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Depth location of the foundations in seismic areas
Rasulov Rustam Khayatovich, Tashkent architecture and construction institute, candidate for technical sciences E-mail: [email protected]
Depth location of the foundations in seismic areas
Abstract: Despite prevailing opinion on the independence the depth location of the foundation structures from seismic area the author brings up the idea to increase the earthquake resistance of buildings to the extent of embedment of the foundation. As a result of conducted studies, the formula for determining the depth of the foundation, taking into account the intensity of earthquakes was proposed. It is noted that the use of the proposed formula helps to ensure the stability of any construction erected in seismic regions.
Keywords: soil, seismic subsidence, dynamic load, depth location, the foundation base, soil adhesion, fictitious depth, limited load.
Stability of constructions erected in seismic region, mostly depends on the strength of the soil which form the bases, their ability to withstand the vibrational motion during earthquakes. But the construction practice shows that even the implementation of this requirement does not always guarantee the seismic stability ofbuild-ings and the normal conditions of operation [1].
These mandatory normal operating conditions of operations and facilities are frequently violated during their seismic subsidence too due to deformation of weak soil differences in the thickness of the base under the influence of dynamic forces. Particularly, quite often such situations occur upon availability of such soils as moisturized loess and loess-like types in the thickness of erections' base.
For a number of reasons the seismic subsidence of constructions-(non-homogeneity of foundation base and characteristics of constituent soils, uneven dynamic load and dissemination of pressure in the soil thickness, etc.) is always in uneven. In particular cases, it is measured in tens of centimetres (sometimes more than 1.0 meters) upon availability of the base with little resistant deposits loess structure [2].
In such cases there is a danger of inclination (tilt) and the warping of the building and its separate elements, as well as derangement of the constructions' sustainability where cracks and breaks occur as a result of the activation over voltage appeared due to deformation. Consequently, in most casesone of the most reliable methods addressing sustainability of foundation soils is the right selection of the depth location of construction's foundation.
The practice of constructions' operations in seismic areas shows that the deepening the buildings' foundations, i. e., the inception of erections' foot to a certain depth from the surface of the thickness will be the simplest and most effective measure to reduce the expected seismic subsidence deformation as a result of the vibrational motion of the base.
The solution to this problem in the interpretation of the author is provided in following manner [2].
Referring to fig.1, which shows a conditional diagram of construction, which was built on the surface of the soil.
Fig.1. Design diagram
In this case, the estimated load on pp soil in the level of foundation base, taking into account the seismic influence is determined by: pp = p0(i+ké )-rh, (i)
where: p0 — pressure of construction's weigh; kc — seismic coefficient; y — density of the soil; h — depth location of foundation.
With regard to current case, the calculation of the foundation's depth location is leads to the following considerations:
ph = ух = y(z + h3), (2)
where: z-dept location considered horizon's bedding is below the level of the actual application of load p0 to the soil, i.e, below the foot of the foundation.
Let us assume that at a depth h from the surface of the soil there is a foot of construction's foundation with an F area which transmits the pressure p0 to the soil. It is required to identify the depth of h foundation, where oscillating foundation will give deformation (seismic subsidence). We shall replace the construction with the pillar
Section 9. Technical sciences
structure ofsoil, with h height which has the same F foot area and p0 weight. Consequently, the pressure transmitting by soil column to the base will be the same this building transmits, which is provided in the following manner considering the inertia forces:
HFy = p0(l + k/M). ^ (3)
Let us assume that the prism split off the soil's column on the surface AB with the sole base F1, and as a result of the destruction of the soil under its base AF it tends to sink into the soil.
The destruction of the stability of the soil under the foundation is represented in the form of a prism AFC (prism collapse), tending to slide down BY AC surface. The prism FCD (resistance prism) will prevent this shift and loaded GFDE soil layer, with height h. Obvious, the collapse prism will put pressure on FC area to the resistance prism through some force Q.
Let's assume that the efforts required to shear resistance prism and lying layer of soil on it is equal to R and Qforce is directed normally to the surface FC. The size of angle fi is determined by assuming that R is the minimum.
Hence, the condition of soil dynamic stability will be determined as per following:
R > Q (4)
Obviously pressure Q will be active and the pressure R passive. The angles of inclination a and fi are defined as:
" (5)
a= 45 ; 2
ß = 45» + -. 2
(6)
Q h R quantity in accordance with the equations and theory of soil's pressure on retaining wall will be defined as: for active pressure:
for passive one:
Q = (^Yz2 +YHz2 (45° - 2"),
R = (^ 2 +Yhz 2 (45° +1).
(7)
(8)
Placing the values (7) and (8) to the condition of limiting equilibrium (4), after appropriate conversion we shall obtain:
Yh = p°(1 + k/M )-
tg2(45° )
tg2 (45° + 2)
(9)
In current formula, p0 as noted above means the pressure of structures equal to the h height of the soil prism pressing with its weight same pressure to the base. Considering that:
-=-= tg2 (45°-*-), (10)
tg2 (45° +|) ctg(45° -|) 2
expression (9) to the desired dept location h of foundation H can be represented as:
Po(1 + keimt )
H = -
(11)
Ytg4 (45° - 2)
It should be noted that the formula (11) was derived without considering the clutch in the soil of the base. This indicator straight
of a soils is easily calculated using famous method in soil mechanics by considering it as the force of friction in the soil to the pressure of a particular fictitious column of the soil with height hc and compactness y , i. e. [3]:
c
h =
Ytg$ '
(12)
where: hc — fictitious depth, defined by fig. 2.
Q 4
Z, 2
h
Angle of internal friction, shatter
Fig. 2. Schedule to define fictitious depth location of foundation h at the value of adhesion c = 0.1-105
c
Considering (12) the formula (11) will be rewrittenin final version:
H = p„(i+kr ) +
Ytg (45„ - 2)
(13)
Table 1. - Seismic subsidence of loess soil in various depth locations of foundation
№ Foundation depth H, m. Maximumloadat the sole of foundation p 10 5 Pa. np Seismic subsidence, mm.
1 1.0 0.33 57
2 2.0 0.48 48
3 3.0 0.63 39
4 4.0 0.78 31
5 5.0 0.93 27
6 6.0 1.08 22
7 7.0 1.23 18
8 8.0 1.38 16
9 9.0 1.53 14
The formula (13) has notability properties. Upon considering it thoroughly we can note that the quantity H is located in direct proportion dependence on p0.
For more evidence of the role of the foundation depth location in ensuring the seismic stability ofbuildings, the seismic subsidence for particular building under various layers of the foundation was calculated. The results of these calculations are summarized in table 1, which implies a significant role of the foundation depth location in the seismic resistance of constructions.
2. 3.
References:
Rasulov H. Z. Threshold of seismic subsidence as a factor ensuring seismic stability of loess soils//"Architecture and construction problems" Journal. - Samarkand: Publishing Office of Samarkand Architecture and Construction Institute, 2008. - № 2. Rasulov H. Z., Sadikov A.H, Rasulov R. H. Landslide dilution in the loess slopes. - Tashkent: Publ. House "Credo Print", 2014. Rasulov R. H., Tashkhodjaev A. U. State of tension of soil thickness in the propagation of seismic waves in it//"Problems of mechanics and construction of transport facilities", Il-International Scientific and Practical Conference. - Almaty, 2015.
