RU2317372C1 - Method to estimate condition of cohesive soil of foundation base and building structure body - Google Patents

Abstract

FIELD: construction, particularly to improve weak base of cohesive ground or to substitute weak ground for solid material and to estimate condition of ground structures under exploitation.

SUBSTANCE: method involves taking ground samples; determining ground density and humidity in natural state, ground particle density, liquid limit density and humidity, as well as density and humidity under different loads from mathematical formulae; dividing underground structure base and body into several layers having the same characteristics; calculating ground weight pressure in each layer; performing ground sample compression when ground sample humidity is close to liquid limit; determining finite dry ground density as a function of pressure for each layer from obtained results and determining base ground and structure body condition from relation between natural and equilibrium density and humidity values. In normally compacted state natural density and humidity reach equilibrium value, that means that ground compaction process is completed. In the case of insufficient ground compaction natural density is less than equilibrium value and natural humidity exceeds equilibrium value, which indicates ground compaction process continuation. In the case of excessive ground compaction natural pressure exceeds equilibrium value and humidity is less than equilibrium value, i.e. as ground is subjected to water action ground may be deformed because of ground swelling.

EFFECT: increased accuracy of equilibrium density and humidity determination with taking into consideration loads caused by ground layer weight.

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Description

The invention relates to the field of construction, in particular to the selection of methods for improving a natural weak base from cohesive soils or for deciding to replace them with a stronger material, as well as when assessing the condition of operating soil structures, for example dams, dams, embankments, etc.

To assess the novelty and inventive step of the claimed solution, we consider a number of well-known technical means of a similar purpose.

Known method of estimating the state of cohesive soils consistency, characterized by melt index I _L, which determines the ratio of the difference humidities, corresponding to the two ground states: natural and W at the boundary rolling W _p, among the plasticity _p I by the formula:

where I _L - flow rate, d.ed; W - natural humidity, d.ed; W _p - rolling border, d.ed; I _p - the number of plasticity, units According to the consistency index, cohesive soils are divided into solid, semi-hard, refractory, soft-plastic, fluid-plastic and fluid, sandy loam - solid, plastic and fluid (GOST 25100-95. Soils. Classification - M .: MNTKS, 1996, p.22).

The main disadvantage of the analogue method is the conditional nature, which allows roughly characterizing and evaluating the physical state of clay soils, because determination of the values included in the formula for calculating the yield index is carried out on samples of clay soils of disturbed composition. In addition, the magnitude of the current load from the structure is not taken into account.

A known field method of dynamic and static sounding, based on determining the resistance of the soil to the introduction of a probe in it, which allows you to establish a section of loose deposits, quantitatively or qualitatively evaluate the engineering-geological properties of rocks and their variability in space (Methodological manual on engineering-geological study of rocks. Volume 11. - M., MSTU, 1968, p. 330-346). The sounding results are used to develop classifications for the consistency of cohesive soils, to determine the bearing capacity of piles based on the semi-quantitative characteristics of the physical properties of cohesive soils.

The disadvantages of the analogue method are the insufficient accuracy of determining the conditional resistance of the soil during dynamic sensing due to friction of the rods against the ground, bending of the rods upon impact, changes in the ratio of the weight of the probe and the anvil, which causes a significant scatter in the data when comparing them with indicators of physical and mechanical properties and the establishment of correlations, the great labor involved in installing the tower in a strictly vertical position to avoid skewing and jamming of the rod in the well.

Closest to the claimed combination of essential features is the method adopted by us for the determination of the technological characteristics of cohesive soils during compaction by mechanisms, including the determination of moisture at the yield point and optimal soil moisture, its particle size distribution and, based on the results, the content and density of fine particles less than 2 mm in size , the density of the sample with moisture at the yield strength, technological characteristics according to the formulas:

- maximum density of the fine fraction

- optimum humidity

where ρ _dmax is the maximum density of dry soil of fine fractions for the selected mechanism, g / cm ³ ;

ρ _s is the density of particles of fine fractions, g / cm ³ ;

ρ _dL is the density of dry soil in a sample of fine fractions with humidity at the yield strength W _L , g / cm ³ ;

σ is the specific sealing pressure under the working body of the mechanism, MPa;

W _opt - optimal moisture content of fine fractions for the selected mechanism, d.ed .;

S _r is the degree of soil moisture, taken equal when compaction with rollers S _r = 0,85-0,90; for ramming plates and vibro-shock vibratory rollers S _r = 0.90-0.95;

ρ _w is the density of water equal to 1.0 g / cm ³

(RF patent No. 2186174, IPC E02D 1/02, publ. 07.27.2002).

