EP3325725B1

EP3325725B1 - Method for improving the mechanical and hydraulic characteristics of foundation grounds of existing built structures

Info

Publication number: EP3325725B1
Application number: EP16738820.6A
Authority: EP
Inventors: Andrea BIRTELE
Original assignee: Thur SRL
Current assignee: Thur SRL
Priority date: 2015-07-17
Filing date: 2016-07-15
Publication date: 2020-10-07
Anticipated expiration: 2036-07-15
Also published as: EP3325725A1; CA2992668A1; US20180209113A1; WO2017013014A1

Description

The present invention relates to a method for improving the mechanical and hydraulic characteristics of foundation grounds of existing built structures.
Any built structure transmits to the ground pressures that produce in the ground deformations deferred over time, known as subsidences. When the subsidences are different at the base of two points of the same built structure, the difference between the measured values is termed differential subsidence.
Structural engineering indicates the extent of the differential subsidences that can be tolerated by different types of built structures.
Geotechnical engineering provides reliable calculation methods suitable to estimate differential subsidences during design.
Usually, differential subsidences of the ground at the base of existing built structures are scarcely significant and do not cause deformations of the structure such as to bring out damage, collapses or malfunctions in general.
However, there are cases in which the differential subsidences of the ground cause displacements of the overlying built structure that exceed the allowable tolerances and are such as to cause failures on the structure which are often not negligible. Reference is intended to cases in which the ground is particularly deformable with respect to the pressures transmitted by the built structures or to cases in which the built structures are not adequate.
Therefore, two approaches for the solution of differential subsidences are distinguished: the intervention aimed at preventing the onset of differential subsidences on built structures during design or on existing built structures that are being extended or converted, and interventions aimed at solving differential subsidences that have already occurred on existing built structures. The method usually used to prevent the forming of differential subsidences on a built structure undergoing construction design provides for adapting the geometry and rigidity of the foundations to the load bearing characteristics of the ground or, in the more complex case of existing built structures, for which extension or conversion work is planned, provides for comparison between the mechanical characteristics of the ground and the additional loads produced by the work for extension or conversion that the existing foundations transmit to the ground.
The method usually used to deal with differential subsidences that have occurred on existing built structures is more complex than the preceding one and provides for a double analysis that relates to the foundation ground and to the structure of the built structure.
The first one assesses the nature and consistency of the ground in the volume in which the differential subsidence has occurred and consequently allows to calculate the resistance and deformability with respect to the loads of the built structure.
The second one reconstructs in details the differential motions of the structure that have generated the cracks that are present on the built structure both in terms of time and in geometric terms.
For ground analysis, the designer can employ traditional geotechnical tests both in situ and in the laboratory. The ground analysis method substantially derives from traditional geotechnics and provides for calculating the resistance and deformability of the soil that is present below the various portions of the built structure starting from the geotechnical parameters obtained from the tests.
The analysis of the built structure, in addition to providing an accurate assessment of the loads according to the faithful reconstruction of the nature of the materials used and an interpretation of the cracking situation that is present on the masonry, is based on measurements of displacements and deformations by means of instruments linked to topography and structural monitoring. Leveling operations with precision instruments are performed often in order to check which part of the built structure has subsided and the extent of the displacement. Topographic readings are then combined with monitoring operations by means of tell-tales, inclinometers, strain gauges, etc., which have the task of checking whether the subsidence is evolving and at what rate it is developing.
After completing the analysis on the foundation ground and on the structure of the built structure, the designer defines the most suitable method for preventing or solving differential subsidences.
There are various systems for preventing or solving differential subsidences.
In particular, a distinction is made between systems that act on the structure and systems that treat the ground.
The former have the task of modifying the manner in which the pressures of the built structure are transferred to the ground by means of work intended to widen the base of the foundation or to extend it deeper into the ground until it encounters more substantial and therefore stronger layers. For this reason, the described methods are usually applied to the entire structure: among these, mention is made for example of micropiles and sub-foundations.
The latter have the task of improving the strength and deformability characteristics of the ground by means of actions aimed at increasing the density of the mass and or at introducing therein materials or mixes that modify physically or chemically the characteristics of the natural ground. These methods can be limited to some portions of the built structure, where the ground has poorer characteristics. This category includes, among others, injections of concrete and synthetic resins.
As specified in the UNI EN 12715 standard, EXECUTION OF SPECIAL GEOTECHNICAL WORK - GROUTING, the injections are distinguished mainly into two categories: injections that do not produce ground displacement and injections that produce ground displacement.
Injections that do not produce ground displacement are limited to alluvial grounds up to a certain value of particle size fineness and are performed by simple permeation.
The parameters that regulate the injectability of a ground by simple permeation are the permeability coefficient of the ground proper and the average diameter of the particles that constitute the mix. The chart of Figure 4 shows the borderline for injections by simple permeation (low-pressure injections), to the left of which ground displacement occurs necessarily.
With low-pressure injections, the mixes tend to fill the interconnecting pores of the ground, without causing hydro-fracturing (or claquage) phenomena that are responsible for significant volume variations of the ground.
During an injection process that does not produce ground displacement, the maximum pressures applied by the pumping systems and/or the maximum expansion pressures of the mixes must therefore remain below a critical value defined by the term P_crack[kPa]. This value depends on many factors, including the weight and the mechanical characteristics of the ground that lies above the injected volume. From a theoretical standpoint, the expression of P_crack[kPa] is as follows: $P_{crack} [kPa] = (γ' * z * v) * (1 + \sin φ) + c$

