Geotechnical ULS Failure Case Study Analysis

Geotechnical ULS Failure Case Study Analysis

Summary

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In this assignment, two major causes of geotechnical failures, namely the “Lower San Fernando dam” failure as well as the Nerlerk Berm failure, are reviewed and analyzedfor the purpose of understanding the key features of the failures and their reason for the occurrence.

The ultimate limit state design is done for a structure in the UK or other European nations correspond to the guidelines of Eurocode 7. The limit state design that is done for the structure corresponds to the states beyond the structure is unpredictable as it does not meet the design criteria. In this essay, two case studies are focussed on which are caused due to the failure in the ultimate limit state in Geotechnical design.

The first case which would be analyzed is of the failure which occurred in the “Lower San Fernando Dam”. The construction of the “Lower San Fernando dam” consisted of an about 40 m in height “hydraulic fill earth dam” that was started in the year 1912 to integrate as a suitablewater reservoir basin based in the area of “San Fernando”, which is located inthe country of US (Castro et al., 1992). The material from the end of the reservoir was excavated as well as evacuated with the help of sluice pipes situated on each end of the dike that is upstream level and downstream level causing parts of sand and silty material in those regions and the presence of silty clay in the central regions of the reservoir. As a part of the treatment, a 3 to 5 m layer of weathered shale was deposited on top of the hydraulic fill as a safety measure. After which, the additional stratum of “roller-compacted fills” was stationed, which ultimately raised the height of the dam to 40 m in 1940, and finally, the “roller compacted berm” was provided as well on the downstream end. After the completion, the dam was able to hold a basin with a “volumetric capacity” of about 25 million cubic metres.

In 1971an earthquake struck on the dam with a relative magnitude of 6.6 Richter scale that severely destabilized the dam, which resulted in the occurrence of major slides in the “upstream slopes” and the upper end of the “downstream slope” of the dam, causing a freeboard of highly unstable freeboard levelconcerning 1.5 m was observed. 

The plan for the structure which was used to build the given structure of the “Lower San Fernando” dam is provided below in the depiction detailing the various aspects of the structure itself.

The hydraulic fill conditions as per the different stratum of the dam and the reservoir hydraulic fill shells and the necessary characteristic performance of each of these layers are depicted by performing suitable laboratory testing to determine their strength.

It was found out after performing corrected laboratory test data involving suitable parameters to determine the actual field strength of the layer at the time of the major slide (Seed et al., 1989). It was found that although the results from the 4 laboratories were in unison agreement, there is a need for a conservative interpretation of resulting data as “steady undrained strength” is largely reactive to void ratio change, and therefore necessary to caution while selecting strength value of the soil with this approach with more rigorous lab testing to determine the actual strength.

The next case which has been considered is of Nerlerk Berm, which was an underwater sand berm that was constructed in order to create an offshore artificial island structure to cater for the need of hydrocarbon exploration in the site of Nerlerk situated in the "Canadian Beaufort Sea". The construction of the underwater berm was undertaken in the year of 1982 to 1983 which were considered as the underwater seasons. The site where the artificial island was due to be built was about 45 m deep. A sand berm was designed to uphold the structure that had side slopes in the range 5H:1V starting from the sea bed level all the way up to -10m with respect to the sea level at the level. To carry out the construction, 1.3 million cubic metres of dredged material was used and dumped in the site via the "bottom dump hopper dredges". After the dredge placement, an additional 1.8 million cubic meters of sand was placed using a floating pipeline directly into the barge (Been et al., 1987). It was during this construction period of 1983 consecutive slide occurred in the berm, and it was found via the “Bathymetric surveys” that a major portion of the berm vanished due to these slides. The final construction of the berm was thus abandoned, incurring losses roughly of about $ 50 million.

The series of slope summary data corresponding to the geotechnical investigation that was carried out on the site after the failure of the berm conformed to the various slope failure criteria observed at the site.

The steady-state parameter as what was derived for the soil layer that was collected from the site drew out important characteristics regarding the failure of the berm with regards to the less overall stability in the layers.

It was found on the basis of the evidence that the positive excess pore pressure existed and therefore the failure of the “Nerlerk berm” was caused by the shearing of the clayeystratum that was highly affected by stress because of the presence of excess “pore pressure” occurring in the sand fill layers (Sladen, 1989). The characteristics prefailure and postailure state parameters of the layer are represented below for further understanding.

Reference List

Journals

Castro, G., Seed, R.B., Keller, T.O. and Seed, H.B., 1992. Steady-state strength analysis of lower San Fernando Dam slide. Journal of Geotechnical Engineering, 118(3), pp.406-427.

Seed, H.B., Seed, R.B., Harder, L.F. and Jong, H.L., 1989. Re-Evaluation of the Lower San Fernando Dam.Report 2. Examination of the Post-Earthquake Slide of February 9, 1971. SEED (H BOLTON) INC ORINDA CA.

Li, X.S. and Ming, H., 2004. Seepage driving effect on deformations of San Fernando dams. Soil Dynamics and Earthquake Engineering, 24(12), pp.979-992.

Been, K., Conlin, B.H., Crooks, J.H.A., Fitzpatrick, S.W., Jefferies, M.G., Rogers, B.T. and Shinde, S., 1987. Back analysis of the Nerlerk berm liquefaction slides: Discussion. Canadian Geotechnical Journal, 24(1), pp.170-179.

Sladen, J.A., 1989. Problems with interpretation of sand state from cone penetration test. Géotechnique, 39(2), pp.323-332.

Wanatowski, D., Chu, J. and Lo, R.S., 2008.Types of flowslide failures and possible failure mechanisms.In Geotechnical Engineering for Disaster Mitigation and Rehabilitation (pp. 244-253).Springer, Berlin, Heidelberg.