Abstract

The significance of the excavatability for the successful completion of constructional projects has been recognized in recent years, but little attention has been paid specifically to rotary pile boring. Given the quantity of geological parameters alone which need to be taken into account, it is apparent that the estimation of the drilling rate is difficult. Based on a case study in extremely abrasive tertiary silcretes (quartzite), the engineering geological influences on the drillability are examined. A minutely detailed documentation of the drilling process was produced, and laboratory testing was conducted to gain a better understanding of the correlation between significant rock properties and wear on drilling tools. The examined silcretes (quartzite) show CERCHAR abrasivity indices (CAI) up to a value of more than 6. It comes as no surprise that this highly abrasive rock causes extensive wear on piling equipment and an extremely high consumption of wear parts. In combination with the drilling velocity of both the drill tools and the casing shoe remaining well below 10 cm/min, it is obvious that the drilling performance is significantly impacted in more than one way. A correlation between the lifespan of the wear parts and the net drilling velocity is developed, allowing for a more reliable performance prediction for bored piles.

Introduction

In recent years, the number of pile foundations for bridges and pile walls has been ever-increasing, due to economical, constructional, and structural reasons. Unavoidably, this increasing demand of pile boring has led to an increase in difficulties during the construction process, often caused by problems regarding the drillability of the ground. Challenging ground conditions are often ascribed to high strength and highly abrasive rocks. As a result, tool wear often exceeds the forecasted amount by far and the drilling velocity remains well beyond the expectations. With these two performance criteria not being met, extensive additional cost is the outcome. This shows the need for the drillability to be considered at early stages in a project to allow for adequate performance predictions instead of encountering problems during the construction stages of a project and solving them along the way.

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