RETAINING 'ALL is the term used for a wall erected to resist the thrust of a Masi of earth, or of a volume of water stored in a reservoir. In consequence of the variable nature of the materials to be sustained, as well as of the materials of which the walls themselves are composed, the practical application of the mechanical laws usually adopted for calculating the thicknesses of this class of works is more empirical than is usually the case iu the ordinary operations of building. The following arc, however, some of the generally received principles adopted by engineers in the construction of these walls.
Navier, who has applied the higher analysis to the investigation of the laws of the stability of buildings, gives a formula for the thickness of retaining walls, and a table of the weights of the materials com monly dealt with. The table is as fellows, in English weights and dimensions :— earth piled against it. In most' cases, also, the resistance of retaining walls is Increased by the introduction of eouuterforts; especially when the length of the embankment, or earthwork, to be retained is con siderable.
Retaining walla used in situations where the earth at their backs is liable to become saturated with water by tidal action, or by the capillary attraction of the earth, must be established on the supposition that their backing becomes in fact a semi-fluid, denser than water, and which consequently has no angle of repose. Careful observations on the state of the walls in the French ports on the Channel, have flown that in many cases those walls have yielded when their mean thickness has oven been as much as : POO; and it therefore would appear that in such cases it Is not safe to keep the ratio of the mean thick ness below or 0.50 to POO. In reservoir walls of masonry, the thickness should be made, practically, double that of ordinary earth retaining walls, as indicated by theory; and especial care should be taken to ensure the adhesion of the wall to its foundations; for it is found that the expression of the resistance of these walls to overthrow a 0.41h, while that of the resistance to longitudinal displacement is
0.50 h. The aummit of the retaining wall of a reservoir should be made at least equal to 1.3rd or 1.4th of the head of water ; and the effect of the surcharge of the masonry above the water-lino may be neglected in estimating the statical resistance.
'rho reader who would desire to study the strict theory of this question would do well to examine the 314moires' by Coulomb, 3layniel, and Prony, ' Sur Is Pouas6e des Terres ;' Nevier'a Lecons do la 316canique; ' l'asley's' Course of Military Instruction ;' Slogreley'a ' Engineering and Architecture ; ' but the most condensed and practical notice upon the subject is to be found in Carr's invaluable 'Synopsis of Practical Philosophy,' under the head Earth.' The following table of the angle formed by the natural elope of the materials enumerated may be found useful in applying Nevier's formula, previously given, when it may be desired to calculate the resistance of a wall rather more closely than it is found necessary to do in practice :— I. Fine dry sand forms with the vertical line a slope, having an angle of 69°; sometimes the angle is, however, = 554, when the mud has been well rammed.
2. Vegetable earth de. of 55 i° 3. Loose shingle do. of 51° 4. Ordinary earth, well dried and pulverised forms an angle of 43" 10' 5. Rubble, small and rough do. of 37I° 6. Do. do. slightly moistened do. of 36° 7. Densest and most compact sand do. of 35' From this table it appears that 0 usually ranges between 35° and 70°; for water and for extremely fluid mud 0 becomes = 9°.