THE STRUCTURAL BEHAVIOUR AND THE SHELTERING EFFICIENCY OF THE SUBMERGED BREAKWATER AT THE ENTRANCE TO THE PORT OF LEIXOES MAINTENANCE CHARGES AND EFFECTS

Abstract

The urgent necessity for providing a system of protective works of the harbour of Leixoes with complementary works to improve the poor con ditions of the shelter in the basin and to protect ships entering the port became evident in 193^* In fact, the frequency of disaster to trawlers and to other ships within the basin during southwestern gales as well as the very dangerous conditions at the entrance under such gales were incompatible with the increasing economic importance of Oporto and its hinterland. These bad conditions culminated in the foundering of the 9000 T Dutch steamship "Orania" at anchor in the basin colliding on the 19th December 193^ with the steamship "Luanda" when entering the harbour under severe hurricane. The construction of the 1000 m long new breakwater was started wit a 15 m wide vertical wall to be set partly on rocky bottom and partly o a sandy bottom. Before leaving the rocky bottom the upper part of the wall was destroyed by a strong gale (Fig. l). For the advancement of t work the contractor and an official commission recommended the adoption of the Casablanca type of slope work. For economical reasons and on those of the sure possibility of its conversion into a conventional type of work, would it become convenient it was decided, at the author's suggestion, and after model study at Lausanne School of Engineer's Laboratory, to adopt a new type of submerged work, to replace the unsuccessful vertical type or the type of Casablanca proposed by the contractor (Fig. 2). The section adopted coi prised a rubble mound core of stones to 1- with the crest at level (-9m00), and berms of rubble to kF, 12 m wide to the sea side and 10 m wide to the port side (Fig. l). T The top and slopes of the mound were protected by an armour of 90 concrete blocks with the corners cut for best accommodation. The cost of the work could be reduced to half of that corresponding to the last of these types of breakwaters and its maintenance charge greatly reducec The estimated cost of the works would be reduced from JL 1.770.000 to £. 880.000 and the maintenance charges foreseen reduced from £ 35*^00 yearly to £ 2.800 yearly (comparison with the Casablanca type).