## 3 Materials and Methods
### Permeability Test  
There exist a variety of methods of permeability measurements using field methods that utilize BAT piezometers [38] and the falling head method with a flow pump [39; 40; 41]. However, a constant head method device was used in this study for estimating the basic characteristics of RCA because of its simplicity and invariable conditions of test. Moreover, the constant head method is one of the most reliable techniques of permeability measurements. In this paper for permeability testing, laboratory equipment was used. A constant head device (called in this study as permeameter) is shown in Figure 2. Permeameter construction consists of inner and outer cylinders made form stainless steel (inner cylinder dimensions: height h = 0.17 m, diameter d = 0.205 m, outer cylinder dimensions: h = 0.27 m, d = 0.19 m), which are connected by a permeability mold also made from stainless steel, where the sample is placed. The permeability mold has a perforated bottom. After placing a sample, a perforated cover is installed on top. The permeability mold is fixed to the inner cylinder by four screws and an o-ring to make sure that no unexpected seepage of water from outer cylinder occurs. The principle on which this device operates relies on communicating vessels, which allow the water to flow from the outer cylinder to the inner cylinder through the soil sample. The hydraulic gradient is simply set by the difference between the outer and inner water table heights. In practice, the inner water table is fixed, and the hydraulic gradient is inflicted by a changeable outer water table height. Tests were conducted when both the inner and outer water tables were in the fixed position. Measurements of the outflow water in time were repeated five times for each test point. Tests were performed on three blends of grain, whose size was 0-8, 0-16, and 0.05-16 mm. The sample presented in Figure 1 were compacted in a permeability mold using [PERSON]'s method. The energy of compaction was 0.59 J/cm3, and the mass of the compaction hammer was 2.5 kg. Compaction was conducted in three layers, where 16 strokes were performed for each layer. The sample volume was 6.34 \(\times\) 10\({}^{-4}\) m\({}^{3}\), with a diameter of 0.116 m and a height of 0.06 m. [PERSON]'s method of compaction was used for the proper simulation of conditions that are present in the compacted layers of a road or embankment. Nevertheless, before compaction in a permeameter mold, a proper Proctor's study has been conducted. The RCA's maximal density and optimal moisture content were estimated as 1960 kg/m\({}^{3}\) and 8.0%, respectively. The procedure of the compaction and testing of the sample was upgraded several times before the main study in order to eliminate common mistakes, as reported by [PERSON] [21]. Figure 2: Permeameter scheme. (1) Compaction was performed with respect to the Proctor method in order to avoid crushing the grains, and this operation was performed in the permeability mold because of the risk of creating voids around the sample and the rigid wall. Compaction of samples was performed in the optimal moisture content, which was 8% [41]. This action was vital in order to simulate the performance of the subbase soil layer or compaction process in earnthen construction. (2) It is obvious that the permeability tests' saturation degree, \(S_{r}\), has to be equal to 1. To solve this issue, after its installation on the permeameter, the sample was filled by aerated water at a constant rate of 0.25 mm/h of the water table, which, for a 6 cm sample, takes 24 h.  
(3) Construction of the permeameter [41] made it possible to avoid hydraulic head loss because of the constant inner and outer water tables. The outflow was measured exactly next to the outflow pipe, which, during the tests, was never fully filled. (4) Moreover, the movement of fine particles was controlled by checking the inside of the cylinder after the tests. Weight installed on the top of the perforated cover prevented the soil skeleton from movement due to the seepage of water.