Engineering and Construction

Tweetear

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.

Tweetear

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...

Tweetear

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.

Tweetear

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.

Tweetear

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.

Tweetear

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.

Tweetear

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.

Tweetear

The function of a foundation is to transfer the structural loads reliably from a building into the ground. Every building needs  a foundation of some kind: A backyard toolshed will not be  damaged by slight shifting of its foundation and may need only  wooden skids to spread its load across an area of the ground  surface suffi cient to support its weight. A wood-framed house  needs greater stability than a toolshed, so its foundation reaches  through the unstable surface to underlying soil that is free of  organic matter and unreachable by winter frost. A larger building of masonry, steel, or concrete weighs many times more than  a house, and its foundations pierce the earth until they reach   soil or rock that is competent to carry its massive loads; on some  sites, this means going 100 feet (30 m) or more below the surface.

Because of the variety of soil, rock, and water conditions that  are encountered below the surface of the ground and the unique  demands that buildings make upon their foundations, foundation design is a highly specialized fi eld combining aspects of geotechnical and civil engineering that can be sketched here only in its  broad outlines.

Tweetear

Certain themes are woven throughout this blog and surface again and again in different, often widely varying contexts. These represent a set of concerns that fall into two broad categories: building performance
and building construction. 

The performance concerns relate to the inescapable problems that must be confronted in every building: fire; building movement of every kind, including foundation settlement, structural defl ections, and expansion and contraction due to changes in temperature and humidity; the fl ow of heat and air through building assemblies; water vapor migration and condensation; water leakage; acoustical per-formance; aging and deterioration of materials; cleanliness; building maintenance; and others.

The construction concerns are associated with the practical problems of getting a building built safely, on time, within budget, and to the required standard of quality: division of work between the shop and the building site; optimum use of the various building trades; sequencing of construction operations for maximum productivity; convenient and safe worker access to construction operations; dealing with inclement weather; making building components fit together; and quality assurance in construction materials and components through grading, testing, and inspection.

To the novice, these matters may seem of minor consequence when compared to the larger and often more interesting themes of building form and function. To the experienced building professional, who has seen buildings fail both aesthetically and physically for want of attention to one or more of these concerns, they are issues that must be resolved as a matter of course before the work of a building project can be allowed to proceed.

Tweetear

Improving Collaboration Among Team Members
The design and construction industry continues to evolve, testing innovative organizational structures and project delivery methods in which designers, builders, and owners assume less adversarial and less compartmentalized roles. Such approaches share characteristics such as:

•  Contractual relationships and working arrangements that foster collaboration between project members
•  Participation of the construction contractor during the design phases of a project
•  Overlapping of design and con-struction activities to reduce the “time to market”
• Expanded definitions of project services to encompass the full life cycle of a project—from its original conception, through planning, design and construction, to postconstruction occupancy—to better serve the
needs of the building owner

The growth of design/build in the construction marketplace is one example of this trend: Between 1980
and 2005 the share of private, nonresidential construction work per-formed as design/build construction increased from roughly 5 percent of the total market to an estimated 30 to 40 percent. Alternatives to traditional design/bid/build project delivery have gained increased acceptance in the public construction sector as well. Other new practice models, with such names as teaming, concurrent design, integrated practice, or alliancing, combine efficient project delivery methods with innovations in team member relationships in a variety of ways, with the aim of aligning all parties’ efforts with the shared goal of a finished product of  the highest possible quality and value to the owner.

Improving Effi ciency in Production
Other efforts within the construction industry focus on improvements in the efficiency of construction methods themselves. Unlike factory production, much building construction takes place outdoors, is performed
within constrained and often physically challenging work areas, and is executed by a highly fragmented workforce. Despite the differences in these production environments, the construction industry is looking
to lessons learned in factory production for approaches to improving the quality and effi ciency of its own processes. Such so-called lean construction methods attempt to:

•  Eliminate wasteful activities
•  Structure the methods of production and the supply chain of materials and products to achieve the quickest and most reliable workfl  ow
•  Decentralize information and decision making so as to put control of construction processes into the
hands of those most familiar with the work and most capable of improving it

Current estimates of labor in efficiency in building construction run as high as 35 to 40 percent, and estimates of materials wastage are 20 percent or more. The challenge of lean construction is to restructure the way in which construction materials and building components are manufactured, delivered, and assembled so as to reduce these inefficiencies and improve the quality of the delivered product.

