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Home My WebLink AboutGRD2007-00012- Independent Geotechnical Review - GRD Engineering / Geo-Tech Reports - 2/12/2009 w �AWE- iadw— KraZan & ASSOCIATES INC . GEOTECHNICAL ENGINEERING • ENVIRONMENTAL ENGINEERING CONSTRUCTION TESTING & INSPECTION February 12,2009 KA Project No. 092-09002 Mr. Earl Iddings Dewatto Bay Development P.O. Box 2755 Belfair,WA 98528 RE: Independent Geotechnical Review Proposed Driveway Access Parcel No.323283100130 Mason County,Washington REF: Grading Plan Addendum by Tahja Engineering,Inc.,dated October 29,2007 Geotechnical Review Letters by Alkai Consultants,LLC,dated November 15,2007,August 6,2007,and September 5,2007 Geotechnical Report,Dewatto Beach Drive,by Geotechnical Testing Laboratory(GTL) dated May 16,2006 Geotechnical Report Addendum Letter by GTL(undated-April 18,2007 by reference) Geotechnical Report Revision by GTL(undated) Unnamed letter regarding nesting areas by Tahja Engineering,Inc.,dated April 11,2008 Dear Mr.Iddings: At your request, we have prepared this letter to discuss our review and opinions regarding the Grading Plan Addendum letter prepared by Tahja Engineering, Inc., dated October 29, 2007 for the proposed driveway and retaining wall at the referenced property. In preparation of this letter, we have observed the steep slope located along the southern property line, reviewed available slope stability and geologic mapping for the area of the site, and we have reviewed the documents listed above. Site Observations During our site visit on January 16, 2009, we observed the current conditions of the bluff portion of the subject property. The bluff area is approximately 45 feet tall and near vertical in many areas. The overall slope magnitude ranges from approximately 100 to 150 percent(Figures 1 and 2). We observed numerous trees with curved trunks along the top of the slope and observed spring/seep activity within the middle to upper portion of the bluff face. We also observed a channel that has been created by erosion behind a block of glacial sediments (Figure 2). The spring/seep is located above numerous thin (approximately 1 cm thick) layers of silt or clay interbeds within the outwash deposits (Figure 3). These Iayers generally dip to the north and east. This kind of situation (soil beds dipping downslope and permeable sediments overlying impermeable silt and clay) is typically very conducive for landslide failures. Offices Serving The Western United States 11715 N.Creel:Parkway S.,#C-106•Bothell,Washington 98011 *(425)485-5519•Fax:(425)485-6837 EXHIBIT A-2 i KA No. 092-09002 February 12,2009 Page 2 of 5 Landslide Potential The area of the site is mapped by the Coastal Zone Atlas as having Intermediate slope stability. This mapping was done in the late 1970's and the Intermediate designation generally describes slope areas that are oversteepened with a moderate chance of slope failure and landslide activity. Small areas to the east and west of the property are mapped as having Unstable Recent Slides. We observed a small slide mass at the base of the bluff within the subject property and locally along the bluff side of the roadway extending to the east(Figures 1 and 2). Typical indicators of unstable slope areas are that they are oversteepened to magnitudes greater than 100 percent coupled with a lack of vegetation. The lack of vegetation indicates that periodic landslide activity occurs. Additional characteristics of potentially unstable slope areas include curved tree trunks, groundwater or spring activity within the slope face, and permeable materials overlying relatively impermeable sediments. We observed all of these conditions during our site visit. Soil Parameters The slope stability analyses by Tahja Engineering, Inc. referenced utilizing soil parameters based on the geotechnical engineering report by Geotechnical Testing Laboratories (GTL) for the project dated May 16, 2006. These soil parameters were based on a Soil Conservation Service soil classification of a gravelly sandy loam(Alderwood soils)with a unit weight of 126 pounds per cubic feet(pcf), cohesion of 200 pounds per square feet (psf), and a shear(friction) angle of 35 degrees. GTL discussed a 37 degree friction angle in setback and slope stability analyses while a graph on page 12 of their report indicates 35 degrees for this material. Tahja Engineering's slope stability output files indicate that cohesion of 700 psf and a friction angle of 37 degrees were used for analysis without providing clear rationale. The Soil Conservation Service mapped unit for the bluff area is Rough broken land and not Alderwood gravelly sandy loam. Alderwood gravelly sandy loam generally correlates with glacial till (silty-sand with gravel) which was observed upslope from the bluff. The materials within the bluff are consistent with glacial outwash deposits and would have different soil parameters than glacial till (Alderwood soils). The referenced soil parameters are more consistent for glacial till. The cohesion value used appears to be aggressive for outwash sand deposits. Although some apparent cohesion can be assumed due to glacial over-consolidation and negative pore pressures, a cohesion value of 700 pounds per square feet(psf)is more typical of very stiff to hard silts and clays. Slope Stability Analyses We conducted global slope stability back analyses for the existing slope configuration assuming that the current static factor of safety for the slope is approximately 1.0. This is based on the fact that recent slide activity has taken place within the bluff area and on similar slopes nearby. A cohesion value of 100 psf, unit weight of 126 pcf, and a friction angle of 37 degrees yielded factors of safety near 1.0 for static conditions. We noted that groundwater surfaces and roadway surcharge loads were not included in the analyses performed by Tahja