Topic 13. Critical Loads to the ground. Initial critical load on the soil. Maximum load for loose and cohesive soils. The influence of soil properties, foundation dimensions and laying depth on the magnitude of the ultimate load of soil foundations.
Critical soil loads.
Phases of soil stress state
The concept of the safety factor for slopes and slopes
All models of general interest can be modeled, such as elastic soil beams, foundation walls, plateaus, palfitates, plinths. Materials, general, isotropic and orthotropic can be taken from the standard archive. Flexural and cutting stiffness can be reduced, even in a differentiated manner for various elements, also in relation to their stressful conditions.
D.; It is also possible to set the most common communication modes. It is also possible to process general sections consisting of several materials, which are determined by a specific drawing procedure and static property calculation, capable of recognizing and interpreting even in this case the external design. The structural model and its properties can be intervened at any time using general selection operations to change geometric properties, properties or loads. Each action is aided by a preview that anticipates the user's effect before final confirmation.
When loads are applied to the soil, compaction processes begin to occur, which can be divided into 3 phases:
1 Compaction phase. The work of the soil during this phase occurs under conditions of ensured strength. The soil is compacted to some extent A.
2 Phase of local shifts. In the edge zones there is a local violation of the strength of the soil. With further increase) at the boundary between phases 1 and 2 R z = R pr called limit.
The graphical environment works in continuous three-dimensional modeling with the removal of hidden surfaces; Obviously, a single image view is also available. Copy, translation, mirror tools allow you to quickly build the entire model and can work with any coordinate system, even local, in cylindrical or spherical Cartesian coordinates.
Critical loads on foundation soils. Phases of the stressed state of soil foundations
You can freely import structural parts designed into individual models. Other useful features are also available for proper modeling. Rigid planes, even on secured areas of the floor and even inclined slopes, allow you to model solo, deputies, in a seismic environment, distribute actions between vertical carriers. Other useful relationships are a rigid body link, which can combine two bodies that are not separated in the simulation but are combined in a structural behavior or connection, which instead forces the nodes to accept the same deformations for their respective directions.
3 Bulging phase. If the increase in load continues, then when it reaches P 5 = P cr - critical load in soils. Shear stress begins to dominate. Sliding surfaces are formed under the structures, along which the soil moves, and the soil bulges out from under the loaded area, accompanied by its subsidence.
Measures to improve the stability of slopes and slopes
In parallel with architectural offsets, which are only valid for drawings, structural offsets can be entered, but only based on calculation results. If the analysis is linear, walls are usually produced using two-dimensional elements to represent the box-like behavior of the building.
Load and Load Combinations
Likewise, contours and possible holes in plates can also be recognized by hinge geometries. Charges stored in a specific bank are divided into activities and categories, imported into the current project and assigned to the load condition. Therefore, the insertion process is direct: import from the bank and attribute a custom load condition.
The mechanical phenomena that occur in soils with increasing local load on them were considered, and two critical loads were established (at pressures on the soil greater than the structural strength): 1-load corresponding to the beginning of the appearance of shear zones in the soil and the end of the compaction phase, when under the edge loads between tangential and normal stresses, relationships arise that bring the soil (first at the edges of the base of the foundations) to a limiting stress state, and 2 - load, at which continuous areas of limiting equilibrium are formed under the loaded surface, the soil comes into an unstable state and its bearing capacity is completely exhausted .
Partial safety factors and load combinations are taken from the bank and can be freely updated. The type of load for all elements is quite general and concerns linear loads, surface forces, concentrated forces, applicable in any direction, local or global, with a general distribution of action.
Initial critical load
Distributing structural loads is facilitated by functions that distribute floor loads to beams, assigning strain, indicating elemental loads, and identifying any floor voids. Automatic functions are also predisposed to other cases, such as hydrostatic pressure distribution or earth walls.
Let's call the magnitude of the first load initial critical load , is still completely safe in the foundations of structures, since before reaching it the soil will always be in the compaction phase, and the second, in which the bearing capacity of the soil is completely exhausted, - ultimate critical load on the ground under given loading conditions.
