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The concept of satellite position fixing commenced with the launch of the first Sputnik satellite by the USSR in October 1957. This was rapidly followed by the development of the Navy Navigation Satellite System (NNSS) by the US Navy. This system, commonly referred to as the Transit system, was created to provide a worldwide navigation capability for the US Polaris submarine fleet. The Transit system was made available for civilian use in 1967 but ceased operation in 1996. However, as the determination of position required very long observation periods and relative positions determined over short distances were of low accuracy, its application was limited to geodetic and low dynamic navigation uses.

In 1973, the US Department of Defense (DoD) commenced the development of NAVSTAR (Navigation System with Time and Ranging) Global Positioning System (GPS), and the first satellites were launched

in 1978.

The system is funded and controlled by the DoD but is partially available for civilian and foreign users. The accuracies that may be obtained from the system depend on the degree of access available to the user, the sophistication of his/her receiver hardware and data processing software, and degree of mobility during signal reception.

In very broad terms, the geodetic user in a static location may obtain ‘absolute’ accuracy (with respect to the mass centre of the Earth within the satellite datum) to better than ±1 metre and position relative to another known point, to a few centimetres over a range of tens of kilometres, with data post-processing. At the other end of the scale, a technically unsophisticated, low dynamic (ship or land vehicle) user, with limited access to the system, might achieve real time ‘absolute’ accuracy of 10–20 metres.

The GPS navigation system relies on satellites that continuously broadcast their own position in space and in this the satellites may be thought of as no more than control stations in space. Theoretically, a user who has a clock, perfectly synchronized to the GPS time system, is able to observe the time delay of a GPS signal from its own time of transmission at the satellite, to its time of detection at the user’s equipment. The time delay, multiplied by the mean speed of light, along the path of the transmission from the satellite to the user equipment, will give the range from the satellite at its known position, to the user. If three such ranges are observed simultaneously, there is sufficient information to compute the user’s position in three-dimensional space, rather in the manner of a three-dimensional trilateration. The false assumption in all this is that the user’s receiver clock is perfectly synchronized with the satellite clocks.

This method is used to give high precision over long baselines such as are used in geodetic control surveys. At its simplest, one receiver is set up over a station of known X, Y, Z coordinates, preferably in the WGS84 reference system, whilst a second receiver occupies the station whose coordinates are required.

Another typical phone call. An owner wants to know how long it will take for the lightweight concrete in the elevated slabs of his new building to dry before he can place the floor covering. The slabs were placed 4 months ago, but tests still show a moisture-vapor-emission rate of 8 pounds per 1000 square feet in 24 hours. The floor-covering manufacturer requires the concrete to be at 3 lbs/1000 sf/24 hrs. Delaying floorcovering installation will delay building occupancy. The owner has never had this problem in other buildings he has constructed. Why won’t this concrete dry? Concrete is concrete, right?

Well, unfortunately it’s not. Many owners and contractors have told us they’ve experienced project delays while waiting for lightweight concrete to dry. Though we couldn’t find any data regarding the drying time of lightweight concrete, field experience tells us that lightweight concrete takes longer to dry than normal-weight concrete. To help fill this information gap, CONCRETE CONSTRUCTION devised a testing program to find out how long it takes lightweight concrete to dry.

Three normal-weight concretes with water-cement ratios of 0.31, 0.37, and 0.40 were delivered to a testing lab in 1-cubic-yard loads. A lightweight concrete mix with a water cement ratio of 0.40 was also delivered to the testing lab. The proprietary mixes were supplied by the ready-mix division of CAMAS Colorado, Denver. The normal-weight concrete moisture-vapor-emission test results were reported in THE CONCRETE PRODUCER (Ref. 1). Here we compare the lightweight concrete moisture-vapor-emission test results with those for normal-weight concrete having the same water-cement ratio. The producer tightly controlled water content and water-cement ratio by closely monitoring aggregate moisture and water left in the drum. The fresh and hardened properties of the lightweight concrete were as follows:

■ 3-inch slump

■ 4.5% air content

■ 124-pound-per-cubic-foot unit weight

■ 48° F temperature

■ 6850-psi 28-day compressive strength

Workers placed and vibrated the concrete in 3-foot-square test slabs 2, 4, 6, and 8 inches thick. After striking off the surface with a 2x4 and floating it by hand, they covered the slabs with plastic sheeting for 3 days. Calciumchloride moisture-emission testing began after the sheeting was removed.

