With the improvement of highways and new artery networks during recent years, long-span cross-strait bridges were planned between Honshu and Shikoku; they have been completed in part or are under construction now. A particular characteristic of cross-strait bridges is that they require vast foundations, constructed on submarine ground. And the ground used for their foundations is granite rock masses.
This report discusses the geological investigation system utilizing borehole measurement, and the procedure for evaluation of its engineering properties that were practiced in the geological investigations of the Honshu-Shikoku Bridge construction project.
The geological investigation of cross-strait bridge foundations for weathered granite is accomplished through a combination of borehole measurement practices that may be broken down broadly into four items:
Core observation, rock classification, and RQD
Physical (velocity, electrical, density, etc.) logging
Borehole load test
Sample core physical and mechanical Test
These measurement items applicable to a wide range of granite weathering from fresh to weathered rocks. Also, the measured values, even though local, subjective, and physical measurements individually, can be converted by said combination of measurement practices into continuous, objective, and mechanical measurements. This makes it easy to evaluate the engineering properties of rock masses and to establish geomechanical models.
Fig. 1 shows the range of adaptation for each measurement item. In the case of foundation ground for the cross-strait bridges, main emphasis was given to those measurement items which were adaptable particularly for measuring the physical properties of heavily weathered rocks and which were effective in the detailed evaluation of engineering properties.
(Figure in full paper)
The various borehole measurement practices may be outlined as follows:
Core observation represents the naked-eye judgment of cores by the geological engineer. The hardness and shape of cores were divided in accordance with Tables 1 and 2 while at the same time the rock masses were classified into 8 grades. RQD measurement, etc., also were carried out.
Physical logging is the continuous measurement of various physical values. This was done by velocity logging (based on sonic logging; borehole proving interval, 100 cm; 20 kHz magnetic strain trembler used as oscillation generator-receiver), electrical logging (normal method; 2-pole method; pole distance, 25 cm, 50 cm, 100 cm), density logging (gamma-gamma ray method; 137Cs-25mm Ci used), and borehole diameter logging. From the measurement results, the velocities (Vps), (Vss) of rock mass elastic waves P and S, true resistivity (Rt), and density (Pd) were derived continuously.
The borehole load test is the local measurement of mechanical values. A high-pressure G-type pressure meter (diem. 52 mm; pressurizing limit, 100 kgf/cm2) was used to determine the deformation modulus Esb of rock masses at an arbitrary depth.
The core test is conducted to learn the basic properties of rocks that constitute a rock mass. The sample cores were given a physical test (density Pc, porosity nc, ignition loss I.L., elastic wave velocities Vpc, Vsc, etc.) and mechanical test (uniaxial compressive strength qu, static elastic modulus Esc, static Poisson ratio vsc, etc.)
