WO2014020620A1

WO2014020620A1 - A method for the forecast and prevention of landslides in cohesive soils

Info

Publication number: WO2014020620A1
Application number: PCT/IT2012/000242
Authority: WO
Inventors: Luigi Coppola
Original assignee: Luigi Coppola
Priority date: 2012-08-01
Filing date: 2012-08-01
Publication date: 2014-02-06

Abstract

It has been observed that the nucleation of cracks in the substratum originates at a depth where the ground presents a permeability coef- ficient value (K) in a range of orders of magnitude between 10^-6cm/ s and 10^-7 cm/s. As such, the insertion of DMS columns at said depth enables the detection of a landslide event long in advance, and thus allows preventative action to be taken.

Description

TITLE

A METHOD FOR THE FORECAST AND PREVENTION OF LANDSLIDES IN

COHESIVE SOILS

Technical field

The present invention concerns the technical field relating to the methodologies of forecasting and preventing of landslides related to argillaceous grounds, the cohesive or mainly cohesive types.

In particular the invention refers to an innovative method of geognostic research in a continuous cycle of a site at risk, in order to be able to forecast and therefore to prevent the landslide event long in advance.

Background art

As is well known, the risk of landslide is a problem that, whether it is underestimated, can cause serious damage to the structures and also to the people.

In that sense, currently, legislation exists, that forbids construction in areas clearly considered as presenting an hydro geologic risk.

However lots of areas exist that, at least apparently, are wholesome and upon which have been built for a long time many houses and whose number is evergrowing. Being able to estimate and monitor such areas in order to understand the real risk of landslide, and possibly to forecast it, is an essential factor for the security .

Currently, however, there are not reliable methods that allow to forecast a landslide event in a predetermined area but, rather, we rely on mathematic modelings implemented by appropriate softwares whose reliability in forecasting is rather weak. The said modeling refers for instance to the said "Limit-Balance Method" that is based on differential mathematic equations which aim to approximate the behaviour of the ground under consideration. It is nevertheless obvious that, besides being extremely complex, such modeling is not much accurate as a multiplicity of factors are known only in an approximate way.

First of all the mechanical behaviour of the ground assimilates to the behaviour of a perfect plastic body and it is considered that in the sliding conditions the state of tension is constant and independent from the deformation and from the time. Such approximation introduces in the calculation various evaluation errors that are not insignificant. Moreover the ground mainly argillaceous is subject to the variations in the regime of the interstitial water pressures caused by the oscillations of the superficial hydric layer, which modify markedly frictional resistance of such groung. Such parameter is also hardly estimable.

Finally in the existing methodology it is not possibile to know the depht of the surface of the ground crack, that is a value extremely necessary to formulate a forecast calculation of a potential mass movement. Therefore without obtaining any indication about the state of deformation of the ground before the collapse it is not possible to have any modeling whose purpose is the a priori knowledge of a mass movement but only after the accident occurred.

All those factors of uncertainty are therefore approximated with parameters difficult to calculate and not much accurate so the final outcome of the forecast is often really deficient.

Even these detection and research methodologies as GPS, LASER, RADAR, etc., that concur in an accurate way to delineate the movements of surface and the effects on the ground, do not provide sufficient information related to the inner causes of the substratum and the factors prearranging the trigger which remain to this day often uncertain, not unambiguous and surely belated from the point of view of forecasting.

Said methodologies can therefore contribute only partially to the prevention of the landslide event as, by and large, they detect macroscopic movements, that is when the landslide event is already ongoing.

Finally, although a method of monitoring in continuous of a site through the insertion of sensors in the ground for the measurement of movement is far-back well-known, such technology is however not reliable as the exact depht at which the sensors should be placed to prevent a state of landslide is not known. The placement of a sensor at a depth minor than the depht of the surface of the rupture implies the missing detection of the movement or, particularly, the detection of a movement indicative of an ongoing state of landslide and so, in fact, too late to be prevented.

Disclosure of invention

The aim of the present invention is therefore to provide an innovative method allowing to solve at least partially said inconveniences.

