«A Design Guide for Earth Retaining Structures Hugh Brooks Civil & Structural Engineer John P. Nielsen Civil & Geotechnical Engineer Basics of ...»
Pilaster Design Pilasters are usually 16" by 16" masonry units, or smaller for lower walls and usually spaced 6’ to 8’ apart. Use conventional procedures for the design. Lateral load reaction to the pilasters from
Retaining walls are broadly defined as either yielding or non-yielding. The former refers to cantilevered walls, which are free to rotate, thereby allowing a lateral displacement at the top which activates the soil wedge concept upon which both Rankine and Coulomb theories are based.
Non-yielding walls are restrained at the top to prevent movement and therefore generate a reaction at the top and reduce moments at the base of the wall. A typical restrained, non-yielding, wall is the so called “basement wall”. The designer must assess whether the wall really is “restrained” at the top against lateral movement. Wood diaphragms may be too flexible.
Lateral restraint at the top can also be accomplished using tie-backs, also called anchored walls, which are another example of restrained non-yielding walls. These walls use drilled and grouted anchors placed into the backfill as the wall is constructed to provide lateral restraint. If multiple levels of lateral restraint are required, such as for a multi-level structure, the design becomes complex due to varying wall moments, shears and reactions. Tie-back forces can also be affected by earth movement.
Dual Function Walls
Often it is desirable to prepare two designs for the same wall. For example a basement wall may be backfilled before the lateral restraint at the top (such a floor or roof diaphragm) is in place. It can first be designed as a conventional cantilever wall as for an assumed depth of backfill, and perhaps lessening the factors of safety because of a temporary condition. This would require a larger footing for overturning and result in a larger moment at the stem base. Then a second design for the final condition when the top restraint is in place and backfill completed. Then you’ve covered both conditions, but only if the contractor placed the backfill to meet your design/soil placement assumption.
If the bottom of a basement wall is fixed at the footing, and assuming a triangular earth pressure against the wall, the base moment will be about one-half the pin-pin positive moment, and the positive moment if fixed at the bottom will reduce to about one-quarter the pin-pin positive moment condition.
“At Rest” Active Soil Pressure If a wall is restrained from movement at the top and therefore the sliding-wedge active pressure cannot be mobilized, the lateral soil pressure is somewhat higher. This is termed the “at rest” pressure, (designated Ko) and is applicable to a wall rigidly restrained at the top, such as a basement wall (but light framing with a flexible diaphragm may be inadequate “restraint” and the active soil wedge may be activated). The at-rest soil pressure is: Ko = 1 – sin , where is the angle of internal friction. For example, if = 34°, Ka = 0.44, as opposed to Ka = 0.28 (assuming level backfill). For sloping backfill, a suggested equation is Ko = (1 – sin )/(1 + sin β).
For a well-drained granular soil, a typical value for Ko = 0.50. For a saturated sandy soil the density could be 125 pcf giving a lateral pressure of 0.5 (125 – 62.5) + 62.4 = 93.7 pcf. Clayey soil can be higher. Some agencies require Ko = 1.0, giving 110 pcf for a soil density of 110 pcf.
ASCE 7-10 specifies a minimum of 60 pcf for “relatively rigid” walls, and states that basement
Description Soldier beam retaining walls are used to temporarily retain soil, such as at a construction site.
They can also serve as permanent retaining walls as shown in Figure 21.3. This concept is illustrated in Figure 21.1. Steel HP (wide flange) beams are driven into the soil a sufficient embedment depth to resist by passive pressure the moment imposed by the retained soil. The soldier beams (also called soldier piles) are usually spaced from six to eight feet apart and can also be dropped into pre-drilled holes and encased in lean concrete. Soldier beams are usually cantilevered, but if space is available, and for retained heights over about 15 feet, tiebacks can be used to reduce the beam size and depth of embedment.
As excavation proceeds on the down-grade side, wood lagging is placed horizontally to support the retained soil. Lagging is supported at their ends by the beam outer flanges.
Design Procedures Consult with the geotechnical engineer for design criteria. This information will include nature of the soil, phi angle, soil density, active and passive allowable pressures, arching factors to use, and any other site-specific recommendations. It is advisable to also consult with the contractor to verify the most economical beam selection and any other concerns he or she may have.
There are numerous design methodologies used and most foundation engineering textbooks propose various design approaches. This text selected a relatively simple procedure which is often used.
This procedure assumes non-cohesive (sandy) soil. If the soil is clay a different passive resistance diagram will apply and the geotechnical engineer should be consulted. It should be noted that although clay is usually assumed to have a zero phi angle, it actually can vary in a range from 6° to 12° or more.
The above photo is a rare occurrence. No building permit, not engineered, minimal reinforcing in ungrouted cells, and other oversights.
“Failure” of a retaining wall does not necessarily mean total collapse as shown above, but rather local signs of impending instability and likelihood of a total collapse. Total collapses are relatively rare. In a total collapse the wall overturns, slides, topples, or otherwise causes a massive letting loose of the retained earth with resulting damage above and below the wall. Such walls cannot be saved – the remedy is rebuilding. The engineer who provided this photo was retained to investigate the deficiencies causing the collapse and to design a new wall.
Fortunately, retaining walls are quite forgiving, nearly always display telltale signs of trouble alerting an observer to call for professional evaluation before a collapse. After an evaluation, and determination of the causes, most walls can be saved.