Nodal loads, such as binding reactions calculated using other structural analysis programs, can be imported automatically. Diagnostics is of strategic importance: early detection and filtering of obvious design defects are identified, and high-level modeling defects are highlighted before structural calculations.
Search, query and presentation tools
At any stage of your work, you can change any data and request input parameters and results of structural analysis. You can transfer one or more properties of an item to others, as well as numerous search and selection methods that allow you to fully master the model. Important are the design control activities performed by the three main functions: exploration, query, and presentation.
Formula N.P. Puzyrevsky.
Let us consider the effect of a uniformly distributed load p on a strip of width b (Fig. 4.6) in the presence of a lateral load q=γh (where γ is the density of the soil and h is the depth of the loaded surface).
Figure 4.6. Scheme of operation of a strip-shaped load
Vertical compressive stress (pressure) from the soil’s own weight with a horizontal bounding surface σ 1 g.
The search identifies elements with the same property. Found elements can then be selected, frozen, rendered, hidden for further event management. Only by searching can you have immediate evidence of some of the "hidden" properties of the model. Query functions highlight both model properties and analysis results by simply pointing to the desired point.
Standard ground pressure. Design foundation soil pressure
Presentation is an effective alternative to printed tablets, which are becoming an increasingly old-fashioned tool. Presentation functions relate to both the input data and the results of analysis and structural measurement, using diagrams or color maps that immediately indicate any cases of suffering or excessive size of the structure.
Let us accept an additional assumption about the hydrostatic distribution of pressures from the soil’s own weight, namely σ 2 g.
For an arbitrary point M (Fig. 4.6), located at a depth z and characterized by the visibility angle α, we find the main stresses taking into account the actions of the soil’s own weight as a continuous load:
Let's substitute the expressions into the limit equilibrium equation:
Linear and nonlinear solvents
The first concerns second-order nonlinearity, which involves updating the elastic stiffness matrix based on the geometric one; thus, the end result of the load acting on the deformed structure rather than on the original undeformed structure can be achieved.
The second question concerns the study of structures in which even traction or compression-resistant elements are present. Seismic analysis can be carried out using a static or dynamic method. Modal dynamic analysis determines vibration modes and spectral response, providing a combination of modes. Appropriate combination of seismic action components and finally overlaying these effects with appropriate static results. The traditional modal analysis results are then processed accordingly by post-processors to induce all sign variations and correlate the associated actions.
if we take z=0, i.e. There will be no zones of limit equilibrium at any point in the ground; the initial critical pressure on the ground will be:
This is the formula prof. N.P. Puzyrevsky for the initial critical load on the ground. The pressure determined by it can be considered completely safe.
As the foundation is loaded, two critical loads are observed: the load corresponding to the beginning of the appearance of shear zones in the soil and the end of the compaction phase, and the load at which continuous areas of limit equilibrium are formed under the loaded foundation, loss of stability of the foundation soil occurs and its bearing capacity is exhausted.
Stability of slopes and slopes
The excited mass is monitored. The percentage of cutting absorbed by pillars and walls is determined, as well as the overall vertical substructure, freely defined by the user. A special case of operation that falls into the so-called “zone 4” is controlled, leading to simplified calculation methods.
Taking into account the influence of filtration forces
For other countries, you can always freely define the response spectrum to apply to the structure. The specific graphical representation performs geotechnical checks, in particular the ultimate pressure and bearing capacity of the soil for floor coverings and foundation beams. Having established geotechnical properties, evaluation criteria and partial safety factors, more representative modes of results are available, including a resistance index that correlates earth pressure with capacity.
The initial critical load corresponds to the case when a limit state occurs at the base under the base of the foundation. This load is still safe in the foundations of a structure, since before it is reached the soil is always in the compaction phase. At loads less than the initial critical one, at all points of the foundation the stress states are at their limit and the deformability of the soil obeys Hooke’s law. Therefore, to determine the initial critical load, solutions to problems in the theory of elasticity can be used
Input results and structural analysis results, even on foreign language, can be freely separated and separated so as to reduce the table value to the required tightness. Instead, a specific design reporting procedure was developed to produce an overall design report. A standard modified report is imported into the project and can then be adapted to suit the context's requirements.
