Developing a Novel Superstructure System for the Ballasted Railways Using RRP_235special Stabilized Clayey Soil

Abstract

The ballasted tracks have been considered due to their benefits such as low construction cost, simple technology, good performance in vibration, and proper drainage. Despite the advantages, low bearing capacity, time-consuming maintenance, low vertical stiffness, vertical and horizontal displacement, and ballast problems are the reasons for the scientist’s effort to enhance the track using different methods. The problems increase in the presence of clay. The Royal Road Product 235 Special (RRP235Special) have used in road construction since 1960s. In this regard, all the layers underneath the sleeper have been replaced with the clayey subgrade stabilized with RRP235Special. A series of static and dynamic lab tests with different additive dosages were carried out to investigate the stabilized soil properties, and an optimal percentage was found. The sample with 0.15 lit/m3 RRP235Special was determined as the suitable dosage for lab experiments. According to the lab experiment results, a track was constructed in Urmia railway station, and field tests such as single tie push test (STPT) and plate loading test (PLT) were carried out. The conventional system is compared to the proposed superstructure. As a result, STPT has no significant difference, and PLT was increased due to the use of RRP235special.

1. Introduction

The traditional ballasted tracks have been used widely in railway transportation infrastructure. These tracks have been investigated in two views in terms of civil engineering, layers underneath the ballast layer (subgrade) and the ballast layer (pavement). Facing clayey soils in the subgrade of railway tracks reduces the bearing capacity, increases water absorption, and as a result, creates horizontal and vertical deformations that are transmitted through the ballast layer to the railway pavement. Construction of ballasted tracks on the clayey soft subgrade causes high settlement and low bearing capacity. In addition, significant maintenance costs and time-consuming operations have been encountered due to the presence of clay. Despite the ballast layer advantages in low construction cost and time, proper drainage, simple technology, and proper damping, it has some disadvantages such as vertical and horizontal displacement, low vertical stiffness, need for time-consuming and expensive maintenance operation, low lateral resistance, slippage, pumping, dirty ballast, ballast breakage and flying ballast in high-speed railways. In this regard, many researches have been done to manage these drawbacks.

It will be a caught-in-crossfire situation when structures fail due to the presence of clayey soils and the need for their microstructural, mechanical, and strengthening properties to be improved before construction.

A huge amount of suitable material and enormous costs is needed in transportation projects. Thus different stabilizing methods have been considered by scientists [1].

Clayey soft soil can damage the transportation infrastructure due to its disadvantages such as low strength and huge volumetric changes that generate expensive maintenance costs [2, 3].

The subgrade of railway track is an important part that cans cause increased maintenance costs [4].

Facing soft subgrade causes a challenge in railway track design. The long-term behavior of soft subgrade under repeated load is important in track design [5].

Extensive costs and structural damages generate in clayey soil due to volumetric change arising from wet-dry conditions [6].

Cement and lime-stabilized subgrade soils are considered environmentally unfriendly. So, as a solution Sodium Alginate Biopolymer have used in pavement construction. The results show an increase in resilient modulus, stiffness and strength depending on the material type and concentration and curing time [7].

Lime-Microsilica has been used as a silty soil stabilizer in the railway subgrade to improve California Bearing Ratio (CBR). The results show an increase due to the use of additives [8].

The railway subgrade has been stabilized with Fly-ash. Results show a significant increase in shear strength, CBR, and cohesiveness [9].

Different methods such as preparing suitable material, using mechanical techniques, and using additives are common in subgrade enhancement. Application of chemical additives has been proposed using reliable research in soft subgrade, some of which are listed below:

Advanced techniques enhanced subgrade bearing capacity instead of the lime treatment method [10].

Application of cement and lime lonely had disadvantages such as insufficient specified properties and environmental impacts. So the combination of them in suitable dosage had an important influence. The optimum proportion was investigated using a compressive strength test, and results show the highest amount [11].

