Determination of base sediment. Causes of Uneven Precipitation
Precipitation is calculated on the main combination of loads, while the temporary train load is not taken into account, because precipitation over the period of its action does not have time to develop. The main calculation conditions:
S S u ……………………………………………………………………
where S is the calculated value of the settlement of the base of the support, cm;
S u - limit (permissible) value of the vertical displacement of the support, cm, determined by the empirical dependence
S u \u003d 1.5, cm ..................... .. ()
where L is the length of the smaller span adjacent to the support, m; received at least 25 m.
Calculation of base sediment is carried out according to the linearly deformable half-space scheme with the condition of limiting the depth of the compressible thickness H c in accordance with paragraph 6 of mandatory Appendix 2 to SNiP by the method of layer-by-layer summation / adj. 5 /,. Using the formula:
S \u003d 0.8 
h i / E i, ………………………………….
where zp \u003d ( zpi + zpi-1) / 2 is the average value of the additional vertical normal stress in the i-th soil layer along the vertical axis passing through the center of the base of the foundation;
h i and E i - respectively, the thickness and deformation modulus of the i-th soil layer;
n is the number of layers into which the compressible thickness of the base is divided.
The decomposition of the compressible stratum into design layers is carried out from ensuring the following conditions:
The boundaries of the layers coincide with the boundaries of lithological differences and with the level of groundwater;
The thickness of the calculated i - th layer is h i 0.4b.
Calculation of draft is carried out in tabular form. The vertical component of the load, reduced to the center of gravity of the base of the foundation (excluding the load from the weight of the train):
……………………… .N II \u003d P 0 + P p + P f + P v + P g
The average pressure on the bottom of the foundation from the normative constant loads:
We determine the values \u200b\u200bof vertical stresses from the own weight of the soil at the level of the basement of the foundation and at the border of each calculated i-th layer:
σ zg \u003d σ zgo + ∑γ i * h i;
where: σ zgo is the vertical stress from the dead weight of the soil at the level of the base of the foundation;
γ i - specific gravity soil of the i-th layer;
σ z p o \u003d Р- σ zgo
The value of the voltage dissipation coefficient is taken from
Table / adj. 5 / depending on \u003d 2Z / b and n \u003d l / b \u003d 2.7; where Z is the depth of the upper boundary of the layer from the bottom of the foundation.
The ordinate values \u200b\u200bof the plot of the distribution of additional vertical stresses in the soil:
σ zpi \u003d αi * σ zp 0;
On the scheme of calculating the foundation sediment, the following are plotted: plot of stresses in the soil from its own weight - z g; auxiliary plot
0.2 zg and plot of sealing stresses - zр \u003d P о;
The position of the GTS can be defined graphically as the intersection point of the plots 0.2 zg and zр
The settlement of each CALCULATED soil layer is calculated by the formula
S \u003d β * (σ zpi cf * h i / E i);
where: σ zpi cf is the average additional vertical stress in the i-th soil layer, equal to the half-sum of the indicated stresses at the upper and lower boundaries of the layer.
β \u003d 0.8 - dimensionless coefficient for all types of soils
→ Foundations
Causes of Uneven Precipitation
A variety of reasons affect the occurrence of uneven deposits.
1. Uneven settlement precipitation.
Under the action of additional stresses transmitted to the base through the foundations, the soil is deformed under the condition ’if these stresses exceed the stresses of the own weight of the soil. Soil deformations mainly develop due to a decrease in the soil pore volume resulting from compaction.
As already noted, residual deformations are much more elastic, therefore, base sediments occurring under the influence of external loads are usually called compaction sediments.
Fig. 3.2. The reasons for the formation of uneven compaction sediments:
1 - load on the foundation; 2 - sediment slowly deforming base; 3 - sediment quickly deformed (sandy) base; 4 - diagrams of the distribution of vertical stresses; 5 - the boundaries of the stress zones of the base; 6 - settlement of the first foundation in time; 7 - load on the second foundation; 8 - settlement of the second foundation in time
Compaction sediments in separate areas of the base under the structure are usually uneven (uneven) due to the heterogeneity of soil conditions and the heterogeneity of the stress state in the base.
