Please enable JavaScript.
Coggle requires JavaScript to display documents.
5.1 Basement excavation methods - Coggle Diagram
5.1 Basement excavation methods
Diaphragm Wall
Sequence
Construction of guide walls
The guide walls direct the clamshell or grabbing equipment to maintain the desired width, keeps vertical alignment and retains bentonite slurry.
Typically, a vertical tolerance up to 1:200 is permissible for diaphragm wall construction.
Excavate and construct 250 mm thick and 2 meters deep guide walls on each side of the diaphragm wall center line.
Trench is excavated to the required depth in alternate sections up to 6 m length.
Excavation is done by digging technique using clamshell or grabs to the desired depth.
Installation of Stop Ends to form joint between panels
Two stop end tubes will be placed at the ends of the excavated trench before concreting.
Withdraw the tubes at the same time of concreting in order that a semi-circular end section is shaped.
Soil is replaced by a bentonite mud slurry.
Bentonite slurry is a mixture of Bentonite and Water
Trench does not collapse as slurry balances hydraulic pressure
On completion of each panel, a prefabricated cage of reinforcement is lowered through the slurry and concrete is placed through a tremie pipe.
As the concrete rises, the bentonite slurry is displaced and can be re-used.
Intermediate panels are then excavated and the wall completed following the same procedures.
Stability of D-Walls achieved by:
struts in the form of floor structure
ties in the form of ground anchors
Hard or rocky soil
Clamshell replaced by cutter when soil is hard and rocky
Uses
Suitable for soft clay soil
Unsuitable for stiff clay soil
Contiguous Bored Pile (CBP)
Characteristics of Contiguous Bored Piles (CBP)
A CBP wall is an earth retention system formed by installing closely spaced bored piles, with a small gap between adjacent piles.
As the name implies, contiguous piles touch each other, or are separated by a small gap.
CBPs are suitable in crowded urban areas, where traditional retaining methods
would otherwise encroach on the adjoining properties.
Diameter and spacing of the piles is decided based on soil type, ground water level and magnitude of design pressures.
Large spacing is avoided as it can result in caving of soil through gaps. Piles vary in diameter from 600 to 15000mm, though piles greater than 1200mm are rarely used.
A small gap 100mm is left between each pile. The size of the gap depends on the soil strength.
Due to the gaps between piles, the wall has limited effect on the control of ground water.
In practice, contiguous piled walls are built to a maximum depth of 25m
CBP wall can only be used where ground water is not a hazard or where
grouting can be used to remedy leakage between the piles.
Capping beams at the top help in pressure distributions in piles.
Separate facing usually provided to improve looks
Supports
Ground anchors can be used on contiguous piled walls to help resist the overturning forces caused by the surrounding strata and the hydrostatic pressure.
Capping beams
Walers
Uses
Suitable for stiff clay soils
Suitable for soft marine clay soil
Secant Piles
Sequence
Secant piles consist of overlapping and interlocking piles. .
Soft piles (primary piles) are bored using Continuous Flight Auger (CFA) drill rigs and then concreted first.
The hard piles (secondary piles) are then drilled, secanting (cutting into) the soft piles.
Characteristics
SBP walls are formed by constructing intersecting piles.
SBP walls are formed by keeping spacing of piles less than diameter.
SBP walls are used to build cut off walls for the control of groundwater inflow and to minimize movement in weak and wet soils.
SBP walls are constructed in the form of hard/soft wall.
Construction is carried out by installing the primary piles (A) and then the secondary piles (B) are
formed in reinforced concrete, cutting into the primary piles.
Diameters can range from 500mm to 1200mm
When the secant wall is in place, the excavated face can be covered with a layer of structural concrete.
The concrete can be either sprayed - shotcrete or cast against the wall, providing a fair faced concrete finish.
Secant pile construction, only hard piles are reinforced to resist the lateral loads.
The system is often used to build deep basement walls.
Secant walls are often considered to be more economical than diaphragm walls.
Soldier Pile walls
Method of construction
Driving steel soldier piles at regular intervals (2 to 3m),
Excavating in small stages and installing the lagging,
Backfilling and compacting the void space behind the lagging.
Moment resistance in soldier pile and lagging walls is provided solely by the soldier piles.
Passive soil resistance is obtained by embedding the soldier piles beneath the excavation grade.
The lagging bridges and retains soil across piles and transfers the lateral load to the soldier pile system.
Soldier Pile Wall Construction
Drive Piles and backfill with weak grout
Excavate and place lagging
Install support
Struts
Ground Anchors
Soil nails
Lagging
Timber lagging (most common for temporary systems)
Precast concrete lagging
Steel Sheeting
Wire mesh with shotcrete
Advantages over other ERSS
Cost
Lower cost compared to other retaining wall systems.
Speed
Fast construction, making them ideal for short-term projects.
Flexibility
Builders have absolute control over the spacing and layout of all materials used.This makes last-minute adjustments much easier during the construction process.
Noise and Disturbance
Although installing soldier piles is a labor-intensive activity, the actual process is relatively quiet. Both the vibrations and noise are minimal, which can be a huge selling point when working in residential areas.
Uses
Suitable for stiff clay soils
Can be used as temporary erss