BASEMENT
CONSTRUCTION
Dincel Construction System Pty Ltd
Level 3, 7K Parkes Street
PARRAMATTA NSW 2150
Phone: (612) 9689 1877 Fax: (612) 9689 2028
Email: construction@dincel.com.au
COPYRIGHT
© Dincel Construction System Pty Ltd
All rights reserved. No part of the information contained in this document may be reproduced
or copied in any form or by any means without written permission from Dincel Construction
System Pty Ltd.
DISCLAIMER
The information contained in this document is intended for the use of suitably qualified and
experienced architects and engineers and other building professionals. This information is not
intended to replace design calculations or analysis normally associated with the design and
specification of buildings and their components. Dincel Construction System Pty Ltd accepts
no liability for any circumstances arising from the failure of a specifier or user of any part of
Dincel Construction System to obtain appropriate professional advice about its use and
installation or from failure to adhere to the requirements of appropriate Standards and Codes of
Practice, and relevant Building Codes.
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SHORING – BASEMENT CONSTRUCTION
The basement excavation for buildings is required for carparking, amenities and services.
The depth of the excavation, depending on the ground slopes can be single or multiple
carparking/storage levels.
The difficulty, hence cost of basement shoring construction depends on:
y
The type of material being excavated.
For example, the majority of rock
excavations, depending on the rock quality and fault planes may not need any
shoring at all, however on the other hand sandy materials or even clayey materials
require shoring systems.
y
The ground water conditions – it is conventionally very costly to build below the
permanent water table.
The majority of basement constructions in clayey types of foundation material can be
excavated without shoring having adequate batters to soil if there is adequate distance
from the boundaries to allow the battering. Battering to avoid excavation shoring systems
may cause the following problems.
h
Massive quantities of earth moving for the materials along the batters.
h
Batters may need to be stepped and stabilised to satisfy work cover requirements.
h
The extent of the batters may restrict the site usage and accessibility.
h
The removed material needs to be back-filled behind the basement walls, usually
with imported granular, free draining material which needs to be placed carefully to
avoid future soil consolidation problems.
All of the above procedures are timely and certainly not cost effective. There are many
cases observed where excavations have been shored rather than having soil battering to
avoid the above issues. The conventional method of shoring is to have discreet piles with
shotcreted walls in between or over the piles for clay-shale types of ground materials.
However, the irony is the entire site containing clayey material is normally shored by piles
plus shotcrete walls for the full extent of the basement height for the sake of loose clayey
material immediately below the natural ground level. The clay required to be shored could
only be a couple of metres high over the stable shale or rock which can be excavated
safely with minimal or no batter at all. It is therefore obvious to the professionals in ground
engineering science that only the loose and unstable materials need to be stabilised by
sheet piling or similar terminating over stable soils.
The most unpredictable ground material for shoring purposes is shale type ground
materials. The shale can have a fault line inclining towards the excavation. This type of
material reported by geotechnical soils investigation and analysis must be retained by
shoring to prevent the mass movement of shale material over the fault line irrespective of
how hard the shale is.
The construction sites containing sand or similar materials needs to be shored for the full
extent of the bulk excavation.
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The conventional shoring systems can be summarised as follows:
1.
DISCREET PILES AND SHOTCRETE WALLS IN BETWEEN THE PILES
This shoring system will be applicable to non-granular, virgin types of cohesive soil
conditions. This type of shoring normally extends the full height of basement
excavation irrespective of the depth of the top soil material required to be shored.
The problems are usually associated with cracking and steel rusting of shotcrete
walls, inadequate waterproofing and unsightly walls.
2.
CONTIGUOUS PILING SYSTEMS
This shoring system is normally associated with granular foundation materials or
loose fill materials when there are no water table issues. The face of piles receives
shotcreting to close the gaps between piles and also to provide a presentable
appearance to the basement walls. Normal construction tolerance is such that
frequent gaps of 75mm or more and misalignment in the exposed faces can occur.
This is a costly and slow operation and certainly not the best way of waterproofing
the walls. There can be risk of sand or backfilled material leaking between the gaps.
Cracking and corrosion of shotcreting is again a common problem.
3.
SECANT-PILE WALL SYSTEM
This shoring system is normally specified if there is a ground water problem. The
soft piles are placed first; the structural and reinforced piles are bored and placed
within the soft piles to provide water tight walls. In most cases, the tightness of the
joint between soft and hard piles cannot be effective to prevent the water egress.
The use of bentonite in soft piles is a better way of making sure that the joints are
watertight. This system requires structural render to bring the piles in reasonable
alignment before the waterproof membrane system is placed onto it. The membrane
system further receives another masonry or concrete wall to protect the membrane.