The heterogeneity of the base is due to the following factors: the presence of pinch out layers (Fig. 3.2, a); lenticular bedding of individual soil layers (Fig. 3.2, b); different thicknesses of the layers (Fig. 3.2, c); uneven density of soil compaction (Fig. 3.2, d); transferring pressure from parts of the structure of various masses to the base soils of different physical and mechanical properties (Fig. 3.2, e); uneven compaction of soils over time, due to different rates of consolidation and creep processes under different parts of the structure (Fig. 3.2, f).
The heterogeneity of the stress state of the soils at the base is formed due to the following reasons: uneven loading of the foundations, in connection with which a more loaded foundation must be done with large sizes soles, which leads to a different stress-strain state in the base and different settlement of foundations (Fig. 3.2, g); the influence of the load of neighboring foundations, that is, the foundations located in the middle part of the structure, and less in the extreme or corner part, experience a greater impact from mutual influence (Fig. 3.2, h); non-simultaneous loading of neighboring foundations in the process; construction and operation (Fig. 3.2, and).
The occurrence of uneven precipitation during the construction process is determined by the different weight of structures that transfer the load to the foundations, and depends on the methods of construction and installation. For example, column sediments during the construction of buildings with an incomplete frame usually lag behind sediments bearing walls, since the latter receive a large load during the construction, and the columns receive most of the load only after the installation of ceilings, wall panels, partitions and equipment installation.
Non-uniform precipitation, which manifests itself in the process of building construction, can be eliminated during operation due to the equalization of precipitation.
2. Uneven precipitation decompression.
They develop when the load from the weight of the building or structure is less than the weight of the soil extracted during the excavation. This is due to the fact that when removing soil during excavation of the pit, soil decompression occurs under its surface as a result of removal of previously existing stresses from the own weight of the soil (soil swelling during unloading). With deep pits, residual plastic deformations from the pressure of the soil located around the bottom of the pit also influence. The development of soil softening is affected by the impact and deformation of the elastic aftereffect.
The presence of these factors leads to a rise in the bottom of the pit (Fig. 3.3) with the subsequent development of uneven sediments resulting from a more significant softening of the soil in the center of the pit than along the edges; different course of decompression process in time under separate foundations; uneven elevation of the bottom of the pit as a result of heterogeneity of the soil of the base.
Fig. 3.3. Raising the bottom of the pit as a result of decompression of the soil
During the construction of urban buildings and structures, the depth of the pit rarely exceeds 5 m, and it is known from construction practice that in this case, decompression precipitations are insignificant and occur mainly in the process of building foundations before the construction of elevated structures. Uncondensed precipitation can have a detrimental effect on a building if the depth of the excavation pit developed for it exceeds 5 m, and the load from the weight of the structure, together with backfill, is significantly less than the soil excavated from the excavation.
To determine the ability of the soil to experience softening sediments, soil samples are tested not only for compression, but also for softening with decreasing pressure. Softening sediments are determined by soil mechanics.
3. Precipitation bulging.
As a result of the development of zones of plastic deformations in the basement soils, bulging sediments occur under the soles of the foundations. As a result of the uneven distribution of pressure under the sole of the foundation, even with small loads, zones of plastic deformations form under its edges. An increase in the load causes a redistribution of contact pressures along the bottom of the foundation, which is explained by a greater flexibility of the soil in the plastic deformation zones, as a result of which the pressure in these areas decreases, and in the middle part of the foundation increases due to the less flexibility of the soil. With a further increase in the external load, the zones of plastic deformations increase and there is a risk of soil bulging out from under the sole.
4. Uneven rainfall When carrying out preliminary measures preceding the construction of foundations (excerpts of foundation pits, etc.), the foundation soils, being exposed, are exposed to various factors that can cause a violation of the natural structure of the soil.
Precipitation due to destructuring of soils, as a rule, is uneven, since destructuring occurs with varying degrees of intensity in different areas of the base. Precipitation depends on many factors: the method of performing excavation work, the duration of the work from the beginning of excavation to the backfilling of the sinuses of the foundations, the nature of the drainage, as well as measures aimed at preserving the natural structure of the soil. Violation of the structure usually occurs as a result of the following reasons: meteorological effects, dynamic effects of mechanisms; adverse effects of groundwater and gas; blunders of builders.
Meteorological impacts cause a violation of the natural structure of soils as a result of freezing or thawing, softening and swelling, drying and shrinkage.
