Excavation for Building Foundation

At least some excavation is required for every building. Organic topsoil is subject to decomposition and to shrinking and swelling with changes in moisture content. It is excellent for growing lawns and landscape plants but unsuitable for supporting buildings.

Often, it is scraped away from the building area and stockpiled to one side for redistribution over the site after construction of the building is complete. After the topsoil has been removed, further digging is necessary to place the footings out of reach of water and wind erosion.

In colder climates, foundations must be placed below the level to which the ground freezes in winter, the frost line, or they must be insulated in such a way that the soil beneath them cannot freeze. Otherwise, a foundation can be lifted and damaged by soil that expands slightly as it freezes.

Or, under certain soil and temperature conditions, upward migration of water vapor from the pores in the soil can result in the formation of ice lenses, thick layers of frozen water crystals than can lift foundations by even larger amounts.

Excavation is required on many sites to place the footings at a depth where soil of the appropriate bearing capacity is available. Excavation is frequently undertaken so that one or more levels of basement space can be added to a building, whether for additional habitable rooms, for parking, or for mechanical equipment and storage.

Where footings must be placed deep to get below the frost line or reach competent soil, a basement is often bargain-rate space, adding little to the overall cost of the building. In particulate soils, a variety of excavating machines can be used to loosen and lift the soil from the ground: bulldozers, shovel dozers, backhoes, bucket loaders, scrapers, trenching machines, and power shovels of every type. If the soil must be moved more than a short distance, dump trucks come into use.

In rock, excavation is slower and many times more costly. Weak or highly fractured rock can sometimes be broken up with power shovels, tractor-mounted rippers, pneumatic hammers, or drop balls such as those used in building demolition. Blasting, in which explosives are placed and detonated in lines of closely spaced holes drilled deep into the rock, is often necessary. In developed areas where blasting is impractical, rock can be broken up with hydraulic splitters.

Excavation Support

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 1). Such construction can take many forms, depending on the qualities of the soil, depth of excavation, equipment and preferences of the contractor, proximity of surrounding buildings, and level of the water table.

Shoring

The most common types of slope support, 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 (Figure 2). 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 3).

Most often, shoring is temporary, and it is removed as soil is replaced in the excavation. 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.

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.

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.

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.

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 prestressed and is produced in sections in a precasting plant, 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.

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 consistency, excavation can begin, with the hardened slurry on the inside face of the wall dropping away from the coated surface as soil is removed.

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. 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. Soil mixing can also serve to stabilize and strengthen areas of weak soil.

Bracing

All forms of slope support and excavation support must be braced against soil and water pressures as the excavation deepens (Figure 4). Crosslot bracing utilizes temporary steel wide-flange 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 instead, bearing against heel blocks or other temporary footings. Both rakers and crosslot bracing, especially the latter, are a hindrance to the excavation process.

A clamshell bucket on a crane must be used to remove the earth between the braces, which is much less efficient 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 maintaining an open excavation.

At each level of walers, holes are drilled at intervals through the sheeting and the surrounding soil into rock or a stratum of stable soil. Steel cables or tendons are then inserted into the holes, grouted to anchor them to the rock or soil, and stretched tight with hydraulic jacks (post tensioned) before they are fastened to the walers (Figure 5).

Excavations in fractured rock can often avoid sheeting altogether, either 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 6). 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 reinforcing 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.

Dewatering

During construction, excavations must be kept free of standing water. Such water may come from precipitation or it may come from groundwater seepage originating from any of a number of sources, such as surface water percolating through the soil, underground streams, perched water moving over impervious soil strata, or adjacent permanently saturated soil areas where the excavation extends below the water table.

Some shallow excavations in relatively dry soil conditions may remain free of standing water without any intervention. But most excavations require some form of dewatering, or extraction of water from the excavation or surrounding soil. The most common method of dewatering is to remove water by pumping as it accumulates in pits, called sumps, created at low points in the excavation.

Where the volume of groundwater fl owing into the excavation is great, or with certain types of soils, particularly sands and silt, that may be softened by constant seepage, it may be necessary to keep ground water from entering the excavation at all. This can be done either by pumping water from the surrounding soil to depress the water table below the level of the bottom of the excavation or by erecting a watertight barrier, such as a slurry wall, around the excavation (Figure 7).

Well points are commonly used to depress the water table. These are vertical sections of pipe with screened openings at the bottom that keep out soil particles while allowing water to enter. Closely spaced well points are driven into the soil around the entire perimeter of the excavation. These are connected to horizontal header pipes leading to pumps that continually draw water from the system and discharge it away from the building site.

Once pumping has drawn down the water table in the area of the excavation, work can continue “in the dry”. For excavations deeper than the 20 feet (6 m) or so that cannot be drained by a suction pump stationed at ground level, two rings of well points may be required, the inner ring being driven to a deeper level than the outer ring, or a single ring of deep wells with submersible force pumps may have to be installed.

In some cases, well points may not be practical: they may have insufficient capacity to ensure that an excavation remains dry; restrictions on the disposal of groundwater may limit their use; reliability due to power outages may be a concern; or lowering of the water table may have serious adverse effects on neighboring buildings by causing consolidation and settling of soil under their foundations or by exposing untreated wood foundation piles, previously protected by total immersion in water, to decay.

In these cases, a watertight barrier wall may be used as an alternative (Figure 7). Slurry walls and soil mixed walls (pages 40–45) can make excellent watertight barriers. Sheet piling can also work, but it tends to leak at the joints. Soil freezing is also possible. In this method, an array of vertical pipes similar to well points is used to continuously circulate coolant at temperatures low enough to freeze the soil around an excavation area, resulting in a temporary but reliable barrier to groundwater.

Watertight barriers must resist the hydrostatic pressure of the surrounding water, which increases with depth, so for deeper excavations, a system of bracing or tiebacks is required. A watertight barrier also works only if it reaches into a stratum of impermeable soil such as clay. Otherwise, water can flow beneath the barrier and rise up into the excavation.