The magnitude of the first load will be called the initial critical load, which is still completely safe in the foundations of the structure, until it is reached, the soil will always be in the compaction phase.
Consider the action of an equal distribution load ron the band is wide. b in the presence of a lateral load. q = γh, where γ - density of soil, h-the depth of the outside. Vert-e compressive stress (pressure) from the self-weight of the graph on the horizontally bounding surface σ 1gr = γ (h + z), z-the depth of the location of the considered point below the plane of application of the load. The problem is to determine the magnitude of the pump load Pcr, at which the shear zone (the zone of equilibrium equilibrium) is only generated under the surface. Since the tangential stresses will be greatest at the edges of the stresses with a strip-like load, then it is natural to expect in these places, with an increase in the inflow of the equilibrium zones. We assume the condition of limiting equilibrium. σ 1 - σ 2 = 2sinφ (((σ 1 + σ 2) / 2 + p e) We find the main stresses taking into account the action of the self-weight gr. As a continuous load:
σ 1 = (p-γh) * (α + sinα) / π + γ (h + r)
σ 2 = (p-γh) * (α-sinα) / π + γ (h + z).
We substitute expressions σ 1and σ 2 in the condition of the limiting equilibrium, and take into account p e= c ctqφ, we obtain: (p-γh) * sinα / π-sinφ ((p-γh) / π * α + γh + γz) = c ctqφ. Get Expressions. can rasmat. as Vp-e of the boundary region of the limiting equilibrium, z as the ordinate of this region. z = (p-γh) * ((cosα / sinφ) -α) / πγ -c / γ ctqφ-h. Let us find z max, dz / dα = (p- γ h) * ((cosα / sinφ) -1) / π γ = 0. Cosα = sinφ or α = π / 2-φ, sin (π / 2-φ) = cosφ. Substituting the obtained values, pcr = π / ctqφ + φ-π / 2 ( γ zmak + yh + c ctqφ) + γ h. The normal pressure on gr.R is such pressure, at which under the edges of the funda- tes, the zone of limiting equilibrium does not spread at a depth greater than z max = b / 4.b-width of the foundation. If not at all to admit at any point of the development of the zones of the maximum equal to the bottom of the fund, then we put in the equation zmax = 0. By calling the greatest pressure, at which there are no zones of the limit at any point of the soil, priming primkRkr. beginning Pcr = π (γh + c ctqφ) / ctqφ + φ-π / 2. This is the formula for the initial critical load on the ground.
The initial critical pressure on the base is the pressure value at which the regions of the ultimate stress state arise in the base soil. At pressures of lower initial critical values at all points of the base, the stress states are pre-limiting, which is absolutely safe for the foundations of structures. In this case, until the initial critical pressure is reached, the soil is in the compaction phase and the approach to its determination is demonstrated for the strip load on the ground.
To find the initial critical pressure, the values of the main stresses σ1 and σ3 are determined taking into account the applied load P = P0-q and the ground weight, respectively.
The vertical compressive stress (pressure) of the self-weight of the soil at the point M (see figure 8.5, c), lying at a depth z from the base of the basement, is determined from expression
Primers of the ground experience two types of pressure:
· Household sb, which occurs in soils under the influence of the weight of overlying layers;
· Additional s arising under the influence of loads from the foundations.
In general, the ordinates of the pressure diagram under the sole of a rigid foundation, under the action of a vertical load, are determined by the formula:
If a horizontal load or a tipping moment acts on the foundation, then in this case a tipping moment created by the horizontal load is found, and formula (6) is written in the form:
Fig. Scheme for calculating pressures under the sole of rigid foundations.
Fig. Ground pressure diagrams under the sole:
a-in clay soils; b-with sandy soils; in the case of an eccentric load, when e< b/6; г–при внецентренной нагрузке, когда е = b/6; д–при внецентренной нагрузке, когда е > b / 6.
These expressions make it possible to give an estimate of the stressed state of the soil, i.e. to determine whether the soil is in a pre-limiting or limiting state, and, consequently, how stable the array is.
The limiting state of the soil corresponds to the point in Fig. 4.1a, where the sediment S goes to infinity, ie. the theory of limiting equilibrium studies only the stressed state of the soil massif and does not make it possible to determine the deformations developing in it.
4.1. Critical loads on the foundation soils. Stress state phases of soil bases
Consider the graph of the dependence in Fig. 4.1, a.
For cohesive soil, the initial plot area Oa will be almost horizontal, the length of this section will be determined by the magnitude of the structural strength of the soil, and the deformation has an elastic character.
With increasing pressure (section ab) the sediment increases, the compaction process develops due to the decrease in the porosity of the soil. The dependence is close to linear, the precipitation tends to a constant value (4.1, b). No limit state is formed at any point of the base. The greatest stress limiting this area is called initial critical load p start cr. , and a load change from 0 to p start cr. characterizes compaction phase.
When the pressure under the base of the foundation changes from 0 to p start cr. at no point in the base does the limiting state arise; Only the compaction of the soil takes place, which is absolutely safe for the foundation.
With further increase in load (section bw Fig. 4.1, a) at points located under the edges of the foundation, the tangential stresses over certain areas become equal to their limiting values. As the load increases, these points are combined into zones, the sizes of which increase. There are shear deformations having a plastic character. The dependence graph deviates more and more from linear. Plot of land bw is called phase of shifts. The end of this phase corresponds to p and, called the critical critical load, at which closed regions of limiting equilibrium are formed at the base, and there is a loss of soil stability, i.e. complete exhaustion of the bearing capacity.
