Some results of rock measurements; residual stresses and strains about cavities

5.5 In situ methods of measuring residual stresses; measuring stresses about cavities

5.5.3 Some results of rock measurements; residual stresses and strains about cavities

Results of measurements on residual rock stresses published in many coun- tries, mainly by the U.S. Bureau of Reclamation, show a wide range of values.

In many cases, if a rough average were taken, Heim's bold assumption (k = 1) would seem to be confirmed. In a few cases k > 1 was found (Portugal, surge tank chamber excavation). In Sweden there is a general overstressing of granite in the horizontal direction, with an average value k == 3 (Hast & Nilsson, 1964).

To supplement the direct measurement of residual stresses, strains around cavities have been measured in some tunnels. Some information is available on Straight Creek tunnel which was excavated in granite (Hartmann, 1966;

C. Jaeger, 1966). Rock displacements have been measured in a radial direc- tion in boreholes drilled in the tunnel soffit at different angles. From pub- lished diagrams it appears that areas of slight tensile stress exist almost everywhere near the tunnel soffit. It is remarkable that in several instances the radial displacements indicate radial tensile stresses in existence beyond the point xx = l-22r, which theoretically is the limit for tensile stresses even in the worst case, k = 0. Tensile stresses appear up to xx = l-8r. The higher the average density of fractures, the larger the ratio xjr.

Theories cannot be developed whilst the available information is so limited, but it is defined that tensile stresses tend to extend in fissured rock beyond the accepted normal, and that the weaker the rock the more they spread. This behaviour of rock under moderate tensile strain and stress is quite different from that under moderate compression stresses and it can possibly be explained by Griffith's theory of rupture of rocks.

In situ methods of measuring residual stresses 99 5.5.4 Special instruments

The modern trend towards lighter rock supports in tunnels and galleries requires precise knowledge of the actual strains and stresses developing in the rock, together with methods for registering minute deformations in the rock masses.

Some of the specialized instruments already available are an indication of how interest has recently shifted from measurements made on the surface of the rock or at shallow depth to borehole measurements at some distance from the rock surface.

(1) Rod type extensometer for boreholes. This is a rod which is fixed to the bottom of the borehole and protected by a pipe or hollow rock-bolt which (if it is attached to the sides of the drill by grouting or anchors) may also provide the reference point for collar measurements. The relative distance between the end of the centre rod and the end of the protective pipe is measured by a dial gauge, strain meter or transducer. The comparative rigidity of the protective pipe and centre rod makes installation of this instrument difficult or impossible in boreholes exceeding one tunnel diameter in length. The usual zone influence around a tunnel opening is more than two diameters, so that both fixed points may be displaced and total displacement is not measured.

(2) Wire type extensometers for boreholes. Wire type extensometers may be tensioned with weights or with springs. They can be either single position which provide information on two separate fixed points contained within a borehole, or the more recently developed multiple position which makes it possible to obtain much more useful and detailed data from a single borehole (fig. 5.11).

/ / ô / / /

T

Fig. 5.11 Schematic arrangement of a three-point borehole extensometer (Terra- metrics).

klx

where A/ = length variation in mm; M = dial gauge reading in mm; k = spring constant in k g / m m ; / = cross-sectional area of measuring wire in mm2 (2 mm steel wire). In the figure a = tension head; b = anchors; c = grouted protection pipe;

d = measuring wires.

With the wire coil-spring type, an invar wire is fixed to the anchors placed inside the borehole. The 'air-end' of the measuring wire is clamped to a tensioning pipe incorporating a spring to apply tension to the wire. The instru- ment head is enclosed in a watertight housing. This extensometer is best suited to long-term measurements.

The four-point coil extensometer consists of three independent spring- tensioned wires, combined in a single housing with anchorages at three different points inside a borehole. Completely waterproof, the thermically insulated sensor head is fixed outside the borehole. The anchor assemblies may be grouted individually, by the use of inflatable air pockets. The annular space between the borehole walls and the plastic pipe protecting the wires is filled with injected grout. The spacer pipes are coated with grease, so the grout does not bond to them except at the anchor assembly points. The measuring accuracy is ±0-01 in (±0-25 mm), and the measuring range is

±0-4 in (±10 mm) but this can be increased to ± l i n (±25 mm). An improved version with eight fixed points and a sensing head capable of remotely measuring their axial displacements is also on the market.

Such measurements, taken successively with time and tunnel face advance, provide the basis for plotting rock mass strain gradients. It is possible to outline stable zones, and those which are being compacted, compressed, loosened, or placed in tension. Active major fracture zones penetrated by the instrument's borehole can also be located.

(3) Borehole deflectometer. The borehole deflectometer (e.g. System Terrametrics, fig. 5.12) moves in a drill hole of size 5 in, inside a 4-in plastic

Fig. 5.12 Borehole deflectometer (Terrametrics) a = deflection arm; b = deflec- tion arm head; c = ball bearing supported ball rollers; D = drill hole diameter = 4 in or larger; a = deflection angle (measuring sensitivity ±0001 inch).

tube. A high degree of protection against water and extreme temperatures must be provided. The borehole deflectometer measures rock deformation normal to the axis of the borehole in any desired measuring plane.

It consists of two flexibly connected parts: the guide housing and the deflection arm. A pivoted joint between the deflection arm head and the guide housing allows the arm to vary its position as the instrument is moved up or down the cased borehole. Ball bearing supported ball rollers are mounted on both housings in a triaxial arrangement. One of each of the three rollers is spring loaded. Two sets of rollers provide longitudinal stability for the guide housing while the deflection arm has only one set. An anchored and

In situ methods of measuring residual stresses 101 tensioned steel wire passes through a knife-edge orifice plate in the guide housing. Deflection of the borehole results in a corresponding bending of the wire and the angular deviation of the wire is measured by electrical trans- ducers. Semifixed multiple-point borehole instruments are also available.

Precise monitoring of rock mass movements normal to the borehole axis at a number of points is thus possible.

More information on measurements of rock deformations in situ will be given in section 10.11 on underground hydro-electric power stations, and in sections 16.1 on Kariba South Bank and 16.3 on Waldeck II pumped storage station.