ACHIEVING TIGHT LIMITS IN INSTRUMENTATION BOREHOLE VERTICALITY

There is a dearth of information on instrumentation borehole verticali ty available within the public domain, This paper describes the result s of measured deviations from vertical of boreholes constructed in ver y close proximity to running underground tube tunnels in London's stif f clays, The three drilling techniques of cable tool boring, rotary dr illing using flushing medium and drag bits, and continuous flight auge ring (CFA) were used to establish the most appropriate technique for a chieving tight borehole verticalities of 1 (horizontal) in 80 (vertica l), Borehole verticality was surveyed by means of an inclinometer prob e passed through an access tube, CFA techniques were found to yield th e highest rate of success in achieving borehole profiles better than 1 in 80, with at least a 40% success rate, this being approximately 25% higher than the success rate achieved in the case of cable tool borin g methods, These success rates correspond to verticalities between 1 i n 80 and 1 in 170 for CFA techniques and between 1 in 80 and 1 in 115 for cable tool boring methods, The success rates for achieving vertica lities better than 1 in 50 were approximately 55% and 65%, respectivel y, However, by excluding results affected by unforeseen obstructions, the rate of success in the case of the former increases to 75%, with 6 2.5% of the total achieving verticalities better than 1 in 80.