Effects of Drilling for Tieback Anchors on Surrounding Ground: Results from Field Tests

Abstract

A full-scale field test program was carried out to investigate the effects of drilling for tieback anchors on the surrounding ground. The test anchors were drilled from the ground surface through a soft clay deposit and into bedrock. Five different drilling methods were compared. All methods caused excess pore pressures in the surrounding clay, up to 70 kPa, extending several meters away from where drilling took place. This impact on pore pressures was for most drilling methods significantly larger than what has been observed for driven piles in clay. High penetration rate combined with water flushing during drilling through soft clay is the main reason for the effects on the pore pressure. Drilling with a down-the-hole hammer and air flushing through a layer of moraine and into bedrock in one of the test areas (Area B) caused significantly larger excess pore pressures and ground settlements than the other drilling methods. Approximately half of the maximum resulting settlements of 12 mm in Area B was most likely caused by reconsolidation of remolded clay around the casing tubes. Drilling with water-driven hammer in Area C had less effect on both pore pressures and ground settlements.

Physical modelling of pile drilling in sand

“…The results generally display a reduced pore pressure in the surrounding ground which could indicate that the air flushing caused an air-lift pump effect as suggested in case studies (Lande et al 2020;Ahlund and Ögren 2016;Bredenberg et al 2014). However, the limited dataset makes the analysis challenging and no clear conclusions can be drawn.…”

“…Other aspects that should be investigated are (1) the effect of drilling in a confined aquifer (e.g. under an impermeable soil layer) as frequently observed in the field (Lande et al 2020;Ahlund and Ögren 2016;Sandene et al 2020), (2) the influence of varying soil density, (3) different degrees of saturation and (4) the impact of different pile rotation rates. It is expected that refined tests will provide further insight into overburden drilling.…”

“…This effect has been observed in the field when drilling with air flushing is carried out at shallow depths or when the flushing pressure is too high (e.g. Lande et al 2020;Sandene et al 2020). Due to these challenges the air flushing tests could not be compared with the water flushing tests.…”

Pore Water Pressure Development in Soft Soil due to Installation and Loading of Controlled Modulus Columns

“…Rahardjio (2017) observed high EPWP even 5 months from the installation of large group of jacked piles. Lande et al (2020) reported approximately 8 kPa EPWP remaining in soft clay 150 days after anchor drilling with uncased system was completed. In order to control the pile driving process, Tefera et al (2013) installed the piezometers in the area of potential failure plains.…”