WO2006051167A1

WO2006051167A1 - Method for attenuation of vibration propagating in the ground and an attenuation barrier

Info

Publication number: WO2006051167A1
Application number: PCT/FI2005/050403
Authority: WO
Inventors: Jouko TÖRNQVIST
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2004-11-09
Filing date: 2005-11-08
Publication date: 2006-05-18
Also published as: FI20045430A0

Abstract

The invention relates to a method and an attenuation barrier for attenuating the amplitude of vibration propagating in the ground as surface waves. The invention is especially suited for use in soft soil layers, such as clay, silt and peat soil. According to the invention, an attenuation barrier (20) comprising at least one, preferably a plurality of, tension resistant profile sheets, is sunk into the ground. The surfaces of the profile sheet function as adhesion surfaces, by means of which the profile sheet becomes bonded to the surrounding layers of soil by adhesion forces. The surface layers of the ground, which vibrate more strongly, are then by the profile sheet coupled together with the soil layers deeper in the ground, which vibrate more weakly, whereby the amplitude of the vibration in the surface layers is decreased. The attenuation barrier can be built as continuous, in which case the edges of the adjacent profile sheets are substantially butted to each other, or piecewise, in which case there remains a gap between at least some of the adjacent profile sheets. The profile sheets can be sunk into the ground by vibrating, pressing or hammering, to a vertical or oblique position.

Description

Method for attenuation of vibration propagating in the ground and an at¬ tenuation barrier

The invention relates to a method for attenuating the amplitude of vibration propa- gating in the ground as surface waves and an attenuation barrier used in the method.

Rail traffic along railway lines causes vibrations in the railway bed, which propa¬ gate in the ground to the areas surrounding the railway. The vibration of the ground reduces the comfort of living in the buildings near the railway and may cause damage to the buildings and structures. The magnitude and propagation of the vibration is especially dependent on the quality of the base of the railway bed and the ground in the vicinity of the railway.

Various methods are known for the prevention of the propagation of vibrations caused by railway traffic in the soil layers. The most efficient method for attenuat- ing the vibrations is stiffening the railway bed and the ground below it by means of piling or stabilization, for example. When the stiffness of the railway bed and the ground increases, the static and dynamic dislocations decrease. The stabilization of the ground under the railway track requires at least partial removal of the rail¬ way bed, and therefore it is not well suited for the prevention of vibration in exist- ing railway sections. Therefore, for cost reasons stabilization is a practicable alter¬ native only when building new railway sections.

It is also possible to build stabilization walls or fields in the ground, which can be placed either on the whole length of the section or only around a certain site to be protected against vibration. The attenuation effect of stabilization walls is based on their ability to reflect, diffuse and refract wave energy. Rigid stabilization walls can be manufactured by mixing cement with natural soil, for example. In order to achieve effective attenuation, the stabilization fields must be sufficiently wide and reach sufficiently deep, which results in high construction costs. Stabilization walls can attenuate the maximum amplitude of the vibrations by 40% at the best, but they can also amplify some frequencies.

The propagation of the vibrations in the ground can also be prevented by building a deep open trench beside the source of vibration. The attenuation effect of the trench is the better the deeper it is. However, for reasons of safety and land use, it is in most cases impossible to use open trenches to attenuate vibrations. Instead of leaving the trenches open, they can also be filled with so-called bubble mats, which consist of air-filled, horizontal tubes attached to each other. This provides approximately the same efficiency of attenuation without the drawbacks entailed by open trenches. The drawbacks of the bubble mat are its high material and in- stallation costs. The excavation of the trench required by the installation work re¬ quires special skills and equipment especially in soft and muddy ground. In addi¬ tion, the bubble mat technique functions inadequately on low frequencies typical of clay ground.

It is an objective of the invention to provide a new method for attenuating vibration propagating in the ground and an attenuation barrier used in the method, by which the drawbacks and disadvantages of the prior art can be reduced.

The objectives of the invention are achieved by a method and attenuation barrier, which are characterized in what is set forth in the independent claims. Some pre¬ ferred embodiments of the invention are presented in the dependent claims.

