Swelling or swelling (expansion of the rock matrix) is due to an increase in volume due to the absorption of water. This phenomenon also induces tensions and changes in the material can lead to time-dependent yielding.

In general, it is associated with the presence of clayey soils or rocks; it can also occur in rocks with anhydrite or a mixture of anhydrite-clay, which is less common but can cause big problems.

Swelling is a combination of a Physicochemical reaction that involves water and a decrease in effort (stress relief). The water reaction is the main contributor to this mechanism, that is, the increase in volume over time, however, it can only be carried out simultaneously or followed by a drop in the state of stress (stress relief). The main physicochemical mechanisms that are involved in water are:

Water is absorbed on the outer surface of the clay minerals and is carried to the inner surfaces of the clay that have expandable layers. The pressure due to “expansion” (swell pressure) depends on the inter-particle distance of the clay particles and the intra-particle distance between the expandable layers. So those rocks with layers of expandable particles swell more than those with particles that absorb water only externally, which translates into greater pressure due to this effect.

The hydration process of the mineral anhydrite (CaSO4 + 2H2O  CaSO4 * 2H2O) produces gypsum, this process involves an increase in volume of up to 60%. In general, this transformation is carried out due to the aqueous dissolution of anhydrite followed by a precipitation process (due to the saturation of the solution), the % increase in volume will depend on the percentage variation of water contained in the initial and final mineral, subject to the availability of water and the amount of anhydrite that is converted to gypsum. In general, massive anhydrites with few fissures swell (swell) less than those that are finely divided (fissured), that is, with a greater specific surface area.

The coexistence of both cases in the rock is also possible, for example the presence of anhydrite grains in shales or shales.

All these physical-chemical processes only take place with a decrease in stress (stress relief) since:

• Causes negative pore pressure which allows water flow. If this water circulation occurs, the volume of water drained increases and so does the volume of the material.

• It can cause fissures (secondary permeability) which facilitates the flow of water and leads to an increase in volume through one of the mechanisms mentioned above.

Other effects that can cause Swelling indirectly are:

• Shear and stress cracks, which increases the area exposed to swelling.

• Weathering and alteration of minerals, for example in aluminosilicates, which initially do not give rise to swelling but their alteration produces clay minerals susceptible to this mechanism.

• Swelling of slabs and filling of joints or minor faults.

Swelling is expressed by an increase in volume and in isotropic or anisotropic materials the deformations are in all directions that are not restricted to movement. If there is any limitation, this will cause an increase in stress in that direction; if there is no possibility of expansion, the stress will be the maximum possible.

The excavations lead to a reduction in the radial stress (if the stresses are observed in polar coordinates) and a variation in the tangential stress close to the periphery of the tunnel. So when the state of stress changes, some of its components will be reduced, allowing the necessary conditions for swelling. A good indicator of the stress condition relevant to swelling is the First stress invariant. To reduce the possibility of swelling, consider keeping the Invariant close to the initial condition prior to excavation.

Contenido

Las pruebas de laboratorio pueden ser pruebas de identificación o pruebas cuantitativas. Con la primera se puede estimar si ocurrirá Swelling en la roca donde se quiere construir una excavación u obtener una idea aproximada del potencial de Swelling que se podría llegar a tener, es decir, la posible magnitud de los desplazamientos y tensiones producidos por el Swelling. Mientras que las pruebas cuantitativas están destinadas a ser utilizadas junto con predicciones analíticas e empíricas.

Identification tests

The best known identification test is the Casagrande test through the Atterberg Limits. There are a number of correlations between Atterberg Limits and Swelling potential. You should know that the Atterberg Limits give us the basic measurements of the critical water content in a fine-grained soil: its shrinkage limit, plastic limit and liquid limit. Most of these relationships are applicable to expansive soils, but Brekke and Howard (1973) provide one for faulting, and Katzir and David (1968) for Swelling in marl rocks. The fact that there are several relationships between the Atterberg Limits and the Swelling potential tells us that the Atterberg Limits values ​​can provide an estimate, but probably cannot be used as a basis for validation relationships. Even more important are the effects of sample preparation such as oven drying, which may be necessary for Atterberg Limits testing which can modify the Swelling behavior in the rock.

