Engineering and Construction

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Normally, the substructure of a building is completed before work begins on its superstructure. If the building  has several levels of basements, however, substructure work can take many months or even years.

In such a case,  up–down construction is sometimes an economical option, even if its first  cost is somewhat more than that of  the normal procedure, because it can  save considerable construction time.

As diagrammed in Figure 2.69,  up–down construction begins with  installation of a perimeter slurry  wall.

Internal steel columns for the  substructure are lowered into drilled, slurry-filled holes, and concrete footings are tremied beneath them. After  the ground floor slab is in place and  connected to the substructure columns, erection of the superstructure may begin. Construction continues  simultaneously on the substructure, largely by means of mining machinery: A story of soil is excavated from  beneath the ground floor slab and a  level mud slab of CLSM is poured.

Working on the mud slab, workers reinforce and pour a concrete structural slab for the floor of the topmost
basement level and connect this floor to the columns. When the slab is sufficiently strong, another story of soil is removed from beneath it, along with the mud slab. The process  is repeated until the substructure iscomplete, by which time the super-structure has been built many storiesinto the air.

Figure 2.69 Up–down construction.

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After the basement walls have been  waterproofed or dampproofed, insulating boards or protection boards  have been applied, drainage features  have been installed, and internal constructions that support the basement walls, such as interior walls and floors,  have been completed, the area around a substructure is  backfilled to restore the level of the ground. (A substructure built tightly against sheeted walls of an excavation needs little or no backfilling.) The backfilling  operation involves placing soil back against the outside of the basement walls  and compacting it there in layers,  taking care not to damage drainage  or waterproofing components or to exert excessive soil pressure against  the walls. An open, fast-draining soil  such as gravel or sand is preferred for backfilling because it allows the perimeter drainage system around the  basement to do its work. Compaction  must be sufficient to minimize subsequent settling of the backfilled area.

In some situations, especially in backfilling utility trenches under road- ways and floor slabs, settling can be  virtually eliminated by backfilling with  controlled low-strength material (CLSM),  which is made from portland cement  and/or fly ash (a byproduct of coal- burning power plants), sand, and water.  CLSM, sometimes called “flowable fill,”  is brought in concrete mixer trucks  and poured into the excavation, where  it compacts and levels itself, then hardens into soil-like material. The strength  of CLSM is matched to the situation:

For a utility trench, CLSM is formulated so that it is weak enough to be  excavated easily by ordinary digging  equipment when the pipe needs ser-vicing, yet as strong as a good-quality compacted backfill. CLSM has many  other uses in and around foundations.

It is often used to pour mud slabs, which  are weak concrete slabs used to create  a level, dry base in an irregular, often  wet excavation. The mud slab serves  as a working surface for the reinforcing and pouring of a foundation mat  or basement floor slab and is often the  surface to which a waterproofing mem- brane is applied. CLSM is also used to  replace pockets of unstable soil that  may be encountered beneath a sub- structure or to create a stable volume  of backfill around a basement wall.

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In  shallow frost-protected foundations, extruded polystyrene foam insulation boards can be used in cold climates to construct footings that lie above the normal frost line in the soil, resulting  in lower excavation costs. Continuous  layers of insulation board are placed  around the perimeter of the building  in such a way that heat fl owing into the  soil in winter from the interior of the  building maintains the soil beneath  the footings at a temperature above  freezing (Figure 2.68). Even beneath  unheated buildings, properly installed  thermal insulation can trap enough  geothermal heat around shallow foundations to prevent freezing.

Figure 2.68 A typical detail for a shallow frost- protected footing.

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Comfort requirements, heating fuel efficiency, and building codes often require that basement walls be thermally insulated to limit the loss of heat from basements to the soil outside. Thermal insulation may be applied either inside or outside the basement wall.

Inside the wall, mineral batt or plastic foam insulation may be installed between wood or steel furring strips, as shown in Figure 23.4. Alternatively, polystyrene foam or glass fiber insulation boards, typically 2 to 4 inches (50 to 100 mm) thick may be placed on the outside of the wall, held by either adhesive, fasteners, or the pressure of the soil. Proprietary products are available that combine insulation board and drainage mat in a single assembly.