Due to rolling stock movement, forces are applied to the track and cause movement in horizontal and vertical planes. Horizontal forces have resisted by longitudinal and lateral resistance of the track. Using the different materials and procedures, resistance against forces has increased. Longitudinal resistance has improved using continuous welded rails (CWR) technology. However, it is impossible to join the rails in tight curves due to higher lateral forces. The material, size, geometry, and dimension of track components affect lateral resistance. Researchers have carried out several studies to enhance lateral resistance. In this regard, changing sleeper shape is a common method. Lateral resistance has influenced by various factors such as environment conditions, applied loads, track components, track geometry, and maintenance procedure. The lateral resistance between ballast and sleeper usually is conducted using the tests with single or multiple sleepers in full-scale or scaled model in laboratory or in situ [12]. The ballast layer geometry and interaction between the ballast and the sleeper is the main factor of lateral resistance [13]. With the introduction of CWR, buckling may occur due to thermal expansion. So, lateral resistance is an important factor in track stability. In this regard, a series of laboratory tests were carried out using STPT and track panel pullout test (TPPT) on different types of concrete sleepers. Results revealed the importance of the shape, the spacing, and the number of sleepers [14]. Determination of participation of each part on total lateral resistance has important to choose a sleeper and designing the components in railway. Lateral resistance of the sleeper achieves from sum of base, crib, and shoulder area. According to the experimental laboratory research on STPT test and corresponding to the material of sleeper, base area resistance of concrete, steel, and wood are 62%, 56%, and 51%, crib area are 28%, 27%, and 18% and shoulder area are 9%, 22%, and 26%, respectively [15].

The sleeper’s shape significantly affects ballasted track’s lateral resistance [16]. Changing the shape and material of the sleeper is recommended by scientists. In this regard, the frictional sleeper is an effective solution. The measurement has been conducted on conventional and frictional B70 sleepers by panel displacement method. Results show an increase in lateral resistance [17]. Based on the experimental test, the lateral resistance of three different frictional sleepers has been evaluated using the STPT test. Results indicated increases in lateral resistance due to frictional sleeper-enhanced interaction between ballast particles and sleeper [18]. A numerical model in finite element software has developed and investigated the effect of shoulder extend, base friction, and ballast layer thickness. Compared with the conventional sleeper, the frictional sleeper led to increased lateral resistance. A decrease in the ballast layer causes an increase in lateral resistance. Increasing in ballast shoulder, results increase in lateral resistance [19]. Y-shape steel sleeper in ballasted track has been investigated based on experimental methods using STPT and lateral track panel test (LTPT). The longitudinal resistance force (LRF) in STPT and LTPT [20]. The lateral resistance of HA110, winged and middle-winged sleeper achieved from STPT test, compared to the conventional B70, increased [21].

Track maintenance is one of the factors that influence lateral resistance. The lateral resistance was reduced significantly due to surfacing and increased by mechanical stabilization following the surfacing [22, 23]. The influence of ballast material type on the interaction between ballast and sleeper has been investigated. So the ballast consisting of crushed and angular particles has more lateral resistance than rounded and crushed angular types [24]. The lateral resistance of polyurethane-mixed ballasted track has been investigated using a discreet element model. Results show increases in lateral resistance due to an increase in the bonding depth of the shoulder ballast [25]. Lateral support is a new method to enhance lateral resistance in ballasted tracks. Field investigation by using STPT and Multi Tie Push Test (MTPT) tests and numerical modeling illustrated that lateral supports significantly increase lateral resistance [26]. Ballast bonding is a way to enhance track performance. Lateral resistance increases in curves due to bonding stabilization technology [27]. Waste tiers have been used in ballasted track foundations, rendering higher lateral resistance [28]. The use of geogrid causes a decrease in lateral deformation [29]. A field and laboratory sample has been made by geogrid stabilized ballast layer. The STPT and track panel displacement tests have been conducted. The results show an increase in lab and field, respectively [30].