Moisture-emission tests were conducted in accordance with the test manufacturer’s instructions. Technicians ran one test on each slab at the 3-day age but thereafter ran two tests on each test slab and reported an average test value. 

The calcium-chloride test kits were left in place for 72 hours on slabs stored inside the test lab at a 70±3° F and a relative humidity of 28±5%. This is the normal indoor environment during the winter in the Rocky Mountain region, where the tests were conducted, and represents the environment found in many buildings without relative-humidity controls. 

The table shows moisture-vapor emission rates for the test slabs after drying for up to 183 days. Drying times are measured from the end of the 3-day curing period to the end of the 72-hour moisture-emission test. Thus the 3-day measurement was taken 6 days after concrete placement. The test data support the following conclusions.

Lightweight concrete dries slower. Regardless of the test slab thickness, the lightweight concrete took about 6 months to dry to a moisture-vapor emission rate of 3 lbs/1000 sf/24 hrs. Normal-weight concrete of the same water-cement ratio took only 6 weeks of drying in laboratory air to reach the same level (Ref. 1). From previous work (Ref. 2), we know that laboratory drying represents the fastest drying time. So field conditions that include wet-dry cycles will increase the actual time for the slab to reach the specified moisture-vapor-emission limits. 

As with normal-weight concrete, the moisture-vapor-emission rates were unaffected by slab thickness. 

Lightweight concrete offers many advantages. However, any owner using lightweight concrete that’s to be covered by a moisture-sensitive floor covering or any architect/engineer specifying this combination should consider the slower drying time as an important part of the construction schedule. Once lightweight concrete is specified, the contractor can’t change its drying characteristics. If owners want the benefits of lightweight concrete and a fast-track schedule, they may need to consider applying a polymer coating or sheet product to reduce moisture emissions.

Reinforced concrete structural systems can be formed into virtually any geometry to meet any requirement. Regardless of the geometry, standardized floor and roof systems are available that provide cost-effective solutions in typical situations. The most common types are classified as one-way systems and two-way systems. Examined later are the structural members that make up these types of systems.

It is common for one type of floor or roof system to be specified on one entire level of building; this is primarily done for cost savings. However, there may be cases that warrant a change in framing system. The feasibility of using more than one type of floor or roof system at any given level needs to be investigated carefully.

A one-way reinforced concrete floor or roof system consists of members that have the main flexural reinforcement running in one direction. In other words, reactions from supported loads are transferred primarily in one direction. Because they are primarily subjected to the effects from bending (and the accompanying shear), members in one-way systems are commonly referred to as flexural members.

FIGURE 1 One-way slab system.

Members in a one-way system are usually horizontal but can be provided at a slope if needed. Sloped members are commonly used at the roof level to accommodate drainage requirements.

Illustrated in Fig. 1 is a one-way slab system. The load that is supported by the slabs is transferred to the beams that span perpendicular to the slabs. The beams, in turn, transfer the loads to the girders, and the girders transfer the loads to the columns.

Individual spread footings may carry the column loads to the soil below. It is evident that load transfer between the members of this system occurs in one direction.

FIGURE 2 Standard one-way joist system.

Main flexural reinforcement for the one-way slabs is placed in the direction parallel to load transfer, which is the short direction. Similarly, the main flexural reinforcement for the beams and girders is placed parallel to the length of these members. Concrete for the slabs, beams, and girders is cast at the same time after the forms have been set and the reinforcement has been placed in the formwork. This concrete is also integrated with columns. In addition, reinforcing bars are extended into adjoining members. Like all cast-in-place systems, this clearly illustrates the monolithic nature of reinforced concrete structural members.

A standard one-way joist system is depicted in Fig. 2. The one-way slab transfers the load to the joists, which transfer the loads to the column-line beams (or, girders). This system utilizes standard forms where the clear spacing between the ribs is 30 in. or less. Because of its relatively heavy weight and associated costs, this system is not used as often as it was in the past.

FIGURE 3 Wide module joist system.

Similar to the standard one-way joist system is the wide-module joist system shown in Fig. 3. The clear spacing of the ribs is typically 53 or 66 in., which, according to the Code, technically makes these members beams instead of joists. Load transfer follows the same path as that of the standard joist system.

Reinforced concrete stairs are needed as a means of egress in buildings regardless of the number of elevators that are provided. Many different types of stairs are available, and the type of stair utilized generally depends on architectural requirements. Stair systems are typically designed as one-way systems.

