Compaction of Course-Clastic Soils at an Experimental Site During the Earth Dam Construction

doi:10.21203/rs.3.rs-3556072/v1

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Compaction of Course-Clastic Soils at an Experimental Site During the Earth Dam Construction

https://doi.org/10.21203/rs.3.rs-3556072/v1

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This paper examines the results of laboratory and field studies on the compaction of coarse-clastic soils. In the laboratory conditions, a 300 mm diameter vibratory unit and a standard compaction device, developed by SRI Soyuzdor, were used for the soil compaction. In the field conditions, the compaction was carried out using a smooth SANY-type roller weighing 27 tons. In the laboratory, to determine the maximum density and optimal moisture content, a dry soil density – moisture content curve was plotted; the maximum density and optimal moisture values were determined according to the maximum of the obtained curve. The paper presents the results of soil compaction in the field conditions at a layer thickness of 50, 70, and 80 cm. The densities in these cases were 2.19, 2.10, and 2.04 t/m³, respectively. The soil compaction was implemented using a 27-ton roller compactor for 6–8 passes along one track. In addition, the effectiveness of the applied method of drilling and blasting in a quarry for obtaining stone material with the required fractional composition is shown

coarse-clastic soil

earth dam

fractional composition

model mixture

pit

In recent years, coarse-clastic soils have been used in a wide variety of elements of earth structures. Their share in industrial-civil, hydrotechnical, water management and road constructionis more than 75% [1,2]. When using them, the main task is to ensure high-quality soil laying [3,4].

In the general case, the quality of laying the materials of the above ground structures is determined by their density, strength, deformability and filtration characteristics [5–7]. To meet these requirements, it is necessary to determine the control parameters for laying soils at each site. They are individual and require extensive experimental work to substantiate these control parameters of laying in the dam body [8, 9].

Coarse-clastic soils are one of the most promising building materials in the field of earth dam construction. Building materials for such structures can more often be destroyed [10–12]. As is known, coarse-clastic soils are widely used for supporting the prism of rock and earth structures and occupy an average of 70% of the total volume of the dam [13–16].

At present, rock-and-earth dams are the most widespread among other types of high earth dams. For high rock-and-earth dams, studies of the behavior of coarse-clastic materials under heavy loads are of particular importance. The choice of characteristics of the soils used in the construction of an earth dam is one of the main stages of the construction, since they directly affect the reliability and efficiency of the structure [17–22].

To determine soil density under laboratory conditions, the standard compaction method was used. In the field conditions on the experimental site, the density was determined by the pit-hole method. The volume of the pit in this case was determined by means of pouring water onto a pre-lined 0.2 mm thick polyethylene film (Fig. 1) [23].

The purpose of the study was to choice and to develop a method for laying and compaction of soil materials for a dam body in natural conditions, providing the materials’ highest density and lowest filtration coefficient.

The tasks were solved by building experimental embankments and performing the necessary minimum of laboratory work.

Before the laying on the experimental sites, preliminary laboratory experiments were carried out, which made it possible to establish the soil’s moisture and density dependence during various types of work. The laboratory compaction, unlike the field conditions, was performed not by rolling, but by compaction of the soil using a standard compaction device. The filling of the experimental embankment in order to save money was carried out directly at the construction site on the left bank of the Pskem River. For these purposes, a horizontal site was cleared of the vegetation, planned and compacted with a 27-ton smooth roller compactor SANY. The layout of the base was implemented subject to possible deviations from the base’s horizon up to 5 cm.

The selected area measuring 100x100 m was carefully compacted with a vibratory roller in 6–8 passes along one track. The prepared site was leveled in the measuring sections every 10 m. The layout of the site was made by an experimental embankment for the thrust prism and filters of the first and second layers. Landmarks (40 pieces of F16ASH fittings 1.5 m long) to designate sites and alignments were installed along the contour of the area. After that, the density of the base was determined by the cutting ring method. The structure of the dam includes: a stone prism, pebble filters of the first and second layers and a loamy soil core. The soil density of the dam’s prism on the experimental site was determined by the following way. The Pskem HPP dam’s prisms are planned to be built from the rock mass. For this purpose, it is planned to use a deposit located in the upper pool on the left bank, 2.0–3.0 km from the dam site. The soil was transported by a heavy dump BELAZ-type truck with a load capacity of up to 40 tons. Laboratory and field compactions of coarse-clastic soils were carried out at the experimental site.

The soil on the experimental site was leveled with a bulldozer in non-compacted layers 50, 70 and 80 cm thick. After the leveling, the soil was watered with water from a water carrier at a rate of 200 liters per 1 m3. The compaction was carried out with a smooth roller weighing 27 tons, turned on at low speed. After every second pass, the soil’s density was determined by the pit-hole method, two pits per each layer.