Figure 23.4 A furred gypsum board fi nish over a concrete block wall. The Z-furring channels are attached to the masonry with powder-driven fasteners. The plastic foam insulation is tucked in behind the fl ange of the channel, and the gypsum board is screwed to the face of the fl ange. Long slots punched from the web of the channel (not visible in this drawing) help to reduce the thermal bridging effect of the Z-furring channel.

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Where building substructures enclose basements, parking garages, or other usable space, groundwater must be kept out. Concrete alone is rarely adequate for this purpose. Moisture can migrate through its microscopic pores, or through other pathways created by shrinkage cracks, form tie holes, utility penetrations, and the joints between concrete pours. To ensure a substructure’s resistance to water entry,
two approaches are used: drainage and waterproofing. Drainage draws groundwater away from a foundation, reducing the volume and pressure of water acting on the foundation’s walls and slabs.

Waterproofing acts as a barrier, stopping water that reaches the foundation from passing though to the interior.

Drainage, consisting of some combination of drainage backfill (well-sorted crushed stone or gravel), drainage mat, and perforated drain piping, is used with almost every building substructure (Figure 2.60). Drainage mat is a manufactured component that may be made of a loose mat of stiff, inert fibers, a plastic egg-crate structure, or some other very open, porous material. It is faced on the outside with a filter fabric that prevents fine soil particles from entering and clogging the drainage passages in the mat. Any subterranean water that approaches the wall descends through the porous material of the mat to the drain pipe at the footing. Perforated drain piping is frequently laid around the outside perimeter of a building foundation. The pipes are 4 or 6 inches (100 or 150 mm) in diameter and provide an open channel in the crushed stone bed through which water can flow by gravity either “to day-light” at a lower elevation on a sloping site, to a municipal storm sewer system, or to a sump pit that can be automatically pumped dry whenever it fills. The pipes are laid at least 6 inches (150 mm) below the top of the basement floor slab tomaintain the groundwater level safely below that of the slab. Perforations in the pipes face downward so that water is drained from the lowest possible level. Where groundwater conditions are severe, rows of perforated pipe may be installed under the basement slab as well (Figure 2.61).

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 diffi culty of depositing the crushed stone and backfi ll soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.61 For a high degree of security against substructure fl ooding, drainage both around and under the basement is required, as seen here in a section view. Above-slab drainage is used in buildings with mat foundations.

On most foundations, some form of water-repelling barrier is also used to protect against the pas-
sage of groundwater. Dampproofing is a moisture-resistant cement plaster or asphalt compound commonly applied to residential basement walls and to other substructures where groundwater conditions are mild or waterproofing requirements are not critical. Cement plaster dampproofing, or parge coating, is light gray in color and troweled on.  Asphalt or  bituminous dampproofing is dark in color and is applied in liquid form by spray, roller, or trowel. Dampproofing is less expensive and less resistant to water passage than true waterproofing.

Waterproofing, unlike dampproofing, can prevent the passage of water even under conditions of hydrostatic pressure. It is used where groundwater conditions are severe or the need to protect subgrade space from moisture is critical. Waterproof mem-branes are most commonly formulated from plastics, asphalt compounds, or synthetic rubbers and come in a great variety of forms.

Liquid waterproofing  is applied by spray gun, roller, or squeegee and then allowed to cure in place. It is easy to install and easy to form around complex shapes. When fully cured, the finished membrane is seamless and fully bonded to the underlying substrate. However, because liquid membranes are formed in the field, they are subject to uneven application, and the surfaces to which they are applied must be clean, smooth, and dry to ensure reliable adhesion of the membrane.

Preformed sheet membrane waterproofing may be adhered or mechanically fastened to substructure walls
or laid loosely over horizontal surfaces (Figure 2.62). Fabricated under controlled factory conditions, sheet membranes are reliably uniform in material quality and thickness.

Figure 2.62 A diagrammatic representation of the  placement of sheet membrane waterproofi  ng 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.

