Mechanism and Internal Stability of Supportive Stone Constructions

Abstract

Natural stone constructions for the protection of slopes, banks and riverbeds are widely used in infrastructure engineering. These structures are made of stacked natural stones, which can be placed loosely on top of each other. Additionally, their bond behavior can be improved by using concrete mortar to fill the joints between the stones. Although such structures are now widely used, there is still a need for research concerning their inner stability and the structural design of such protective stone structures. In this study, experimentally, investigations were made to determine the force transmission and the interaction between rock and concrete mortar by deriving characteristic values of the adhesion strength and friction angle at different scales. A method for the determination of shear parameters from direct shear testing is used, considering the interaction between vertical and horizontal forces in the joint. In the course of these investigations, the roughness of the rock surface was recorded using conventional visual methods using the joint roughness coefficient (JRC) as well as via laser imaging. By applying laser scanning, a theoretical roughness factor could be derived. Furthermore, the properties of the rocks of the concrete mortar (fresh and hardened concrete mortar properties as well as a durability characteristic) were investigated in detail. It could be shown that different types of concrete mortar result in different bond strengths-expressed as tensile and shear strengths-when applied to a stone surface. The roughness of the stone surface has a positive influence on the tensile and shear strength between the stone and the mortar. Based on the test results, a failure description based on the Mohr-Coulomb fracture criterion could be determined, which can be used to calculate characteristic parameters for the design of stone support bodies. It was also shown that the stone's compressive strength is being exceeded through load due to very punctual contact areas. Moreover, concrete mortar differs significantly from conventional concrete in terms of its mechanical properties due to the on-site installation conditions, which allow no dynamic compaction.

Keywords: adhesion; concrete mortar; friction angle; pitched slopes; rock-to-mortar interface; rock-to-rock interface; shear tests; stone pitching; supportive stone construction.

Figure 1

Front view of piled (a) and stacked (b) stone pitching types.

Figure 2

Manufacture of a piled mortared stone support body (a) and a piled supportive stone construction (b).

Figure 3

Temperature curve for the freeze–thaw tests in accordance with the specifications provided in [15] (a) and test performance in the climate chamber ((b); model Weiss WK1000).

Figure 4

Drill core sampling from the existing supportive stone body (a) and extracted drill core for laboratory testing (b).

Figure 5

Piled stones to investigate the contact area using graphite paper (a) and prints of the contact areas, scale bar in cm (b).

Figure 6

Determination of the adhesive tensile strength (a) and adhesive shear strength (b) after failure on mortared limestone drill cores.

Figure 7

Installing stones for model tests (a), application of a constant vertical load and increasing the horizontal force until the failure of the structure (b).

Figure 8

Scanning of the stone surface to determine the roughness: test configuration with the scanner (a), stone surface and resulting scanner image (b).

Figure 9

Results of the freeze–thaw tests according to [15] with 250 cm³/m²-criterion.

Figure 10

Weathering impact of 28 freeze–thaw cycles with defrosting salt: low weathering of the concrete XC4/XW2/XD3/XF4/XA1L (a), high weathering of the concrete XC2 (b); FTW = number of freeze–thaw cycles.

Figure 11

Results of the determination of the contact area between two stones in non-mortared constructions (considering an average stone mass of 1700–2100 kg).

Figure 12

Boxplot of the adhesive tensile strengths between stone and concrete mortar for different concrete recipes.

Figure 13

Boxplot of the adhesive shear strength between stone and concrete mortar for different concrete recipes.

Figure 14

Comparing the adhesive tensile strength with the stone surface roughness JRC.

Figure 15

Comparison of the adhesive shear strength with the stone surface roughness JRC.

Figure 16

Comparison of the joint roughness coefficient JRC [16] with roughness factor f_r of two different types of limestone.

Figure 17

Determination of shear parameters and failure straight of a model stone structure.