Engineering and Construction

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A caisson, or drilled pier (Figure 2.37), is similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded. It differs from a column footing in that it extends through strata of unsatisfactory soil beneath the substructure of a building until it reaches a more suitable stratum. A caisson is constructed by drilling or hand-digging a hole, belling (flaring) the hole out at the bottom as neces- sary to achieve the required bearing area, and fi lling the hole with concrete. Large auger drills (Figures 2.38 and 2.39) are used for drilling caissons; hand excavation is used only if the soil is too full of boulders for the drill. A temporary cylindrical steel casing is usually lowered around the drill as it progresses to support the soil around the hole. When a fi  rm bearing stratum is reached, the bell, if required, is created at the bottom of the shaft either by hand excavation  or by a special  belling bucket on the drill (Figure 2.40). The bearing sur- face of the soil at the bottom of the  hole is then inspected to be sure it  is of the anticipated quality, and the  hole is fi lled with concrete, withdrawing the casing as the concrete rises.

Reinforcing is seldom used in the  concrete except near the top of the  caisson, where it joins the columns of  the superstructure.

Figure 2.37 Deep foundations. Caissons are concrete  cylinders poured into drilled holes. They reach through weaker soil (light shading)  to bear on competent soil beneath. The end bearing caisson at the left is belled as  shown when additional bearing capacity is required. The socketed caisson is drilled  into a hard stratum and transfers its load primarily by friction between the soil or  rock and the sides of the caisson. Piles are driven into the earth. End bearing  piles act in the same way as caissons. The friction pile derives its load-carrying  capacity from friction between the soil and the sides of the pile.

Figure 2.38 A 6-foot- (1828-mm)-diameter auger on a telescoping 70-foot (21-m) bar brings up a load of soil from a caisson hole. The auger will be rotated rapidly to spin off the soil before being reinserted in the  hole.

Figure 2.39 For cutting through hard material, the caisson drill is equipped with a carbide-toothed coring bucket.

Figure 2.40 The bell is formed at the bottom of the  caisson shaft by a belling bucket with retractable cutters. The example shown  here is for an 8-foot- (2.44-m)-diameter shaft and makes a bell 21 feet (6.40 m) in  diameter.

Caissons are large, heavy-duty foundation components. Their shaft diameters range from 18 inches  (460 mm) up to 8 feet (2.4 m) or  more.  Belled caissons are practical  only where the bell can be excavated  in a cohesive soil (such as clay) that  can retain its shape until concrete is  poured. Where groundwater is present, the temporary steel casing can  prevent flooding of the caisson hole  during its construction. But where  the bearing stratum is permeable, water may be able to fi ll the hole from  below and caisson construction may  not be practical.

A  socketed caisson (Figure 2.37) is  drilled into rock at the bottom rather than belled. Its bearing capacity
comes not only from its end bearing, but from the frictional forces between  the sides of the caisson and the rock  as well. Figure 2.41 shows the installation of a rock caisson or drilled-in caisson, a special type of socketed  caisson with a steel H-section core.

Figure 2.41 Installing a rock caisson. (a) The shaft of the caisson has been drilled through softer soil to the rock beneath and cased with a steel pipe. A churn drill is being lowered into the casing to begin advancing the hole into the rock. (b) When the hole has penetrated the required distance into the rock stratum, a heavy steel H-section is lowered into the hole and suspended on steel channels across the mouth of the casing. The space between the casing and the H-section is then fi lled with concrete, producing a caisson with a very high load-carrying capacity because of the composite structural action of the steel and the concrete.

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Most shallow foundations are simple concrete  footings. A  column footing is  a square block of concrete, with or  without steel reinforcing, that accepts  the concentrated load placed on it  from above by a building column and spreads this load across an area of soil large enough that the allowable bearing stress of the soil is not exceeded.

A wall footing or  strip footing is a continuous strip of concrete that serves the same function for a loadbearing wall (Figures 2.30 and 2.31).

Figure 2.30 A column footing and a wall footing of concrete. The steel reinforcing bars have  been omitted from this illustration for clarity.