Another relatively simple identification procedure is to soak a sample in water until it is 100% wet. Brekke and Howard (1973) were able to correlate the amount of water absorbed and the Swelling pressure.

Also in certain well-defined specific lithological zones or subzones, it is possible to relate the unconfined compressive strength to the Swelling potential.

Quantitative tests for Swelling potential

There are mineralogical texture tests that serve to identify minerals that can potentially exhibit the Swelling phenomenon and, in many circumstances, to provide a quantitative estimate of the expected Swelling pressures. The mineralogical texture test is the quantitative basis of Swelling pressures. Through X-ray diffraction which is ideal for mineralogical tests. It is not only possible to identify minerals with Swelling potential, but also to determine their quantitative proportion. Sometimes it is not only knowing the existence and relative quantities of minerals that are prone to Swelling, but also knowing the distributions of their textures (for example, the intercalation of anhydrite and clay minerals can significantly increase Swelling). Additionally, the parallelism of the clay particles can have a significant effect on increasing Swelling. Investigations with the electron microscope are very suitable for these purposes.

Probably the best-known tests in the area of ​​quantitative testing for Swelling potential are "Swelling tests" in the strict sense, that is, they are tests that involve intact samples obtained from ground or pulverized material, which are subjected to a particular combination of water and applied stresses and the associated volume change of the sample is observed.

The Free Swelling Test and the Maximum Axial Stress Test are tests intended to provide a rapid assessment of Swelling potential with respect to both deformation and stresses. They allow us to verify if Swelling will be a problem and gives us an idea of ​​how long it will take to develop. If these tests indicate that Swelling is important, so-called "Axial Swelling Stress Tests versus Axial Deformation" should be performed. Since, in reality, the conditions are not completely restricted in the Free Swelling Test or the Maximum Axial Stress Test.

Bozberg is a 2.5 km long two-track railway tunnel located in the Jura Mountains in Switzerland, where there is presence of marl, clays and anhydrite and was built between the years of 1871-1875. During the construction of the tunnel, a pillar converged due to Swelling, causing considerable damage. The abutments of some sections of the tunnel had to be rebuilt several times and inverted arches were also built in the most affected sections; However, these arches were quickly destroyed by the highly sulfated water. Drainage channels were frequently destroyed, causing floods that increased the degree of swelling and the vibrations induced by traffic; extreme cases of rock explosion due to high swelling were recorded.

El Squeezing es una deformación en función del tiempo, que ocurre en las cercanías de la excavación, en esencia está principalmente asociado al “creep” (reptación o arrastre) causado por exceder el esfuerzo de corte de la roca circundante. Sin embargo en materiales dilatantes puede estar también asociado a un incremento de volumen del material.

Esto ocurre bajo condiciones de altos esfuerzos, lo cual en rocas de comportamiento competente llevaría a “rock Burst” mientras que en similares condiciones de esfuerzos la presencia de rocas no competentes produce el squeezing.

Como se mencionó previamente este mecanismo puede ocurrir ( en suelos y rocas) en presencia de esfuerzos inducidos (debido al estado original de esfuerzos y la excavación ) y que las propiedades del material lleven a algunas zonas en la superficie del túnel al esfuerzo cortante ( o tangencial) necesario para que se produzca “creep” (reptación).

Submechanisms

Squeezing is a time-dependent tangential displacement of material which causes the periphery of the tunnel to move, decreasing its section (an inward movement). The squeezing mechanism consists of one or a combination of submechanisms called “creep”.

Creep (is the expression of viscous behavior) in mineral particles or grains in intact rock/soil, this individual mechanism of the particles is due to the viscous behavior of the crystalline structure or unstable cracks. Along the interfaces between particles of the material

Along discontinuities (large scale) such as foliation or bedding surfaces, joints and faults. In these mechanisms, the following components shown in the graph are noticeable.

Usually this mechanism is viscoplastic in nature, but in cases of low stress the deformations can be temporary, translating into viscoelastic behavior.

Creep generally begins at stress levels below the shear strength of the material.

So Creep then Squeezing can occur without a volume change. In cases of dilating behavior it may be associated with increased volume.

