FOUNDATIONS OF WALLS. 



67 



passing, that it is anything but advisable 

 to build walls so close to shelter trees, as to 

 admit of their roots reaching the borders ; 

 for, where this is practised, half the utility 

 of the walls is completely lost. Hence it 

 is always better to surround the garden 

 with slips of ground, themselves enclosed 

 by ha-ha's, hedges, or other fences, and 

 thus leave both surfaces of the walls all 

 round clear for the training of trees. 

 Inverted arches may be used where the 

 foundation is bad ; but where this is the 

 case, it almost naturally follows that the 

 site is not a suitable one for a garden. 



The best of all foundations is concrete ; 

 and such foundations, unless for the pur- 

 pose of preventing the roots of shelter 

 trees from passing under them, need not, 

 in almost any soil, be more than 2 feet 

 deep, as they form one solid piece through- 

 out the whole length, and are not liable 

 to settlement. Concrete foundations are 

 of great antiquity, as well as concrete 

 walls, and are, if properly prepared, the 

 strongest and most durable of any. 



It is somewhat remarkable that this 

 most valuable discovery of modern en- 

 gineering is, after all, only the revival of 

 an ancient practice, well known to the 

 Romans and the Moors. Abundant in- 

 stances of foundations thus formed by 

 them exist to the present day. Modern 

 attention was first directed to this sub- 

 stance so recently as the building of 

 Waterloo Bridge. In excavating for the 

 foundation of the piers, the workmen 

 came upon a solid mass, which they de- 

 scribed as a block of granite, surrounded 

 with the loose sand that forms the bed of 

 the river, and which was so hard as almost 

 to resist all their efforts to break it up. 

 This mass was found to have originated 

 by the accidental sinking of a barge of 

 lime some time before, the cargo of which 

 had cemented together the loose gravel 

 into a solid mass. The engineer, Mr 

 Rennie, drew the attention of Sir Robert 

 Smirke to the circumstance : he availed 

 himself of the hint, and afterwards laid 

 most of his foundations with concrete, 

 not one of which has ever been known to 

 fail. He even, when the Customhouse 

 of London was in danger of falling, from 

 the insecurity of the piling which was 

 used for a foundation, underset the whole 

 of the walls with concrete to the depth of 

 15 feet, or until he came to a solid foun- 



dation. The late Mr Atkinson suggested 

 to Brunei, the engineer of the Thames 

 Tunnel, to lay a bed of concrete formed 

 of Mulgrave cement, lime, and gravel, 

 shot out of barges, across the bed of the 

 river, above the line of the tunnel ; and 

 there is little doubt that, if this idea had 

 been acted upon, the tunnel would have 

 been constructed at much less expense, 

 and without the disastrous consequences 

 which attended it. 



The following is the process for forming 

 these foundations. The trench being dug 

 out to the desired width and depth, 

 " coarse and fine gravel is thrown in, 

 just as it comes from the pit, to the 

 thickness of about 4 inches ; it is then 

 grouted with thin hot lime, just enough 

 to bind the gravel together, and after- 

 wards rammed quite hard. Course after 

 course must then be laid, and so treated, 

 till the mass reaches within about 6 inches 

 of the ground line. The proportion of 

 hot lime to the gravel is about one-eighth 

 part only. Others use lime in the pro- 

 portion of one to five of loamy gravel, 

 but much depends on the quality of the 

 lime. In countries where gravel is 

 not common, dry brick rubbish, broken 

 stones, flints, or any material that will 

 bind into one mass, will answer." — Archi- 

 tectural Magazine, vol. i. p. 284. 



A better plan, however, is to mix the 

 concrete on the surface in convenient- 

 sized heaps, and to erect a scaffolding, so 

 that the concrete may be carried in hods, 

 or wheeled up to the platform in barrows, 

 and thrown into the trench from a height 

 of 8 or 10 feet, and merely levelled on the 

 surface, to prepare it for the first course 

 of bricks. Falling from this height con- 

 solidates the mass better than ramming 

 it, as above recommended. Where the soil 

 is loose, or where the foundation is to be 

 above the surface-level, a framework of 

 strong planking should be provided, in 

 the form of a trough, 30, 40, or more feet 

 in length, and of the depth and breadth 

 required for the foundation. In a day 

 or two these planks may be removed, and 

 carried forward until the whole is com- 

 pleted. When the concrete is fully set, 

 the operation of building on it may be 

 commenced. 



The concrete employed for the founda- 

 tions of the Crystal Palace was com- 

 posed of large stones mixed with one- 