The invention relates to a method and an attenuation barrier for attenuating the amplitude of vibration propagating in the ground as surface waves. The invention is especially applicable for use in soft soil layers, such as clay, silt and peat soil. According to the invention, an attenuation barrier with at least one tension resis¬ tant profile sheet is sunk in the ground. Both surfaces of the profile sheet function as adhesion surfaces, by means of which the profile sheet becomes bonded to the surrounding soil layers by adhesion forces. The surface layers of the ground, which vibrate more strongly, are then by the profile sheet coupled with the layers of soil deeper in the ground, which vibrate more weakly, whereby the amplitude of the vibration of the surface layers is decreased. The thickness and profile shape of the profile sheet have been designed such that they withstand the stress during installation and use without buckling and harmful deformations.

In a preferred embodiment of the invention, several profile sheets are sunk side by side in the ground for forming an elongated attenuation barrier. The attenuation barrier can be built as continuous, in which case the adjacent profile sheets are substantially butted to each other at the edges or overlapped, or piecewise, in which case there remains a gap between at least some of the adjacent profile sheets. The profile sheets can be sunk in the ground by vibrating, pressing or hammering either to a vertical or oblique position. Oblique installation especially provides the possibility to utilize the phase differences of the propagating wave motion for decreasing the amplitude of the vibration. The invention has the advantage that it is an economical method for attenuating vibrations. The manufacturing and material costs of the attenuation barrier are low, and its installation in place takes place quickly and easily without excavation work.

In addition, the invention has multiple uses, because it is equally suited for attenu¬ ating the vibration caused by elongated sources of vibration, such as railways, and small point-like sources of vibration, such as machine bases and crushers. The invention is also well suited for attenuation measures carried out afterwards at sites which are found to have problems. When required, the attenuation barrier can also be used for the support and foundation of railway related facilities and equipment, noise walls and the like.

In addition, the invention has the advantage that it is especially well suited for at¬ tenuating the vibratory motion at sites having soft soil, such as clay, silt and peat soil, in which it is difficult to use other attenuation methods. There are hundreds of kilometres of such sites in Finland alone.

Another advantage of the invention is the small amount of space required by it. Therefore it suitable for use in narrow streets and sites where other methods, such as stabilization, cannot be used because of lack of space.

Yet another advantage of the invention is the fact that it also effectively attenuates vibrations of low frequency.

In the following, the invention will be described in detail. Reference will be made to the accompanying drawings, in which

Figure 1a shows a partially completed attenuation barrier according to the inven¬ tion as a cross-sectional drawing. Figure 1b shows examples of alternative placements of an attenuation barrier ac¬ cording to the invention.

Figure 2a shows the propagation of the vibrations to the environment from the source of vibration as an illustrative drawing,

Figure 2b shows an exemplary cross-section of the path of a single particle of soil in different depths during the propagation of the vibratory motion, Figure 2c shows an exemplary cross-section of the forces having an impact on the attenuation barrier according to the invention during the propagation of the vibratory motion, and

Figure 2d shows an exemplary cross-section of the change caused by the method and attenuation barrier according to the invention on the amplitude of the vibration and the path of a single particle of soil.

Fig. 1a shows, by way of example, an attenuation barrier 20 according to the in¬ vention partially completed. The attenuation barrier according to the invention is constructed of profile sheets 10, which are sunk in the ground in the longitudinal direction of the sheet in a way that the profile sheet forms a wall structure to re¬ main hidden in the ground. The profile sheets are manufactured of a material hav¬ ing a high modulus of elasticity and good installation properties. A preferred mate¬ rial for the profile sheet is steel, but other metals or some composite materials can also be practicable. The cross-sectional geometry of the profile sheet is chosen such that the profile sheet has sufficient bending resistance, tensile strength and the desired cohesion area for each linear metre of the barrier. The profile sheet of steel has preferably a trapezoidal cross-section having a cross-sectional height of approximately 70-200 mm and sheet thickness of 2-6 mm. The method and at¬ tenuation barrier according to the invention is mainly used at sites having soft soil, such as clay, silt or peat. In such a soft ground, the measures of the above men¬ tioned profile sheet enable sinking the profile sheet into the ground by some prior art method, such as pressing, vibrating or hammering without the danger of buck¬ ling of the profile sheet.

