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The Present Study Situation of Seismic Isolation by Sand Gravel

Underlayer and Several Questions Concerned

JIANG Xiaomin1, YAO Xuequn2

1. Capital Construction Department of South-west Petroleum University, Chengdu, Sichuan, P.R.China,

610500

2. Urban & Country Construction and Engineering Management Department of Kunming University,

Kunming, Yunnan, 650216

Abstract: Expatiate the application of sand gravel underlayer for seismic isolation and Productions

about these that have been studied. Offered disagreement about the calculation mode of sand cushion for

some scholars. Suggested it should allow a certain extent sliding displacement take place of

constructions building on sand cushion. Also suggested the content of define the vibration reduction

ratio.

Keywords: sand gravel underlayer, seismic isolation, calculation mode, sliding displacement responses

spectrum, vibration reduction ratio

1 Introduction

Now we have recognized that traditional seismic techniques used by "Ying Kang" measures, that is used

to strengthen the structure, bold elements section, even be reinforced components, and other methods to

improve structural rigidity to resist earthquakes, ultimately because of the greater stiffness , the greater

the earthquake, not the economy, not necessarily safe. The traditional seismic design "minor earthquake,

the earthquake can repair, earthquake is not down" of the seismic fortification goal, the structure may be

under strong earthquakes seriously damaged and can not guarantee the time of severe earthquake safety

of life and property. The use of seismic isolation, energy dissipation and other control techniques are

consistent with current performance-based seismic design, a better guarantee strong earthquake occurs,

personnel, equipment, apparatus safety, has been showing good prospects for the development and

application .

2 Ground Isolation Technology Brief

Isolation and energy dissipation technology, the idea is that the seismic effect of earthquakes from the

"anti" to "guide", is not relying on seismic structure itself, but through the addition of some of the device

isolation, energy dissipation or damping.

Isolation unit set up by different parts, it will be isolated and divided into: isolation, base isolation,

foundation isolation. At present the use of more mature technology is the basis for isolation. "Seismic

Design of Buildings" (G B50011-2001) The isolation method is used in base isolation method. Isolation

mainly by setting the bottom and the foundation of the building between the top surface of the isolation

damper, seismic energy absorption and dissipation, and thus significantly reduce the earthquake

response of buildings to isolate the transmission of seismic energy structure of the Department up to

reduce the vibration of the superstructure to prevent earthquake damage. Research and application

programs over a wide range of base isolation are: isolation rubber program (including natural rubber, the

standard multi-layer rubber mat, high-damping rubber pads and lead multi-layer rubber mat, etc.) and

use of talcum powder, flexible graphite , PTFE sheet, lubricating materials such as steel sliding friction

sliding material hysteretic isolation programs and steel ball bearing support, double roller bearings, ball

bearing sliding isolation box columns and other programs.

Ground isolation is the basis by setting the building and the soil between the bottom of sand, stone

cushion, or composite materials, or other ground handling to reduce the seismic energy to pass up the

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structure of the Department of isolation purposes, are mattresses layer isolation of the area, its isolation,

damping mechanism and base isolation method is different compared to the current standard

recommended by the base isolation rubber bearing isolation method, sand, stone cushion method

isolated more low cost, construction simple, although some of China's ancient architecture (such as the

famous arch bridge) by this method successfully to resist occurred several times in history, earthquakes

and other natural disasters, and stand tall, but the sand cushion the vibration theory currently In the

study, only in some rural towns have houses built in the use of sliding sand cushion technology has not

been adopted by national norms, only the formulation of Shaanxi and Liaoning provinces on the

corresponding sliding technology, local standards, but also enrich the theory, also need more validation

experiments and engineering practice.

3 Gravel Cushion Isolation, Damping Mechanism and Research

3.1 The basic principle of sand and gravel bedding isolation

Gravel cushion isolation, need to lay the building foundation and a certain thickness between the

foundation, density and water content of sand or stone (also available on the sand mixed with gravel)

cushion, sand and gravel are coarse grained soil, the nature widely distributed, abundant, because of its

compaction properties, absorb water, filling density, shear strength, subsidence deformation is small,

capacity of advanced engineering features is an efficient natural building materials. Isolated use of sand

and gravel bedding sand friction coefficient is high and easy to sand movement, the characteristics of

tensile stress can not be passed through the large gravel surface deformation to the impact of energy

consumption, reduce the ground motion on structures to ensure the safety of buildings. From the

perspective of mechanics, due to the superstructure stiffness, and stiffness of the foundation soil,

earthquake, the entire system will be part of the deformation of soil borne by the foundation, while the

upper structure and the occurrence of cushion with a degree of rigid flat action. In this way, sand and

gravel stratum and soil constitutes a natural isolation, seismic energy through gravel and soil

deformation is absorbed, thereby reducing the seismic structure of the transfer of energy to top, to

achieve the effect of isolation.