The most common sign of distress is excessive deflection of the wall – tilting out of plumb – caused by a structural overstress and/or a foundation problem. Some structural deflection is to be expected and a ruleof-thumb is 1/16th inch for each foot of height, which is equivalent to one-half inch out-of-plumb for an eight foot high wall. More than that is suspect. It’s easy to check with a plumb bob.
Here are Twelve Things That Can Go Wrong and Signal Distress:
1. Reinforcing not in the right position. If the stem shows sign of trouble (excessive deflection and/or cracking) the size, depth, and spacing of the reinforcing should be verified. Testing laboratories have the devices (usually a magnetic field measuring Pachometer) which can locate reinforcing and depth with reasonable accuracy, up to about 4 inches depth. For exact verification you can first locate the reinforcing then chip out to determine its exact depth and bar size. More elaborate devices are also available if needed – check with your testing laboratory, they’ll come to you jobsite. Unbelievably, cases have occurred where the reinforcing was placed on the wrong side of the wall, either through a detailing error, or contractor error. When the actual reinforcing size, location, and spacing are determined, and perhaps a core taken to verify strength of stem material, a design can be worked backwards to determine actual design capacity and thereby guide remedial measures.
22. WHY RETAINING WALLS FAIL & COST EFFECTIVE FIXES
23. CONSTRUCTION TOPICS AND CAVEATSVertical Control Joints Vertical joints along the length of the wall are intended to control cracking and are largely a matter of judgment. Shrinkage in a wall cannot be eliminated. As the adage goes, concrete shrinks and ice cream melts, or “if it ain’t cracked, it ain’t concrete.” We can attempt to control where the cracks form by forming crack control joints and by increasing the horizontal reinforcing. With a little more than minimum reinforcing there are few reports of problems when control joints are 100 feet or more for masonry, and somewhat less for concrete. The more horizontal reinforcing, the less likely vertical cracks will be obvious, and the further apart joints may be spaced. In the case of a concrete wall, a ratio of 0.002Agross is suggested; for masonry 0.0013Agross is suggested (#5 bars at 32" o.c. for an 8" CMU wall).
Vertical joints for both concrete and masonry should be “cold joints”, allowing for movement, but it is suggested that some horizontal dowels extend into the adjacent wall to assure out-of-plane alignment. Usually one end of horizontal dowels are wrapped, sleeved, or greased to prevent bonding.
Improper drainage causing water seepage into the backfill is the leading cause of retaining wall problems. Lateral earth pressure design is usually based upon drained soil. Saturated soil can substantially increase pressures. Therefore it is important to have weep holes at the base of the wall for any percolating water to escape. In concrete walls drain holes are 3" to 4" in diameter to facilitate cleaning and spaced five or six feet on center. Gravel should be placed along the inside base for any water to freely flow, otherwise the only thing coming out of a weep hole will be grass.
“Weep holes” in masonry walls can be provided by leaving the head joints open at alternate blocks (no mortar in end joints at 32" on center).
In lieu of weep holes, or for basement type walls, horizontally placed perforated Sch 40 pipe should be laid along the base of the heel adjacent to the stem, slopped to an outlet, and encased in a generous amount of coarse gravel. It is also recommended to lay a filter fabric over the gravel to keep out soil fines.
The most important drainage control is to keep water off the top slope as much as possible. This can be done by slope control, paved swales, paving, or other means. Preventing water from entering the backfill is critical important because it changes the soil characteristics and increases lateral pressures.
Backfill Backfill material should be sandy non-cohesive material. Clayey soil are to be avoided because clay swells when wet, causing additional lateral pressure. An excellent practice is to fill the soil wedge with gravel.
Compaction Compact the gravel behind the wall with care. You don’t want settlement to occur later. Place the gravel in layers about one foot thick and start compacting at the face of the wall and work away from the wall. Gravel is best compacted with multiple passes of a vibrating plate compactor.
Inspections If a consultant was employed, he or she will verify that the footings are excavated into the anticipated soil and indicate any corrections deemed necessary. They can also approve the backfill material.
Placement of reinforcing dowels projecting from the footing into the wall are critical to the design, and the Engineer-of-Record (EOR), or a deputy inspector, should verify that the dowels were properly placed. Several retaining wall failures were attributable to the dowels being on the wrong face of the wall!
Other inspections may be required by the building official, or by the EOR.
The Investigation The geotechnical report for a project will nearly always have recommendations for site preparation (e.g. if fill is present or there is a liquefaction problem) in addition to design criteria information. This investigation report is usually a part of the contract documents and should be carefully reviewed and observed.
If a problem is evident, or suspected, an independent engineer may be retained to investigate the problem. This will involve a review of the design, particularly to determine if the site conditions match the design criteria (e.g. a wall designed to retain eight feet, and actually retaining ten feet).
The plans will be reviewed for clarity and conformance with the design intent and applicable building codes. The wall will be measured, deflection checked, and testing done to determine positioning of reinforcing and material strengths. Cores are often taken to determine both concrete strength and grout penetration into cells. The geotechnical report will be reviewed and perhaps more soil samples recommended.
When the cause of the problem is discovered, the most economical solution acceptable to the owner should be determined. This can be contentious, particularly if opposing parties offer different solutions. Hopefully the issues can be resolved equitably and with civility without resort to litigation. If an impasse, mediation can be a very effective and less costly resolution of a dispute.