WO1988009412A1

WO1988009412A1 - Method for forming road and ground constructions

Info

Publication number: WO1988009412A1
Application number: PCT/SE1987/000264
Authority: WO
Inventors: Björn RINGESTEN; Olav Berge; Leif Berntsson
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-11-29
Filing date: 1987-05-29
Publication date: 1988-12-01
Anticipated expiration: 1989-11-29

Abstract

Method for forming road and ground constructions with a load-bearing function for static and dynamic loads on ground with low carrying capacity, in which connection the said construction is designed as rigid elements comprising a supporting part (1) with an upper wearing course (3) and, below it, light material (5) with a lower bulk density than the bed. The elements (1) are designed so rigid that point loads are distributed over the supporting surface of the bed. The element is founded in an excavation in the bed, which is adapted in depth such that the weight of the stripped, masses of the bed corresponds essentially to the weight of the element founded in the excavated area. The bulk density of the element is adapted, by combination of heavier material in the supporting part (1) and the light material (5), so as not to exceed the average bulk density of the stripped masses, so that the load on the bed by way of the weight of the element and some of the dynamic loads absorbed by the element are compensated by the weight reduction by means of the stripped masses. The excess part of the dynamic load is temporarily absorbed by pore water pressure present in the bed.

Description

Title:

Method for forming road and ground constructions

Technical field:

The invention relates to a method for forming the foundation of ground and road constructions on beds having a low carrying capacity such as clay, peat, mud, and also water. The constructions are adapted and designed in such a way that a complete compensated foundation is permitted. In this connection drainage of the ground layers as a result of overload is avoided and, thus, the accompanying settling in the subsoil, particularly with respect to short-term loads. The supporting construction forms a composite construction of floating bodies with a continuous beam grid whose rigidity and carrying capacity are adapted to the properties of the ground, the magnitude of the load and the carrying capacity of the floating bodies. Point load stresses are balanced out and stress concentrations are reduced by means of the rigidity of the construction and by means of spreading the load via the floating bodies to the ground layers. Moreover, since the construction is heat-insulating, freezing and frost damages are avoided in the underlying earth.

Prior art:

Road and ground onstructions consist essentially of, on the top, a wearing course and, below it, a base course of well-defined sand or gravel material of varying thickness. In cases of particularly low carrying capacity of the ground layers, subbases. can be added, these too being of defined composition. A characteristic feature of road and ground surfacings is that the latter can only absorb small tensile stresses. The function of the base course is essentially one of load distribution or, in other words, increasing the influence surface of the point loads to an acceptable level. The tensile stresses which are formed a r e absorbed as friction in the earth mass. Conventional road surfacings are made up of base courses and wearing courses whose bulk density is at least equally as great as that of the underlying ground. Considerable variations can occur for different soils. For example, well-graded, packed, sandy gravel has a bulk density of 1,800-2,000 kg/m³, clay 1,500-1,600 kg/m³, and peat

1,000-1,100 kg/m³.

Bitumen stabilization is used to increase the tensile strength of base courses and, especially, to absorb shortterm loads. Various solutions, for example with fibre fabric mats, increase the tensile strength of both the base course and the earth masses. Cement stabilization or lime stabilization of the underlying ground, or similar, is primarily intended to heighten the rigidity. At the same time the tensile strength also increases. Other measures for increasing the carrying capacity of the base courses and for transferring tensile stresses are the laying-out of horizontal piles with end-anchors or grillages of wood. Concrete is also used as a construction material, plain or reinforced. The concrete constitutes wearing courses, but also contributes to distributing point loads on the underlying ground layers. By virtue of the reinforcement, the tensile strength of the concrete is considerably improved. Even if the density of the concrete is 2,300-2,400 kg/m³, the result is a reduction in the load of the construction, since the thickness of the base course can be reduced. Plastic-moulded concrete tends to shrink in time, in which connection uncontrollable crack formation occurs. Concrete surfacings are generally provided with joins intended to function as indications of cracking. In such joins the capacity of the wearing course to absorb tensile stresses caused by bending moment is reduced. In order to prevent extensive settling as a consequence of the function of the seams, the base course is chosen relatively thick. The load can be reduced to some extent by producing the wearing course from light ballast concrete. In the USA, for example, light ballast concrete with a density of down to 1,600 kg/m³ has been used with good results. Concrete of even lower density has too little abrasive resistance and is quickly worn down by traffic load.