Depending on the depth of foundation of the basement d/ b The outlines of the marginal equilibrium regions have a different character (Figure 4.2).
Loads corresponding to p start cr. and p andcalled critical loads, they are determined by the methods of the theory of limiting equilibrium.
The initial critical load corresponds to the case when the limit state appears at the base under the base of the foundation at a single point under the face of the foundation.
We choose in the base point M (Figure 4.3) and determine such a contact voltage r, at which a limiting stress state arises at this point.
In the model of a linearly deformed medium, the total stresses at the point M are determined as
We write down the relationship for the depth of the lowest point at which the limit state from the basement bottom is possible.
By definition, with p zmax = 0. Then at a single base point under the face of the foundation, the condition of limiting equilibrium will be fulfilled:
The foundation, designed so that the voltage under its sole does not exceed the initial critical load ( pp star), will be in an absolutely safe state. However, as practice has shown, the base soils will have a significant reserve of bearing capacity.
4.1.2. Standard resistance and design pressure
If we assume a base under the sole of a centrally loaded foundation of width b the development of zones of extreme equilibrium to depth, the load-carrying capacity of the base remains assured. In this case, the precipitation decays in time and tends to a constant value, and the dependence is fairly close to linear. Consequently, under these conditions, the formulas of the theory of linear deformation of soils can be used to calculate the deformation of the base.
Normative resistance of the foundation soil R n corresponds to the greatest average compressive stress under the base of the foundation at which, on the base of the foundation, it is allowed to develop the regions of the limiting state to a depth equal to b/4 .
here M γ , M q, M c- some functions of the angle φ.
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The values of these coefficients are given in SNiP 2.02.01-83 *.
Further studies made it possible to further push aside the practical limit of the average stress under the basement foundation, where we also assume the calculation of sediments taking into account the linear deformation of the base soils. This value, according to SNiP 2.02.01-83 *, received the name design basis resistance R (4.11).
In this case, formula (4.9) has a somewhat more complicated form (taking into account basements, taking into account the heterogeneity of the ground, etc.) and will be considered in the course "Foundations and foundations".
4.1.3. Critical load limit
corresponds to the stress beneath the base of the basement, at which the bearing capacity of the base soils is exhausted (Figure 4.1), which leads to extrusion of the soil from under the foundation and its huge sediment (Figure 4.2). The load corresponding to p and leads to a complete loss of stability of the foundation soil and is absolutely unacceptable for the proposed structure.The solution of this problem was dealt with by L. Prandl, K. Terzachi, V. V. Sokolovsky, M. V. Malyshev.
In Fig. 4.3. one (left) region of limiting equilibrium and two "families" of slip lines, which form slides with certain angles of inclination of the lines.
The most complete solution to this problem was obtained in 1952 by VV Sokolovsky.
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where N γ , N q, N c= f(φ , δ ) - tabulated dimensionless coefficients.
The above solutions are valid for relatively small depths of foundations and a homogeneous structure of the base; therefore, in practical calculations, engineering methods are used, which to some extent take into account the rigorous solutions of the theory of limiting equilibrium.
4.2. Practical ways of calculating the bearing capacity and stability of bases
Principles for calculating the foundations of foundations according to the I limiting state (on the strength and load-bearing capacity of soils).
According to SNiP 2.02.01-83 * the bearing capacity of the foundation is considered to be secured when the condition is fulfilled:
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F - resultant of the calculated force (load) applied to the base;
F u - the ultimate resistance force (the resultant of the ultimate load);
γ from - coefficient of working conditions, depending on the type of soil.
γ n - coefficient of reliability according to the purpose of the structure.
4.3. Stability of slopes and slopes
A slope is an artificially created surface, bounding a natural soil massif, a notch or a mound. Slopes are formed during the erection of various types of embankments (dams, earth dams, etc.) and excavations (excavations, trenches, canals, etc.). A slope is a slope formed by the natural way and limiting the mass of soil of natural build.
The main reasons for the loss of stability of slopes and slopes are:
The device is inadmissible steep slope or pruning of the slope, which is in a state close to the limit;
Increase in external load (erection of structures, storage of materials on a slope or near its edge);
Change in internal forces (change in the specific gravity of the soil with a change in its moisture content);
Wrong designation of design characteristics of soil strength or reduction of its resistance to shear due to higher humidity and other reasons;
The manifestation of hydrodynamic pressure, seismic forces, various kinds of dynamic influences (traffic, pile driving, etc.).
4.3.1. The concept of the coefficient of stability of slopes and slopes stability
The coefficient of stability is often taken in the form:
, (4.13)
where φ, s - calculated values of the ground shear resistance characteristics adopted in the design; φ ', c' - the same, corresponding to the limiting state of the slope or slope.
Stability of slope or slope is considered to be provided if, where = 1,1 ... 1,3 - the normative coefficient of stability.
The groups of methods used to calculate the stability of slopes and slopes:
Elementary solutions;
Strict solutions;
Engineering methods;
Numerical methods.
In this case, two types of tasks are analyzed:
1). Estimation of stability of a slope or a slope of a given slope
2). Determination of the optimal steepness of the slope or slope for a given.
4.4. The simplest methods of calculating stability
4.4.1. Stability of slopes in ideally free-flowing soils (φ ≠ 0, с = 0)
There is a slope with an angle of laying α , for a given φ for the sand composing the slope (Fig.4.4, a). Let us consider the equilibrium of a particle lying freely on the surface of a slope; the soil has only internal friction, then stability will be ensured if T≤T '.