In general, the analyzes consider that the body deforms over time under a constant stress, when in reality, as the section of the tunnel decreases due to squeezing, the movement restrictions increase (even more so if the perimeter is curved), which can change the stress condition and the squeezing process. As previously mentioned, stress anisotropy due to low strength materials can lead to failure.

Squeezing in the boxes predominates under conditions of greater vertical principal stress (before the excavation) than the horizontal one, on the contrary, if the horizontal one is greater, it will be observed in the ceiling and floor of the excavation. As the material increases its movement restrictions due to squeezing in the tunnel, it can become more severe depending on the original stress state (in situ). Supports in general produce a counterstress in reaction to displacement due to squeezing; if this effect generates an increase in pore pressure, it can be counterproductive.

Rehearsal for Squeezing

The appropriate tests for this purpose are triaxial tests, where the stresses are kept constant and the deformation over time is observed. The good thing about triaxial tests is that they simulate the in-situ condition where the sample was, the consolidation, the pore pressure and the external conditions. In this way we will have good results and be able to predict the behavior of the rock as a function of time.

To mitigate the effects of these instabilities (Squeezing and Swelling) in the design of tunnels we have, from the point of view of Geomechanics, the fortification that can be active and passive.

When we talk about a passive fortification it is one in which, for operational and safety reasons, no external load is applied at the time of installation and only works when the rock mass experiences some deformation. The most used methods are wood, metal frames, reinforced concrete, shotcrete, mesh.

In the case of an active fortification, it is aimed at restoring the original balance of the different efforts and, at the same time, structurally modifying the rock to make it self-supporting. The most used methods are anchor bolts in rocks (punctual or distributed).

In general, the way to mitigate the effect of these deformations for both cases is similar (Squeezing and Swelling). When we have an anchoring system, the prestressed rock arch serves to inhibit deformations that may occur in the contour of an excavation, if a ceiling-to-floor anchoring system is possible, since the floor level of a gallery must be maintained. In the case of using a support system, a lining can be used around the tunnel with which it generates an internal pressure that counteracts the deformations in the contour. In this case, the lining materials to be used and the elasticity model of thick-walled pipe can help us perform a calculation and thus be able to determine the appropriate thickness to carry out the support design.

Swelling or swelling (expansion of the rock matrix) is due to an increase in volume due to the absorption of water. This phenomenon also induces tensions and changes in the material can lead to time-dependent yielding.

In general, it is associated with the presence of clayey soils or rocks; it can also occur in rocks with anhydrite or a mixture of anhydrite-clay, which is less common but can cause big problems.

Swelling is a combination of a Physicochemical reaction that involves water and a decrease in effort (stress relief). The water reaction is the main contributor to this mechanism, that is, the increase in volume over time, however, it can only be carried out simultaneously or followed by a drop in the state of stress (stress relief). The main physicochemical mechanisms that are involved in water are:

Water is absorbed on the outer surface of the clay minerals and is carried to the inner surfaces of the clay that have expandable layers. The pressure due to “expansion” (swell pressure) depends on the inter-particle distance of the clay particles and the intra-particle distance between the expandable layers. So those rocks with layers of expandable particles swell more than those with particles that absorb water only externally, which translates into greater pressure due to this effect.

The hydration process of the mineral anhydrite (CaSO4 + 2H2O  CaSO4 * 2H2O) produces gypsum, this process involves an increase in volume of up to 60%. In general, this transformation is carried out due to the aqueous dissolution of anhydrite followed by a precipitation process (due to the saturation of the solution), the % increase in volume will depend on the percentage variation of water contained in the initial and final mineral, subject to the availability of water and the amount of anhydrite that is converted to gypsum. In general, massive anhydrites with few fissures swell (swell) less than those that are finely divided (fissured), that is, with a greater specific surface area.

The coexistence of both cases in the rock is also possible, for example the presence of anhydrite grains in shales or shales.

All these physical-chemical processes only take place with a decrease in stress (stress relief) since:

• Causes negative pore pressure which allows water flow. If this water circulation occurs, the volume of water drained increases and so does the volume of the material.

• It can cause fissures (secondary permeability) which facilitates the flow of water and leads to an increase in volume through one of the mechanisms mentioned above.

Other effects that can cause Swelling indirectly are:

• Shear and stress cracks, which increases the area exposed to swelling.