FIGURE 4 Two-way beam supported slab system.

A two-way beam supported slab system is illustrated in Fig. 4. The slab transfers the load in two orthogonal directions to the column-line beams, which, in turn, transfer the loads to the columns. Like a standard one-way joist system, this system is not utilized as often as it once was because of cost.

Since the start of the formal approaches and procedures for carrying out the structural design, there have been many developments in the underlying principles and the implicit and explicit design objectives. Starting with putting limits in the allowable (working) stresses in various materials to achieve indirect safety factors, the design process slowly evolved within last few decades to more explicit consideration of different load and capacity factors. The recognition of the difference between brittle and ductile failure, and the introduction of capacity-based design approaches, led to the more comprehensive performance design using high level of analysis sophistication, and more explicit linkage between demand and performance. The most recent emphasis is on risk-based design, and a more integrated and holistic approach within the framework of consequence-based engineering. This section discusses a brief account of the progression of these design approaches and their impact on the cost, performance, and the final objective of public safety.

Structural design is the process of proportioning the structure to safely resist the applied forces and load effects in the most resource-effective and friendly manner. The term “friendly” refers to the aspect of design dealing with environmental friendliness, sustainability, ease of construction, and usability that are not explicit part of the strength consideration. Resources refer to the use of material, labor, time, and other consumables that are used to construct and maintain the structure. Ideally, the role of structural design is straight forward. It is the transformation of the effects of various environmental and man-made actions (including constraints on materials, dimensions and cost etc.) into the appropriate material specifications, structural member sizes, and arrangements (Fig. 1).

The basic objective is to produce a structure capable of resisting all applied loads without failure and excessive deformations during its anticipated life. The very first output of any engineering design process is a description of what is to be manufactured or built, what materials are to be used, what construction techniques are to be employed, and an account of all necessary specifications as well as dimensions (which are usually presented in the form of drawings). The second output is a rational justification or explanation of the design proposal developed based on either full-scale tests, experiments on small physical models, or the mathematical solution of detailed analytical models representing the behavior of real structures.

The process of structural design has passed through a long and still continuous phase of improvements, modifications, and breakthroughs in its various research areas. The structural analysis and design philosophies for new and existing buildings have a fascinating history. Perhaps, the first ever achievement in the history of structural design was the “confidence” by virtue of which early builders were able to convince themselves that the resulting structure could, indeed, be built and perform the intended function for the entirety of its intended life. Hence, the job of the very first engineers can be thought of as “to create the confidence to start building”. Over the course of the history, various scientists, mathematicians, and natural philosophers presented revolutionary ideas which resulted in improved understanding of structures and built environment. With the developments in different areas of practical sciences, the task of building design was gradually divided among more and more professionals depending upon aesthetic considerations, intended functions, materials, optimum utilization of space, lighting, ventilation, and acoustic preferences. The visual appearance, sense of space, and function (or the architecture) became a distinct concern during the 15th and 16th centuries. About a century later, designers first began to think about the load bearing aspects of structures in terms of self-weight and other sources of expected loading. Thinking separately about the role of individual materials and resulting structures grew during the late 17th and 18th centuries following Galileo’s work. The idea that the aesthetics should be given proper importance independent of the materials and load-bearing characteristics of the structure prevailed during the late 19th and the early 20th centuries.

Table 1 presents a brief timeline of some of the major developments which led to modern techniques and methodologies for analyzing and designing structures.

It is worth noting that historically an understanding of how structures work was never a phenomenon that required detailed knowledge of mathematical procedures and laws of mechanics. A common misconception is that various new structural forms and shapes were first devised by mathematicians (and experts of geometry) and later taken up by builders and engineers. In fact, the opposite is true, with perhaps just one exception, i.e., the hyperbolic paraboloid (whose structural properties were discovered in 1930s). In the last few centuries, artists, sculptors, and builders have displayed a remarkable understanding and skill of converting materials into structures (some of which are still standing today remarking the testimony of their expertise).

The word structure (pronounced as “strək(t)SHər”) is a noun as well as a verb. As a noun, it means “the arrangement of and the relations between parts or elements of something complex,” and as a verb it refers to the process to “construct or arrange according to a plan; give a pattern or organization.” This term is used extensively in literature, and many disciplines signify these properties and processes.