Improving Information Management
Developments in information technology also are infl uencing the way buildings are designed and con-
structed. Most notable is  building information modeling (BIM), the computerized, three-dimensional modeling of building systems. Unlike the two-dimensional representation of building systems characteristic of
conventional  computer-aided design (CAD), BIM involves an intelligent model. Components are not only represented geometrically, but are also linked to data describing their intrinsic properties and their relationships to other components. Originally developed for use in highly capital-intensive industries such as aerospace and automobile manufacturing, this modeling technology is now fi  nding increased application in the design and construction of buildings.

BIM has the potential to impact many aspects of the building life cycle. It can aid the design team with the visualization and realization of complex geometries. It can improve coordination between design
disciplines––for example, searching out “collisions” between mechanical system ductwork and structural framing or other such physical interferences between systems––and it can facilitate the modeling of building energy use and the performance of other building systems. For the builder,  BIM can be used to improve coordination of trades, to drive the automated fabrication or preassembly  of building components, and to integrate cost and schedule data more closely with building design. For the building owner, information accumulated in the model during design and construction can be carried forward for use with postconstruction building operations and facilities planning.

As the implementation of BIM matures, it is expected to have a profound impact as a communication tool used to improve the coordination and sharing of information among all of the parties to a project.

As the integrated building model is shared across the traditional boundaries of disciplines and project phases, the boundaries of responsibility between the designers, constructors, and owners will also blur, and
new, more integrated relationships between these parties will likely be required to fully enable the potential of this technology.

Tweetear

Once a construction project is underway, the general contractor or construction manager assumes responsibility for day-to-day oversight  of the construction site, management of trades and suppliers, and communications between the construction team and other major parties, such as the owner and the designer.

On projects of any signifi cant size, this may include responsibility for filing  construction permits, securing the project site, providing temporary power and water, setting up office  trailers and other support facilities, providing insurance coverage for the work in progress, managing personnel on site, maintaining a safe work environment, stockpiling materials, performing testing and quality control, providing site surveying and engineering, arranging for cranes and other construction machinery, providing temporary structures and weather protection, disposing or recycling of construction waste, soliciting the work of subtrades and coordinating their efforts, submitting product samples and technical information to the design team for review, maintaining accurate records of the construction as it proceeds, monitoring costs and schedules, managing changes to the work, protecting completed work, and more.

Tweetear

Constructing a building of any signifi  cant size is a complex and costly endeavor, requiring the combined  efforts of countless participants and the coordination of myriad tasks.

Managing this process requires an in-depth understanding of the work required, of the ways in which differ-
ent aspects of the work depend upon each other, and of the constraints on the sequence in which the work must be performed.

Figure 1.11 captures one moment in the erection of a tall building. The process is led by the construction
of the building’s central, stabilizing core structures (in the photograph, the pair of concrete towerlike structures extending above the highest fl  oor levels). This work is followed by the construction of the surrounding fl  oors, which rely, in part, on the pre-viously completed cores for support.

Figure 1.11 In this photo, the construction sequence of a tall building is readily apparent: A pair of concrete core structures lead the construction, followed by concrete columns and fl oor plates and, fi nally, the enclosing curtain wall.

Attachment of the exterior skin can follow only after the fl oor plates are in place and structurally secure. And as the building skin is installed and fl  oor areas become enclosed and protected from the weather, further operations, such as the roughing in of mechanical and electrical systems, and eventually, the installation of fi  nishes and other elements, can proceed in turn. This simple example illustrates considerations that apply to virtually every aspect of building construction and at every scale from a building’s largest systems to its smallest details: Successful construction requires a detailed understanding of the tasks re-
quired and their interdependencies.

The construction project schedule is used to analyze and represent construction tasks, their relation-
ships, and the sequence in which they must be performed. Development of the schedule is a fundamental part of construction project planning, and regular updating of the schedule throughout the life of the project is essential to its successful management.

In a Gantt chart, a series of horizontal bars represent the duration of various tasks or groups of tasks that make up the project. Gantt charts provide an easy-to-understand representation of construction tasks and their relationships in time. They can be used to provide an overall picture of a project schedule, with only a project’s major phases represented (Figure 1.10), or they can be expanded to represent a larger number of more narrowly de-fi  ned tasks at greater levels of project detail (Figure 1.12).