However, they can be more difficult to form around complex shapes, and the seams between sheets, which are sealed in the field, may be subject to lapses in quality. Sheet membranes that are loosely laid or mechanically fastened can be used over substrates that will not bond with liquid-applied or adhered sheet membranes. They are also a good choice where substrate cracking or movement may be expected, because such movement is less likely to stress or damage the membrane. An advantage of adhered membranes (both sheet and liquid) is that in the case of a defect, water cannot travel far under the membrane, limiting the extent of water damage that may occur and simplifying the tracing of leaks.

Bentonite waterproofing is made from sodium bentonite, a naturally occurring, highly expansive clay. It is most often applied as preformed sheets consisting of dry clay sand-wiched within corrugated card-board, geotextile fabric, or plastic sheets (Figure 2.63). When bentonite comes in contact with moisture, it swells to several times its dry volume and forms an impervious barrier to the further passage of water. Bentonite sheets can be placed directly on the soil under a concrete slab on grade or mechanically attached to uncured, damp concrete walls. In slurry form, bentonite can  be sprayed even onto highly irregular, rough stone walls. The swelling behavior of bentonite clay also allows it to adjust to cracking and  movement in the substrate.

Figure 2.63 Waterproofi ng in progress on a  concrete foundation. Leftmost: the bare foundation wall remains exposed.  Middle: bentonite waterproofi ng panels are fastened in place. These panels are  lined on the outer face with a black- colored, high-density plastic that adds to  the waterproofi ng qualities of the panel. Right: drainage mat has been installed  over the waterproofi ng. The mat’s outer face of fi lter fabric is lightly dimpled,  telegraphing the egg crate structure of the underlying molded plastic panel.  The top edge of the mat is secured in place with an aluminum termination bar  that holds the panel in place and keeps dirt and debris from falling behind the  panel. Lower right: white perforated drain piping can be seen, temporarily  supported on wood blocking and running alongside the footing. (Photo by  Joseph Iano)

Integral waterproofing includes cementitious plaster or crystalline admixtures for concrete or mortar
that react chemically to stop up the pores of these materials and render them watertight. It may be applied
to the surface of existing concrete or masonry or used as an admixture in new concrete. Unlike most other waterproofing materials, many integral waterproofing materials can be applied as  negative side waterproof-
ing, that is, applied to the inner side of a concrete wall acting to resist water passage from the opposite side.

Blind-side waterproofing is installed prior to the pouring of concrete walls. This occurs most commonly when a substructure wall is built close to a property’s edge, and excavation cannot be enlarged beyond the property line to permit workers access to the outer face of the wall after its construction.

Drainage matting is first applied directly to the excavation sheeting, and then any of a number of possible waterproofing membranes are applied over the drainage mat. Later, the concrete wall is poured against the membrane. The sheeting remains permanently in place (Figure 2.64).

Figure 2.64 Blind-side waterproofi ng is used where there is no working space between a sheeted excavation and the outside of the foundation wall. The drainage mat and waterproof membrane are applied to the sheeting; then the basement wall is poured against them.

Joints in construction require special attention to ensure water-tightness. Preformed waterstops made of plastic, synthetic rubber, or metal can be cast into the mating concrete edges of both moving and nonmoving joints to block the passage of water (Figures 2.65 and 2.66). Waterstops for nonmoving joints such as between concrete pours of a wall or slab can also be made of strips of bentonite or mastic that are temporarily adhered to the edge of one pour. After the adjacent pour is complete, these stops remain embedded in the joint, where they form a watertight barrier (Figure 2.67).

Figure 2.65 A synthetic rubber waterstop is used to seal against water penetration at movement joints and at joints between pours of concrete in a foundation. The type shown here is split on one side so that its halves can be placed fl at against the formwork where another wall will join the one being poured. After the concrete has been poured and cured and the formwork has been removed from the first wall, the split halves are folded back together before the next wall is poured.

Most waterproofing  systems are inaccessible once building construction is complete; they are ex-
pected to perform for the life of the building, and even small defects in installation can allow the passage of large volumes of water.