Having specified the weight of a particle P and considering that the coefficient of internal friction of the soil, we get;
at α=φ in ideally free-flowing soils, the angle of natural slope - α is equal to the angle of internal friction of the soil.
4.4.2. Accounting for the influence of filtration forces
If the groundwater level is above the bottom of the slope, a filtration flow appears on its surface, which leads to a decrease in the stability of the slope.
In this case, when considering the equilibrium of a particle, it is necessary to add the hydrodynamic component of D.
The hydraulic gradient at the outlet point of the flow is:
γ w is the specific weight of water;
n - porosity.
Considering that the weight per unit volume of soil P=γ V, where V=1.
The equation of the limiting state is written as:
Angle of embankment at a given normative coefficient of stability:
4.4.3. Stability of vertical slope in ideally connected soils (φ = 0; s ≠ 0)
If the height of the slope folded by the cohesive soils does not exceed the limiting value h 0, then the cohesive soil can hold a vertical slope.
The most unfavorable stress state occurs at the bottom of the slope in tA (Fig. 4.1, c) It is here that the state of limiting equilibrium begins to form.
The maximum principal stress at this point is equal to the natural one, i.e. . Since the slope is bounded by a free vertical surface, the minimum principal stress in T. is zero, i.e. .
The condition of limiting equilibrium has the form:
Taking into account that here φ = 0 (by the condition of the problem), and also substituting here σ 1 and σ 3, after the transformation we have: u
It is assumed that the loss of stability of the slope (slope) can occur as a result of rotation of the soil massif relative to some center ABOUT(Figure 4.5, a) .
The essence of the method consists in analyzing the stability of the slope against shear over a number of possible slip surfaces, represented by an arc of a circle with a radius r and the center in t. ABOUT.
The ground mass compartment, bounded by a free surface and a sliding surface, is divided by vertical lines into nelements in such a way that it is possible to take the base of each compartment flat, and the strength characteristics are constant.
The displacing array is considered as an undeformable compartment, all points of which participate in the general motion.
.Usually a series of such calculations is performed for different positions of the centers of rotation and values of r.
Most often the most dangerous sliding surface passes through the lower point of the slope (slope). In addition to weak soils with minimal φ and from.
4.5.2. Measures to increase the stability of slopes and slopes
One of the most effective ways to increase the stability of slopes and slopes is to flatten them or create a concave profile with the formation of horizontal platforms (berms) along the height of the slope.
At a relatively low height of the slope, the sole is effectively loaded in its lower part or the retaining wall device supporting the escarpment. Fixing the surface of the slope can be carried out by paving stone, overnovka, stacking concrete slabs.
The most important measure is the regulation of the hydrogeological regime of the slope or slope. The device of ditches for interception of surface waters, drainage of water from berms, the device of a drainage.
Constructive measures such as cutting a potentially unstable massif of soils with a system of pile or ramming piles, anchoring in cooperation with retaining walls or pile structures.
4.6. The concepts of the interaction of soils with enclosing structures (rest pressure, active and passive pressure)
Fencing structures are designed to keep from collapsing the underlying massifs behind them. Such structures include the retaining wall, as well as the walls of the cellars and buried parts of the building, walls of underground structures, etc. There are massive (or gravitational) and thin-walled retaining walls (Figure 4.6). By the nature of the work are divided into rigid and flexible (sheet piling).
Coefficient. lateral pressure; ν is the coefficient. Poisson.
When the wall is displaced under pressure from the backfill by an amount u a (Fig. 4.7, a) in the soil of the backfilling, a ground collapse zone is formed, the boundary of which is called the slip surface, and the area itself is a collapse prism. The pressure transmitted by the collapse prism on the face of the wall is called active pressure, and its result is denoted by E a.
When the wall is displaced toward the ground under the influence of some forces in the backfilling, sliding surfaces are also formed, with a displacement value of + u n, the prism of the ground protrusion is formed (Fig. 4.7, c). In this case, the reaction of the soil reaches its maximum value and corresponds to the passive pressure (repulsion) of the soil, the resultant of which is denoted E P.
4.6.1. Determination of the active pressure on the vertical face of the wall for free-flowing soils and cohesive soil, recording of the load on the surface of the backfill E
The ordinates of the passive pressure at a depth z from the backfill surface at the origin of coordinates at the point 0 ':
At the same depth from the backfill surface of the ordinate, the passive pressure diagrams are substantially larger than the ordinate of the active pressure diagram. The width of the prism of the bulb l’ =htg (45 ° + φ / 2).
The first critical load on the ground (ultimate load on the ground)
Puzyrevsky defined the first critical load for a cohesive soil, and Gersevanov for a disconnected one.
P - uniformly distributed load
g- lateral loading (γ is the specific weight of the soil, h = d is the depth of the loaded surface)
z- depth of the location of the point under consideration below the plane of application of the load.
z max is the maximum value of the shift zones.
2β is the angle of visibility
- the equation of limiting equilibrium
The task is to determine such a load P 1, at which the shear zones (zones of limiting equilibria) are only nascent under the loaded surface.
Since in the case of a strip load (plane problem) the tangential stresses will be greatest at the load edges, then it is natural to expect the emergence of critical equilibrium zones in these places with increasing load. Suppose that a continuous load of intensity q acts.
with in - own weight
Since the point M is located in the zone of shifts, where the soil is in a plastic state and the pressure in all directions is the same, we assume an additional assumption about the hydrostatic distribution of pressures from the soil's own weight.
