«A Design Guide for Earth Retaining Structures Hugh Brooks Civil & Structural Engineer John P. Nielsen Civil & Geotechnical Engineer Basics of ...»
Descriptions Gabion walls consist of steel wire baskets filled with rock and stacked as units to form gravity retaining walls. Similar baskets have been used since ancient times and the word “gabion” does not refer to an inventor but rather to Italian and Latin words meaning “cage”. Today, the cages are manufactured, generally, in three foot by three foot by three foot steel wire panel sides which at the job site are unfolded to form a cage, which are filled with rock, tied together, and assembled into the retaining walls. Since mesh openings are generally 3 inches square, the rock infill should be 3 inch to 8 inch clean hard stone. Perpendicular to the plane of the wall the wythes can be 1, 2, 3 or more units deep and can be stacked in successive courses to a height usually not more than about 15 feet.
Note: The masonry term “wythe” means one vertical section of wall one unit in thickness.
Similar in concept, precast large concrete blocks, which are commercially available from a number of vendors and concrete plants, can be laid one or more blocks deep (wythes) and stacked to retain soil to 12 feet or more. Such blocks can be laid with the front exposed side flush or with successive blocks stepped back.
If the front face is flush, it is customarily tilted into the soil about 6° for aesthetics.
Design Methodology The cages are wired together and due to their mass they are considered one rigid cohesive mass for design purposes. Gabion walls are designed or analyzed in the same manner as gravity walls.
Resisting moments are taken about the front lower corner of the first row and overturning moments are applied to the back face using the Coulomb method for calculating Ka. Density of the gabion units is usually taken as 120 pcf. Refer to Figure 15.1 for conceptual example of a flush-face wall.
Overview Segmental block retaining walls (SRWs) are composed of dry-stacked masonry blocks usually manufactured as proprietary products. They have gained wide acceptance for high earth retention condition and are seen everywhere: leaning against hillsides alongside highways, behind shopping centers, providing tiered grade changes for developments, highways, railroads, bridge abutments and other applications. See Figure 16.1.
Figure 16.1 Example segmental block wall Advantages include relatively fast construction; at the footing consists of just a gravel levelling pad, and the units are dry-stacked without mortar, steel reinforcing, or grouting.
The designer has a choice of block sizes, textures, colors and configurations, from a variety of vendors. Retained heights of 40 feet or more can be achieved (using geogrids) far exceeding economical limits of conventional masonry or concrete retaining walls. These do, however, have limitations. If a segmental retaining wall requires geogrids for stability, this requires an available space behind the wall of approximately 70% or greater of the wall height for the placement of the geogrid reinforcement layers. If space is unavailable, a segmental wall is not an option. Buried utility lines or drain lines in the backfill zone may also be constraints for a segmented wall. However, using the segmental blocks as a fascade is also feasible for soil not walls or soldier beam and wood-logging walls.
Segmental walls are of two types: pure gravity walls where stability depends solely upon the resisting moment of the stacked blocks to exceed the overturning moment of the lateral soil pressure. This stability problem limits the height to four or five feet, although some vendors offer larger blocks enabling greater retained heights.
Higher walls, the more common type of segmental walls use layers of geogrids placed in the backfill for soil reinforcement as the wall is constructed. This results in a mass of reinforced soil (also termed Mechanically Stabilized Earth, MSE) which can be used en masse to improve resistance to overturning and sliding. To be effective, each layer must be properly connected to
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the block facing by engaging the geogrid within block joints, and extending behind the wall and beyond the failure plane a distance sufficient for anchorage. The vertical separation between geogrid layers is usually two- to three blocks, but varies with design requirements. The length of the reinforced zone is usually a minimum of 60% to 70% of the wall height.
For many engineers, designing segmental retaining walls is a niche market. Their design can be quite complex, particularly for higher walls using geogrids, and other improvements that are to be accommodated in the geogrid reinforced zone, such as caissons, drain liners, shallow foundations for light weight structures. Consultation with a selected block vendor is recommended and many offer design software.
Segmental Blocks are concrete blocks with compressive strength of 3,000 psi or greater, and, in the US, they are manufactured per proprietary designs at licensed local plants. The blocks come in many choices of texture, color, sizes, and configurations. The blocks vary in size, with the most commonly used blocks being 8-inch high with depths varying from 10” to 24”. The block width for the most commonly used blocks is 18 inches. Blocks with dimensions smaller than these are available for non-engineered landscape applications for retaining heights of about three feet or less. All of these blocks weigh between 30 and 110 lbs each. So called “big blocks” are also available from some vendors, weighing two tons or more and placed by small cranes.
The blocks are designed to allow construction of walls with vertical batter -- angle of the wall face to the vertical -- to as much as over 15 degrees from vertical. To control batter most segmental blocks have offset lips or other means, such as pins between units, to control the offsets as successive courses of blocks are placed.