Practical methods for calculating the bearing capacity and stability of foundations
Not just as a cosmic force; just as a country is only the principle of limitation and not its limits. However, on the plane of archetypes, the “two meanings” are unified. Even though living is not a matter or a component, it is an elemental way of creating and moving.
It should be borne in mind that the initial critical load corresponds to the limit of proportionality between soil stresses and deformations, and a pressure equal to or less than the initial critical pressure is considered safe.
No. 3 EMTIKHAN TICKETS/EXAMINATION TICKET
1. Structural connections between soil particles
Critical soil loads
The earth moves towards formlessness: the earth lives because the elementary movement is manifested by the loss of form, a sense of wholeness. Only with aquatic life does the earth acquire the ability to take the form or its natives. Enraged, he becomes the earth - the mother-animated; damped, formed into a raft-like material, a solid mass suitable for shaping and molding, suitable for the manufacture of bricks, ornaments, molds and slabs. Only shooting - the effect of fire and air - takes the form that the earth has captured or assumed, the condition, strength, durability, elasticity that we attribute to the Earth itself.
Structure- these are the size, shape, quantitative ratio of the particles composing the soil and the nature of the connections between them, determined by the entire prehistory of the soil. The connections between particles and aggregates of particles are called structural connections . Due to the high strength of the particles themselves, the bonds between particles determine the deformability and strength of the soil.
This whole impoverished country is really a manifestation of its internal structure and structure, the result of a special arrangement of its “turbulence”, which is the result of the loss of formality caused by the decoupling of water by fire and air. For strength, firmness, stubbornness and resilience, again the “dead end” is responsible: “deadly”, broken when the team meeting track lands with other lives. On the other hand, their serenity and uncertainty - "living", blind, unlimited, unlimited influence of others in earthly form is in vain: air causes destruction and exploitation, water of fog and erosion, flames of ash and dust.
Based on the nature of structural connections, non-rocky soils are divided into liaisons And incoherent (bulk ). Cohesive soils include silty-clayey soils; non-cohesive soils include coarse-grained and sandy soils. Cohesive soils are able to withstand low tensile stresses; non-cohesive soils tensile stresses are not perceived.
The earth is the base and base: it provides a place to lay, build, sit, give support to what is built, placed and planted, preserve and teach what is inserted or inserted into it. As the website points out, the land is a place of communal living.
"Churrik" the expression of land as a space or location is always specific - locally, "locally". It is characterized by a unique relationship to the sky, to time and to depth. The earth, as a landscape, represents a horizon - a horizon that gives it an edge and a limit. What is before the horizon can be reconciled with the vicious region of our vision: we do not see its edge, so it is given to us as a whole, which loses nothing, but all of which points to what lies beyond. The Sky line gives the Earth a page of Immanence and Transcendence.
Resistance to the mutual movement of particles of loose soil is determined by the frictional forces of the contacting surfaces. This mechanism of interaction between particles of granular soils is called internal friction of the soil.
Structural bonds in clayey soils are of a much more complex nature and are determined by electromolecular interaction forces between particles, as well as particles and ions in pore water. They determine the cohesion of clay soils. The intensity of these bonds depends on the distance between the particles, the charges on their surface, the composition and content of ions in the pore water.
Clay soil with very high moisture content is essentially a fine suspension located in fluid condition. In this case, there are practically no connections between particles. With increasing concentration of the dispersed phase (decreasing humidity W) thickening of the suspension occurs, as a result of which the distance between solid particles decreases. When clay particles approach a distance of several hundred and thousand angstroms, forces of molecular attraction (van der Waals forces) appear between them. These forces are caused by the interaction of surface molecules of solid particles as a result of periodic vibrations of electronic shells and atomic nuclei, during which instantaneous dipoles are formed. The approach of particles is prevented by the repulsive forces between their similarly charged hydration-ion shells, therefore molecular bonds are realized in the corners and on the edges of the particles, where the shells are thinner.
Molecular forces play a significant role in the formation of the strength properties of clayey soils at the initial stages of lithogenesis (transformation into rock), the stage of sedimentation (formation of sediments), during coagulation and formation of sediments, as well as at the stage of diagenesis (transformation of sediments into solid rocks).
