The present paper reports on the stability problem of excavated slopes as follows: 1) In Japan, slope excavation works have often been performed at the site of narrow and steep topography. 2) In foreign countries, an emphasis has been put on the problem of progressive failure of soft rocks. 3) Two methods for predicting stability of excavated slopes are introduced; one is based on the ultimate strength theory and the other is based on the stress increments procedure. 4) It is shown, by an example of computation by the stress increments procedure, that the govering factors of stability of slopes are mainly stress path and residual strength. 5) An example of measurement of deformational behavior of excavated slope is shown.
Le compte rendu presente rapporte sur le problème de la stabilite de l'excavation de talus comme ci-dessous: 1) Au Japon, les travaux de l'excavation de talus avait ete souvent fait dans l'endroit de la topographie etroite et pente. 2) Dans les pays d'etranger, l'accentuation etait soulignee sur le problème de rupture progressive des roches faibles. 3) Deux methodes pour predicter stabilite de talus excaves sont introduisees; une se base sur la theorie de resistance finale et l'autre se base sur le procède de contraintes augumentees. 4) Par une example du calcul de la deuxième methode, facteurs importants concernant la stabilite sont principalement le passage des contraintes et la resistance residuelle. 5) Le comportememt observe se presente.
Dieser Beitrag behandelt die Standsicherheitsprobleme zur ausgehöhlten Böschungen. ie: 1) In Japan ist es nicht selten,daβ die Böschungsaushub an Orter mit schmalen und steilen topographischen Verhaltnissen ausgefuehrt werden. 2) Im Ausland dagegen handelt es sich um die fortpflanzenden Einstuerzen lockerer Gebirge. 3) Zwei Methoden fuer die Voraussage nach Standsicherheiten der eingeschnitten Böschungen sind vorgestellt. ie: Die erst Methode basiert auf der Bruchfestigkeitstheorie, die zweite auf stufenweisen Spannungszunahmen. 4) Aus mittels dieser gefuehrter Rechnungsbeispiel ergab sich,daβ Spannungsgesicht und residuale Festigkeiten spielen eine groβe Rolle. 5) Eines Beispiel von Deformationsbeobachtungen wurde angezeigt.
It seems that progressive failure plays an important role in the stability of the slopes excavated in the soft rock masses such as weathered or weak bonded rocks. The mechanical background of the progressive failure is investigated on the basis of past studies on that in heavily overconsolidated, fissured clayly shales by Skempton, Bjerrum, Bishop and others. In reference to the results mentioned above, it becomes very important to clarify how the stress path at the time of excavation influences the distribution of stresses within an excavated slope and how the brittleness of materials influences the stability of slope. In order to approach these hilightened problems, the F.E.M.is applied as the method in which non-linearity and residual strength of materials within slope can easily be taken into account. As the consequence of the numerical approaches mentioned above, it is found out that the stresses distributed within a slope are not significantly affected by the difference of the values of Young's moduli between those obtained by the triaxial tests under the condition of decreasing stresses which are simulated to the stress path within excavated slopes and those obtained by normal triaxial tests under the condition of increasing stresses and that the residual strength makes a great influences upon the stability of slopes which are subjected to progressive failure. The numerical method presented here must be verified in such a manner as that the computational reults may be approximately coincident with the observed results. Even though within the low stress level, it can be said that observed upheaval displacements caused by excavation work are about 80 percent of computational ones. Further studies should be carried out, especially in case of critical state of stresses. In such severe condition, it is better to obtain informations by an experimental laboratory test. Model tests from the view point, are now going on. These results will be published in near future.
In the case of such a ground as heavily over-consolidated clay, the date of the failure is often registered several decades after excavation and therefore safety factor, which is usually calculated by using peak-strength under the condition of drainage, was more than 1.0 at the time of excavation. This can not explain the actual failure. Skempton described by showing many cases of failed slopes that in most cases of delayed failure the strength of materials within the slope was actually lowered from peak strength to residual one. And finally, the importance how to estimate the strength as a design condition was emphasized, especially in such a case when the peak strength τp, to a certain degree exceeds the residual strength τr.
