Underground Construction - January 2019 - 37

Figure 7: Shear stress change after rapid drawdown

Figure 8: Monitoring the resistivity changes in the earth dam during rapid drawdown

Conclusions
Based on this experimental and analytical study, the following conclusions are advanced:
* Unconfined compressive strength was affected by the density, particle
size and gradation. Strength of grouted sands increased when particles were finer. The compressive strength of the grouted sands varied
from 240 kPa to 675 kPa after 7days of moist curing. Vipulanandan
p-q model predicted the stress-strain and piezoresistivity behavior of
acrylamide grouted sands.
* The permeability of grouted sand was 10-10 cm/sec and was not affected by the grain size distribution and particle size of the sand.
* The acrylamide grouted sand with and without conductive filler was
piezoresistive. Adding 0.1 percent conductive filler into the sand
increased the sensitivity of the grouted sands. The average change in
resistance at 400 kPa compressive stress was 21 percent compared to
the compressive strain of less than 0.8 percent, over 25 times magnification.
* For the embankment configuration investigated in this study, monitoring the shear stress change due to drawdown of water level in the
dam on the upstream side was possible using the piezoresistive smart
grouted sand column. The resistivity changes were more than 25
times higher than the compressive strains.

New Vipulanandan Failure Model for Rocks
When tunneling through rocks to place the required infrastructure, there
is a need for better quantification of the failure criteria for the rocks.
Also, deep foundations are socketed in rocks to carry very high loads, and
it is important to know the failure strength of the rocks under multiaxial stresses. For the hydraulic fracturing of rocks to produce oil and gas
in a cost-effective way, the drilling, installation of wells and fracturing
processes must be designed using rock properties, including the tensile
strength and failure criteria.
At present, the Mohr-Coulomb model is used to characterize the failure of the rocks. However, the model over-estimates the tensile strength
and has no limit on the maximum shear strength tolerance for the rocks.
In addition, there is inadequate property correlation in the literature for
the rocks. Hence, there is a need for developing improved failure criteria
for the rocks.
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz or feldspar because these are the most common minerals in the Earth's crust.
Rock formations that are primarily composed of sandstone usually allow
the percolation of water and other fluids and are porous enough to store
large quantities, making them valuable aquifers and petroleum reservoirs.
Fine-grained aquifers, such as sandstones, are better able to filter out
pollutants from the surface than are rocks with cracks and crevices, such
as limestone or other rocks fractured by seismic activity. Shale is a finegrained sedimentary rock composed of mud that is a mix of flakes of clay
minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.
Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms, such as coral, with the major minerals of
calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Limestone has been used for many construction applications such as, Portland cement concrete aggregate, asphalt pavement
aggregate, asphalt surface treatments, road base, structural fill, railroad
ballast, riprap, and drainage and erosion control.
The objective of this study was to verify the new Vipulanandan failure
model and compare the predictions to the Mohr-Coulomb model for
sandstone, shale and limestone rocks, using more than 750 data collected from the literature and tests performed in the CIGMAT laboratory.
Methods & Analyses
The Mohr-Coulomb failure model represents the linear relationship between the shear strength ( ) of a rock and the applied normal stress ( )
on the failure plane. This relation is as follows:
where {\displaystyle \tau }ooooo is the cohesion and the angle {\displaystyle \phi }ff fffis called the angle of internal friction.

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