated granular soils, to
a depth of many feet, lies a uniformly consolidated cemented Till soil.
The unconsolidated surface granular soils drain rapidly. At neighboring houses,
we found that downspout water dissipates/infiltrates into the soil immediately,
with no need for more than a small splash trough to dissipate the flow.
2. Ground Water Conditions:
a. Upslope Water Bodies & Wetlands:
The Attachment 2 Topography Map shows that the upslope tributary drainage
area is extremely small. No water bodies or wetlands exist within this small
tributary area.
b. Surface Water Runoff:
Given the very small upslope tributary area and the rapid infiltration of the
upslope soils, no surface water runoff is expected and no evidence of it was
3
found.
c. On-Site Springs:
No-on site spring(s) exist.
d. Depth to Groundwater:
(i) Well Log:
No well log exists near this property since the Lots are served by a
Public Water System.
(ii) Soils Test Pits:
The Septic & Pump tank placement open excavation and the Dartmoor
Drive & Driveway cut-slopes did not encounter groundwater. No
evidence of Groundwater/Springwater can be found when examining the
northerly Dartmoor Drive drainage ditch. No indication of groundwater
was found in any of them. The footings for the new residence will be
placed at an elevation well above the bottom of the adjacent excavation
made to place the Septic System Tanks. Therefore, groundwater will not
be a slope stability issue at this site.
3. Depth to Suitable Soil:
a. Foundation Bearing Soil:
The above mentioned excavations confirm that firm organic free granular soil,
suitable for footings, exists at depths of 12" to 14" below the surface.
b. Till, Hardpan or Bedrock:
The above mentioned excavations confirm that the Alderwood Till is present
and that this Till controls the stability of the slopes.
4. Visible Signs of Unstable Slopes:
a. Ground Cracks or Terraced Topography:
No cracks or terracing were found.
b. Bowed or Arched Trees:
No trees were found to be bowed by earth movement. See the trees in the
above Photos #1 .
5. Landslide History:
a. Coastal Zone Atlas:
The Coastal Zone Atlas does not include this site. No other history of landslide
activity exists on or near this site.
4
b. Stereo Aerial Photo Review:
Available Stereo Aerial Photos were reviewed for the years of 1992 and 2003.
They show no signs of land movement on or near the site. Attachment 4 shows
a copy of the 7/21/03 photo.
6. Close By Landslide Hazard Areas or Associated Buffers:
No such areas are close to the property.
7. Should Geotechnical Report be Required:
No, not at this site, given the Alderwood soils, natural slopes less than 40%, and
the large number of single family residences safely constructed within the
neighboring area.
8. Prepare Delineation Map(s) Showing Areas Identified in 6. above Within 300':
No Map preparation required since no areas were found in 6. above.
9. Site Plan Map:
The Site Plan Map is Attachment 5. It along, with the Attachment 6 Elevation Views
(Section A-A, Section B-B, & C-C), show the proposed site development.
10.Drainage, Septics, Water, Placement:
Drainage at this site is not a problem where ground cover remains intact since no
measurable offsite runoff arrives at the Site and precipitation can readily infiltrate
where no impervious surfaces have been placed.
The Septic System has been constructed in accordance with the approved design
(SWG2002-00408).
The roof drains for the proposed home gutter downspouts can simply discharge
onto Splash Troughs that dissipate the flow and spread it onto open ground, without
visible problems.
A Public Water System exists to serve the residence. The service connection exists
at the SE corner of the Lot.
11.Erosion Control Plan:
Since no runoff currently exists and no offsite runoff is tributary to the site, no
special erosion control plan is necessary.
The driveway and parking pad will be covered with several inches of crushed rock.
All ground surfaces, where large portions of bare earth remain (outside the
5
residence and driveway/parking areas), shall be covered with 3" to 4" of loose
straw/hay or coarse beauty-bark to prevent erosion, until/unless planted grass or
other natural vegetation can be established to resist winter rains.
The remaining Understory vegetation should remain undisturbed as much as
practical when installing the remaining Single Family Improvements.
A row of staked straw/hay bales or a silt fence shall be placed as shown on the
attached Site Plan Map, to prevent erosion until all construction is completed and
Revegetation/Landscaping is accomplished.
This completes this Geotechnical Analysis Report for the above reference property.
I, Kenneth W. Martig, Jr., PE, of Martig Engineering have completed the required
Geotechnical Assessment Report, as documented within this letter Report and its
attachments. I am familiar with this site, having frequently driven past it since the 60's,
having worked within this Community for decades, and having visited and inspected the
site and its surrounds. As documented, potential landslide hazards have been
overcome and no significant environmental impacts will result if this proposal is
permitted as discusse bove.
Ce ified b
Z�W• mil/
eth artig r., PE ��'4(,0f Wa �ti
Attachments: 1. Vicinity Map; i
2. Topography Map; Q7
3. Soils Map; °� 9FcrsT�P``o
4. 7/21/03 Aerial Photo; �s's�0NAL ENG
5. Site Plan;
6. Elevation Views (Section A-A, B-B, & C-C); EXPIREs 1-9- o
7. Published Angle of Repose Values. -7 1.6 v 7
6
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ATTACHMENT 7
EARTH PRESSURE THEORY AND APPLICATIGii
Published
. Angle of Repose Values
APPROXIMATE ANGLES OF REPOSE FOR SOILS
Soils will stand at some natural slope, the angle of repose, unless
acted upon by some external force, or unless it is subjected to an
internal change of composition, such as a change in water content.
FIGURE 9 depicts some repose angles for various materials.
A slope of 1:1 corresponds to Type B soil per CAL/OSHA.
A slope of 1.5:1 corresponds to Type C soil per CAL/OSHA.
A running soil by Cal/OSHA defintion would have a repose angle of less
than 2:1.
/ \ t,�vCov,go l tGEa�c
5ot!S
A B C D
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A. Solid Rack, Shale or Cemented Sand and Gravels . . . . . . (90*) U 2r''�
B. Compacted Angular Gravels ... .... . . . . . . .. . .
C. Reconmended Slope for Average Soils ..:... . ....
D. Compacted. Angular Sand . . . .. . . .... . . . ... . . .. . . .. .
E. Well Rounded Loose Sand .. . .. .. . 2 1
i iGM 9
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