The comprehensive method that increase majority of ballasted track disadvantages considering economic impact rarely proposed. A method is needed that, while improving the desired properties in terms of mechanical, physical, durability, environment impact, construction time, maintenance operation and dynamic, is also economic and constructive. Slab-track is the method that developed recently due to its advantages compared to ballasted track. But the economic issues and complicated construction technology are the challenges to prevent comprehensive extension. So the middle method is containing majority advantages of both of methods is needed.

2. Measuring tests

Using road construction experience with Royal Road Product (RRP_235Special), as an innovative method for the first time, the layers underneath the sleeper have been replaced with the clayey subgrade stabilized with RRP_235special. To investigate reaction against applied forces, a series of static and dynamic lab and field experiments such as maximum compaction test, California bearing ratio, unconfined compressive strength, Brazilian indirect tensile test, direct shear strength, uniaxial cyclic tests, and single tie push test (STPT) are needed. All the tests were done using the international codes according to Table 1.

Test name	Type of test	Standard code
Maximum compaction test	Laboratory	ASTM D 698-78 [31]
Uniaxial compression test	Laboratory	ASTM C39 [32]
Brazilian indirect tensile test	Laboratory	ASTM C496 [33]
Direct shear strength	Laboratory	ASTM D3080 [34]
California bearing ratio (CBR)	Laboratory	ASTM D1883 [35]
Ballast box	Laboratory	—
Plate loading test (PLT)	Field	DIN 18134 [36]
Single tie push test (STPT)	field	—

Table 1.

Table of tests.

The samples were made in different RRP_235special dosages according to Table 2.

Sample number	RRP235special dosage (lit/m^3)
1	0
2	0.09
3	0.15
4	0.21
5	0.27

Table 2.

Dosage of RRP235special in different samples.

3. Royal road product 2-3-5 special (RRP_235special) introduction

The RRP_235special has been made in Germany. The additive is acidic liquid and affects clayey soil through the chemical–mechanical process. Figure 1 shows the RRP_235Special with the R symbol and its chemical reaction.

Figure 1.

Chemical reaction of RRP235Special [37].

As shown in Figure 2, the additive acts chemical process in three stages dissociation, ion exchange, and neutralization. The additive is diluted with water to achieve dissociation. Then, the mixture is pure to the soil, and the exchange of ion starts. The reaction time is different depending upon the type of soil, fine grain fraction, and the chemical elements of the soil. Finally, the neutralization is done by exodus of water.

Figure 2.

Description of the ion exchange at the colloids [37].

The soil particles can perform the chemical reaction with water and other elements only when the fine grain fraction is less than 0.06 mm. Large part of soil (sand and gravel) cannot produce a chemical connection with water. So particles smaller than 0.06 have a key role in clayey soil stabilization with RRP_235special.

4. Clayey sand subgrade

As mentioned in the last paragraph, the percentage of fine particles smaller than 0.06 indicates RRP_235special dosage. The soil was brought from Urmia railway station. Typically the region soil has enough clay.

The primitive soil tests such as particle size distribution were done, and the type of soil was specified. Results revealed 40–50% of soil was smaller than 0.06 mm (Figure 3).

Figure 3.

Particle size distribution of the soil.

5. Sample preparation

The results of tests must achieve the least acceptable amounts. As mentioned in Table 1, laboratory and field experiments are needed to investigate additive performance. The similarity of samples must be considered. In this regard, all the specimens have equal water content and compaction energy.

The specimens were made with different additive dosages according to ASTM D 698 standard. The compaction energy for all the specimens was same and determined from eq. (1):

$$E=\frac{N^{\ast }{L}^{\ast }{W}^{\ast }h}{V}$$E1

where E is compaction energy (kJ/m³), N is the number of blows, L is the number of layers, W is hammer weight (N), h is the height of falling (m), and V is the mold volume (m³).