Figure 1.10 In sequential construction, construction does not begin until design is complete. In phased construction, design and construction activities overlap, with the goal of reducing the overall time required to complete a project.

Figure 1.12 In a Gantt chart, varying levels of detail can be represented. In this example, roughly the top three-quarters of the chart is devoted to a breakdown of preconstruction and procurement activities such as bidding portions of the work to subtrades, preparing cost estimates, and making submittals to the architect (a). Construction activities, represented more broadly, appear in thebottom  portion (b).

The critical path of a project is the sequence of tasks that determines the least amount of time in which a project can be completed. For example, the construction of a building’s primary structural system is commonly on the critical path of a project schedule. If any of the tasks on which the completion of this system depends—such as design, shop drawing production and review, component fabrication, materials de-livery, or erection on site—are delayed, then the fi nal completion date of the project will be extended.


On the other hand, other systems not on the criti-cal path have more fl exibility in their scheduling, and delays (within limits) in their execution will not necessarily impact the overall project schedule.

The  critical path method is a technique for analyzing collections of tasks and optimizing the project schedule to minimize the duration and cost of a project. This requires a detailed breakdown of the work involved in a project and the identifi cation of dependencies between the parts (Figure 1.13). This information is combined with considerations of cost and resources available to perform the work, and then analyzed, usually with the assistance of computer software, to identify optimal scheduling scenarios. Once the critical path of a project has been established, the elements on this path are likely to receive a high degree of scrutiny during the life of the project, since delays in any of these steps will directly impact the overall project schedule.

Figure 1.13 The critical path method depends on the detailed analysis of work tasks and their relationships to generate an optimal construction schedule. Shown here is a schematic network diagram representing task dependencies. For example, task 6 cannot begin until tasks 1, 4, and 5 are completed, and tasks 7 and 9 cannot begin until task 6 is fi nished. The dashed lines on the diagram trace two of many possible paths from the start to the end of the diagram. To determine the critical path for this collection of tasks, all such paths must be identifi ed and the time required to complete each one calculated. The path requiring the most time to complete is the critical path, that is, the sequence of activities that determines the least time in which the collection of tasks as a whole can be completed.

Tweetear

In sequential construction (Figure 1.10), each major phase in the design and construction of a building is completed before the next phase begins.

Sequential construction can take place under any of the project delivery methods described previously. It
is frequently associated with design/bid/build construction, where the separation of design and construction
phases fi ts naturally with the contractual separation between design and construction service providers.

Phased construction, also called fast track construction, aims to reduce the time required to complete a project by overlapping the design and construction of various project parts (Figure 1.10). By allowing construction to start sooner and by overlapping the work of design and construction, phased construction can reduce the time required to complete a project.

Figure 1.10 In sequential construction, construction does not begin until design is complete. In phased construction, design and construction activities overlap, with the goal of reducing the overall time required to complete a project.

However, phased construction also introduces its own risks. Since construction on some parts of the project
begins before all design is complete, an overall cost for the project cannot be established until a signifi  cant portion of construction is underway.

Phased construction also introduces more complexity into the design process and increases the potential for costly design errors (for example, if foundation design does not ad-equately anticipate the requirements
of the not yet fully engineered structure above). Phased construction can be applied to any construction
delivery method discussed above. It is frequently associated with design/build and construction management project delivery methods, where the early participation of the construction entity provides resources that are
helpful in managing the complex coordination of overlapping design and construction activities.

Tweetear

With  fixed  fee or  lump sum compensation, the contractor or other construction entity is paid a fixed dollar amount to complete the construction of a project regardless of that entity’s actual costs. With this compensation method the owner begins construction with a known, fixed construction cost and assumes minimal risk for unanticipated increases in cost. On the other hand, the construction contractor assumes most of the risk of unforeseen costs but also stands to gain from potential savings. Fixed fee compensation is most suitable to projects where the scope of the construction work is well defi ned at the time that the construction fee is set, as is the case, for example, with conventional design/bid/build construction.

As an alternative, compensation may be set on a  cost plus a fee basis, where the owner agrees to pay the  construction entity for the actual costs of construction—whatever they may turn out to be—plus an
additional fee. In this case, the construction contractor is shielded from most cost uncertainty, and it is the
owner who assumes most of the risk of added costs and stands to gain the most from potential savings. Cost
plus a fee compensation is most often used with projects where the scope of construction work is not fully known at the time that compen-sation is established, a circumstance most frequently associated with con-struction management or design/build contracts.

