Development of an algorithm to detect hydraulic jacking in high pressure rock mass grouting and introduction of the PF index

Abstract

Pre-excavation rock mass grouting is a common procedure for reducing water ingress into tunnels during construction in Norwegian tunnelling. The level of grouting pressure is a disputed subject and the knowledge of how the rock mass responds to the high pressure and how this inflicts on the grouting results is sparse and little investigated. For this reason, it is of interest to use data from high pressure grouting performed in Norwegian projects to investigate these matters. This paper presents the development of a method for identifying hydraulic jacking during rock mass grouting and the making of an algorithm to perform computerized detection of hydraulic jacking in grouting logs. The algorithm is the base for a larger study, where screening for hydraulic jacking is performed in over hundred grouting rounds, distributed on several Norwegian tunnels excavated in rock mass. The relation between grout flow and grouting pressure has shown to be vital for the understanding of the grouting progress and events occurring during the grouting. Interpretation of pressure and flow as two separate variables, which are affected by aliasing, caused by low and irregular sampling frequency is a challenging task, and it was found to be helpful to create a parameter to represent this relationship, named the PF index (Pressure Flow index). This parameter has also shown to be useful in other applications such as monitoring the grouting progress on site.

Simplified Hydraulic Jacking Test to Assess Fracture Normal Stress for Unlined Pressure Tunnels: A Field Experiment Using the Rapid Step-Rate Test

“…As a potential extension to the original intended scope of the RSRT, it could be interesting to try out this test during grouting operations, as a potential way to delineate the "safe" grouting pressures during pre-excavation grouting in tunnels. This is particularly relevant for infrastructure projects where high-pressure pre-excavation grouting is performed, known to quite frequently produce undesirable hydraulic fracturing events (Strømsvik et al 2018).…”

Investigation and assessment of pre-grouted rock mass

“…This force makes the fracture open, which means that the aperture of the fracture is increasing. A detailed discussion of this process can be found in Stille (2015) and Strømsvik et al (2018). Stille (2015) draws attention to the following negative consequences of HJ during pre-grouting:…”

“…The pressure and flow data from the grouting rigs were screened for occurrences of HJ. For the grout rounds at chainage 155 and 164, the algorithm described in Strømsvik et al (2018) was used. It was not possible to perform such analysis on the grouting rounds at chainage 105 and 10.…”

“…At chainage 105 the log interval was over 10 s, and at chainage 10 the log from the grouting was not possible to locate. The data from the grout round at chainage 105 was analysed by visual inspection of the pressure flow charts, using the general principals suggested in Strømsvik et al (2018).…”

Study on Seepage Simulation of High Pressure Grouting in Microfractured Rock Mass

“…Especially under high pressure grouting conditions, microfractures will deform under the action of grouting pressure, which will cause changes in the seepage characteristics of the slurry [28][29][30][31]. Existing grouting seepage simulations lack descriptions of fracture deformation and slurry seepage characteristics under high pressure grouting conditions and are not suitable for the simulation of microfractured high pressure grouting slurry seepage characteristics [32][33][34]. Therefore, studying the seepage characteristics of high pressure grouting on microfractures considering fluid-solid coupling is of great significance to the study of grouting in microfractured rock masses [35].…”