Geotechnical Properties of Washed Mineral Waste from Grit Chambers and Its Potential Use as Soil Backfill and Road Embankment Materials

Abstract

The current practice of managing washed mineral waste from grit chambers under national legislation focuses primarily on its disposal, generating high costs for wastewater treatment plants. Other ways are being sought to enable its use, especially as a by-product in the construction industry. This paper presents the results of laboratory tests of the geotechnical, physical and mechanical parameters of washed mineral waste from grit chambers. Research samples were taken from the largest, in terms of maximum daily capacity, wastewater treatment plant “Czajka” in Poland. The washed mineral waste was characterized by organic matter content (0.36% by Tyurin’s method or 1.04% by the loss on ignition method), fraction content (sand fraction was at least 90%; it corresponds in grain size to uniform-grained medium sand), specific density of solids (2.55 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$), dry density, void ratio and porosity corresponding to the state of the loosest and densest possible composition of soil grains and particles (1.54 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, 0.656, 0.396 and 1.87 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, 0.364, 0.267, respectively), sand equivalent (93), passive capillarity (0.20 m), maximum dry density (1.78 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$), optimal moisture content (11.23%), degree of saturation after compaction (0.66) and permeability coefficient ($6.22·{10}^{-3}\mathrm{c}\mathrm{m}/\mathrm{s}$). The mechanical parameters determined included internal friction angle (35.5°) and apparent cohesion (14.27 kPa). The possibility of using washed mineral waste as soil for the backfill of installation trenches, abutments and retaining structures, as well as road embankment material, was evaluated considering current standards and legislation. It was found that the values of the determined parameters of washed mineral waste coincide with the values of the geotechnical parameters of sand, and there is a possibility of using this waste as a material in the indicated applications after fulfilling the appropriate conditions.

1. Introduction

Grit chambers are one of the places in a wastewater treatment plant where mineral waste can be obtained [1]. Grit chambers are facilities belonging to the first stage of wastewater treatment, i.e., mechanical treatment. Depending on the technologies used during mechanical treatment, mineral waste recovered from them is most often transported to washing separators, which ensure the rinsing and separation of organic parts from the mineral sand pulp. Such a solution improves the final parameters of mineral waste, minimizing or practically completely removing the odor from the waste, reducing the content of organic matter (even to a level below 2%) and the hydration of the waste. Due to the source of the generation of the washed mineral waste from grit chambers, it has a waste code, 19 12 09—“minerals (e.g., sand, stones)”, is classified as non-hazardous and inert waste [2,3], and belongs to the group of industrial waste [4]. Due to the predominant percentage of the sand fraction (Figure 1), this waste is also called sandy waste, by which it is most often compared to soil or aggregate [1].

Current legislation on managing washed mineral waste recovered from Polish wastewater treatment plants distinguishes three ways of dealing with it [1]. These include

• Disposal by processing.

• Disposal by landfilling.

• Change of waste status to a by-product.

The first two options for dealing with washed mineral waste generated at wastewater treatment plants are commonly used. However, they make costs for the wastewater treatment plants and are associated with administrative procedures for waste management. According to available information, the cost of washed mineral waste disposal by processing in 2021 amounted to PLN 700/ton and the cost of disposal by landfilling in 2019 amounted to PLN 170/ton [1]. Currently, due to price inflation in Poland, the cost of washed mineral waste disposal by processing has increased to as much as PLN 2000/ton [5]. It is also supposed to increase the cost of waste disposal by landfilling. The procedure for changing the status of waste, undertaken as part of the third option for dealing with washed mineral waste generated at wastewater treatment plants, using the concept of by-product, is described in [6]. After fulfilling the appropriate conditions, it is possible to use washed mineral waste recovered from wastewater treatment plants as a full-value product in construction solutions. Ref. [1] presents a number of practical applications of washed mineral waste from wastewater treatment plants.

Changing the status of washed mineral waste from mechanical wastewater treatment to a by-product for construction applications could become another example of implementing a circular economy in Poland [7]. Moreover, such an action proves once again that wastewater treatment plants are facilities where the assumptions of the circular economy model can be systematically implemented [8], for example, by recovering resources such as nutrients, water and energy from wastewater [9], or recovering organic compounds, nutrients and energy from sewage sludge generated in wastewater treatment processes [10], or even recycling ashes from sewage sludge combustion and using them as a by-product, such as in road embankments [11].

The possibility of the industrial management of washed mineral waste recovered from the process of the mechanical treatment of wastewater would impact the reduction in the amount of waste for disposal, and thus provide financial savings for the wastewater treatment plant, both in terms of the cost of processing and landfilling of waste, as well as the purchase of construction materials necessary for the repair and construction of water and sewage networks. Thus, this solution could contribute to saving natural sand resources, a low carbon footprint and achieving selected sustainable development goals [12,13].