The load on underlying earth masses can be reduced in several ways. Material with low bulk density such as slag, haydite and cellular plastic has been used to reduce the weight of road embankments. Driving piles can be another possibility of transferring load from the roadway down to deeper-lying earth layers with higher carrying capacity and rigidity than those lying above. The piles can be provided with pile helmets, or a reinforced, continuous concrete slab can be cast, which is then supported by the piles. The base course and wearing course are then above the slab. The carrying capacity of the underlying earth layers is not utilized, and the construction can be compared to the mode of action with a bridge construction which absorbs all loads in the support. At the same time loading and drainage of the ground results in a packing effect. The porosity decreases, as does the pore water pressure. On ground with low carrying capacity a pipedraining of the upper ground layers is often carried out, together with an early loading by means of applying subbases and base courses. Vertical drainage is also performed in order to shorten the consolidation time upon loading. In this way the extensive settling occurs before the wearing course is laid. The construction is provided with pipes for leading surface water away and .for preventing a rise in the groundwater. In many cases where the ground is extremely inhomogeneous, the worst earth masses are removed before pre-loading is applied. With pre-loading and simultaneous drainage there is the possibility of detecting those areas which result in particularly extensive settling. Deformations can also occur upon freezing, so-called frost damages. These can appear where frost-protection material is located under road and ground constructions. The damage occurs when the groundwater is conveyed in capillary fashion in fine-grained earth up to the feezing zone where an accumulation takes place and ice lenses are formed. Freezing occurs more easily when the ground surfaces are exposed, as with snow-ploughed roads with insulating snow-banks along the sides. The material in the roadway has little insulating power, and the freezing is concentrated in the areas under the roadway itself. In order to prevent frost damage, the frost-susceptible material must be removed and the ground drained under the construction by pipe drainage. In order to improve the heat-insulating power of the roadway, insulating material such as haydite and slag can be used.

Technical problem:

The ability of a soil type to absorb loads with subsequent deformations depends on the particle size, particle distribution, degree of compaction and pore water pressure of the soil type in the intermediate space between the particles. In loose soil types, such as clay and peat, the particle structure itself can only bear a load for which the soil layer has previously reached an equilibrium, the pre-consoIidation pressure. When the load increases beyond this, the excess load is initially absorbed by the pore water pressure. This pressure is dependent on time, and the change depends on the permeability of the soil, i.e. the dewatering rate. The squeezing-out of a certain volume of water results in corresponding deformations or settling in the ground layers. This settling is irreversible. The carrying capacity and rigidity of the earth masses increase with increasing depth. These properties are determined by the earth being loaded, at a certain depth, by overlying layers which have been compacted and dewatered with time. A so-called consolidation has taken place.

If a certain critical load is exceeded, the deformations accordingly increase quickly. When designing constructions on cohesive soils, either the load must be below this load, or else the load must be transferred via piles down to soil layers of greater carrying capacity. Since ground layers are heterogeneous, the critical load tends to vary from place to place.

Since the level of road and ground constructions lies above the surrounding ground in order to permit water runoff, loads are applied to underlying ground layers. The pore water between the particles in the ground is drained off, and remaining deformations appear. Settling in the ground layers is more pronounced with low pre-consolidation pressure. Such ground layers require, moreover, thicker base courses in order to increase the influence area of the point loads. This in turn results in greater loading and increased settling.

Solution:

The present invention is based on achieving, in road and ground constructions, load distribution by means of introducing increased rigidity in the upper part of the construction. In this way stresses and deformations in the subsoil are reduced. Moreover, the pore water pressure is used in order to absorb loads such as loads of dynamic character and other short-term loads with concentrated distribution. Designing the constructions so light that a complete compensated foundation is possible means that underlying soil layers do not acquire any additional load and the pore water pressure is maintained unchanged. In this connection drainage must be avoided, and it is no disadvantage to seek to obtain a high groundwater level. By means of the abovementioned foundation principle deformation or settling is considerably reduced both for short-term and long-term loading. In addition, the constructions are designed to be heat-insulating, so that the underlying ground is prevented from freezing, in which connection frost damage is also avoided in frost- protection soils.

