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Title:
A METHOD AND STRUCTURE TO ALLEVIATE THE EFFECT OF FROST HEAVE AROUND MANHOLES
Document Type and Number:
WIPO Patent Application WO/2000/017457
Kind Code:
A1
Abstract:
Frost heave of pavements around sanitary and storm sewer manholes (10) and similar installations disposed in an under the pavement (12), is a major and costly problem in cold climates. As a result of the frost heave, a difference in level is created between the manhole cover (32) and the pavement (12) around it. Due to traffic, pavement cracks are formed around the manhole (10) necessitating costly repairs. It is proposed to alleviate the problem to a substantial degree by providing a frost-heave mitigating structure in a direct vicinity of a manhole (10) or a similar installation. To this effect, a heat-transfer zone (16) filled with large granulate is formed around the manhole (10) under the pavement (12), and openings (26) are formed in the wall (24) of the manhole (10) in a way enabling heat transfer between the warmer interior of the manhole (10) and the colder heat-transfer zone (16). The heat that flows to the zone warms up the surrounding soil in the vicinity of the zone thus preventing its freezing and consequently the frost heave approximately above the zone.

Inventors:
Svec, Otto J. (4797 Massey Lane Gloucester, Ontario K1J 8W8, CA)
Application Number:
PCT/CA1999/000778
Publication Date:
March 30, 2000
Filing Date:
August 24, 1999
Export Citation:
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Assignee:
NATIONAL RESEARCH COUNCIL OF CANADA (Intellectual Property Services Building M-58, Room EG12 Montreal Road Ottawa, Ontario K1A 0R6, CA)
Svec, Otto J. (4797 Massey Lane Gloucester, Ontario K1J 8W8, CA)
International Classes:
E02D29/12; E02D31/14; (IPC1-7): E02D31/14; E02D29/12
Foreign References:
US4358223A
GB2184477A
Attorney, Agent or Firm:
Szereszewski, Juliusz (National Research Council of Canada Intellectual Property Services Building M-58, Room EG12 Montreal Road Ottawa, Ontario K1A 0R6, CA)
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Claims:
CLAIMS
1. A structure for mitigating frost heave of ground surface or pavement in the vicinity of a sewer manhole or a similar installation disposed in and under the surface or pavement, the structure comprising: a wall encompassing the manhole or a similar underground formation and defining a barrier between the manhole and its exterior, the wall having means for heat transfer between the manhole and the exterior thereof, a zone adjacent to and surrounding said wall at least partially, the zone disposed beneath said surface or pavement in thermal communication with said heat transfer means and containing predominantly a material enabling heat transfer between the manhole and the zone.
2. The structure according to claim 1 wherein said heat transfer means are openings in said wall.
3. The structure according to claim 2 wherein said openings are disposed such as to enable the heat transfer between the manhole and said zone.
4. The structure according to claim 2 wherein said openings are provided with screens to prevent ingress of particulates to the manhole.
5. The structure according to claim 1 wherein said zone is separated from adjoining ground formation with a barrier.
6. The structure according to claim 1 wherein said material is a solid porous material.
7. The structure according to claim 6 wherein said material is stone aggregate.
8. A method of mitigating the effect of frost heave in the area of a sewer manhole or a similar installation having a side wall, the method comprising providing heat transfer means in the side wall, providing a heat transfer zone adjacent to and at least partly surrounding said side wall, said zone being in thermal communication with said heat transfer means to enable heat transfer between said manhole and said zone, and placing road pavement over said heat transfer zone.
9. The method of claim 8 further comprising the step of providing a barrier between said space and adjacent ground or pavement.
10. A method of mitigating the effect of frost heave in the area of a sewer manhole or a similar installation disposed in and under a road pavement, the method comprising removing any pavement and soil adjacent to a wall of the manhole to create a space having a predetermined depth below the level of the pavement and a predetermined horizontal diameter or area, providing heat transfer means in the wall of the manhole at a level below the pavement, filling the space with a solid, heat convection enabling material such that the heat transfer means are in thermal communication with the material thus enabling heat transfer from said manhole through said heat transfer means into and through the space, and depositing road pavement over said space.
11. The method of claim 10 further comprising the step of providing a barrier between said space and adjacent ground or pavement.
Description:
A METHOD AND STRUCTURE TO ALLEVIATE THE EFFECT OF FROST HEAVE AROUND MANHOLES Field of the Invention This invention is concerned with the mitigation of frost heave effects, particularly with a method to alleviate the problems associated with the frost heave around road manholes and with a structure designed to that purpose.

Background of the Invention Frost heave of asphalt pavements around sanitary and storm sewer manholes and similar installations disposed in and under the pavement, has been a major and costly problem in cold climates since the utilities were first constructed. The problem is dependent on (below-freezing) temperature, frost susceptible (suction-generating) soil type and availability of water. Frost heaving in soils is caused not only by freezing of in-situ pore water but also by freezing of migratory water. As the 0° C (32 deg. F) isotherm reaches frost-susceptible soil such as clay or silt below the road pavement (asphalt, concrete, etc.), a water suction potential develops in the"freezing fringe"between approximately-0.2 ° C to-0.4 ° C isotherms. This causes ice lenses to form and grow and can in turn lead to a substantial uplift of the pavement surface, in the order of 2 to 4 inches or even more, around manholes. The manholes themselves, being built on a deep foundation, do not usually rise with the pavement.

