QUANTIFYING THE IMPACT OF ROCK DOWEL SUPPORT IN A CONTINUUM MECHANICS MODEL

Cement grouted rock dowels are a commonly used support element in underground excavations. Though the mechanism by which this rock reinforcement method stabilizes discreet sets of rock blocks is fairly well understood, a quantifiable understanding of the impact these support elements have on materials more closely representing a continuum is unavailable.

In an attempt to develop that understanding, a 15 m zone beneath the bottom of a 10 m by 20 m elliptical shaft was "pre-doweled" and instrumented. Pre-doweling was included in the shaft design in an effort to minimize the short-term and long-term heave effects suspected to be encountered once the excavation progressed from the much stiffer and stronger Austin Chalk limestone into the low strength clay shale of the Eagle Ford Shale formation. The pre-doweling was performed after excavating the elliptical shaft to a depth of 60 m in the Austin Chalk, after constructing a 2.4 m thick shaft liner key and after pouring a 10 cm concrete mud mat at the base of the liner key.

From this same location, holes for two heave gages (one exactly in the shaft center, and one centered along the longer shaft axis but approximately 7.5 m from the shaft center) were drilled and the heave gage casings were installed and grouted in place. Data from these instruments was recorded following each subsequent 1.5 to 2.0 m excavation stage. The data recorded clearly revealed the impact the rock dowels had on reducing the amount and rate of vertical deformation beneath the shaft bottom.

This paper focuses on comparing the field data and the results of the finite difference numerical modeling to quantify in a continuum model the influence rock dowels have on deformations and on the stability of the rock mass beneath the shaft bottom.

Results of analyses indicate that there is considerable quantitative benefit to the installation of rock dowel reinforcement in a continuum. Combined field and numerical modeling evidence indicates, that the patterned rock dowels appear to be increasing the cohesion of the material and preserving it's integrity during shearing.