Figure 2.31 These concrete foundation walls for an apartment building, with their steel formwork not yet stripped, rest on wall footings. For more extensive illustrations of wall and  column footings

To minimize settlement, footings are usually placed on undisturbed soil.  Under some circumstances, footings
may be constructed over engineered fill,  which is earth that has been deposited under the supervision of a soils engineer. The engineer, working from the results of laboratory compaction tests on samples taken from the soil used for filling, makes sure that the soil is deposited in thin layers at a controlled moisture content and compacted in accordance with detailed procedures that ensure a known loadbearing ca-pacity and long-term stability.

Footings appear in many forms in different foundation systems. In  climates with little or no ground frost, a concrete  slab on grade with thickened edges is the least expensive foundation and floor system that one can use and is applicable to one- and two-story buildings of any type of construction (see Chapter 14 for further
information on slabs on grade). Or, in colder regions the edges of a slab on grade may be supported with deeper wall footings that bear on soil below the frost line. For floors raised above the ground, either over a  crawlspaceor a  basement, support is provided by concrete or masonry foundation walls supported on concrete strip footings (Figure 2.32). When building on slopes, it is often necessary to step the footings to maintain the re-quired depth of footing at all points around the building (Figure 2.33). If soil conditions or earthquake precau-tions require it, column footings on steep slopes may be linked together with reinforced concrete  tie beams to avoid possible differential slippage between footings.

Figure 2.32 Three types of substructures with shallow foundations. The slab on grade  is the most economical under many circumstances, especially where the  water table lies near the surface of the ground. A crawlspace is often used under  a floor structure of wood or steel, and gives much better access to underfloor  piping and wiring than a slab on grade. Basements provide usable space for  building occupants.

Figure 2.33 Foundations on sloping sites, viewed in a cross section  through the building. The broken line indicates the outline of the superstructure. Wall footings are stepped to maintain the necessary distance between the bottom of the footing and the surface of the ground. Separate column foundations, whether caissons, as shown here, or column footings, are often connected with reinforced concrete tie beams to reduce differential movement between the columns. A grade beam differs from a tie beam by being reinforced to distribute the continuous load from a bearing wall to separate foundations.

Footings cannot legally extend beyond a property line, even for a building built tightly against it. If the outer
toe of the footing were simply cut off at the property line, the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail. Combined footings and cantilever footings solve this problem by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized (Figure 2.34).

Figure 2.34 Either a combined footing (top) or a cantilevered footing (bottom) is used when columns must abut a property line. By combining the foundation for the column against the property line, at the left, with the foundation for the next interior column to the right in a single structural unit, a balanced footing design can be achieved. The concrete reinforcing steel has been omitted from these drawings for the sake of clarity.

In situations where the allowable bearing capacity of the soil is low in relation to the weight of the build-
ing, column footings may become large enough that it is more economical to merge them into a single mat or  raft foundation that supports the entire building. Mats for very tall buildings may be 6 feet (1.8 m) thick
or more and are heavily reinforced (Figure 2.35).

Figure 2.35 Pouring a large foundation mat. Six truck-mounted pumps receive concrete from a continuous procession of transit-mix concrete trucks and deliver this concrete to the heavily reinforced mat. Concrete placement continues nonstop around the clock until the mat is finished to avoid “cold joints,” weakened planes between hardened concrete and fresh concrete. The soil around this excavation is supported with a sitecast concrete slurry wall. Most of the slurry wall is tied back, but a set of rakers is visible at the lower right.

Where the bearing capacity of the soil is low and settlement must be carefully controlled, a  floating foundation is sometimes used. A floating foundation is similar to a mat foundation, but is placed beneath a building at a depth such that the weight of the soil removed from the excavation is equal to the weight of the building above. One story of ex-cavated soil weighs about the same as five to eight stories of superstructure, depending on the density of the soil and the construction of the building  (Figure 2.36).

Figure 2.36 A cross section through a building with a floating foundation. The building weighs approximately the same as the soil excavated for the substructure, so the stress in the soil beneath the building is the same after construction as it was before.

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It is convenient to think of a building as consisting of three major parts: the superstructure, which is the above-ground portion of the building; the substructure, which is the habitable below-ground portion; and the foundations, which are the components of the building that transfer its loads into the soil (Figure 2.29).

