Experimental Evaluation of the Effects of Dynamic Pressure on Improving Cement-based Grout Penetrability : A study performed with the short slot

The increasing need for watertight underground works, such as tunnel excavations, has sparked an interest in a number of research studies with grout penetrability being the focus. The research has so far contributed to a deeper understanding regarding the assessment of a successful grouting operation from different perspectives such as choice of equipment, material properties, phase planning and performance. It is well established that several crucial factors influence the penetrability of grouts with pressure type and magnitude being two of them. Researchers have examined this issue from different perspectives and with different instruments. Previous research has shown that the increase of pressure has yielded better grout penetration both in the laboratory and the field. According to studies that dealt with dynamic pressure application, superimposed oscillatory pressure of high frequency resulted in grouts with decreased viscosity and better penetration. However, there is still need for further investigation in terms of lower frequencies, different pressure profiles and magnitudes and possible alternatives regarding the filtration and erosion mechanism. The present study aims to examine the penetrability of grouts under dynamic conditions of predefined peak and rest pressure intervals with frequencies lower than 0.5 Hz. The mechanism of action assumes active decomposition of any partially built plug in the vicinity of a narrowing in a rock fracture caused by the change in flow type before the constriction. The chosen method included the development of a laboratory grout injection system with a pneumatic valve and a pressure control rig. Tests were performed with both constant and varying pressure utilizing different periods of peak and rest intervals. The results showed an overall improvement of penetrability under dynamic conditions compared to the static. The decrease in peak – rest durations from [2 – 2] to [4 – 8] s yielded almost 11 times more material penetrating the 30μm aperture size. The developed method utilized a pneumatic injection system which culminated in better penetration when tuned at [2 – 2] s of [peak – rest] durations with specific material properties. This study presents an indication that further research towards the direction of dynamic pressure application could be proven beneficial in the grouting industry