As a result, a difference in level is created between the manhole cover and the pavement around it. Unless repaired, at a significant cost and labor, this level difference, or vertical offset, may cause the pavement to crack. It may also increase damage to vehicles, noise level, road vibration and associated effects. Vehicles passing over the manhole, particularly snow-clearing equipment, can deteriorate the road surface around manhole to a significant degree because of the level difference.

It is desirable to alleviate the effects of the frost heave in the vicinity of manholes and similar structures where the difference in road level due to frost heave

can create the above problems. More specifically, it is desirable to move the zero (water freezing) isotherm upwards, above the level of frost-susceptible soil.

Summary of the Invention It has been found that the above-identified problem can be alleviated to a substantial degree by providing a frost-heave mitigating structure in a direct vicinity of a manhole or a similar installation. The structure comprises an underground wall encompassing a manhole or a similar underground formation and defining a barrier between the manhole and its exterior, the wall having means for heat transfer between the manhole and the exterior, a zone adjacent to the wall and surrounding it at least partially, the zone containing predominantly a solid material enabling heat convection through the zone.

Preferably, the heat transfer means are perforations in the manhole wall, sized and disposed such as to enable the flow of air and other gases and thus heat transfer between the interior of the manhole and the zone and further through the zone. The zone and the perforations or other heat transfer means are disposed at a level below the road pavement.

The zone may vary in size. It may be disposed concentrically around the manhole or have a different shape dependent on the desired area of heave mitigation.

The zone may surround the manhole entirely or partly, the latter case when the manhole is disposed close to an existing structure.

In accordance with another aspect of the invention, there is provided a method of manufacturing a structure for mitigating the effect of frost heave in the area of a sewer manhole or a similar installation disposed in and under a road pavement, the method comprising removing any pavement and soil around a wall of the manhole to create a space having a predetermined depth below the level of the pavement and a predetermined horizontal diameter or area, providing heat transfer means in the wall of the manhole at a level below the pavement, filling the space with a solid, heat

convection enabling material such that the heat transfer means are in thermal communication with the material thus enabling heat transfer from said manhole through said heat transfer means into and through the space, and depositing road pavement over said space.

Preferably, the heat transfer means are openings in the side wall of the manhole. The openings are preferably disposed at the level of the zone and protected against plugging with particulate material e. g. soil or small stones. The heat convection enabling material is typically a coarse granulate. It is preferable to separate the material from surrounding soil etc. material of smaller particle sizes with a barrier to prevent the filling of the pores of the coarse granulate with the finer material which would lead to a loss of heat transfer capability of the granulate. The barrier may also have heat isolating capabilities.

The structure, when properly designed and constructed will alleviate to a substantial degree the frost heave of frost-susceptible soil and thus the heave of the pavement thereabove. The heave-mitigating effect extends only, approximately, to the area disposed above the zone. Still, the provision of the structure, properly designed, may reduce or totally eliminate the vertical offset between the manhole cover and the directly surrounding pavement which is, as discussed above, the most serious problem associated with the frost heave.

Brief Description of the Drawings The invention will be described in more detail by way of the following description to be taken in conjunction with the drawings in which Fig. 1 is a schematic cross-sectional view of an embodiment of the frost-heave mitigating structure of the invention designed around a sewer manhole; Fig. 2 is a side view of the perforated ring of Fig. 1, with a screen covering the openings;

Figs. 3-8 are graphs representing ground isotherms within the structure of the invention (sanitary sewer manhole) during tests in February 1999; and Figs. 9-11 are graphs representing ground isotherms within a comparable conventional structure (sanitary sewer manhole) during comparable tests in February 1999.

Detailed Description of the Invention In sanitary and storm sewer manholes or similar underground installations, there is usually an amount of stagnant or moving sewer liquid at the bottom of the manhole. This liquid represents an amount of sensible and latent heat. In a conventional manhole, the heat dissipates partly through the manhole cover and also to a some degree becomes conducted sideways through the wall of the manhole.

The invention utilizes the heat to raise the temperature of a zone disposed adjacent the wall of the manhole, below the road pavement. Due to the elevated temperature of the zone, the water-freezing isotherm in the area of the zone is prevented, to a substantial degree, from penetrating into the frost-susceptible soil beneath which would cause its freezing and heave. In other words, the isotherm is "pushed"upward thus alleviating the frost-heave in the area of the zone.

As seen in Fig. 1, the sewer manhole, generally designated as 10 extends downwards from the road pavement 12 made of hot mix asphalt. The pavement lies over a conventional aggregate base 13 and an aggregate subbase 14, which in turn overlies a layer 16 of large aggregates. The layer 16 is deposited over natural soil 18 which in the specific embodiment is a frost-susceptible soil e. g. silt.

