18 January 2022
Markov V.O. - Head of the GEXA Engineering Center
Materials by TM "Geospan" are used in road structures to reinforce the bases, which in the practice of GK "Gexa" regional and urban, there are already 500 and more than 60 federal ones, and in a number of regions such road structures are accepted as standard type, which have proven their effectiveness over a decade of operation.
To solve the problems of erecting embankments on soft soils and substantiating the reinforced soil structure, our engineers perform calculations of the stability and stability of the subgrade, calculations of settlement and deformation of the embankment and the timing of its consolidation, including with full and partial replacement of weak soils. Based on the results of the calculations, the optimal variant of reinforcing and the technology of work are determined, for example:
Replacement of weak soil that does not have sufficient bearing capacity;
Reinforcement of the base of the embankment according to the type of reinforced ground envelope or geoplatform;
Surcharge to prevent subsequent settlement of the embankment;
Vertical drainage (pressing vertical drainage elements into waterlogged soil);<
Embankments on piles with flexible geosynthetic grillage;
Reinforcement of slopes with a steepness of more than 1: 1, reinforced soil structures, retaining walls.
Such an engineering approach gives confidence in the correct use of reinforcing geomaterials and obtaining an economic effect, which consists in reducing the use of traditional road building materials and increasing the operational reliability and service life of the road structure.
Let us consider successful examples of the use of reinforcing geomaterials in complex engineering and geological conditions.
The design institute of Soyuzdorproekt OJSC provides for the use of a reinforced soil geoplatform in a problem area with weak soils (clayey fluid-plastic gray silt in the form of a lens of variable depth 5-15m, E=0.45MPa).
As part of the geoplatform, a Geospan ORP 30/20 volumetric geogrid was used in a holder made of high-strength woven geotextile Geospan TN-80. The computational analysis of the problem area and the technical and economic efficiency of the soil geoplatform was carried out by engineers in the PLAXIS geotechnical calculation program (modeling of reinforced soil structures of embankments on weak foundations using the finite element method).
The results of the calculation of the reinforced soil embankment showed:
decrease in settlement values compared to the settlement of the embankment without reinforcement by 56 cm (54%) and decrease in uneven deformation;
increase in slope stability coefficient from Kst=1.18 to Kst=1.72 at the required value of Ktr.=1.3. The stability of the reinforced soil embankment with the use of a geoplatform with Geospan materials is ensured with a margin of 35%;
reduction of consolidation terms by 44%.
When developing the working documentation for the project, the Geo-Project Institute identified a problem area. At the base of the site, peat was found, weakly and strongly decomposed, wet and saturated with a thickness of 3.0-4.1 m, which is unsuitable as the base of the embankment of the road and the underlying layer of soft-plastic loam with a deformation modulus equal to E = 4 MPa.
The computational analysis of the problem area (PK77 - PK87) was carried out by the engineers of the Gexa Group of Companies together with the design department of JSC VAD. Based on the analysis of calculations of the settlement of the embankment and the timing of consolidation, the stability of the base, it was decided to reinforce the base of the embankment with Geospan TN-80 woven geotextile with laying sheets in the transverse direction in the form of a closed envelope 1 m high and complete replacement of peat (up to 4 m).
As a result of the application of the design solution, the stiffness in the lower part of the embankment was increased, the deformation unevenness associated with the heterogeneity of engineering and geological conditions decreased, and the consolidation period was reduced.
The reconstruction project provides for the construction of a road bed in problem areas with a weak base (soils with a deformation modulus below 5 MPa), which is represented by plastic sandy loam and soft-plastic, fluid-plastic loams of different thicknesses with a deformation modulus in a water-saturated state E = 1.4 MPa. At the same time, engineering-geological conditions are complicated by a high level of groundwater and unsecured drainage - flooding of the embankment with both surface and groundwater. Additionally, it was required to ensure the stability of the embankment, taking into account the presence of a weakening zone at the contact of the existing and arranged subgrade; reinforcement of the interface zone due to possible deformations on a weak base.
