THE ULTIMATE STATE CONCEPT APPLIED TO TUNNEL SUPPORT

Iz kritičkog osvrta na uobičajeni način proračuna interakcije stijene i podgradc slijedi da proračun podgradc koji se temelji na njihovim zajedničkim deformacijama nije pouzdan. Ako se, nakon šo je ugrađena podgrada, deformacije stijene povećavaju (»konvergenci¬ja«), će biti neminovno opterećena do njezine granične nosivosti. Budući da je takvo granično opterećenje vjerojatno predlaže se inver-zan postupak, tj. umjesto da se podgrada optereti aktivnim tlakom stijene ili tla, opterećuje se stijena mogućim reaktivnim tlakom pod-grade. Proračun reaktivnog tlaka slijedi iz odabranih dimenzija pod¬gradc; debljine luka, njegovog radijusa, te čvrstoće betona i eventualne armature luka. Takvo opterećenje stijene prouzrokuje stanje napre¬zanja, koje se mora kontrolirati jednim od kriterija loma stijene. Ako stijena opterećena reaktivnim tlakom i odabrane podgradc zadovolja¬va kriterij loma znači da su dimenzije podgradc adekvatne. Iterativnim postupkom mijenjanja debljine luka podgradc i njegovog geome¬trijskog oblika mogu se naći dimenzije podgradc koje osiguravaju stabilnost stijene sa zadanim kriterijem loma. Kod proračuna granice nosivosti podgradc i kriterija loma stijene primjenjuje se pravila Euro-koda 7 za geotehničke konstrukcije.The most questionable are the values of pressures between rock and support resulting from common deformations on the contact area between rock and support. Therefore the modelling and design of the tunnel support is not reliable, if it is based on active rock pressure resulting from this common deformations. The inversion of the design procedure is proposed. Instead of the active extreme pressure of the rock on support, the influence of ultimate reaction of the support on the rock has to be analysed. This procedure can be performed using the ultimate load principle, as proposed by Eurocodc 7 (Geotechnies). Normally, the rock has the tendency to increase the common conver¬gence until the support reaches its ultimate state. So, loading of profile boundary with the ultimate possible reaction of the support is very plausible. The reactive support pressures have to be probable and itself in equilibrium. The ultimate reactive load has to be reduced by Euro-code safety factor for structural elements and applied on the rock with given properties, or alternatively (as proposed by Eurocode 7) the soil or rock properties have to be diminished and calculated with full ultimate support pressures. If the rock with given (or proposed) pro¬perties and loaded with ultimate reactive pressures resulting from supposed support, satisfy its failure criterion, then is the compound system support-rock verificatcd. By this procedure, the number of relevant material properties is reduce to the primary stress ratio and the constants defining the failure criterion. The verification can be performed by any of numerical methods, but we prefer here used boundary elements method