Masonry Magazine April 1966 Page. 27
construction
Place footings for retaining walls on firm, undisturbed soil. In areas where freezing temperatures are expected, place the base of footings well below the frost line.
Where soil under the footing consists of soft or silty clay, place 4 to 6 inches of well compacted, sharp grained sand, or sand and gravel, under the footing before pouring. Where backfill exceeds 6 feet in height, provide a key under the base of the footing to resist the tendency of the wall to slide horizontally. Dowels must be provided to connect the wall to the footing. Locate them at vertical wall rods and as near to the back (tension face) of the wall as possible to assure maximum reinforcement efficiency. Suggestion: place a small longitudinal rod along the dowel line near the top of the footing so dowels can be accurately spaced and securely tied in the correct position.
Lay the first course of block on the footing in a full mortar bed. Remaining courses may then be laid with mortar coverage on vertical and horizontal face shells only, and on cross-web of cells to be grouted.
Take care in laying up the wall to preserve at least 2 inches of unobstructed continuity of the hollow cells in which reinforcement will be placed. Since mortar extruding from the joint into the cell spaces will restrict the flow of the grout, remove this mortar as each unit is placed, or break free with a long rod and remove through clean out openings at the base before attempting to fill the cells, (see figs. 1 and 3). If mortar protrusions are not removed, the grout material tends to bridge at these locations and an excessive amount of puddling will be required to assure complete filling of the cavity. Metal lath may be placed over cells not containing reinforcement to prevent filling with grout.
It is necessary to provide some horizontal steel reinforcement to distribute stresses which occur when the wall tends to expand or contract. If bond beams are used, (figs. 1 and 4), the steel is placed in the beams as the wall is constructed. If desired, horizontal joint reinforcement may be placed in each joint (8 o.c.) and the bond beams omitted. Temperature reinforcement indicated in these designs should prove satisfactory in most areas.
Vertical reinforcement is usually secured in place after the masonry work has been completed and before grouting. Be sure to accurately position and tie at vertical intervals not exceeding 160 bar diameters, (see fig. 5).
Allow masonry walls to set at least 24 hours after completion before grouting. Pour grout in layers of 4 feet allowing one hour between successive layers. Compact grout thoroughly by puddling or vibration.
Provision should be made to prevent accumulation of water behind a retaining wall. Water accumulation causes increased soil pressures, seepage, and in areas subject to frost action, expansive forces of considerable magnitude near the top of the wall. In most instances, four inch diameter weep holes located at 5 to 10 foot spacing along the base of the wall should be sufficient, (see fig. 2a). Place about one cubic foot of gravel or crushed stone around the intake of each weep hole. Where heavy prolonged rains are anticipated, a continuous longitudinal drain, (fig. 2b), along the back of the wall may be used in lieu of weep holes.
Climate, soil conditions, and type of construction will determine the need for waterproofing the back face of the retaining walls. Waterproofing is recommended: (1) in areas subject to severe frost action, (2) in areas of heavy rainfall, (3) when backfill material is relatively impermeable, and (4) when wall is constructed of porous masonry units. Of course, the top of masonry retaining walls should be capped or otherwise protected to prevent the entry of water into unfilled hollow cells and spaces.
Long retaining walls should be broken into panels 20 to 30 feet in length by means of vertical control joints. Joints should be designed to resist shear and other lateral forces while permitting longitudinal movement, (see fig. 6).
Take care in backfilling against retaining walls. Backfilling should not be permitted until at least 7 days after grouting. Where heavy equipment is used in backfilling walls designed to resist earth pressure only, such equipment should not approach closer to the top of the wall than a distance equal to the height of the wall. Care should also be taken to avoid exerting large impact forces on the wall as could be caused by a large mass of moving earth.