11_Chester_Creek_Levee_Certification_Report_Revised_8-29-16
GEOTECHNICAL EVALUATION
LEVEE EVALUATION AND CERTIFICATION
4403 SOUTH DISHMAN-MICA ROAD
SPOKANE COUNTY, WASHINGTON
Inland Pacific Engineering Company Project No. 14-037
February 12, 2015
Revised August 29, 2016
IPEC
Inland Pacific Engineering Company
Geotechnical Engineering and Consulting
GEOTECHNICAL EVALUATION
LEVEE EVALUATION AND CERTIFICATION
4403 SOUTH DISHMAN-MICA ROAD
SPOKANE COUNTY, WASHINGTON
Inland Pacific Engineering Company Project No. 14-037
February 12, 2015
Revised August 29, 2016
Prepared for:
NAI Black
Spokane, Washington
IPEC
Inland Pacific Engineering Company
Geotechnical Engineering and Consulting
Geotechnical Evaluation Project No. 14-037
Levee Evaluation and Certification 4403 South Dishman-Mica Road
Spokane County, WA February 12, 2015
Inland Pacific Engineering Company Page 5
slopes at these locations be protected with erosion matting or rip rap. We also recommend
erosion matting or rip rap from the Thorpe Road bridge to River Station 21600 due to higher
flow velocity. Also, all trees on or adjacent to the levee on the levee side of the creek will need
to be removed.
3.5 Embankment and Foundation Stability
We evaluated the embankment and foundation stability for conditions described in EM 1110-2-
1913, “Design and Construction of Levees, by the US Corps of Engineers dated April 30, 2000,
Chapter 6. We analyzed the levee embankment for the following cases:
1. CASE I, End of construction.
2. CASE II: Sudden drawdown.
3. CASE III: Steady state seepage from full flood stage.
We performed slope stability analyses for each case. We analyzed the levee embankment with
2.3:1 slopes. This configuration is considered to have the lowest factor of safety. For our analyses,
we used XSTABL software which is based on a software program developed at Purdue University.
For these cases, we calculated the minimum factors of safety as shown in the following table.
CASE Minimum Factor of Safety
I 1.58
II 1.50
III 1.55
For stability, a minimum factor of safety of 1.5 is generally considered acceptable. Based on this
analysis, it is our opinion that the levee will be stable with respect to global slope stability provided
the recommendations of this report are followed.
3.6 Settlement
The average depth of fill is approximately 10 feet. This would result in a loading increase of
approximately 1,250 pounds per square foot (psf) on the bearing soils. Based on the data
obtained from the borings, the levee was constructed above loose to medium dense sands.
Settlement in these soils would have occurred shortly after construction. Also, given the age of
the levee, it is our opinion that significant additional long term settlement will not occur.
We did analyze the silty clay layer encountered in Boring B-5 with a 1-foot raise in grade should
it be necessary to maintain minimum freeboard. For our analysis, we used a unit weight of 125
pounds per cubic foot (pcf) for the embankment fill soils and a compression index of 0.15 for the
silty clay and assumed total saturation of the clay layer. Based on these parameters, we
estimated the additional settlement to be less than 0.35 inches or 0.03 feet.