3.2 The Results of Present Research

Liu Xiaoli, etc. on the sand cushion thickness and substrate pressure on the seismic response of the

ground has been studied[1], when used in the study seismic safety evaluation of engineering software

(ESE) in the SLRA module seismic dynamic response of foundation soil Analysis of the module to the

soil shear force method of equivalent linearization analysis of observed soil depth of the seismic

response of any. With thickness lm, shear wave velocity is 200m / s of the sand cushion, base pressure is

50kPa, 100 kPa,, 200 kPa three levels, input EI-Centro wave (peak acceleration is 200gal), results show

that: Sand Pad basal layer in different seismic response of the average pressure similar to, that is, the

average pressure and substrate size. Thickness for the sand cushion the impact of seismic response on

the ground doing the earthquake simulation, the shear wave velocity of sand cushion as 200m/s, taking

into account the actual situation of construction and economy, changes in sand cushion thickness 0.5, 1.

0, 1. 5, 2. 0, 3. 0m total of five sections, enter the EI-CentrO wave, moving Bo and Parkfield wave,

adjust their peak acceleration were 200ga1 (1gal = 1cm/s2), 100ga1 and 200gal, seismic response results

show that, compared to the corresponding results of the whole clay section, when the input EI-Centro

wave and the Parkfield wave, the general trend for the sand blanket thicker, the better shock absorption.

But the input move Bo, the damping effect is not clear, it is clear the input moving Bo's case, the

thickness of sand cushion the impact on the value of acceleration is not.

Liu Xiaoli, etc. in the input ground motion on the sand cushion the impact of isolation[2], enter the three

types of seismic waves peak acceleration of 50, 100, 200, 300 and 400gal, seismic intensity

corresponding to 6 degrees, 7 degrees, 8 degrees and 9 degrees. The results show that: the role of small

earthquakes under the sand cushion and no sand cushion the acceleration response time close to the

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foundation; in the role of a major earthquake under the sand cushion significant damping effect. Enter

the EI-Centro, wave, equivalent to 8 degrees in the intensity of earthquake situation best, and maximum

shock absorption 23%. However, the acceleration peak 50gal, enter the EI-Centro wave acceleration

slightly enlarged, but add no more than 4% of earthquakes. Bo and Parklield wave input to move,

regardless of the size of peak acceleration, can play a role in reducing the earthquake.

Liu Xiaoli, etc. Also on the sand cushion stiffness on seismic response of the ground has been studied[3],

the study will cushion thickness on the value, change the stiffness of cushion (to cushion the shear wave

velocity measured value) , in 120, 150, 180,200, 230, 250, 280, 300m / s shear wave velocity of the 8

cases, the input EI-Centro wave, moved to Bo and Parkfield wave (peak acceleration, respectively

200ga1, 100ga1 and 200ga1), Under the assumption in its model to calculate the stiffness of sand

cushion in less than 200 ~ 250m / s (in secret), the acceleration value increases with the stiffness of sand

cushion the increase, over this range but decreased after the acceleration , then curve to flatten (300m / s

to drop by less than 5%). Also calculate the dense sand cushion under different damping ratio, on

average, concluded that the loose sand cushion in the state (120m / s) under the damping effect is better,

especially in Parkfield damping effect of wave action best, 39.8%; sand blanket in the dense state

(150-250m / s) and dense state (280-300 m / s) under, EI-Centro wave damping effect of the best,

followed by Parkfield wave, moving Bo worst.

Douyuan Ming and other properties of the sand cushion vibration isolation table test[4], confirm with the

sand cushion density (sand cushion stiffness) increases, damping effect has been reduced; input

EI-centro wave, sand cushion thickness, vibration reduction increased input sine wave, the thickness has

little effect on the damping effect.