• Weathering and alteration of minerals, for example in aluminosilicates, which initially do not give rise to swelling but their alteration produces clay minerals susceptible to this mechanism.

• Swelling of slabs and filling of joints or minor faults.

Swelling is expressed by an increase in volume and in isotropic or anisotropic materials the deformations are in all directions that are not restricted to movement. If there is any limitation, this will cause an increase in stress in that direction; if there is no possibility of expansion, the stress will be the maximum possible.

The excavations lead to a reduction in the radial stress (if the stresses are observed in polar coordinates) and a variation in the tangential stress close to the periphery of the tunnel. So when the state of stress changes, some of its components will be reduced, allowing the necessary conditions for swelling. A good indicator of the stress condition relevant to swelling is the First stress invariant. To reduce the possibility of swelling, consider keeping the Invariant close to the initial condition prior to excavation.

Contenido

Las pruebas de laboratorio pueden ser pruebas de identificación o pruebas cuantitativas. Con la primera se puede estimar si ocurrirá Swelling en la roca donde se quiere construir una excavación u obtener una idea aproximada del potencial de Swelling que se podría llegar a tener, es decir, la posible magnitud de los desplazamientos y tensiones producidos por el Swelling. Mientras que las pruebas cuantitativas están destinadas a ser utilizadas junto con predicciones analíticas e empíricas.

Identification tests

The best known identification test is the Casagrande test through the Atterberg Limits. There are a number of correlations between Atterberg Limits and Swelling potential. You should know that the Atterberg Limits give us the basic measurements of the critical water content in a fine-grained soil: its shrinkage limit, plastic limit and liquid limit. Most of these relationships are applicable to expansive soils, but Brekke and Howard (1973) provide one for faulting, and Katzir and David (1968) for Swelling in marl rocks. The fact that there are several relationships between the Atterberg Limits and the Swelling potential tells us that the Atterberg Limits values ​​can provide an estimate, but probably cannot be used as a basis for validation relationships. Even more important are the effects of sample preparation such as oven drying, which may be necessary for Atterberg Limits testing which can modify the Swelling behavior in the rock.

Another relatively simple identification procedure is to soak a sample in water until it is 100% wet. Brekke and Howard (1973) were able to correlate the amount of water absorbed and the Swelling pressure.

Also in certain well-defined specific lithological zones or subzones, it is possible to relate the unconfined compressive strength to the Swelling potential.

Quantitative tests for Swelling potential

There are mineralogical texture tests that serve to identify minerals that can potentially exhibit the Swelling phenomenon and, in many circumstances, to provide a quantitative estimate of the expected Swelling pressures. The mineralogical texture test is the quantitative basis of Swelling pressures. Through X-ray diffraction which is ideal for mineralogical tests. It is not only possible to identify minerals with Swelling potential, but also to determine their quantitative proportion. Sometimes it is not only knowing the existence and relative quantities of minerals that are prone to Swelling, but also knowing the distributions of their textures (for example, the intercalation of anhydrite and clay minerals can significantly increase Swelling). Additionally, the parallelism of the clay particles can have a significant effect on increasing Swelling. Investigations with the electron microscope are very suitable for these purposes.

Probably the best-known tests in the area of ​​quantitative testing for Swelling potential are "Swelling tests" in the strict sense, that is, they are tests that involve intact samples obtained from ground or pulverized material, which are subjected to a particular combination of water and applied stresses and the associated volume change of the sample is observed.

The Free Swelling Test and the Maximum Axial Stress Test are tests intended to provide a rapid assessment of Swelling potential with respect to both deformation and stresses. They allow us to verify if Swelling will be a problem and gives us an idea of ​​how long it will take to develop. If these tests indicate that Swelling is important, so-called "Axial Swelling Stress Tests versus Axial Deformation" should be performed. Since, in reality, the conditions are not completely restricted in the Free Swelling Test or the Maximum Axial Stress Test.

Bozberg is a 2.5 km long two-track railway tunnel located in the Jura Mountains in Switzerland, where there is presence of marl, clays and anhydrite and was built between the years of 1871-1875. During the construction of the tunnel, a pillar converged due to Swelling, causing considerable damage. The abutments of some sections of the tunnel had to be rebuilt several times and inverted arches were also built in the most affected sections; However, these arches were quickly destroyed by the highly sulfated water. Drainage channels were frequently destroyed, causing floods that increased the degree of swelling and the vibrations induced by traffic; extreme cases of rock explosion due to high swelling were recorded.

