CPI-6 piles

Piles which are drilled and concreted in situ, and in which bentonite slurry, or synthetic polymers, specially designed for the purpose, are used to support the walls drilled.

Execution

Before starting the excavation, an auxiliary tube is put into the ground, leaving part of it sticking out, to ensure that the support fluid always stays above the level of the excavation.

Excavation in soft and medium ground is completed using short propeller drills. In cases with harder ground, a tungsten carbide tipped drill is needed. In very hard, rocky ground a circular tungsten carbide tipped crown drill is used.

In this group of micro piles, support fluid is introduced whilst the excavation is being drilled, keeping the level always above the grade line.

Once the desired depth is reached, the bottom of the excavation is cleaned using a scoop (‘bucket´). The presence of suspended solids in the support fluid causes them to settle to the bottom during the placement of the re-enforcement and concreting. To avoid this, it is necessary to limit their presence as much as possible. For this reason, different methods are used to separate the fluid and leave it as clean as possible. This stage is essential in these types of micro piles to ensure the correct execution.

Alter clearing the base steel reinforcement is introduced with the help of additional equipment (a crane). To guarantee the minimum necessary coverage it is raised 20cm above the base of the excavation and separators are used for correct centring.

Alter placing the steel frame the concreting begins. To pour the concrete into the hole properly, avoiding segregation or bleeding, a tremie tube is used. The tube is fed through the steel frame until the base of the hole is reached. Then the concrete is pumped, which should be of a fluid consistency. This being denser than the support fluid (bentonite slurry of polymer) which stays at the base, pushing the concrete to the surface. .

Whilst the concreting progresses, the tremie tube is simultaneously raised, taking care to always keep it two metres below the level of the fresh concrete. This ensures that the only part of the concrete contaminated with mud comes up with the pile head. When the concrete reaches the surface level the concreting finishes and any remaining support fluids are eliminated. Finally, the tops of the piles are removed, where the first few centimetres of the concrete could be mixed with remains of dirt or mud.

Support fluids

Bentonite slurries consist of a mix of bentonite and water that has thixotropic properties, which means that whilst it is being mixed it behaves as a fluid, but whilst stable it becomes cohesive. It acts on the walls of the excavation by forming a mixture with the ground known as “cake”, reducing the permeability of the walls. This produces a membrane effect that can transmit the hydrostatic pressure of fluid to the excavation walls.

Bentonite slurries are considered toxic waste, and under current regulations must be disposed of at an authorised landfill site after use.

A viable alternative to bentonite slurries exists; the use of synthetic polymers, of which two groups can be distinguished:

Acrylic polymers. Consists of a fluid that contains polymers formed by a positively charged carbon chain. These react with the negatively charged particles found in the natural terrain, creating a cohesive effect on it.

Third generation system. Consists of a fluid containing several types of synthetic polymers. They consist of bipolar carbon chains (containing both types of charges) and therefore are capable f generating membranes that interact with the ground, forming a “cake”. The support of the walls of the excavation is achieved both because of the cohesive effect of the soil particles and by transmitting hydrostatic pressure to the walls of the excavation.

Their primary advantage is that they are biodegradable and it is not necessary to transport them to a landfill site.

The use of polymers reduces the amount of solids in the liquid and decreases their settlement in the base of the excavation.