Experience in applying the Geocomposite method on soft soils in Moscow

. The pace of construction of industrial and civil buildings in Russia is increasing every year. In this regard, most of the undeveloped territories are composed of weak soils, which in turn leads to an increase in construction costs and the need for additional measures. Such measures include changing physical and mechanical characteristics of soils and increasing their bearing capacity. One of the more promising methods of improving the properties of soils is the method of "Geocomposite". This method allows to form stronger artificial structural bonds between particles in the soil. When fixing soils with Geocomposite, injectors are arranged and chemical or microcement solution is supplied under pressure [4]. This method of mortar supply helps to break up weakened soil areas and fill voids. It is important to note that when consolidating soils using the Geocomposite method, it is necessary to build a test site to select the optimal pressure of solution supply and to determine the characteristics of the consolidated soil.


Introduction
One of such sites, composed of weak soils, was encountered in Moscow when designing a multi-storey residential building with maximum axis dimensions of 71.04m and 31.52mrespectively.The building foundation is a monolithic foundation slab.
Determination of soil properties was carried out by laboratory studies and field tests by static probing method up to the depth of 80.0m.The four upper layers are composed of technogenic soils, which will be reinforced in the future.
Characteristics of soils of technogenic origin are given for the main aggregate and may differ from the properties of soils in the massif, as they have a large number of various inclusions.Removal of such a large volume of anthropogenic soil or cutting through it with piles will lead to a significant increase in construction costs.Having evaluated various options, the decision was made to consolidate the foundation soils with Geocomposite.

Methods
Using the Geocomposite method in this case should be based on: 1. Calculation substantiation; 2. Field tests; 3. Numerical calculations.Taking all the data into account when consolidating soils will allow to determine with sufficient accuracy the increase in VAT and characteristics of the consolidated soil after the works [6].Strengthening of foundation soils is carried out in the following sequence: -in the first stage, the excavation is carried out to a mark three meters deeper than the bottom of the foundation slab of the buildings.
-In the second stage, the excavation is backfilled up to the basement bottom level with sandy soil with compaction coefficient not lower than 0.97, with Е not less than 18MPa.
-In the third stage, injection wells are drilled from the mark of the sand embankment and cuff columns are installed, filling the annular space with casing solution.
-In the fourth stage, injection of the bulk soils is carried out through the cuff columns.
-At the fifth stage the building construction is carried out with backfilling of the excavation slots.
Soil reinforcement is carried out throughout the entire thickness of the bulk soils.

Results
In the process of calculations to determine the settlement, settlement calculations were performed for all layers [1,2,4,6].
When selecting the optimal characteristics of the consolidated soils it was obtained that for IGE-1 the deformation modulus E=30000MPa, for IGE-2 E=25000MPa; for IGE-3 E=25000 and for IGE-4 E= 22000MPa.After the calculation of the fixed soil it was obtained that the building settlement is equal to 7.74 cm and does not exceed the maximum allowable value of foundation deformations, according to Annex D, "SP 22.13330.2011"[1], which is 15 cm for this type of structures.
To conduct field tests, a pilot site was created, delineated by 16 wells of the barrier row, forming a closed space in plan.Inside the site there were 11 working wells for consolidation of the bulk soil mass.
According to the results of static sounding before consolidation the value of deformation modulus of bulk soils of the experimental site averaged: at the depth of 3.0-12.0m -10.7-16.4MPa, at the depth of 12.0-26.5m -10.9-12.0MPa.
To evaluate the results of the performed consolidation of bulk soils at the experimental site, 2 engineering-geological wells were drilled, static sounding was carried out in 4 points for the entire thickness of bulk soils and in 3 points to a depth of 15 m, as well as drilling of engineering-geological excavations and carrying out 6 stamp tests.
Analysis of static sounding data after soil consolidation and their comparison with the data before consolidation allowed us to draw the following conclusions: -up to the depth of 12 m the massif became more compacted, no weakened zones (less than 1 MPa) were registered in the test points; -in the depth interval of 12-20 m the massif is not so significantly compacted as before the depth of 12 m, weakened zones with the thickness of about 30-40 cm are registered in some places; -weakened non-compacted zones were registered below the depth of 20 m; -average values of the frontal resistance of the probe after fixing the embankment increased from 5.3 MPa to 9.2 MPa, i.e. 1.7 times.
The given data show that after consolidation the number of weak layers decreased significantly, and the values of the frontal resistance of the probe increased on average by 1.7 times, which is shown in the following tables [1,5,14].Comparing the obtained data of analytical calculation, experimental site and calculation scheme we can conclude that the data of experimental site showed the highest convergence with real data.When calculating in the software complex there is a difficulty in modeling the injection of mortar in time.
During the construction the injection of mortar was carried out through the collars, located at a spacing of 1.0m in height.

Figure 3 .
Figure 3. Settlement at the stage of complete construction of the building (with modeling of the tongue and reinforcing elements)