According to the manufacturer’s guidance, additive usage amounts were classified into five dosages of 0, 0.09, 0.15, 0.21, and 0.27 lit/m³. The chemical process needs at least 8 hours for ion exchange.

6. Tests

According to Table 1, the tests were done to investigate the proposed method. First, the lab experiments were done for different additive dosages, and the optimal amount was found. Using optimal dosage, the track with 50 meters length in 4 RRP_235special stabilized subgrade layers was constructed in Urmia railway station. Then the field tests were done on this site.

6.1 Maximum compaction test

The maximum compaction is a common test that gives useful information about optimum water content and maximum density of soil. The test can be done in the standard proctor and modified proctor method. The results are criteria of sample preparation and field controls and guides (Figure 4).

Figure 4.

The mold of maximum compaction test.

6.2 Uniaxial compression test

The samples were made in a cubic vessel with dimensions of 15 by 15 by 15 cm. To investigate the effect of time on additive performance, six different ages was selected. Figure 5 depicts some of the prepared samples. The last series of specimens were tested at the age of 46 days and visually completely dried.

Figure 5.

a) sample with 0.15 lit/m³ additive tested in 7 days of age and b) sample with 0.21 lit/m³ additive tested in 28 days of age.

6.3 Indirect Brazilian tensile test

This test is an indicator of soil tensile strength. The samples were made in a standard mold with different dosages of an additive, according to Table 2. The age of 46 days was selected to compare the latest uniaxial compressive test time. Figure 6 shows the sample with 0.15 lit/m³ in the Brazilian Indirect Tensile Test device before failure.

Figure 6.

The sample with 0.15 lit/m³ additive in Indirect Brazilian Tensile Test device.

6.4 Direct shear test

As the additive affects surface energy of colloids, shear parameters need to investigate. Based on the manufacturer’s statements about the possibility of loading the route and releasing traffic immediately after construction, the shear test age was conducted on the day after sample preparation (Figure 7) [37].

Figure 7.

Direct shear test device.

6.5 Uniaxial cyclic test

A series of dynamic tests were carried out to investigate proposed track properties. Each track component has a specific role, and the performance depends on its type and geometry. Any change in the specification of a component leads different reaction. The important factors of track under dynamic loads were investigated using the Ballast box test. The Ballast box device simulates applied forces from rolling stock to track.

6.6 Cyclic test setup

As depicted in Figure 8, the ballast box device is used at IUST University to simulate rolling stock forces applied to the track. The device includes power supplier, pump, and piston to provide vertical forces, computer software, sensors to measure displacements and calculate them, and a box to hold ballast specimens. The dimension of ballast box and piston is 0.7 m (l) × 0.45 m (h) × 0.3 m (w) and 0.22 × 0.22 m, respectively. Using the hydraulic pump power controlled by software in terms of applied force and frequency, the piston applies vertical force to specimen in box. This process continues to 100,000 cycles. In this research, the vertical force of 27 kN and frequency of 3 Hz in 100,000 cycles were applied to the specimens.

Figure 8.

a) Ballast box test apparatus; b and c) ballast box dimensions; d) LVDT location [38].

6.7 California bearing ratio (CBR)

The CBR is a well-known test in transportation projects. The CBR is a criterion of acceptance of structural layer proficiency. The specimens in CBR test were tested at 1, 3, and 7 days of age in wet situations (Figure 9).

Figure 9.

The CBR device.

6.8 Plate loading test (PLT)

This test aims to help determine important parameters such as Young’s modulus, deformation, and strengths characteristics, and modulus of subgrade reaction. Different methods are used according to the type of structure. Due to the frequency of loading and unloading cycles in transportation engineering, incremental and cyclic loading can be used. The test can be done at surface level or subgrade depth (Figure 10).

Figure 10.

PLT test device.