Further approach of soil particles begins to be hampered by the repulsive forces of similarly charged surfaces of particles and diffuse layers of bound water, therefore further approach of particles is possible only with the expenditure of additional effort, for example, as a result of compaction of the soil under load or drying it out. Soil compaction brings particles closer together and strengthens bonds, and ion-electrostatic forces become significant. The determining factor for their formation is the presence of exchangeable cations in the diffuse layer. If you bring another charged particle closer to one, then the cations of the diffuse layer will interact simultaneously with two particles and an ion-electrostatic bond will form between the latter (Fig. 1.4). This bond appears at distances between particles of several tens of angstroms, but its strength is several orders of magnitude higher than the strength of bonds caused by van der Waals forces.
According to the classification developed by academician P.A. Rebinder, professors N.Ya. Denisov, N.A. Maslov and others, you noted
The above structural connections relate to water-colloidal . The presence of hydration shells of particles gives these bonds a mobile, reversible character. They are preserved during deformation: kneading a wet piece of clay does not disrupt its overall coherence. The state of clay soil in which it is capable of changing its shape under the influence of external forces without breaking continuity and maintaining the newly obtained shape for a long time is called plastic .
Drying of the soil leads to a decrease in the thickness of the hydration-ion shells and an increase in water-colloidal bonds between particles. On the contrary, an increase in soil moisture and saturation of diffuse layers has a wedging effect on soil particles, which leads to a weakening of water-colloidal bonds and an increase in soil mobility.
A small amount of water in the soil corresponds to a very high strength of water-colloid bonds; clay soil at low humidity values is in hard condition.
Along with water-colloidal bonds in soils that preserve their natural structure, there may be cementation communications. They are formed during a long geological period of formation and existence of soils due to the precipitation of salts dissolved in pore water, cementing individual solid particles with each other.
They can be less strong and water-resistant bonds formed by gypsum, calcite, and more durable and water-resistant ones, such as oxides of iron, silicon, etc. In contrast to water-colloidal bonds, cementation bonds are rigid and irreversible, not restored when the natural structure of the soil is destroyed. .
Mutual spatial arrangement of particles in the soil (texture) depends on the conditions of their deposition: whether the sediment is formed in an air or water environment, stationary or flowing water, etc.
During the deposition of relatively heavy sand and dusty particles, for which gravitational forces prevail over electromolecular interaction forces, a granular system of particle composition is formed (Fig. 1.5, a).
At the initial stage of sedimentation of clay particles in still water, a scattered (dispersed) system is formed (Fig. 1.5, b), the particles are in a suspended state.
Over time, clay particles can come into contact with each other and form a flocculation system of particle arrangement (Fig. 1.5c). Dispersed and flocculation systems are characteristic of newly formed clayey soils (loose, highly compressible silts and silty soils).
As a result of the action of the weight of the overlying layers of sediment, their lower layers are compacted, and at the same time reorientation of particles occurs in them. They receive an ordered, oriented system of relative position (Fig. 1.5, d).
The natural structure of soils, their composition and condition mainly determine the deformation-strength properties of soils and their work as foundations and environments for structures, and a very important characteristic will be the structural strength of soils and the stability of structural connections of water under the influence of external influences.
