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In this testing water under pressure flows outwards from a test hole. The quantity taken in test periods indicates relative permeability which is taken in conjunction with the type of water flow to enable decisions about the grouting.
There are two main types of water testing in relation to grout holes: EXPLORATORY TESTING, where permeability exploration of a site is carried out for design of the grouting, and GROUT HOLE TESTING, where each stage of every grout hole is water tested before grouting it. The grout hole testing is a simpler version of the exploratory test method. This programme shows both types.
WATER TESTING IN GROUT HOLES:
APPLICABLE TO BOTH TYPES OF TEST:
WATER TESTING IN EXPLORATORY HOLES:
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The Lugeon unit of permeability is the most popular and relevant unit for grouting purposes. It is based on the type of water test just shown.
Definition of the lugeon unit:
or put more formally:
or
In terms of "velocity"-type permeability units, very approximate calculated relationships are
To give a sense of proportion for the unit:
Most water testing is done under different conditions to those. The length of the hole tested is usually more than 1 metre, and the definition pressure is frequently too high to use safely in weak rock foundations prone to move. Compensating adjustments are therefore made when calculating Lugeon values from water testing results. The formula is
Lugeon units = litres/metre/minute
x 10 (bars)
actual pressure (bars)
or
Lugeon units = 0.01076 cu ft/ft/minute
x 142 (psi)
actual pressure (psi)
When the recommended test pressure of 15 psi [1 bar] is used, and measurements are in cubic feet and feet:
This value is usually read off tables.
When the test pressure is 1 bar and measurements are in litres and metres:
The overall concept is that the test method must indicate:
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These five different pressure runs are done immediately after each other. They can be denoted as A-B-C-B-A.
The lugeon value is then calculated for each of the pressure runs. Then the five values are inspected to determine the flow regime or testing event. In the examples shown below, bars are used to pictorially convey relative magnitudes, but once the concept is understood, the process of deciding which lugeon value to report for the test can be done entirely mentally.
Graph plots are also shown in the following boxes for illustrative purposes, but are not really necessary.
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On the diagram the pressure increments are shown in hatched bars. The manner in which pressure rises to a peak in the third run and then reduces, in a symmetrical pattern, can be noticed. The bars shown solid on the right portray the lugeon values calculated for each of the pressure increments. In this example, they are more or less the same value for each of the pressures. The implication of this is that the flow was generally laminar (i.e., smooth flow along cracks, without turbulence, usually the result of quiet flow in finer cracks), but not exclusively, of course, considering the mix of crack sizes and conditions met with in most test stages.
When flow is of this type, any one of the five lugeon values can be accepted as the result of the test and reported as such, or their average used.
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In this example the lugeon values for the various pressure increments are no longer the same. A symmetrical distribution with the lowest value occurring on the highest pressure is indicative of turbulent flow (i.e., rough, eddying, turbulent motion usually the result of fast flow in wider cracks). This lowest value should be reported as the result of the test. Note the considerable error that would result from averaging the values.
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The pattern of values shown here is somewhat the reverse of the previous one. The peak pressure produces a much higher lugeon value than before or after it. This pressure has been sufficient to locally dilate cracks by compression of softer materials or by closure of adjacent parallel cracks. The dilation has been temporary, and for this reason the reported permeability of the stage should be that obtained for the lowest pressures, or alternatively for the medium pressures if these are less than for the low pressures, indicating that turbulent flow was occurring prior to the dilation.
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This shows permeabilities increasing as the test proceeds. They do not decrease when the pressure does. This indicates permanent washing out of joint filling materials, or movement of rock caused by the testing, followed by "propping" where rock fragments fall into joints and prevent closing after relaxation of pressure. Too much of this sort of thing indicates that test pressures are too high. The reported permeability can be taken as that measured for the final run (at low pressure); this presumes that the peak pressure bears some resemblance to eventual in-service pressures, which would produce somewhat similar results if not grouted.
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This is the reverse of the previous pattern. The lugeon values are progressively reducing, indicating that empty voids, cracks, and so on are being filled and that water cannot then progress further. Prudent action is to carry the test further (at low pressure) until the reducing trend ceases, indicating that all empty voids have become filled. Even better action is to saturate such a foundation before starting the test. The reported permeability should be the lowest measured; this is an indication of the flowthrough permeability once the voids are filled.
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A form for recording the results of this type of water testing is available here in TRADITIONAL UNITS or in METRIC UNITS.
The data is automatically analysed on the form and a representative lugeon value for adoption as the result of the testing is suggested.
This is an abbreviated extract from Version 8.0 of the series of ROCKGROUT grouting programmes. The version is on DVD but is not yet publicly available.
The DVD contains the book Construction and Design of Cement Grouting together with the contents of this Web site and some additional matter.