36 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the clay being either the upper or the lower bed, the space was 

 filled with concrete. It may appear that the order of taking 

 the compartments should have been reversed, and that those in 

 which the boils were situated should have been dealt with first; 

 but boils had appeared in all the compartments, and, without 

 doubt, if the boil were stopped in one compartment before that 

 adjoining it had been made good, it would have reappeared in 

 the latter place. Sections of old bore-holes were exposed in the 

 foundations choked up with fine gravel or sand ; but time was 

 not allowed for the water to clear out these vents, though 

 within a distance of a few yards it was rising from the same 

 strata with which the bore-hole communicated, to a height of 

 15 feet above the bottom of the foundation. The compartments 

 on the north side of the lock were filled with concrete without 

 accident ; and all went well with those on the south side until 

 the last length to the westward was being excavated, when a 

 boil burst up .by the side of the second row of piling ; a small 

 stank or dam was formed round this boil by driving 4-inch 

 sawn planks on three sides, the piling forming the fourth side. 

 The concreting was then completed in this compartment, with 

 the exception of the part surrounded by the stank, round which 

 clay was heaped to check the flow of water. In order to 

 facilitate the excavation of the easternmost of the centre com- 

 partments, two rows of sheet-piling were driven across it at 

 right angles to the centre line of the lock, dividing it into 

 three spaces 15 feet, 40 feet, and 18 feet wide respectively, the 

 latter being to the westward and partly underlying the sill. 

 The space to the eastward was first excavated to a depth of 

 13 feet 6 inches below the sill level and filled with concrete. 

 In the centre, beneath the apron, no attempt was made to reach 

 the clay, as the whole of the upper bed had been removed, and 

 it would have required the excavation to be carried to a depth 

 of 20 feet below the sill level to reach the surface of the lower 

 bed of clay. Accordingly the concrete was carried to a depth 

 of 10 feet only below sill level. In the third compartim-nt, 

 which would form the foundation of the back part of the sill, 

 the bottom bed of clay was reached over one third of the aivn, 

 and the old rails of one of the waggon roads were found at a 

 depth of 19 feet below sill level. In the remaining part of the 

 space, the excavation was carried to the same d<-pth, but the 

 clay bottom was not found, nor was there any trace of 



