EURAL ENGINEERING. 685 



The theory of loads on pipes in ditches, and tests of cement and clay drain 

 tile and sewer pipe, A. Marston and A. O. Anderson (Iowa Engin. Expt. Sta. 

 Bui. 31, 1913, pp. 181, figs. 40). — Exhaustive investigations of drain tile and 

 sewer pipe in ditches indicate that there have been a large number of failures 

 by cracking in ditches, and that there is a wide prevalence of cracked pipe 

 in existing sewers and drains, which is generally confined to pipes larger than 

 15 in. in diameter. It is stated that the principal cause of cracking of drain 

 tile and sewer pipe in ditches is that sizes larger than 15 in. in diameter, 

 as at present manufactured, are very generally too weak to carry the weight 

 resting upon them for more than a few feet depth of the ditch filling and 

 that many failures are caused by carelessness in bedding, refilling, and tamp- 

 ing. An exhaustive treatise on the theory of loads on pipes in ditches is fol- 

 lowed by the results of actual tests, a comparison demonstrating the correctness 

 and reliability of the theory of loads as developed. 



Standard methods for testing drain tile and sewer pipe are described and 

 specifications for drain tile and sewer pipe and pipe laying are given. The 

 results of Iowa standard tests of over 1,000 specimens of cement and clay 

 pipe, including sizes from 4 to 42 in. internal diameter, are given in tabular 

 form. From these tests the following results and conclusions are stated : 

 The moduli of rupture are often very high for small cement pipe as compai'ed 

 with the transverse strength of ordinary concrete beams several inches thick, 

 but they average somewhat lower than those computed from transverse tests 

 of curved beams cut from the shell of the same pipes. These curved beams 

 show quite a large variation in the values of the modulus of rupture from point 

 to point in the shell. The modulus of rupture is apparently higher for small 

 thicknesses of cement tile than for large thicknesses. On account of the 

 variation in the modulus of rupture with thickness, ordinary mathematical 

 formulas for strength are not reliable for diameters of less than 18 in, in 

 computing the increase in strength of cement pipe which may be secured by 

 increasing the thickness of shells. For diameters of 18 in. and over the in- 

 crease in strength, due to thicker cement pipe shells of the same quantity, 

 should be a little less in proportion than the ratio of the squares of the 

 thicknesses. The modulus of rupture of double strength vitrified clay sewer 

 pipe of the large sizes is often much lower than the modulus of rupture of 

 single strength pipe. 



The absorption limits for drain tile and sewer pipe are given as follows: 

 For farm tile 3 ft. deep, of cement, from 8 to 11 per cent maximum allowable 

 absorption; farm tile 3 ft. deep, of clay, 8 to 16 per cent; large tile drains, 

 of cement, 6^ to 9 per cent ; large tile drains, of clay, 6 to 7 per cent ; and 

 for clay sewer pipe 4 to 5 per cent. 



In many cases drain tile and sewer pipe will break under permanent loads 

 appreciably smaller than the breaking strength developed in laboratory tests 

 in which the entire load is applied within a comparatively short time. Material 

 loss of strength in cement pipe is caused by thorough wetting, as is also the 

 case in some clay pipe. 



A factor of safety of 1.65 is recommended for the required bearing strength 

 of drain tile and sewer pipe. In the case of large pipe and fairly deep ditches, 

 and where tests indicate that the strength of pipe is insufficient to prevent 

 danger of cracking, the engineer should require either pipe of specially high 

 strength or that the pipe be bedded in concrete. In case the ditches have hard 

 bottoms the pipe-laying contractor should be required to shape the bottom of 

 the ditch carefully to fit the lowest 90° of the pipe surface, and to bed care- 

 fully the pipe for this distance in sand or granular soil so as to secure a firm 

 uniform bearing. 



