Distinct-Element Modeling of the Use of Reinforced Shotcrete Facings for Soil–Nail Retrofitting of Masonry Retaining Walls

Abstract

One method for retrofitting existing masonry retaining walls is the use of tieback systems such as soil nailing. Ordinarily, reinforced shotcrete is used as a facing for soil-nail walls. However, the use of shotcrete for the soil-nail retrofitting of masonry retaining walls is unknown and there is no method of calculation for the requirement of the use of a shotcrete facing in front of masonry block systems. The present study evaluated the use of a shotcrete facings for soil-nail retrofitting of existing masonry retaining walls. A series of distinct element method (DEM) parametric studies were done using the universal distinct element method element code (UDEC). The effect of the wall thickness and height, block height, nail span and cohesion between blocks on the requirement for the use of a shotcrete facing were evaluated. The results showed that, for a short, retrofitted wall (<7 m) having a nail span of less than 2.5 m, a shotcrete facing is not required. For short nail spans (<1 m), the blocks act as a permanent facing that prevents arching stress between the nails meaning that the use of a shotcrete facing will not be required, even for high walls. For a nail span of greater than 2.5 m, the existing blocks cannot act as a facing; thus, the use of shotcrete becomes necessary. For intermediate nail spans of 1 to 2.5 m, the need for a shotcrete facing along the entire height of wall or in the lower part should be determined based on the wall geometry. Tables have been provided to assist in this. Although the present study was based on common ranges for parameters in practice, the mechanisms emphasize that a shotcrete facing could be neglected depending the characteristics of the soil-nail retrofitted masonry retaining wall. The observed mechanisms show that a shotcrete facing could be neglected for retrofitting existing masonry retaining walls by soil nailing, depending on the characteristics of the soil-nail retrofitted masonry retaining wall. When a short wall (H<7 m) is retrofitted with soil nails spaced less than 2.5 m apart, no shotcrete facing is necessary. For a nail span of 1 m, the blocks acts as a permanent facing and no shotcrete facing is required for the cohesion of ordinary mortar. However, if the mortar cohesion is very low (less than 100 kPa), the use of a shotcrete facing is recommended. For wide nail spans (more than 2.5 m), the blocks are unable to act as a facing and a shotcrete facing will be necessary. However, if large block sizes are used (more than 0.75 m), no shotcrete facing is required. For nail spans of 2 m, a shotcrete facing is required only for tall walls (H=16 m) or when the mortar has low cohesion (less than 100 kPa). In all other cases, the blocks should be sufficient for use as a permanent facing.