CC BY
36
Rasulov Rustam Khayatovich, Tashkent architecture and construction institute, candidate for technical sciences E-mail: [email protected]
Seismic subsidence deformation of moisturised loess
Abstract: The results of experimental studies the influence of seismic subsidence on deformation of loess at different accelerations are being described by the author. The methods of laboratory and field studies on soil's seismic subsidence are provided. The factors affecting the seismic subsidence of the soil deformation are identified.
Keywords: Loess, seismic subsidence, porosity, comparative deformation, module subsidence, vibro — compression curvilinear, acceleration of fluctuation.
Additional subsidence of loess occurring under vibration is called "seismic subsidence" and its record is important especially when assessing the stability of structures in seismic regions.
The seismic subsidence of moisturised loess may have a catastrophic effect on the stability of the structure in specific cases.
The seismic subsidence natural soil can be estimated using the natural porosity coefficient ^ in the form of:
P = -(1)
n
where nnp — the natural porosity of the soil; n — the porosity of the sample.
When np = n the coefficient is /3 = 0 and the soil will be in equilibrium state. In case of P > 0 the soil is considered to be unstable seismically in the greater extent rather than /. In case of P < 0 the soil is characterized by insufficient density and has a tendency under certain conditions when it is shaken to de compaction.
Vibration (a) ■ . ,,
1 ' , Time t, month
0'
10
s 70
S:
r 30
<1)
F
<D 40
Cl)
1/1 50
60
70
1 a 1
! b II
c i i i
'h
III » «
V V ¿L-: IV
J _ g
Fig. 1. Typical curve seismic subsidence of construction on loess soils
Fig. 1 shows the different possible cases of subsidence of loess soils structures on the same initial porosity. The curve pattern I corresponds to the construction of buildings on the thickness of the dry loess moisture about 4-9 %. In this condition, the soil is characterized by high strength and low compressibility, and the total subsidence of construction with insignificant.
The curve pattern II corresponds to the construction of buildings on loess thickness with high humidity (about 18-23 %). Due to the higher level of moisture, the sustainability of soil decreases, while increasing its compressibility. The settlement n2 increases and that often lead to cracks in buildings.
The curve III corresponds to the condition when the thickness of loess is exposed to dynamic stress while moistening with water. In this case, the soil humidity reaches its maximum, the sustainability reached its minimum and compressibility increases dramatically.
It is obvious that the sludge structure in these conditions significantly greater than in the first two cases.
The curve IV describes the condition of dry forests. In this case the soil sustainability remains stable in the range of 10 - tc. The moment tc corresponds to the start soil's tremor and further subsidence of construction will be characterized by three stages (branches): Oa, ab and Br. In the first stage (a branch of Oa) IV curve coincides with the curve 1 (dry loess). In the third stage (branch vg) IV curve is slightly above the curve III (soaked loess). The sharp jump of subsidence at the beginning of the second stage (branch ab) explains to the influence of vibration on the soil's deformation. The magnitude of this jump determines the seismic subsidence of the construction.
Thus, the seismic subsidence structure in this case is associated with a transition of soaked loess from step I in stage III under fluctuating. When seismic subsidence occurs, usually there are cracks and distortions in erections; sometimes they can completely collapse. To ensure the smooth operation of facilities, in such circumstances, actions which need a good amount of financial expenditures are often used with proper design and use ofspecial methods of construction.
It should be noted that in the project being examined, the loess which due to any reason was soaked during the construction activities is considers to be of minimum reliability. And the loess which occurs in natural conditions below the soil water is more reliable.
The quantity of seismic loess (rf ) based on other conditions depends on the thickness of the layer that lies above the subsoil water level. With increasing capacity of thickness, the quantity np increases in almost same proportion. It is also natural that the quantity is determined by the greater or lesser capacity of the rocks composing this thickness, causes the seismic subsidence phenomenon.
Seismic subsidence of loess, as well as the compressibility of the soil is more easier and graphically depicted by the comparative deformation e. In terms of this quantity in parts per mille, we will operate with seismic subsidence module ep, which corresponds to the sediment in millimetres of meter layer of loaded soil under a tremor acceleration a.
c
The degree of seismic subsidence is different for varieties of loess, as well as others from the type of clay and determined by their composition and condition.