The topic of sand removal from wastewater is not widespread in the literature. Moreover, most papers concern only mineral waste characterization and grit chamber performance evaluation, with a significant proportion being site-specific [14]. Hence, the main purpose of studies is usually to optimize the design guidelines of grit chambers for mechanical wastewater treatment and their technological processes [14,15,16,17,18,19]. The papers [15,19] also present an additional purpose, which is to consider the possibility of processing and using mineral waste in construction. The conducted research focuses on determining the physical properties of mineral waste in terms of dry matter content, the content of organic and inorganic substances, granulometric composition, specific density and the settling velocity of particles in grit chambers [14,15,16,17,18]. The sieving and sample pretreatment methods used to characterize wastewater sand particles are also included in [17]. The authors of papers [14,15,16,17,18,19] state that the measurements of the physical properties of mineral waste have been neglected for a long time.

The newest issue in the literature, which currently constitutes a research gap, is the presentation of the physical properties of mineral waste obtained in wastewater treatment plants from devices that additionally enable the rinsing of this waste. Washed mineral waste obtained in wastewater treatment plants is a relatively new, insufficiently recognized material [1], and its basic physical properties have been presented so far in several papers [1,5,20]. There is a lack of legislation (regulations, standards, guidelines, approvals or technical conditions) regulating its applicability in domestic construction. The assessment of waste suitability for engineering purposes should be preceded by appropriate studies confirming its applicability, depending on where it is built into the structure and the purpose of the construction object. The identification and classification of waste in geotechnical terms should be carried out in accordance with current standards [21,22]. In addition, the use of withdrawn classification standards [23,24] is recommended as a supplement.

The papers present considerations on the management of washed mineral waste generated in wastewater treatment plants [20], its characteristics as a raw material [5] and a review of the possibilities of using it in construction [1]. Grit chambers and sewer chambers are identified as the source of mineral waste. Ref. [1] shows the average test results of two samples of washed mineral waste in terms of the content of organic substances and granulometric composition. Ref. [5] presents the average test results of six samples of washed mineral waste, taking into account the dry matter content and content of inorganic substances. Ref. [20] exposes the test results of four samples of washed mineral waste with various degrees of rinsing within the scope of testing given in [5], and the moisture content of them is also determined. In the studies of [1,20], the source of the washed mineral waste is not indicated in relation to specific samples, while in [5] it is adopted that samples of waste from grit chambers were marked as WWTP and samples of waste from sewer chambers as WN.

Washed mineral waste is characterized by moisture content ranging from 0.47 to 19.98% and dry matter content from 80.02 to 99.53% [20], while the dry matter content in the waste present in [5] is in the range of 97–98%. The content of the inorganic substances in washed mineral waste is in the range of 85.56–99.22% [20], and for waste from grit chambers and sewer chambers, respectively, it is 98.2% and 99.2% [5]. Washed mineral waste is characterized by an organic substance content in the range of 1.02–1.06% [1], 1.8% and 0.8%, respectively, for waste from grit chambers and sewer chambers [5], and in the range from 0.78 to 14.44% [20]. In [20], it is noted that the rinsing process had a key impact on the quality of waste. The least rinsed waste, containing almost 20% water, had an organic substance content of 14.44%, while in the case of thorough rinsing, the almost completely dehydrated waste (moisture content of 2.64%) reaches an organic substance content of 0.78% [20].

Based on tests of the granulometric composition of washed mineral waste [1] and the standard [24], it is determined that waste corresponds to the grain size of medium sand. However, in [5,20], the study of the granulometric composition is used to determine the sand fraction of waste in order to indicate the possibility of its use. The sand fraction content (in the range of 0.063–2 mm) in waste [20] ranges from 85.7 to 94.8%, while in [5], it is 98% and 90%, respectively, for washed mineral waste from grit chambers and sewer chambers. The grain-size distribution curves of two washed mineral waste samples characterized by the lowest organic substance content (sample no. 1—1.30% and sample no. 3—0.78%) [20] and the grain-size distribution curves of washed mineral waste from grit chambers and waste from sewer chambers [5] are presented in Section 3.1.2, in Figure 2 and Figure 3, for comparison purposes.

The authors of [20] state that the best washed mineral waste meets most of the requirements of grade II gentrified sand according to the standard [25], but they did not present the indicated criteria in the paper. Based on the content of the sand fraction in washed mineral waste [5] and the standard [26], the authors of the study classify washed mineral waste as fine aggregate. Moreover, they find that this waste could be stabilized with cement.

As indicated by the authors of [1,5,20], the modernization of the wastewater treatment technological system, including the installation of a system ensuring the rinsing and separation of mineral waste, depriving it of a significant amount of organic and anthropogenic pollutants, seems to be crucial in terms of the possibility of using this waste. It allows to conclude that sand recovered in wastewater treatment plants, perceived as waste, can be an alternative for the global economy to obtain material for various applications in construction (e.g., as fine-grained aggregate in the production of concrete) and replace natural raw materials extracted for industry.

The purpose of this paper is to present the geotechnical parameters of washed mineral waste obtained from grit chambers to the extent that it is possible to indicate potential directions for its further use in earthworks. The scope of laboratory tests performed included determining the physical and mechanical properties of the washed mineral waste. The following parameters were determined:

• The content of organic substances.

• Granulometric composition (fraction content).

