324 The Shrhikage of Soils 



3. The normal relation between contraction and water loss in 

 these circumstances appears to be that the cubical contraction observed 

 to take place is equal to the volume of water evaporated from a given 

 quantity of soil, provided that contraction proceeds unrestricted by 

 internal friction among the soil particles. 



4. In normal soils contracting in this way a point is reached at 

 which internal friction among the soil particles ofiers sufficient resistance 

 to the contracting pull due to the gel as to cause a progressively 

 increasing departure from the normal relationship. The point at which 

 this departure occurs and the magnitude of the total shrinkage observed 

 appears to be a function of the amount of colloidal clay contained in 

 any particular example. 



5. The relation between cubical and linear contraction is in accord 

 ance with the equation 



„ _ / 3a- a 



^ ^ V 10"2 + 10* 



PART 11. 



THE RELATIONSHIP BETWEEN CONTRACTION AND THE 

 PERCENTAGE OF COLLOIDAL CLAY. 



Additional information concerning the process of shrinkage can 

 be obtained from the study of the amounts of internal pore space 

 existing in blocks of soil prepared in the manner already indicated, 

 which have been allowed to contract to the full extent, and from which 

 any residual water has been removed by subsequent drying to constant 

 weight at 110° C. 



Experimental. 



The method adopted in performing this measurement consisted in 

 covering the dried and weighed blocks with a coating of paraffin wax 

 applied in a series of thin layers by means of a brush until the blocks 

 had been rendered thoroughly waterproof. The blocks were then 

 weighed again in air, the difference between the dry weight of the blocks 

 and that of the waxed blocks giving the weight of the wax coating. 



Subsequently the blocks were suspended from the hook of a balance 

 and weighed in distilled water at a known temperature, and from the 

 difference between the weight in air and the weight in water the volume 

 of the waxed blocks calculated. 



