GENERAL PRINCIPLES OF DESIGN. 233 



weight of the masonry, the width of the foundation, and the cross-section of the wall. The most 

 economical section for a masonry retaining wall is obtained when the back slopes toward the 

 filling. In cold localities, however, this form of section may be displaced by heaving due to the 

 action of frost, and it is usual to build retaining walls with a slight batter forwards. The front of 

 the wall is usually built with a batter of from i in. to I in. in 12 in. In order to keep the center 

 of gravity of the wall back of the center of the base it is necessary to increase the width of the 

 wall at the base by adding a projection to the front side. Where the wall is built on the line 

 of a right of way it is sometimes necessary to increase the width of the base by putting the pro- 

 jection on the rear side, making an L-shaped wall. The weight of the filling upon the base and 

 back of the wall adds to the stability of the wall. Where the wall is built to support an em- 

 bankment expensive to excavate, it is often economical to make the wall L-shaped, with ah the 

 projection on the front side. 



In calculating the thrust on retaining walls great care must be exercised in selecting the 

 proper values of w and </>, and the conditions of surcharge. It will be seen from the preceding 

 discussion that the value of the thrust increases very rapidly as decreases, and as the surcharge 

 increases. Where the wall is to sustain an embankment carrying a railroad track, buildings, 

 or other loads, a proper allowance must be made for the surcharge. 



The filling back of the wall should be deposited and tamped in approximately horizontal 

 layers, or with layers sloping back from the wall; and a layer of sand, gravel or other porous 

 material should be deposited between the filling and the wall, to drain the filling downwards. 

 To insure drainage of the filling, drains should be provided back of the wall and on top of the 

 footing, and "weep-holes" should be provided near the bottom of the wall at frequent intervals 

 to allow the water to pass through the wall. With walls from 15 to 25 ft. high, it is usual to use 

 "weepers" 4 in. in diameter placed from 15 to 20 ft. apart. The "weepers" should be connected 

 with a longitudinal drain in front of the wall. The filling in front of the wall should also be 

 carefully drained. 



The permissible point at which the resultant thrust may strike the base of the foundation 

 will depend upon the material upon which the retaining wall rests. When the foundation is 

 solid rock or the wall is on piles driven to a good refusal, the resultant thrust may strike slightly 

 outside the middle third with little danger to the stability of the wall. When the retaining wall, 

 however, rests upon compressible material the resultant thrust should strike at or inside the center 

 of the base. Where the resultant thrust strikes outside of the center of the base, any settlement 

 of the wall will cause the top to tip forward, causing unsightly cracks and local failure in many 

 cases, and total failure where the settlement is excessive. Where extended footings are used it 

 may be necessary to use some reinforcing steel to prevent a crack in the footing in line with the 

 face of the wall. 



Plain masonry walls should be built in sections, the length depending upon the height of the 

 wall, the foundation and other conditions. 



Under usual conditions the length of the sections should not exceed 40 ft., 30 ft. sections 

 being preferable, and in no case should the length of the section exceed about three times the 

 height. Separate sections should be held in line and in elevation, either by grooves in the masonry 

 or by means of short bars placed at intervals in the cross-section of the wall, fastened rigidly in 

 one section and sliding freely in the other. The back of the expansion joints should be water- 

 proofed with 3 or 4 layers of burlap and coal tar pitch. The burlap should be about 30 in. wide, 

 and the pitch and the burlap should be applied as on tar and gravel roofs. The joints between 

 the sections of a retaining wall on the front side should be from fc to J of an in. in width, and 

 should be formed by a V-shaped groove made of sheet steel and fastened to the forms while the 

 concrete is being placed. Where there is danger of the water in the filling percolating through 

 the wall or in an alkali country, the surface of the back of the wall should be coated with a water- 

 proof coating. The most satisfactory waterproof coating known to the author is a coal tar 

 paint made by mixing refined coal tar, Portland cement and kerosene in the proportions of 1 6 

 parts refined coal tar, 4 parts of Portland cement and 3 parts of kerosene oil. The Portland 