The main disadvantage of the prototype method is its limited use for homogeneous soils, compacted by mechanisms, when the soil reaches a homogeneous composition during the development of natural deposits in the quarry and subsequent mixing during filling and leveling before compaction by mechanisms.

The invention solves the problem of expanding the use of assessing the state of the foundations and the body of a structure from cohesive soils with a layered texture, increasing the accuracy of determining the equilibrium density and humidity, taking into account the existing loads from the dead weight of the soil layers.

To achieve the named technical result in the proposed method, which includes taking a soil sample, determining the density and humidity in the natural state, the density of soil particles, density and humidity at the yield point, density and humidity at various loads according to the formulas:

- density

- for soils with a layered texture, depending on pressures equal to the vertical at the base of the layer,

- humidity

where ρ _dmax is the maximum density of dry soil for each step pressure σ, g / cm ³ ;

ρ _s is the density of soil particles, g / cm ³ ;

ρ _dL is the density of dry soil in the sample at humidity at the yield strength W _L , g / cm ³ ;

σ is the specific pressure at the base of the layer, MPa;

W _opt - optimum soil moisture for each pressure stage, d.ed .;

S _r is the degree of soil moisture, taken equal to 1.0;

ρ _w is the density of water equal to 1.0 g / cm ³ .

Additionally, the depth of the foundation and the body of the structure are divided into layers with uniform characteristics, the pressure from the own weight of the soil of each selected layer is calculated, the compression of the soil sample is carried out at a moisture content close to the yield point, the dependence of the final density of dry soil on pressure for each is established layer and determine the soil condition of the base and body of the structure by the ratio of natural and equilibrium values of density and humidity: when normally compacted with state - the natural density and humidity reach an equilibrium value, which means the completion of the soil compaction process, when the state is under-compacted, the natural density is less, and the humidity is more than the equilibrium values, which indicates the continuation of the soil compaction process, when the state is densified, the natural density is more and the humidity is less equilibrium values, i.e. swelling deformations are possible upon contact of soils with water, for example, when filling a reservoir, raising groundwater levels, etc. Moreover, for homogeneous soils when determining the equilibrium density ρ _dσ = ρ _dmax at pressures equal to the vertical at the base of the layer, and when determining the equilibrium humidity W _σ = W _opt for S _r = 1.0.

In addition, the proposed technical solution has an optional feature characterizing its particular case: when determining the equilibrium densities ρ _dσ and humidity W _{σ, the} additional pressure to the weight of the overlying layers of soil is taken into account, transmitted to the base and body of the structure, for example, from an automobile / or railway transport, as well as from existing buildings, warehouses of materials, etc.

The advantage of the invention in comparison with the prototype consists of the possibility of its use in assessing the state of natural bases and the body of the structure from cohesive soils with a layered texture.

Distinctive features of this method are the determination of the pressure from the own weight of the soil of each selected layer, the compression compression of the soil sample at a moisture close to the yield point, the dependence of the final density of dry soil on pressure for each layer, and the determination of the condition of the soil of the base and body of the structure by the ratio of natural and equilibrium density and humidity.

Numerous experiments with cohesive soils of different particle size and mineralogical composition have established that the equilibrium density is ρ _dσ = ρ _dmax and depends on the value of the acting pressure σ and does not depend on how it is applied cyclically, dynamically during compaction by mechanisms, or static under compression. The optimum moisture content at which the maximum density depends on the pressure application process in the formula (2) is determined during compaction rollers with S _r = 0,85-0,90, for tamping plates under vibro compaction S _r = 0,90-0 , 95, and with compression compaction in the case of determining the equilibrium humidity S _r = 1,0 (Recommendations for laboratory determination of the maximum density of cohesive soils in relation to compaction by rollers. P 50-90 / VNIIG. - L .: VNIIG named after B.E. Vedeneeva, 1974, v.105, p.102-115).

