Excavated Piles with Large Diameter and / or Diaphragm Pile
Excavated piles or Diaphragm Piles are piles executed circularly or rectangularly, cast on site and made with underwater concreting. Although relatively recent (its development occurred in the late 1960's), the process impacted the excavation and foundations technique.
Its success is based on several reasons, such as the multiplicity of its applications, the development of excavation equipment, slurry processing plants and the availability of bentonite for industrial use.
Excavated piles and diaphragm pile have great strength and small deformability, which makes them the most suitable solution for supporting excavations.
Advantages
Foundation elements, transmitting loads to deeper layers.
Execution without vibrations and noise when driving foundation piles or shoring.
Possibility to pass through layers of great resistance.
Rapid execution.
EXECUTION METHODOLOGY AND CARE IMPLEMENTATION
Execution Phases
The execution sequence comprises the following phases:
1. Placement of the guide shirt or execution of the guide wall
2. Drilling, with simultaneous filling with bentonite slurry or mud
3. Reinforcement placement after sand removal or changing the bentonite slurry
4. Concreting
5. Disposal of bentonite slurry
STABILIZING ACTION OF SLURRY
The slurry is produced in the trench during the excavation process and must keep it stable until it is filled with the concrete or mortar provided. The slurry used is a suspension of bentonite (sodium montmorillonite), which has thixotropic properties, that is, it has a fluid behavior when stirred, but is capable of producing a gel when at rest. The use of bentonite in oil well drilling is quite old and its first application in foundation engineering was in pile drilling. For use in the process of molded walls in the soil, its characteristics need to be controlled immediately after mixing, during its supply in the trench and immediately before concreting. The Federation of Piling Specialists (FPS, 1975) recommends the following control of bentonite slurry for supply in the trench:
| ITEM TO BE MEASURED | RESULTS LIMITS AT 20º | TEST METHOD |
| Specific weight | Less than 1,02 and 1,03 g/ml | Slurry Density Scale |
| Viscosity | 33 to 35s | Cone Marsh |
| Shear Strength (10-minute gel resistance) |
1,4 – 10 N/m² |
Shearometer |
| pH | 9,5 – 12,0 | Indicator Paper |
Before the concreting phase, it is important to check the contamination degree of the slurry by suspended soil particles. This contamination causes an increase in the specific weight of the slurry, affecting the concreting quality. It is recommended that, on this occasion, the specific weight measured on the slurry scale does not exceed 1.3 g / ml.
One of the basic functions of the slurry is to create an impermeable film on the walls of the excavation, called 'cake', formed by the penetration of the slurry into the voids in the soil. This film allows the slurry to push against the excavation walls to stabilize it. The thickness of the penetration will depend on the difference between the slurry level in the trench and the water level on the ground (must be at least 1.5 m), the soil permeability and the slurry viscosity. The excessive penetration of the slurry is inconvenient for the stability of the trench and, in soils with very high permeability (k > 1 cm / s), there may be loss of this material, making it impossible to use the process of the molded walls in these soils. In intact clays, there is no 'cake' formation, however, it does not influence the stabilization process. The purpose of the slurry is to prevent the release of sand grains from the walls of the trench.
PILE DETAILS
The execution of piles and molded diaphragm piles in the soil requires a very specialized technique, involving great mechanization. Thus, it is important that the designer is familiar with this knowledge and that be discussed all aspects of the work with the developer, so that the project leads to a simplicity of operations at the construction site. Even if these recommendations are met, there are frequent changes in the project, as a result of the development of the works.
Next, some aspects of the technique of piles and diaphragm pile in the soil are discussed, with regard to their details in the projects.
As horizontal arching represents an important factor in the stability of the open trench, the length of the panels will depend on the characteristics of the soil and the proximity of overloads.
a) Diaphragm Piles - the width of the panel must be that of the Clamshell (0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 1.00, 1.20 m) and the minimum length of a panel is limited by its open length (2.50 m to 3.20 m).
b) Piles – are dug through buckets (Ø 60 to Ø 170 cm) placed on the rotating table rod, which is a drill rig attached to the main crane.
It is necessary that the entire panel be concreted continuously, without interruption. Thus, the ability to supply concrete needs to be considered when choosing the length of the panel.
CONCRETING
The underwater concreting is made through a tube that reaches the bottom of the panel, equipped with a mouth funnel. The concrete must be quite plastic and expel the slurry from the bottom of the pile due to its higher density. The tube is made up of threaded elements that are dispensed as the filling develops. No mechanical method is used in compacting the concrete.
The concrete to be used must be highly workable (19 to 21 cm in the 'slump-test') and a high cement consumption (not less than 350 kg / m³) to enable resistances of 10 mpa in specimens with 7 days and 18 mpa with 28 days.
REINFORCEMENT
The particularities that are present in the arrangement of the reinforcement bars, and, at times, the deficient adhesion between the reinforcement and the concrete released under bentonite suspension will require a careful anchoring metal structure. The vertical bars should be on the outside and the horizontal bars on the inside. The free spacing between the irons must be at least 10cm between the irons' faces and at most 20cm.
The concentration of irons should be avoided. In principle, steel bars with dents and projections should be used, as they offer better adhesion.
The reinforcement (cages) must be appropriate to the stress they will undergo. In these conditions, the builder must take great care in making the cage, especially in the suspension irons (handles) so that they have the proper positions (indicated in the project). In doing so, the cage will have the necessary verticality for its easy penetration in the excavation.
The steel to be used must be weldable, with quality assurance (CA-50A). For these special steels, the limit values established in the technical standards must be obeyed safely and verified through a certificate of analysis for each batch. Each steel starting must have a label, indicating the batch number, and be stored separately.
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