Innovative Approaches to Assessing Unknown Pile Lengths: Ensuring Clearance and Stability for Urban Tunneling Projects-Paper

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The paper examines how to estimate unknown pile lengths when urban tunneling or pipeline construction passes beneath existing structures. Because older foundations often lack reliable as-built records, uncertainty about pile toes can make it difficult to assess tunnel clearance and risks to structural stability.<br /><br />Three non-destructive testing (NDT) methods are evaluated: Parallel Seismic (PS), which tracks stress waves from a foundation using receivers in an adjacent borehole; Length Inductive Testing (LIT), which detects changes in the magnetic response near metallic piles; and surface Guided Wave testing, which estimates length from wave dispersion. The methods were applied in three projects: drilled shafts beneath a Toronto condominium, helical piles supporting a pedestrian bridge in the Greater Toronto Area, and H-piles beneath an Edmonton transmission tower.<br /><br />Results varied by method and site. For the condominium shaft, PS estimated 6.5 m, while Guided Waves estimated 8.0–9.2 m; the estimates were treated as lower and upper bounds. For the bridge piles, PS indicated approximately 10.5 m, consistent with an LIT signal interpreted as a helix at about 10–10.5 m. For the transmission tower, PS estimated 19 m and Guided Waves 20.5 m, while LIT results were inconclusive.<br /><br />The authors conclude that NDT can provide valuable information when exposing foundations is impractical. PS can be accurate but requires an access borehole and suitable impact access. LIT performance depends on close borehole proximity and may be inconclusive. Guided Waves require little pile-head exposure and can help validate lengths, though steel piles may produce more signal noise. Combining methods and adapting them to site conditions improves the reliability of clearance assessments.

Keywords

pile length estimation

non-destructive testing

parallel seismic

length inductive testing

guided wave testing

urban tunneling

foundation assessment

pile toe location

structural clearance

subsurface investigation