When applied to the physical and built environment, the term “structure” means an assemblage of physical components and elements, each of which could further be a structure in itself, signifying the complexity of the system. The discipline of “Structural Engineering” refers to the verb part of the definition, dealing with the ways to arrange and size a system of components for construction according to a plan and serving the intended purpose. The primary purpose of any structure is to provide a stable, safe, and durable system that supports the desired function within the physical environment, of which the structure is a part of. The role of the structural engineer, therefore, is to “conceive, analyze, and design” the structure to serve its purpose.

There is hardly any aspect of our built environment or human activity that does not rely on physical structures. Buildings that provide useful places to live and work, bridges that provide us means to move across obstacles, factories that are needed to manufacture almost everything we need, dams to store water to generate power and irrigate lands, transmission towers to distribute electricity; all require structures to function, and structural engineers to design such structures. The role and importance of structural engineering and structural engineers is often underestimated and misunderstood. While a well-conceived and well-designed structure is the backbone of our built environment, a poorly conceived, designed, or constructed structure poses a serious hazard to the safety and well being of people and property. Collapse or failure of structures can claim a large number of lives and result in extensive economic loss. The role of structural engineers is, therefore, critical for overall economic development as well as for improving the community resilience to disasters.

The physical structure, or structure for brevity, can be assembled in infinite ways using very few basic element types or forms, some of which are shown in Fig. 1.1. As is evident, these are derived from or are consistent with basic geometric primitives such as line, curve, plane, surface, and solid. This compatibility can often be used to blend the form, function, and structure.

The assemblage of these components can be of many types and configurations. Some are made entirely of the skeleton-type members, some from surface-type members, and some from solid-type members, but most structures are a combination of more than one member type. Based on the member types, the structures can be broadly categorized as

• cable structures
• skeletal structural
• spatial structures
• solid structures, and
• a combination of the aforementioned categories.

Cable Structures: Using Cables as the Main Member Type

These structures primarily transfer forces and internal actions through tension in individual cables or a set of cables. The shapes or geometry of cable profile often govern the behavior. Examples of such structures are:

• Cable nets and fabric structures
• Cable stayed structures
• Cable suspended structures

Skeletal Structures: Using Beam-Type Members

These structures are composed of bar members that mainly resist the loads and forces through a combination of tension, compression, bending, shear, torsion, and warping. Such skeletal members are often called ties, struts, beams, columns, and girders. Typical application of skeletal structures can be found in the following:

• Truss structures
• Framed structures
• Grid and grillage structures

Spatial Structures: Using the Membrane/Plate/Shell-Type Members

These are structures created by spatial or surface type of elements and they transfer loads through a combination of bending, compression, torsion, and in-plane and out-of-plane shear of the element surface. The cross-sections of such members are generally rectangular. Examples of such structures include:

• General shell structures
• Dome-type structures
• Slab, wall structures
• Silos, chimneys, and stacks
• Box girder bridges

Sometimes, surface members and structures can be created by using a large number of skeletal members, covered by a skin or cladding, combining the fabric, cable, and bars to create surfaces.

Solid Structures: Using the Solid-Type Members

These structures comprise of solid bodies or members in which the forces or loads are transferred through the member bodies. Such members or structures do not have a cross-section in the conventional sense. Some of the structures are:

• Dams, thick arches, thick tunnels
• Pile caps, thick footings, thick slabs, pier heads, large joints, etc.

Mixed Structures: Using One or More of the Basic Element Types

Most often, the real structures are composed of one or more types of basic elements. For example, a typical building is made of columns and beams (skeletal), slabs and walls (shell), footings (solid) structures, etc.

There are several other types of structures that are either a combination of the basic forms or especially developed for a particular application or usage. These include stressed ribbon bridges, fabric structures, skeleton spiral structures, floating offshore structures, pneumatically inflated structures etc.

The slabs-on-ground does not require high protection against water penetration and moisture in arid regions that does not have soils or have any sort of drainage problems.

This precaution is not necessary if the area does not have irrigation activities going on nearby. This is the case unless the building code of that regions requires treatment.

But for all site conditions other than mentioned above, it is essential to have water proofing or damp proofing for the slabs constructed on grounds.

As per ACI 302.IR-89, there is no need for damp proofing or water proofing nor vapor retarders on sites that are well drained or those sites under the following conditions:

1. The site has water table level very low from the ground surface
2. A substratum of coarse aggregate that is freely draining is installed
3. A floor covering is provided to the slab that is not affected by the moisture.

