198 tbbzaghi. SOFT-GROUND TUNNELING [Ch. 11 



filled, the swelling ceases; but the pressure on the support increases 

 first at an increasing, later at a decreasing, rate, and it approaches 

 an ultimate value which may be considerably greater than the over- 

 burden pressure. In tunnels through clay this is not possible un- 

 less the initial horizontal pressure in the clay is considerably greater 

 than the vertical pressure. The most common cause of such stress 

 conditions is intense compaction under the weight of superimposed 

 strata which are subsequently removed by erosion. 



• Settlement due to loss of ground 



A few weeks 

 Swelling clay cj >. after mining 



-Swelling 

 i ' ' H /Compression . 



Original water content 



T 



-Zone of swelling 

 Before mining ""~ Zone of compression 



Fig. 3. Diagram illustrating the mechanics of the swelling of stiff clay in a tunnel. 

 The change in the distribution of the water content caused by the mining opera- 

 tions is shown on the right-hand side. 



According to a widespread belief the water that causes the swelling 

 comes out of the air contained in the tunnel. Therefore many at- 

 tempts have been made to prevent the swelling by covering the ex- 

 posed surfaces in tunnels with a bituminous coating. Most of these 

 attempts have failed because, as a rule, the water that causes the 

 swelling comes out of the ground and not out of the air. The mechanics 

 of this process is illustrated by Fig. 3. The left-hand side of the dia- 

 gram shows the distribution of the water content of the clay over a 

 horizontal section through the center line of a proposed cylindrical 

 tunnel in a heavily precompressed clay prior to excavation. Excava- 

 tion relieves the confining pressure on the clay adjoining the walls of 

 the tunnel. According to the theories of soil mechanics, local re- 

 moval of the load on a saturated porous material draws water out of 

 the loaded portions of the material toward the zone of pressure re- 

 laxation. In the tunnel the zone of pressure relaxation has the shape 

 of a cylindrical mantle surrounding the tunnel bore. On account of 