• Sand equivalent.

• Passive capillarity.

• Specific density of solids.

• Quantities characterizing the limiting states of compaction (dry density, void ratio and porosity corresponding to the state of the loosest and densest possible composition of soil grains).

• Maximum dry density, optimal moisture content and degree of saturation after compaction.

• Water permeability (permeability coefficient).

• Mechanical parameters describing shear strength (internal friction angle and apparent cohesion).

2. Materials and Methods

2.1. Waste Characteristics

The washed mineral waste for testing was taken from the “Czajka” Wastewater Treatment Plant in Warsaw.

Washed mineral waste from grit chambers is a product of the technological process of mechanical wastewater treatment. During this stage of treatment, mineral suspended solids, which are a mixture of sand, gravel and other mineral solids, as well as suspended solids of other origins that have not previously been retained on the screens, fall to the bottom of the tanks. Then, they are collected using mechanical equipment, transported to the washing separators and finally sent to the storage tank or temporary waste storage site [1].

Testing of the properties of washed mineral waste obtained from grit chambers using the macroscopic method was realized in accordance with the standard [23]. Samples for the test were taken from the temporary waste storage site, located in the open area. Due to waste transport from separators to the temporary storage site, samples were characterized by a disturbed structure. At the time of taking samples, the material was moist. Due to the moisture content of the samples, their state was determined to be medium dense. The color of the moist waste (Figure 1A) was black and light yellow gray, while in the air-dry state (Figure 1B), it became light brown. When the waste is air-dried, it forms unbound grains and particles, classifying it as a cohesionless material. The individual grains were visible from a distance of about 1 m and resembled mainly grains of medium sand. In addition, small amounts of gravel grains and organic substances such as wood particles or fruit stones could be seen in the waste, as well as particles of anthropogenic materials, including glass particles or small amounts of contaminated fabrics. The approximate calcium carbonate content of CaCO₃ was determined to be above 5% (class IV carbonate content according to standard [23]).

According to the European classification [21], washed mineral waste from grit chambers can be considered as anthropogenic soil (soil in which the solid phase was not formed by natural processes). Moreover, due to the technology of storing this waste at the wastewater treatment plant site, i.e., placing anthropogenic soil without engineering control (laying layers without a certain thickness and degree of compaction), it can be classified as an uncontrolled embankment. In addition, due to the type of material from which the anthropogenic soil was formed (waste has displaced or processed natural materials), the soil should be designated as mineral soil.

A similar classification of waste is presented in [27] (p. 191). Industrial waste is defined as soil-like waste, that is, a granular dispersed material for which the principles of soil mechanics and foundation are allowed to be applied and the methods recommended in soil testing according to soil standards.

2.2. Methodology

Based on the presented classification, the methodology used in soil testing to determine the physical and mechanical properties of washed mineral waste recovered from a wastewater treatment plant was adopted.

The methodology for the laboratory testing of the physical and mechanical parameters of washed mineral waste from grit chambers is shown in Table 1.

3. Results

3.1. Physical Parameters

3.1.1. Content of Organic Substances

Based on the conducted research, the average value of the organic substance content of washed mineral waste from grit chambers was determined as $I_{om}=0.36\mathrm{\%}$ (Tyurin’s method) and $I_{ż}=1.04\mathrm{\%}$ (loss on ignition method).

According to the European classification [21,22], washed mineral waste from grit chambers can be treated as mineral soil with organic substance content.

Concerning the standard [24], the average value of the organic content obtained using Tyurin’s method allows the classification of washed mineral waste obtained from grit chambers as mineral soil. According to [32] (pp. 194–195), the results obtained using the loss on ignition method are generally higher than those obtained using Tyurin’s method and only allow for an approximate determination of the organic content. It is not recommended to use the results obtained by the loss on ignition method to classify soil according to the standard [24].

3.1.2. Granulometric Composition (Fraction Content)

The grain-size distribution curves of washed mineral waste samples developed on the basis of our own research with the grain-size distribution curves presented in [5,20] according to the European [21] and Polish classifications [24] are shown, respectively, in Figure 2 and Figure 3.

The percentage content of each fraction of washed mineral waste recovered from wastewater treatment plants according to the European [21] and Polish [24] classifications is shown in Table 2. In addition, the dimensions of the effective diameters read from the grain-size distribution curves are included, and the grain-size coefficients are calculated. Information about the dry organic matter of each sample is also included.

According to the European classification [21,22], the washed mineral waste retained in grit chambers determined during our own research corresponds in grain size to coarse-grained soils. More than half of the waste particles and grains by weight correspond to the particle- and grain-size range of sand (≥0.063 mm and <2 mm), mainly medium sand (>0.20 mm and ≤0.63 mm), constituting the primary fraction. The secondary fraction of the waste is gravel, the weight of which does not exceed 10% of the particles and grains of the test samples. In the case of washed mineral waste from grit chambers, it also seems necessary to indicate the tertiary fraction, which can be highly heterogeneous due to the characteristics of the waste. The fragments of organic parts and other anthropogenic materials are accepted as tertiary fractions. According to the classification [21], washed mineral waste obtained at the wastewater treatment plant can be described as medium sand with a small amount of gravel and fragments of organic parts and other anthropogenic materials.

