It is the more common practice in driving headings through soft soils to use inclined poling-boards to support the roof. Fig. 23 shows one method of doing this. The method of operation is as follows: Assuming the poling-boards _a_ and _b_ to be in place, and supported by the frames _A_, _B_, _C_, as shown, the first step in continuation of the work is to insert the poling-board _c_ over the crown bar of frame _C_, and under the block _m_. Excavation is then begun at the top, and as fast as the soil is removed ahead of it the poling-board _c_ is driven ahead until its rear end only slightly overhangs the crown bar of frame _C_. The remainder of the face of the heading is then excavated nearly to the front end of the poling-board _c_, and another frame is set up. By a succession of these operations the heading is advanced. The poling-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-boards is shown by Fig. 24. Here the poling-boards run transversely, and are supported by the arrangement of timbering shown. The chief advantage of using these inclined poling-boards, particularly in the manner shown by Fig. 23, 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 quicksand, peat, and laminated clay, box strutting is commonly employed. 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 planks 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 the new face of the excavation, which supports the planks so that the remainder of the earth included by them may be removed. These two frames, with their plank lagging, 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 face of the heading in order to prevent it from caving in. This is generally done by setting plank vertically, and bracing them up by means of inclined props whose feet abut 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 excavation. =Full Section Timber Strutting.=--For strutting the full section two forms of timbering are employed, known as the polygonal system and the longitudinal system.