When sampling the stone materials and processing the measurement results, the method described below was used. The sampling was carried out from the layer surface as follows:

- a carefully leveled horizontal site 1.5x1.5 m is prepared; a metal frame 1.1x1.1 m is installed within the site and a “pit-hole” is passed to the depth of the density determination horizon;

- the material extracted from the pit was weighed and dispersed into fractions. The volume of the pit was measured by pouring water onto a pre-lined polyethylene film.

The soil’s volumetric weight is determined as a quotient of the sample’s weight and the pit’s volume:

$$\frac{w}{pd}=\frac{P}{V}$$

where p – weight of the soil, kg; V – volume, l; w/pd – volumetric weight of the wet soil.

Considering that the humidity of the stone material slightly affects its density, it can be ignored. If the content of fine earth (< 5 mm) is more than 5%, it is necessary to determine its moisture content and make a correction for all the samples, i.e. the amount of water in the material must be subtracted from the total weight of the sample.

The moisture content of the extracted material is determined by sampling 2 kg of the material with a fractional composition of less than 20 mm and its drying in a drying cabinet.

The sample representativeness is determined by the volume of the hole depending on the maximum size of the inclusions and is regulated by State Standard 28514-90 [24].

The overall assessment of the laid natural soil quality is made according to the total curve of the grain composition, determined by sifting on the experimental site. The use of coarse-clastic soils requires reliable knowledge of their physical and mechanical properties and construction characteristics. The use of these soils is complicated by a number of specific factors: labor intensity, large particle sizes and sample volumes, and the need to apply large-scale instruments.

A preliminary assessment of the compactibility of coarse-clastic soils is carried out by the standard compaction method using a standard compaction device in accordance with regulatory requirements. The size of individual fractions sometimes reaches 700–1000 mm. When studying coarse-grained soils in laboratory conditions, this circumstance forces us to proceed to modeling the grain compositions of natural soil and to perform experimental determinations using these model mixtures.

The task of modeling the grain composition of the soil is carried out in such a way that the model mixtures can be used in laboratory instruments of acceptable dimensions. At the same time, the characteristics of the model mixtures should best match the characteristics of natural soil.

A generalization of the results obtained at the studying coarse-clastic soils allows us to conclude that a ratio of the device’s diameter to the maximum fraction’s size should not be less than 5: dd ≥ 5dmax. The maximum particle size for a standard instrument with a diameter of 300 mm should be 60 mm.

When determining the model mixtures’ grain composition, it is recommended to keep in them the percentage of fractions less than 5 mm or 10 mm. Thus, two points are fixed on the model mixtures’ grain composition graph: the fine earth content and the maximum fraction content (Fig. 2). This coarse-grained soil modeling technique was developed by HydroProject JSC.

Using the coarse-clastic soil modeling method, in which the percentage of fractions less than 5 mm in the model and natural soils should be the same, the maximum fraction is determined by the minimum size of the container used. Thus, there are two points on the graph of the grain composition of the model mixture: the contents of fine earth and maximum fraction. In this case, the modeling method is applied that excludes the arbitrariness of the content of the fractions ranging in size from 5 to 60 mm. Intermediate points on the graph are determined by a proportional decrease in the fractions’ content in the natural soil, which are calculated according to the formula:

$${P}_{i}-P<5$$

$$\frac{m}{Pd}=\left(100-p<5\right)+p<5$$

where: m/Pd – the percentage of fractions in the model mixture;

p < 5 – the percentage of fraction less then 5 mm;

p – the percentage of the fraction in the natural soil.

Model mixtures are made for each material type that is tested on a standard compaction unit. Before the compaction, the mixtures are moistened with water in such a way that the moisture content of the fine earth, which is part of the mixture, is at least 5–6%. The mixture is loaded into a container of a vibrator unit (Fig. 3) and leveled, and then the distance between the soil surface and the top of the device is measured at five points with a measuring ruler. The bar of the measuring ruler rotates 90° and measures five more points on the ground surface.

The average value of 10 points determines the position of the sample surface relative to the top of the instrument, and the difference between the position of the bottom and the ground surface determines the sample height.

After the installation is assembled, the vibrator fixed at the bottom is turned on, and the container vibrates for 8 minutes. Then the load and the rubber gasket are removed, and the distance from the upper edge of the container to the surface of the layer is measured at ten points. The obtained data is used to calculate the volume and maximum density of the compacted sample.

