Investigation of compacted sandy soils properties at base of buildings and structures

. Based on the data of physical and mechanical properties of soils obtained at various construction sites in Moscow, the article discusses in detail the issues of soil compaction and determination of bearing capacity. The article describes various methods of soil compaction, which must be taken into account to determine the value of the physical and mechanical characteristics of the soils laid down in the project. It is especially important at the initial stage to determine the necessary parameters of the compacted base. The described dependences of the deformation modulus and the compaction coefficient for the artificial base were carried out in comparison with engineering and geological data obtained from real objects in the city of Moscow. The analysis of the results obtained for sands with different granulometric composition was carried out and graphs of the dependencies of the data obtained were constructed. The article also describes the controlled parameters when using a compacted sand cushion as an artificial base.


Introduction
Currently, it is common to build buildings and structures on the ground of the base, which is pre-compacted in various ways.At the same time, during the compaction process, the values of the physical and mechanical characteristics of the foundation soils laid down in the project are achieved.Often, in addition to mechanical characteristics (angle of internal friction, specific adhesion, and modulus of deformation), the project includes the values of the compaction coefficient of the artificial foundation of buildings and structures [1].
When installing an artificial foundation, the question arises before design organizations what parameters of the sand cushion should be set.In addition, during the construction of a sand base, it is necessary to maintain the specified parameters laid down in the project [2].
This article presents the dependences of the deformation modulus obtained during compaction and the soil compaction coefficient.
In practice, when installing compacted artificial sand bases, the method of layer-bylayer compaction with rollers and vibratory rollers has become widespread.During the production of works, the results of compaction should be monitored, for which control engineering and geological studies of the compacted soil of the base are carried out by various methods (determination of the compaction coefficient, deformation modulusby stamp tests, probing, in laboratory conditions in odometers and stabilometers, determination of physical characteristics: soil density, soil particle density, humidity, etc.) In general, the concept of compacted soil is extremely vague.For each individual construction site, the soil can be considered compacted or not, depending on the design loads that the compacted artificial base will perceive.So, during the construction of a fivestory building, the sand cushion should be compacted to one deformation module, and during the construction of, for example, a seventeen-story building-to another (larger), since the bearing capacity of the soil should be greater for the safe operation of the artificial foundation.
For several years, studies of the physical and mechanical characteristics of compacted sandy soils of the base of buildings and structures have been carried out at various construction sites in Moscow.
Since it is impossible to present a large volume of the obtained research results within the framework of the article, this article will present part of them on the example of one object.

Compaction coefficient and deformation modulus of compacted sandy soils.
In Moscow, territories that were previously unsuitable for construction have recently become increasingly used for construction.So, as a vivid example, we can cite the construction of several residential blocks in the Nekrasovka micro-district.In 2011, it was decided to reclaim the areas of the Lyubertsy aeration fields, which had been performing their functions since 1964.The territory of more than 500 hectares was heavily silted up and, for the construction of residential quarters, the soil was excavated with its replacement by filling with sandy soils with their subsequent compaction.
Soil compaction was carried out by surface methods: Vibro-plastering and rolling with rollers weighing from 14-24 tons [3].
In Moscow, territories that were previously unsuitable for construction have recently become increasingly used for construction.So, as a vivid example, we can cite the construction of several residential blocks in the Nekrasovka micro-district.In 2011, it was decided to reclaim the areas of the Lyubertsy aeration fields, which had been performing their functions since 1964.The territory of more than 500 hectares was heavily silted up and, for the construction of residential quarters, the soil was excavated with its replacement by filling with sandy soils with their subsequent compaction.
Soil compaction was carried out by surface methods: Vibro-plastering and rolling with rollers weighing from 14-24 tons [3].
According to the project, the sand cushion under the building should have the following characteristics: -compaction coefficient Kcom = 0.97; -modulus of deformation E = 25 MPa; -the angle of internal friction φ = 30°; -soil density q=1.54 g/cm3.

