The “Shape Factor” in Estimation of Tunnel Inflows
By Dr Steven Pells
Why I wrote this paper
At bedtime one day my 12-year-old son asked me to recall my life’s most terrifying experience. Coincidently, it occurred when I was also 12 years old, when I was asked to do a walk-through inspection of the Skitube tunnel in Australia’s Snowy Mountains during its construction. Being 12, the PPE didn’t fit, and anyway, asking for equipment would have alerted the construction supervisors to my presence. So, without communication or lights I would run up the side of the tunnel to grasp a service pipeline to allow trucks to pass, hopefully undetected. There was blasting underway at the other end of the tunnel, sending a shock wave that nearly bowled me over. I was terrified. Such was life as the son of a tunnelling engineer in the 1980’s.
A decade later, as an undergraduate engineer I conceded to the same ruling powers to trudge alone through the muddy Eastern Distributor tunnel construction in Sydney to document the locations of monitoring instrumentation.
Despite all this, a further decade later I found myself as a principal engineer at the company founded by the chief protagonist – my father – concerning myself mostly with tunnel inflows and seepage pressures. I have provided estimates of tunnel inflows over the years, both from analytical methods and 2D and 3D numerical modelling. Along the way I had done some research of various analytical methods (like the Goodman, 1965 methods, and many that followed it) and compared it to inflows from 2D and 3D numerical modelling, and had developed methods and insights that I thought were useful – such as what “shape factors” to apply to a tunnel and how these shape factors vary over time (i.e. transient effects).
The following paper sets out these methods and findings, which I think are useful to industry both in inflow estimation at the planning stage but also in guiding methods during construction, such as probing.
These days I can be found at Pells Consulting (www.pellsconsulting.com.au), working on tunnels and dams and other civil construction projects.