On this basis, formulas (1) and (2) are used to determine the values of the maximum equilibrium density and moisture of soils during static compression compaction under the influence of their own weight of the layers of overlying soil.

The proposed method is illustrated by graphs: on figa and b shows a comparison of the weighted average values of humidity W (a) and density ρ _d (b) of the soil of the base in the gap and the equilibrium values for loads from its own weight; on figa and b is a graph of changes in moisture content W and density ρ _d loam in depth in the body of the dam.

The method is as follows.

Samples of soil of undisturbed composition are taken, the density ρ _d and moisture W in the natural state, the density of soil particles ρ _s , and the soil moisture at the yield strength W _{L are} determined. Then, compression compression of the sample is carried out at a moisture close to the yield point, measuring the precipitation of the sample for each pressure stage, and the results establish the dependence of the final equilibrium density of dry soil on the pressure for each stage, and the equilibrium humidity is determined by the formula (2), taking the degree humidity S _r = 1,0. For homogeneous soils (not a layered texture), the equilibrium density can be determined by the formula (1).

The base and body of the structure are divided in depth into layers with uniform characteristics and determine the pressure from the own weight of the soil at the base of each selected layer.

The soil condition of the base and body of the structure is determined by comparing the natural and equilibrium values of density and humidity, and the following types are distinguished:

- normally compacted - natural density and humidity have reached equilibrium values, which means the completion of the compaction of soils;

- underconsolidated - the natural density is less, and the humidity is more than the equilibrium values, which indicates the continuation of the compaction of the soil;

- overconsolidated - the natural density is greater and the humidity is less than the equilibrium values, i.e. swelling deformations are possible upon contact of soil with water, for example, when filling a reservoir, raising the level of groundwater, etc.

If necessary, take into account the additional pressure to the weight of the overlying layers of soil transmitted to the base and body of the dam from a motor vehicle and / or railway transport passing along the crest, buildings, warehouses of materials, etc.

This is a set of essential features that provide a technical result for all cases to which the present invention applies.

Example 1. It is necessary to determine the condition of soft soils in the basement of the flood protection complex in St. Petersburg.

Soil samples are taken from boreholes after 1 m in depth, particle density, density and moisture in the natural state are determined, and the weighted average values are calculated from the results obtained. Based on the materials of engineering and geological surveys in this section, the following characteristic layers are distinguished: bulk sand with a thickness of 2.7 m, tape dusty clays (layer 25 d), characterized by the alternation of horizontal thin interlayers of fine clay and fine sand, layer thickness 8.0 m and non-layered loams (layer 26), the explored thickness of the layer is 3.8 m. The moisture values at the yield strength W _{L are} determined.

According to SNiP 2.02.01-83 ^* "Foundations of buildings and structures" determine the pressure in the base of the selected layers of base soil. The results are shown in table 1.

Next, compression compression of soil samples of the selected layers is performed at the following pressure levels σ: 0.01; 0.02; 0.03; 0.05; 0.10; 0.15; 0.20 and 0.25 MPa. Determine the equilibrium density ρ _dσ for each load stage after stabilization of the deformations. The equilibrium moisture content for each load stage is determined by the formula (2) with the degree of soil moisture S _r = 1,0. The results are shown in table 2.

According to the graph in figure 1, which shows the changes in natural, weighted average and equilibrium values of the density and humidity of the selected layers in depth, taking into account the existing pressures from the own weight of the soil, it was found that the natural density is less and the humidity is more than the equilibrium values in the soil of the base, which allows you to determine their condition as uncompressed. In this regard, when completing the dam, it is necessary to take measures to strengthen the foundation, either remove the weak layer or fill the dam with the method of squeezing a weak soil layer with strict adjustment of the work rate in accordance with the increase in pore pressure and sedimentation in the soil of the base, providing for breaks in the filling of the soil to reduce pore pressure to a safe level.

