2011-01547 (zoning- stairway to lk)

Home Browse Search

r , Civil Design Professionals (CDP) was contracted to evaluate the global stability analysis prepared by Gray Engineering to verify if their procedures and conclusions were accurate. Discussion: As previously discussed, the issue of slope stability is highly sensitive to the strength of the materials that make up a given slope. Just a slight modification in the assumed strength of the soil within the slope can radically change its resultant safety factor. Therefore determining the soil strata and having material testing data on the site soils is critical in determining the most realistic safety factor of a slope. Upon review of Gray's slope stability analysis, two items stood out that could have a large impact on the accuracy of their resultant safety factor and corresponding conclusions: 1. The boring depths are too shallow. It is critical to have borings extend close to the bottom of the slope if possible to determine the actual soil of the slope. Soil and its associated strength can vary radically from top to bottom of a slope. Therefore potential failure planes can extend deep into the slope along weaker layers of soil strata if they exist. The borings used in Gray's analysis fail to extend to even 1/3 of the overall slope height In Grays's defense, they failed to approve the slope, but had they assumed a higher shear strength of the material in the top 10', they likely would have approved the slope for they assumed the same soil strata from top to bottom of the slope as shown in their stability analysis. 2. Gray made a conservative assumption of the strength of the soil in the slope from the simple hand auger borings. Hand augers are useful in certain conditions to determine soil strata. However, hand augers are not only limited in how far they may extend, but they do not provide blow counts, a very useful parameter in assessing soil strength. A blow count reading provides an estimate of the density or compaction of the soil. The denser the soil, the higher the correlating strength values. There are publications available that can be used to estimate soil strength from associated blow counts. However, it is virtually impossible to make an educated assumption of a soil strength from a simple classification only. Therefore Gray was forced to make a very conservative assumption of the strength, resulting in an obvious failure in their slope stability analysis. It would be impossible to have any degree of safety on a 35° slope comprised of soil having only a strength of cp = 28° and C = 50 psf. The existing slope, timber stairway, and timber walls have remained in a relatively stable condition for over 20 years, yielding no evidence of external or rotational movement throughout the area. There is no evidence of previous slope failures, and from my understanding there were also no known slope failures in the vicinity. There are very large mature trees at the top of the slope, and trees on the slope that provide additional stability to the slope with their root systems. The global stability results of 1.03 from Gray's repart appear extremely low given all these conditions. Gray's conservative assumption for the strengths of the soil were however reasonable given their lack of available information. CDP therefore arranged to have soil borings and in-situ soil samples taken to better approximate both the soil strata and the strength of the site material. Civil Design Professionals • 8609 Lyndale Ave. 5.,Suite 200 Bloomington, MN 55420 • Office 952-303-5312 • www.cdp-us.com 3