The size of the effective diameter $d_{90}$, below which the waste contains 90% of its mass, as read from the grain-size distribution curves (Figure 3), does not exceed 2 mm. Hence, regarding the standard [24], washed mineral waste from grit chambers corresponds in grain size to fine-grained cohesionless soil. The size of the effective diameter $d_{50}$, below which the waste contains 50% of its mass, as read from the grain-size distribution curves (Figure 3), is in the range corresponding to medium sand (0.25 mm < $d_{50}$ ≤ 0.50 mm).

Waste granulation is characterized by two coefficients calculated from the shape of the grain-size distribution curve. According to the European [22] or Polish classifications [24], these are the uniformity coefficient ($C_U$) or ($U$) and curvature coefficient ($C_C$) or ($C$), respectively.

Based on the results obtained from the calculation of the uniformity and curvature coefficients (Table 2), it can be concluded that the washed mineral waste from grit chambers, determined within the framework of our own research, is a uniform-grained material, i.e., mono-fractional (the uniformity coefficient does not exceed a value equal to 3 [22] or 5 [24], while the curvature coefficient reaches a value close to 1, according to the standard [22]).

According to the Unified Soil Classification System [33], washed mineral waste from grit chambers corresponds in grain size to poorly graded sand. More than half of the waste particles and grains by weight passed a No. 4 (4.75 mm) sieve and were retained on or above a No. 200 (0.075 mm) sieve. Moreover, this waste contains less than 5% fines, and its uniformity coefficient is less than 6, while the curvature coefficient is less or slightly greater than 1.

In addition, the content of fine particles and grains of ≤0.075 mm and ≤0.02 mm in the tested washed mineral waste from grit chambers is trace and does not exceed 0.39% and 0.14% of its weight, respectively (Figure 2 and Figure 3).

3.1.3. Sand Equivalent

Based on the completed tests, the average value of the sand equivalent for washed mineral waste from grit chambers was determined to be ${SE}_4=93$.

3.1.4. Passive Capillarity

The average value of passive capillarity for washed mineral waste obtained in grit chambers, determined on the basis of our own research, was $H_{kb}=0.20 \mathrm{m}$.

3.1.5. Specific Density of Solids

Based on the realized research, the average value of the specific density of solids for washed mineral waste recovered from grit chambers was ${\rho }_s=2.55 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$.

3.1.6. Quantities Characterizing the Limiting States of Compaction

Based on the completed tests of washed mineral waste from grit chambers, the following values were obtained:

• Minimum dry density: ${\rho }_{dmin}=1.54 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, which corresponded to the void ratio: $e_{max}=0.656$ and porosity: $n_{max}=0.396$.

• Maximum dry density: ${\rho }_{dmax}=1.87 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, which corresponded to the void ratio: $e_{min}=0.364$ and porosity: $n_{min}=0.267$.

3.1.7. Maximum Dry Density and Optimal Moisture Content

The compaction curve of the tested waste and the theoretical curve of maximum compaction (at the degree of saturation: $S_r=1.0$) are shown in Figure 4.

The compaction curve (Figure 4) shows that the highest compaction of washed mineral waste from grit chambers, ${\rho }_{ds}=1.78 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, was obtained at the optimal moisture content $w_{opt}=11.23\mathrm{\%}$. The washed mineral waste from grit chambers at maximum compaction achieved a degree of saturation of $S_r=0.66$.

3.1.8. Permeability Coefficient

Tests on the permeability coefficient of washed mineral waste from grit chambers were conducted at a constant hydraulic gradient of i = 0.60 and i = 0.64. These values are within the recommended range of the hydraulic gradient of i = 0.3 ÷ 0.8 for this study.

Based on the research, the average value of the permeability coefficient for washed mineral waste obtained in grit chambers was determined as

$k_{10}=6.22·{10}^{-3} \mathrm{c}\mathrm{m}/\mathrm{s}=5.37 \mathrm{m}/\mathrm{d}.$

3.2. Mechanical Parameters—Shear Strength

Many literature sources provide information that the cohesion of cohesionless soil is zero. However, the presence of capillary water in the pores of compacted cohesionless soil can cause an increase in the soil shear strength by forming additional soil cohesion (the so-called apparent or capillary cohesion) [34]. Apparent cohesion $c_p$ can reach values approaching 16 kPa depending on the soil type, its degree of compaction and the degree of saturation [35].

For cohesionless soil, the shear strength under capillary forces can be expressed by Equation (1) [34]:

(1)$\tau =\tan \phi · \sigma +{\mathrm{c}}_{\mathrm{p}}=\tan \phi · \sigma +\tan \phi · {\sigma }_{\mathrm{k}}=\tan \phi · (\sigma +{\sigma }_{\mathrm{k}}) \mathrm{[kPa]}$

where

$c_p$—apparent cohesion [kPa],

$\sigma$—normal stress [kPa],

$\phi$—internal friction angle [$°]$,

${\sigma }_k$—capillary pressure [kPa].