γ'= weight of immersed volume of the ground;
z = depth to which injection is performed with respect to the plane of site;
v = Poisson coefficient;
ϕ = angle of shear strength of the ground;
c = ground cohesion

The theoretical value of P_crack[kPa] is then validated in the various sites by means of preliminary injectability tests.
During an injection process that does not produce ground displacement, the pressures must increase gradually, since the progressively increasing presence of the mix or of the synthetic resin in the ground tends to reduce intergranular flow spaces.
The increase proceeds until the pressure P_crack[kPa] is reached which determines a displacement of the ground and/or of the overlying built structure.
The field of application of this type of injection is limited to highly permeable and uniform grounds and the method requires very long times.
Among the various known methods for solving differential subsidences by means of injections that do not produce ground displacements, mention is made for example of the TMG (Trevi Multi Grouting) method applied by the Trevi company, which uses for each perforation a bundle of single-valve pipes that are connected selectively to a plurality of pumps with very low flow-rates and pressures.
The main limitation of this type of technology resides in that in the absence of systems for sensing the overlying structure as described above, reaching the pressure P_crack[kPa] is avoided intentionally, limiting the effectiveness of the intervention.
The injections in fact are usually stopped when a previously determined injection pressure is reached or by injecting a mix with a limited expansion pressure.
These pressures are far lower than the pressure P_crack[kPa], while the effectiveness of the intervention is maximum indeed when the pressure is increased gradually to the vicinity of this value.
Another limitation of this type of injection consists in that the injected mixes can drift away from the desired point and reach less confined regions, where the weight and the characteristics of the overlying soil are different, to the point of causing unwanted displacements of the ground or of the structure in regions of the built structure that are far from the volume of ground that one intends to treat.
All other types of injection of mixes in foundation grounds, which are not permeation only, necessarily generate differential displacements in the overlying structure, which are caused by the injection pressure produced by the pump or by the expansion pressure of the mix.
Among the various known methods for solving differential subsidences by means of injections that produce ground displacements, mention is made for example of the method disclosed in EP0851064 , which provides for an increase in the load-bearing capacity of foundation grounds for buildings, by means of the injection of a substance that expands as a consequence of a chemical reaction. The disclosed method uses laser receivers that are fixed to some points of the built structure that lies above the injected volume and which, connected to an emitter, indicate the vertical displacements of the built structure as a consequence of the expansion of the substance in the ground.
There are other methods that provide for the injection of mixes of a different kind which cause ground displacement. Among these, mention is made of the Soilfrac technology of the Keller company, which provides for the use of cement mixes, including expanding ones. The method provides for the creation in multiple steps of fractures in the ground on the part of the mix injected by means of a pump that generates medium-high pressures. In this case also, monitoring of the displacements of the overlying building is provided by means of level meter systems that allow to observe the relative displacement of some points of the built structure with respect to others, utilizing the principle of communicating vessels.
Therefore, in known methods, which provide for injections that produce ground displacement, during work some points of the built structure on which the laser receivers or the level meter cups or other systems, which are in any case localized, may undergo a worsening of the cracking situation due to excessive differential displacements of some portions of the structure, such as to represent a risk for the entire built structure and without a clear awareness of the phenomenon on the part of those who perform the work.
In known cases, in fact, monitoring of the displacements of the built structure during the injection step is performed in a localized manner, usually by means of a laser level or level meter chains, which measure vertical or relative displacement between points.
One of the best-known methods for checking the effectiveness of an injection intervention consists in observing an initial rise of the portion of built structure that lies above the injection point. The initial rise of the structure bears witness to the fact that the mechanical and hydraulic characteristics of the ground have been increased, since the injected ground not only withstands the pressure induced by the overlying load but also withstands the dynamic pressures that are generated upon lifting.
By following this type of verification, in known methods that provide for injections that produce ground displacement, during work the monitoring systems may be anchored to portions of the structure that are not loaded, for example portions of the structure located on built structure portions located beneath wall damage. In such cases, the operator, by observing a displacement of the built structure by means of the localized monitoring system (optical level; laser level, level meter system; etc.), decides to end the injection process before the portion of structure that lies above the injection has actually moved and therefore before the injection has produced a sufficient improvement of the characteristics of the conditions of the foundation ground.
In particular, the method disclosed in EP0851064 provides for each individual injection of synthetic mix to be interrupted when a displacement of the overlying structure is detected. During work, the displacements detected at each injection are added and can produce displacements that cannot be withstood by the structure. It might also be indispensable to interrupt the work before the entire portion intended has been treated, in order to avoid damage to the structure.
In known methods, moreover, the displacements are measured along a single direction and displacements in the other directions are not detected. Therefore, the injection process may produce unwanted movements that damage the structure, following directions that are not monitored by the sensing systems.
The aim of the present invention is to solve the problems described above, by providing a method that is capable of providing criteria for verifying the increase of the mechanical and hydraulic characteristics of the ground and of preserving the built structure against excessive distortions that might be produced during execution of work adapted to solve differential subsidences.
Within this aim, an object of the present invention is to provide a method that integrates or replaces localized monitoring systems.
Another object of the present invention is to provide a method that is simple and quick to perform.
This aim, as well as these and other objects that will become better apparent hereinafter, are achieved by a method according to appended claim 1.
Further characteristics and advantages of the present invention will become better apparent from the description of some preferred but not exclusive embodiments of the method according to the invention, illustrated only by way of nonlimiting example in the accompanying drawings, wherein:

