Drainage by Gravitation. 29 



a contingency from which no banks are free. Some settle- 

 ment, or weak place in construction, or burrow made by mole, 

 rat, or rabbit, which may have been in existence for some time 

 unknown, is finally discovered by a flood a few inches higher 

 than usual, the water finds its way through, the bank bursts, 

 and a whole level is inundated. 



Great care is required in making the foundations of sluices to 

 prevent the water from finding its way under the floor at times 

 when, owing to the head on the outside during high tides, the 

 power of the water to penetrate through the ground 1% very 

 great. The lands enclosed generally consisting of deposits of 

 alluvial matter, and the site of the sluice being on the sea- 

 shore or on a river, the soil Is frequently nothing but silt 

 or sand, and frequently no material of a more tenacious 

 character can be reached within a distance that would 

 warrant the foundations being carried down to the solid 

 stratum. When the soil consists entirely of silt, the danger 

 to guard against is that of the water finding its way under the 

 floor of the apron and invert of the culvert Several cases 

 have come under the author^s experience where this has 

 occurred, and the sluices, otherwise well built, have been left 

 standing on the bearing piles with the material entirely washed 

 away from under the concrete below the floor. The only 

 reliable course to pursue in dealing with foundations in sand 

 or silt is to build the sluice on bearing piles, and completely 

 to box in the whole of the site covered by the foundation with 

 sheet piling, driven eight or ten feet below the bed of the 

 channel into which the sluice discharges. Within the box a 

 solid bed of cement concrete to be placed, and on this the 

 planking for the floor. Wings should be carried out with box 

 piling for some distance each side, both at the inner and 

 outer ends, to prevent the water making its way round outside. 

 The brick-work of the piers and for the culvert will rest on 

 the planking, bearing piles being driven to carry the walls ; 

 the whole upper part being surrounded with puddled clay. In 