There are two basic types of foundations: shallow and deep.  Shallow foundations are those that transfer the load to the earth at the base of the column or wall of the substructure. Deep foundations, either piles or caissons, penetrate through upper layers of incompetent soil in order to transfer the load to competent bearing soil or rock deeper within the earth. Shallow foundations are generally less expensive than deep ones and can be used where suitable soil is found at the level of the bottom of the substructure, whether this be several feet or several stories below the ground surface.

Figure 2.29 Superstructure, substructure, and foundation. The substructure in  this example contains two levels of basements, and the foundation consists  of bell caissons. (In some buildings, the substructure and foundation may be partly  or wholly the same.)

The primary factors that affect the choice of a foundation type for a building are:

•  Subsurface soil and groundwater conditions
•  Structural requirements, including foundation loads, building configurations, and depth

Secondary factors that may be important include:

•  Construction methods, including access and working space
•  Environmental factors, including noise, traffic, and disposal of earth and water
•  Building codes and regulations
•  Proximity of adjacent property and potential impacts on that property
•  Time available for construction
•  Construction risks

The foundation engineer is responsible for assessing these factors and, working together with other members of the design and construction team, selecting the most suitable foundation system.

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During construction, excavations must be kept free of standing water.
Such water may come from precipitation or it may come from ground-water 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 sat-urated soil areas where the excavationextends 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 ground-water flowing 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 surround-ing 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 2.26).

Figure 2.26 Two methods of keeping an excavation dry, viewed in cross section. The water sucked from well points depresses the water table in the immediate vicinity to a level below the bottom of the excavation. Watertight barrier walls work only if their bottom edges are inserted into an impermeable stratum that prevents water from working its way under the walls.

 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 al-lowing 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” (Figures 2.27 and 2.28). 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.

Figure 2.27 Well point dewatering. Closely spaced vertical well points connect to the larger-diameter header pipe.

Figure 2.28 An excavation is kept dry despite the close proximity of a large body of water. Two dewatering pumps are visible in the foreground.  A pair of header pipes and numerous well points can be seen surrounding the excavation.

In some cases, well points may not be practical: they may have insufficient capacity to ensure that an ex-
cavation 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 2.26). 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 hy-drostatic 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.

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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.

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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 devel-oped 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...

⇒ Dewatering   During construction, excavations must be kept free of standing water. Such water may come from precipitation or it may come from ground-water seepage originating from any of...

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Building sites should be selected and developed so as to protect and conserve natural habitats and resources, to promote biodiversity, to preserve quality open space, and to minimize pollution and unnecessary energy consumption.

Site Selection
Buying and renovating an existing building rather than building a new one saves a great deal of building material and energy. If the existing building has been scheduled for demolition, its renovation also avoids the disposal of an enormous quantity of material in a landfill.

Building in an urban area with existing infrastructure, rather than in undeveloped land unconnected to other community resources, protects open space, natural habitats, and natural resources.

Building on a damaged or polluted site, and designing the building so that it helps to restore the site, benefits
the environment rather than degrades it.

Avoiding construction on prime agricultural land prevents the permanent loss of this productive use of the land.

Avoiding construction on undeveloped land that is environmentally sensitive protects the wildlife and natural habitats such land supports. This includes fl  oodplains, land that provides habitat for endangered or threatened species, wetlands, mature forest lands and prairies, and land adjacent to natural bodies of water.

Avoiding construction on public parkland or land adjacent to bodies of water that support recreational use prevents the permanent loss of public resources.

Selecting a building that is well connected to existing networks of public transportation, and to pedes-trian and bicycle paths, pays environmental dividends every day for the life of the building by saving fuel, reducing air pollution from automobiles, and minimizing commute times.

Site Design
Minimizing the building footprint and protecting and enhancing portions of the site with natural vegetation protects habitat and helps maintain biodiversity.

Appropriate landscape design and the use of captured rainwater, recycled wastewater, or other nonpotable sources of water for landscape irrigation minimize wasteful water consumption.