6.9 Lateral resistance

Due to recent development in train speed and freight and the use of CWR technology, lateral resistance is a key parameter to provide track safety and stability and counteract buckling. Lateral resistance of railway track measures using four methods mentioned below [39]:

1. single tie push test (STPT),

2. panel displacement test,

3. mechanical track displacement test, and

4. continues dynamic measurements of lateral resistance.

In this study, the STPT test was selected and used (Figure 11). In ballasted tracks, the most important components that affect lateral resistance are sleeper and ballast layer specifications such as shape, material, geometry, thickness, and interaction between them.

Figure 11.

STPT test devices.

7. Lab tests result

7.1 Maximum compaction test

With due attention to part 3 and the principle of additive chemical act, further use of RRP_235special causes more water release using ion exchange process and decreases optimum water content. The more use of RRP_235special, the more release of interlayer water. Also, the additive arranges the colloid placement. Releasing interlayer water and colloids arrangement increases congestion and maximum dry density. The results in Figure 12 reveal that maximum dry density increases and optimum water content decrease by 4, and 38 percent, respectively.

Figure 12.

Maximum dry density curves.

7.2 CBR

The CBR ratio limitation for subballast, subbase, and base are 25, 30, and 80, respectively [40, 41]. Application of RRP_235special increases CBR value in general. But the strengthening is not proportion to the additive dosage. As shown in Figure 13, the CBR ratio increases from 0 to 0.15 lit/m³ additive dosage and then is reduced and then by further use of the additive, is increased and strength again. In the optimum dosage of additive (0.15lit/m³), the CBR increased more than 400% compared to the sample with no additive.

Figure 13.

The effect of RRP235 special on CBR value.

Clay colloids absorb a certain amount of additives, and the excess remains between the colloids. A large amount of excess material seems to reduce the soil’s physical properties and make the situation glide colloids. Due to the concentration of the ions, the zeta potential decreases, and cations and anions are liberated from the diffuse double layer; thereupon, the swelling properties of soil reduce [42].

7.3 Compressive test results

To place the sleepers on the stabilized layer, the minimum strength of soil is needed to be evaluated by compressive test. Similar to the CBR test results, the use of additives gives higher strength to the specimens. The strengthening process ascends from 0 to 0.15 lit/m³ and then descends. This pattern is repeated at different ages. As indicated in Figure 14, the use of RRP_235special had caused a 46 and 300 percent increase in compressive strength compared to the sample with no additive. Therefore, the optimal required amount of additive, according to Figure 14 is 0.15lit/m³.

Figure 14.

RRP-stabilized soil compressive strength with different amounts over time.

7.4 Indirect Brazilian tensile test results

The tensile strength has improved using RRP_235Special. Like the previous sections, increasing the additive usage up to 0.15 lit/m³ increases the tensile strength and then descends. As shown in Figure 15, the sample with 0.15 lit/m³ additives has the best result and is selected as the optimum dosage. Compared to the sample with no additive, the strength improves by more than 20%. Comparing Figures 14 and 15 reveals that more compressive strength mirrors more tensile strength.

Figure 15.

Tensile strength of samples with different RRP235 special at 46 days of age.

7.5 Direct shear test results

As depicted in Figure 16, the maximum shear stress was obtained from Mohr–Coulomb diagrams.

Figure 16.

Shear stress–strain diagram (the sample with 0.15 lit/m³ additives).

To evaluate shear stress, different samples with RRP_235special were made and tested. The additive has a positive role in the shear parameter. As illustrated in Figure 17, same as in previous tests, the strengthening pattern ascends up to 0.15 lit/m³ additive and then decreases, and further use causes increases again. The 0.15 lit/m³ amount is determined as the optimum value among the used additive dosages.

Figure 17.

Maximum shear stress diagram.

Good compaction and high density of soil increase the shear strength. When the reaction has occurred, less water can accumulate in the soil than was originally possible. As a result, the swelling capacity is reduced, the internal moisture of the soil is also reduced, and complete compaction to zero content of air-tilled voids becomes possible because of the space that has become available from the expelled pore water. Subsequent additions of water cannot reverse this process once the latter has been accomplished (the swelling capacity is destroyed and the shearing strength increased) [43].