Damp Proofing of Slabs–on–Ground

The damp proofing of slabs constructed on ground can be done either below or above the surface. Mostly, the damp proofing is installed below the surface of the slab. Mostly plastic film of vapor retarders is employed below the slab surface as a damp proofing membrane.

The figure-1 below shows the installation of a vapor retarder over the surface of gravel substrate. The vapor retarders are sealed to the foundation wall as shown. This will provide continuity in the placement.

In order to avoid the penetration of sheet, brick chairs are employed other than the wire chairs.

Fig.1. The plastic vapor retarders are placed above the grave substrate that is tamped.

When wood flooring has to be provided above the slabs on ground, the damp proofing is done over the surface of the floor. The damp proofing over the slab top surface is permitted only under the following conditions, when:

1. The water table is at least 12 inches below the slab surface
2. Installation of footing drains is carried out

The slabs-on ground with wooden flooring on the top will ask for damp proofing unless the above conditions are satisfied.

The figure-2 below shows the installation of wooden flooring over the slabs that are constructed on ground.

The wood flooring span sleepers will separate the surface from the bottom slab as shown in figure-2. The damp proofing can be applied to the slab only under the sub soil conditions mentioned above.

Fig.2.Wooden Flooring Above Slabs-on-Ground

In the figure-2, an air space is created between the wooden flooring and the slabs constructed. This must be vented at the junctions so that the water penetration due to water vapor migration is avoided. This will help in maintaining an equal relative humidity both above and below the surface of the wooden flooring provided.

The top surface damp proofing of the slabs constructed on ground will be interrupted by the partitions and the columns present. Vapor retarders constructed will be interrupted by these extra elements bringing a loss of integrity of the damp proofing system.

The figure -3 below shows the same. Under such situations, the best alternative is to have damp proofing or water proofing under the slab.

Fig.3. The column interrupting the vapor retarder placed above a slab-on-ground

Risk in Using Vapor Retarders as a Waterproofing Solution

The substitution of vapor retarders instead of water proofing will welcome risks. The performance of the vapor retarder is dependent on the integrity of the film as well as the way it is seamed. The maintenance of a vapor retarder seam integrity in field condition is very difficult to attain in a slab on ground construction.

The use of wooden or vinyl coverings too make the vapor retarders into risk. Other risk parameters are the use of wall – sensitive adhesives. Under these situations, it is most preferred to have a heavy-duty panel or water proofing to be installed than waiting for the loss caused by the complete replacement of moisture damaged floor.

As shown in figure-2, the attachment of sleepers to the slabs-on-ground must be carried out with the help of water resistant adhesives instead of mechanical anchors.

The use of nails and other mechanical anchors will result in puncture of vapor retarders that is located on the top surface of the slab.

Considerations in Damp Proofing Slabs on Ground

When the top surface of the slab is subjected to damp proofing, the slab should have a substratum that is of a granular material. This layer of properly graded aggregate will help in preventing the rise of moisture (mainly by the principle of capillary action).

It is advised not to have a vapor retarder below the slab surface by using suspenders and other belts along with having a top surface damp proofing. The residual moisture in the concrete slabs can result in vapor pressure that can disband the vapor retarder and result in rupture.

It is not appropriate to have too many floor finishes and coatings. This is because the concrete is moisture sensitive. Moisture sensitive adhesives can be used as a mode of moisture resistance.

Liquid applied coatings in concrete slabs on ground have resulted in moisture in concrete. The following conditions can result in high level moisture contents:

1. For those slabs that are cast on a grade with no proper functioning of the under-slab vapor retarders.
2. Conditions when the suspended slabs are cast over the non – vented steel formworks.
3. The conditions where the suspended slabs are constructed over the occupancies that possess higher relative humidity. These includes elements like commercial kitchen and swimming pools.
4. The installation of suspended slabs over the crawl spaces that are unvented.
5. The slabs with lightweight aggregates.
6. The slabs that used a water cement ration in excess of 0.55 (w/c = 0.55).
7. The slabs that cured in a period less than 90 day.

The manufactures of the floor finishes or damp proofing coatings highly follow the requirement of testing the concrete slab before the installation of any membranes. This testing will ensure that the moisture content in the slab is within the susceptible limits and won’t result in any failure.

The Calcium Chloride Test as specified by ASTM F1869 is used to determine the Moisture Emission Rate (MVER). This is measured in pounds per thousand square feet within 24 hours. The accepted range of MVER is three to five pounds.

