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The Cambridge Insitu Constant Flow Self Boring Permeameter
This system uses the self boring technique to place a perforated metal sleeve at some depth in the ground, and then forces water through the sleeve in a controlled manner. The pressure of the water in the sleeve and the flow of water are known, and these two pieces of information are used to derive the horizontal permeability coefficient of the soil, Kh.
Because the probe is self bored into position the structure of the soil sees minimal disturbance and it is possible to establish the permeability in a matter of hours. Other methods of obtaining this parameter disturb the soil so severely that many weeks must elapse before any sensible data are obtained.
The permeameter shown is a conversion to a standard Cambridge Insitu Self Boring Pressuremeter (SBP). The outer sheath and the cutting parts are removed from the SBP and in their place a set of parts for the permeameter is substituted. This raises the diameter of the probe to 100mm. The membrane of the SBP stays in place, and the two pore pressure cells provide a means for measuring the water pressure in the soil for the permeameter test. All the usual SBP data logging hardware and software are used.
Water is supplied from the surface by a constant flow system where a syringe is driven by a stepper motor. The pulses for the motor are supplied by a control unit that allows a number of different flow rates to be supplied.
The picture above shows a typical test in London clay where pressure is plotted
against time. Six different flow rates can be seen, and the corresponding
pressure for the steps of flow are identified.
Flow is then plotted against head of water. Using well known formulae it can be shown that the slope of this plot is a function of the horizontal permeability. Note also that the intercept on the pressure axis gives the ambient head of water.
The instrument is undergoing development and future plans include allowing the length of the pressurised part to be readily altered, so permitting the vertical permeability Kv to be inferred.
Further information
Mk2
method. This is a link to a report on this web site describing an alternative
approach using self boring methods to derive insitu permeability .
Sang. This is a link to the web site of Cambridge University Engineering Department and the work of Sang Ratnam, who is making a detailed study of the feasibility of self boring technology for deriving the permeability properties of soil.
Chandler. This is a link to the reference section of this web site. The paper by Chandler et al is probably the best known reference for using a self boring pressuremeter to measure lateral permeability insitu .