2 Protection of pits from flooding
| · To protect foundation pits from flooding, the following groups of methods are used: - water reduction; - anti-filtration curtains;- a combination of the first two methods. · The choice of one or another group of methods depends on: - type groundwater ;- UPV (UGV); - soil properties;- features of their bedding; : - depth, size and shape of the pit in plan;- other factors. · In all cases, no matter what method we choose, it is necessary to exclude disruption of the natural structure of the soil at the base, ensure the stability of the pit slopes and the safety of nearby buildings.→ Water reduction is carried out using: - deep water reduction; - open drainage 1.(LIU) are used to lower the groundwater level to a depth of 4...5 m in sand. At greater depths, wellpoints are placed in several tiers (Fig. 14.9. b) or special ejector wellpoints are used (water-jet pumps that create a vacuum around the filter element, which increases suction), which makes it possible to reduce the groundwater level to a depth of 25 m. - LIU is used in sand large, medium-sized and small - Ejector wellpoints, as more powerful ones, are used in silty sands and sandy loams with k f >0.1 m/day. Bituminization consists of supplying (injecting) bitumen heated to a liquid state into soil with fractures (fissured rocks) with a large influx of water. Due to this, a continuous waterproof wall is formed. Along with bitumen injection, cement mortar or synthetic resins are used. Injecting any material into the soil in order to eliminate its permeability is called |
3 tamponage
.End of form Soil pressure on enclosing structures
- Soil pressure
- on the enclosing surface depends on many factors:
- method and sequence of soil filling;
- natural and artificial compaction;
- physical and mechanical properties of soil;
- random or systematic ground shaking;
settlement and movement of the wall under the influence of its own weight, soil pressure;
type of associated structures. All this significantly complicates the task of determining soil pressure. There are theories for determining soil pressure that use premises that allow solving the problem with varying degrees of accuracy. Note that the solution to this problem is carried out in a flat formulation. always takes place. Taking into account the friction of the soil against the wall using the dependencies arising from the Coulomb theory gives at φ = 15-20° a significant error in the direction of exaggeration compared to the existing solution. The theory proposed by SV gives more accurate results. Sokolovsky, built on the basis of the general theory of the limiting stress state of a granular medium. There are various interpretations of this theory, including the well-known graphic interpretation by S.S. Galushkevich.
Most engineering calculations use results obtained from Coulomb theory; in cases where the results need to be clarified, correction factors are used, introduced on the basis of exact solutions and experimental data. The following types of lateral soil pressure are distinguished:
- resting pressure (E a), also called natural (natural), acting in the case when the wall (enclosing surface) is stationary or the relative movements of the soil and the structure are small ( rice. 10.7);
- active pressure ( E a) that occurs during significant movements of the structure in the direction of pressure and the formation of slip planes in the soil corresponding to its limiting equilibrium ( rice. 10.8). ABS - base of the collapse prism, prism height - 1 m;
- passive pressure ( E r), appearing during significant movements of the structure in the direction opposite to the direction of pressure and accompanied by the beginning of “soil uplift” ( rice. 10.9). ABS - bulging prism base, prism height -1m;
- additional jet pressure ( E r), which is formed when the structure moves towards the ground (in the direction opposite to the pressure), but does not cause “soil uplift”.
The interaction of the enclosing structure with the soil mass is complex and depends on the rigidity of the structure, its displacements and deflections. In an absolutely stationary state of the soil mass, retaining wall the so-called resting pressure. When the wall is displaced from the soil mass behind the retaining wall, active pressure. When the wall moves onto the soil mass it holds, it realizes passive pressure. Graphically, these three types of pressure are represented as a relationship
No. 4 EMTIKHAN TICKETS/EXAMINATION TICKET
1. Geological structure of foundations
Typically, there are several types of soils at the base. In this case, in addition to assessing the properties of each soil, an equally important task arises - schematizing the geological structure of the foundation, i.e. highlighting the boundaries between them. Engineering-geological elements form geological bodies in the soil massif (Fig.)
A layer is an internally homogeneous geological body bounded within the region under consideration by two non-intersecting surfaces: the base and the roof. The distance between the base and the roof is called the thickness of the layer. A lens is an internally homogeneous geological body bounded within the region under consideration by a closed surface. If a geological body enters a geological section from one side and ends in it, then this is called pinchout of the layer. A very thin geological body bounded by two non-intersecting surfaces is called an interlayer. A vein is an internally homogeneous geological body that is extended and intersects layers. A zone is the area of transition from soils with one properties to soils with other properties.
When determining the structure of the soil strata, it is necessary to remember that the structure of the soil strata is determined by interpolation of data obtained from individual verticals (wells, geological survey data), and the reliability of the data obtained will depend on the number of verticals, as well as the distances between them.
Structurally unstable soils are soils that in their natural state have structural bonds, which, under certain influences, reduce their strength or are completely destroyed. These effects may consist of a significant change in temperature, humidity, or the application of dynamic forces.
Structurally unstable soils include soils: loess soils, the structure of which is disrupted when soaked under load; Frozen and permafrost, the structure of which is disrupted during thawing; loose sands that become sharply compacted under dynamic influences; silts and sensitive clays, the deformation and strength properties of which change sharply when their natural structure is disturbed. Special soils also include: swelling soils, which, when moistened, can significantly increase in volume even under load; peats and peaty soils, which have very high compressibility and low strength; rocky and semi-rocky soils, which, as a rule, have high strength and low deformability.