The internal relative humidity in the structure is obtained by means of a hygrometer by placing probes in concrete. This is as per ASTM F710 and the established rate is 75 percent.

This result can be achieved within 1 month of drying time per inch of the concrete element tested. It recommends the installation of vapor retarders in the direction of concrete pouring.

This brief article explains the work flow procedure for site engineeers to execute steel sheet piling works. The procedure details various steps and checks that are recommended to be followed for such activities.

• Piling contractors must be requested to provide a written method statement as appropriate for the piling operations. Find out what induction training and information specific to a method statement is provided by immediate site supervisors to piling operatives.

• Note that cranes must be selected and used in accordance with CP3010 and with the Construction (Lifting Operations) Regulations 1998. A firm level base of adequate bearing value must be provided, or crane mats used.

• Check that cores of pendant/bridle ropes are not fractured.

• Any crane used for raising or lowering operatives must be fitted with a dead man’s handle and the descent must be effectively controlled; the latter may be achieved by power lowering. Properly constructed man-carrying cages, which are unable to spin or tip, must be used. The cages should be regularly and carefully inspected.

• All lifting appliances and gear must carry appropriate certificates of test and examination, and must be adequate for the job, paying particular attention to the risk of damage to gear by sharp edges.

• All personnel working on piling operations must wear safety helmets (helmets appropriate for piling work are now available with retaining strap and smaller peaks). Ear and eye protection should be provided.

• Piling machine operators must be at least 18 years of age, trained, competent, medically fit and authorised by site management to operate the machine.

• When piling from a pontoon or adjacent to water, personnel must wear life jackets. Rescue equipment (e.g. a safety boat and lifebuoys with lifelines attached) must be kept ready for immediate use and enough operatives must know how to use it.

• When splitting bundles of sheet piles, use chocks. If large quantities of piles are handled, the use of purpose-made strops and grips is advised.

• Piles should not be stacked too high or in a cantilever position. Use spacers and chocks where necessary. Tubular piles should not be stacked more than four high and should be properly chocked.

• When lifting piles or piling hammers, use hand lines to control the load. Give due consideration to wind speed during the operation.

• Check the dimensions and alignment of clutches. If necessary perform trial clutching of piles. This is advisable for Z-shaped pile sections where the clutch is less positive than on trough sections (e.g. Larssen piles).

See Fig 1.

• Positioning (pitching) and driving sheet piling is usually done by using a temporary supporting structure (gate). This is made up of heavy steel or timber H-frames supporting horizontal-heavy H-beam guides. The H-frames can be supported on heavy steel spreaders or specially cast concrete blocks. Ensure an adequate foundation under these frames and bases to prevent subsidence and overturning during piling operations. This is particularly applicable during work in rivers, etc.

• If using concrete blocks ensure that they are suitably reinforced to withstand loads from lifting and shock loading. Vertical steel columns should have a good bottom fixing. Vertical timber should not be cast into the block but should be wedged and bolted. Where doubt exists over stability use guy lines or raking steel props.

• All horizontal gates with platforms over 2m high, or over any potentially dangerous areas, must be provided with adequate guardrails, toeboards and correct ladder access.

• Ladders must be secured and extend at least lm above staging.

• If using a cantilever system, use a tie-back where possible, as well as kentledge to provide safe anchorage.

• When piling is progressing and temporary piles are used to support the gate system, use purpose-made brackets and bolt them to the piles. Any welding necessary should be carried out by competent welders.

• If shackle holes have to be burned in the pile, remove sharp burrs to prevent damage to shackle pins.

• Use quick-release shackles wherever possible:

– the sheet pile must not be lifted vertically without first checking that the pin is properly engaged through the sheet

– do not use a pull rope less than 5mm diameter

– the length of rope used must be less than the length of the pile, to prevent the extra rope snagging and pulling the release for the shackle

– secure the rope around the sheet pile to prevent snagging

– if a special lifting eye is to be welded to the pile for angled pitching, the weld should have a factor of safety of at least 2.

• Pitch long sheet piles with a pile threader, following the manufacturer’s guidance for use. Where this is not possible, use a pile pitching cage. The cage is normally hung from an adjacent pile, the operatives wearing safety harnesses hooked to the adjacent pile before the crane hook is removed from the cage.

• When feeding sheet piles through the top and bottom gates, use wood blocks or a bent bar. Never use a straight pinch bar, as fingers can easily be trapped. Use a spacer block between the guides to keep the leading free clutch in its correct alignment.