Results and discussion
In the course of studying the sandy soils of the base at various construction sites in Moscow, various experimental studies were carried out (stamp tests, static probing, express method for determining density with density meters, laboratory studies in compression devices and triaxial compression devices, standard compaction) [4].A significant number of experiments made it possible to compare the values of the deformation modulus and the obtained values of the compaction coefficient for various types of sandy soils [5].
The dependences are shown on the graphs of the dependence of the Deformation Modulus on the compaction coefficient for sandy soils in Fig. 1.
In the course of the work, compacted sand pillows of various granulometric compositions were studied.Thus, Figure 1 shows data for large, medium, small sands and for sands with dusty and clay particles.After analyzing the results obtained, we were able to determine that at the same value of the compaction coefficient, the deformation modulus for sands of different granulometric compositions will be different.In addition, as the values of the compaction coefficient approach unity, a large increase in the values of the deformation modulus is not observed.
As a rule, in practice, in design solutions, the values of the compaction coefficient (Kcom) do not reach one, nevertheless, for the completeness of the experiment, soil compaction was carried out, including up to indicator 1.In parallel, control engineering and geological studies were conducted, the deformation modulus was determined [6][7][8].
Based on this graph, we can conclude that the medium-sized sands that are the basis of our structure, with a compaction coefficient of 0.97, should have a deformation modulus equal to E = 43 MPa.
In particular, engineering and geological surveys were carried out at the construction site to determine the deformation modulus of bulk sandy soils [9].At the construction site of the building, static sounding at 3 points and stamp tests of soils to the entire depth of the sand cushion were performed [10].According to the results of these tests, we obtained an average soil deformation modulus equal to E = 42 MPa, which corresponds to the deformation modulus obtained from the graph in Figure 1.
Based on the results of engineering and geological surveys [11] and drawings of building structures in the Plaxis 2D software package, a calculation scheme was created, shown in Figure 2. In the same software package, the calculated draft was determined at the deformation modulus E = 43 MPa, which was 79.41 mm.
In addition to the calculated value of the predicted precipitation, geotechnical monitoring was carried out at the construction site.Monitoring was carried out both during the construction of the building and during the year after the completion of construction [12].It became possible to compare the calculated precipitation values obtained using the Plaxis 2D software package with the actual precipitation obtained as a result of geotechnical monitoring.Fig. 3.A sedimentary geodetic mark was installed on the wall of the building.
For the organization of geodetic instrumental observations (geodetic monitoring) in September 2013, a network of wall stamps in the amount of 12 pieces was laid on the supporting structures of the building under construction of block 9 of block 7 [13].
Figure 3 shows a sedimentary geodetic stamp installed on the wall of the building.According to the results of geotechnical monitoring, the precipitation of the building at the end of the measurements stabilized and amounted to 80.73 mm.The obtained results of the actual precipitation and the value of its calculated value have obtained good convergence [14,15].

Conclusions
1. Thanks to various and numerous studies conducted at various construction sites in Moscow, we can say that reliable characteristics of compacted soils were including the dependence of the value of the deformation modulus on the value of the compaction coefficient.2. The obtained graphs of the dependence of the deformation modulus on the value of the compaction coefficient allow you to set these parameters before the start of filling and compaction of the sand cushion.3. When installing a compacted sand cushion as an artificial base, it is possible to control the specified value of the deformation modulus by determining the value of the compaction coefficient.At the same time, it is possible to obtain the values of the compaction coefficient more quickly, and the performance of such works is less expensive.4. The calculated value of the draft modulus of deformation obtained from the graphs has a good convergence with the actual draft of the building.5.When determining the modulus of deformation to the compaction coefficient, the granulometric composition of sandy soils should be considered.6.When the value of the compaction coefficient approaches the unit, the changes in the value of the deformation modulus are not significant after the indicator Kcom=0.97...0.98.

Fig. 1 .
Fig. 1.Graphs of the dependence of the deformation modulus on the compaction coefficient for sandy soils.

Fig. 4 .
Fig. 4. A graph of the development of the average precipitation values from time to time.