Underline or mark the main ideas of Text 10A and retell it in English.

19. Skim Text 10B "Foundations of Residential and Industrial Buildings" and try to understand what it is about and what information is already known to you.

• TEXT 10B

Foundations of residential and industrial buildings

The foundations in residential and industrial buildings support considerably heavy loads. Floor loadings range from 450 to 1,500 kilograms per square metre, and the full range of foundation types is used for them. Spread footings are used, as are pile foundations, which are of two types, bearing and friction.

A bearing pile is a device to transmit the load of the building through a layer of soil too weak to take the load to a stronger layer

of soil some distance underground; the pile acts as a column to carry the load down to the bearing stratum. Solid bearing piles were originally made of timber, which is rare today; more commonly they are made of precast concrete, and sometimes steel H-piles are used. The pile length may be a maximum of about 60 metres but is usually much less. The piles are put in place by driving them into the ground with large mechanical hammers. Hollow steel pipes are also driven, and the interiors are excavated and filled with concrete to form bearing piles; sometimes the pipe is withdrawn as the concrete is poured.

An alternative to the bearing pile is the caisson. A round hole is dug to a bearing stratum with a drilling machine and temporarily supported by a steel cylindrical shell. The hole is then filled with concrete poured around a cage of reinforcing bars; and the steel shell may or may not be left in place, depending on the surrounding soil. The diameter of caissons varies from one to three metres. The friction pile of wood or concrete is driven into soft soil where there is no harder stratum for bearing beneath the site. The building load is supported by the surface friction between the pile and the soil.

When the soil is so soft that even friction piles will not support the building load, the final option is the use of a floating foundation, making the building like a boat that obeys Archimedes' principle — it is buoyed up by the weight of the earth displaced in creating the foundation. Floating foundations consist of flat reinforced concrete slabs or mats or of reinforced concrete tubs with walls turned up around the edge of the mat to create a larger volume.

If these buildings do not have basements in cold climates, insulated concrete or masonry frost walls are placed under all exterior nonbearing walls to keep frost from under the floor slabs. Reinforced concrete foundation walls for basements must be carefully braced to resist lateral earth pressures. These walls may be built in excavations, poured into wooden forms. Sometimes a wall is created by driving interlocking steel sheet piling into the ground, excavating on the basement side, and pouring a concrete wall against it.

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Deeper foundation walls can also be built by the slurry wall method, in which a linear series of closely spaced caisson-like holes are successively drilled, filled with concrete, and allowed to harden; the spaces between are excavated by special clamshell buckets and also filled with concrete. During the excavation and drilling operations the holes are filled with a high-density liquid slurry which braces the excavation against collapse but still permits extraction of excavated material. Finally, the basement is dug adjoining the wall, and the wall is braced against earth pressure.

Give a brief overview of the structure and contents of Text10B.

Relate each heading to the corresponding paragraph of Text 10B.

a) Caissons.

b) The slurry wall method.

c) Bearing piles.

d) Reinforced concrete foundation walls.

e) Some specific features of foundations.

f) Floating foundations.

22. Read Text IOC "Deep Foundations" and answer the questions. Discuss your answers with your groupmates.

a) What is the difference between a deep foundation and a shallow foundation?

b) Why is a deep foundation preferred over a shallow foundation?

c) What are the other names of a deep foundation?

d) What are driven foundations characterized by?

e) What is the structure of pile foundation systems?

f) How are boring techniques employed for drilled piles?

g) What do dry boring methods consist in?
h) What is specific of wet boring?

• TEXT IOC

Deep Foundations

A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including piles, drilled shafts, caissons and piers. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and pre-tensioned concrete. Deep foundations can be installed by either driving them into the ground or drilling a shaft and filling it with concrete, mass or reinforced.

Prefabricated piles are driven into the ground using a pile driver. Driven piles are either wood, reinforced concrete, or steel. Wooden piles are made from trunks of tall trees. Concrete piles are available in square, octagonal, and round cross-sections. They are reinforced with rebar and are often prestressed. Steel piles are either pipe piles or some sort of beam section (like an H-pile). Historically, wood piles were spliced together when the design length was too large for a single pile; today splicing is common with steel piles, though concrete piles can be spliced with difficulty.

Driving piles, as opposed to drilling shafts, is advantageous because the soil displaced by driving the piles compresses the surrounding soil, causing greater friction against the sides of the piles, thus increasing their load-bearing capacity.

Foundations relying on driven piles often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear. Pile caps and isolated piles are typically

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connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams while heavier elements bear directly on the pile cap.

Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site, in particular, whether boring is to be undertaken in diy ground conditions or through water-logged but stable strata, i.e. wet boring. Boring is done until the hard rock or soft rock layer is reached in the case of end bearing piles.

If the boring machine is not equipped with a rockauger, then

socketing of the hard rock layer is done with the help of a heavy

chisel which is dropped from a height of about 1.5 metres by

suspending it from a tripod stand attached to a winch crane. The

socketing is carried out until the desired depth within the rock layer

has been attained. The depth within the rock layer is considered to

be equal to the diameter of the pile in hard rock layers and is taken

to be equal to 2.5 times the diameter of the pile in soft rock layers.

Dry boring methods employ the use of a temporary casing to

seal the pile bore through water-bearing or unstable strata overlying

suitable stable material. Upon reaching the design depth, a

reinforcing cage is introduced, concrete is poured in the bore and

brought up to the required level. The casing can be withdrawn or

left in situ.

Wet boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by a tremie pipe.