«A Design Guide for Earth Retaining Structures Hugh Brooks Civil & Structural Engineer John P. Nielsen Civil & Geotechnical Engineer Basics of ...»
When to Use Piles or Piers?
The recommendation to use piles or piers to support a retaining wall will usually come from the geotechnical engineer. Conditions which would suggest using piles include poor or compressible underlying soil, the need for greater lateral resistance, space limitations when a conventional footing may be too large, or other site-specific concerns. Single-row drilled cast-in-place piers, aligned under a retaining wall, are probably more commonly used. Single rows of piers are relatively easy to install, penetrate to better soil, and resist both the vertical and lateral loads imposed from the wall above. With higher walls a double row of staggered piers is common practice. The staggering provides from greater overturning resistance and use of smaller diameter piers. Small implies diameters less than 24”, as opposed to large diameter piers that might be needed for overturning moment or high retaining walls.
Design criteria for piers and piles is usually provided by the geotechnical engineer because IBC ’12, 1803.5.5 requires a foundation investigation for deep foundations “unless sufficient data upon
which to base the design and installation is available”. This investigation generally includes:
recommended type of piles or piers suitable for the site; allowable capacity curves for the various alternates, including lateral design criteria; minimum spacing; driving and installation requirements; testing requirements and related recommendation that include site-specific precautions.
10. PIER AND PILE FOUNDATIONS Page 73 Basics of Retaining Wall Design To aid the geotechnical engineer, the designer should provide the total vertical load imposed by the retaining wall (weight of stem, footing, soil, surcharges, and any additional axial loads) and the total base shear (lateral force imposed by the retaining wall). Using the recommendations of the foundation investigation report the designer can then select the proper size and penetration of the pier or pile, and provide the appropriate specifications, referencing the foundation investigation report. It is important that the owner retain the geotechnical engineer to observe all aspects of the installation for conformance with the recommendations of the geotechnical report.
Pile Design The structural design requirements for piles are covered in IBC ‘12 Chapters 1808 through 1812.
Lateral stability is an essential consideration for any retaining wall. To resist a lateral force piles may be either battered or the lateral force can be resisted by bending in vertically aligned piles. In the latter case, passive and active pressures can be used to determine pile/pier depth.
Consider possible site clearance problems and consult the installing subcontractor for suitability of your design when using battered piles. Generally, a batter flatter than 1(H): 4 (V) should be avoided. Combining lateral pile bending with battered pile resistance is not recommended.
Where multiple piles are used the code requires interconnected lateral restraint at the top of the piles. However for retaining walls this is achieved by the footing which also serves as the pile cap.
Pile Design Example This example assumes the same vertical load and horizontal force as Design Example #1.
Use two rows of piles, space 4 ft. apart laterally, centered under footing, and, say, 8 ft. on center longitudinally.
Reduce footing width to 7 ft. and increase thickness to 24", therefore footing weight about the same.
Description The decision to use either a buttress or counterfort depends up site restraints, such as property line locations, and aesthetics. A “counterfort” wall should not be confused with a “buttressed” wall.
The two are different. A counterfort wall has the stiffening element on the inside of the wall, within the retained earth. See Figure 11.1. A buttress wall has the counterforts on the outside exposed side of the wall. Although most counterfort walls are cast-in-place concrete, masonry can also be used. The design procedures are essentially the same.
Figure 11.1 Typical counterfort wall Proportioning The spacing between counterforts for economical design is usually one-half to two-thirds the wall height.
The width of the footing will usually be about two-thirds the wall height, or larger for surcharges or sloped backfill.
Design Overview The design of a counterfort wall can be somewhat complex because the number of components which must be designed differently than for a conventional cantilevered wall. The steps in the
design of a reinforced concrete counterfort wall are as follows (each step will be discussed later):
1. After establishing the retained height, select a spacing for the counterforts, usually one-half to two-thirds of the retained height. Determine the footing width required and soil bearing at both the toe and heel because you will need these dimensions to establish the counterfort dimensions, and for stability calculations design as if the wall is a continuous cantilevered wall. You can add an estimated weight of the counterforts prorated as a uniform longitudinal axial load.
2. Design the wall as described in the following section as a two-way slab, fixed at the base and at the counterfort crossings and free at the top.
3. Design the footing toe as a cantilever from the wall.
4. Design the heel as a longitudinal beam spanning between counterforts.
5. Design the counterfort. It will be a tapered trapezoidal shaped tension member.
6. Check the final design for stability, overturning, sliding, and soil pressures.
Tilt-up concrete construction is a growing segment of the concrete industry and now accounts for over 50% of all low-rise commercial buildings and about 90% of industrial and warehouse buildings. Tilt-up yard walls, trash area enclosures, dock walls, and retaining walls are now commonplace and the use of this technique can be advantageous for retaining walls in general.
