The behavior of grout and rock mass response to grouting is described, among others, by the models GIN1 and RTGC2. However, these models do not fully explain some phenomenon observed in Norwegian grouting projects. In GIN and RTGC the criterion for jacking is determined by the spread of grout and the grouting pressure (see Kobayashi et al. 2008) and (Perkins et al. n.d.). When the force from the injected grout on the fracture surface gets large enough the models predict either fracture dilation due to compression of the surrounding rock mass (see Gothäll et al. 2009) or expansion due to lifting of the rock mass (see Kobayashi et al. 2008).
Norwegian rock mass practice uses a high grouting pressure. Stop criterions can be as high as 80 bars with only 15 meters of rock cover (see Olav et al. 2010). With the models for hydraulic jacking available reaching these pressures should be unobtainable as the grouting pressure far exceeds the criterions for jacking. It is also observed that even after jacking, higher pressures can be achieved in the same hole. How can this be when the criterion for jacking has evidently been met?
This paper is based on Haugsands master thesis (see Haugsand 2018) where an explanation is presented. The explanation is based on operator experience which stated that the risk of jacking is linked to grout flow. The presented reason for why flow is a factor lies in the description of grout as a fluid in the models. The models assume the pressure distribution from the injection hole to the grout front to be linear and uniform. However, the flow properties of cement can be described as both fluid-like and solid-like depending on both the cement properties and the flow rate (see Yahia et al. 2016). With the cement being more solid-like at low flow speeds. This is suggested will affect how the pressure is distributed in the grouted fracture and can thus explain why jacking is avoided in high pressure grouting.
The explanation is backed with the argument that it ties the models with what is observed without discarding the models or the observations. Circumstantial evidence with data from THIGTs3 work package 74 is presented where pressure connections between injection holes is shown to be lost after short stops in grouting. Indicating that the pressure distribution in the fracture is not uniform or always recoverable. A colloquial understanding for how the pressure is distributed in the fracture can be summed with the phrase "the pressure follows the flow".