" In case the pipe is bedded in loose material, the effect of the settlement will be to compress 

 the earth immediately under the bottom of the pipe more completely than will be the effect at 

 one side, with the result that the pressure will not be uniformly distributed horizontally. 

 Similarly, in a sewer trench, if loose material is left at the sides and the material at the 

 extremity of the horizontal diameter is loose and offers little restraint, the pressure on the earth 

 will not be distributed horizontally and the amount of bending moment will be materially different 

 from that where careful bedding and tamping give an even distribution of bearing pressure over 

 the bottom of the sewer. 



"In case a small sewer in a deep trench, the load upon the sewer may be materially less than 

 the weight on the earth above, where the earth forms a hard, compact mass and is held by pres- 

 sure and friction against the side of the trench. 



" In case a culvert pipe is laid in an ordinary embankment by cutting down the sides slopingly, 

 it is evident that the load which comes upon the pipe will be materially less than the weight of 

 the earth immediately above it. If a culvert pipe replaces a trestle and the filling is allowed to 

 run down the slope, the direction and amount of the pressure against the pipe will differ con- 

 siderably from that which obtains in a trench or in the case of a level filling. It is possible in 

 the latter case that the small amount of settlement of the earth directly over the culvert pipe, 

 due to the greater depth of earth on the adjacent sections, may allow a greater proportion of the 

 load to rest upon the culvert pipe than would ordinarily be assumed. 



"Attention should be called to the fact that the distribution of the pressure by means of earth 

 under and over a ring assumes that the earth is compressed in somewhat the same way as when 

 other material of construction is given compression. Unless the earth has elasticity, the 

 distribution of the pressure cannot occur. To secure the uniform distribution assumed the ring 

 itself must give enough to allow for the movement of the earth which takes place under pressure. 

 This is especially true with reference to the presence and utilization of lateral restraint, and a 

 ring which does not give laterally, as for example a plain concrete ring, will not develop lateral 

 pressure in the adjoining earth under ordinary conditions of moisture and filling to any great 

 extent. As the conditions of earth and moisture produce mobility and approach hydrostatic con- 

 ditions, the necessity for this elasticity and movement do not exist, but here the lateral pressure 

 approaches the vertical pressure in amount and the bending moments become relatively smaller. 



77 