• Where access and work is carried out from ladders:

– Clutching: the ladder must be placed in the valley of a previously placed pile; the ladder must be footed and when at the top of the ladder both hands are required for clutching, a safety belt must be worn and secured to the pile using a manlock.

– Wedging: the ladder must be placed against the H-beam and footed wedges should be pre-placed on the beam. A 4 lb. lump hammer must be used as this can be swung with one hand but if two hands are required, a safety belt must be used with the lanyard wrapped around the H-beam or used with a manlock.

Note: At all times safety helmets and footware must be worn. When working at height the helmet should be secured with retaining strap.

• Changes in work method:

– The work method must not be changed without consultation of the senior site representative responsible for the piling operation.

– If windy conditions (e.g. over 30 mph gusts) make the handling of the sheet piles difficult, stop work until the senior site representative responsible for the piling operation has been consulted and a safe method of continuing the work has been devised.

A shallow foundation is often selected when the structural load will not cause excessive settlement of the underlying soil layers.


In general, shallow foundations are more economical to construct than deep foundations. Common types of shallow foundations are listed in Table 1 and described later:

These types of shallow foundations are probably the most common types of building foundations. Figure 1 shows various types of shallow foundations.

Figure 2 shows various types of mat foundations. Based on economic considerations, mat foundations are often constructed for the following reasons (NAVFAC DM-7.2, 1982):

i. Large individual footings. A mat foundation is often constructed when the sum of individual footing areas exceeds about one-half of the total foundation area.

ii. Cavities or compressible lenses. A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlement caused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition.

iii. Shallow settlements. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

iv. Unequal distribution of loads. For some structures, there can be a large difference in building loads acting on different areas of the foundation. Conventional spread footings could be subjected to excessive differential settlement, but a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.

v. Hydrostatic uplift. When the foundation will be subjected to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.

Posttensioned slab-on-grade is common in southern California and other parts of the United States. The most common uses of posttensioned slab-on-grade are to resist expansive soil forces or when the projected differential settlement exceeds the tolerable value for a conventional (lightly reinforced) slab-on-grade. For example, posttensioned slabs-on-grade are frequently recommended if the projected differential settlement is expected to exceed 0.75 in. (2 cm).

The Post-Tensioning Institute (1996) has prepared installation and field inspection procedures for posttensioned slab-on-grade. Posttensioned slab-on-grade consists of concrete with embedded steel tendons that are encased in thick plastic sheaths. The plastic sheath prevents the tendon from coming in contact with the concrete and permits the tendon to slide within the hardened concrete during the tensioning operations. Usually tendons have a dead end (anchoring plate) in the perimeter (edge) beam and a stressing end at the opposite perimeter beam to enable the tendons to be stressed from one end. However, it is often recommend that the tendons in excess of 100 ft (30 m) b stressed from both ends.

Because posttensioned slab-on-grade perform better (i.e., less shrinkage related concrete cracking) than conventional slab-on-grade, they are more popular even for situations where low levels of settlement are expected. Posttensioned slab-on-grade has become common for situations where it is desirable to limit the amount and width of concrete shrinkage cracks.

If the expected settlement for a proposed shallow foundation is too large, then other options for foundation support or soil stabilization must be evaluated. Some commonly used alternatives are as follows:

i. Grading. Grading operations can be used to remove the compressible soil layer and replace it with structural fill. Usually the grading option is only economical if the compressible soil layer is near the ground surface and the groundwater table is below the compressible soil layer or the groundwater table can be economically lowered.

ii. Surcharge. If the site contains an underlying compressible cohesive soil layer, the site can be surcharged with a fill layer placed at the ground surface. Vertical drains (such as wick drains or sand drains) can be installed in the compressible soil layer to reduce the drainage paths and speedup the consolidation process. Once the compressible cohesive soil layer has had sufficient consolidation, the fill surcharge layer is removed and the building is constructed.

iii. Densification of soil. There are many different methods that can be used to densify loose or soft soil. For example, vibro-flotation and dynamic compaction are often effective at increasing the density of loose sand deposits. Another option is compaction grouting, which consists of intruding a mass of very thick consistency grout into the soil, which both displaces and compacts the loose soil.

iv. Floating foundation. A floating foundation is a special type of deep foundation where the weight of the structure is balanced by the removal of soil and construction of an underground basement.