According to clause 7.44 of SP 34.113330.2012, when arranging embankments on weak foundations, special measures should be taken based on calculations to ensure the possibility of using weak soils in the foundation - reinforcement of embankments with woven geosynthetic materials. For each interval, calculations were made of the overall stability of the subgrade, calculations of the settlement and deformation of the embankment and the timing of its consolidation, including with partial replacement of weak soils and weights to prevent subsequent settlement of the embankment. The calculations were performed both by analytical methods in the Credo Slope, Settlement and GEO-5 programs, and by numerical analysis using the finite element method in the Plaxis program. The brand and type of geosynthetic material for reinforced soil structures were selected according to the calculation of the required strength, taking into account coefficients that take into account the influence of individual factors on the deterioration of properties (damage, creep, frost resistance, UV and chemical resistance).
Based on the results of the calculations, the optimal options for reinforcing, the strength characteristics of the geomaterial and the technology of work were selected:
Reinforcement of the base of the embankment without replacing weak soil by the type of reinforced soil casing made of woven polypropylene geotextile Geospan TN-80, which performs the combined function of reinforcement and separation (A+P). This made it possible to increase the rigidity of the lower part of the embankment and reduce the difference in settlements along the axis and at the edge of the cross interval of the embankment, which reduced the time of consolidation (the period of consolidation of the reinforced embankment to the degree of 0.9xUTP was 6 months);
Reinforcement of the body of the embankment in areas with slopes from 1:1.5 to 1:1 to provide the required Kst = 1.3 with interlayers of high-strength polyester geotextile Geospan TNPE-300 with a tensile strength of 300 kN/m in the longitudinal direction.
Deformations of the existing subgrade were observed in this interval of the excavation with a weak foundation. This led to the formation of cracks in the coating and its premature destruction. The design organization of CJSC "MK Indor" (city of Irkutsk) provides for a soil replacement device and a working layer device at the excavation site with weak soils. The replacement was carried out with rocky soil to a depth of 1 meter from the bottom of the working layer according to the "enveloped soil" type. The reinforcing woven geotextile Geospan TN-80 was adopted as the geosynthetic material of the reinforced soil casing. Construction work on this site has been carried out by Bratskdorstroy CJSC since June 2014.
The use of a reinforced soil casing made it possible to increase the design strength indicators of the structure, which in the future should increase the overhaul periods of operation.
The design institute "Voronezh branch of Giprodor NII OJSC" provides for a protective and reinforcing layer of woven polypropylene geotextile Geospan TN-40 or an analogue at the border of the crushed stone and sand underlying layer in the pavement structure. Road pavement structure (asphalt concrete - 5 + 6 + 12 cm, bearing base of C-6 mixture - 40 cm, protective and reinforcing layer Geospan TN-40 (TN-50), additional base layer of medium-sized sand - 50 cm, earthen soil pavement - fine sand) with the accepted design data meets the criteria of ODN 218.046.-01, which is confirmed by the calculations of the road pavements in the Credo Radon program.
Geospan TN-50 woven geotextile was used throughout the entire length of the interval between the underlying sand layer and the carrier layer of crushed stone sand mixture (SCHPS) to increase the bearing capacity of the road clothing as a combined layer with the function of reinforcement and separation (A+P). Construction work was carried out by JSC VAD.
One of the memorable objects of 2011 is the reconstruction of the Sayanogorsk - Mainskaya HPP - Cheryomushki highway, which ensured the unhindered and reliable transportation of heavy loads (up to 300 tons) for the restoration of the Sayano-Shushenskaya HPP technological equipment after an accident. The designer considered dozens of pavement design options using innovative materials.
As a result of the discussion, to increase the bearing capacity at the border of the subgrade and the layer of crushed stone mixture, a protective and reinforcing layer of this geotextile Geospan tn-50 was used. After the launch of the Sayano-Shushenskaya HPP and three years of operation of the road, monitoring of the pavement condition was carried out, which confirmed the correctness of the chosen solution.
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