TIMBERING OR STRUTTING TUNNELS 47 



the heading is advanced. The idling-boards at the sides of 

 the heading are placed in a similar manner to the roof poling- 

 boards. A second method of using inclined poling-boanls is 

 shown by Fig. 25. Here the poling-boards run transversely, 

 and are supported ly the arrangement of timliering shown. 

 The chief advantage of using these inclined poling-boards, 

 particularly in the manner shown by Fig. 24, is that the 

 excavators work under cover at all times, and are thus safe 

 from falling fragments or sudden cavings. 



Box Strutting. In very treacherous soils, such as quick- 

 sand, peat, and laminated clay, box strutting is commonly em- 

 ployed. The method of building this strutting is to set up at 

 the face of the work a rectangular frame, and use it as a guide 

 in driving a lagging or boxing of horizontal plunks into the 

 soft soil ahead. These planks have sharp edges, and are driven 

 to a distance of 2 ft. or 3 ft. into -the face of the heading, so as 

 to inclose a rectangular body of earth. This earth is excavated 

 nearly to the ends of the planks, and then another frame is 

 inserted close up against tin- new lace of the excavation, which 

 supjiorts the planks so that the remainder of the earth included 

 by them may be removed. These two frames, with their plank 

 lading, constitute a "box;" and a series of these boxes, one 

 succeeding another, form the strutting of the heading. 



Strutting the Face. In some cases it is found necessary 

 to strut the fare of the heading in order to prevent it from 

 caving in. This is generally done by setting plank vertically, 

 ami bracing them up l>y means of inclined props whose feet 

 almt against the sill of the nearest cross frame. This strutting 

 is erected while the workmen are placing the side and roof 

 strutting, and is removed to permit \ .ivation. 



Full Section Timber Strutting. I-'oi -strutting tin- full section 

 two forms of timbering are employed, known as the jxilygonal 

 system and the longitudinal system. 



Longitudinal strutting consists of a timlter structure 

 arranged as to have all the principal mrml>ers supjMirting the 