Zhao Shaowei, also using sine wave and the EI-Centro wave in the shaking table test of[5] found that the

relative effect of a single particle size of the isolation is better graded, also found that a single particle

size than in the 5.0mm 2.5mm of the effect, regardless of the changes in the thickness of sand cushion,

5.0 mm diameter sand cushion can always be ideal isolation effect. analyze the reasons is the larger size

of sand cohesion between the poor's sake, If you use pellets sand screening, isolation may have better

results.

Liu Darong Isolation of sand cushion in its[8], considering the load and the isolation and other factors

made the isolation of sand cushion design for non-collapsible region isolation design of some reference

effect.

4 Questions for Further Study

4.1 Calculation of model sand and gravel bedding problem isolation

Liu Xiaoli in the literature[3], the simplified model assumed in calculating Figure 1, that take into

account the flexibility of buildings and infrastructure is much smaller than sand cushion, it can be

omitted and the quality of them as a focus; the other hand , the quality of sand cushion than the quality

of buildings and infrastructure is much smaller, it can also be omitted, but the sand cushion as spring;

sand cushion itself based on internal friction and the friction between and around the constraints from

the damping effect, they could be together as a damper. Will be placed in the sand cushion on the basis

of quality as a single degree of freedom (the basis of quality) of a spring system (spring on behalf of the

elastic nature of the sand cushion, spring constant k) of a damping system (damping on behalf of the

damping properties of sand cushion, damping coefficient c).

Fig.1 Sketch Map Of Seismic Isolation of Sand Cushion

The quality of foundation

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I believe that this assumption the model is problematic, perhaps because reference on Maximum

Qing-Shan Li, etc. carried out on the sand cushion closed underground explosion measured[9] The

reasons for the model. Single degree of freedom of the system to represent the spring with a sand

cushion of flexibility, the prior condition is fixed the upper sand cushion can provide the basis and level

of force, in fact, easy flow of sand can not pass the level of tensile stress, while the literature[9] in the

shock isolation bed system, is composed of viscous dampers and spring isolation devices can not be

simply applied to the calculation of sand cushion isolation building model. Sand cushion damping

system represents the actual damping force most of the internal friction of sand cushion and the base

itself and the friction between the surrounding constraints. Should be simplified model[6] in the model,

shown in Figure 2. The equation of motion should be used as reference[7]: 0)( 0 =++− Fxxm &&&& .

Where F is frictional force, mgfxF k ⋅⋅−= )sgn( , kf is dynamic sliding friction coefficient,

)sgn(x is sign function.

Fig.2 Sliding Isolation System of SDOF Fig3 Sliding Isolation System of MDOF

Reference[6] also presented a model of multi-mass system, shown in Figure 3, the equation of motion

should reference[7] using the equation expressed in matrix form and instructions.

4.2 Isolation of sliding sand and gravel bedding deformation

Many scholars believe that the basis of a simple sliding can not solve the excessive slip deformation and

the problem can not be reset, so isolation device should be equipped with adequate damping (such as

lead rubber bearings, such as mild steel dampers ) to provide resilience against the unpredictable effect

of strong earthquakes occurred in a large displacement.

However, these results show that the sand cushion under strong earthquake has obvious damping effect,

and the other by the peak in the literature[10], using the equivalent linearization method, given by the

stochastic model of rigid frictional base isolation Structure slip response spectrum (see Figure 4). ma is

maximum acceleration of seismic ground mean, we can see even more than 9 degrees corresponding to

the ground acceleration, to 750cm/s2, the dynamic friction coefficient of sand cushion the maximum slip

value of 0.2 responses (single-particle displacement relative to ground), but about 7cm , while the

calculations described in the evidence with great credibility, such as[11] to the proof in the epicenter of

the 1976 Tangshan earthquake intensity of the urban culture of 10 degrees the same way two three-story

brick building, a collapse, one does not collapse, This is a building foot of the wall did not collapse the

entire building has a level of office crack, cracks can be seen clearly in the sliding traces of brick walls

between the upper and lower part of the residual displacement 6cm. The author believes that as long as

the structural integrity after the earthquake, resulting in less than 15cm slip displacement on such a

small building used has little impact on the general structure of architectural fashion and the peak to

some extent to allow the horizontal displacement across sand cushion earthquake should also be allowed

housing housing relative to some extent sand cushion slip. This is also the future research in this area

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and the development of standards or norms should be added to the content.