Based on the research, the equation of the approximating straight line representing the shear strength of washed mineral waste obtained in grit chambers using the direct shear method (Figure 5) was determined as follows:

(2)$\tau =0.7131 · \sigma +14.27 \mathrm{[kPa]}$

Based on Equation (2), the average value of the internal friction angle $\phi$ and the apparent cohesion $c_p$ of the washed mineral waste from grit chambers were determined and are shown in Figure 5. It reached $\phi =35.5°$ and $c_p=14.27 \mathrm{k}\mathrm{P}\mathrm{a}$.

The obtained laboratory test results are discussed in Section 4.2.

In further research work, the results obtained by the direct shear method will be compared with the results by the triaxial shear method.

4. Discussion

4.1. Physical Parameters

4.1.1. Granulometric Composition (Fraction Content)

In terms of grain size, the washed mineral waste from this study has a similar granulometric composition to the mineral waste presented in [5,20]:

• All the mineral waste samples shown in Figure 2 can be classified as medium sands with small amounts of gravel according to the standard [21]. Similarly, in Figure 3, they are classified as medium sands according to the standard [24];

• All the mineral waste samples shown in Table 2 are characterized by a low percentage of the silt and clay fraction, not exceeding a maximum of 0.20% of their weight, indicating a limited effect of these particles on waste properties;

• Based on the uniformity and curvature coefficient results, all the mineral waste samples shown in Table 2 can be considered as uniform-grained (mono-fractional) materials.

4.1.2. Specific Density of Solids

The specific density of solids ${\rho }_s$ of the washed mineral waste is lower than that of mineral soil of a similar grain size, which is ${\rho }_s=2.65 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$ [11]. According to [36] (p. 173), the lower value of ${\rho }_s$ is supposed to be caused by the content of relatively lighter orthoclase or admixtures of organic parts in the waste. The determination of organic matter in the tested washed mineral waste confirmed its presence, which could affect the specific density of solids. As indicated in [18], the specific density of mineral waste obtained in grit chambers is reduced due to the formation of organic material layers covering mineral particles and, as a result, may vary from 1.10 to 2.65 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$.

The obtained value of the specific density of solids ${\rho }_s$ is within the range of the specific density values for organic sands and silts (${\rho }_s=2.30÷2.64 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$) and organic muds (${\rho }_s=2.15÷2.60 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$) [32] (p. 62).

4.1.3. Quantities Characterizing the Limiting States of Compaction

The range of values of sand dry density, with a specific density ${\rho }_s=2.65 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$ [32] (p. 62) and a range of void ratios $e=0.3-1.0$ [37] (p. 75), is 1.33–2.04 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$.

The range of values of the dry density of uniform-grained sands, with a specific density ${\rho }_s=2.65 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$ [32] (p. 62) and a range of porosity $n=0.258-0.476$ [37] (p. 75), is 1.39–1.97 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$.

The washed mineral waste from grit chambers corresponds to uniform-grained sand, in terms of the obtained results of dry density, void ratio and porosity.

4.1.4. Maximum Dry Density, Optimal Moisture Content and Degree of Saturation after Compaction

Considering the research of fine-grained cohesionless soil presented in [27] (p. 77), the optimal moisture content $w_{opt}$of sands can reach values from 8.0 to 13.5% with a maximum dry density ${\rho }_{ds}$ from 1.65 to 2.10 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, and the degree of saturation $S_r$ corresponding to the maximum compaction ${\rho }_{ds}$ at optimal moisture content $w_{opt}$ is in the range of 0.4 to 0.7.

The results of the research on the washed mineral waste obtained from grit chambers belong to the given ranges for sands.

4.1.5. Permeability Coefficient

According to [36] (p. 254), the range of approximate values of the permeability coefficient for medium sands is between 5 and 30 $\mathrm{m}/\mathrm{d}$, and the result of research on the washed mineral waste obtained from grit chambers belongs to this range.

4.2. Mechanical Parameters—Shear Strength

The obtained average value of the internal friction angle $\phi$ is within the range of the characteristic values of the internal friction angles of fine and silty sands in the dense and very dense state (33–36°) and for coarse and medium sands in the medium-dense state (34–37°) [37] (p. 192).

Considering the degree of saturation of maximally compacted washed mineral waste from grit chambers ($S_r=0.66$), the resulting cohesion value ($c_p=14.27 \mathrm{k}\mathrm{P}\mathrm{a}$) is within the range of the apparent cohesion values for dense and very dense fine sands [35].

4.3. Material Requirements

The analysis of the research results of washed mineral waste obtained from grit chambers regarding its potential for use in earthworks was based on standards [38,39]. The standard [38] specifies the requirements and tests for road earthworks. It applies to the design, execution and acceptance of earthworks related to the construction, reconstruction and maintenance of roads, streets, squares, parking lots and airports. Special attention is given to the requirements for embankments and the backfills of engineering structures. In turn, the standard [39] specifies the technical conditions for constructing open trenches for water supply and sewerage pipes in areas unaffected by mining damage, including requirements for backfilling trenches.

