Some thoughts on the breach of the Conservancy Dam
Stabroek News
November 16, 2001

Dear Editor,

Over the last week I have read with interest the opinions expressed regarding the possible reasons for the failure of a section of the East Coast Conservancy Dam which resulted in severe flooding of adjacent residential property and farmlands. Opinions provided include digging at the toe of the dam, poor sub-soil foundation, seepage, slope stability failure, removal of vegetation, lack of new vegetation, cracks in the surface of the dam, inadequate consolidation of material used to increase the height of the dam, erosion by wave action, poor construction, lack of attention to historic information and views of nearby residents, and possible vandalism.

If we rule out vandalism, the remaining opinions can be considered valid reasons that could have contributed singularly or in combination to the failure that occurred. The committee in determining an unbiased reason for the failure faces a difficult task. One of the outcomes of the ensuing investigation may be a set of guiding rules for the safe design, operation and maintenance of this dam so that such failures may not occur in the future.

From a technical viewpoint, design considerations for raising of the height of an existing dam invoke both hydraulic and geotechnical (soils and foundation) principles. It is expected that the design would have taken into consideration seepage, hydraulic pressures, foundation types and their characteristics, characteristics of the existing material of which the dam is composed, characteristics of the materials to be used to raise the dam, the existing cross-section and profile of the dam, local input from nearby residents, and historic evidence.

Another important issue is the constructability of the design. How was construction expected to be undertaken to achieve the proposed design?. In some circumstances to achieve what is required by the design may result in construction practices that could have an adverse effect on the existing structure. This is often very common when undertaking retrofitting of an existing structure. Hence, in the case of this dam that was already operational, was constructability conceived at the design stage? If so, what considerations were invoked?

Many times we expect that the Contractor will have all the answers and apply good construction practices, but very often also adjustments to the design have to be made in the field as practical issues are missed in the design. Likewise, the field supervision team has to be ever vigilant to ensure that the construction practice being adopted by the Contractor would not impair the functionality of the existing structure. Here, the supervision team has to make field decisions, which are compatible with the intent of the design. Such decisions are often based on discussions with the designer, and on changes agreed upon and approved by the designer.

Since the opinions expressed so far on the reasons for the breach weigh heavily toward the realm of the geotechnical discipline, it would be important that the committee examines the opinions raised regarding slope and foundation stability in their deliberations. This could be challenging, as geotechnical assessments are sometimes more of an art than science and clear-cut answers are not often available. For example, what is the strength of pegasse soil and what is the strength expected to be in time if we are to place a foundation on such a soil. How much of an extra load can the dam structure and the sub-soil accommodate after 32 years?

While this incident is indeed unfortunate, such failures do provide a lesson, allowing us to examine our approach to such works in the future. As pointed out by others, two important aspects that must be addressed are historic information and information from nearby residents. These are free and often invaluable input.

Yours faithfully,

Vishnu Diyaljee, Ph.D., P.Eng.