2β is the angle of visibility from point M
We substitute σ1 and σ3 in the equation of limiting equilibrium:
From this equation, we express z (the depth of the location of the point M within the shift zone).
For the first critical load, it is necessary that the requirement is fulfilled for the shift zones to have point dimensions. This condition will be satisfied if the maximal size of the zone of shifts z max = 0.Z max we obtain by investigating the z function at the maximum. In this case it is necessary to find the first derivative equal to 0, to determine those variables that would correspond to z max and substitute them in z.
- the formula of Puzyrevsky
Normative pressure on the ground. Estimated ground pressure.
z max is the maximum value of the shift zones (its maximum value is b / 4).
Until 1962, when calculating soil bases, the condition was accepted that the actual average pressure over the base of the foundation p should not exceed the first critical load P 1. (p ≤ P 1). In 1962 the first SNiP was published.
P cr -\u003e A cr -\u003e S calc.
The criteria was the actual observation of precipitation S fact. The actual precipitation was much less than the calculated S fact<
R n - was obtained as the first critical load, but not at z = 0, and at z = b / 4.
P 1, when z = 0, there are no plastic zones.
R n, with z = b / 4 - there are plastic zones.
R n\u003e P cr, P Observations continued and in 1975 another SNiP-R-estimated pressure on the ground came out: Observations increased the pressure due to the coefficients m 1 - coefficient, depending on the type of soil base m 2 - coefficient, depending on the type of soils and the structural scheme of the building (structure) k n is the reliability factor as the foundation is loaded, two critical loads are observed: the load corresponding to the beginning of the formation of shear zones and the end of the compaction phase in the soil, and the load at which solid marginal equilibrium regions form under the loaded foundation, the stability of the base soils is lost, and its bearing capacity is exhausted. The initial critical load corresponds to the case when the limit state appears in the basement under the foot of the foundation. This load is still safe in the foundation of the structure, since before its reaching the soil is always in the compaction phase. At loads less than the initial critical value, at all points of the base, the stressed states are pre-limiting and the deformability of the soil obeys Hooke's law. Consequently, to determine the initial critical load, solutions of problems in the theory of elasticity It should be borne in mind that the initial critical load corresponds to the limit of proportionality between stresses and deformations of the soil, and the pressure equal to or below the initial critical pressure is considered as safe. № 3 EMTIHAN BILETI / EXAMINATION TICKET 1. Structural bonds between particles of soil
Structure- this is the size, shape, the quantitative relationship of the soil constituents and the nature of the relationships between them, due to the entire prehistory of the soil. The connections between particles and aggregates of particles are called structural links
.
Because of the high strength of the particles themselves, the bonds between the particles determine the deformability and strength of the soil. The non-local soils according to the nature of the structural bonds are divided into connected
and disconnected
(loose
). To the connected are the silty-clay soils; to cohesive - coarse-grained and sandy soils. Cohesive soils are capable of perceiving small tensile stresses; disconnected soils of tensile stresses are not perceived. The resistance to the mutual displacement of particles of loose soils is determined by the frictional forces of the contacting surfaces. Such a mechanism of interaction between particles of loose soils is called internal friction of the soil. Structural bonds in clay soils have a much more complex nature and are determined by the electromolecular forces of interaction between particles, as well as particles and ions in the pore water. They also determine the cohesiveness of clay soils. The intensity of these bonds depends on the distance between the particles, the charges on their surface, the composition and the content of ions in the pore water. Clay soil with a very high moisture content is essentially a fine dispersion suspended in flowingcondition. In this case there are practically no connections between the particles. With an increase in the concentration of the dispersed phase (moisture decrease W)there is a thickening of the suspension, as a result of which the distance between the solid particles decreases. When approaching clay particles a distance of the order of several hundred and thousands of angstroms, the 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 oscillations of electron shells and atomic nuclei, at which instant dipoles are formed. The approach of the particles is impeded by repulsive forces between their like-charged hydrate-ion shells, so molecular bonds are realized in the corners and on the edges of the particles, where the shells are thinner. Molecular forces play an important role in the formation of strength properties of clayey soils at the initial stages of lithogenesis (transformation into rock), sedimentation (precipitation), coagulation and sediment formation, and also in the stage of diagenesis (conversion of sediments into hard rocks). Further rapprochement of soil particles is prevented by the repulsive forces of the like-named charged particle surfaces and diffuse layers of bound water, so further approach of the particles is possible only with the additional effort, for example, as a result of compaction of the soil under load or its drying. Soil compaction leads to the approximation of particles and the strengthening of bonds, while ion-electrostatic forces become significant. The determining factor for their formation is the presence in the diffuse layer of exchange cations. If another particle is approached to one charged particle, the cations of the diffuse layer will interact simultaneously with the two particles and an ion-electrostatic bond will form between the particles (Figure 1.4). This relationship manifests itself at distances between particles of several tens of angstroms, but its strength is several orders of magnitude higher than the strength of the bonds due to van der Waals forces. According to the classification developed by academician PA Rebinder, professors N. Ya. Denisov, NA Maslov, etc., you marked structural links relate to water-colloidal
.