We will use the term of seismic subsidence module ep as per the analogy of accepted subsidence which features the relative soil compaction under the influence of tremor in the form of:
< = ^, (2) p h
where: Vh — absolute value of compaction of the sample under tremor; h — first height of the soil sample, compressed in the compression by p.
Module seismic subsidence is a dimensionless quantity. In practice, it may be used in its absolute value or its expression in
Seismic subsidence deformation of moisturised loess
percentage ( %) or, as per the recommendations of N. N. Maslov in pro mille (mm/m) [1].
The last method is especially useful. In this form it is called as a module of seismic subsidence and is indicated as ecp. The index p indicates the quantity of static load p, which is attached to the soil, and c index meets the conditions of seismicity of seismic subsidence module definition.
Under this condition, the module el of seismic subsidence rep
flects the amount of deformation, i. e. the amount of compression in millimeters of soil in the 1.0 meter high column in case of tremor under its load p .
Here the formula (2) will have following from:
Vh
(3)
ecp = 1000-p h
The indicator ecp has a specific engineering meaning and is very simple calculations.
Seismic subsidence of soil under dynamic loading is studied in compression conditions and vibro compression curves are based on the results of experiments (fig. 2).
120
<= 100
7Î SO
60
40
20
c ep tl C
Com pression
<0 u
yj M
Brai ich A Co mj iress a Oil Comme icement af vibrat a
1000 2000 3000 4000 5000 6000 Acceleration of fluctuation, mm/s2
Fig. 2. Vibro consolidation of moisturized loess under seismic influence
Unlike conventional compression tests these experiments are implemented without vibration of soil the initial stage of the load. This stage is usually characterized by low soil compaction value (branch A). Furthermore the sample is subjected to vibration. The soil under this vibration gives a sharp drawdown (branch B). Then the deformations of the sample after the vibration (branch C) are observed.
Important exponents loess seismic subsidence are the porosity and moisture (table 1). The lower the natural porosity, the more they are sealed under tremor, therefore, seismic subsidence of the construction will be higher.
Table 1. - Seismic subsidence deformation of loess with differ porosity
Degree of soil's Porosity, % Module of seismic
deformation subsidence, mm/m
Not deformed 35-40 0
Hardly deformed 40-45 10
Deformed 45-50 50
Strongly deformed 50-55 100
Dramatically deformed > 55 > 100
Field experiments should be conducted for the greater reliability of loess' seismic subsidence. At the same time observing more experienced subsidence under the stamp (plate) with varying
degrees of vibration of soaked soil is continued. Stamp 50 x 50 or 100 x 100 cm. is established on a layer of sand in the pit. Pit is filled with sand to prevent bulging of the soil beneath the stamp.
After rainfall attenuation the water is supplied through pipes to the bottom of the pit in the sandy layer for soaking the loess strata. Next dynamic load is put on the stamp and its drafting observed. The stamp gives a sharp slump in case of seismic subsidence phenomenon.
To control the before and after the experiment, the samples are taken out of the stamp and initial of n0 and final porosity n1 modules are determined. The seismic subsidence module is determined via following [2]:
" (4)
X
1.1D
where: X — sediment stamp, mm., which occurred after water supply into experienced pit and dynamic load applications; D — side of square stamp m.
Control is being carried out with the formula:
n„ - n
e =1000-
1 - n
(5)
where: ec — module of seismic subsidence soils mm. per meter of thickness; n0 — initial porosity of the soil; nl — soil porosity after seismic subsidence.
The following are the results of experimental researches conducted to study the factors affecting the module of seismic subsidence at different vibration effects.
The role ofgradation ofthe soil as per the module of seismic subsidence is seen from the graph illustrated in fig. 3, where the results of experiments with different contents of particles per size are reflected.
<L>
3
o
1
Soil
Soil № 7
Soil №1
.> More than 40 40 30 20 10
The content of clay particles (0.005 mm)
Fig. 3. Dependence of module seismic subsidence of loess on the content of clay particles
The analysis of such kind of experiments showed that the homogeneous soil is in unfavourable condition in dynamic proportion heterogeneity. On the other hand the content in the soil clay particles results in a reduction of loess deformation due to the emergence of internal consistency between the grains.