The disadvantage of this system is that it is extremely costly and a slow system to
build. It is normally difficult to obtain a guarantee from the piling contractor for the
straightness of long piles if used for water cut-off and assist dewatering purposes.
There could also be water related problems depending on how well the membrane
system is applied.
4.
SHEET-PILING AND SHOTCRETE WALL
Steel sheet pile wall systems can be suitable for many situations but have tended to
be rejected due to a number of problems including:
y
Noise of installation.
y
Possible vibration damage to nearby properties during driving.
y
Availability and cost of suitable sheet pile wall sections.
y
Risk of leakage through the clutches and holes in the sheets below the water
table.
y
Disturbance to surrounding area during extraction of sheets.
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This shoring system can be considered as an alternative to Item Nos: 2 and 3 above.
The sheet piling’s inner face normally is reinforced with bars and/or mesh prior to
concreting. This way concrete cladding to the sheet piling has various concrete
depths between each rib of the sheet piling. The concrete cracks at the position
where the concrete thickness varies between the ribs. Since the water tightness of
the joints of the sheet piling cannot be 100% prevented, water or moisture will attack
the reinforcing steel at the cracked concrete. Depending on the type of soil, water
and minerals, contaminants in the environment, the life of the shotcrete concrete can
even be less than the builder’s liability period. The rectification of this type of
problem will prevent the already occupied building use and hence costly. On the
other hand steel piling itself must have adequate extra steel thickness for corrosion
problems hence sheet piling itself becomes more expensive.
5.
DIAPHRAGM WALLS
These are usually excavated in panels by a specialist contractor with a grab and the
excavation supported by drilling mud until reinforcement is placed and the panel
concreted. Panels are generally 3 – 6m long and 0.5 to 0.8m wide. Work is carried
out on an alternate panel system coming back later for the infill panels. Stop ends
are provided in the first set of panels to form the ends, and various other patented
devices have been developed to reduce the risk of leakage through the joints
between panels. It is generally normal to construct guide walls to ensure location
control. The finished walls are usually smooth and of a high standard suitable for a
basement. Construction of diaphragm walls is expensive and requires a specialist
experienced contractor who has the appropriate equipment and expertise.
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THE BASEMENT CONSTRUCTION WITH
As it can be seen from the above conditions there is currently no available basement
shoring system that is:
h
Considerably cheaper and faster to build.
h
Waterproof, maintenance free, does not rust, decay or corrode or crack.
h
Ready finish.
h
Elimination of slab edge formings and slab edge beams, more economical floor slab
solutions.
h
Long term corrosion free reliable structural connections between wall and floor slabs.
h
Provisions for ready safety handrails.
can be used:
(i)
(ii)
In association with sheet piling with or without the presence of ground water.
will offer the following advantages.
h
The use of cold formed sheet piles, i.e. cheaper sheet piles.
h
Sheet piling without corrosion allowance, i.e. cost effective sheets.
h
Cheaper sheet piling can be sacrificed, hence eliminates the costs and risks for
sheet extractions.
h
Much quicker sheet piling erection procedure since the water tightness of the
sheet joints are not prime importance, as long as dewatering is manageable.
In association with site excavations consisting battered ground conditions.
h
Elimination of sub-soil drainage and imported backfilling behind the basement
.
perimeter
h
Elimination of conventional footings where allowable ground bearing pressures
are suitable.
Note: The latest studies indicate that aggregate lines behind walls contribute to
bacteria problems.
The basement conditions and shoring methodologies may be generalised as shown by
Figures A and B below.
FIGURE A represents sand, sandy soils, full height of basement shored by anchored sheet
piles and permanent water table above the lowest level.
FIGURE B covers the most common cases of basement excavation. The excavations are
either battered or sheet piled and the lowest basement level above the permanent water
table.
The sheet piles in association with
are treated as temporary shoring purposes
as they can be either removed or left in place to rust away. The supply of sheet piling is
determined by its thickness and how well the joints of the sheet piling will retain the water.
The sheet piling suppliers in association with
do not need to consider the
additional corrosion allowances for sheet thicknesses. Their sheet thicknesses can be
determined by the drivability issue rather than corrosion. However, drivability issues can
be easily overcome by pre-drilling the ground where required. The joints of the sheet piling
can be allowed to leak as long as the leakage is within the de-watering limits. Today’s
sheet piling technology is well advanced and reasonably stiff clays, even shale can be
shored with the assistance of vibrationless hydraulic rams pushing into the soil, where
required ground is pre-drilled to assist the drivability.
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eliminates the need for sheet piling to be treated as permanent structural
element hence reducing its relative costings and risks. As illustrated on Figure A
provides a permanent water-proofed wall to water charged ground, is reliable
and liability free, and the most cost efficient rate.
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