|Title of Invention||
"COMPOSITE SCOUR PROTECTION DEVICE"
|Abstract||Scour around a bridge pier is one of the major possible causes of the hydraulic failures of a bridge. A typical sediment-water structure interaction at the upstream junction of the bridge pier initiates a vortex system, the primary vortex of which is responsible for local scour around the pier. The growth of the primary vortex can be arrested by retaining it on a rigid surface as on a collar plate. A collar plate of size 3.0D or larger when placed at the average bed level assures 100% scour reduction while a collar plate of size 2D placed 0.1D below the bed does not allow scour depth to go beyond it. Anticipating degradation of general bed level, a group of multiple collar plates of smaller size were employed at the laboratory scale to reduce the scour considerably. A group of three collar plates of size 1.5D having an inter-plate spacing of D/6 to D/4 with the top plate initially placed at average bed level reduces scour by about 80% in comparison to the scour depth of an unprotected pier. The composite device, apart from being axis symmetric, is highly suited to degrading river beds.|
|Full Text||Title of invention:
COMPOSITE SCOUR PROTECTION DEVICE
Field of Invention:
This invention relates to scour countermeasures around bridge piers. More particularly, one embodiment of the invention relates to a device comprising at least three circular collar plates.
Background of the invention
Bridges across rivers are considered to be the life line of transportation network of a country as almost all civilization including the most ancient ones had their roots on the banks of the rivers. Till date all kinds of strip development has been taking place along the major rivers. In today's age, rightly termed as the age of fast communication, bridges occupy a place of pride as they facilitate movement of men and material safely and quickly, across rivers. In a country like India, the failure of a bridge on a major railway track or a national highway even for a day can upset many a balance and the economic activities in that part of the country comes to a virtual standstill. The damages associated with a bridge failure can be significant.
Rivers are natural flowing systems, which cannot be controlled by manmade structures. Construction of a bridge pier across a river obstructs the flow. The flowing water in the river scours the sediment around the bridge piers. Thus, all bridges and structures associated with waterways are potentially at risk of failure due to hydraulic scour.
The major possible causes of bridge failures are those due to structural, foundation and hydraulic failures. In an article titled "Update: Bridge Scour" by Huber (1991), the author has stated that since 1950 over 500 bridges have failed in the USA and that a large majority of the failures were due to hydraulic conditions, i.e., scour, Garde (1995). Where the rivers scour, the foundation should be taken deeper than the possible maximum scour and placed on firm stratum. The Indo-Gangetic belt of the Indian sub-continent is interwoven with mighty rivers like the Indus, Ganga, Brahmaputra and their innumerable tributaries. The alluvial deposit is so deep that in some cases even upto a depth of 100metres, no rock strata is available. Moreover the river beds are highly erodible.
It has been estimated that 60% of all river bridge failures is attributable to undermining of pier foundations following scouring or erosion of the channel bed. During 1961 to 1976, out of 86 major bridge failures, 46 failed as a result of excessive scour near piers (Murillo 1987). Breusers et al. (1977) present a state of the art report on the mechanism and prediction of scour depth around a bridge pier. The bridge failures of Ganga Pul at Makamah (River Ganga) and on river Tapi are examples of major bridge failures due to scour in India. Reports of Ministry of Railways, India (1967, 1968, and 1972) describe the history of work carried out in India in the direction of bridge pier scour.
In order to protect the bridge piers against scouring the foundations have to be taken very deep. In the bridge across river Ganga at Varanasi on Ramnagar-Mugalsarai Bye-pass, the design maximum depth of scour was estimated to be as deep as 60 metres below the highest flood level (Ref. Thakkar, 1992). That is a very expensive proposition.
Knowledge on the extent of scoured area and the equilibrium scour depth is very important in the safe as well as economic design of foundations of bridge piers. In India, Lacey-Inglis method of estimating equilibrium scour depth below the highest flood level is recommended for use by Indian Railways and Indian Road Congress. However, the range of variables and conditions under which this empirical method was devised is narrow and specific than the general case for which it has been recommended. Similar deficiencies can be found to occur in other methods of scour depth prediction as will be observed later in the literature review. And it becomes imperative to investigate the mechanism of scour and in particular the characteristics of horseshoe vortex as it develops and is sustained in a scour hole.
The major cause of scouring leading to exposing of the pier is a system of vortices. It commences with the formation of adverse pressure gradient at the upstream front of pier, which results in the formation of this vortex. This vortex wraps around the pier in the form of a horse shoe and dislodges sediment from the bed and releases it downstream.
