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Tutorial One answers

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Construction Technology 2 (Substructure) (300721)

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School of Built Environment

2020 300721

CONSTRUCTION TECHNOLOGY 2

Tutorial One answers

  1. How do most foundations fail? Foundations do not compress easily. This can only happen if the ground is subjected to massive vibration. In nature, this form of compaction occurs in an earthquake. It is known as Local Bearing Failure.

Most foundation soils fail by General Bearing Failure. Underneath the building, soil “flows” sideways and then upwards, so that the building settles (often unevenly!) into the ground. General bearing failure is resisted by two properties of the soil:

In sandy soils, friction between sand particles is important. As the soil flows sideways, the particles rub against each other. The friction resists the flow of soil. The frictional resistance depends on the type of soil. Gravelly soils are more efficient interlocking than sandy soils, so they generate more frictional resistance [The gravel angle of repose or coefficient of friction is higher than the sand coefficient]. The frictional characteristics of a coarse soil (gravel or sand) are enhanced by overburden. Deep in the ground, coarse particles are literally pushed together. In clayey soils, chemical bonding between particles is important. Clayey soils will not flow sideways until the cohesive strength is reached. At that point, catastrophic failure can occur, as the chemical bonds are broken. Cohesive strength does not change with depth. Deep in the ground, the same chemical bonds must be broken. Real soils have both cohesion and frictional properties, because they contain gravel, sand and clay particles.

  1. Detail how excavations are made safe in clayey soils? And in sandy soils? In clayey soils, trenches are safe, until a particular depth is reached. The safe working depth will depend on the cohesive properties of the soil. Heavy clay soils can be self-supporting, down to a depth of 2 metres. They appear deceptively safe. However, if the excavation proceeds past the safe working depth, then the trench can cave-in without warning. After the safe working depth, the cohesive strength has been exceeded and tonnes of soil can impact any workmen or equipment in the trench with tragic consequences. It is important to support the sides of trench with shuttering and struts.

Figure 1 Excavation shoring (from roadplates)

As you can see from Figure 1, the shuttering does not need to be continuous, provided the clayey soil has sufficient strength to span between the shutters. The trench can be excavated below a shutter, provided a safe spacing of the shutters (related to the safe working depth) is not exceeded.

Shuttering does not work for sandy soils. These soils have not cohesive strength, so sand will continuously fall into the trench, below the shutter. The site must be benched at the angle of repose of the soil, as shown in Figure 2.

Figure 2 Excavation benching (from roadplates)

Table 2 Mohr-Coulomb plot for test

At a normal stress of zero, only the cohesive properties of the soil are important. Hence the

cohesion c is found by extending the line-of-best-fit back to the vertical axis (c = 4 kN/m 2 ).

The frictional properties of the soil determine the slope of the line. Highly frictional soils will have a steep slope. Clayey soils have no frictional properties; the line-of-best-fit will be horizontal. For this soil, choose an arbitrary run (10 to 60, say) and determine the corresponding rise (9 to 32)

the slope of the line is rise over run:

. 
32 5 9

60 10= 0 = tan  ;  = tan-1(0) = 25.

  1. Calculate the amount of weight that a circular footing (Figure 3) can safely withstand.

Figure 3 Circular pad footing

The overburden pressure increases from zero at the surface. At the base of the footing, the distributed weight (pressure) of the overburden constraining the shearing of the footing is:

q =   z

Using the data provided:

q = 15  0 = 10 kN/m 2

The ultimate bearing capacity is given by the equation:

qu  13. cN qNc 0 3. qN

where B is the diameter of the footing

From the Table on page 8 [Module 2],

N = 2 Nq = 4 Nc = 11.

Installing Terzaghi’s factors in the equation, the ultimate bearing pressure, qu, is determined

by adding the effects of cohesion, over-burden and unit weight (for a particular ):

qu = 1  27  11 + 10  4 + 0  15  0  2.

= 414 + 42 + 5.

= 461 kN/m 2

A footing cannot be loaded to its ultimate capacity – There may have been sources of weakness that were not revealed during the site investigation. The ultimate bearing pressure is factored down by a safety factor of 4 to obtain the allowable bearing pressure:

Allowable pressure = 461/4 = 115 kN/m 2 (or kPa)

Figure 5 Enlargement of Terzaghi’s zones

In wider footings, all of Terzaghi’s zones are enlarged. Zone I is not significantly affected,

since the zone is merely elastically compressed. The volume of earth in Zone II is increased.