Under the influence of negative temperatures, the soil of the bases can freeze to a depth, which is determined according to the Construction Norms and Rules. During freezing, a substantial increase in the volume of highly moistened silty-clayey and watery silty and fine sandy soils as a result of the development of frost heaving forces is possible. When heaving in the foundations, significant internal stresses develop, which in some cases can exceed the stresses from the external load under the sole of the foundations of buildings and structures and lead to significant vertical deformations. Puching can have an adverse effect not only during fragments of the foundation pit and foundation, but also during the construction of the building and its subsequent operation. It is not possible to completely avoid the harmful effects of frost heaving forces, even if the sole of the foundation is placed below the freezing zone, since in this case tangential forces of frost heaving develop along the lateral surface of the foundation. In the presence of a heated basement as a result of the development of lateral freezing of the soil, horizontal displacement and a roll of the foundation are possible.
To reduce the negative influence of the heaving forces, they cover the lateral surfaces of the foundations with bitumen dissolved in fuel oil or solar oil, and backfill the sinuses with non-porous materials (sand is usually used).
In the case of thawing of frozen soil, it is able to experience subsidence, which can be even more dangerous for buildings and structures. The process of thawing the soil proceeds non-uniformly; on the south side of the building, thawing occurs more intensively than on the north side, the base sections under the outer parts of the building more quickly thaw and slower under the inside. Since thawing is accompanied by a sharp violation of the structure, the soil acquires greater compressibility, therefore, it is not allowed to freeze the soil below the bottom of the pit, even if the soil is thawed before the foundation is installed.
Softening and swelling of soils occur when some types of dusty-clay soils below the bottom of the pit are moistened as a result of atmospheric precipitation (Fig. 3.5, c). This process is especially intense in sandy loam, silty loam, layered and fractured clays. The ability to experience swelling deformations increases with increasing clay content. Clay soils with pores filled with air and communicating with the atmosphere are most susceptible to softening. As a result of swelling and softening, the physical and mechanical properties of soils deteriorate, which is accompanied by uneven precipitation. In order to preserve the natural structure of soils, in this case they resort to artificial removal of surface water from the construction zone, and the lower soil layer to be developed is left as a protective layer and it is removed from the foundation pit only immediately before the foundation works.
Fig. 3.5. Violation of the structure of the soil during meteorological impacts:
1 - atmospheric moisture; 2 - call softening in shrinkage;
In some areas with a hot climate, dusty clay soils lying below the bottom of the pit, under the influence of the intensive drying process as a result of solar insolation, can decrease in volume, experiencing shrinkage (Fig. 3.5, b). When the original moisture is restored, the soil that has shrunk will experience swelling, causing the foundations to rise.
Adverse effects of groundwater and gas disrupt the soil structure as a result of the influence of hydrostatic pressure on the weight of a column of water, hydrodynamic pressure on its movement, mechanical and chemical suffusion, filtration, expansion and evolution of gas dissolved in water.
Deformations and even destruction of the layer of waterproof soil remaining in the pit occur when the hydrostatic pressure in the water-permeable layer of the underlying soil exceeds the stresses of the remaining part of the layer of waterproof soil in the pit (Fig. 3.6, a). This phenomenon occurs especially intensively with a layered texture of soils, when water permeability along the layering is tens of times greater than across (Fig. 3.6, b). To reduce hydrostatic pressure, water reduction is used.
When water moves through the permeable layer of the rune, hydrodynamic pressure is formed (Fig. 3.6, c), due to the strength of the hydraulic effect of the filtration stream, which, acting on the soil particles, causes it to swell. To reduce hydrodynamic pressure, they resort to water reduction or arrange sheet piling around the foundation pit, immersed to layers of relatively waterproof soil.
Mechanical suffusion refers to the movement under the influence of a filtration flow of smaller particles of soil through pores formed by larger particles, which leads to an increase in porosity and permeability.
Filtration head is the movement of soils upward under the influence of filtration forces.
Mechanical suffusion and filtering support lead to the removal of particles to the surface of the creep or the bottom of the pit, forming cones of removal with a diameter measured in tens of meters and a height of tens of centimeters (Fig. 3.6, d).
Fig. 3.6. Structuring of soils under the influence of groundwater
Chemical suffusion refers to the process of dissolution of mineral aggregates of the skeleton of the soil, causing in some cases a sharp deterioration in the physical and mechanical properties of the bases.