7.6 Uniaxial cyclic test results

7.6.1 Settlement

Vertical and horizontal displacement of structural layers is an important problem in ballasted railways that cause noticeable geometry change, reduce maintenance intervals, and consequently increase maintenance costs. The RRP_235special stabilized clayey soil can replace the ballast underneath the sleepers. This led a significant reduction of displacement in vertical and horizontal directions. The specimens with different dosages of additives were made in special boxes and tested using a ballast box device. Figure 18 shows the specimens after 100,000 cycles of loading.

Figure 18.

Ballast box samples after testing (specimens from left to right correspond to 0, 0.09, 0.15, 0.21, and 0.27 lit/m³ of RRP235 Special).

The ballast box test was carried out on the samples, and as rendered in Figures 19 and 20, the additive is an influential factor in the strengthening of clayey soil. Reverse to the static tests pattern, the sample with 0.15 lit/m³ has the least amount (2 mm), and the specimen with no additive has the largest amount (4.7 mm) of settlement. Use of RRP_235special causes more than 57% reduction in vertical displacement. Therefore, the dosage of 0.15 lit/m³ was selected as the optimal amount in terms of settlement in the ballast box test.

Figure 19.

Settlement diagram.

Figure 20.

Settlement of samples in cycle 100,000.

The exodus of interlayer water due to ion exchange gives improved compaction to the stabilized soil and is the main factor of strengthening, but increased additive results in excess of free ions around the colloids, which renders negative properties [43].

7.6.2 Stiffness

The stiffness of the track is affected by the materials used. Any change in material type causes a change in stiffness. Using the ballast box test, the stiffness of clayey soil with different additive dosages has been evaluated. According to the RRP_235special action, the results are predictable. As shown in Figure 21, the sample with 0.15 lit/m³ RRP_235Special has the highest stiffness, and the sample with no RRP_235Special has the lowest. Low and high stiffness limits have been shown in Figure 21 by 30 and 80 kN/m, respectively [40]. The specimen with 0.15lit/m³ has the best result and is selected as the optimal value for stiffness.

Figure 21.

Stiffness of samples with different dosages of RRP235Special.

7.6.3 Damping ratio

Comparing different amounts of RRP_235Special, the damping ratio of each sample, which is representative of the lost energy, divided by the energy input in a cycle, has been determined using eq. (2), proposed by Jacobsen [44];

$$\xi =\frac{\mathit{\Delta E}}{2\pi {\mathit{Kx}}^2}$$E2

Where ΔE is the dissipated energy, and k and x refer to the stiffness and deflection of samples, respectively.

Figure 22 shows the force–displacement relationship of the sample with 0.15lit/m³ RRP_{235 Special} at the final cycle (100,000th). The red zone on this graph indicates the dissipated energy. In order to calculate the damping ratio of the samples, the area of this zone should be divided by that of the loop.

Figure 22.

Calculation of damping ratio by means of the force–displacement.

Figures 23 and 24 show the force–displacement loops of all samples and their damping ratio values, respectively.

Figure 23.

Piston Force–displacement graphs for five different samples (a, b, c, d, and e are the sample with 0, 0.09, 0.15, 0.21, and 0.27 lit/m³ additive, respectively).

Figure 24.

Damping ratio of samples with different percentages of RRP235 special.

The settlement, stiffness, and damping ratio are interdependent. The damping ratio and settlement are opposite of the stiffness. The specimen with high stiffness has a low settlement and damping ratio. Greater concentration of colloids with the use of additive and ion exchange causes vibration transmission and low damping ratio. So the results shown in Figure 24 confirm the relation between the specifications. The sample with a high damping ratio behaves better facing dynamic forces and reduces transmitted energy than others. The sample with 0.15 lit/m³ has the lowest, and the sample with no additive has the highest damping ratio. The sample with no additive was selected as optimal dosage for damping ratio. But the preference for stiffness and settlement cause the challenge of optimal dosage selection. So the dosage of 0.15 lit/m³ has been selected as an optimal value for dynamic tests.

