132 LECTURE 



the water-way by sucli masses is not of importance. Another plan is 

 to make strong timber cribs and to ^ink them, filled with stone, to 

 the bottom, permitting them to reach just to the surface of the water, 

 a method largely adopted in this country with good results. 



The neatest, though much most expensive way, is to build a water- 

 tight wall or coffer-dam around the place of the pier, and after pump- 

 ing out the water so as to lay bare the bed of the stream, to build up 

 the foundations to the surface with concrete or with regular masonry. 

 The coffer-dam is usually made by driving two rows of piles, one 

 outside of the other, around the space to be laid dry, and then by 

 packing clay in between them so as to render the enclosure imper- 

 vious to water. Pure clay is not as good for the packing as that 

 which has a slight admixture of fine gravel or sand. AVhen the 

 stream is deep it will be very difficult to prevent the water from 

 soaking through the clay puddling, and a steam pump is often re- • 

 quired t-o keep the space free from leaks and breaks which greatly 

 delay the work and add to its cost; but it is quite practicable, with 

 proper precautions, to lay foundations in this way in very deep and 

 rapid rivers. 



We have so far supposed that the natural bottom is suited to bear 

 the weight of the structure, but frequently we find soil of such a na- 

 ture as not to be able to do so, especially if the pier is thin, and there- 

 fore presents only a small bott9m surface; in such cases it will be 

 necessary to provide some means of solidifying the bottom, or of dis- 

 tributing the weight over a surface sufficiently large to prevent its 

 sinking. Where the bottom is nearly firm enough, a platform of wood, 

 somewhat larger than the pier, carefully laid, will be sufficient to pre- 

 vent any sinking. Such a platform will last under water for cen- 

 turies, many having been found in a good state of preservation under 

 old Roman bridges. 



In place of a wooden platform a bed of concrete or beton may be 

 employed, composed of a mixture of mortar made, if necessary, of 

 hydraulic lime and gravel or broken stones, with some sharp sand, 

 which is thrown into the space excavated to receive it, rammed and 

 levelled, and allowed to harden into a solid mass, upon which the 

 masonry is built. Concrete should not, as a general thing, be ex- 

 posed directly to the action of running water, since it may be rapidly 

 worn away, but with the proper precautions it forms the best of 

 foundations. 



Should the soil of a particular locality not prove sufficiently strong 

 for this, however, recourse must be had to piling ; that is, to driving 

 posts or piles of timber to such a depth that they will not settle any 

 further, and laying a platform upon their heads on which the masonry 

 will rest. The piles, with the bark stripped off", the points sharpened 

 or shod with iron, and the heads defended by iron rings, which ar& 

 afterwards removed, are driven by a heavy weight falling upon thein 

 until they will go no further, or rather only a fraction of an inch with 

 several strokes. They are placed in rows, at distances apart depend- 

 ing upon the nature of the ground, and then their tops being sawed 