The presence of hydrated shells of particles gives these bonds a mobile, reversible character. They are preserved under deformation: the recession of a wet piece of clay does not violate its general coherence. The condition of the clay soil, in which it is capable of changing its shape under the influence of external forces without breaking the 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 hydrate-ion shells and to the enhancement of water-colloidal bonds between the particles. On the contrary, the increase in soil moisture and the saturation of diffuse layers exert a wedging effect on the soil particles, which leads to a weakening of the water-colloidal bonds and an increase in the mobility of the soil. A very high strength of water-colloidal bonds corresponds to a small amount of water in the soil; Clayey soil at low moisture values is in solid
condition. Along with water-colloidal bonds in soils that preserve the natural structure, there may exist cementing
connection. They are formed during a long geological period of formation and existence of soils due to precipitation of salts dissolved in pore water, cementing individual solid particles with each other. They may be less strong and water-resistant bonds formed by gypsum, calcite, and more durable and waterproof, such as iron oxides, silicon, etc. In contrast to water-colloid, cementation bonds are rigid and irreversible, not restored when the natural structure of the soil is destroyed . Mutual spatial arrangement of particles in the ground (texture)
depends on the conditions of their precipitation: whether a sediment is formed in the air or water environment, fixed or flowing water, etc. When depositing relatively heavy sand and dust particles, for which the gravitational forces prevail over the electromolecular forces of interaction, a granular system of addition of particles is formed (Fig. 1.5, a). In the initial stage of precipitation of clay particles in immobile water, a scattered (dispersed) system is formed (Figure 1.5, b), the particles are in a suspended state. Over time, clay particles can contact each other and form a flocculation system for the arrangement of particles (Figure 1.5, c). Dispersed and flocculation systems are characteristic for freshly formed clay soils (loose, highly compressible silt and muddy soils). As a result of the weight of the overlying layers of sediments, their lower layers are densified, and in this case a reorientation of the particles takes place. They receive an ordered, oriented system of mutual arrangement (Figure 1.5, d). The natural structure of the soils, their composition and state basically determine the deformation-strength properties of soils and their work as bases and environments for structures, and the structural strength of soils and the stability of structural water connections due to external influences will be a very important characteristic. 2 Protection of pits from flooding
3 Pressures of soils on enclosing structures
Soil pressureon the enclosing surface depends on many factors: All this significantly complicates the task of determining the ground pressure. There are theories of determining ground pressure, using the prerequisites, which allow performing the solution of the problem with varying degrees of accuracy. We note that the solution of this problem is satisfied in a flat formulation. The theory of Coulomb, proposed in 1776, is based on the consideration of the limiting equilibrium of the prism of the soil, limited by straight planes of collapse (bulging). A more rigorous solution to the limiting equilibrium shows that the actual outline of these sliding surfaces is curvilinear. However, the values of the active soil pressure on vertical or near vertical, rigid, smooth and rough walls, determined according to Coulomb and according to the exact procedure, differ by 2-3%, which, undoubtedly, can be considered a satisfactory result from the engineering point of view. Passive ground pressure is very dependent on the friction of the soil against the wall, which always takes place under real conditions. Allowance for the friction of the soil against the wall with the use of dependences arising from the Coulomb theory gives, at φ = 15-20 °, a significant error in the direction of exaggeration in comparison with the available solution. More accurate results are provided by the theory proposed by the SV. Sokolovsky, built on the basis of the general theory of the ultimate stress state of a granular medium. There are various interpretations of this theory, including well-known graphical interpretation of SS. Galushkevich. Most engineering calculations use results obtained on the basis of Coulomb's theory; in those cases where the results should be clarified, correction factors introduced on the basis of exact solutions and experimental data are used. There are the following types of lateral earth pressure: The interaction of the enclosing structure with the ground mass is of a complex nature and depends on the rigidity of the structure, on its displacements and deflections. With the absolutely immobile state of the soil massif behind the retaining wall, the so-called resting pressure. When the wall is displaced from the soil mass over the retaining wall, active pressure. When the wall moves on the soil mass held by it, it is realized in it passive pressure. Graphically, these three kinds of pressure are represented in the form of dependence № 4 EMTIHAN BILETI / EXAMINATION TICKET 1. Geological structure of bases
Typically, there are several types of soils at the base. In this case, in addition to evaluating the properties of each soil, an equally important task arises - schematization of the geological structure of the base, i.e. separation of boundaries between them. Engineering geological elements form geological bodies in an array of soils (Fig.) A layer is called an internally homogeneous geological body, bounded within the area under consideration by two disjoint surfaces: a sole and a roof. The distance between the sole and the roof is called the thickness of the layer. A lens is an internally homogeneous geological body, bounded within a given area by a closed surface. If the geological body enters from the one side into the geological section and ends in it, it is called the wedging out of the layer. A very thin geological body, bounded by two disjoint surfaces, is called an interlayer. Residential is called internally homogeneous geological body, extended and intersecting layers. Zone refers to the area of transition from soils with some properties to soils with other properties. When determining the structure of the earth's strata, it must be remembered that the structure of the soil sequence is determined by interpolation of data obtained from individual verticals (wells, geological exploration data), and the reliability of the obtained data will depend on the number of verticals, as well as the distances between them. Structurally unstable soils are soils that have structural bonds in the natural state, which under certain influences reduce their strength or completely break down. These effects can consist in a significant change in temperature, humidity, the application of dynamic forces. Structurally unstable soils include soils: loess soils, the structure of which is disturbed by soaking under load; frozen and friable, the structure of