Figure 1 is a schematic view of a built structure on which work is necessary;
Figure 2 is a schematic view of the first sensing step;
Figure 3 is a schematic view of the injection step and of the second sensing steps;
Figure 4 is a chart related to the injectability of the grounds as a function of the properties of the mix and of the ground.

With reference to the figures, the present invention relates to a method for improving the mechanical and hydraulic characteristics of foundation grounds of existing built structures.
The method comprises:

a first step of two-dimensional or three-dimensional sensing of at least one portion 2 of the built structure 1;
a step of identifying at least one region of intervention 3 in the foundation ground beneath the at least one portion 2 sensed in the first sensing step;
a step of injecting, through a plurality of holes 4 provided at least at a part of the intervention region 3, a cement or synthetic mix;
second steps of two-dimensional or three-dimensional sensing, mutually spaced in time, of the at least one portion 2 during the injection step.

In particular, there is a step of interrupting the injection step on the basis of the information gathered during the second steps of two-dimensional or three-dimensional sensing of the at least one portion 2.
In greater detail, the step of interrupting the injection step is performed if the two-dimensional or three-dimensional sensing of the at least one portion 2 sensed in the second sensing steps finds, between two successive sensings, as a function of the intervention type:

a. an overall displacement of at least one part of the at least one portion 2 that lies above the intervention region 3; or
b. a differential movement of parts of the at least one portion 2 that substantially corresponds to the limit of allowable deformation of the at least one portion 2; or
c. the reaching, on the part of the portion that lies above the intervention region 3 of the built structure 1, of a position that was predefined during design.

