10.5 Interpretation of the results
(1) The results of cylindrical dilatometer tests can be used to determine the deformation properties and the creep properties of the in situ rock at the test location in intact rocks.
(2) In fragile or clayey rocks and in fractured or closely jointed formations, where core recovery is poor or inadequate for purpose of obtaining representative samples for laboratory testing, the cylindrical dilatometer test can be used for rapid index logging of boreholes and for comparisons of relative deformabilities of different rock strata.
(3) The interpretation of cylindrical dilatometer tests requires a knowledge of the Poisson's ratio of the tested rock.
(4) In hard and non fractured rocks and when the applied pressure and corresponding borehole dilation are measured directly against the borehole wall the dilatometric modulus of deformation Ed of a borehole test section can be determined from the slope of the straight portion of a pressure-strain graph for the range of pressures greater than the seating pressure as shown in figure 10.2.
(5) The value of the secant dilatometric modulus Ed1 expressed in MPa and corresponding to a pressure increment Δp1c can be determined from the graph shown in figure 10.2 or using the following relationship:
Ed1 = (1 + νr) × D × (Δp1c/ΔD)
where:
Δp1c is the pressure applied to the bore hole wall, in MPa;
ΔD is the bore hole dilatation, in m;
D is the borehole diameter prior to testing, in m.
(6)P When the applied pressure is measured inside the probe the value of p1c shall be determined from the corresponding value of pi corrected to account for membrane stiffness before determining the values of Ed and Ed1.
(7)P When borehole dilation is determined from measurements against the internal wall of the membrane, these measurements shall be corrected to account for membrane compression before determining the values of Ed and Ed1.
(8)P When the pressure applied to the borehole wall and the corresponding borehole dilation are determined from measurements of fluid pressure and of volume of displaced fluid taken at ground surface these measurements shall be corrected to account for hydraulic system stiffness, membrane stiffness and membrane compression.
10.6 Reporting of the results
(1)P In addition to the requirements given in 2.6 the test report shall include the following information:
- borehole diameter and length;
- detail of the drilling program including method and equipment used;
- a geotechnical log of the drill core showing rock type and properties, ground water level, location of test sections and of cased and cemented sections, if any;
- details of equipment and procedure for testing and calibration together with results of calibration including ambient air temperature at time of calibration;
- tabulated test readings of the applied pressure and corresponding volume of displaced fluid or of the measured borehole dilation for each transducer obtained at each test depth within the boreholes and for each pressure range and time interval;
- plots of pressure-dilation curves obtained during calibration and testing for each test depth and for each displacement transducer inside the probe when relevant;
- plots of pressure-dilation curves obtained during calibration and testing for each test depth corrected for system stiffness, membrane stiffness and membrane compression when relevant.
10.7 Derived values of geotechnical parameters
(1) The results of dilatometer tests may be used to check against the serviceability limit state of spread foundations on rocks through a deformation analysis.
(2) When performing a deformation analysis the Young's modulus E may be taken equal to Ed on the assumption that the rock is linearly elastic and isotropic.