Minimizing impervious ground surface (such as for vehicle parking) and providing a surface drainage system
that conducts water to areas on the site where it can be absorbed into the ground works to replenish natural aquifers, avoid overloading of storm sewer systems, and reduce water pollution.

Grading the site to appropriate slopes and planting vegetation that holds the soil in place will prevent erosion.

Large existing trees cannot be replaced except by growing new ones over a period of many decades. Planting of new trees is good, but preservation of existing trees is even better.

Distinctive site features such as rock formations, forests, grasslands, streams, marshes, and recreational paths and  facilities, if destroyed for new construction, can never be replaced.

Providing shade, vegetated or refl ective roof surfaces, refl  ective paving materials, and open pavement systems reduces heat island effects and creates an improved microclimate for both humans and wildlife.

Minimizing nighttime light pollution is a benefit  to humans and nocturnal wildlife.

Avoiding unnecessary shading of adjacent buildings protects those buildings’ sources of natural illumination and useful solar heat, minimizing their unnecessary consumption of electricity and heating fuel.

Siting a building for best exposure to sun and wind maximizes solar heat gain in winter and minimizes it in
summer to save heating and air conditioning fuel. It also allows utilization of daylight to replace electric lighting.

In general, a building should be designed in such a manner that the site does the heaviest work of environmental modification through good orientation to sun and wind, trees that are placed so as to provide shade and wind-breaks, and use of below-grade portions of the building for thermal mass. The passive shell of the building can do most of the rest of the work through orientation of windows withrespect to sunlight, good thermal insulation and airtight-ness, utilization of thermal mass, and energy-efficient windows. The active heating, cooling, and lighting equipment should serve only to fine-tune the interior environment, using as little fossil fuel and electricity as possible.

Construction Process
It is essential to protect trees and sensitive areas of the site from damage during construction.

A building should comply with all local conservation laws relating to soils, wetlands, and stormwater.

Topsoil should be stockpiled carefully during construction and reused on the site.

It is important to guard against soil erosion by water and wind during construction, as well as the sedimentation of streams and sewers, or the polluting of air with dust or particulates that can result.

Vehicle tires compact soil so that it cannot absorb water or support vegetation. Thus, it is important to develop minimal, well-marked access routes for trucks and construction machinery that minimize soil compaction, as well as minimizing noise, dust, air pollution, and inconvenience to neighboring buildings and sites.

Construction machinery should be selected and maintained so that it pollutes the air as little as possible.

Surplus excavated soils should be reused either on the site or on another site nearby.

Construction wastes should be recycled as much as possible.

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Generally, soil groups listed toward the top of Figure 2.2 are more desirable for supporting building foundations than those listed further down. The higherlisted soils tend to have better  soil engineering properties, that is, they tend  to have greater loadbearing capacity,  to be more stable, and to react less to
changes in moisture content. Rock is  generally the best material on which  to found a building. When rock is too deep to be reached economically, the designer must  choose from the strata  of different soils that lie closer to the  surface and design a foundation to per- form satisfactorily in the selected soil.

Figure 2.5 gives some conservative values of  loadbearing capacity for  various types of soil. These values give only an approximate idea of the  relative strengths of different soils;  the strength of any particular soil is
also dependent on factors such as  the presence or absence of water, the  depth at which the soil lies beneath  the surface, and, to some extent, the manner in which the foundation acts  upon it. In practice, the designer may  also choose to reduce the pressure  of the foundations on the soil to well  below these values in order to reduce  the potential for building settlement.

Figure 2.5 Presumptive surface bearing values of various soil types, from the 2006 IBC. Classes 3, 4, and 5 refer to the soil group symbols in Figure 2.2.

The stability of a soil is its ability to retain its structural properties under  the varying conditions that may occur  during the lifetime of the building. In  general, rock, gravels, and sands tend  to be the most stable soils, clays the least  stable, and silts somewhere in between.

Many clays change size under chang-ing subsurface moisture conditions,  swelling considerably as they absorb  water and shrinking as they dry. In the  presence of highly expansive clay soils,  a foundation may need to be designed  with underlying void spaces into which  the clay can expand to prevent structural damage to the foundation itself.

