ART. 39. VITRIFIED PIPE SEWERS

Salt-glazed vitrified clay sewer-pipes that meet standard specifications are non-porous, not attacked by acids, smooth of surface, and have sufficient strength to resist crushing under ordinary conditions. The so-called "double-strength" pipes are somewhat thicker than the standard and are recommended where there is any probability of heavy pressure. (The doublestrength pipe have a thickness of shell the diameter.) Some engineers, however, question whether pipe more than 30 or 36 inches in diameter have sufficient strength against crushing. But the best quality of double-strength pipe of any size made is probably amply strong for ordinary circumstances.

In many instances where vitrified clay pipe has been crushed in the ground it has been found that this was probably due to the fact that the pipe had a bearing on the bottom at only one or two points instead of along its entire length, or that stones or frozen earth were thrown upon it in back-filling. If earth is well tamped under and around a vitrified clay pipe it will not usually collapse, even when broken, although it may leak. Such pipe ordinarily breaks along four lines-at top, bottom, and each side-into pieces of almost equal size. For this reason fire-cracks and slight imperfections which do not cause the rejection of a pipe should be placed at a point about 45 degrees below the horizontal in laying, and not at the top.

Tests made by the Brooklyn, N. Y., Bureau of Sewers on several hundred pipes gave an average breaking load of about 350 pounds per foot of length for each inch diameter, the minimum ranging from about 200 to 300 for the different sizes. In these, the pipes were bedded carefully in a box of sand and the pressure applied through a strip of wood placed along the top.

The exact amount of pressure brought to bear upon a sewer by back-filling is uncertain. For a few feet of depth it probably bears the entire weight of the earth immediately above it. With granular material the proportion of pressure to weight of backfilling probably decreases but little, while with other soils it decreases more or less rapidly after the depth equals the width of

the trench. But it is probable that, while clayey material gives an almost vertical pressure, sand acts more as a fluid, pressing normally to the surface of the sewer, and is not so liable to crush it. Little, however, is known on this point. If the depth of covering is small, there is danger that outside weight from roadrollers or even heavy wagons may crush it. But this danger appears to be very slight when the depth of covering equals or somewhat exceeds double the width of trench.

After several years of experimental research on this subject, the Engineering Experiment Station of Iowa State College developed a theory of the pressure received by a sewer in a trench from the back-filling, which is expressed by the formula W = CFB2, in which W is load on pipe in trench, in pounds per lineal foot; C is a coefficient; F is weight of trench-filling material, in pounds per cubic foot, and B is breadth of ditch, in feet, measured the H diameter of the sewer below its top. C varies with the ratio

B'

in which H is the height of fill above top of pipe. Values of C H

for the most commonly used values of

diagram, Fig. 26.

can be taken from the

B

The maximum load, using clay at 130 pounds per cubic foot, trench 2 feet wide at the pipe and 8 feet deep above the pipe, would be 1380 pounds per lineal foot. Of twenty 12-inch standard-thickness vitrified clay pipe tested, the weakest cracked under a load of 2230 pounds per lineal foot, and the average cracking load was 2890 pounds. A factor of safety of 1 is recommended, which would give a safe load for such pipe of 1927 pounds, which is the calculated load from the heaviest back-fill in a trench 2 feet wide and 15 feet deep. If the trench is deeper or wider, double-strength pipe should be used. Similar tests of standard-thickness 18-inch pipe gave a safe load for such pipe of 2050 pounds per lineal foot, and for 24-inch pipe, 2100 pounds. The last would be sufficient for only 8 feet depth (above the pipe) of a 3-foot trench; while double-strength pipe would suffice for about 13 feet depth, both assuming the heaviest saturated clay back-filling.

If sheeting is left in thetrench, but not the rangers and braces, the pressure on the pipe may be increased 8 to 15 per cent.

Additional pressure upon the pipe is caused by loads upon the surface of the back-filling, as of vehicles. The greatest propor

0

1

2 3

4

For saturated clay

Ordinary May, for clay

Mas, for saturated top soil
Max, for sand and gravel

Min. for granular materials without cohesion

FORMULA, W = CWB2

W=Weight on pipe in pounds per foot of length.

C-Coeficient from diagram.

w=Weight of filling material in lbs. per cu. ft.

B= Breadth of ditch a little below top of pipe, in ft.

INSTRUCTIONS FOR USING DIAGRAM

1. Determine H & B by field measurement and calculate

H

2. Select the most probable value of C by inspection of diagram.

3. Measure the value of w, or assume 120 lbs; per cu. ft. for "safe loads" 4. Calculate W by the formula.

5

6 7

8

9

1 1

ட 1 L 1

10 11 12 13 14 15 Ratio,Height of Fill above Pipe to Breadth of Ditch a little Below Top of Pipe.

FIG. 26.-DIAGRAM FOR COEFFICIENT IN LOAD FORMULA.

tions of such loads that reach the pipes are estimated, from the Iowa State College experiments, to be about as follows:

GREATEST PROPORTIONS OF SURFACE LOADS ON BACK-FILLING THAT REACHES PIPE IN TRENCHES

"Three-point" Bearings

Thus, for a trench 2 feet wide and 8 feet deep, a wheel carrying a load of 1 ton would transmit to the buried pipe about 240 to 280 pounds, depending upon the nature of the soil. (A pavement, especially one with a concrete base, would presumably relieve the back-filling of much or all of this pressure.)

The joints of vitrified clay pipe sewers are generally made of the bell-and-spigot pattern, as shown in Fig. 28. Other joints, including the ring joint, have been used, but none during recent years to the author's knowledge. The bell-and-spigot joint is made by filling the annular space between bell and spigot with a material that should be durable, watertight and not too expensive nor too difficult to apply properly. Cement mortar is most 'commonly used, generally one part Portland cement to one part (sometimes two parts) of sand. This is applied by hand, rubber gloves being used, or with trowels. The latter is not recommended, as it is difficult to obtain a perfectly filled joint by troweling. A twisted strand of oakum or similar material is generally placed at the back of the joint (often soaked in cream of cement before use) to hold the spigot so elevated above the bell as to bring the inverts of the two pipes to the same grade, and to completely fill the joint space and prevent the mortar from being squeezed into the inside of the pipe. Joint makers are apt to slight che bottoms of joints-the most important part-because they are out of sight and sometimes difficult to get at. Water rising in the trench before the mortar has set will soften the mortar, causing it to slide out of the joint. Sand, mud or other dirt is apt to find its way into the joint after the pipe is set but before the joint is made. These must all be guarded against by careful inspection and use of experienced and conscientious joint makers.

Some engineers have practiced tying a band of cheesecloth around each mortar joint to prevent it from sliding out of the bell. Special forms have been used that permit pouring cement grout into the joint, thus insuring that it is completely filled at the bottom. More common is the use of material other than cement, with the aim of more surely obtaining watertight joints. Pitch pine tar and cement kneaded together has been