222 TOWERS AND TANKS FOR WATER- WORK'S. 



posure of the foundation-pit to atmospheric influences should 

 be as limited as possible, as a sudden rain may change a good 

 foundation to a quagmire. Excavations in clay should be 

 made immediately in advance of the actual masonry con- 

 struction. 



When wet, the bearing value of clay can be artificially in- 

 creased and improved by incorporating with it, according to 

 its plasticity, layers of sand or gravel, or both, or by spread- 

 ing layers of concrete. 



The tendency of the veins of the clay to transport water 

 results in the discovery of springs of water of more or less 

 volume in a number of foundation-pits, and these springs are 

 a source of embarrassment and trouble, as they prevent the 

 masonry from setting, or ooze or stream through the sides or 

 bottom of the completed work. 



Their treatment is largely a matter of personal experi- 

 ence, but the less troublesome varieties may be suppressed 

 by plugging the water-bearing crevice with dry sand arid 

 Cement, dry cement or concrete, either directly or upon some 

 fibrous material, such as yarn, which will absorb the moist- 

 ure until the cement has an opportunity to set, or upon some 

 impervious material, such as tarred or oiled cloth ; or by set- 

 ting a tube over the aperture, and plastering about its foot 

 with pipe-clay, or some plastic material, allowing the water 

 to rise in the tube, or be drawn away through the tube while 

 the masonry is being constructed. After the masonry has 

 set the tube may be plugged with concrete below the face of 

 the foundation, and then either cut off or withdrawn. These 

 are only general suggestions, experience being the only safe 

 guide in such emergencies. 



Dry Sand. Dry sand makes one of the best subfounda- 

 tions if its status as such can be fully determined, for it is an 

 almost incompressible body; is not affected by exposure to 

 any extent, and its bearing power is therefore very great. 