There is more earth shearing, so the bearing capacity of the zone increases. Zone III also

enlarges. More earth must be pushed out of the way before the footing fails.

In deeper footings, only Zone III enlarges – increasing the bearing capacity of the footing.

The frictional resistance of Zone II is also increased, because there is increased overburden

pressure on the base of the footing. The cohesive resistance of Zone II does not change.

  1. Detail the pad footing reinforcement required to resist the following structural actions: Explain why the reinforcement has been provided.

Figure 6a Cantilever bending (reinforcement in orange)

Loads from the superstructure push the centre of the pad footing into the ground (while the edges of the pad footing lag behind). So the pad footing is distorted, as shown in Figure 6a. The distortion is grossly exaggerated to illustrate the effect. An unreinforced pad footing could

never deform to this degree, without completely failing. Steel reinforcement would limit the deflection of a reinforced pad footing as well.

Steel reinforcement (orange) is placed close to the bottom face of the footing. This face is in tension, while the top face is in compression. Concrete is weak in tension and strong in compression. So the steel reinforcement is best placed, where it will improve the properties of the concrete.

Figure 6b Punching shear

Heavily-loaded pad footings will crack along the dotted lines, shown in Figure 6b. The most efficient reinforcement involves placing steel bars (orange) at right angles to the cracks. However, reinforcement is usually placed horizontally, where it can also guard against bending (cantilever) failure.

Deep pad footings cannot fail in bending (cantilever action). The stresses on the tensile face (bottom) of the pad footing are too small to cause the concrete to crack. However, heavily- loaded deep pads can still fail by punching shear.

  1. One strategy is to ensure that the rigid elements can move freely with ground movements. For instance, brickwork can be articulated with regular joints (shown in blue), along the wall. Each brick panel is allowed to move in relation to the others. The footing beams do not need be so strong to resist ground movement.

  2. If bedrock is reasonably close to the surface, the footing beam can be supported by piers (Figure 7). Movement of the superstructure is limited by the span between the piers. On very heavy clay soils, compressible liners must be provided under the footing beam. Under conditions of ground swell, the highly reactive clay can force the footing beam off the piers, if the liner is not provided.

Of course, there is a limit to the depth of the piers. If the piers are very deep, they will require a lot of concrete. Very long piers must be reinforced or they buckle. Any cost savings in the footing beam may be eliminated by the cost of the piers.

  1. Why is the continuity of a strip footing important? Explain how this relates to the detailed design of step-downs and penetrations.

Unlike a slab footing, Strip footings do not underlie the whole building (obviously!). However, like a Slab footing, the entire network of strip footings must be designed to act together........ to jointly support the building. Corners and step-downs are points of potential weakness.

  1. It is important that strip footings are continuous across a corner. In particular, the footing reinforcement from both sides of a corner must overlap.
  2. Step-downs must be able to act in bending, just as a straight length of strip footing would. Hence, the step-downs are reinforced, as shown in Figure 8.

Figure 8 Step-down details of strip footings (from AS2870-2011)

The continuity of the strip footing network can also be compromised by a single poorly-placed penetration, for a sewer pipe or electrical cable. For instance, if a service penetrates through the tension side of a footing beam, the beam has no strength at that point and the entire footing network is compromised.

It is best to avoid either the top or bottom of a footing beam. Since the strip footing can sag or hog at any point, it is almost impossible to determine whether the top or bottom is the tension half of the beam. It is best to place service penetrations through the centre of the beam (within a duct that allows the footing to move in relation to the service main). The centre of the beam is known as the Neutral Axis, because it is neither in compression or tension.

Placing services within a footing system is unconventional. The standard practice is to place the service main 75 mm below the base of the footing. Chasing a service through the footing system should only be done as a last resort.

Figure 9 Penetration details of strip footings

  1. Where are end-bearing piles used? And friction piles?

End-bearing piles are used to transfer building loads down to a stable strata. Generally, the piles are driven or bored to bedrock, although this is not always necessary. As we saw with shallow footing systems, the bearing capacity of a foundation increases dramatically with depth. When the end of the pile is placed deeper in the ground, overburden pressures and frictional resistances are magnified.

