Tunnel Face Stability & New CPT Applications - Geo-Engineering

2.1. Introduction 19GC45 ◦ + φ/2s(z t )B ′BFigure 2.17: Balthaus’ model for the safety against blow-outChDFigure 2.18: Drying of soil due to expanding air bubbleAccording to Babendererde [16] this type of blow-out is most common in soils with low permeability(10 −2 m/s) air willescape almost unhindered and erosion of the surface soil and flow channels might occur, but abuild-up of air pockets is not likely to occur. He also states that the different combinations ofsoil permeability and air loss mechanism show different characteristics of air consumption overtime, which can be used to assess the safety against blow-out during the actual tunnelling works.Instead of lifting the soil as a whole, the fluid pressure can be large enough to force individualsoil particles apart and form cracks in the soil. The fluid will propagate into the cracks and as thepressure loss along the crack is often negligible, the process will continue to elongate the crack.This process is known as soil fracturing. The flow channel created in this way can be eitherhorizontal or vertical and these channels tends to propagate quickly. Due to its sudden occurrence,speedy propagation and resulting loss of large amounts of support medium, fracturing is difficultto recognize in time and is as hazardous to the boring process as a blow-out.Mori [121] defines the pressure at which (vertical) fracturing must occur for a normallyconsolidated soil ass f = K 0 σ ′ v + p + q u (2.21)with q u the unconfined compressive strength. In general fracturing will occur somewhat earlier,