Engineering and Construction

If the site is sufficiently larger than the area to be covered by the building, the edges of the excavation can be sloped back or benched at an angle such that the soil will not slide back into the hole. This angle, called the angle of repose, can be steep for cohesive soils such as the stiffer clays, but it must be shallow for frictional soils such as sand and gravel. On constricted sites, the soil surrounding an excavation must be held back by some kind of  slope support or  excavation support capable of resisting the pressures of earth and groundwater (Figure 2.10). Such construction can take many forms, depending on the qualities of the soil, depth of excavation, equipment and preferences of the contractor, proximity of sur-rounding buildings, and level of the water table.

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.

Shoring
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.

Slurry Walls
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 2.16 Steps in constructing a slurry wall.  (a) The concrete guide walls have been installed, and the clamshell bucket  begins excavating the trench through a bentonite clay slurry. (b) The trench is  dug to the desired depth, with the slurry serving to prevent collapse of the walls  of the trench. (c) A welded cage of steel reinforcing bars is lowered into the  slurry. (d) The trench is concreted from the bottom up with the aid of a tremie.  The displaced slurry is pumped from the trench, filtered, and stored for reuse.  (e) The reinforced concrete wall is tied back as excavation progresses.

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.

The primary advantages of a precast slurry wall over a sitecast one are better surface quality, more accurate wall alignment, a thinner wall (due to the structural efficiency of prestressing), and improved watertightness of the wall because of the continuous layer of solidified clay outside.

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.

Figure 2.20 Soil-mixed excavation support.  (a) Excavation proceeds after completion of soil mixing. Bracing  consists of soldier piles, walers, and tiebacks. The soldier piles are inserted  during soil mixing, before the soil/ cement mixture hardens. The walers and  tiebacks are installed later, as excavation progresses. (b) Fully excavated soil- mixed sheeting. This excavation support system must be strong enough to resist  the soil pressures caused by the adjacent  buildings.

Bracing
All forms of slope support and excavation support must be braced against soil and water pressures as the excavation deepens (Figure 2.21).

Figure 2.21 Three methods of bracing shoring drawn  in cross section. The connection between the waler and the brace, raker, or tieback  needs careful structural design. The broken line between rakers indicates  the mode of excavation: The center of the hole is excavated first with sloping  sides, as indicated by the broken line. The heel blocks and uppermost tier of rakers are installed. As the sloping sides are excavated deeper, more tiers  of rakers are installed. Notice how the tiebacks leave the excavation totally free  of obstructions.

Crosslot bracing utilizes temporary steel wide-fl ange columns that are driven into the earth by a piledriver at points where braces will cross. As the earth is excavated down around the sheeting and the columns, tiers of horizontal bracing struts, usually of steel, are added to support walers,  which are beams that span across
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 2.22 Three steps in the installation of a  tieback to a soil anchor. (a) A rotary drill bores a hole through the sheeting  and into stable soil or rock. A steel pipe casing keeps the hole from caving in  where it passes through noncohesive soils. (b) Steel prestressing tendons are  inserted into the hole and grouted under pressure to anchor them to the soil.  (c) After the grout has hardened, the tendons are tensioned with a hydraulic  jack and anchored to a waler.

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.