With fi xed fee compensation, the builder assumes most of the risk related to unanticipated construction costs; with cost plus a fee compensation, the owner assumes most of this risk. Between these two extremes, many other fee-structuring arrangements can be used to allocate varying degrees of risk between the two parties.

Tweetear

In conventional design/bid/build project delivery (Figure 1.8), the owner first hires a team of architects and
engineers to perform design services, leading to the creation of drawings and technical specifi cations, referred to collectively as the construction documents, that comprehensively describe the facility to be built. Next, construction firms are invited to bid on the project. Each bidding firm  reviews the construction documents and proposes a cost to construct the facility.

The owner evaluates the submitted proposals and awards the construction contract to the bidder deemed most suitable. This selection may be based on bid price alone, or other factors related to bidders’ qualifications may also be considered. The construction documents then become part of the construction contract, and the selected firm proceeds with the work. On all but small projects, this firm acts as the  general contractor, coordinating and overseeing the overall construction process but frequently relying on smaller, more specialized subcontractors to perform significant portions or even all of the construction work.

Figure 1.8 In design/bid/build project delivery,  the owner contracts separately with the architect/engineer (A/E) design team  and the construction general contrac- tor (GC). In a design/build project, the  owner contracts with a single organiza- tional entity that provides both design  and construction services.

During construction, the design team continues to provide services to the owner, helping to ensure that the facility is built according to the requirements of the documents as well as answering questions related to the design, changes to the work, payments to the contractor, and similar matters. Among the advantages of design/bid/build project delivery are its easy-to-understand organizational structure, well-established legal prec-edents, and ease of management. The direct relationship between the owner and the design team ensures that the owner retains control over the design and provides a healthy set of checks and balances during the construction process. Also, with design work completed before the project is bid, the owner starts construction with a fixed construction cost and a high degree of confidence regarding the final costs of the project.

In design/bid/build project delivery, the owner contracts with two entities, and design and construc-
tion responsibilities remain divided between these two throughout the project. In  design/build project de-
livery, one entity ultimately assumes responsibility for both design and construction (Figure 1.8). A design/
build project begins with the owner developing a conceptual design or program that describes the functional or performance requirements of the proposed facility but does not detail its form or how it is to beconstructed.

Next, using this conceptual information, a design/build organization is selected to complete all remaining as-
pects of the project. Selection of the designer/builder may be based on a competitive bid process similar to that described above for design/bid/build projects, on negotiation and evaluation of an organization’s qualifi  cations for the proposed work, or on some combination of both. Design/build organizations themselves can
take a variety of forms: a single fi rm encompassing both design and construction expertise, a construction management fi rm that subcontracts with a separate design fi rm to provide those services, or a joint venture between two fi rms, one specializing in construction and the other in design.

Regardless of the internal structure of the design/build organization, the owner contracts with this single entity throughout the remainder of the project, which assumes responsibility for all remaining design and construction services. Design/build project delivery gives the owner a single source of accountability for all aspects of the project. It also places the designers and constructors in a collaborative relationship, introducing construction expertise into the design phases of a project and allowing the earliest possible consideration of constructability, cost control, construction scheduling, and similar matters. This delivery method also readily accommodates fast track construction, a scheduling technique for reducing construction time that is described below.

Other delivery methods are possible: An owner may contract separately with a design team and a construction manager. As in design/build construction, the construction manager participates in the project prior to the onset of construction, introducing construction expertise during the design stage. Construction management project delivery can take a variety of forms and is frequently associated with especially large or complex projects (Figure 1.9). In  turnkey construction, an owner contracts with a single entity that provides not only design and construction services, but financing for the project as well. Or design and construction can be undertaken by a  single-purpose entity, of which the owner, architect, and con-tractor are all joint members. Aspects of these and other project delivery methods can also be intermixed, allowing many possible organizational schemes for the delivery of design and construction services that are suitable to a variety of owner requirements and project circumstances.

Figure 1.9  In its traditional role, a construction manager (CM) at fee provides project  management services to the owner and assists the owner in contracting directly  for construction services with one or more construction entities. A CM at fee  is not directly responsible for the con- struction work itself. A CM at risk acts  more like a general contractor and takes on greater responsibility for construction  quality, schedule, and costs. In either case, the A/E design team also contracts  separately with the owner.