None of the standards [38,39] describe the direct use of washed mineral waste from grit chambers in the facilities indicated in the standards. The standard [38] allows for the use of anthropogenic soil, such as municipal waste, in the construction of road earthworks. However, no specific provisions relate to the values of the geotechnical parameters for structures made from this waste. Therefore, in the following part of this paper, an analysis of the obtained results of the geotechnical parameters of washed mineral waste from grit chambers was performed in relation to the values of the geotechnical parameters of soil required for individual objects described by both standards [38,39]. Table 3, Table 4 and Table 5 present, respectively, the requirements for soil for trenches and backfills based on the standards [38,39], the criteria for classifying soil as frost-unsusceptible according to the standard [38] and the requirements for soil for road embankments based on the standard [38]. Table 3 and Table 5 comment on meeting the use criterion by washed mineral waste obtained from grit chambers regarding the indicated use.

An analysis of the requirements for soil for trenches, backfills and road embankments, developed on the basis of standards [38,39] and presented in Table 3, Table 4 and Table 5 shows that

• The requirements for the use of soil are mainly limited to the physical properties (mineral composition, fraction content, sand equivalent, frost susceptibility, permeability coefficient and maximum dry density of the soil), while special attention is paid to the value of the compaction degree and the technical conditions for achieving the required compaction and thus the correct execution of the objects.

• The tested washed mineral waste from grit chambers meets the basic and both additional criteria for classifying it as a frost-unsusceptible soil.

• The tested washed mineral waste from grit chambers only did not meet one requirement of soil intended for the backfilling of abutments, retaining structures and road embankments (the requirement for the uniformity coefficient). However, the standard [38] assumes that it is possible to use conditionally embankment soil with a lower uniformity coefficient, provided that preliminary field compaction tests demonstrate the ability to achieve the required compaction. Taking into account the requirements for the uniformity coefficients for road embankments, backfills for abutments and retaining structures, the tested washed mineral waste from grit chambers can be classified as an unsuitable material for their construction, at least until the required compaction is verified during preliminary field compaction tests or its granulometric composition is modified with a material with a grain size that ensures an increase in uniformity coefficients to the demanded values.

5. Conclusions

Based on the research and analysis, the following conclusions were made:

• Washed mineral waste, with waste code 19 12 09, can be classified as a medium sand with small amounts of gravel and fragments of organic parts and other anthropogenic materials, as uniform-grained and frost-unsusceptible soil.

• Based on laboratory tests carried out on washed mineral waste recovered from grit chambers, it can be concluded that the values of the determined parameters coincide with the values of the geotechnical parameters for sands.

• It is possible to use washed mineral waste, with waste code 19 12 09, as a material for

  • Filling (backfilling) of trenches to frost depth.

  • Protective layers of trench backfills after separating grains larger than 20 mm.

  • Backfilling of abutments and retaining structures after modification of waste granulometric composition with a material that ensures an increase in the uniformity of grain composition to the demanded value of coefficients while maintaining the other parameters.

  • Construction of road embankments after verification of the possibility of obtaining the required compaction in the field.

• Due to the absence of legislation (regulations, standards, guidelines, approvals or technical conditions) regulating the applicability of washed mineral waste with waste code 19 12 09 in the construction industry, further tests should be carried out and expanded to include field tests and chemical and microbiological tests to confirm the suitability of washed mineral waste for engineering purposes.

Author Contributions

Conceptualization, J.K., P.P. and A.D.; methodology, J.K.; validation, J.K., A.D. and P.P.; formal analysis, J.K., A.D. and P.P.; investigation, J.K.; resources, P.P., A.D. and J.K.; writing—original draft preparation, J.K., A.D. and P.P.; writing—review and editing, A.D. and P.P.; visualization, J.K.; supervision, P.P. and A.D.; project administration, J.K. and P.P.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Washed mineral waste from grit chambers, which is a mixture of mineral, organic and anthropogenic parts: (A) taken from a temporary waste storage site at the wastewater treatment plant; (B) in the air-dry state [photo: J. Kostrzewa].

Figure 2. Grain-size distribution curves of washed mineral waste samples from our own research, 1, 3 from [20] and WN, WWTP from [5] according to the European classification [21].

Figure 3. Grain-size distribution curves of washed mineral waste samples from our own research, 1, 3 from [20] and WN, WWTP from [5] according to the Polish classification [24].

Figure 4. Compaction curve of washed mineral waste from grit chambers and theoretical curve of maximum compaction (at the degree of saturation: [Forumla omitted. See PDF.]).

Figure 5. Determination of the internal friction angle [Forumla omitted. See PDF.] and apparent cohesion [Forumla omitted. See PDF.] of washed mineral waste from grit chambers using the direct shear method.

Table 1

Methodology for the laboratory testing of the physical and mechanical parameters of washed mineral waste from grit chambers.