Determining the limiting densities allows us to build a graph of the dependence of two parameters: content of fine earth in a mixture with a model grain composition “m” and density “рd”.

The graph has the density extremum at a certain content of fine earth in the composition of the mixtures under study. Testing the mixtures with different content of fine earth makes it possible to determine its optimal content (Fig. 4).

Table 1

The results of compaction of model mixtures, their limiting densities and maximum densities of natural soil
Mixture #	Content of fraction < 5 mm, %	Content of fraction < 20 mm, %	Density of model mixture, t/m³		Natural soil ρ_d^max, t/m³	Required density ρ_d^mp, t/m³
Mixture #	Content of fraction < 5 mm, %	Content of fraction < 20 mm, %	ρ_d^mix	ρ_d^max	Natural soil ρ_d^max, t/m³	Required density ρ_d^mp, t/m³
1	25	35	1,74	2,06	2,25	2,18
2	18	64	1,68	1,98	2,13	2,02
3	10	33	1,63	1,88	2,00	1,84

Using the results of Table 1, we obtain the relative densities of the stone used for preparing the grain compositions containing fine earth from 10 to 25%.

Table 2

Experimental results
Fine earth content, %	Relative density, t/m³
Fine earth content, %	I_d= 0,70	I_d= 0,75	I_d= 0,80	I_d= 0,85
25	1,95	1,97	1,99	2,00
18	1,88	1,90	1,91	1,93
10	1,80	1,81	1,82	1,84

The results of determining the soil density of the prism on the experimental site, depending on the number of passes of the roller and the thickness of the layer, are shown in the following graphs (Figs. 5–7).

Thus, the prisms were laid out of stone rock mass in layers of 50, 70, and 80 cm, moistened with water from a water carrier and compacted with a 27-ton roller compactor in 6–8 passes. The average density was 2.19, 2.10, 2.04 t/m³, respectively. The data were obtained on the experimental site for the grain mixtures with a maximum fraction of up to 200 mm (Fig. 8).

When developing stone quarries using explosions, it is possible to estimate the grain composition of the resulting stone material using experimental curves. At the same time, one can also judge the effectiveness of the applied blasting method to obtain the stone material with the required grain composition. To obtain complete information on the grain composition of the stone material, experimental blasting should be carried out, which will determine the explosion method to obtain the needed fractions.

According to the data obtained, the most rational and providing a relatively high soil density is the method of laying stone material in layers no more than 1 meter thick with mandatory wetting with water at a flow rate of 150–300 l/m³ and compaction with a long-range vibrating mechanism.

The laying of stone material is recommended to be carried out by high-capacity dump trucks with a more or less uniform distribution over the surface of the layer and subsequent leveling with a bulldozer.

As can be seen from the results of determining the granulometric compositions and densities of the stone material laid in the experimental embankment, the relative shrinkage of the stone material layer in the embankment decreases with an increase in fine fractions (less than 5 mm) in the soil composition. At loads of 4.0 MPa, the shrinkage decreases from 8.6 mm to 6.2 mm; with an increase in the content of the fine fraction in the rock mass, it is quite high and the deformation of the material is significant. The most optimal value of fine earth content in the soil composition is 18–25%; in this case, the shrinkage will not exceed 6 mm.

Based on the results obtained, the following conclusions can be drawn:

1. The dam’s prisms from the rock mass were laid in layers of 50, 70, and 80 cm, followed by moistening with water and compaction with a 27-ton smooth roller compactor in four to six passes. In this case, the soil density was 2.19 t/m3, 2.10 t/m3, and 2.04 t/m3 for the layer thickness of 50, 70, and 80 cm, respectively. The data were obtained on the experimental site when using the grain composition with a maximum fraction of up to 200 mm.

2. When developing stone quarries using explosion, the effectiveness of the applied method of conducting drilling and blasting operations can be judged by the results of obtaining the stone material with the required grain composition.

3. To obtain a relatively high density of the laid soil, it is recommended to apply the laying of stone material in layers no more than 1.0 meter thick with mandatory moistening with water at a flow rate of 150-300 l/m3 and compaction with a long-range vibrating mechanism.

4. To prevent delamination, it is

metodov ekspress-kontrolya dlya otsenki kachestva uplotneniya krupnooblomochnykh recommended to lay the stone material with large-capacity dump trucks with a more or less uniform distribution of the material over the surface of the layer and its subsequent leveling with a bulldozer.

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No competing interests reported.

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Compaction of Course-Clastic Soils at an Experimental Site During the Earth Dam Construction

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1. Introduction

2. Materials and methods

3. Results and discussion

4. Conclusion

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