which is disturbed during thawing; loose sands, sharply condensed under dynamic influences; silt and sensitive clays, the deformation and strength properties of which sharply change when their natural structure is disturbed. Also, special soils include: swelling soils, which, when wet, can increase significantly even under load; peat and ground soils with very high compressibility and low strength; rock and semicollar soils, which, as a rule, have high strength and low deformability. Not taking into account the specific properties of these soils can lead to a violation of the stability of buildings and structures, to excessive deformation. Structurally unstable soils are often referred to regional types of soils 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 can practically be absent in others. The features of the deformation of soils are differently manifested in different types of soils and depend significantly on the state of the soil and the intensity of the existing loads. Monolithic rocky ground with loads arising from the construction of industrial and civil structures, can usually be regarded as practically undeformable bodies. However, the fractured rock and the demountable rock ground have some deformability. The destroyed structural bonds in rocky soils do not recover over time. 2 Protection of foundations from groundwater and dampness
Penetrating moisture in building structures is a serious cause of their destruction. Protection against water penetration (waterproofing) is an important factor in the preservation and durability of buildings. At a high level of standing of ground waters there is a danger of their penetration into the basements, the formation of leaks and damp spots on the walls. Capillary moisture, rising along the pores in the foundation massif and the basement from the wet ground, can spread into the masonry of the walls of the lower floors. In case of aggressive groundwater, the foundation materials and underground parts of the building may collapse. To protect the building from groundwater, measures are envisaged to combat the movement of groundwater and the penetration of atmospheric precipitation into the ground of the base and arrange a protective waterproofing from the penetration of ground moisture into the structure of the building. To prevent the penetration of rainwater and meltwater into the underground parts of the building, plan the surface of the plot for building, creating the necessary bias for the removal of surface water from the building. Around the building along the outer walls arrange a blind area of dense waterproof materials (asphalt, asphalt concrete, etc.). To protect against penetration of soil moisture in the structure of a building during new construction, external insulation of structures from the side of water is usually performed, and for the old building, internal waterproofing is applied in basements. There are three types of waterproofing, corresponding to the types of water exposure - non-pressure, anti-head and anti-capillary. Non-pressure waterproofing is performed to protect against temporary exposure to moisture from atmospheric precipitation, seasonal perch, as well as in drained floors and ceilings. Anti-waterproofing - to protect enclosing structures (floors, walls, foundations) from the hydrostatic backwater of groundwater. Anti-capillary - for insulation of building walls in the area of capillary rise of ground moisture. The device of waterproofing of cellars is determined by the nature of the impact of water, the peculiarity of drained structures and materials, as well as the functional requirements for premises for operation, purpose and permissible humidity. This affects the choice of the type and material of insulation, determined by the necessary parameters for water permeability, water resistance, vapor permeability and durability. The opportunities of contractors, the season and the pace of work should also be taken into account when selecting waterproofing materials. There are various methods of waterproofing: the main ones are oklechnye, painting, coating, plastering, sheet (coffered) and clay, as well as special ones - injection, penetrating (penetration), geomembrane impregnation, seam, underwater, liquidation of active leaks, 3 Practical methods for calculating finite deformations of foundation foundations
Calculation of sediments by the method of layerwise summation. This method (without the possibility of lateral expansion of the soil) is recommended by SNiP 2.02.01 - 83 and is the main for calculating the sediment of foundations of industrial buildings and civil structures. Below we consider the order of auxiliary constructions and the sequence of calculations applied to the design scheme in the figure. Initially, the foundation is fixed to the geotechnical base situation, that is, the alignment of its axis. Under known loads from the structure, the average pressure on the base is determined from the base of the base p. Then, according to the rules given in § 5.4, starting from the surface of the natural relief, a natural pressure plot is plotted along the axis of the foundation. Knowing the natural pressure in the level of the basement foundation, determine the additional vertical stress in the plane of the basement foundation. In accordance with the foregoing, a diagram of additional stresses along the foundation axis is constructed on the same scale. Having constructed the diagrams of natural pressure and additional stress, they find the lower boundary of the compressible strata. This operation is conveniently carried out graphically, for which the natural pressure diagram, reduced by 5 or 10 times (depending on the constraint condition of the compressible stratum), is combined with an additional stress diagram. The intersection of the lines bounding these diagrams will determine the position of the lower boundary of the compressible stratum. The compressible base thickness is divided into elementary layers so that within each layer the soil is uniform. Usually the thickness of each elementary layer is not more than 0.5. Knowing the additional stress in the middle of each elementary layer, determine the compression of this layer. The norms allow to take the values of the dimensionless coefficient p equal to 0.8. № 5 EMTIHAN BILETI / EXAMINATION TICKET 1. Deformability of soils