During drainage in water-saturated soils, the hydrostatic pressure in the pore water decreases, as a result of which the volume of closed gas bubbles in the water increases, and, as a result of the pressure drop, part of the dissolved gases begins to be released from the water. These factors cause a violation of the natural structure of weakly filtering soils, such as silt, sandy loam and loam.
Dynamic effects of mechanisms often cause a significant violation of the natural structure of the soil base. This phenomenon is particularly susceptible to water-saturated silty sands. Violation of the soil structure is possible as a result of using earthworks shock mechanisms (wedge-women, rammers, piling hammers, etc.). To preserve the natural structure of soils, their development is carried out by light mechanisms moving along the edge of the pit, at the bottom of which a protective layer is often left, which is subsequently removed using light earth-moving machines.
The most typical mistakes of builders leading to disruption of the soil structure include: soil digging during excavation and its improper re-laying, excavation of foundation ditches without their immediate use for the construction of foundations, development of deep foundation ditches near already constructed buildings with a lower foundation laying depth, penetration in the pits of industrial or household water.
Uneven settlement formed as a result of soil structuring is very difficult to predict using calculation methods, therefore, the main requirement put forward in the construction of foundations is to maximize the preservation of the natural structure of the soil of the base.
5. Uneven precipitation formed during the operation of structures. They can be divided into five types.
Soil compaction of foundations after the start of operation of structures. This compaction occurs as a result of the continuation of the process of consolidation and creep deformations of the soil.
The precipitation of buildings and structures located on foundations with a predominance of dusty clay soils sometimes continues for several decades, with a tendency to gradual attenuation. In sandy foundations, most precipitation occurs during the construction period and in the first months of operation. Additional precipitation during the operation of buildings should be taken into account when designing structures.
In the construction of storage facilities, where the payload has a major impact on the functioning of the building, since its share may be significantly higher than the weight of the building itself (capacity of metal and reinforced concrete tanks, elevators, silos, etc.), in some cases with dusty clay bases in the first year of operation allowed the application of not more than 50% of the payload. This limits the rapid increase in precipitation and reduces the harmful effects due to the sharp development of its unevenness.
Groundwater level change. Significant lowering of the groundwater level in some cases can have a detrimental effect on the operating building, expressed in the manifestation of uneven precipitation due to the formation of additional stresses in the soil of the bases as a result of an increase in its own weight due to the removal of the weighing effect of water.
An increase in the level of subterranean water can reduce the strength of soils, since as a result of wetting, the adhesion forces between soil particles are reduced. Bases capable of experiencing the phenomenon of swelling, increasing in volume during wetting caused by rising groundwater levels, will lead to additional uneven precipitation of the foundations. Loess soils as a result of wetting can receive significant subsidence. The rise in groundwater levels most often occurs as a result of the penetration of atmospheric moisture into the ground, as well as household and industrial waters. Rising groundwater above the base of the foundation can cause corrosion of the reinforcement. This phenomenon becomes especially dangerous when it is possible to form an aggressive environment in water. In the absence of proper waterproofing in the basement, water, penetrating into the basement, requires pumping, which can cause mechanical suffusion in the soil of the base.
Uneven precipitation can become emergency as a result of soil erosion caused by the breakthrough of pressure head water mains. In some cases, underground water along with soil can enter into faulty sewer collectors, sometimes with significant pressure, as a result of which a discharge funnel is formed, within which the soil receives significant displacements. Floating soils are particularly susceptible to this phenomenon, therefore, when erecting buildings and structures on similar types of soils near pressure pipelines and deep collectors, it is necessary to place foundations outside the limits of a possible soil removal and erosion funnel.
The weakening of the underground and excavation pits. The construction of foundations in modern urban development is complicated by the additional weakening of soils as a result of the construction of subway lines during tunneling, the construction of sewer collectors and other underground workings, which causes subsidence of the soil along with buildings and structures located on its surface within the so-called settlement trough (Fig. 3.7).
Fig. 3.7. Underground mining trough
When developing deep pits near existing buildings and structures, which often require urban construction conditions, it is necessary to exclude possible horizontal displacements and shifts of the foundation soils together with previously constructed buildings, which is achieved by means of special fastening of the walls of trenches and pits.
