140 cleaves. HIGHWAY ENGINEERING PROBLEMS [Ch. 7 



these areas, and their foundation excavations were carried beneath 

 the former mine workings and beneath the underclay when it was 

 present. No subsidence has occurred under any of these structures 

 where such work was carried out. Where the coal underlay the high- 

 way at depths greater than 40 feet, the coal beneath the right-of-way 

 was reserved, and the wisdom of such practice was observed when in 

 one stretch coal at a depth of over 100 feet was permitted to be re- 

 moved. As a result, serious damage to an underpass and substantial 

 settlement of the road resulted within a few months. 



In one small mine beneath the highway, dry "slushing" was re- 

 sorted to. Here the areas beyond the road berms were bratticed off 

 and the confined area filled with agricultural slag which was blown 

 into position by compressed air. Sand, when available, may be 

 handled in the same manner. 



Anticipated subsidence beneath the piers of an important railway 

 structure in the Middle West was taken care of in a similar manner. 

 The mined-out sections 165 feet beneath the viaduct were bratticed 

 off outside the calculated limits of the angle of draw, and an 8-inch 

 well-drilled hole was driven from the surface into the critical area. 

 Hoses were led down this cased hole, and lean cement grout was placed 

 in the danger area, completely filling it. Sand or agricultural slag 

 would have accomplished the filling as well and would have been 

 cheaper. 



Sinkhole regions present a serious problem when highways must 

 pass above them. In country where the limestones are relatively 

 flat-lying or massively bedded, it may be necessary to skirt such 

 features where major caverns occur. However, in many areas the 

 sinks are of limited extent, sometimes having considerable lineal ex- 

 tent in response to bedding or major joint direction, but limited lateral 

 extent. In these, filling with field stone to preserve the natural sub- 

 surface drainage and topping off with graduated stone and soils in 

 the road's base courses may be sufficient. In such a case a reinforced- 

 concrete slab gives added security. Nevertheless, in some places ac- 

 tual bridging of sinks may be required. The most serious conditions 

 develop when the overburden of soils conceals the cavernous condi- 

 tion and construction proceeds with no anticipation of the cave-ins 

 likely to occur later. It is possible, when such caverns are relatively 

 large, that an electrical resistivity survey would accurately determine 

 the position of them. 



Compaction of sediments is due to an assortment of causes, but com- 

 mon ones are drainage of swamp and marsh lands and the construc- 

 tion of fills on such ground. The same results arise from lowering 