For these reasons, waterproofing membranes are inspected carefully during construction and horizontal  membranes are often  flood  tested (submerged for an extended period time while leak-checking is  performed) to detect the presence of defects while repairs can still be easily made. Once inspection and testing are complete, membranes are covered with a  protection board, insulation board, or drainage matting to shield the membrane from prolonged exposure to sunlight and  to prevent physical damage during  soil backfilling or subsequent construction operations.

Figure 2.66 A rubber waterstop ready for the next pour of a concrete wall, as diagrammed in Figure 2.65. (Courtesy of Vulcan Metal Products, Inc., Birmingham, Alabama)

Figure 2.67 A bentonite waterstop is adhered to a concrete footing prior to casting of the concrete wall above. Later, if groundwater seepage occurs, the bentonite will swell to fully seal the joint between the two pours. The waterstop is positioned to the side of the steel reinforcing bars closer to the wall’s exterior, also protecting the reinforcing from moisture and corrosion. However, because of bentonite’s expansive force, the waterstop must not be positioned too close to the surface of the wall, or when it swells, it could cause portions of concrete to split away or spall.

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Geotextiles are flexible fabrics made of chemically inert  plastics that are highly resistant to deterioration in the soil. They are used for a variety of purposes relating to site development and the foundations of buildings.

As described in the accompanying text, in earth reinforcement  or  soil reinforcement, a plastic mesh fabric or grid is used to support an earth embankment or retaining wall. The  same type of mesh may be utilized in layers to stabilize engineered fill beneath a shallow footing (Figure A), or to stabilize marginal soils under driveways, roads, and airport runways, acting very much as the roots of plants do in preventing the movement of soil particles.

Another geotextile that is introduced later in this chapter is drainage matting, an open matrix of plastic filaments with a feltlike filter fabric laminated onto one side to keep soil particles from entering the matrix. In addition to providing free drainage around foundation walls, drainage matting is often used beneath the soil in the bottoms of planter boxes and under heavy paving tiles on rooftop terraces, where it maintains free passage for the drainage of water above a waterproof membrane.



Synthetic filter fabrics are wrapped over and around subterranean crushed stone drainage layers such as the one frequently used around a foundation drain. In this position, they keep the stone and pipe from gradually
becoming clogged with fine soil particles carried by the groundwater. Similar fabrics are used at grade during construction, acting as temporary barriers to filter soil out of water that runs off a construction site, thus preventing contamination of lakes, streams, or stormwater systems.

Special geotextiles are manufactured to stake down on freshly cut slopes to prevent soil erosion and encour-age revegetation; some of these are designed to decay and disappear into the soil as plants take over the function of slope stabilization. Another type of geotextile is used for weed control in landscaped beds, where it allows rainwater to penetrate the soil but blocks sunlight, preventing weeds from sprouting.

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Underpinning is the process of  strengthening and stabilizing the  foundations of an existing building.

It may be required for any of several reasons: The existing foundations may never have been adequate to carry their loads, leading to excessive settlement of the building over time. A change in building use or
additions to the building may over-load the existing foundations. New construction near a building may disturb the soil around its foundations or require that its foundations be carried deeper. Whatever the cause, underpinning is a highly specialized task that is seldom the same for any two buildings. Three different alternatives are available when foundation capacity needs to be increased: The foundations may be enlarged; new, deep foundations can be inserted under shallow ones to carry the load to a deeper, stronger stratum of soil; or the soil itself can be strengthened by grouting or by chemical treatment. Figures 2.53 and 2.54 illustrate in diagrammatic form some selected concepts of underpinning.

Figure 2.53 Two methods of supporting a building while carrying out underpinning work beneath its foundation, each shown in both elevation  and section. (a) Trenches are dug beneath the existing foundation at intervals, leaving the majority of  the foundation supported by the soil. When portions of the new foundations have been completed in the trenches, using one of the types of underpinning shown in Figure 2.54, another set of trenches is dug between them and the remainder of the foundations is completed. (b) The foundations of an entire wall can be exposed at once by needling, in which the wall is supported temporarily on needle beams threaded through holes cut in the wall. After underpinning has been accomplished, the jacks and needle beams are removed and the trench is backfi  lled

Figure 2.54 Three types of underpinning. (a) A new foundation wall and footing are constructed beneath the existing foundation. (b) New piles or caissons are constructed on either side of the existing foundation. (c) Minipiles are inserted through the existing foundation. Minipiles do not generally require excavation or temporary support of the building.