No.	Study	Research Methodology
Physical Parameters
1.	Content of organic substances	Standards [23,28]: Tyurin’s method and loss on ignition method
2.	Granulometric composition (fraction content)	Standard [23]: sieve method
3.	Sand equivalent	Standard [29]
4.	Passive capillarity	Standard [30]
5.	Specific density of solids	Guidelines described in [27] (pp. 22–23), a modified test method presented in the standard [23]
6.	Quantities characterizing the limiting states of compaction (maximum and minimum dry density)	Standard [23]
7.	Maximum dry density and optimal moisture content	Standard [23]: I normal Proctor method
8.	Permeability coefficient (1)	Standard [31]
Mechanical Parameters
9.	Shear strength (internal friction angle and apparent cohesion) (1)	Guidelines described in [27] (pp. 41–43), a modified test method presented in the standard [23]: direct shear method

⁽¹⁾ The test was carried out on samples compacted to the parameters obtained during the compactability test by the I normal Proctor method (No. 7).

Table 2

Percentage content of the fractions constituting washed mineral waste recovered from wastewater treatment plants [21,24].

No.	Sample no.	Source	Standard	Fraction Content [%]			Effective Diameters [mm]					Grain-Size Coefficients		Dry Organic Matter [%]
				Gravel	Sand	Silt + Clay	D90 [21], d90 [24]	D50 [21], d50 [24]	D60 [21], d60 [24]	D10 [21], d10 [24]	D30 [21], d30 [24]	CU [21], U [24]	CC [21], C [24]	Iom/Iż
1.	W4	Own research	[21]	5.69	94.17	0.14	1.10	0.41	0.48	0.22	0.31	2.18	0.91	0.36/1.02
			[24]	5.69	94.21	0.10
2.	W1.4		[21]	9.42	90.44	0.14	1.70	0.40	0.44	0.20	0.30	2.20	1.02	0.36/1.06
			[24]	9.42	90.48	0.10
3.	1 (1)	[20]	[21]	5.20	94.60	0.20	1.00	0.37	0.41	0.19	0.30	2.16	1.16	1.30
			[24]	5.20	94.66	0.14
4.	3 (1)		[21]	8.60	91.30	0.10	1.70	0.41	0.47	0.21	0.31	2.24	0.97	0.78
			[24]	8.60	91.35	0.05
5.	WN (2)	[5]	[21]	10.00	90.00	0.00	2.00	0.40	0.50	0.23	0.32	2.17	0.89	0.80
			[24]	10.00	90.00	0.00
6.	WWTP (2)		[21]	2.00	98.00	0.00	0.57	0.30	0.35	0.19	0.26	1.84	1.02	1.80
			[24]	2.00	98.00	0.00

⁽¹⁾ The authors of the paper did not specify the source of the washed mineral waste: whether the samples indicated are mineral waste retained in grit chambers (waste code: 19 08 02) or sludge from cleaning sewage collectors (waste code: 20 03 06—waste from sewer chambers). ⁽²⁾ The authors determined that the WN sample refers to mineral waste from sewer chambers, while the WWTP sample refers to mineral waste retained in grit chambers.

Table 3

Soil requirements for trenches and backfills based on [38,39] concerning the tested washed mineral waste obtained from grit chambers.

Purpose	Requirement	Meeting the Requirement (YES/NO)	Comments on the Requirement
Requirements for soil for trenches to frost depth [38]	Frost-unsusceptible soil (1)	YES
Requirements for soil for backfilling of installation trenches up to 30 cm above the top of the conduit or its casing [38]	Sandy soil or till with grains not larger than 20 mm (2)	YES (3)	Backfilling of narrow-space cross-cuts through roadways, regardless of the road traffic category, should obtain to a depth of 1.2 m a degree of compaction of at least 1. At a greater depth, a compaction degree of 0.97 is allowed, provided that measures are taken to mitigate the effects of settlement (e.g., the use of well-compacted aggregates, embedding of geotextile reinforcement, mechanical or binder improvement) [38].Caution should be taken not to cause displacement of the cable. Backfill to a height of 1 m above the cable casing should be compacted only with light equipment [38].
Requirements for soil for protective layers of trench backfill [39]	Non-rocky soil, without clod and stones, cohesionless, fine or medium grain according to [24] (4)	YES	The material used and the method of backfilling the conduit should not cause damage to the laid conduit, objects on the conduit, and waterproof, moisture-proof and thermal insulation. The thickness of the protective layer of the hazardous-zone backfill above the top of the conduit or pipe should be at least 0.5 m [39].Up to the ground level or the required ordinate, the backfilling of the conduit should be carried out while maintaining soil compaction according to the design. If the compaction degree is not specified, it should be at least 1 [39].If earthworks are carried out on an existing road with an improved pavement and it is difficult to achieve a soil compaction degree of at least 1, the top layer of backfill should be replaced with a reinforced road substructure [39].
General requirements for soil for backfill of abutments and retaining structures [38]	Gravels, tills and sands of at least medium grain with a uniformity coefficient of not less than 5 ($C_U\ge 5$)	NO	It is permissible to make light backfill, for example, from ash mixtures or useful artificial aggregates, provided that it is protected from becoming wet and from contact with ground water [38].The required compaction degree of the backfill is $I_S\ge 1.0$, except for the slopes of the cones at the wings and the frontal slopes of the openwork abutments and embedded in the embankment, where the compaction degree should be no less than 0.95. The soil backfill should be laid evenly and compacted in layers with a thickness to achieve the required compaction degree [38].
	Permeability coefficient$k_{10}\ge {10}^{-5} \mathrm{m}/\mathrm{s}$ (5)	YES
Requirements for soil for the upper layers of the backfill of abutments and retaining structures (6) [38]	Cohesionless soil	YES
	Frost-unsusceptible soil	YES
	The uniformity coefficient should be at least 5 ($C_U\ge 5$)	NO
	Permeability coefficient $k_{10}\ge 6·{10}^{-5} \mathrm{m}/\mathrm{s}$	YES