Example 2. During the construction of a dam from boggy loam (layer 38), it was poured into water 5 m deep. There was a fear that a liquefied layer would form in the lower layer of the dam. And 18 years after construction, wells with sampling after 1 m were drilled from the dam ridge and the natural values of density, humidity, density of soil particles ρ _s = 2.72 g / cm ³ , humidity at the yield point W _L = 0.22 were determined as well as the equilibrium density values according to formula (1) and humidity according to formula (2) at S _r = 1.0 for fine earth (fractions less than 2 mm in size) and when 15% of large fractions in the soil (larger than 2 mm) are contained. The test results are shown in table 3 and in the graphs of figure 2.

In this case, the natural density is greater, and the humidity is less than the equilibrium values, which allows us to determine the overconsolidated state of the soil of the body of the dumped dam. Given the lack of ability of soils to swell, the dam body is assessed as stable, dense and monolithic.

Table 3 Equilibrium values of density ρ _dσ and humidity W _σ at various pressure levels calculated according to formulas (1) and (2) for fine earth and when the content of large fractions is 15% in the soil The content of large fractions P,% σ, MPa 0 0.01 0.02 0,03 0.05 0.10 0.15 0.20 0.25 0.30 one 2 3 four 5 6 7 8 9 10 eleven 12 0 (fine earth d <2 mm) ρ _dσ , g / cm ³ 1,61 1,64 1,67 1,69 1.72 1.76 1.78 1.80 1.81 1.82 W _σ 0.253 0.242 0.231 0.224 0.214 0.201 0.194 0.188 0.185 0.182 fifteen% ρ _dσ , g / cm ³ 1.71 1.74 1.77 1.79 1.82 1.86 1.88 1.90 1.91 1.92 W _σ 0.223 0.213 0.204 0.198 0.189 0.178 0.172 0.167 0.165 0.162

The proposed method for assessing the state of cohesive soils of the base and body of the structure allows to reduce the complexity of the process, to expand the scope for bases of layered soils, to increase the accuracy of determining the equilibrium values of density and humidity, taking into account the existing loads from the dead weight of the soil layers.

Claims (2)

1. A method for assessing the state of cohesive soils of the foundation and body of a structure, including taking a soil sample, determining the density and moisture in the natural state, the density of soil particles, density and humidity at the yield point, density and humidity at various loads using the formulas density

for soils with a layered texture, depending on pressures equal to the vertical at the base of the layer, humidity

where ρ _{d max} - the maximum density of dry soil for each pressure step σ, g / cm ³ ; ρ _s is the density of soil particles, g / cm ³ ; ρ _dL is the density of dry soil in the sample at humidity at the yield strength W _L , g / cm ³ ; σ is the specific pressure at the base of the layer, MPa; W _opt - optimum soil moisture for each pressure stage, d.e .; S _r is the degree of soil moisture, taken equal to 1.0; ρ _w is the density of water equal to 1.0 g / cm ³ , characterized in that, in addition, the base and body of the structure are further divided into layers with uniform characteristics, the pressure from the own weight of the soil of each selected layer is calculated, the compression of the soil sample is carried out at a humidity close to the yield point, the dependence of the final density of dry soil on pressure for each layer and determine the condition of the soil of the base and body of the structure according to the ratio of natural and equilibrium values of density and humidity: at in a densely compacted state — natural density and humidity reach an equilibrium value, which means the completion of the soil compaction process, in an underconsolidated state — the natural density is less, and moisture is greater than the equilibrium values, which indicates the continuation of the soil compaction process, in a re-compacted state — the natural density is greater, and humidity is less than equilibrium values, i.e. swelling deformations are possible upon contact of soils with water, for example, when filling a reservoir, raising the level of groundwater, and for homogeneous soils when determining the equilibrium density ρ _dmax = ρ _dσ at pressures equal to the vertical at the base of the layer, and when determining the equilibrium humidity W _opt = W _σ at S _r = 1.0. 2. The method for assessing the state of cohesive soils of the base and body of a structure according to claim 1, characterized in that when determining the equilibrium density ρ _dσ and humidity W _{σ, the} additional pressure to the weight of the overlying layers of the soil is transferred to the base and body of the structure, for example, from passing along the crest of the dam of automobile and / or railway transport, as well as from existing buildings, warehouses of materials.

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