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In areas where strong earthquakes are  common, buildings are sometimes placed on base isolators. When signifi cant ground movement occurs, the  base isolators flex or yield to absorb a  significant portion of this movement;  as a result, the building and its sub- structure move significantly less than  they would otherwise, reducing the  forces acting on the structure and  lessening the potential for damage.

A frequently used type of base isolator is a multilayered sandwich of rubber and steel plates (Figure 2.52).

The rubber layers deform in shear  to allow the rectangular isolator to  become a parallelogram in cross section in response to relative motion  between the ground and the building. A lead core deforms enough to  allow this motion to occur, provides  damping action, and keeps the layers  of the sandwich aligned.

Figure 2.52 Base isolation.

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Piles may be made of timber, steel, concrete, and various combinations of these materials (Figure 2.47). Timber piles have been used since Roman times, when they were driven by large mechanical hammers hoisted by muscle power. Their main advantage is that they are economical for lightly loaded foundations. On the minus side, they cannot be spliced during driving and are, therefore, limited to the length of available tree trunks, approximately 65 feet (20 m). Unless pressure treated with a wood pre- servative or completely submerged below the water table, they will decay (the lack of free oxygen in the water prohibits organic growth). Relatively small hammers must be used in driving timber piles to avoid splitting them. Capacities of individual timber piles lie in the range of 10 to 55 tons (9000 to 50,000 kg).

Two forms of steel piles are used, H-piles and pipe piles.  H-piles are special hot-rolled, wide-flange sections, 8 to 14 inches (200 to 355 mm) deep,  which are approximately square in cross section. They are used mostly in end bearing applications. H-piles displace relatively little soil during driving. This minimizes the upward displacement of adjacent soil, called heaving, that sometimes occurs when many piles are driven close together. Heaving can be a particular problem on urban sites, where it can lift adjacent buildings.

Figure 2.47 Cross sections of common types of piles. Precast concrete piles may be square or round instead of the octagonal section shown here and may be hollow in the larger sizes.

 H-piles can be brought to the site in any convenient lengths, welded together as driving progresses to form any necessary length of pile, and cut off with an oxyacetylene torch when the required depth is reached. The cutoff ends can then be welded onto other piles to avoid waste. Corrosion can be a problem in some soils, however, and unlike closed pipe piles and hollow precast concrete piles, H-piles cannot be inspected after driving to be sure they are straight and undamaged.
Allowable loads on H-piles run from 30 to 225 tons (27,000 to 204,000 kg).

Steel pipe piles have diameters of  8 to 16 inches (200 to 400 mm). They  may be driven with the lower end either open or closed with a heavy steel  plate. An open pile is easier to drive  than a closed one, but its interior  must be cleaned of soil and inspected before being filled with concrete,  whereas a closed pile can be inspected  and concreted immediately after driving. Pipe piles are stiff and can carry  loads from 40 to 300 tons (36,000 to 270,000 kg). They displace relatively  large amounts of soil during driving,  which can lead to upward heaving of  nearby soil and buildings. The larger  sizes of pipe piles require a very heavy  hammer for driving.

Minipiles, also called  pin piles or micropiles, are a lightweight form of  steel piles made from steel bar or  pipe 2 to 12 inches (50 to 300 mm) in  diameter. Minipiles are inserted into  holes drilled in the soil and grouted in  place. When installed within existing  buildings, they may also be forced into the soil by hydraulic jacks pushing downward on the pile and upward on the building structure. Since no hammering is required, they are a good  choice for repair or improvement of  existing foundations where vibrations from the hammering of conventional piles could damage the existing structure or disrupt ongoing activities within the building (Figure 2.54).