Failure to take into account the specific properties of these soils can lead to a violation of the stability of buildings and structures and to their excessive deformations.
Structurally unstable soils are often classified as regional soil types because these soils are often grouped within certain geographic and climatic zones, in certain regions of the country, i.e. predominate in some regions and may be practically absent in others.
Features of soil deformation manifest themselves differently in various types soils and significantly depend on the condition of the soil and the intensity of the existing loads.
Monolithic rocky soils under loads arising from the construction of industrial and civil structures can usually be considered as practically non-deformable bodies. However, fractured rock and collapsible rock have some deformability. Destroyed structural connections in rocky soils are not restored over time.
2 Protection of foundations from groundwater and dampness
Moisture penetrating into building structures is a serious cause of their destruction. Protection against water penetration (waterproofing) is an important factor in the safety and durability of buildings. At high standing level groundwater There is a danger of them penetrating into the basement, causing leaks and damp spots on the walls.
Capillary moisture rising through the pores in the foundation and plinth mass from the wet soil can spread into the masonry of the walls of the lower floors. In case of aggressive groundwater, foundation materials and underground parts buildings may collapse. To protect a building from groundwater, measures are taken to combat the movement of groundwater and the penetration of precipitation into the foundation soil and protective waterproofing is installed against the penetration of ground moisture into the building structure.
To prevent the penetration of rain and melt water into the underground parts of the building, they level the surface of the building site, creating the necessary slope to drain surface water from the building. A blind area made of dense waterproof materials (asphalt, asphalt concrete, etc.) is installed around the building along the outer walls. To protect against the penetration of ground moisture into building structures during new construction, external insulation of structures from the side exposed to water is usually performed, and for old buildings it is used internal waterproofing in basements.
There are three types of waterproofing, corresponding to the types of exposure to water - free-flow, anti-pressure and anti-capillary. Non-pressure waterproofing is performed to protect against temporary exposure to moisture from precipitation, seasonal high water, as well as in drained floors and ceilings. Anti-pressure waterproofing - to protect enclosing structures (floors, walls, foundations) from hydrostatic backwater of groundwater. Anti-capillary - for insulation of building walls in the area of capillary rise of ground moisture.
The design of basement waterproofing is determined by the nature of the impact of water, the characteristics of drained structures and materials, as well as the functional requirements for the premises for operation, purpose and permissible humidity. This affects the choice of type and material of insulation, determined by the required indicators for water permeability, water resistance, vapor permeability and durability. The capabilities of contractors, season and pace of work should also be taken into account when selecting waterproofing materials. There are various methods of waterproofing: basic - pasting, painting, coating, plastering, sheet (caisson) and clay, as well as special - injection, penetrating (penetration), geomembrane impregnation, suture, underwater, liquidation of active leaks, etc.
3 Practical methods for calculating the finite deformations of foundations
Calculation of sediment using the layer-by-layer summation method.
This method (without the possibility of lateral expansion of the soil) is recommended by SNiP 2.02.01 - 83 and is the main one when calculating the settlement of foundations of industrial buildings and civil structures. Below we discuss the order of auxiliary constructions and the sequence of calculations in relation to the calculation scheme in the figure.
First, the foundation is tied to the engineering-geological situation of the foundation, i.e., its axis is aligned. With known loads from the structure, the average pressure on the base along the base of the foundation is determined. Then, according to the rules given in § 5.4, starting from the surface of the natural relief, a diagram of natural pressure is constructed along the axis of the foundation. Knowing the natural pressure at the level of the base of the foundation, the additional vertical stress in the plane of the base of the foundation is determined. In accordance with the above, a diagram of additional stresses along the axis of the foundation is constructed on the same scale.
By constructing diagrams of natural pressure and additional stress, the lower limit of the compressible thickness is found. It is convenient to perform this operation graphically, for which the diagram of natural pressure, reduced by 5 or 10 times (depending on the condition of limiting the compressible thickness), is combined with the diagram of additional stresses. The point of intersection of the lines limiting these diagrams will determine the position of the lower boundary of the compressible thickness.