The series of experiments to determine the effect of soil porosity (n) the quantity of seismic subsidence module ( ep) were conducted on loess soils, taken from different depths of the strata. Thus, the module of seismic subsidence ecf with average porosity of the upper layer (1-3 m.) n = 48 % under tremor with acceleration of a = 3000 mm/s 2 exceeds 118 mm/m, which should be considered more than significant value.
The nature of the possible dependence of the seismic subsidence module (ec) on the porosity of the Tashkent loess at a
constant intensity fluctuations is seen on fig. 4. As we can see, here is a very weak dependence ep = f (n), although at relatively low values of the a. The picture changes with increasing acceleration of the vibration motion.
SO
E 70
S GO
sz
Q)
50
i/i
_Q _>
t/i 40 o
Ï2 30
20
<s>
'at
i/i
o
jy
Z5
■a o
10
a =3000 mm/s
So 1 №2
s. 5il №7
S uil №d!
Soi №5
n -
43 44 45 46 Soil porosity, %
47 48
Fig. 4. Changes of seismic subsidence module depending on porosity of soil under continuous acceleration of fluctuations
40
30
U6t5% h-V-45,8% a=22Q( -2500 mm/s2
Soil №4
^Soil №7 44,6 43,
43,0 1 42,
5 Pa
0,5
1,0 1,5 2,0
External load, 105 IIa
3,0
Fig. 5. Dependence of seismic subsidence module on external load under fluctuations with acceleration 2200-2500 mm/s2
Fig. 5 illustrates a graph of seismic subsidence module ( ep) dependence on external load of sample which shows a sharp decrease of module ( ecp) in unit values with increasing load. Also intensive decrease ecp is considered in the initial stages of the external load application. So, for the loess of upper horizon (1-3 m.) with
a porosity n = 47.8 % through increasing the load on the soil of 0.5-10 5 Pa. to 2.5-10 5 Pa. the quantity of seismic subsidence module decreases from 112 mm/m.
We can therefore conclude that the role of the load during the deformation of the soil is not only favourable, but also highly effective, particularly when designing anti seismic subsidence events.
Influence of intensity and nature of the dynamic load in the speed of seismic subsidence deformation (seismic subsidence module) of loess also was the object of research. Experiments conducted for this purpose on different loess provided the opportunity to establish directly proportional dependence of loess seismic subsidence from the fluctuations' intensity.
So,
a = 500 mm/s 2; ec = 4,0 mm/m;
p
a = 1000 mm/s 2; ecp = 8,8 mm/m;
, 2.
a = 1500 mm/s 2; ecp = 20,0 mm/m; a = 2500 mm/s 2; ep = 30,0 mm/m.
As an example, the fig. 6 shows a schedule in form of dependence ecp = f (a), which means that speed of deformation of soil's seismic subsidence increases with the increment of intensity of the measured acceleration [3] of the vibration motion.
r
Soil №8 (r =48,8%)
/7
S. Soil №8 (r =42,2%)
J / / Soil №£
/ / O ** is Nl 1(1=41, !Ï3~n~ 44,71 %) 6)
k ^ Soil №13 n=40J%)
500 1000 1500 2000 2500 3000
Acceleration of fluctuations, mm/s2
Fig. 6. Dependence of ecp= f (a) on loess soils of various compactness
The last one is important in assessing the stability of the soil subjected to seismic influences. It was noted that such dependence ecp = f (a) is observed up to so-called limit of acceleration for concrete soil, above which the seismic subsidence of soil deformation becomes progressively increased. The increase of soil deformation with increment of acceleration of fluctuations occurs due to the intense vibrations of the destruction of the structural links (primarily power connectivity between the particles) of soil under vibrations.
During processing the results of researches showed the increment of the module of soil's seismic subsidence under fluctuation of the soil with high frequency that is vital for regions characterized by high-frequency earthquakes.
References:
1. Maslov N. N. Basics of Engineering geology and soil's mechanics. - Moscow: «High school», 1982. - 571 p.
2. Rasulov H. Z. Seismic resistance of soil foundations. - Tashkent: Publishing office «Uzbekistan», 1984. - 192 p.
3. Djuraev A. Influence of subsidence of loess soils of Tashkent territory on seismic effect under Tashkent Earthquake in 1966. Conference materials «Scientific and practical basics of solving actual problems of seismology». - Tashkent, 2006. - P. 282-285.
