Basement

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  • Topic: Concrete, Cement, Grout
  • Pages : 13 (2010 words )
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  • Published : March 27, 2013
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3 - basement

3 - Basement
1

Quick Revision

1.1 Problems arising from basement construction
a.

Excavation method.

b.

Surface and ground water control

c.

Lateral stability of basement excavation.

d.

Stability of adjoining building.

1.2 Ground water control - dewatering
a.

Open Sump Pumping

b.

Wellpoint System.

c.

Shallow-Bored Well System

d.

Deep-Bored Well System

1.3 Basement Excavation
a.

Open excavation

b.

Sheet pile cofferdam
Supported with raking struts
Supported with struts and walings
Supported with ground anchors

Ground support and ground water control for deep excavation
Cut off wallings:
c.

Thin grouted diaphrage

d.

Mixed in place wall

e.

Ground treatment

f.

Circular cofferdam

g.

Diaphrage wall

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3 - basement

2.

Cut-off wallings

2.1

Thin grouted diaphragm

A series of touching universal beams
or columns are driven into the ground
to the required depth. A grout
injection pipe is fixed to the web the
section and this is connected to a
group pump at the ground. As the
sections are withdrawn the void
created is filled with cement grout to
form the thin membrane.
This is a non-structural wall and
services only as a cut-off wall. It is
suitable subsoil of silt, sand and
gravel.

2.2

Thin grouted diaphragm (Source: R. Chudley)

Mixed-in-place wall

A hollow churn drill is drilled into the ground.
Intrusion grout is pumped through the hollow kelly
bar during the drilling operation. When the required
depth is reached, the auger rotation is reversed and
withdrawn while grout injection continues. The
grout is thus mixed with the existing soil to form a
mixed-in-place pile. The above process is repeated
with the piles being cast against each other until the
whole wall is formed.

Mixed-in-place pile
(Source: Jufri & Wellmen)

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3 - basement

2.3

Ground treatment

The major disadvantage of pumping is causing settlement of surrounding ground. In urban areas, settlement would serious affect the stability of adjacent properties. In this situation, grouting method can be used to control ground water. The basic method is to inject the soil or rock with fluids which, on setting, seal or reduce the permeability of the material. There are various types of grouts. The choice of grouts, the pattern and spacing of the injection pipes will depend upon the particle sizes of the soil or the size of fissures in the rock.

Cement Grouting (Source: R. Chudley)

2.3.1

Cement grouting

Cement grout is suitable for injecting into coarse materials which have a high permeability. This method not only can form an impermeable barrier to ground water but also solidify loose soils which prevents from collapsing during excavation. It is usual to commence grouting with a batch of thin grout and then to increase the viscosity of the grout as the process continues, by reducing the water-cement ratio. The grout used may be composed of neat cement grout, or a mixture of cement and

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3 - basement

sand in the ratio of 1 : 4. The adding of sand reduces the cost of material but it also reduces the workability.
On the other hand, pulverized fuel ash (PFA) may be used to replace part of the cement (up to 50%) to reduce the cost of the material. The spherical particle shape of PFA improves the flow quality of the grout without reducing its strength.

2.3.2

Bentonite grouting

Bentonite grouting is used in ground conditions where the particles of the soil are too small for cement grouting. While bentonite grout adds little strength to the soil, it has a high resistance to water flow and therefore produces an excellent barrier. Bentonite is produced from montmorillionite clay, which has thixotropic properties: when it coagulates it forms a gel, and if mixed with certain additives, such as Portland cement or soluble silicates, the barrier formed will be permanent....
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