Samples with different dosages of additive were made, and an optimal percentage was found. As a result, the sample with 0.15 lit/m³ RRP_235Special was determined as the suitable dosage for mechanical and physical tests, while only in the Maximum Compaction test, by increasing the additive, the optimum water content decreased.

8. Field tests result

8.1 STPT

The lateral resistance was determined using STPT device in conventional and RRP_235special stabilized tracks at Urmia railway station. All the characteristics are the same between tracks except ballast thickness under the sleeper.

Regarding the results, there is no significant difference between conventional and RRP_235special tracks. The difference has two reasons. First, the ballast layer compaction in conventional tracks was more than RRP_235special tracks, leading to higher side and end resistance. The second reason comes from the placement position of the ballast under the RRP_235special track. When the roller compacts the ballast to the last stabilized layer, most ballast particles have rotated in a position that surfaces parallel to the sleeper’s bottom side (Figure 25). So this led to the uniformity of layer decreases and caused less load transfer (Figure 26). Of course, this action has a beneficial side that causes increase in the contact area between ballast particles and the sleeper bottom side.

Figure 25.

The ballast particles placement under the sleeper in RRP235special method.

Figure 26.

Conventional and RRP235special tracks comparison.

8.2 PLT

The PLT test was conducted in the Urmia railway station RRP_235special stabilized track, conventional track, and in the part of the Urmia-Oshnaviyeh road track constructed by RRP_{235stabilized} clayey soil. The EV was determined, and there are significant differences between conventional tracks and RRP_235special stabilized tracks. The results show an approximately 232% increase in EV.

9. Conclusion

RRP_235special is an acidic additive that makes a situation in which colloid surface energy changes, and consequently, water is released, and using the physical process, permanent compaction is created. The result of lab and field experiments indicated improvement of stabilized clayey soil properties.

1. The RRP_235special stabilized clayey soil has less optimum water content and much maximum dry density.

2. Static tests include compressive strength, tensile strength, shear stress, and CBR, has improved due to stabilization with additives. The sample with 0.15 lit/m³ has been selected as the optimal dosage, and the pattern of strengthening is the same.

3. The RRP_235special influenced the soil positively in terms of dynamic tests. The results render significant enhancement in stabilized soil properties. RRP235special stabilized clayey soil has the highest stiffness and lowest settlement and damping ratio. Additives increase the parameters of clayey soil to acceptable limit values. The sample with 0.15 lit/m³ has been selected as the optimal dosage in terms of dynamic tests.

4. Stabilizing clayey soil with RRP completely differs from ordinary materials such as cement and lime. RRP changes the properties of colloids and gives permanent changes to the soil, whereas cement and lime give their own properties to the soil. RRP-stabilized soil properties depend on the chemical–physical process, so there is no limitation to compaction energy. According to the manufacturer’s statement, the additive makes colloids hydrophobic, creating a waterproof layer and consequently preventing capillarity.

5. In term of methodology, the STPT test in the field was conducted, and lateral resistance was measured in both of conventional and RRP_235special method. Generally, the lateral resistance of the tracks is almost the same. There is no need to use other methods to increase lateral resistance. Therefore, the RRP_235special method can be recommended because of lateral resistance.

6. The PLT test was conducted, and the results showed significant increases in EV.

Acknowledgments

The author acknowledges SAINA and RRP Gmbh Company for supplying RRP_{235 Special} material. The research team also would like to thank Dr. Y. Eghbali Afshar for his technical support.

Conflict of interest

This research did not receive any specific grant from funding agencies in the public, commercial, or nonprofit sectors.