The deformability of clay soils is due mainly 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 the sandy soils both processes of reorientation and mutual motion of particles occur and their destruction. The deformations are divided into volume and shape changes. Volume compressibility of clay two-phase soils is possible only when water is squeezed out of the ground. Since the pores of the soil are small, the free water is squeezed out slowly, and the process of deformation of the soil, depending on its volume, is stretched often over a long period of time. The increased viscosity of the cohesive water also slows the process of deformation (volumetric and deformation). The process of deformation of clay soil in time is described by the theory of consolidation of soils, some simplified provisions of which are set out in 12.3. In sandy soils, deformation processes under the action of a static load proceed quickly, so they are usually not treated in time. In coarse-grained soils, the processes of contact collapse, their destruction, the rearrangement of the structure due to the destruction of individual grains, and the redistribution of the load between the particles often take up a long time, even in clay soils, although the mechanism of the process is different in time. Since the soil consists of solid particles and pores that are partially or completely filled with water, then under the action of an external load in the ground, such deformations occur: Mutual displacement of particles and aggregates of particles with a more dense repackaging; Destruction of particles and their aggregates; Pressing water and air from the soil pores; Deformation of water films at points of contact of ground particles; Compression of air in closed pores of the soil; Elastic deformation of mineral particles. After de-stressing, some deformations are restored. They are called elastic deformations. These are deformations of ground particles, bound water films, elastic compression of trapped air bubbles and pore water. Such deformation of the soil is, as a rule, many times less than the deformation due to the shifts of ground particles, the squeezing of water and air from the pores, which are called residuali.e. not recoverable after removal of the load. As a result, residual deformations lead to soil compaction. 2 Foundations on weak clayey water-saturated and ground soils
To weak water-saturated soils are highly water-saturated soils saturated with water, which lose their strength at the usual rates of application of loads on the base, as a result of which their shear resistance decreases and compressibility increases. A weak clayey soil is a dispersed structured system with a coagulation type of structural bonds, capable of breaking from a solid to a liquid state. The current state of the soil is determined by the degree of disruption of structural bonds. When calculating the sediment of highly compressible water-saturated clay bases, it becomes necessary to take creep, nonlinear deformability and permeability into account. The cyclic application of loads, for example, in elevators, changes the strength and deformation properties of the foundation soils in time. Uneven loading of individual silos leads to significant uneven deformations. Experts recommend carrying out a uniform primary loading and unloading of elevators. Often, clayey soils (silt, belt clay soils, water-saturated loess macroporous and ground soils, etc.) are classified as weak water-saturated ones E≤ 5 MPa and s r≥ 0.8, φ = 4 ... 10 °, from= 0.006 to 0.025 MPa. The values of the filtration coefficients in the vertical and horizontal directions differ up to 10 times. The total sediment is divided into the part described by the theory of filtration consolidation and the part described by secondary consolidation processes. When designing foundations of shallow laying, it is necessary to limit: Mean precipitation by limiting values; Relative differences of sediments of adjacent foundations by limiting values; The rates of sediment flow are permissible. When seismic waves pass through a weak waterproof ground, pore pressure appears and the strength characteristics of the soil decrease. In these conditions it is recommended to use piles with full cutting of weak soils and support on strong. In addition, it is possible to use sand cushions, drainage slots with loading mounds, calcareous piles, and then compacting the soils with heavy tampers. In the case where the methods of compaction and hardening do not give an effect, and the sediment exceeds the limiting value, constructive measures are necessary. These include: increasing the rigidity of buildings by cutting sedimentary seams into separate blocks; Increase of rigidity of each block by the device of monolithic reinforced concrete or prefabricated-monolithic foundations; the device of reinforced concrete or metal belts or reinforced seams; the device of rigid diaphragms, for example, horizontal from plates; Increasing the flexibility and flexibility of flexible buildings and structures. The sedimentation of foundations is calculated using the design schemes in the form of a linearly deformed space or a linearly deformable layer. The boundary of the compressible stratum is determined at a depth where the additional stresses are 3 kPa for silt, and for grounded soils at a depth where the additional pressure to the natural pressure is equal to the structural strength. Additional sedimentation of foundations on the bases folded by water-saturated or organo-mineral soils due to the decomposition of organic inclusions is allowed to be ignored, if during the service life of the structure, the groundwater table will not fall 3 General requirements for the design of foundations and foundations
There are no standard projects for construction-ground systems. The system is calculated as a whole. The calculation is carried out according to the maximum loads. In the constructive scheme of the foundation, characteristic sections are chosen. In them, the interaction of the foundation and the foundation is calculated. The dimensions of the structure must correspond to all the calculations for the limiting states. Based on the results of calculations, fragments are constructed in each section, and then a foundation for the entire building is constructed. The bases and foundations are designed according to the variant method. The calculation is made for several sections and structures in general. Then a comparison is made for a number of technical and economic indicators, and the most economical option is chosen. For him, a project is being developed, including architectural and construction drawings and PPR. When choosing a variant, the following are taken into account: In the design of foundations, calculations are made for strength, crack resistance and resistance to deformation. № 6 EMTIHAN BILETI / EXAMINATION TICKET 1. Water permeability of soils Water permeabilitycalled the property of water-saturated soil under the influence of the difference in pressure to pass through its pores a continuous flow of water. In this case, a continuous flow of water is understood to be its inseparable motion (filtration) along the entire section of active soil pores, that is, that part of the pores that is not filled with bound water. Water permeability of soils depends on their porosity, granulometric and mineral composition, pressure gradient. 2 Foundations on saline soils