Conveniently, the portion 2 comprises at least one part of a building or of a built structure, such as for example a vertical wall, a face or a floor.
Preferably, the first sensing step and/or the second sensing steps are performed by using at least one device for the optical acquisition of the two-dimensional or three-dimensional portion.
Advantageously, the acquired images are of the digital type.
Preferably, the optical acquisition device 20 comprises a 3D laser scanning device, which, placed at a suitable distance from the built structure, is capable of emitting laser beams along all directions and of obtaining the exact position of a cloud of points that lie on the built structure being considered.
The data thus acquired can be displayed in real time on the device proper or also on a computer, so that they can be examined more easily.
The first sensing step is adapted to sense two-dimensionally or three-dimensionally a portion from the outside or from the inside of the building.
The second sensing steps are adapted to detect two-dimensionally or three-dimensionally a portion from the outside or from the inside of the building.
Conveniently, the first sensing step and/or the second sensing step substantially consist in placing the optical acquisition device 20, which comprises for example a laser scanning device such as a 3D scanner laser detector, in the vicinity of the building, in a point that allows to sense the entire face or a part thereof (or part of the floor) below which the steps of injection in the ground of cement or synthetic mixes will be performed.
Nothing prevents the first sensing step and/or the second sensing steps from being performed by other types of sensing devices.
By way of example, it has been found that it is particularly effective to perform the first sensing step and/or the second sensing steps by means of a radar device.
Conveniently, the radar device is of the interferometer type.
It is further possible to provide for the first sensing step and/or the second sensing steps to be performed by a device for emitting/receiving electromagnetic waves and/or acoustic waves or by similar devices.
The first sensing step can provide for one or more scans of the built structure to determine the exact position, and specifically of the intervention region 3, prior to the beginning of the injection step.
The method continues with the provision of a plurality of holes in the ground beneath the intervention region 3, even through the foundation of the built structure.
Typically, the diameter of the holes varies between 6 mm and 200 mm.
The depth of the holes is a function of the dimensions of the foundation ground and their center distance is usually comprised between 0.50 m and 3.0 m.
Pipes are then accommodated in the holes and the cement or synthetic mixes are injected into the ground through such pipes.
The non-expanding mixes or synthetic resins are injected into the ground by means of pressure pumping systems, which force the entry of the mixes or synthetic resins in the intergranular voids or, in the presence of grounds having a finer texture, produce hydro-fracturing, i.e., local breakup of the ground and the forming of lattices of mix which, once set, improve the mechanical characteristics of the mass. The pumping systems for the non-expanding mixes or synthetic resins deliver flow-rates on the order of 5-30 liters per minute and usually generate pressures comprised between 10 and 30 bars.
These pressures are capable of forcing the penetration of the cement or synthetic mixes in the intergranular voids of sandy and gravelly grounds and to allow access of the cement or synthetic mix in silty or clayey grounds by means of local ruptures known as hydro-fractures.
The non-expanding mixes or synthetic resins, moreover, can be injected into the ground by means of high- or very high-pressure pumping systems (200 bar to 400 bar), which break up the ground in place and allow the stirring of the matrix with the mix. This last system is known as jet grouting.
The expanding synthetic or cement mixes are injected into the ground through low-pressure pumping systems.