The road and ground constructions are made up of prefabricated elements for assembly on site. The design depends on the element type which can be chosen both for the full width of the construction and for parts thereof. The elements are made up of cellular plastic or equivalent which, during the casting, forms mould sides for the beam grid which is intended to form the upper part of the ele ment. More precisely, the construction is designed as rigid elements comprising a support part with an upper wearing course and, below it, light material with a lower bulk density than the bed. It is characterized in that the elements are designed so rigid that point loads are distributed over the supporting surface of the bed, in that the element is founded in an excavation in the bed, which is adapted in depth such that the weight of the stripped masses of the bed corresponds essentially to the weight of the element founded in the excavated area, the bulk density of the element being adapted, by combination of heavier material in the supporting part and the light material, so as not to exceed the average bulk density of the stripped masses, so that the load on the bed by way of the weight of the element and at least some of the dynamic loads absorbed by the element are compensated by the weight reduction by means of the stripped masses, while the excess part of the dynamic load present, and in particular that part which exceeds the elastic deformation range for the element, is temporarily absorbed by pore water pressure present in the bed.

Description of figures:

In the attached drawings Fig: 1 shows a cross section along the line I-I in Fig.2 of an element used in the foundation method according to the invention; Fig.2 shows the element in longitudinal section along, the line ll-ll in Fig.1; and Fig.3 shows a diagram of the vertical stress in a bed.

Preferred embodiment:

The invention is described herinafter on the basis of the elements used in the foundation method. The concrete construction itself (Figures 1 and 2) is designed as a beam grid 1 which is covered, on its upper edge, by a continuous concrete slab 2. The beam grid and the slab are reinforced so that the necessary rigidity and carrying capacity are obtained in the finished construction. The concrete in the beams of the beam grid and the slab is made up of light ballast concrete in the density range of 800- 1,400 kg/m³ and whose compression strength lies between 5 and 25 MPa. In particular 3L concrete and X concrete, which are both of the structural light ballast concrete type, have good frost resistance and provide good protection against reinforcement corrosion and are, in both these respects, fully comparable to high-quality normal concrete. However, the abrasive resistance of these types of concrete is low, for which reason they are unsuitable for wearing courses. Where required, for the prefabricated elements of the construction, the ground excavation is carried out where the stripped earth mass corresponds to the magnitude of the applied load. The depth of the excavation is usually from several decimetres up to half a metre, if the height of the roadway is not determined by other considerations.

The excavation can be carried out in principle in a conventional manner, without support walls on the sides. The ground does not have to be strengthened or drained, but only evened out with, for example, sand or gravel. Furthermore, base courses or subbase courses do not have to be added, and measures for preventing frost damage are also unnecessary .

When the elements have been laid in place in the excavation by cranes or other lifting arrangements, the elements a r e locked in such as way that they can function as a base for positioning of subsequent elements or as a base for the lifting arrangement. It should also be possible to adjust the height of the positioned elements. A 20-60 mm thick reinforced concrete layer 3 is cast on top of the mounted elements. The concrete is to be of the high- strength type with compression strengths preferably within the range of 50-200 MPa. This concrete is used also for casting together the joints 4 between the elements. By using, for example, welded steel nets of great anchoring capacity, the construction can be designed without joins in the top surface. The shrinking of the wearing layer leads to many fine cracks which are of no importance for the functioning of the construction or which can, in some way, adversely affect its stability. It is also possible to use fibres and fibre mats, both of steel and of glass or polymer material. The high-strength concrete is workable on the top surface in order to produce surface grooves to provide vehicles with the necessary gripping power. In connection with the groove design there is also the possibility of running-off of water on the roadway. Damage which has occurred is easy to repair, inter alia with the high-strength concrete.

The properties of the concrete in the construction are adapted so as to give low intrinsic weight and, at the same time, maximum carrying capacity and rigidity. The high-strength concrete, which is also used for the wearing course, functions as a compression zone within the areas in which the bending moments are at their greatest. The light ballast concrete is then situated in the tension zone and affects neither the moment capacity nor the rigidity. The high- strength concrete on the upper edge of the construction increases the punch resistance when this zone is compressed. The rigidity and carrying capacity of the lower part of the elements, which consists of cellular plastic 5 or the like, are so great that it can absorb the deformations from the superstructure and spread these out to the underlying ground.

The load on the underlying ground can be made small compared to that from a conventional superstructure. Dimensioning in order to obtain the load-bearing capacity of the roadbed is based on load transfer of wheel pressure from, for example, asphalt layers to underlying ground according to the elasticity theory. The greater the modulus of elasticity of an overlying layer, the less the stress and deformation on the underlying ground. The stress is a function of the quotient E₁/E₂, Figure 2. E₁ and E₂ are the elasticity moduli for two subsequent layers.