The compressible thickness of the base is divided into elementary layers so that within each layer the soil is homogeneous. Typically, the thickness of each elementary layer is taken to be no more than 0.5. Knowing the additional stress in the middle of each elementary layer, the compression of this layer is determined. The standards allow the values of the dimensionless coefficient p to be equal to 0.8.
No. 5 EMTIKHAN TICKETS/EXAMINATION TICKET
1. Soil deformability
The deformability of clayey soils is mainly due to the mutual movement of solid soil particles. In coarse-grained soils, the main factors of deformability are the collapse of contacts and the destruction of solid particles under load. In sandy soils, processes of reorientation and mutual movement of particles, as well as their destruction, occur. Deformations are divided into volumetric and shape changes.
Volumetric compressibility of clayey two-phase soils is possible only when water is squeezed out of the soil. Since the pores of the soil are small, the squeezing out of free water occurs slowly, and the process of soil deformation, depending on its volume, often extends over a long period of time. The increased viscosity of cohesive water also slows down the process of deformation (volume and shape change).
The process of deformation of clayey soil over time is described by the theory of soil consolidation, some simplified provisions of which are set out in 12.3.
In sandy soils, deformation processes under the influence of static load occur quickly, so they are usually not considered over time.
In coarse-grained soils, the processes of contact collapse, their destruction, re-arrangement of the structure due to the destruction of individual grains and redistribution of load between particles often take a long time, just like in clayey soils, although the mechanism of the process in time is different.
Since the soil consists of solid particles and pores, which are partially or completely filled with water, the following deformations occur in the soil under the action of an external load:
Mutual displacement of particles and particle aggregates with their more dense repacking;
Destruction of particles and their aggregates;
Squeezing water and air out of soil pores;
Deformation of water films at the points of contact of soil particles;
Air compression in closed soil pores;
Elastic deformation of mineral particles.
After the load is removed, some deformations are restored. They are called elastic deformations. These are deformations of soil particles, films of bound water, elastic compression of pinched air bubbles and pore water. Such soil deformations, as a rule, are many times smaller than deformations due to shifts of soil particles, squeezing out water and air from pores, which are called residual, i.e. not recovering after removal of the load. As a result, residual deformations lead to compaction of the soil.
2 Foundations on weak clayey, water-saturated and peaty soils
Weak water-saturated soils include highly compressible soils saturated with water, which at normal rates of application of loads on the foundation lose their strength, as a result of which their shear resistance decreases and compressibility increases. Weak clay soil is a dispersed structured system with a coagulation type of structural bonds, capable of transitioning from a solid to a liquid state if they are disturbed. The fluid state of the soil is determined by the degree of disruption of structural bonds. When calculating the settlement of highly compressible water-saturated clay foundations, it becomes necessary to take into account creep, nonlinear deformability and permeability. The cyclical application of loads, for example, in elevators, changes the strength and deformation properties of foundation soils over time. Uneven loading of individual silos leads to significant uneven deformations. Experts recommend uniform initial loading and unloading of elevators.
Clay soils (silts, banded clay soils, water-saturated loess macroporous and peat soils, etc.) are often classified as weakly water-saturated soils. E≤ 5 MPa and s r≥ 0.8, ϕ = 4 … 10°, With= 0.006 ... 0.025 MPa.
The values of filtration coefficients in the vertical and horizontal directions differ up to 10 times. The total sediment is divided into a part described by the theory of filtration consolidation and a part described by secondary consolidation processes.
When designing shallow foundations, it is necessary to limit:
Average precipitation limits;
Relative differences in settlement of adjacent foundations with limiting values;
The sediment flow rates are acceptable.
When seismic waves pass through weak waterproof soil, pore pressure arises and the strength characteristics of the soil decrease. In these conditions, it is recommended to use rack piles with complete cutting through soft soils and resting on strong ones. In addition, it is possible to use sand cushions, drainage slots with loading embankments, lime piles, followed by soil compaction with heavy tampers.