The manhole is made up of several standard concrete rings 20,22, built over a foundation, not shown in the drawing, and of a modified ring 24, also shown in Fig. 2, that has eight longitudinal, vertically elongated through-holes 26. In the embodiment illustrated herein, the holes are 2 in. in width and 10 in. in height.

The large aggregates, typically clean 2 in. stone, have enough porosity to allow the convection of heat transferred from the inside of the manhole through the holes 26 into the layer 16. Needless to say, the aggregate layer 16, when deposited, should be practically non-compressible to avoid a cave-in of the pavement.

The holes 26 are advantageously covered with a screen 28 to prevent the ingress of any small particulate, sand etc. into the holes 26 and into the manhole.

The layer 16 is separated from the adjacent soil 18 and subbase 14 by means of a barrier made of so-called geotextile 30 which is a polymeric foil. The barrier serves primarily as a mechanical screen preventing the ingress of the surrounding soil into the heat transfer zone 16. It also serves partly as a thermal curtain between the zone 16 and the surrounding formations.

The manhole is covered with an insulated cover 32 to reduce heat losses through the cover. The heat is generated by the flow of sanitary or storm sewage, not shown, at the bottom of the manhole 10.

To manufacture an actual structure of the invention, the existing pavement and soil were removed approximately two metres around the 1 m diameter manhole thus creating a circular hole 5 m in diameter. The depth of excavation was approximately 1 m. During excavation, one of the existing conventional rings was replaced with a ring 24 having eight longitudinal slots 26. Galvanized steel screen 28 was placed around the ring 24. The bottom and sides of the resulting cavity were covered with a layer of geotextile 30. The cavity was then filled with clean 2 in. stone aggregates and another layer of geotextile was placed over the layer of the stone aggregates thus creating a"bag", or a heat-transfer zone 16. As can be seen in the drawing, the placement of the slots 26 is selected to allow heat transfer from the manhole 10 into the bag 16 and back, through the slots 26. The direction of heat transfer within the manhole and through the slots 26 is shown schematically with arrows in Fig. 1.

While not part of the invention and not shown in the drawings, several temperature sensors were placed within the cavity during the construction of two

manholes. The sensors were thermocouples with accuracy approximately +/-0.25 ° C. Temperature readings were taken during February 25,1999 in Nepean, Ontario, Canada. The outside temperatures varied, the lowest being about C.

The readings of the sensors were compiled and presented in Figs. 3-5 (manhole #1) and Figs 6-8 (manhole #2) as isotherms at various depths within the layer 16. For comparison, sensors were also installed and readings taken on the same day on unmodified, conventional, comparable sewer manhole. The temperature readings from the conventional manhole are presented on Figs. 9-11. It will be noted that the structure shown on Figs. 9-11 is devoid of the heat transfer zone 16 and the geotextile layer 30, and that no slots 26 are present.

The time of the measurement is identified on each drawing.

In the actual example, the diameter of the manhole was about 1m and the depth about 4m. The openings 26 were located between about 70 and 95 cm depth below the surface of the road. The zone 16 extended about 2 metres from the wall of the manhole.

It will be seen that the freezing isotherm (zero deg. C isotherm) and below- freezing isotherms in the conventional manhole (Figs. 9-11) run through the frost- susceptible soil 18, causing the heave of the pavement in the vicinity of the manhole.

In the modified manholes of the invention, the freezing isotherm is moved upwards into the heat-transfer zone 16 ("bag"of large aggregates) which, owing to the heat flowing into the zone from the manhole through the openings 16, is immune to ice- lens formation. The frost susceptible soil 18 is maintained in above-freezing temperature whereby the frost heave of the pavement above the area delineated by the "bag"is substantially prevented. Thus, while the pavement beyond the zone, i. e. farther away from the manhole, is not protected from the effects of the frost heave, at least the vulnerable area around the manhole cover is largely protected from the effect of the heave. This prevents, to a significant degree, the deterioration of the pavement in the vicinity of the manhole cover.

It will be understood that the above-described method of manufacturing the structure of the invention applies to existing conventional manholes in need of a retrofit to mitigate the effect of frost heave. In the case of a new road where the structures of the invention are to be designed, there would be no need to remove existing pavement around the manhole. Instead, the manhole would be provided with openings or equivalent heat transfer means in its wall, and a heat-permeable zone would be installed around the manhole, preferably at the level of the openings, so that heat transfer could take place between the interior of the manhole and the heat- permeable zone. Standard road pavement would be placed over the zone to complete the road construction.

It will be appreciated that while asphalt pavement is the most popular in many countries, the heave problem is also associated with other surfaces. Therefore, the invention applies to any surface or pavement that is susceptible to frost heave.

It will be understood that various modifications are possible within the scope and spirit of the invention. For example, it is feasible to provide other methods of heat transfer than convection between the interior of the manhole and the heat-transfer zone. Such other method may be, conceivably, conduction of heat from the manhole by installing conducting elements between the interior of the manhole and the heat transfer zone. Such modifications are intended to be part of the invention which is to be defined only by the appended claims.