⁽¹⁾ The criteria for classifying soil as frost-unsusceptible according to the standard [38] are shown in Table 4. ⁽²⁾ Slag, stony soil or other materials that could damage the conduit should not be used for backfilling. ⁽³⁾ The tested washed mineral waste obtained from grit chambers after sifting it on a 20 mm sieve meets the requirement. ⁽⁴⁾ The backfill material within the hazardous zone should be compacted with a rammer on both sides of the pipe or hydraulically in the case of backfilling with cohesionless material [39]. ⁽⁵⁾ Culverts backfilled with soil with such a permeability coefficient should be protected against piping by constructing sealing screens placed, for example, behind the wings of the culvert inlet. ⁽⁶⁾ In case of building into the top backfill layer (at least 0.5 m thick). In the absence of such soil, the top layer should be improved with a binder (cement, lime or active ashes). Difficult-to-reach areas of the backfilled space may be filled with soil stabilized with cement. It is unacceptable to fill them with liquefied cohesionless soil.

Table 4

Criteria for classifying soil as frost-unsusceptible based on [38].

Criterion	Requirement		Meeting the Requirements by Washed Mineral Waste Retained from Grit Chambers (YES/NO)
Basic	Particle content ≤ 0.075 mm	<15%	YES
	Particle content ≤ 0.02 mm	<3%	YES
Additional no. 1	Sand equivalent	>35	YES
Additional no. 2	Passive capillarity	<1.0 m	YES

Table 5

Soil requirements for road embankments ⁽¹⁾ based on [38] concerning the tested washed mineral waste obtained from grit chambers.

Purpose	Requirement	Meeting the Requirement (YES/NO or N/A)	Comments on the Requirement
General requirements for soil for embankments	The uniformity coefficient of the soil should be at least 3 ($C_{U }\ge 3$)	NO	Soil with a lower uniformity coefficient can be used conditionally if preliminary tests on the compaction field demonstrate the ability to achieve the required compaction.
	The maximum dry density of the soil should be at least 1.6 g/cm3 (${\rho }_{ds}\ge 1.6 \mathrm{g}/{\mathrm{c}\mathrm{m}}^3$)	YES	Except for slag and ash, soil with a maximum dry density less than 1.6 g/cm3 is considered to be difficult to compact and not recommended for use.
	Clay and other cohesive soil with a liquid limit of more than 60% ($w_L>60$%)	N/A	It is assumed that such soil types are unsuitable for embankment construction.
	Organic soil (with organic content: $I_{OM}>2\mathrm{\%}$), except for organic sands with $I_{OM}\le 5\mathrm{\%}$	N/A
Requirements for soil for the upper layers of embankments in the frost depth	Cohesive soil with a liquid limit of more than 35% ($w_L>35$%)	N/A	It is assumed that such soil is unsuitable for constructing the upper layers of embankments.
	The California Bearing Ratio (CBR) when using fine sands should be at least 10 ($CBR \ge 10$)	N/A	When embedded in a zone up to 0.5 m below the ground level of the earthworks.
	Cohesionless soil	YES	When embedded in the upper layer of the embankment (at least 0.5 m thick).In the absence of such soil, the top layer should be improved with a binder (cement, lime or active ashes).
	Frost-unsusceptible soil	YES
	The uniformity coefficient should be at least 5 ($C_U\ge 5$)	NO
	Permeability coefficient$k_{10}\ge 6·{10}^{-5} \mathrm{m}/\mathrm{s}$	YES
Requirements for filling soil in embankments made of stony soils or coarse industrial waste	Cohesionless soil	YES
	Grain size up to 5 mm	YES (2)
	The sand equivalent should be at least 40 (${SE}_4\ge 40$)	YES

⁽¹⁾ The soil should be compacted immediately after embedding. The required moisture content of the compacted material, the compaction procedure and the thickness of the layers should be determined experimentally during testing compaction with the equipment used. The soil layers should be compacted in strips from the edges towards the axis of the embankment. The next layer of soil can be laid after it is determined that the required parameters of the already-laid layer have been achieved. For soil not improved with binders, in embankments and trenches, the required compaction degree $I_S$ should be adopted according to the standard [38], depending on the expected traffic category and the layers’ level. ⁽²⁾ The tested washed mineral waste obtained from grit chambers after sifting it on a 5 mm sieve meets the requirement.

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