Discussion on Papers 8 and 9

Paper 8

DR F. B. J. BARENDS, Delft Soil Mechanics Laboratory

What were the bad soil conditions for the breakwater at Sibari that the authors of Paper 8 showed? In many parts of the world the subsoil is too poor to put a breakwater on top.

DR L. FRANCO

We agree that the breakwaters should not have been built on the poor subsoil at Sibari. The original design (1964) was based on very superficial information of the sea bed characteristics and the initially constructed rubble mound structure surprisingly sank (was swallowed) under the surface sandy layer. The following investigations showed that this fine sand cover has a limited variable thickness (3-5 m at the offshore tip, up to 25-30 m at the root of the breakwaters) and stands on a large soft silty clay layer which rests on a gravely and sandy formation situated 75 m below sea level. Moreover an exceptional artesian pressure of 35 m exists under the muddy layer. The main physical characteristics of the mud are a low permeability coefficient (K » 10 -10 cm/s), a plasticity index I between 15% and 25% and an average dry density y^ = 1-3 E/m . Tests on the mud material showed a high compressibility and a low shear resistance. Further details on the soil characteristics can be found in ref. 1. On the basis of these conditions a preloading was proposed and a sand bank with a crest width of 50 m (at +2.0 m) and side slopes of 1/10, and the revised design introduced caissons to lighten the core of the most critical breakwater sections (Fig. 6 in the text).