When the profile sheet is sunk into the ground, a groove-like anvil piece 14, the length of which is essentially equal to the width of the profile sheet, is placed against the end surface 12 of the profile sheet. The anvil piece distributes the forces caused by the vibration and strokes over the whole width of the profile sheet and thus prevents the collapsing and local deformations of the sheet. A suitable, optimal shape of the profile and thickness of the sheet can be defined for the profile sheet for each installation site according to the penetration resistance of the ground and the installation equipment available.

The attenuation barrier 20 can be required to have such a great height that it is not possible or economically feasible to manufacture profile sheets having the height of the whole attenuation barrier. In that case, the attenuation barrier can be built of profile sheets of a suitable length, such as 6 metres, which are fastened to each other at the place of installation by self-tapping screws or corresponding fastening means. The best way to extend the profile sheets is to fasten the new sheet to the end of the former when the former sheet sunk in the ground is 0.5-1.0 metres visi¬ ble above the surface of the ground 24. The size, number and locations of the fas- tening screws are determined by conventional design methods so that the joint 18 of the sheets withstands the stress it is exposed to during installation and later during use. When a continuous attenuation barrier is constructed, the adjacent profile sheets are placed side by side with their edges overlapping. Adjacent pro¬ file sheets need not be fastened to each other. The profile sheets can be installed in a vertical or oblique position.

An edge guide 16, which supports the edge of the sheet during the penetration into the ground, can be used on one edge or both edges of the profile sheet. The edge guide becomes useful especially when the height of the attenuation barrier increases and when the attenuation barrier is installed in a oblique position. The edge guide is an elongated, bar-like part, which is fitted around the edge region of the profile sheet 10 in a way that it forms a strengthening part supporting the edge region. The edge guide is preferably a U-section of metal or an opened box sec¬ tion with a wall thickness of 3-5 mm. The size and cross-sectional shape of the edge guide have been selected such that the edge part of the profile sheet can be fitted inside the edge guide. The edge guide can be sunk into the ground in ad¬ vance before installing the profile sheet or simultaneously with the installation of the profile sheet. When a continuous attenuation barrier is built, the profile sheets already installed in place function as guides for profile sheets to be installed later, and so the edge guide is generally needed only in connection with the installation of the first profile sheet of the attenuation barrier. However, when a continuous, uniform attenuation barrier is constructed, it may be necessary to use an edge guide to support the free edge of the profile sheet. The edge guide can be left in place in the ground or it can be removed after the profile sheet has been installed in place.

Examples of alternative placements of an attenuation barrier 20 according to the invention are shown in Fig. 1b. The source of vibration shown in the drawing as an example is a railway 100, but the method and attenuation barrier according to the invention can also be used for attenuating vibrations in the ground caused by other sources of vibrations, such as machine bases, crushing plants or streets. The distance of the attenuation barrier from the source of vibration is in each case selected on the basis of the source of vibration, the sites to be protected against vibration and the properties of the ground in the area.

In Fig. 1a, various alternative placements of the attenuation barrier are denoted by Greek letters. The attenuation barrier can be continuous α or piecewise β. A con- tinuous barrier is uniform in the longitudinal direction of the barrier, i.e. the adja¬ cent profile sheets are essentially butted to each other or overlapping. A piecewise attenuation barrier is constructed of several adjacent parts of attenuation barrier with a clear gap between them. Each part of attenuation barrier and each gap be¬ tween the parts can have the width of one or more profile sheets. An "overlapping" attenuation barrier γ is constructed of two parallel, piecewise barriers at a distance from each other, in which the gaps between the parts of the barrier are located at different places in the longitudinal direction of the attenuation barrier. The attenua¬ tion barrier can be installed in the ground in a substantially vertical position or to a desired angle of inclination. Instead of a single attenuation barrier, it is also possi- ble to build two parallel attenuation barriers, in which case they are called a recur¬ ring attenuation barrier. In a recurring attenuation barrier, the distance between the parallel attenuation barriers is preferably half of the wavelength of the most harmful vibration. The different placement alternatives described above can also be used in various combinations. The properties of the attenuation barrier itself can also be varied by changing the shape of the profile, surface structure, length and/or material of the profile sheets. In a preferred embodiment of the invention, two profile sheets are installed tightly against each other, and some anti-friction material, such as teflon, soot or lubricant, is added between the sheets for at least part of the length of the profile sheet.

