Quantcast Bluff-Top Failures

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 47
Where marine erosion allows a fairly rapid retreat of the lower bedrock unit (primarily by
block falls along joints and faults within the various middle Eocene-age units), the upper-
bluff Pleistocene sands are undermined, causing a relatively steep to near-vertical upper
bluff, more susceptible to continuous sloughing. Traditional engineering stability analyses
have only limited usefulness for this type of profile, because the upper bluff terrace sands
continually slough and ravel to re-attain a stable angle of repose (a natural geomorphic
process). This natural geologic Aflattening@ process reduces the driving force from a
hypothetical failure geometry, and renders the original stability analyses invalid. Further,
marine erosion at the seacliff continues to undermine the upper bluff from the basal
contact up, starting the whole process over again. In summary, and from a practical
standpoint, proper determination of the appropriate bluff-top setback must include an
analysis of both the rate of marine erosion of the lower cliffed portion of the bluff, and of
the effect of that rate in creating an Aartificially@ oversteepened upper bluff.
7.4
Bluff-Top Failures
For given values of soil strength, and assuming homogeneous conditions within the
geologic units, the stability of the bluff top can be shown to be a function of the slope and
the thickness of the upper terrace deposits, along with the height of a vertical scarp in the
terrace deposits at the Eocene contact. The development of a vertical scarp at the base of
the terrace deposits above the Eocene contact occurs subsequent to the development and
collapse of a notch at the base of the seacliff. Assuming a 45 degree upper slope
inclination, the failure of a ten-foot-deep notch in the Eocene unit results in a ten-foot
vertical scarp above the contact.
In order to assess the stability of the upper bluff, slope stability analyses were performed
using soil strengths for the upper terrace deposits as follows (USCOE, 1996):
φ = 33 degrees
c = 300 psf
γt = 124 pcf
A terrace thickness of 50 feet was analyzed for various slope inclinations and lower vertical
scarp heights. The results are reported on Figures 27 and 28. Critical failure geometries



 


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