粘土性岩における押し出し性-膨潤性トンネル地圧のメカニズムと実測例

抄録

This paper discusses the mechanism of squeezing-swelling rock pressure of mudstone and presents the quantitative data of rock pressure measured during tunnel construction in the field together with the associated measnrements of physical and mechanical properties of the rocks.
Wire-strain-gauges were used to determine the axial force N, bending moment M and the shearing force Q and the resultants of external forces P and S due to rock pressure were calculated at selected points using the method originally proposed by Prof. Murayama14), thus the pressure distribution concentric with the tunnel axis being obtained (Fig. 14).
Three of the tunnels of the main irrigation water line for Noshiro Reclamation Project in Akita Prefecture provide the data for this paper. These three tunnels No.3, No.4 & No.6 were all constracted in massive, occasionally faulted tertiary mudstone named as Fujikotogawa Formation.
The phenomena encountered during the construction of the tunnel No.4 were very different from those of the tunnels No.3 and No.6.
In the case of the tunnel No.4, the heavy rock pressure caused an excessive deformation of the first placed light steel supports, resulting in the necessity to change to the heavier supports with invert struts.
However, in the case of the tunnels No.3 and No.6, the rock pressure was considerably smaller in spite of the fact that the mudstone encountered in these tunnels was apparently similar to those of the tunnel No.4.
To investigate the reason for this and to assist in the deeper understanding of the mechanism of squeezing and swelling rock pressure, tests were made to determine the unconfined compressive strength qu of the rocks, the swelling characteristics of the artificial mudstone or remoulded mudstone**, clay mineral identification and the pF water content relationships of the rock powder (Fig.10, 15, 16, 11, 19).
As shown in Fig.11, expansive clay minerals (montmorillonite) were contained in the mudstone of the three tnnnels but as shown in Fig. 10 the strength qu, of the rocks sampled from the tunnel No.4 was approximately one half to one third (or even less) of those sampled from the tunnel No.3 and No.6 and nearly equal to or less than two times the initial vertical stress induced by the weight of the rock itself at the side wall, indicating that the mudstone around the tunnel No.4 was broken and plastic zone of failure was formed (Fig.4), whereas the mudstone of the tunnels No.3 and No.6 underwent little breakage by the stress of the same magnitude.
On top of this, the swelling capacity of the remoulded mudstone sampled from the former tunnel was conspicuous (Fig.15) and much larger than that of the latters, indicating that the strength parameters and the unconfined compressive strength qu of the mass of broken rocks of tunnel No.4 reduced considerably in the course of time, resulting in the much increase of σi as is clear from Eq. 14 and Fig. 8.
These facts not only explain the reasons of the difference of the rock pressure phenomena but also the mechanism of the squeezing-swelling rock pressure of mudstone.