A cut slope deformation occurred during road improvement work on a rock mass slope, therefore, instruments were immediately installed. A rock mass of 60,000 m3 collapsed 40 days after installation. The time of collapse had been forecast accurately by the Saito method. Also, on the forecast time of rock failure, continuous photographing through video cameras at two fixed points was carried out for detailed analysis as to the mechanism of collapse and the form of motion involved.


Une deformation d'un talus de deblai a eu lieu lors de travaux d'amelioration d'une route sur un talus à fond rocheux. Par consequent, des instruments ont ete immediatement installes. Un fond rocheux de 60,000 m3 s'est affaisse 40 jours apres l'installation. La date de la rupture avait ete predite precisement par la methode Saito. De plus, pour prevoir la date de la rupture du fond rocheux, une photographie en continu utilisant des enregistrements video en deux points fixes avait ete realisee pour effectuer une analyse detaillee du mecanisme de rupture et de la forme de mouvement impliquee.


Wahrend Straβenausbesserungsarbeiten an einer Gebirgskörperböschung trat eine Baugrubenböschungsdislokation auf. Aus diesem Grunde wurden sofort Meβgerate installiert. 40 Tage nach der Installation stuerzte ein Gebirgskörper von 60,000 m3 ein. Der Bruchzeitpunkt wurde durch die Saito-Methode genau vorhergesagt. Des weiteren wurden zum Vorhersagezeitpunkt zur ausfuehrlichen Analyse des Bruchmechanismus und der auftretenden Bewegungsfonn unter Verwendung von Video-Bandaufnahmen an zwei festen Punkten fortlaufend Aufnahmen gemacht.


A rock mass slope under improvement construction work on National Highway of Route 327, Miyazaki Prefecture, collapsed at 15.15 hours, May 31,1990. On April 19, before said collapse, part of the cut slope under construction showed such deformations as cracking and push-out, therefore, the road traffic in use in the neighborhood was immediately stopped, then extenso meters (S-3, R-1) were set on tension cracks of the slope head, and light wave mirrors (M-l, M-3) in the slope, to observe their behavior while a collapse time was forecast using the Saito method, etc. On the forecast time of slope failure, continuous shooting by video camera was done from fixed points to observe and analyze the progress of rock fall and collapse as sign phenomena and the form of motion. And the mechanism and motion of collapse were analyzed. The cut slope was found to be about 70m in height, 59° in cut inclination, and about 200m in total length. The geology consists of crackful slate and sandstone that belong to the Hyuga formation group, Mesozoic Shimanto stratum. The formation was found inclined as a reverse dip structure to the slope, but five principal faults were distributed in an interfinger form on the cut slope (see Figure 1), their block and direction of crack forming a wedge-shaped rock body on dip slope structure.

Behavior observation by extenso-meter

A cumulative variation curve from the observation results of invert wire extenso-meters (S-3, R-l, Invarline length 10m) set on the slope head is shown in Figure 2. While the curve after instrument installation indicates the form of a tertiary creep, a secondary creep varying at a constant strain rate tends to increase in speed with each of four rainfalls until May 23, and the form of a tertiary creep that increases in creep rate gradually after that date. To compare the behaviors of S-3 and R-1, the difference in variation is shown in Figure 3.

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