The paper describes some experiences during the construction of the rock mass reinforcement in the paleogene flysch deposits for Adriatic motorway near city of Rijeka, Croatia. During the construction, experiences pointed at several important factors with reinforcing of flysch rock mass with rock bolts. The bond between rock mass and grout mixture plays the crucial role in rock mass reinforcement. There is the need for using grout mixtures with lower deformation modulus to achieve gradual load transfer from rock mass to grout mixture and finally to the rock bolt. The structures, which are used to transfer the load to the rock mass, should be much stiffer from relatively thin supports made of reinforced shotcrete, which are commonly used. The support structures should be installed as soon as excavation is made, to reduce the deformations until the activation of the support system and load transfer. Because of flysch rock mass heterogeneity, the supervisor engineer should be competent to determine the rock mass condition in the field of reinforcement regarding the designed assumptions. The designer in active design approach, based on the rock mass conditions and monitoring results, could intervene with adequately changes in support system.
During construction of cuts in rock mass, usual method for sustaining of stability is reinforcement of the rock mass with rock bolts (Arbanas 2002, 2003). Flysch rock mass is significantly weaker than other sediment rocks (Hoek et al., 2002) regarding strength and deformability parameters. Because of distinct heterogeneity, flysch rock mass is harder to describe with usual classification tests (Bieniawski 1989, Hoek et al. 1998, Marinos & Hoek 2000, Marinos & Hoek 2001, Marinos et al. 2005). Result of that is that the reinforcement of flysch rock mass with rock bolts is significantly more complicated. Because of relatively low strength parameters of flysch rock mass, which are lower then grout mixture strength, and especially than rock bolts material, loosening mechanism of the bolts starts by breaking of bonds between the rock mass and grout mixture. Rock mass deformation modulus is significantly lower than modulus of used grout mixtures, and activation of the bolts requires significantly higher deformation rock mass.As a result of these deformations, comes to the breaking of bonds between grout mixture and rock mass at the beginning of the bolts, because of large difference in deformation modulus of grout mixture and rock mass. Because of this transfer of the load (Windsor 1996, Windsor&Thompson 1996, 1997), this often leads to breaking of supporting system, although there is no total failure of the slope. The paper describes experiences during the construction of reinforcements in the paleogene flysch deposits during construction of Adriatic motorway near city of Rijeka, Croatia. Flysch deposits in this area mainly consist of siltstones and marl, and rarely with layers of sand. Experiences during the constructions, pointed at several important factors concerning reinforcement of flysch rock mass with rock bolts. The major role in the reinforcement with rock bolts plays the bond between flysch rock mass and grout mixture.