This method is particularly advantageous for long walls allowing repetitive use of panels.
The primary advantage of the use of tilt-up concrete is speed of construction and the elimination of expensive formwork necessary for cast-in-place walls. However, because a crane is necessary during erection, and because a casting bed is required, provision must be made for stacking panels on the site. Connections must also be made for joints between panels.
After preparing a 3” to 4” thick concrete casting slab (later discarded), edge forms are set, a bond breaker is sprayed on the bed to prevent bonding of the wet concrete to the casting bed, reinforcing is placed, and the concrete for the wall is placed. To save casting area, panels can be stacked on top of each other, separated by a bond breaker, up to five or six slabs high as desired.
Unique to using tilt-up panels for free-standing or retaining walls, a trench for the foundation is first excavated and the panels set on temporary concrete setting blocks and the panel is temporarily braced. Dowels project from the bottom of the panels into the footing excavation to provide a moment connection after the concrete is placed.
These retaining walls are usually constructed by the homeowner or landscape contractor and rarely exceed three or four feet soil retention. These consist of wood posts embedded into the soil a sufficient depth to restrain the lateral soil pressure imposed by wood lagging spanning between the posts. The design is often based upon do-it-yourself books for wood retaining walls. Wood retaining walls are advantageous for economical construction of low walls (about five feet maximum earth retention). Such walls do require excavation into the uphill side and the low side of the wall can be used for planting. An illustration of a wood retaining wall is shown in Figure 13.1.
Calculating Lateral Pressures To design the horizontal lagging, then the cantilevered support posts, the lateral soil pressure can be determined using the Rankine equation as described in Chapter 5. For example, the lateral pressure at a depth of three feet, with a soil density of 110 pcf, phi angle of 34°, and a level backfill, would be 94 psf acting horizontally at the lower most lagging. The lagging at that depth would be designed for that lateral force along the entire span between posts. The code prescribed minimum lateral earth pressure for a level backfill is 30 psf/ft, but increases if a sloped backfill and for different soil types. See IBC ‘12, Table 1610.1.
Lagging Design Lagging usually consists of planks with a nominal thickness of 2”, 3”, 4”, or 6”. Thicker planks are generally not economical but may be necessary for higher walls. Allowable stresses are based upon the species selection and given in National Design Standards (NDS), 2005 Edition. All stresses should be based upon long-term loading and wet conditions of use. Spacing between posts is usually determined by how far a 3” plank for a given depth will span – or 6” if necessary.
For the above example, a 3” x 12” plank (dressed dimensions 2-1/2” x 11-1/4”) has a section modulus (weak axis) of 11.7. in3. Assuming an allowable stress of 900 psi this plank could safely Page 87
13. WOOD RETAINING WALLS
14. GRAVITY WALLS Overview Gravity walls depend upon bulk weight for stability, as opposed to a cantilevered retaining wall fixed to a foundation. Some of the many types of gravity retaining walls were described in Chapter 1. Most gravity retaining walls are relatively low, such as used in landscaping, and do not require engineering per se – the design is intuitive to the astute builder. Most landscaping walls do not have a footing, rather are founded on a gravel base.
Note that retaining walls not over four feet from bottom of footing to retained height, and if without a surcharge, do not require a building permit per IBC ‘12, 105.2(4).
Gabion walls, crib walls, and large-block gravity walls are discussed in Chapter 15.
The design of the more common types of gravity walls composed of rubble, stones, and mass concrete is discussed in this chapter.
Design Procedure The design of a gravity retaining wall of concrete or bonded (mortar/grout) stone involves seven
1. Calculate the dead weight of the wall, including all components and any superimposed surcharge or axial load, plus tributary earth weight over the base.
2. Based upon (1) compute the resisting moment about the front edge of the base.
3. Determine the lateral soil pressure and its line of action. The Coulomb Equation (see Chapter
5) should be used because it includes backfill slope, batter of the wall, and the soil friction angle at the wall interface. You may consider the use of the vertical component of the active pressure, which is assumed to act vertically at the back edge of the wall footing. The line of action for the resultant lateral force is assumed to be the wall friction angle plus the inclination angle of the wall batter. Alternatively use the Rankine equation with the force diagram in Figure 14.1.
4. Check stability by computing overturning moment, resisting moment (per above), and determine factor of safety (1.5 minimum).
5. Check that soil bearing is within code allowable.
6. Check sliding. Coefficient of friction is generally 0.25 to 0.40. If soil is clay, cohesion would control.
7. Verify that little or no flexural tension exists in the wall. Check at several locations by calculating the section modulus of the wall and lateral moment at each selected height.