WATER JET CONFERENCE - Waterjet Technology Association

the water jet. This method was not necessary for the cohesive soils 3 and 4 since the hole
or slot excavated by the jet remained open for direct measurement of the depth of
penetration and the diameter of the hole using a set of steel rods.
Variables Investigated
Available information on the cutting of rock using high speed water jets suggests
that, other than grain size, perhaps the most important material properties which control
the depth of cut are the void ratio or porosity, the permeability, and the strength of the
target material. To provide control over the same properties for the soils tested during the
course of this investigation, tests were conducted while carefully monitoring and varying
the water content (or degree of saturation) and the dry unit weight of each soil.
The penetration of a material target by a water jet is not an instantaneous
phenomenon; for stationary jet and target a certain amount of time is required to achieve
the maximum depth of penetration which cannot be significantly increased by continued
application of the water jet. Accordingly, the first series of tests for each soil type was
conducted by maintaining constant all the other variables and allowing the time of
application of the jet to vary so that a lower limit was obtained for the necessary time of
application of the jet on each soil type to achieve maximum penetration.
Having established the necessary time for application of the water jet and selecting
as a convenient jet pressure for conducting the tests the pressure of 1.000 psi two series of
tests were conducted for each soil type with stationary jet and target. In one series only the
degree of saturation was varied from as low as l0% to 15% (dry for soils 1 and 2) to
almost 100% while dry density was maintained at a constant value. In the other series only
the dry density was varied; moisture content remained constant. Finally, a series of tests
was conducted for each soil type by maintaining constant all the other variables and
allowing the target to traverse under the jet at velocities ranging up to 1.0 inch/ second.
Thus, a total of seventy-six soil cutting tests were conducted.
RESULTS AND DISCUSSION
The effectiveness of a water jet in cutting a given material is usually evaluated in
terms of the depth of jet penetration into the target material or in terms of energy required
to excavate a unit volume of the target material (specific energy). For the purpose of this
investigation, jet efficiency was assumed to be represented by the depth of penetration into
the various soil targets, and appropriate graphs were prepared showing the variation in the
depth of penetration as a function of jet duration, soil dry density, degree of soil
saturation, and traversing velocity. The results obtained and the observations made during
the experimental phase of this investigation are presented and discussed next.
Penetration as a Function of Time
For the purpose of this investigation, all samples of each of the soils tested were
required to be prepared at the same dry density and degree of saturation. Due to
difficulties in compacting fully saturated samples of the cohesive soils, all tests were
conducted on partially saturated samples. Accordingly, the dry density had values of 102
lb/ft 3 , 130 lb/ft 3 , 109 lb/ft 3 , and 102 lb/ft 3 for Soil 1,2,3 and 4, respectively; the degree of
saturation had values of 50%, 53%,75%, and 62% for Soils 1,2,3 and 4,respectively.
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