Fig. 2a is a simple illustration of the propagation of the vibrations from the source of vibration to the environment. In the immediate vicinity of the source of vibration 100, the propagation pattern of the vibration is complicated, but when the distance increases slightly, the vibrations propagate to the environment mainly as surface waves 22. Therefore, the vibration of the ground caused by railway traffic propa- gates to its environment almost entirely as so-called Rayleigh waves (R-wave). In clayey ground, the propagation speed of an R-wave is 30-100 m/s. In the fre¬ quency range 3-10 Hz, which is the most problematic frequency range with regard to the building of small houses, the wavelength of the vibration is thus 3-30 m. The penetration depth to the ground of the R-wave can be considered to be the same as the wavelength, because approximately 90 % of the wave energy is transmitted in this zone. Fig. 2b shows an exemplary cross-section of the path of a single particle of soil in different depths during the propagation of the vibratory motion. When the R-wave propagates in the ground, the single particle of soil moves along an elliptical path, i.e. the vibratory motion of the soil particle has a horizontal and vertical amplitude. Near the surface of the ground, the horizontal amplitude is normally 60-80% of the vertical amplitude. When the depth increases, the horizontal amplitude de¬ creases rapidly, and in the depth of 0.18 * wavelength λ, the value of the horizon¬ tal component of the vibration is zero. When the depth further increases from this, the sign of the horizontal component of the vibration changes, in which case the direction of rotation of the soil particle on the elliptical path changes to the oppo¬ site.

Fig. 2c is an exemplary cross-section illustrating the forces having an impact on the attenuation barrier 20 according to the invention during the propagation of the vibratory motion. The relation between the horizontal and vertical component of the vibration changes when moving in the depth direction. Vibration components of different frequency also have a different propagation speed at different levels of depth. For these reasons, there are differences in the phase, direction and ampli¬ tude of the vibratory motion in different depths. The different forms of the vibratory motion in different depths are presented on the right side of Fig. 2c.

The attenuation barrier installed in place becomes bonded to the surrounding ground as a result of adhesion. Due to the profile shape, the attenuation barrier has a large "adhesive surface area" against the ground, and therefore the adhe¬ sion force is also relatively high. Thus the attenuation barrier in a way binds to¬ gether the soil layers on top of each other by a tension-resistant bond. With regard to the operation of the attenuation barrier, it is important that its adhesion to the surrounding soil layers is strong enough. Sufficient adhesion is achieved by the profile shape of the profile sheets, which provides a large adhesive surface area. It is also important that the tensile strength and modulus of elasticity of the profile sheets are sufficiently high so that a sufficiently strong and rigid bond between the soil layers is achieved. These properties are achieved by manufacturing the profile sheets of steel, which has a high modulus of elasticity, and by designing the sheet thickness and the number of joining elements at the joints appropriately.

When the vibration propagates in the ground as surface waves, the vertical ampli¬ tude of the vibration in deeper soil layers is smaller than the vertical amplitude of the vibration in the surface layers. When the upper soil layers, which vibrate more strongly, are bound by the attenuation barrier to the lower soil layers, which vibrate more weakly, the vertical amplitude of the upper soil layers decreases. In addition, the phase differences of vibrations propagating in different soil layers increase the attenuation capacity of the attenuation barrier. As seen in Fig. 2c, the soil particles can be moving upwards in the upper soil layers at the same time as the soil parti- cles in the lower soil layers move downwards. Due to the phase difference of the vibrations, the lower soil layers thus "draw" the attenuation barrier downward and the upper soil layers simultaneously upward. So the force effects of the vibrating soil layers partly cancel out each other. In Fig. 2c, the force effect on the attenua¬ tion barrier and its magnitude is depicted by the darkened surface area limited by the barrier. The direction of the force impact is depicted by an arrow.

Fig. 2d is an exemplary cross-section of the change caused by the method and attenuation barrier 20 according to the invention in the amplitude of the vibration and the path of a single soil particle in different depths. An attenuation barrier 20 according to the invention, sunk in the ground, is shown in the middle of the draw- ing. The propagation direction of the vibratory motion is depicted by arrow A. The form of the vibration propagating on the surface of the ground as wave motion is depicted by graphs B and B'. The loop-like, elliptical figures illustrate the move¬ ment of a single soil particle in different depths.

