Method of Construction

Overview

The piles are bored with rotary drilling rigs (so that virtually no vibration is transmitted to the structure), by rotating and feeding down a temporary steel casing which has a special cutting bit as its leading end.

A drilling fluid (usually water) is circulated through the casing so that it cools the cutting bit and returns outside the casing, transporting the drilling spoil. During boring, short lengths of casing are coupled together to enable the drilling rigs to operate in limited headroom. In granular soils the borehole is temporarily lined throughout its depth so that no collapse can occur. In cohesive soils, augers or drag-bits are used, whereas in rocks or other hard strata, tricone bits or down-the-hole hammers are used.

Once the borehole is formed to the required depth, the bore is flushed with clear water, and sand/cement grout pumped via a tremmie tube extending to the bottom of the bore so that any water present is displaced by the rising grout. When the whole borehole is filled with good quality grout, the reinforcement is placed through it extending the full depth of the pile. Reinforcement takes the form of either a single bar, a cage, or a tube. In certain circumstances the reinforcement is placed prior to grouting, the grout tremmie tube being inserted with the reinforcement.

When the reinforcement has been correctly located the temporary drill casing is withdrawn, the level of the grout being maintained by re-tremmieing as each section of casing is withdrawn Because the density of the fluid grout is greater than that of the surrounding strata, it exerts a positive pressure against the wall of the borehole and partially penetrates and cements the surrounding ground. By this action, necking of the pile is precluded, washing out of the ground cannot take place and high frictional bond between the pile and the ground is achieved.