A collar plate skirting around a pier forms a device that arrests the sinking of horseshoe vortex into sediment bed by virtue of the plate providing an impermeable, unpenetrable solid surface in the local region around the pier. A definition sketch indicating details of a collar plate mounted on a pier and its associated mechanism of shielding scour has been shown in Drawing 1.
A collar plate being axis-symmetric forms a very good device, subject to the following limitations: i. The device may be rendered ineffective in the eventuality of lowering of bed level.
ii. For the device to be effective, the size of the plate required is large, thus offering possibility of generation of vibrating/pulsating forces (Garde and Kothyari 1995).
iii. The difficulty of installation of large size plate.
US Pat. No. 6851889, Feb. 8, 2005, Buchanan discloses a reinforced interlocking retention panel for prevention and/or elimination of shoreline erosion and/or scour beneath marine structures. Other forms of scour reduction like providing a streamlined shape of pier, a slot through a pier, sacrificial piles or vanes etc. suffer from the drawbacks of being non-axis symmetric to the approach flow and hence may be rendered ineffective when the flow is skewed with respect to the axis of the pier/scour prevention device. In the absence of a scour protection device, rip-rap, (i.e., dumping of stones into the scour hole around the pier) has been a quite common method of scour protection. However, this requires continual maintenance and may itself become a potential hazard if a few stones scoop out of the dump. The composite device of multiple collar plates is axis-symmetric as well as highly suited to degrading river beds.
Object of invention:
Bridge piers are known to fail under the action of excessive scour developing at the sediment - water - pier interaction junction. The object of the invention is to develop a composite scour protection device which guards the pier in a manner such that not only does the strength of the horseshoe vortex reduce but also gives an additional factor of safety in the event of lowering of bed level.
Statement of invention:
A composite device comprising of three to five circular collar plates of size 1.5 times the size of the pier having an inter-plate spacing of one-sixth to one-fourth of size of pier with the top plate initially located at the average bed level.
A summary of invention:
A scour protection device comprising of a single collar plate is too large to be effective in preventing scour. Also, in the eventuality of lowering of general bed level, the device is rendered ineffective. A composite device comprising of at least three or
five circular collar plates of size 1.5 times the size of the pier having an inter-plate spacing of one-sixth to one-fourth of size of pier with the top plate initially located at the average bed level is capable of reducing scour by more than 80% in comparison to the scour depth of an unprotected pier.
1. A brief description of the accompanying figures:
Figure 1: Flow mechanism around a pier with collar plate.
Figure 2: Behavior of single and multiple collar plates in the eventuality of lowering of general bed level
Figure 3: Flow mechanism around a pier with multiple collar plates.
Figure 4: Variation of maximum scour depth Hs with number of collar plates, Nc (Above
the average bed level), (D = 62 mm). Figure 5: Variation of maximum scour depth Hs with number of collar plates, Nc (Below
the average bed level), (D = 62 mm). Figure 6: Comparison of the static scour profile with and without multiple collar plates
after a run of 300 minutes.
Detailed description of the invention:
Various aspects of the invention provide for a set of multiple collar plates which guard the pier in a manner such that not only does the strength of the horseshoe vortex reduce but also gives an additional factor of safety in the event of lowering of bed level.
Figure 1 illustrates a collar plate skirting around a pier forms a device that arrests the sinking of horseshoe vortex into sediment bed by virtue of the plate providing an impermeable, unpenetrable solid surface in the local region around the pier. A definition sketch indicating details of a collar plate mounted on a pier and its associated mechanism of shielding scour has been shown in this figure.
Figure 2 illustrates a collar plate being axis-symmetric forms a very good device, subject to the following limitations:
i. The device may be rendered ineffective in the eventuality of lowering of bed
level, ii. For the device to be effective, the size of the plate required is large, thus offering
possibility of generation of vibrating/pulsating forces, iii. The difficulty of installation of large size plate.
The suggested solution has been obtained by using a series of vertically arranged collar plates, henceforth being referred to as multiple collar plates. Figure 2 illustrates how a single collar plate would be rendered ineffective in the event of lowering of bed level. The figure also gives the details of multiple collar plates mounted around a bridge pier.
Figure 3 illustrates how multiple collar plates guard the pier. The scouring vortex is retained on the impregnable collar plate and is not allowed to grow and also its strength is reduced. The series of plates also provides the additional factor of safety in the event of lowering of bed level.
Figures 4 and 5 illustrate the behaviour of series of plates above and below the general bed level under laboratory conditions. Initially, experiments were performed on a series of equi-sized fifteen collar plates of diameter Dc = 1.5D. The plates were interspaced D/4 and D/6, with ten collar plates positioned above the bed, one at bed and four below the bed. For spacing between the plates, Cs of D/4 and D/6, the maximum scour depth that was obtained accounted for a performance potential of 85% and 86% respectively. In a bid to curtail the number of collar plates, it was necessary to delineate the effect of collar plates above and below the bed.
