Ch. 7] SUBSIDENCE 139 



of "permafrost," and kettle-hole subsidence in recently glaciated areas. 



As problems relating to construction in "permafrost" areas are de- 

 scribed in Chapter 14, they will not be considered here. It will suffice 

 to say that no subsidence problems involving loss of bearing values 

 in soils are more serious than those encountered in arctic "perma- 

 frost" regions. See Muller (1947) and Hardy (1946). 



Subsidence in kettle-hole areas, because of their limited size and 

 restricted regional distribution in glaciated areas, is of minor im- 

 portance. Settlement is ascribed to melting of ice layers or lenses, 

 insulated for long periods after general ice retreat by overlying ma- 

 terials. Settlement over metal mines is not uncommon, but that 

 over coal mine areas is of serious concern to railway and road con- 

 struction, especially in the eastern part of the United States. In Illi- 

 nois, where the coal layers are relatively flat-lying, subsidence has 

 usually resulted in long, gentle sags. Break-throughs to the surface 

 are rare, but ponding in the depressed areas is common. Here com- 

 paction of the overlying sediments causes a squeezing of the water to 

 the surface and annually results in considerable property loss, particu- 

 larly in agricultural land. Structural damage to buildings occurs, and 

 regfading of railways and highways is sometimes necessitated. Prob- 

 ably nowhere in the United States has more study been given to this 

 problem than in Illinois; see Rice (1940), Cady (1921), Herbert and 

 Rutledge (1927), and Young and Stolk (1916). 



In Pennsylvania, cave-ins in the anthracite areas, where whole 

 buildings and even sections of roads drop into the steeply dipping 

 mine workings, are common. The bituminous coal fields in the same 

 state have not caused such catastrophic conditions; nevertheless mine 

 settlement areas and cave-ins have resulted in various construction 

 safety measures. In the Pittsburgh region special grouting techniques 

 have been developed to afford safety for major building construction. 

 Not only are void areas present in the form of open rooms and gal- 

 leries, but also in areas of total roof collapse and in "mine-gobbed" 

 sections and in the cracked zones above settlement sections, different 

 techniques, grout mixes, and variations in dry "slushing" are success- 

 fully used. In the line of the original Pennsylvania Turnpike, where 

 mined-out coals showed pitted cave-in areas, it was the practice to 

 remove the coal, pillars, gobbed rooms, and collapsed roof debris to 

 the full width of the highway and backfill with suitable material. 

 The depth to which excavation extended rarely exceeded 40 feet. Gal- 

 leries outside the line of the roadway were bratticed off, and, when 

 necessary, transverse drains permitting the original mine drainage to 

 function were installed. A number of bridges were also located in 



