|Figure 2.10 On a spacious site, an excavation can |
be benched. When excavating close to property lines or nearby buildings, some
form of slope support, such as sheeting, is used to retain the soil around the excavation.
The most common types of slope sup-port, or shoring, are soldier beams and lagging, and sheet piling. With soldier beams and lagging, steel columns called H-piles or soldier beams are driven vertically into the earth at small intervals around an excavation site before digging begins. As earth is removed, the lagging, usually consisting of heavy wood planks, is placed against the flanges of the columns to retain the soil outside the excavation (Figures 2.11 and 2.12). Sheet piling or sheeting consists of vertical planks of wood, steel, or precast concrete that are placed tightly against one another and driven into the earth to form a solid wall before excavation begins (Figures 2.13 and 2.14). Most often, shoring is temporary, and it is removed as soil is replaced in the ex-cavation. However, it may also be left in place to become a permanent part of the building’s substructure. This may be necessary, for example, where shoring is located extremely close to a property line and there is no practical way to remove it after completion of construction without disturbing adjacent property or structures.
|Figure 2.11 Soldier beams and lagging, seen in horizontal section.|
|Figure 2.12 Soldier beams and lagging. Lagging planks are added at the bottom as excavation |
proceeds. The drill rig is boring a hole for a tieback to brace a soldier beam.
|Figure 2.13 Horizontal sections through three types of sheet |
piling. The shading represents the retained earth.
|Figure 2.14 Drilling tieback holes for a wall of steel sheet |
piling. Notice the completed tieback connections
to the horizontal waler in the foreground. The
hole in the top of each piece of sheet piling
allows it to be lifted by a crane.
Slope support may also take the form of pneumatically applied concrete, also called shotcrete, in which excavation proceeds first and then the sloped sides are reinforced with a relatively stiff concrete mixture sprayed directly from a hose onto the soil.
This method works well where the soil is sufficiently cohesive to hold a steep slope at least temporarily.
The hardened concrete reinforces the slope and protects against soil erosion. (Figure 2.15).
|Figure 2.15 Where slope support turns the corner in this |
excavation and the soil can be sloped at a lesser
angle, less expensive shotcrete takes the place of
soldier beams and lagging.
A slurry wall is a more complicated and expensive form of excavation support that is usually economical only if it becomes part of the permanent foundation of the building. The first steps in creating a slurry wall are to lay out the wall’s location on the surface of the ground with surveying instruments and to define the location and thickness of the wall with shallow poured concrete guide walls (Figures 2.16 and 2.17). When the formwork has been removed from the guide walls, a special narrow clamshell bucket, mounted on a crane, is used to excavate the soil from between the guide walls. As the narrow trench deepens, the tendency of its earth walls to collapse is counteracted by filling the trench with a viscous mixture of water and bentonite clay, called a slurry, which exerts pressure against the earth walls, holding them in place. The clamshell bucket is lowered and raised through the slurry to continue excavating the soil from the bottom of the trench until the desired depth has been reached, often a number of stories below the ground.
Slurry is added as required to keep the trench full at all times.
|Figure 2.17 Constructing a slurry wall. (a) The guide walls are formed and poured in a shallow trench. (b) The narrow clamshell bucket discharges a load of soil into a waiting dump truck. Most of the trench is covered with wood pallets for safety.|
Meanwhile, workers have welded together cages of steel bars designed to reinforce the concrete wall that will replace the slurry in the trench. Steel tubes whose diameter corresponds to the width of the trench are driven vertically into the trench at predetermined intervals to divide it into sections of a size that can be reinforced and concreted conveniently. The concreting of each section begins with the lowering of a cage of reinforcing bars into the slurry. Then concrete is poured into the trench from the bottom up, using a funnel-and-tube arrangement called a tremie. As the concrete rises in the trench, it displaces the slurry, which is pumped out into holding tanks, where it is stored for reuse. After the concrete reaches the top of the trench and has hardened sufficiently, the vertical pipes on either side of the recently poured section are withdrawn from the trench, and the adjoining sections are poured. This process is repeated for each section of the wall. When the concrete in all the trenches has cured to its intended strength, earth removal begins inside the wall, which serves as sheeting for the excavation.
In addition to the sitecast concrete slurry wall described in the preceding paragraphs, precast concrete slurry walls are built. The wall is pre-stressed and is produced in sections in a precasting plant (see Chapter 15), then trucked to the construction site. The slurry for precast walls is a mixture of water, bentonite clay, and portland cement. Before a section is lowered by a crane into the slurry, its face is coated with a compound that prevents the clay–cement slurry from adhering to it (Figure 2.18). The sections are installed side by side in the trench, joined by tongue-and-groove edges or synthetic rubber gaskets. After the portland cement has caused the slurry to harden to a soillike con-sistency, excavation can begin, with the hardened slurry on the inside face of the wall dropping away from the coated surface as soil is removed.
|Figure 2.18 Workers apply a nonstick coating to a |
section of precast concrete slurry wall as
it is lowered into the trench.
|Figure 2.19 Soil mixing.|
Soil mixing is a technique of adding a modifying substance to soil and blending it in place by means of augers or paddles rotating on the end of a vertical shaft (Figure 2.19). This technique has several applications, one of which is to remediate soil contaminated with a chemical or biological substance by blending it with a chemical that renders it harmless. Another is to mix portland cement and water with a soil to create a cylinder of low-strength concrete in the ground. A linear series of these cylinders can serve as a cutoff wall against water penetration or as excavation support (Figure 2.20). Soil mixing can also serve to stabilize and strengthen areas of weak soil.
All forms of slope support and excavation support must be braced against soil and water pressures as the excavation deepens (Figure 2.21).
the face of the sheeting. Where the excavation is too wide for crosslot bracing, sloping rakers are used in-
stead, bearing against heel blocks or other temporary footings.
Both rakers and crosslot bracing, especially the latter, are a hindrance to the excavation process. A clam- shell bucket on a crane must be used to remove the earth between the braces, which is much less effificient and more costly than removing soil with a shovel dozer or backhoe in an open excavation.
Where subsoil conditions permit, tiebacks can be used instead of braces to support the sheeting while main- taining an open excavation. At each level of walers, holes are drilled at in-tervals through the sheeting and the surrounding soil into rock or a stra-tum of stable soil. Steel cables or ten-dons are then inserted into the holes, grouted to anchor them to the rock or soil, and stretched tight with hy-draulic jacks (posttensioned) before they are fastened to the walers (Figure 2.22 , Figure 2.24).
|Figure 2.24 Slurry wall and tieback construction used |
to support historic buildings around a
deep excavation for a station of the Paris Metro.
Excavations in fractured rock can often avoid sheeting altogether, ei- ther by injecting grout into the joints of the rock to stabilize it or by drilling into the rock and inserting rock anchors that fasten the blocks together (Figure 2.25).
In some cases, vertical walls of particulate soils can be stabilized by soil nailing. A soil nail is similar to a
rock anchor: It is a length of steel re- inforcing bar that is inserted into a nearly horizontal hole drilled deep into the soil. Grout is injected into the hole to bind the soil nail to the surrounding soil. Large numbers of
closely spaced nails are used to knit a large block of soil together so that it behaves more like weak rock than particulate soil.
Bracing and tiebacks in excavations are usually temporary. Their function is taken over permanently by the floor structure of the basement levels of the building, which is designed specifically to resist lateral loads from the surrounding earth as well as ordinary floor loads.
|Figure 2.25 Rock anchors are similar to tiebacks but |
are used to hold jointed rock formations
in place around an excavation.