It is seen from Fig. 2d that the attenuation barrier 20 according to the invention does not have an effect on the magnitude of the horizontal component of the vi¬ bration, but it significantly decreases the vertical component of the vibration, i.e. the vibration on the surface of the ground and in the surface layers of the ground. The wave motion meeting the attenuation barrier is polarized into the average ver¬ tical vibration of the barrier. The decreasing of the vertical component of the vibra- tion is especially due to the phase difference at which the vibratory motion meets the attenuation barrier in different depths (see Fig. 2c). In general, the ground is stiffer in the deeper layers. When the stiffness increases, the propagation speed of the vibratory motion also increases, and so phase differences also occur in nearly homogeneous layers of clay. When required, the occurrence of a phase difference can be ensured by installing the attenuation barrier in an oblique posi¬ tion. By preliminary calculations it has been proved that the reduction of the verti¬ cal component of the vibration in homogeneous ground can even be of the order of 50-70%. By the method and attenuation barrier according to the invention it is thus possible to achieve even better results than by the ground stabilization meth- ods, which are more expensive. Some preferred embodiments of the method and attenuation barrier according to the invention have been described above. The invention is not limited to the above described solutions only, but the inventive idea can be applied in many ways within the scope defined by the claims.

Claims

1. A method for attenuating the amplitude of vibration propagating in the ground as surface waves, characterized in that at least one tension resistant profile sheet (10) having an adhesion surface for bonding the profile sheet to the soil layers by adhesion forces is sunk into the ground, whereby the surface layers of the soil, which vibrate more strongly, are coupled by means of the profile sheet to the soil layers deeper in the ground, which vibrate more weakly.

2. A method according to Claim 1 , characterized in that a plurality of profile sheets (10) are sunk into the ground side by side for forming an elongated at¬ tenuation barrier (20).

3. A method according to Claim 2, characterized in that the profile sheets (10) are sunk into the ground in a way that the edges of the adjacent sheets are sub¬ stantially butted to each other or overlapping so as to form a continuous attenua- tion barrier (20).

4. A method according to Claim 2, characterized in that the profile sheets (10) are sunk into the ground in a way that there remains a gap between at least some of the adjacent profile sheets so as to form a piecewise attenuation barrier (20).

5. A method according to any one of the Claims 1 to 4, characterized in that the profile sheets (10) are sunk into the ground in a vertical or oblique position.

6. An attenuation barrier (20) according to any one of the Claims 1 to 5, char¬ acterized in that the profile sheets (10) are sunk into the ground by vibrating, pressing or hammering.

7. An attenuation barrier (20) for attenuating the amplitude of vibration propa- gating in the ground as surface waves, characterized in that it comprises at least one tension resistant profile sheet (10) sunk in the ground and having adhesion surfaces for bonding the profile sheet to the soil layers by adhesion forces.

8. An attenuation barrier (20) according to Claim 7, characterized in that it comprises a plurality of profile sheets (10) arranged side by side for forming an elongated attenuation barrier.

9. An attenuation barrier (20) according to Claim 8, characterized in that the edges of the adjacent profile sheets (10) are essentially butted to each other or overlapping so as to form a continuous attenuation barrier (20).

10. An attenuation barrier (20) according to Claim 8, characterized in that there is a gap between at least some adjacent profile sheets (10) for forming a piece- wise attenuation barrier.

11. An attenuation barrier (20) according to any one of the Claims 7 to 10, char¬ acterized in that the profile sheets (10) have been sunk into the ground to a verti- cal or oblique position.

12 An attenuation barrier (20) according to any one of the Claims 7 to 11 , char¬ acterized in that the profile sheets (10) have been sunk into the ground by vibrat¬ ing, pressing or hammering.

13. An attenuation barrier (20) according to any one of the Claims 7 to 12, char- acterized in that the profile sheets (10) are made of steel.

14. An attenuation barrier (20) according to any one of the Claims 7 to 13, char¬ acterized in that the cross-sectional form of the profile sheets (10) is corrugated, preferably trapezoidal.

15. An attenuation barrier (20) according to any one of the Claims 7 to 14, char- acterized in that the height of the cross-section of the profile sheets (10) is 70-

200 mm and the thickness of the sheet 2-6 mm.