Dear Editor,

What happened with the breach of the conservancy dam [ please note: link provided by LOSP web site ] seems to have been a slope stability failure. The surface of sliding must be circular.

Reasons for this can be:

1) The soil used as fill was not suitable. Pegasse is definitely not suitable. The soil must not be plastic, i.e. shrink and form cracks in hot weather. It should be a firm, sandy, dryish clay which does not swell or shrink, and has no organic content (mud). The water content on placement should be of the order of 16% (not more). Most important of all the soil fill has to be compacted layer by layer (about 1-2 ft. thick layers) to a standard called 94% Standard Proctor (at the very minimum). 97% SP would be far better. Compaction rollers must be used and this is a must. If sand fill is used vibration rollers must be used. With clay vibration has no effect and sheep's foot rollers are more suited.

2) The foundation soil cannot bear the weight of the soil fill. Pegasse is very soft and has no strength. By removing at least the upper part of the pegasse and replacing it with firm soil a reasonable project might be possible. Lots of calculations are needed.

3) A water-filled crack is very destabilising for a slope and can cause slope failure. At Cane Grove what probably happened is the other way around. The slope probably started failing gradually and the cracks appeared.

4) Vegetation has a very beneficial effect on a slope because the roots reinforce the soil. Planting long rooted grass on a slope is always very advisable.

5) Raising the height of the slope can trigger failure. Digging behind the dam is catastrophic.

What worries me most is the condition of the "rip-rap" sea-walls constructed on the E.C.D since 1993. Are they in a similar condition to the conservancy dam?

I beg to ask that an unfettered examination be made into the "rip-rap" sea-walls built recently. Let us be constructive. Let us not bash each other on the head. We must calmly gather together what expertise we have and constructively face the task of checking things out.

And we must do things better in the future.