The penetration of the cement or synthetic mixes in the intergranular voids of coarse grounds or the hydro-fracturing of grounds having a finer texture occurs by means of the pressure that is generated during the expansion step, which usually occurs by chemical reaction, reaching values comprised between 0.5 bar and 150 bar.
In the presence of grounds having a finer texture, the hydro-fracturing process is produced not only by the injection pressure but also by the expansion pressure of the cement or synthetic mix. Subsequent hardening of the mix diffused in the ground produces the improvement of the geotechnical characteristics.
In all of the cases cited above, both by pumping into the ground under pressure non-expanding synthetic or cement mixes and by pumping into the ground at low pressure expanding synthetic or cement mixes, inevitably the injection treatment produces a significant volume variation of the ground.
This significant volume variation of the ground produces a displacement of the adjacent and overlying volumes of ground that have not been injected, which, as the injection proceeds, necessarily entail evident displacements of the overlying built structure and therefore of the intervention region 3. The pressure generated in the ground by the injection process, be it performed by means of non-expanding synthetic or cement mixes or by means of expanding synthetic or cement mixes, exceeds the pressures transmitted to the ground by the built structure.
For this reason, during the entire injection step one proceeds with the second sensing steps (two-dimensional or three-dimensional scanning) of the entire built structure or a portion thereof.
The second sensing steps repeated during the injection step provide operators with a complete picture of the built structure and indicate in real time any critical regions that might generate angular distortions that are not allowable for the structure.
This monitoring system, in addition to providing information regarding safety against displacements of the structure during the injection step, is used to return indications as to the overall response of the built structure and therefore the effectiveness of the step of injection into the ground.
With the introduction of the first sensing step and of the second sensing steps by means of the device 20 for two-dimensional or three-dimensional optical acquisition of the portion (for example by means of a 3D scanner laser monitoring device, or by means of a radar device or the like), the function of controlling the effectiveness of the injection, typically performed by traditional laser monitoring topographic systems, as disclosed extensively in EP 0851064 , improves significantly, since it does not merely monitor some points of the structure but it extends the observation to a two-dimensional or three-dimensional portion of the built structure.
The injection step proceeds until the device 20 for optical acquisition (by radar or by means of similar devices) provides indications of a global displacement of the portion 2 of built structure that lies above the intervention region 3 that is detectable but as small as desired (a displacement on the order of magnitude of the tolerance of the instrument used). In this manner one of the best-known criteria for verifying the effectiveness of an intervention for injection into the ground is upheld.
For example, the displacement is global when it affects a certain number of points (from a few tens to several thousand) that are distributed preferably evenly on the portion of built structure that is the subject of the intervention.
If necessary, as an alternative, the injection step can proceed beyond the minimal global displacement and can produce the lifting or in general the displacement of the built structure.
There is a second category of interventions for which injection proceeds until the optical acquisition device 20 detects on any portion of the built structure the forming of angular distortions that are proximate to the allowable tolerances for the structure.
The angular distortions are defined as the ratio between the differential vertical displacement between two points of the same built structure (differential subsidences or differential rise) and their minimum distance.

