**1. Introduction**

Concrete and masonry as base materials for post installed anchors are well considered by different regulations [1,2]. For concrete, in [3], the load-bearing behaviour was investigated and transferred into a design concept. The load-bearing behaviour and the design of bonded anchors were investigated by means of numerical investigations in [4]. More detailed numerical investigations of bonded anchors were carried out in [5]. Therefore, for engineers drawing conclusions about the load-bearing behaviour and capacity in advance, the base material is predefined and well known. In contrary, intact rock (e.g., rock slopes or walls) as a base material is not considered by regulations, not least because of the numerous possible variations of the rock properties. A large number of different installation parameters, such as existing joints, joint condition, uniaxial compressive strength, or weathering degree, induce uncertainty regarding the rock quality. In other words, due to the inhomogeneity of rock, these varying factors lead to insufficient information about the behaviour of post-installed anchors. Therefore, a number of experimental investigations concerning the load-bearing behaviour and capacity of post-installed anchors in rock have been performed in [6–8]. Contrafatto and Cosenza [6,7] were able to investigate the behaviour of chemical anchors in different rock types in the laboratory [6] and to test the applicability of concrete prediction models for natural stone [7]. For this purpose, anchors were installed in blocks of different rock types in the laboratory and tested until failure. Three embedment depths were selected depending on the anchor diameter. The aim was to define the minimum embedment depth at which steel failure occurs, since this can be determined by calculation. Subsequently, theoretical models for determining the load-bearing capacity in concrete were checked for their applicability to natural stone. According to Contrafatto and Cosenza [6,7], the applicability is only given if the mechanical rock characteristics are similar to those of concrete.

**Citation:** Lamplmair, S.; Zeman, O.; Voit, K. Factors Influencing the Load-Bearing Capacity of Rock as Base Material for Post-Installed Anchors. *Materials* **2021**, *14*, 5130. https://doi.org/10.3390/ma14185130

Academic Editor: Krzysztof Schabowicz

Received: 17 July 2021 Accepted: 2 September 2021 Published: 7 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Tóth et al. [8] were able to identify the compressive strength, flexural strength and porosity as the main influencing factors, although the rocks investigated in [8] appeared to be relatively homogeneous rock types. It should be noted that the investigations in [6–8] were carried out in a laboratory environment using prepared blocks containing few joints. Hence, it must be assumed that the rock properties are more homogeneous than in a nonlaboratory environment. Therefore, the investigations in [6–8] are considered to be relevant for natural stone masonry. To further consider inhomogeneous characteristics in intact rock, in [9], the authors examined different geologies in Eastern Austria in a non-laboratory environment to gain experience when considering the load-bearing capacity of anchors. From this contribution [9], the following main conclusions can be drawn:


The conclusions from [9] did not consider further investigations of rock properties in the immediate vicinity of the anchor position and this remains as an open question. Also rock compressive strength and joint condition are expected to have an influence on the load-bearing capacity.

Therefore, this article examines the following research questions for intact rock including inhomogeneities: (i) Which small scale influencing parameters (e.g., rock compressive strength, joint quantity, joint weathering) have an impact on the load-bearing capacity of post installed anchors? (ii) Is it possible to find a method to determine areas with good rock quality for post installed anchors in advance? For this purpose, experiments on post-installed anchors in rock for small embedment depths were performed. In parallel, the mechanical rock properties were investigated in detail to determine the correlation between the load-bearing capacity and the mechanical rock properties. These experiments were performed in different geologies in Eastern Austria, focusing on small-scale rock parameters, like point load index, rebound values, compressive strength, and joint properties.

#### **2. Materials and Methods**

#### *2.1. Test Program*

The conducted test campaign is listed in Table 1 and considers different testing parameters. The test program has already been used to investigate how post-installed anchors with low embedment depths behave in rock and which rock parameters have a significant influence on the load-bearing behaviour and capacity [9]. In addition to the previous evaluations in [9], this paper shows how small-scale investigations can be used for an a priori estimation of the anchor behaviour. As Table 1 indicates, the test program is divided in geological investigations for rock classification of the different geologies and small-scale investigations of the rock in the immediate vicinity of the anchor position. In total, 100 pull-out tests were performed in different geologies in Eastern Austria. Of these, 55 tests were conducted in disturbed (influence by joint given) and 45 were conducted in undisturbed (no influence by joints expected) base material. Injectable adhesive anchors as well as mechanical bolt anchors with an effective embedment depth of 70 mm and a rod

diameter M12 were used. Small-scale investigations carried out next to the anchor position are listed in the last row of Table 1. Pull-out tests were carried out using a hydraulic handpump without displacement measuring, as shown in Figure 1. In order to include the load bearing behavior of the rock, a wide support bridge was used.

**Table 1.** Overview of the conducted test program.


(1) due to rock properties not possible as no undisturbed areas have been observed; (2) two times setting failure occurred.

**Figure 1.** Test setup with hydraulic handpump, wide load bridge and scanline.

#### *2.2. Examined Geology*

Geological characterization was performed on the geologies described in Table 1 and is described in detail in Section 2.2.1.

#### 2.2.1. Engineering Assessment of Examined Geology

In order to understand the load-bearing behavior and capacity of post-installed anchors in rock the experimental campaign as listed in Table 1 was performed. For rock mass classification (rock mass quality) scanlines were carried out (see upper part of Figure 1).

Scanlines are performed by determining joint location, quantity, condition and orientation whereby the rock structure is recorded in detail. These data are subsequently used as input parameters for the rock mass rating (RMR) according to [17] and the geological strength index (GSI) according to [18]. The RMR represents a rock mass classification system, in which rock strength, joint distances and conditions, in addition to water influence are included as parameters. The GSI serves as a rock classification system based on a visual survey, in which rock structure and surface conditions are assessed. Considering this, deformability and rock strength can be estimated. Scanlines and the resulting rock mass classifications RMR and GSI allow a comparison between different rock types. Scanlines were performed also to record the joint structure (joint frequency, roughness, etc.). The uniaxial compressive strength (UCS) was estimated indirectly by a rebound hammer "Schmidt hammer" (Type N) according to [19] and the point load index according to [20]. Furthermore, the uniaxial cylindrical compressive strength was also determined on diamond drilled cores from the specific study area. In Table 2, the results from the above described testing program are listed. These are reflected in the rock mass quality, which represents the potential of the rock being used for fastenings [9]. In other words, it represents the best-case load-bearing capacity of the base material. Taking into account the findings from [6], it remains uncertain which small scale rock parameters are influencing the load-bearing capacity of post installed anchors. In order to improve the understanding of these parameters, small-scale investigations of rock parameters next to the anchor position were also performed.



In a first step, the previous the pull-out test rebound values (R) around the anchor position were measured, concluding the influence of rock strength. As stated by [21], rebound values indirectly describe the rock compressive strength. According to [22], 1.5 × hef is considered to be decisive for determining the tensile load-bearing capacity of anchors in concrete. Hence, rebound values were taken at a distance of 1.5 times the effective embedment depth (1.5 × hef). In the following, "R" is used for the measured rebound values. As mentioned above also joint weathering and quantity are considered to have an influence on the load-bearing capacity [9]. Therefore, after performing pull-out tests, joint weathering and the quantity of critical joints were determined.
