This guide outlines the procedures for subsurface geological and geotechnical investigations for power tunnels and pressure shafts. It covers the different stages of investigation, from reconnaissance to detailed exploration, specifying the methods and extent of work required. The objective is to gather sufficient data on rock/soil conditions, groundwater, and geological hazards for a safe and economical design.
Lays down guidelines for subsurface investigations specifically for power tunnels and pressure shafts, covering geological, hydrological, and geotechnical aspects.
Quick Reference Values
Preliminary drill hole spacing along tunnel300 to 500 m
Detailed drill hole spacing along tunnel100 to 300 m
Minimum drill hole depth below tunnel invertOne diameter of the tunnel
Minimum core recovery in rockGenerally not less than 85 percent
Minimum depth of exploration for shafts1.5 times the base width below foundation level
Practical Notes
The primary focus during investigation should be on identifying major geological discontinuities like faults, shear zones, and areas of high groundwater ingress.
The scope defined in this guide is a minimum; complex geological conditions or critical projects may require a significantly more intensive investigation program.
Accurate logging and preservation of drill cores as per IS 4464 are crucial for correct geological and geotechnical interpretation.