Dynamic effects on the soil base. During the operation of buildings, vibration from industrial or other equipment located inside the building can cause compaction of sandy or poorly connected dusty clay soils. A certain level of vibrations is able to increase the deformability of soils due to the manifestation of the vibration creep process. The work of the forging press and punching equipment, the movement of vehicles, the driving of dowels, piles and other construction work related to dynamic, including shock loads, intensifies additional draft. They also have a detrimental effect on mine development by explosions and seismic activity.
The activity of geological processes. The development of karst cavities of landslides and earthquakes causes significant non-uniform precipitation, which in some cases entails the complete destruction of structures. Prediction of geological processes is carried out using engineering geology methods and is presented in the corresponding course.
The factors listed above complicate the task of designing the foundations of buildings and structures, provided that the requirements for uniformity of precipitation are met.
Building sagging is the displacement of a building caused by the compression of soil in the base under the building. This is a normal process. It is important that the building sludge runs evenly throughout the base. For this, it is necessary to calculate it at the design stage.
Subsidence subsidence - rapid non-uniform vertical deformation associated with the soaking of subsidence soils (for example, loesses) in the base or with thawing of the frozen soil thickness.
Reasons for the development of uneven precipitation:
1. Wedging out of individual soil layers within the contour of the building
2. Lenticular occurrence of certain types of soil
3. Unequal thickness of the soil layers lying in the base
4. Unequal soil density or uneven distribution in the soil of various inclusions (peat, etc.)
5. Unequal loads on individual foundations and different sizes of foundations with equal contact pressures
6. Unequal influence of neighboring foundations on the settlement of foundations in the middle and extreme parts of the structure
7. The simultaneous loading of foundations during the construction of the structure
8. Loading individual foundations with a load less than design
The impact of meteorological factors:
9. Freezing and thawing of soil at the base when building foundations and building construction;
10. Swelling and softening of the soil in the bases when moistened with atmospheric water;
11. Drying of the soil in the base under the influence of solar radiation and wind.
Groundwater exposure:
12. destruction of soil layers by hydrostatic pressure;
13. The destruction of the soil as a result of hydrodynamic effects;
14. Suffusion of soil by the flow of groundwater into the pit or pits.
Dynamic effect on water-saturated, very porous dusty and clay soils:
15. When moving mechanisms along the bottom of the pit;
16. When the earth-moving machinery hits the ground (for example, when developing hard or frozen ground);
17. When performing blasting operations near the construction.
2. Determination of precipitation of the base by the method of layer-by-layer summation.
The essence of the method is to determine the sediment of the elementary layers of the base within the compressible thickness of the additional vertical stresses σZP arising from the loads transferred to the structures.
Since this method is based on a calculation model of the base in the form of a linearly deformable continuous medium, it is necessary to limit the average pressure on the base to a limit at which the areas of plastic deformations only slightly affect the linear deformability of the base, i.e. it is required to satisfy the condition:
To determine the depth of the compressible thickness Нс, stresses from the dead weight σZq and additional stresses σZP from the external load are calculated.
The lower boundary of the compressible thickness of the base aircraft is taken at a depth of z \u003d Hs from the bottom of the foundation, where the condition is satisfied:
those. additional stresses make up 20% of the own weight of the soil.
In the presence of the following soil depth with a deformation modulus of E≤5 MPa, the condition must be met:
Calculation of precipitation is conveniently carried out using graphical constructions in the following sequence:
1) build a geological section of the construction site in place of the calculated foundation;
2) the dimensions of the foundation are applied;
3) stress diagrams are constructed from the self-weight of the soil σZg and additional σZP from the external load;
4) the compressible thickness Hs is determined;
5) Hc is divided into layers of thickness hi≤0.4b;
6) determined by the settlement of the elementary layer of soil according to the formula:
Then the full precipitation can be found by simple summation of the sediment of all elementary layers within the compressible stratum from the expression:
where β is a dimensionless coefficient, depending on the coefficient of relative transverse deformations, taken equal to 0.8; hi is the height of the i-th layer; Ei is the deformation modulus of the i-th soil layer;
is the average voltage of the ith elementary layer.
The method of layer-by-layer summation allows determining the draft not only of the central point of the base of the foundation. Using it, you can calculate the draft of any point within or outside the foundation. To do this, use the method of corner points and plot the stresses of a vertical one passing through a point that requires settlement calculation.
Thus, the method of layer-by-layer summation is mainly used in the calculation of small foundations of buildings and structures and in the absence of very dense low-compressible soils at the base of the layers.