Engineering. Krazan&Associates,Inc. Offices Serving The Western United States w KA No.092-09002 February 12,2009 Page 3 of 5 The addendum states that"the modeling does not rely on the blockwork providing any structural strength to the retained slope". However, it appears that the assumption is made that the blockwork,wall backfill, and geosynthetic fabric have the same soil parameters as the native soils. For analysis, granular structural fill has little to no cohesion and Mirafi 18ON is a filter fabric and is not typically designed to be used as reinforcement in mechanically stabilized earth walls. It is typical to use uniaxial geogrids for this type of wall reinforcement. Furthermore, strength and load calculations are not provided to evaluate the adequacy of the fabric. A separate wall design and analyses for static and seismic conditions for the stability of the wall system would be needed to comment on the design. These analyses would also need to indicate how the global slope stability of the bluff will not decrease due to the proposed driveway/wall construction. Erosion Control The Rough broken land unit exists in areas with slope magnitudes generally ranging from 45 to 90 percent and consists of gravelly loam, very gravelly loam, and very gravelly sandy loam. Rough broken land has a "Severe" to "Very Severe" erosion potential. The Alderwood gravelly sandy loam unit is mapped upslope from the bluff area and is found in areas with slope magnitudes of generally less than 30 percent. These soils have a more moderate erosion potential and can be managed through the use of typical erosion control measures. On the second page of the addendum, it is stated that "the proposed driveway includes a riprap lined ditch which is intended to capture runoff from a portion of the driveway and from a portion of the slope from the top of the hill down to the County Road elevation," and that the riprap "provides a filtering mechanism to trap silt and sediment delivered to the ditch." Due to the steepness of the proposed driveway, it is our opinion that riprap will reduce the velocity of stormwater runoff and provide limited filtering of suspended sediment in the stormwater. The catchbasin should be adequately sized based on the volume of runoff expected in a 100 year storm event and recommendations for periodic maintenance and cleaning of trapped sediment should be included. It is also stated that "the final measure to minimize silt and erosion from escaping the site will be the permanent stabilization of the completed slopes and graded areas with vegetation." It is our experience that is almost impossible to plant vegetation on slopes greater than 150 percent in magnitude. Natural vegetation may slowly grow onto and into these areas; however,continued sloughing and sliding of these materials would likely limit the growth. Including permanent slopes in the impervious area calculations may be warranted. Krazan&Associates,Inc. Offices Serving The Western United States i KA No.092-09002 February 12,2009 Page 4 of 5 Conclusions It is our opinion that additional engineering analyses are needed for the proposed driveway and retaining wall construction in order to comment on the adequacy of the project design. As a minimum these need to include: • Slope stability analyses for the existing bluff topography under seismic and static loading with realistic soil parameters • Slope stability analysis for the retaining wall, roadway,and reinforced fill design under static and seismic conditions with representative groundwater conditions and expected roadway surcharge loads with realistic soil parameters for each material • A reinforced wall design with factor of safety calculations under static and seismic loading conditions with an appropriate roadway surcharge • A mitigation plan for groundwater during and after construction The soil parameters used for the slope stability analyses appear to be based on general assumptions about the subsurface soil conditions at the site. Subsurface explorations and laboratory testing specific to this site and application may be warranted to demonstrate the aggressive soil properties and groundwater conditions that were used in design and analysis. It is our opinion, based on our observations of the current bluff conditions, that the probability for landslide activity to occur along the bluff area to be relatively high without proper engineered remediation. It is also our opinion that the site is considered to be within a Landslide Hazard Critical Area due to the recent landslide activity, slope steepness, and geologic and groundwater conditions within the slope face. Therefore, a Critical Areas Report and mitigation plan for landslide activity may be warranted as a prudent measure of safety. Limitations The geotechnical information presented herein is based upon professional interpretation utilizing standard engineering practices and a degree of conservatism deemed proper for this project. It is not warranted that such information and interpretation cannot be superseded by future geotechnical developments. We emphasize that this report is valid for this project as outlined above, and should not be used for any other site. Our report is prepared for the exclusive use of our client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. Krazan&Associates,Inc. Offices Serving The Western United States KA No.092-09002 February 12,2009 Page 5 of 5 If you have any questions, or if we may be of further assistance, please do not hesitate to contact our office at(425)485-5519. Respectfully submitted, KRAZAN&ASSOCIATES,INC. �0 j Washy ^* a � co Gp?AIAS�N i Enpmeenng Get 0"j-5t.•�'� �• i< 251332985 7S e d G e o\ / `��/� -/ �� 9 /STER PHILLIP HA E 1AA-] NAL Phil Haberman,P.G.,P.E.G. Gopal A. Singam,P.E. Senior Engineering Geologist Geotechnical Division Manager PH/gs Krazan&Associates,Inc. Offices Serving The Westem United States