4.3 Definition of vibration reduction ratio

Different scholars in their research, to facilitate their research defines the different damping rates, such

as Liu Xiaoli in the literature[3] definition of damping rate a = [baseline profile output maximum

acceleration - a sand cushion the maximum output acceleration] / baseline profile of the maximum

acceleration of output x 100%, while Zhao Shaowei in the literature[5] definition of damping ratio β =

[input acceleration absolute - the absolute value of the output acceleration] / input acceleration absolute

value, I believe that what indicators to use how scientific and reasonable measure of the damping effect

of sand cushion is worth studying.

Fig.4 Sliding Displacement Responses Spectrum of Base Isolation Rigid Structure

WANG Yayong in the literature[12] discussed the design acceleration response spectra for seismic design

of the limitations of the energy method using velocity response spectrum and response spectrum

analysis of the feasibility of displacement. This is because the building used for functions and

earthquake damage, it has been generally agreed that the structure of the deformation or displacement by

force than the structures more important. Do not fall down and decided to housing more directly the

deformation or displacement, rather than internal forces, while the displacement and energy (potential

energy) linked to, force and acceleration are related. The paper quoted the analysis shows that single

degree of freedom system in the different energy distribution of seismic waves in the elastic stage, the

maximum instantaneous input energy and maximum displacement has a good relationship, seismic wave

energy distribution and structure ductility have little impact, structural plastic into more complicated

The mean of the maximum slip

response(cm)

The mean of the maximum slip

response(cm)

The mean of the maximum slip

response(cm)

The mean of the maximum slip

response(cm)

Dynamic friction coefficient Dynamic friction coefficient

Dynamic friction coefficient Dynamic friction coefficient

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situation after the seismic wave energy distribution and structure of the natural cycle has an impact on

this relationship are, for the energy on the edge of the seismic waves of short period structure and

energy distribution of seismic waves (such as EI-Centro and artificial waves) under the long-period

structure, the maximum instantaneous input energy and maximum displacement has a good relationship.

For the analysis of single degree of freedom system has proven that a relationship exists. Because the

instantaneous energy reflects the characteristics of seismic waves, in theory, should also be multi-degree

of freedom system has a corresponding relationship exists.

In particular, the earthquake, the isolated structure because most of the deformation occurred in the soil

and the cushion or slip on the deformation of the structure itself is relatively small, making the general

structure of the elastic state.

From this analysis, the future of various isolation or energy dissipation structure of the damping effect,

with a shock absorber damping measures, measures and no two structures, or internal forces of change

in acceleration compared to measure the damping effect is inadequate Department, and the change of

displacement is more desirable in the literature[10] for the rigid structure of sliding friction base isolation

displacement response spectrum in the future research of great value, I believe that the rate and vibration

damping and non-isolation of the maximum rate of change of displacement between layers is more

realistic.

5 Conclusion

This paper reviews the use of isolation technology isolated sand cushion some of research results; in the

analysis of the sand cushion of isolation mechanism, the computational model of some scholars

questioned whether the right; and based on seismic design and slip displacement spectra data have been

proposed in the research and development of isolated sand cushion the housing standards or norms

should be allowed to have some sand cushion housing relative displacement, a measure of how scientific

and reasonable damping sand cushion damping rate effects and the definition put forward their own

proposals. As sand, stone cushion method isolated more low cost, simple construction methods, in the

future the use of natural materials in isolated areas of the foundation needed to continue in-depth, state

standards have yet to be formed.

References

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pressure on the seismic response of the ground. [J] North China Institute of Aerospace

Engineering in June 2004 Volume 14, No. 2 (in Chinese)

[2]. Liu Xiaoli, historical Court, Song Yimin, Douyuan Ming, Liu Chih-Yuan. Input ground motion on

the sand cushion the impact of isolation. [J] North China Institute of Aerospace Engineering in

September 2004 Volume 14 No. 3 (in Chinese)

[3]. Liu Xiaoli, Zhang Chunmei, Douyuan Ming, Liu Chih-Yuan. Sand cushion stiffness on seismic

response of the ground. [J] North China Institute of Aerospace Engineering in December 2004

Volume 14 No. 4 (in Chinese)

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Chinese)

[6]. Isolation Systems group. Structural base isolation system for research. [J] East China Jiaotong

University in August 2004 Volume 21 No. 4 (in Chinese)

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