- Piling Foundations
- Rotary bored piles
- Continuous flight auger piles
- Earth Retaining Structures:
- Continuous pile walls
- Secant pile walls
- Diaphragm walls
- Ground anchors
- Sheet piles
- Berlin wall
- Piling work
- Bored Cast-In-Situ Piles (Bored Piles)
Alsocalled drilled piers or Cast-in-drilled-hole piles (CIDH piles). 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 ‘dry’ ground conditions or through water-logged but stable strata – i.e. ‘wet boring’.
‘Dry’ boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying suitable material. Upon reaching the design depth, a reinforcing cage is introduced; concrete is poured in the bore through tremie pipe and brought up to the required level.
- Continuous Flight Auger Piles (CFA Piles).
The Continuous Flight Auger (CFA) process is virtually vibration free and one of the quietest forms of piling, making it ideal for environmentally sensitive areas, the method enables piles to be formed in water-bearing strata, without the need for casing or bentonite. It is suitable for constructing piles in most strata: gravels, Sands, silts, clays and soft rocks and in mixtures of strata.
CFA piles are formed by drilling to the required depth using a hollow stem continuous flight auger. After reaching the designed depth, a high slump concrete is then pumped through the hollow stem.
While the concrete is being pumped, the auger is withdrawn at a controlled rate, removing the soil and forming a shaft of fluid concrete extending to ground level.
A reinforcing cage is then inserted into the fluid concrete; Reinforcing cages can be installed with the assistance of cage vibrators.
A secant pile wall consists of a series of primary and secondary piles casted alternatively. The secondary piles are installed by cutting a portion of the concrete from the adjacent primary piles on both sides forming a water tight wall.
Typically this technique is suitable to retain stiff and cohesive sub soils and where ground water levels are below the eventual depth of excavation.
Pile diameters range between 450mm and 900mm and are generally installed at pile centers of between 500 and 1000mm respectively thereby leaving gaps between the piles between 15 and 100mm.
This pictorial guide illustrates the construction sequence of a Contiguous Bored Pile (CBP) wall.
Diaphragm walls are concrete or reinforced concrete walls constructed in slurry-supported, open trenches below existing ground. Concrete is placed using the Tremie installation method or by installing pre-cast concrete panels (known as a pre-cast diaphragm wall). Diaphragm walls can be constructed to depths of 100 meters and to widths of 0.45 to 1.50 meters.
Diaphragm wall construction methods are relatively quiet and cause little or no vibration. Therefore, they are especially suitable for civil engineering projects in densely-populated inner city areas.
Due to their ability to keep deformation low and provide low water permeability, diaphragm walls are also used to retain excavation pits in the direct vicinity of existing structures.
Diaphragm wall panels are also used in deep, load-bearing soil layers as foundation elements to carry concentrated structural load in the same way as large drilled piles do.
These foundations elements are known as "Barrettes".
Tiebacks are small diameter drilled elements consisting of steel strands or thread bar members grouted into place and tensioned to provide a preloaded condition to the underlying soil. Tiebacks, also referred to as ground anchors, are used for permanent or temporary shoring support, retaining walls, dam stabilization, and landslide repair. They can provide a cost effective alternative to conventional shoring structures. The tieback anchor will reduce the deflection and bending moment of the shoring wall..
Static load tests were performed during the test phase of contract to verify the design assumptions and load-carrying capacity of the piles. Telltale rods installed at various depths within the piles were used to evaluate the load transfer behavior of the piles with regard to the surrounding soil and bearing stratum.
The bearing capacity is depend on the size, shape and type of pile and on the properties of the soil in which it is embedded. The ultimate bearing capacity is the load at which the resistance of the soil becomes full mobilized. At the load greater than bearing capacity, this causes the head of the pile to settlement 10% of pile diameter.
General pile load test on Preliminary (Non-working) and Working piles are to be carried out as per requirements of the specification.
The test load on Preliminary Pile and Working Piles are defined in the specifications.
The test assembly consists of the following:
-Pressure Gauge and Load cell
-Settlement Measuring Gauge
-Counter weight Kent ledge, Ground Anchors or Reaction Piles.
PDA Testing utilizes the Pile Driving Analyzer (PDA) and is based on the theory of Stress Wave Propagation on Piles (CASE Method). Force and acceleration measurements taken near the top of a pile during impact provide the necessary information to determine:
- Activated bearing capacity
- Hammer Performance
- Maximum driving stresses
- Pile Integrity.
A sonic logger such as the Cross-Hole Analyzer is used to detect defects in cast-in-situ piles. Cross Hole Sonic Logging provides information on:
- Honeycombing due to inadequate vibration
- Segregation due to over vibration and improper concrete placement methods
- Washout of cement due to ground water flow
- Cracks in pile shaft due to shrinkage
- Inclusion of foreign material causing contamination of concrete
- Necking of pile due to collapse of side walls during withdrawal of temporary casings
The Pile Integrity Tester (PIT) is used to for bored piles, driven concrete shafts, concrete filled piles or drilled shafts. It detects potentially dangerous defects such as major cracks, necking, soil inclusions or voids.