El Squeezing es una deformación en función del tiempo, que ocurre en las cercanías de la excavación, en esencia está principalmente asociado al “creep” (reptación o arrastre) causado por exceder el esfuerzo de corte de la roca circundante. Sin embargo en materiales dilatantes puede estar también asociado a un incremento de volumen del material.

Esto ocurre bajo condiciones de altos esfuerzos, lo cual en rocas de comportamiento competente llevaría a “rock Burst” mientras que en similares condiciones de esfuerzos la presencia de rocas no competentes produce el squeezing.

Como se mencionó previamente este mecanismo puede ocurrir ( en suelos y rocas) en presencia de esfuerzos inducidos (debido al estado original de esfuerzos y la excavación ) y que las propiedades del material lleven a algunas zonas en la superficie del túnel al esfuerzo cortante ( o tangencial) necesario para que se produzca “creep” (reptación).

Submechanisms

Squeezing is a time-dependent tangential displacement of material which causes the periphery of the tunnel to move, decreasing its section (an inward movement). The squeezing mechanism consists of one or a combination of submechanisms called “creep”.

Creep (is the expression of viscous behavior) in mineral particles or grains in intact rock/soil, this individual mechanism of the particles is due to the viscous behavior of the crystalline structure or unstable cracks. Along the interfaces between particles of the material

Along discontinuities (large scale) such as foliation or bedding surfaces, joints and faults. In these mechanisms, the following components shown in the graph are noticeable.

Usually this mechanism is viscoplastic in nature, but in cases of low stress the deformations can be temporary, translating into viscoelastic behavior.

Creep generally begins at stress levels below the shear strength of the material.

So Creep then Squeezing can occur without a volume change. In cases of dilating behavior it may be associated with increased volume.

In general, the analyzes consider that the body deforms over time under a constant stress, when in reality, as the section of the tunnel decreases due to squeezing, the movement restrictions increase (even more so if the perimeter is curved), which can change the stress condition and the squeezing process. As previously mentioned, stress anisotropy due to low strength materials can lead to failure.

Squeezing in the boxes predominates under conditions of greater vertical principal stress (before the excavation) than the horizontal one, on the contrary, if the horizontal one is greater, it will be observed in the ceiling and floor of the excavation. As the material increases its movement restrictions due to squeezing in the tunnel, it can become more severe depending on the original stress state (in situ). Supports in general produce a counterstress in reaction to displacement due to squeezing; if this effect generates an increase in pore pressure, it can be counterproductive.

Rehearsal for Squeezing

The appropriate tests for this purpose are triaxial tests, where the stresses are kept constant and the deformation over time is observed. The good thing about triaxial tests is that they simulate the in-situ condition where the sample was, the consolidation, the pore pressure and the external conditions. In this way we will have good results and be able to predict the behavior of the rock as a function of time.

To mitigate the effects of these instabilities (Squeezing and Swelling) in the design of tunnels we have, from the point of view of Geomechanics, the fortification that can be active and passive.

When we talk about a passive fortification it is one in which, for operational and safety reasons, no external load is applied at the time of installation and only works when the rock mass experiences some deformation. The most used methods are wood, metal frames, reinforced concrete, shotcrete, mesh.

In the case of an active fortification, it is aimed at restoring the original balance of the different efforts and, at the same time, structurally modifying the rock to make it self-supporting. The most used methods are anchor bolts in rocks (punctual or distributed).

In general, the way to mitigate the effect of these deformations for both cases is similar (Squeezing and Swelling). When we have an anchoring system, the prestressed rock arch serves to inhibit deformations that may occur in the contour of an excavation, if a ceiling-to-floor anchoring system is possible, since the floor level of a gallery must be maintained. In the case of using a support system, a lining can be used around the tunnel with which it generates an internal pressure that counteracts the deformations in the contour. In this case, the lining materials to be used and the elasticity model of thick-walled pipe can help us perform a calculation and thus be able to determine the appropriate thickness to carry out the support design.