Angle of wall batter = tan-1 [(offset per block) / (block height)] Most blocks have interior voids which are infilled with granular backfill material. Weight per square foot of wall surface is often assumed to be 130 pcf for both block weight and infill.
All vendors have web sites for more information and technical data. Best source: a Google search for “segmental retaining walls”.
Segmental Gravity Wall Design For segmental gravity walls to be stable, the resisting moment should exceed the overturning moment by a factor of safety of at least 1.5. This limits the height of gravity segmental walls to about four feet, depending upon the batter of the wall and block type. For larger blocks that are in the market, the gravity wall height can be greater.
The design procedure for gravity walls follows these steps:
Select the block vendor for texture, color, size and configuration desired. This is often dependent upon proximity to distributors.
Swimming pools are constructed in a wide variety of shapes, sizes, curvatures and designed to fit a specific terrain and soil conditions. One thing nearly all have in common is shotcrete or Gunite walls and bottoms sprayed over a shaped excavation, and encasing the reinforcing. Plaster or tile is used to provide a smooth, aesthetic finish.
The terms “shotcrete” and “Gunite” are used interchangeably, but the former refers to wet-mix spraying whereas the material is mixed in a hopper before exiting the nozzle, whereas the latter is a “dry-mix” where the material reaches the nozzle dry where water is injected. Shot Crete (we’ll use the generic term) sticks to the earth and self compacts because of the velocity of application, thereby permitting it to be used against vertical surfaces. Shot Crete is covered in IBC ‘12, section 1910.
Designing the walls of a pool is unique because not only does the wall usually curve as it descends, but the strength of the cantilevered wall must resist the greater of earth pressure acting inward with the pool empty, or the water pressure outward if the exterior grade is lower or of poor soil. The design task is made further tedious because of the number of cross sections which must be checked (shallow end, deep end, and intermediate points).
The typical controlling condition is when the pool is empty and earth pressure from the outside governs the design. However, the condition is often reversed, such as for “infinity pools” or architectural features where the outside grade is substantially lower, or slopes downward lessening its lateral support value.
There also may be lateral support from of a surrounding deck at or near the top of the wall. All these conditions must be considered and the walls of the pool designed for the most critical combination of conditions that may occur. Lateral loading from a surcharge or increased soil pressures because of expansive soil must also be considered.
Design of swimming pools is a specialty for some engineers and they have developed software (usually spreadsheets) to make the task less tedious.
The walls and bottom are generally at least 4” thick, generally 5” for floors, and may be more depending upon design requirements. Typically, #3 bars are used because of the relative ease in bending and securing to curved surfaces. Number 4 bars can also be used, but #5 bars are difficult to bend and place.
Shot Crete strength is typically 2500 psi minimum, and a low slump suitable for pumping and spraying.
Minimum reinforcing for flexural members is 200 / fy, = 0.0033 for Grade 60 reinforcing. Thus, for a 4" wall the minimum would be #3 at 9", however, the typical practice pattern is 12" on center each way.
Under slab drainage is recommended on sites with expansive soil with special reinforcement and/or thickened slab required for sites with expansive soil to protect from uplift along the bottom of the shallow end.
The classic method of designing swimming pool walls has been to draw to scale (or CAD generated) a cross section at each location to be investigated. Then divide the wall into segments, usually 12" high.
You can then determine the bending moment and shear at the bottom of each segment by constructing a table (spreadsheet) showing the active pressure from either earth or water acting at the bottom of each segment, and the additive (or deductive) moment due to the vertical weight of the segments above acting at their eccentricity from a reference point. This is illustrated in Figure 17.1. This is a tedious process but yields satisfactory results for design. Reinforcing is usually placed in the center of the wall, but for higher moments thicker walls may be needed and off-center reinforcing as the design may require.
Shown in Figure 18.1 is a retaining wall with spaced pilasters and masonry filler walls. Such walls can be economical for low retaining or freestanding walls. The filler walls, usually 6" or 8" masonry, span horizontally between pilasters and the pilasters cantilever up from the footing.
Filler Wall Design The filler wall spans horizontally between pilasters and those walls usually control the spacing of the pilasters. Freestanding walls are designed for wind and, if applicable, a seismic force. That horizontal reinforcing is placed in the center of the wall because lateral wind and seismic loads can be from either direction. To take advantage of continuity, it may be more economical to place the horizontal reinforcing at the center and design for the controlling positive (mid-span) or negative (at pilasters) moments, generally use w (L)2 / 12.
If the filler wall retains earth, some or all of the courses will be subjected to lateral earth pressures and this controls the thickness of the filler wall. In that case, vertical reinforcing should be placed on the earth face between pilaster supports. Reduce reinforcing higher up the wall as moment decreases. The first step would be to determine the lateral pressure at the base of the wall, as select a wall thickness and vertical reinforcing, and then the reinforcing to span between pilasters.
A minimum amount of horizontal and vertical reinforcing should be used. The combined total area should be.0002bd, with not less than.0007 in either direction. Vertical reinforcing is often #4 bars at 32" o.c. or 48" o.c.