The person skilled in the art is always capable of determining the allowable tolerances with the aid for example of tables that list the allowable values and the limit values for the angular distortions as a function of the type of building. By way of nonexhaustive example, the most significant are given hereafter:

Limiting angular distortions according to Bjerrum (1963)

Category of potential damage	tanβ
Limit beyond which problems can arise in machines that are sensitive to subsidences	1/750
Danger limit for lattice structures	1/600
Safety limit for buildings in which no cracking is allowed	1/500
First cracking in panel walls and difficulty in using bridge cranes	1/300
Limit beyond which tilting of tall buildings can be visible	1/250
Considerable cracks in panel walls and load-bearing brick walls	1/150
Safety limit for load-bearing brick walls with h/L<1/4	1/150
Limit beyond which structural damage to the buildings is to be feared	1/150

Allowable angular distortions according to Sowers (1962)

Type of structure	tanβ
Multistory load-bearing masonry	0.0005÷0.001
Single-story load-bearing masonry	0.001÷0.02
Damage to plasters	0.001
Reinforced concrete frames	0.0025÷0.004
Walls of structures with reinforced concrete frame	0.003
Steel frames	0.002
Simple steel structures	0.005

The allowable values of the angular distortions for the built structure being studied are defined during design.
Finally, a third category of possible interventions which is intermediate between the two described earlier is pointed out in which injection might be interrupted when the portion of built structure that lies above the intervention region 3 reaches a position that has been predefined during design.
This is the case of lightweight built structures, such as flooring or roads, which do not offer a sufficient contrast to the injection pressure or to the expansion pressure of the cement or synthetic mixes. In most of these cases, the overall displacement of the built structure portion that lies above the intervention region may be insufficient to verify the effectiveness of the intervention and it is therefore preferable to determine during design the desired displacement as a function of the characteristics of the ground and of the built structure.
Another example of intervention that lies within this category relates to industrial or civil flooring that has significant hollows, such as to prevent its normal use. The design in this case might provide for the local lifting of the flooring to a level that is deemed sufficient to regain its planarity but in any case much higher than the tolerance of the sensing instrument used (for example on the order of centimeters), while remaining well below the limit of allowable deformation of such flooring.
Other examples of interventions that lie within this category relate to historical buildings or built structures that are close to collapse and cannot tolerate significant displacements and for which the injections are sized appropriately in terms of quantity of mix to be injected and in terms of injection pressures. Or very heavy buildings for which the sensing of an overall displacement, especially with the first injections, might require quantities of cement or synthetic mixes that exceed those strictly necessary in order to improve the mechanical and hydraulic characteristics of the ground.
In these cases, the injection step is interrupted upon reaching predefined quantities of mix during design although the above cited criterion of effectiveness has not been upheld in every injection point.
If the design requires that the built structure must not undergo significant displacements, the injections will be interrupted when the displacement sensing system detects a minimal displacement, on the order of instrument precision, even in a single point of the built structure.
The injection step can also be performed by using alternately or in succession mixes of different types.
For example, in order to reduce the costs of the intervention, there might be a first step of injection of cement mixes followed by the injection of synthetic mixes.
Otherwise, if the foundation ground has nonuniformities in the intervention region, different types of synthetic mix might be used in order to optimize consumptions and the obtained results.
The injection step can also be performed by using simultaneously a plurality of injection pumps. In this case, the injections can be performed by limiting the angular distortions that are induced on the structure, allowing the injection of more cement or synthetic mix before the limit of allowable deformation is reached, thus achieving a better result.
In practice it has been found that the method according to the invention achieves fully the intended aim, since it allows, in a simple, quick, effective and final manner to preserve the built structure against excessive distortions that might be produced during execution of work for improving the mechanical and hydraulic characteristics of the grounds, replacing or integrating spot monitoring systems with a system for two-dimensional or three-dimensional monitoring of portions of the building.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

A method for improving the mechanical and hydraulic characteristics of foundation grounds of existing built structures, characterized in that it comprises:
- a first step of two-dimensional or three-dimensional sensing of at least one portion (2) of the built structure (1);

- a step of identifying at least one region of intervention (3) in the foundation ground beneath said at least one portion (2) sensed in said first sensing step;

- a step of injecting, through a plurality of holes (4) provided at least at a part of said intervention region (3), a cement or synthetic mix;

- second steps of two-dimensional or three-dimensional sensing, mutually spaced in time, of said at least one portion (2) during said injection step;

- a step of interrupting said injection step on the basis of the information gathered during second steps of two-dimensional or three-dimensional sensing of said at least one portion (2).
The method according to claim 1, characterized in that said step of interrupting said injection step is performed if the two-dimensional or three-dimensional sensing of said at least one portion (2) sensed in said second sensing steps reveals, between two successive sensings, as a function of the intervention type:
a. an overall displacement of at least one part of said at least one portion (2) that lies above said intervention region (3); or

b. a differential displacement of parts of the at least one portion (2) that substantially corresponds to the allowable deformation limit of the at least one portion (2); or

c. the reaching, on the part of the portion that lies above the intervention region (3), of a position that was predefined during design.
The method according to one or more of the preceding claims, characterized in that said portion (2) comprises at least one part of a floor structure.
The method according to one or more of the preceding claims, characterized in that said portion (2) comprises at least one part of a vertical wall.
The method according to claim 1, characterized in that said portion (2) comprises at least one part of a building.
The method according to one or more of the preceding claims, characterized in that said first sensing step and/or said second sensing steps are performed by at least one optical acquisition device (20).
The method according to one or more of the preceding claims, characterized in that said optical acquisition device (20) comprises a laser scanning device.
The method according to one or more of the preceding claims, characterized in that said first sensing step and/or said second sensing steps are performed by a radar device.
The method according to one or more of the preceding claims, characterized in that said radar device is of the interferometry type.
The method according to one or more of the preceding claims, characterized in that said first sensing step and/or said second sensing steps are performed by a device that emits/receives electromagnetic and/or acoustic waves.
The method according to one or more of the preceding claims, characterized in that said first sensing step and/or said second sensing steps are adapted to sense a portion (2) from the outside or from the inside of said building.
The method according to one or more of the preceding claims, characterized in that said injection step is not limited to permeation only.