Ch. 11] TREATMENT OF SWELLING GROUND 199 



the pressure relaxation, water is drawn toward this zone out of the 

 clay located at a greater distance from the tunnel. Therefore the water 

 content of the clay adjoining the walls of the tunnel increases and the 

 clay swells, whereas beyond this zone the water content decreases. On 

 the right-hand side of Fig. 3 this change of the water content is in- 

 dicated by shaded areas. 



The theory illustrated by Fig. 3 has been verified repeatedly by the 

 results of water-content determinations on samples from the proximity 

 of the walls of tunnels in the "Argile plastique" of the region of Paris, 

 France, a stiff Eocene clay with a plasticity index of 60 to 80 percent. 

 The clay is similar in many respects to the London clay. It rests on 

 the Cretaceous shales of Meudon. It is overlaid by Eocene limestones, 

 and the entire formation is gently folded. The thickness of the clay 

 stratum varies between about 100 and 200 feet. The state of precom- 

 pression was produced by the weight of superimposed Tertiary and 

 younger strata which were later removed by erosion. The temporary 

 load due to the weight of these strata was of the order of magnitude 

 of 25 tons per square foot. A general description of the physical 

 properties of the clay was published by Langer (1936). One of the 

 subway tunnels of Paris was excavated in this clay. The average 

 initial water content of the clay was 56 percent. Within a few weeks 

 after exposure the water content of the clay at the walls of the tunnel 

 had increased to values between 90 and 130 percent. With increasing 

 distance from the walls the water content decreased, and it had a 

 minimum of 46 percent at a distance of about 13 feet from the walls 

 (observations by Langer, reported by Terzaghi, 1936). 



Because the water that causes the swelling of stiff clay comes out 

 of the clay, the swelling of the clay can be prevented only by a tube- 

 shaped tunnel support which is strong enough to sustain the swelling 

 pressure. However, the ultimate pressure on the tube can be reduced 

 considerably by providing a clearance between the walls of the tunnel 

 bore and the extrados of the tunnel support, or else by using tunnel 

 supports which can yield considerably under pressure without being 

 crushed. Experience indicates that a clearance of about 6 inches 

 commonly serves the purpose. A description of the different systems of 

 yielding steel supports used in Belgian and German coal mines for re- 

 ducing the ultimate load on the supports was given by Ernould (1934) . 



Application of a watertight coating can do no more than prevent 

 relatively harmless surface processes such as the raveling of jointed 

 clay or shale due to desiccation. 