In the case where compaction and strengthening methods do not produce an effect, and the settlement exceeds the limit, constructive measures are necessary. These include: increasing the rigidity of buildings by cutting sedimentary joints into separate blocks; increasing the rigidity of each block by installing monolithic reinforced concrete or prefabricated monolithic foundations; installation of reinforced concrete or metal belts or reinforced seams; installation of rigid diaphragms, for example, horizontal ones made of slabs; increasing the flexibility and pliability of flexible buildings and structures.
Foundation settlements are calculated using design schemes in the form of a linearly deformed space or a linearly deformable layer. The boundary of the compressible strata is determined at a depth where the additional stresses are equal to 3 kPa - for silts, and for peat soils at a depth where the additional pressure to the natural one is equal to the structural strength.
Additional settlement of foundations on foundations composed of water-saturated or organic-mineral soils due to the decomposition of organic inclusions may not be taken into account if during the service life of the structure the groundwater level does not decrease
3 General requirements to the design of foundations and foundations
Typical projects does not exist for structure-foundation systems. The system is calculated as a whole. The calculation is carried out based on maximum loads.
In the structural design of the foundation, characteristic sections are selected. They calculate the interaction between the foundation and the foundation. The dimensions of the structure must comply with all limit state calculations. Based on the calculation results, fragments are constructed in each section, and then the foundation for the entire building.
Foundations and foundations are designed using the variant method. The calculation is made for several section designs and the structure as a whole. Then a comparison is made based on a number of technical and economic indicators, and the most economical option is selected. A project is being developed for it, including architectural and construction drawings and PPR.
When choosing an option, the following are taken into account:
- site relief;
- physical and mechanical properties of soils;
- geological structure of the massif;
- hydrogeological conditions of the site;
- space-planning solution of the building;
- features of work when laying the foundation;
- features of the use of the structure and possible factors leading to changes in soil properties during its operation.
When designing foundations, calculations are made for strength, crack resistance and resistance to deformation.
No. 6 EMTIKHAN TICKETS/EXAMINATION TICKET
1. Water permeability of soils
Water permeability is the property of water-saturated soil to allow a continuous flow of water to pass through its pores under the influence of pressure differences. In this case, a continuous flow of water is understood as its continuous movement (filtration) throughout the entire cross-section of the active pores of the soil, i.e., that part of the pores that is not filled with bound water. The water permeability of soils depends on their porosity, granulometric and mineral composition, pressure gradient.
2 Foundations on saline soils
When calculating foundations composed of saline soils according to the second group of limit states, the foundation settlement should be determined taking into account deformations caused by external load, as well as possible deformations from subsidence as a result of swelling, shrinkage and suffusion settlement.
Foundations composed of saline soils must be designed taking into account the following factors: the possibility of formation of suffusion sediment as a result of water filtration with subsequent leaching of salts; reduction in strength characteristics as a result of changes in physical and mechanical properties during the leaching process; possible subsidence or swelling during soaking, as well as increased aggressiveness of groundwater in relation to materials of underground structures, possible as a result of the dissolution of salts contained in the soil.
The calculated resistance of the base R with the possibility of long-term soaking and leaching is determined by formula (4.10) taking into account the values of the characteristics. If the calculated deformations of the base exceed the maximum permissible according to the standards or the bearing capacity turns out to be insufficient, it is necessary to carry out water protection measures that exclude soaking, and if this is not possible: carry out structural measures aimed at reducing the adverse effects of uneven precipitation; resort to partial or complete cutting of layers of saline soils and replacing the latter with cushions of silty-clayey soils; apply pile foundations with cutting through layers of saline soils; use consolidation and compaction of soils, as well as preliminary desalinization with special substances that react with salts.
If a set of measures is envisaged aimed at preventing prolonged soaking and leaching of soils, or the possibility of the latter is completely absent, foundation sedimentation is determined as for non-saline soils at complete water saturation.
When calculating foundations in undermined areas, the calculated relative horizontal deformations, the radius of curvature of the deformation of the earth's surface and the indicator of the total deformations developing within the length of the building are taken into account.
Foundations in mined areas should be designed taking into account the uneven subsidence of the soil surface, accompanied by horizontal deformations from the displacement of soil masses as a result of mining operations and the movement of soil into the goaf during mining.