10th Asian Regional Conference of IAEG (2015)

Washout of clay-rich gouge in a pre-grouted fault zone and increase in groundwater
inflow during tunnel excavation in Neogene siliceous mudstone (Horonobe, Japan)
Eiichi ISHII(1), Yuta HASHIMOTO(2) and Daisuke INAGAKI(3)(1) Horonobe Underground Research Center, Japan Atomic Energy Agency, Japan
E-mail: ishii.eiichi@jaea.go.jp(2) Japan Atomic Energy Agency, Japan, (3) Japan Atomic Energy Agency (present address: Tokyo ElectricPower Company), JapanAbstract
This manuscript describes an unpredicted groundwater inflow that occurred duringtunnel excavation through a permeable fault zone comprising clay-rich gouge, despite thefault zone being pre-grouted. Observations indicate that the gouge was increasingly washed
out into the tunnel through the rockbolts penetrating the gouge and the boundary between
the shotcrete and the gouge on the sidewall during excavation. The resultant piping/erosionof the gouge probably accelerated groundwater flow from the outer aquifer of the
pre-grouted zone to the tunnel. After an excavation face exposing the fault zone wastemporarily shotcreted, a major inflow occurred from the newly formed flow channel in thegouge, breaking the shotcrete. When a fault zone including such clay-rich gouge ispre-grouted, washout of the gouge during tunnel excavation should be fully preparedbecause the gouge itself is not cemented by pre-grouting due to its low porosity.
Keywords: clay-rich gouge; fault zone; grout; tunnel excavation
1.

Introduction
Reducing groundwater inflow is an important
issue during the construction and maintenance ofunderground facilities. At the Horonobe site in Japan(Fig. 1), an underground facility for research anddevelopment into high-level radioactive waste

disposal has been constructed (Fig. 2). The amount ofFig. 2 Diagram of the Horonobe UndergroundResearch Facility. Red-colored shafts andgalleries have already been excavated as of April
2015. This layout may be changed depending on
the results of future investigations.

Fig. 1 Map showing the location of Horonobe.

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drain water is limited to 750 m3/day under anagreement with the local community. Neogene
siliceous mudstone occurs around the facility andhighly permeable fault zones, which typicallycomprise fault breccia, have been identified byborehole investigations in the siliceous mudstone(Ishii, 2012, 2015). The groundwater is saline andcontains dissolved methane gas.
This paper reports an unpredicted groundwater
inflow that occurred during tunnel excavation anddiscusses the cause and recommendations for futureexcavation.
2.

Groundwater inflow
Tunnel excavation was carried out while
pre-grouting the fault zones; however, when one of
the fault zones (the S1 Fault) comprising clay-richgouge (mainly smectite; Figs 3 and 4; Table 1) waspre-grouted at the 350 m East Loop Gallery (Fig. 5),
unusual muddy material exhibiting the same results

Fig. 3 Result of XRD analysis of gouge (whole-rock) of the S1 Fault.

Table 1 Minerals detected by XRD analysis of thegouge.

Fig. 4 Results of XRD analysis of gouge (<2 µmfraction) of the S1 Fault.

Fig. 5 Layout of boreholes drilled for pre-grouting at the 350 m East Loop Gallery (horizontal map).

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10th Asian Regional Conference of IAEG (2015)of XRD analysis as the gouge of the S1 Fault wasFig. 6 Muddy material ejected from a boreholeduring drilling.

Fig. 7 Occurrence of clay-rich
pre-grouted on an excavation face.

gouge

ejected from boreholes during drilling (Fig. 6).
Excavation of the S1 Fault revealed clay-rich gouge(~10 cm thick) and a surrounding damage zone (a few
meters thick). However, the gouge itself was notcemented by grout material (Fig. 7) whereas thegouge-bounding fractures and damage zone fractures
were cemented. Intensive spalling occurred at the S1Fault. During excavation of the fault, the claymaterials were increasingly washed out into thetunnel through the rockbolts penetrating the fault andalong the boundary between the shotcrete and thegouge on the excavation sidewall (corresponding tothe spalling zone and possibly not fully shotcreted) ata flow rate of <1 m3/hr (Fig. 8a and 8b). After an
excavation face exposing the S1 Fault wastemporarily shotcreted, a major groundwater inflow
suddenly occurred, breaking
the shotcretecorresponding to the gouge (Fig. 8c). As a result, theinflow rate temporarily increased to five tens of cubic
meters per hour. Observations of the broken shotcrete
and at the excavation sidewall revealed that the gouge

Fig. 8 (a) Excavation face exposing the S1 Fault
in the tunnel. (b) Inflow of muddy groundwaterfrom the boundary between the shotcrete and thegouge of the S1 Fault on the excavation
sidewall. (c) Major inflow from broken shotcretecorresponding to the gouge of the S1 Fault.
was largely eroded away from the S1 fault (no photos
due to the regulation of the scene). This sudden
inflow required post-grouting for more than 1 month.
3.

Discussion
Theaboveobservations
indicatethatpiping/erosion in the clay-rich gouge of the S1 Fault
occurred by washout of the clay materials into thetunnel through rockbolts penetrating the S1 Fault andalong the boundary between the shotcrete and the310th Asian Regional Conference of IAEG (2015)

gouge on the sidewall during excavation (Fig. 9).
Clay is commonly resistant to liquefaction; however,in this case the groundwater flow into the tunnel andthe gas/fluid pressure build-up due to shotcretingprobably accelerated liquefaction or fluidization andthereby caused washout of the gouge. When a fault
zone including such clay-rich gouge is pre-grouted,
washout of the gouge during excavation should befully prepared because the gouge itself is notcemented by pre-grouting due to its low porosity,
meaning that flow channels may form bypiping/erosion. For example, expanding the grouting
area and/or reducing the number of rockbolts might

be effective in preventing the washout of clay-richgouge.
References
Ishii, E. (2012): Microstructure and origin of faults insiliceous mudstone at the Horonobe UndergroundResearch Laboratory site, Journal of StructuralGeology, Vol. 34, pp. 20–29.
Ishii, E. (2015): Predictions of the highest potential
transmissivity of fractures in fault zones from rockrheology: preliminary results, Journal ofGeophysical Research, Vol. 120, pp. 2220–2241.

Fig. 9 Sketches showing the processes assumed to have caused the major inflow during tunnel excavation(horizontal sections). (a) The gouge-bounding fractures and damage zone fractures of the S1 Fault
intersecting the boreholes were cemented by pre-grouting. (b) During excavation, the gouge was washed
out into the tunnel through the rockbolts penetrating the gouge and along the boundary between theshotcrete and the gouge on the excavation sidewall. The resulting piping/erosion of the gouge induced
groundwater flow from the outer aquifer of the pre-grouted zone. (c) After an excavation face exposing
the S1 Fault was temporarily shotcreted, gas/fluid pressure in the S1 Fault zone built up. (d) A major
inflow occurred, breaking the shotcrete.

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