Where vertical space is limited, such as when working in the basement of an existing building, minipiles can
be installed in individual sections as short as 3 feet (1 m) that are threaded end-to-end as driving progresses.

Minipiles can reach depths as great as 200 feet (60 m) and have working capacities as great as 200 to 300 tons (180,000 to 270,000 kg). 

Precast concrete piles are square, octagonal, or round in section, and in large sizes often have open cores to  allow inspection (Figures 2.47–2.49).

Figure 2.48 Precast, prestressed concrete piles. Lifting loops are cast into the sides of the piles as crane attachments for hoisting them into a vertical position.

Figure 2.49 A driven cluster of six precast concrete piles, ready for cutting off and capping.

Most are prestressed, but some for smaller buildings are merely rein-forced (for an explanation of prestressing, see pages 544–548). Typical cross-sectional dimensions range from 10 to 16 inches (250 to 400 mm) and bearing capacities from 45 to 500 tons (40,000 to 450,000 kg).

Advantages of precast piles include  high load capacity, an absence of corrosion or decay problems, and, in
most situations, a relative economy of cost. Precast piles must be handled carefully to avoid bending and cracking before installation. Splices be-tween lengths of precast piling can be made effectively with mechanical fastening devices that are cast into the ends of the sections.

A  sitecast concrete pile is made by driving a hollow steel shell into the ground and filling it with concrete. The shell is sometimes corru-gated to increase its stiffness; if the corrugations are circumferential, a  heavy steel mandrel (a stiff, tight-fitting liner) is inserted in the shell  during driving to protect the shell  from collapse, then withdrawn before concreting. Some shells with  longitudinal corrugations are stiff  enough that they do not require  mandrels. Some types of mandrel- driven piles are limited in length,  and the larger diameters of sitecast piles (up to 16 inches, or 400 mm)  can cause ground heaving. Load ca- pacities range from 45 to 150 tons  (40,000 to 136,000 kg). The primary  reason to use sitecast concrete piles  is their economy.

There is a variety of proprietary  sitecast concrete pile systems, each  with various advantages and disad- vantages (Figure 2.50). Concrete  pressure-injected footings (Figure 2.51)  share characteristics of piles, piers,  and footings. They are highly resistant to uplift forces, a property that  is useful for tall, slender buildings in  which there is a potential for overturning of the building, and for ten- sile anchors for tent and pneumatic  structures. Rammed aggregate piers and  stone columns  are similar to pressure- injected footings, but are constructed  of crushed rock that has been densely compacted into holes created by  drilling or the action of proprietary  vibrating probes.

Figure 2.50 Some proprietary types of sitecast concrete piles. All are cast into steel casings that have been driven into the ground; the uncased piles are made by withdrawing the casing as the concrete is poured and saving it for subsequent reuse. The numbers refer to the methods of driving that may be used with each: 1. Mandrel driven. 2. Driven from the top of the tube. 3. Driven from the bottom of the tube to avoid buckling it. 4. Jetted. Jetting is accomplished by advancing a high-pressure water nozzle ahead of the pile to wash the soil back alongside the pile to the surface. Jetting has a tendency to disrupt the soil around the pile, so it is not a favored method of driving under most circumstances

Figure 2.51 Steps in the construction of a proprietary pressure-injected, bottom-driven concrete pile footing. (a) A charge of a very low-moisture concrete mix is inserted into the bottom of the steel drive tube at the surface of the ground and compacted into a sealing plug with repeated blows of a drop hammer. (b) As the drop hammer drives the sealing plug into the ground, the drive tube is pulled along by the friction between the plug and the tube. (c) When the desired depth is reached, the tube is held and a bulb of concrete is formed by adding small charges of concrete and driving the concrete out into the soil with the drop hammer. The bulb provides an increased bearing area for the pile and strengthens the bearing stratum by compaction. (d, e) The shaft is formed of additional compacted concrete as the tube is withdrawn. (f) Charges of concrete are dropped into the tube from a special bucket supported on the leads of the driving equipment.

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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.