Multiple Collar Plates above the Bed Level
A series of collar plates interspaced by D/2 to D/4 were experimented upon for varying number and combinations of plates above and below the average bed level. The size of plates was maintained at 1.5D. The scour depth associated with a plate group of interspacing D/12 was very high. It was probably due to the reason that a very close spacing made the plate group behaves like a solid pier of larger diameter. Satisfactory results were obtained with an interspacing of D/4 and D/6. Commencing from the average bed level onwards, plate groups of nine plates each, interspaced by D/4 and D/6 were showing scour depths of 0.72D and 0.57D, which meant a scour reduction of
approximately 32% and 46%, respectively. However, when the number of plates was reduced to only one at the bed level, there was only a marginal reduction in the maximum scour depth. It clearly meant that most of the collar plates were redundant when provided above the general bed level. Figure 4 illustrates a comparative view of different number of collar plates along with bar chart presentation of the scour depth results.
Multiple Collar Plates below the Bed Level
Except for the number of collar plates, plate groups with similar parameters as for above the bed level were tried below the average bed level. To begin with, number of collar plates was only two with one at and another below the bed level. For both types of interspacing, the scour reduced by 52% (Cs = D/4) and 58% (Cs = D/6). With one plate unchanged at the average bed level, further increase in the number of plates by one increased the figures of scour reduction only marginally to 60% and 63%, respectively. The scour depth values under the two cases stood at 0.41D and 0.38D thereby exposing both the plates below the bed. Interestingly however, the scour depth was not able to cross the lowest plate placed at D/2 below the bed. The results of maximum scour depth along with the schematic of plate groups are presented in Figure 5.
Analyzing the results of plate groups above and below the average bed level, it was found that a plate group of three plates of size Dc = 1.5D at an interspacing of D/4 or D/6 with the middle plate placed at the average bed level was the best providing a performance potential of over 80%. However, it was decided to provide groups of five equi-sized (Dc = 1.5D) collar plates, with the top most plate placed at D/4 above bed, the lowest at 3/4D below bed and the other three plates provided in between at an interspacing of D/4. The arrangement would not only assure a very high performance potential but would also provide the additional factor of safety in the eventuality of lowering of general bed level. Should the bed lower by even D/2, the performance of the arrangement would remain unaltered.
Figure 6 illustrates that a group of three collar plate arrangement is particularly effective and advantageous in case of lowering of the general bed level. A comparison of the scour profile with and without multiple collar plates has been presented in this figure. While the maximum scour around the pier without the plates stands at 1.1 D, it is only 0.25 D with the configuration of three plates.
1. Composite scour protection device for guarding a bridge pier against scour comprising
of odd numbers of circular collar plates of diameter one to three times the size of the pier
having an inter-plate spacing of one-tenth to one-half of size of pier, around a circular
pier, top plate is located at average bed level and at least one plate is located at half the
diameter of the pier below the bed level for reducing scour by more than 80%.
2. Composite scour protection device as claimed in Claim 1 wherein the said add numbers i.e. three to five collar
plates (Cost effectiveness considered) are provided for reducing maximum scour.
3. Composite scour protection device as claimed in calim1 wherein plates are circular in shape.
4. Composite scour protection device as claimed in claim1 wherein diameter of the plates has been maintained at 1.5 times the size of the pier for maximum efficacy.
5. Collar plates as claimed in Claim 1, wherein the plates are having an inter-plate spacing
of one-sixth to one-fourth of size of pier for reducing more than 90% of scour.
6. Composite scour protection device as claimed in Claim 1 wherein initially the top plate
is located at average bed level and at least one plate located at D/2 below the bed level
for being capable of reducing scour by more than 80% in the event of lowering of
general bed level.
7. Composite scour protection device as claimed in Claim 1 where plates are made of any
corrosion proof material (e.g. reinforced cement concrete, steel, etc.) of equivalent
8. Composite scour protection device as described and illustrated with the help of
|Indian Patent Application Number||1375/DEL/2005|
|PG Journal Number||31/2009|
|Date of Filing||30-May-2005|
|Name of Patentee||BALDEV SETIA|
|Applicant Address||ASST. PROFESSOR IN CIVIL ENGINEERING, NATIONAL INSTITUTE OF TECHNOLOGY KURUKSHETRA-136119(HARYANA)|
|PCT International Classification Number||E02B 3/12|
|PCT International Application Number||N/A|
|PCT International Filing date|