Once piles reach an aspect ratio of 3 (length/diameter), side-wall friction/adhesion effects become significant. Friction between the side-wall of the pile and sandy soil can be an important source of bearing for a pile. Adhesion between a pile and a clayey soil can also be a source of bearing strength. However, this cannot be relied on, because the ground can become saturated.

Eventually, at aspect ratios above 10, side-wall friction becomes the dominant factor in the strength of a pile. In cities on river deltas (as many of the world’s largest cities are........... New York, Shanghai, Melbourne, etc.) can only rely on side-wall friction. For instance, the

Vibratory or impact piling cannot be used in a built-up area. Vibration can be transmitted through the ground, like a seismic wave, and damage surrounding buildings. Moreover, built- up areas tend to have noise restrictions.

Bored piers can be installed wherever there is access for a piling rig.

iii. under an isolated domestic house on loose sandy soil (on the south coast of NSW) If bedrock is close to the surface, unreinforced piers are preferable.

If bedrock is far from the surface, driven piles or reinforced piers cannot be justified on economic grounds. For light superstructure loads (domestic buildings or light industrial/commercial buildings), screwed piles can be used. Screwed piles act like friction piles, without the extensive surface area.

  1. Figure 11 illustrates the piling system under the Petronas Towers in Kuala Lumpa, Malaysia. What are the advantages of this system?

Figure 11 Pile groups under the Petronas Towers (from briscoesblog.files.wordpress)

The weight of a skyscraper (without the contents and people), for the Petronas Towers, 300,000 tonnes, is spread out over its base and transferred to micro-piles under the building.

On first impression, you might think that a forest of micro-piles would de-stabilise the foundation. But each pile actually stabilises the adjacent piles. Driven piles actually compact the soil within 5 diameters of the pile. Even cast in-situ piles stabilise their neighbours. Loads

on each pile cause a generalized movement of soil away from the pile. This increases frictional forces on the other piles.

The load-carrying capacity of the pile group is still limited by the strength of each pile. An over-loaded pile can fail and cause the progressive failure of the structure as the load is transferred to other working piles.

However, if all the piles are within their working range, then the pile group can be thought of as acting like a single mega-substructure. The mega substructure has a combined pressure bulb that extends far into the soil; much further than a single pile acting in isolation.

Figure 12 Pressure bulbs for single piles and pile groups (from Curtins, 1994)

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Tutorial One answers

Course: Construction Technology 2 (Substructure) (300721)

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Students shared 18 documents in this course

University: Western Sydney University

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School of Built Environment
2021 300721
CONSTRUCTION TECHNOLOGY 2
____________________________________________________________________________________________
Tutorial One answers
____________________________________________________________________________________________
1. How do most foundations fail?
Foundations do not compress easily. This can only happen if the ground is subjected to
massive vibration. In nature, this form of compaction occurs in an earthquake. It is known as
Local Bearing Failure.
Most foundation soils fail by General Bearing Failure. Underneath the building, soil “flows”
sideways and then upwards, so that the building settles (often unevenly!) into the ground.
General bearing failure is resisted by two properties of the soil:
In sandy soils, friction between sand particles is important. As the soil flows
sideways, the particles rub against each other. The friction resists the flow of soil. The
frictional resistance depends on the type of soil. Gravelly soils are more efficient
interlocking than sandy soils, so they generate more frictional resistance [The gravel
angle of repose or coefficient of friction is higher than the sand coefficient]. The
frictional characteristics of a coarse soil (gravel or sand) are enhanced by overburden.
Deep in the ground, coarse particles are literally pushed together.
In clayey soils, chemical bonding between particles is important. Clayey soils will not
flow sideways until the cohesive strength is reached. At that point, catastrophic failure
can occur, as the chemical bonds are broken. Cohesive strength does not change
with depth. Deep in the ground, the same chemical bonds must be broken.
Real soils have both cohesion and frictional properties, because they contain gravel,
sand and clay particles.
2. Detail how excavations are made safe in clayey soils? And in sandy soils?
In clayey soils, trenches are safe, until a particular depth is reached. The safe working depth
will depend on the cohesive properties of the soil. Heavy clay soils can be self-supporting,
down to a depth of 2 metres. They appear deceptively safe. However, if the excavation
proceeds past the safe working depth, then the trench can cave-in without warning. After the
safe working depth, the cohesive strength has been exceeded and tonnes of soil can impact
any workmen or equipment in the trench with tragic consequences. It is important to support
the sides of trench with shuttering and struts.
Tutorial One answers Page 3

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