When calculating the bases composed of saline soils in the second group of limiting states, the base sediment should be determined taking into account deformations caused by the external load, as well as possible deformations from subsidence as a result of swelling, shrinkage and suffusion. The bases formed by saline soils should be designed taking into account the following factors: the possibility of formation of a suffusion precipitation as a result of water filtration followed by leaching of salts; reduction of strength characteristics as a result of changes in physical and mechanical properties during the leaching process; possible sagging or swelling with soaking, as well as increased aggressiveness of groundwater in relation to the 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 prolonged soaking and leaching is determined according to the formula (4.10), taking into account the values of the characteristics. If the calculated deformations of the base exceed the permissible limits according to the norms or the bearing capacity is insufficient, it is necessary to carry out waterproofing measures that exclude soaking, and in case of impossibility: measures to reduce the adverse effects of uneven precipitation; resort to partial or complete cutting of layers of saline soils with the replacement of the latter with cushions from silty-clay soils; use pile foundations with cutting through layers of saline soils; use consolidation and compaction of soils, as well as preliminary desalination with special substances that react with salts. If a set of measures is envisaged to prevent long soaking and leaching of soils, or the possibility of the latter is completely absent, the base sediments are determined both for non-saline soils with full water saturation. When calculating the bases in the territories under cultivation, account is taken of the calculated relative horizontal deformations, the radius of curvature of the deformation of the earth's surface, and the index of total deformations developing within the length of the building. The foundations in the work-in-progress areas should be designed taking into account the uneven subsidence of the ground surface, accompanied by horizontal deformations from the shear of the soil masses as a result of mining operations and moving the soil into the mined area during mining operations. Topic 13. Critical loads on the ground. Initial critical load on the ground. Limit load for loose and cohesive soils. Influence of properties of soils, the sizes of the base and depth of a laying on size of limiting load of the earth bases. Critical loads on the ground. Phase of stressed state of soil When soil loads are applied to the ground, compaction processes begin to occur in it, which can be divided into 3 phases: 1 The phase of compaction. The work of the soil during this phase occurs under conditions of sufficient strength. The soil is compacted to a certain value A. 2 Phase of local shifts. In the marginal zones, there is a local disturbance of the strength of the soil. With further increase) on the boundary between 1 and 2 phases Р з = Р пр is called the limit. 3 The stage of bulging. If the load increase continues, then when P 5 = P cr -critical load in soils. The shifting stress begins to predominate. Under the structures, slip surfaces are formed, along which the soil is displaced, and the ground protrudes from under the loaded platform, accompanied by its immersion. The mechanical phenomena that occur in soils with increasing local load were considered, and two critical loads were established (at ground pressures, high structural strength): 1-load corresponding to the onset of occurrence in the soil of the shear zones and the end of the compaction phase, when under the edge Loads between the tangents and normal stresses cause the relationships leading to the ground (first at the edges of the foot of the foundations) to the limiting stress state, and 2 is the load at which under the loaded surface solid areas of extreme equilibrium are formed, the soil comes to an unstable state and its bearing capacity is completely exhausted. The value of the first load will be called initial critical load
, which is still completely safe in the foundations of the structures, since, before reaching it, the soil will always be in the compaction phase, and the second, at which the full bearing capacity of the soil is exhausted, critical load
on the ground under the given loading conditions.
· The following groups of methods are used to protect trenches from underflooding: - dewatering; - anti-filtration curtains; - a combination of the first two methods. · The choice of a particular group of methods depends on: - the type of groundwater; - OLA (UGW); - properties of soils; - features of their bedding; - the depth, size and shape of the excavation in the plan; - other factors. · In all cases, whichever way we choose, it is necessary to exclude the violation of the natural structure of the soil at the base, to ensure the stability of the excavation slopes and the safety of the nearby buildings. → Water depletion is carried out by means of: - deep water reduction; - open water drainage 1. Outdoor water spill - the easiest way. The water is pumped directly from the pit. To be more exact, from the groove network arranged at the bottom of the pit, a depth of 0.3 ... 0.6 m, along which water is diverted into the sump (sump), where it is pumped systematically by pumps. - Open water drainage is used only in low-washable soils and rocks (fractured rocks, pebbles, gravel, large sands), and also where there is little direct water supply. 2. Deep water lowering Excludes seepage of groundwater through the slopes and bottom of the excavation. It consists in the artificial lowering of the UGW in the pit area. It is carried out with the help of: - wellpoints; or - pumping water from deep tubular wells (in the case of a large inflow of water). :
Filters consists of a steel pipe d = 38 ... 50 mm, the lower end has a filtering device, through which the suction and pumping of water is carried out. The filter is designed in such a way that the particles can not be removed. The resulting water movement (from the bottom of the excavation to the IHF) Fig. 14.9a, hydrodynamic pressure promotes compaction of soils a ... - improvement of their structural properties. · Light wellpoints (LIU) serve to lower the level of groundwater to a depth of 4 ... 5 m in the sands. At greater depths, the wellpoints are placed in several tiers (Figure 14.9b), or special ejector needles are used (water jet pumps that create a vacuum in the filter element, which contributes to an increase in absorption), which makes it possible to reduce the UWW to a depth of 25 m. - LIU is used in sands coarse, medium size and fine - Ejector needlepoints, as more powerful are used in dusty sands and sandy loam with k\u003e 0,1 m / day. Bituminization consists in feeding (injecting) into the ground, which has a fracture (rock fractured rocks) with a large influx of water heated to the liquid state of the bitumen. Due to which, a continuous waterproof wall is formed. Along with the injection of bitumen, cement mortar, or synthetic resins, is used. Injection of any material into the ground in order to eliminate its permeability is called tamponage. The end of the form
The formula of N.P. Puzyrevsky.
Consider the effect of a uniformly distributed load p on a strip of width b (Figure 4.6) with side loading q = γh (where γ is the density of the ground and h is the depth of the loaded surface).
Fig. 4.6. Scheme of action of the strip load
Vertical compressive stress (pressure) from the self-weight of the soil with a horizontal bounding surface σ 1 gr.
Let's take an additional assumption about the hydrostatic distribution of pressures from the self-weight of the soil, namely σ 2 g.
For an arbitrary point M (Figure 4.6) located at a depth z and characterized by an angle of visibility α, we find the main stresses taking into account the effects of the intrinsic weight of the soil as a continuous load:
We substitute expressions in the equation of limiting equilibrium:
if we assume that z = 0, i.e. at no point in the ground there are zones of ultimate equilibrium, 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 on it can be considered as absolutely safe.