## 3 Failure Characteristics and Numerical Analysis
### Field Investigation  
The failures of the shotcrete and the floor concrete in LPL5-1 aroused considerable attention from the builders. To determine the cause of failure, a field investigation was carried out by means of drilling and geophysical prospecting. Along the axis of LPL5-1, four survey sections were selected and boreholes were drilled on the upstream sidewall, the upstream spandrel and the downstream sidewall, as shown in Figure 6a. Ten boreholes were arranged with depths of 17.45-19.33 m, in which acoustic wave tests and digital camera observations were conducted.  
Via the digital borehole camera technique, the dynamic response of the surrounding rock mass during the construction phase can be clearly observed, such as the initiation, propagation and accumulation of cracks in the rock mass. The acoustic wave test can be employed to assess the quality of the surrounding rock mass. With the combined application of these two techniques, the degree of damage and the range of excavation damage zones in the surrounding rock mass can be identified. Figure 6b presents the borehole camera image and the acoustic wave test result of CZK47-2 at the upstream spandrel of section ZC0+81.2 m. The image shows that the rock mass cracked within a depth of 0.6 m and that cracks were distributed densely and were approximately parallel to the free face. These dense cracks indicated that the surrounding rock mass in this range  
Figure 6: Field investigation and results: (**a**) layout of survey sections and boreholes from top view and axial view and (**b**) digital borehole camera images, cores and acoustic wave velocity curve at the upstream spandrel of drainage gallery LPL5-1.  
was damaged. According to the results of the acoustic wave test, the average acoustic wave velocity in the rock mass within a depth of 0.6 m was 3755 m/s, which was the lowest level in CZK47-2. The cracks and damage in the surrounding rock mass accounted for the low wave velocity in this range. In general, the wave velocity curve of CZK47-2 appeared as a step shape. The wave velocity within a depth of 1.2 m was lower, while that at depths over 1.2 m was higher. Thus, we could determine that the range of the excavation damage zone (EDZ) at the upstream spandrel in this section was within a depth of 1.2 m. The surrounding rock mass in the EDZ obviously cracked, while that beyond the EDZ was relatively intact.  
The borehole camera image of CZK47-2 showed that breakout, a typical high in situ stress failure, appeared at depths of 7.0-8.2 m and 11.2-11.9 m. Another phenomenon related to high in situ stress, core discing, also occurred in CZK47-2. Conversely, there was no core discing on the downstream side. Breakout and core discing in CZK47-2 proved the high stress around the upstream arch of LPLS-1. Based on the results of field investigation, preliminary conclusions can be drawn as follows:  
1. The section size of LPL5-1 was relatively small. The surrounding rock mass was hard and most of the rocks were classified as category III. The in situ maximum principal stress was 19-23 MPa. Given all this, only mild spalling would have taken place after excavation. By virtue of systematic support, the surrounding rock mass could have remained stable. The continuous cracking of shotcrete may not be caused by the excavation of LPL5-1; it was possibly related to the stress adjustment induced by the main powerhouse excavation.
2. High stress was proven to exist at the upstream arch by core discing and breakouts there. This conclusion agreed with the fact that only the upstream side of LPL5-1 suffered shotcrete failures.
3. The floor concrete failure was basically on the surface layer, while the underside concrete did not encounter failure. This result suggested that the floor concrete failure was a result of the uplift and squeezing of the floor, together with the low bond strength between the floor concrete and the underside part.