Figure 3 shows the vertical stress, in a two-layer bed of varying depth, immediately under the load as a function of the quotient between the elasticity moduli of the layers. The area designated A relates to the stress in the upper layer and B to the lower layer. C indicates the boundary surface between the layers, the x axis shows the vertical stress, and the thickness of the layers in the depth-wise direction is given along the y axis.

The vertical stress of the upper layer of the two- layer bed can be read off for various depths along the upper curve, and the lower curve relates to the lower layer. The stress peaks from traffic load are dependent on irregularities in the roadway. In cases of superimposed stresses or a long load period plastic deformations and a reduced modulus of elasticity can also occur which, over and above the breakdown from the surface, impair the function of the roadbed. In the conventional roadbed there are also local differences in the properties and thickness of the layers as a consequence of differences in material and shortcomings in the laying technique. The asphalt layer is fatigued with time by dynamic loads, which accelerates the breakdown process. The dimensioning criteria are in principle the same for the present invention. Breakage under wheel load is not a dimensioning criterion in this construction, and the tyre pressure can in principle be increased. The modulus of elasticity on the upper layer is to be high and not altered with time. The high-strength concrete layer satisfies these conditions. Factors such as wearing, breakdown, handling and material inadequacies are of secondary importance, and the road and the construction acquire a good service index, i.e. a high PSI number.

The constructions are dimensioned with respect to movements of temperature and temperature gradients. The stresses which occur from prevented deformation can be absorbed without the function of the constructions being impaired.

Loads from the road or ground construction are thus transferred to the ground, and the long-term loads are essentially of the same magnitude as prevailed in the undisturbed earth. The spread of point loads from the rigid superstructure is distributed such that the stress in the subsoil is below the critical value by a good margin. In conventional road constructions the great intrinsic weight has considerably reduced the margin from the critical load. Exceptional loads in the form of short-term loads which exceed the critical load, the pre-consolidation pressure, a r e absorbed by the pore water pressure in the ground material. Since the earth has low permeability for water flow, this is unaffected by dynamic load. To this load category there also belongs short-term positioning of, for example, vehicles, in contrast to conventional road constructions.

The lower parts of the constructions which consist primarily of cellular plastic or the like, are heat-insulating, for which reason damage in connection with frost is avoided. Further advantages are that the ground does not have to be drained and ditch drainage is avoided. The road follows the ground movements, and a levelIing-out on account of local differences is achieved.

Claims

Patent Claims:

1. Method for forming road and ground constructions with a load-bearing function for static and dynamic Loads on ground with low carrying capacity, in which connection the said construction is designed as rigid elements comprising a supporting part (1) with an upper wearing course (3) and, below it, light material (5) with a lower bulk density than the bed, characterized by the combination of the elements (1) being designed so rigid that point loads are distributed over the supporting surface of the bed, that the element is founded in an excavation in the bed, which is adapted in depth such that the weight of the stripped masses of the bed corresponds essentially to the weight of the element founded in the excavated area, in which connection its bulk density is adapted, by combination of heavier material in the supporting part (1) and the light material (5), so as not to exceed the average bulk density of the stripped masses, so that the load on the bed by way of the weight of the element and at least some of the dynamic loads absorbed by the element are compensated by the weight reduction by means of the stripped masses, while the excess part of the dynamic load present, and in particular that part which exceeds the elastic deformation range for the element, is temporarily absorbed by pore water pressure present in the bed.

2. Method according to Claim 1, characterized in that the supporting part (1) of the element is made from a prefabricated beam grid in the longitudinal and transverse directions of reinforced concrete, preferably reinforced light ballast concrete with a bulk density of 800-1,400 kg/m³, and in that the light material (5) of the element consists of cellular plastic or equivalent under a completely covering reinforced concrete slab cast solidly in the beam grid.

3. Method according to Claim 2, characterized in that a number of the said elements are positioned and mounted in the said excavation to form a continuous construction on top of which is cast a further complete and reinforced concrete layer (3) with a thickness of between 20 and 60 mm.

4. Method according to Claim 3, characterized in that the said concrete layer (3) is designed with a compression strength of 50-300 mPa, preferably 60-150 mPa, which is strengthened and reinforced with nets or fibres of steel, glass, carbon, polymers or the like.

5. Method according to Claims 2, 3 or 4, characterized in that, for the light material (5), a heatinsulating material is used with a bulk density of 10- 500 kg/m³, preferably 20-100 kg/m³, in the form of continuous solid blocks or loose particulate material with low water absorption.