When wet clay is put under pressure,  water can be slowly squeezed out of  it, with a corresponding gradual reduction in volume. In this circumstance, long-term settlement of a foundation bearing on such soil is a risk that must  be considered. Taken together, these properties make many clays the least predictable soils for supporting buildings. (In Figure 2.2, the fine-grained  soil groups indicated as having a liquid  limit greater than 50 are generally the  ones most affected by water content,  exhibiting higher plasticity (moldability) and greater expansion when wet  and lower strength when dry.

In regions of significant  earth- quake risk, stability of soils during seismic events is also a concern. Sands and  silts with high water content are particularly susceptible to liquefaction, that
is, a temporary change from solid to  liquid state during cyclic shaking. Soil  liquefaction can lead to loss of support  for a building foundation or excessive  pressure on foundation walls.

The drainage characteristics of a  soil are important in predicting how  water will fl ow on and under building  sites and around building substructures. Where a coarse-grained soil is composed of particles mostly of the
same size, it has the greatest possible volume of void space between particles, and water will pass through it most readily. Where coarse-grained soils are composed of particles with a diverse range of sizes, the volume of void space between particles is reduced, and such soils drain water less eficiently. Coarse-grained soils consisting of particles of all sizes are termed well graded or poorly sorted, those with a smaller range of particle sizes are termed  poorly gradedor well sorted, and those with particles mostly of one size are termed uniformly graded (Figure 2.6).

Figure 2.6 Two gravel samples, illustrating differences in range of particle sizes or grading. The left-hand sample, with a diverse range of particle sizes, comes from a well-graded sandy gravel. On the right is a sample of a uniformly graded gravel in which there is little variation in size among particles.

Because of their smaller particle size,fine-grained soils also tend to drain water less efficiently:  Water
passes slowly through very fine sands and silts and almost not at all through many clays. A building site with clayey or silty soils near the surface drains poorly and is likely to be muddy and covered with puddles during rainy periods, whereas a gravelly site is likely to remain dry. Underground, water passes quickly through strata of gravel and sand but tends to accumulate above layers of clay and fine silt. An excel-lent way to keep a basement dry is to surround it with a layer of uniformly graded gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of
this porous layer, from where it can be drawn off in perforated pipes before it accumulates (Figures 2.60–2.62). It does little good to place perforated drainage pipes directly in clay or silt because water cannot fl  ow  through the impervious soil toward the pipes.

Figure 2.60 Two methods of relieving water pressure around a building substructure by drainage. The gravel drain (left) is hard to do well because of the dificulty of depositing the crushed stone and backfill soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.62 A diagrammatic representation of the placement of sheet membrane waterproofing around a basement. A mud slab of low- strength concrete was poured to serve as a base for placement of the horizontal membrane. Notice that the vertical and horizontal membranes join to wrap the basement completely in a waterproof enclosure.

Rarely is the soil beneath a building site composed of a single type. Beneath most buildings, soils of various
types are arranged in superimposed layers (strata) that were formed by various past geologic processes. Frequently, soils in any one layer are themselves also mixtures of different soil groups, bearing descriptions such as well-graded gravel with silty clay and sand, poorly graded sand with clay, lean clay with gravel, and so on. Determining the suitability of any particular site’s soils for support of a building foundation, then, depends on the be-haviors of the various soils types and how they interact with each other and  with the building foundation.

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For the purposes of foundation design,  earth materials are classified  according to particle size, the presence  of organic content, and, in the case of finer grained soils, sensitivity to moisture content:

• Rock is a continuous mass of solid mineral material, such as granite  or limestone, which can only be re- moved by drilling and blasting. Rock  is never completely monolithic, but is  crossed by a system of joints  (cracks)  that vary in quantity and extent and that divide the rock into irregular blocks. Despite these joints, rock is generally the strongest and most stable material on which a building can be founded.

• Soil is a general term referring to any earth material that is particulate.
•  If an individual particle of soil is too large to lift by hand or requires two hands to lift, it is a boulder.
•  If it takes the whole hand to lift a particle, it is called a cobble.
•  If a particle can be lifted easily between thumb and forefinger, the soil is  gravel. In the Unified  Soil

Classification System (Figure 2.2), gravels are classified as having more than half their particles larger than 0.19 inch (4.75 mm) in diameter but none larger than 3 inches (76 mm).

Figure 2.2 The Unifi ed Soil Classifi cation System, from ASTM D 2487. The group symbols are a universal set of abbreviations for soil types.

•  If individual soil particles can be seen but are too small to be
picked up individually, the soil is sand. Sand particles range in size from about 0.19 to 0.003 inch (4.75–0.075 mm). Both sand and gravel are referred to as coarse-grained soils.
•  Individual silt particles are too small to be seen with the unaided eye and range in size from 0.003 to 0.0002 inch (0.075–0.005 mm). Like coarse-grained soil particles, silt particles are roughly spherical, or equidimensional, in shape.
• Clay particles are plate-shaped rather than spherical (Figure 2.3) and smaller than silt particles, less
than 0.0002 inch (0.005 mm) in size.

Figure 2.3 Silt particles (top) are approximately equidimensional granules, while clay particles (bottom) are platelike and much smaller than silt. (A circular area of clay particles has been magnifi ed to make the structure easier to see.)

Both sands and silts are also referred to as fine-grained soils.

•  Peat, topsoil, and other  organic soils are not suitable for the support of building foundations. Because of their high organic matter content, they are spongy, they compress easily, and their properties can change over time due to changing water content or biological activity within the soil.

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All foundations settle to some extent as the earth materials around and beneath them adjust to the loads of the building. Foundations on bedrock settle a negligible amount. Foundations in other types of soil may settle
much more. As an extreme example, Mexico City’s Palace of Fine Arts has settled more than 15 feet (4.5 m) into the clay soil on which it is founded since it was constructed in the early 1930s. However, building foundation settlement is normally limited to amounts measured in millimeters or fractions of an inch.

Where  foundation settlement occurs at roughly the same rate through-out all portions of a building, it is termed  uniform settlement. Settlement that occurs at differing rates between different portions of a building is termed  differential settlement. When all parts of a building rest on the same kind of soil, and the loads on the building and the design of its structural system are uniform throughout, differential settlement is normally not a concern.

However, where soils, loads, or structural systems differ between parts of a building, different parts of the building structure may settle by substantially different amounts, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may not work properly (Figure 2.1). Most foundation failures are attributable to excessive differential settlement. Gross failure of a foundation, in which the soil fails completely to support the building, is extremely rare.

Figure 2.1 Uniform settlement (b) is usually of  little consequence in a building, but differential settlement (c) can cause  severe structural damage.

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A building foundation must support different kinds of loads:

•  Dead load, the combined weight of all the permanent components of the building, including its own structural frame, floors, roofs, and walls, major permanent electrical and mechanical equipment, and the foundation itself
•  Live loads, nonpermanent loads caused by the weights of the building’s occupants, furnishings, and movable equipment
•  Rain and snow loads, which act pri-marily downward on building roofs
•  Wind loads, which can act laterally (sideways), downward, or upward on a building
•  Seismic loads, horizontal and vertical forces caused by the motion of the ground relative to the building during an earthquake
•  Loads caused by soil and hydro-static pressure, including  lateral soil pressure loads, horizontal pressures of earth and groundwater against basement walls; in some instances, buoyant uplift forces from underground water, identical to the forces that cause a boat to float; and in others, lateral force flood loads that can occur in areas prone to flooding
•  In some buildings, horizontal thrustsfrom long-span structural systems  such as arches, rigid frames, domes,  vaults, or tensile structures

A satisfactory foundation for a building must meet three general requirements:

1. The foundation, including the un-derlying soil and rock, must be safe against a structural failure that could result in collapse. For example, the foundation for a skyscraper must sup-port the great weight of the building above on a relatively narrow base without danger of overturning.

2. During the life of the building, the foundation must not settle in such a way as to damage the structure or impair its function. (Foundation settlement is discussed more fully in the next section.)

3. The foundation must be feasible, both technically and economically,  and practical to build without adverse effects on surrounding property. For example, New York City’s tallest buildings tend to cluster on the central and southern portions of Manhattan Island, where the un- derlying bedrock is closest to the surface and foundations for such buildings are easiest and least expensive to construct.