In particular the aim of the present invention is to provide an innovative method allowing to forecast the landslide event before it evolves in an irreversible manner permitting thus a work of prevention that enables to stop said event.

These and other aims are therefore obtained with the present monitoring method of a site for the prevention of landslide events according to claim 1.

The present invention provides, therefore, a research and analysis methodology which allows to forecast and to prevent, long in advance, a landslide event through a well known monitoring system in continuous to measure the kinematics of the substratum and to verify the geostatic behaviour of the same in relation to the different stress of the hydric layer.

In particular, the following phases are foreseen:

- a) The making of one or more drilling holes (30) in the ground;

- b) Insertion in the drilling hole (30) of one or more sensor devices (45, 50) for detecting a deformation of the ground;

Altough the monitoring in continuous through sensors is well known, the said monitoring has become at present really efficient and functioning (thus reliable) , simply knowing the exact depth at which have to be inserted said sensors to detect long in advance a landslide movement.

In particular, the holes are dug at such a depth so that said one or more sensor devices (45, 50) are placed within a band dx of the substratum in which the ground presents a permeability coefficient value (K) , excluding its constant E, variable in a range between 10^~6 cm/s and 10^"7 cm/s.

As a matter of fact it has been surprisingly found out that the nucleation of the crack (see for instance Figure 1) in the substratum originates where the ground presents such K values.

According to such method, therefore, the sensors are placed exclusively at the depth where the start of a fracture of extension can occur and thus in a band of substratum which intercepts the real line of a potential fracture of the landslide long in advance. In such a manner, in a simple and reliable way, it is possible to monitor in a continuous cycle a predetermined area of potential trigger of the crack and to intervene should it be measured a movement indicative of the reaching of a first earlier prodromal state of cracking of the landslide. The time of propagation of such cracks is very long, even years, so that once they have been detected it is possibile to intervene conveniently, for instance through drainages, to stop their further propagation.

In such a way, the development of the first signals of deformation of the substratum at said depth, are registered in surface in real-time and sent through the Internet also to be consulted from remote.

This solution allows therefore to forecast long in advance the cracking avoiding to insert sensors excessively in depth (high costs for drilling and risk of interception of the crack when its speed of propagation is already too high) or too much on the surface (detection of the crack when it is already in a phase of advanced propagation) .

If the sensor detects deformations of pre-rupture corresponding to a determined piezometric height then a phase of consolidation of the ground is foreseen.

Such operation consists in the making of a drainage of the hydric layer in order to lower promptly the piezometric level of pre-rupture to a height lower than the former height measured (transversal height) . In such a way the piezometric column is maintained permanently below the hydric pressure of rupture.

Advantageously, the selected sensor device (45, 50) foresees a sensor (45) for the detection of the deformation and a piezometric device (50) to measure the overhanging water column which causes said deformation.

Advantageously, the placement is foreseen in the hole (30) of the sensor (45) for the detection of the deformation and, in the same hole or in an adjacent hole, of the piezometric device (50) to measure the overhanging water column which causes said deformation. The piezometric device can be for instance an "Early Warning DMS" for the automatic managing of the alarms from remote .

Advantageously, a further phase c) is foreseen of monitoring of the site through the sending of the deformation and/or water column data measured to an electronic processor (100) placed in communication with the sensor (45) and/or with the piezometric device (50).

Advantageously, phase c) includes furthermore the operation of sending through the Internet the data measured and uploaded in the processor (100) in such a way that the said data can be downloaded and/or consulted from remote .

Advantageously, the depth at which said sensor device (45, 50) is placed is that in which the permeability coefficient value (K) is, excluding its constant E, in the order of magnitude of 10^~7 cm/s.

Advantageously, the insertion in the same hole (30) of a column (40) comprising both the sensor (45) and the piezometric device (50) is foreseen.

Advantageously, the sensor (45) and the piezometric device (50) are coupled into two distinct holes substantially adjacent one to the other.

Advantageously, the sensor (45) used in phase b) is an inclinometer (45) able to incline itself in response to a movement of the surrounding ground.

Advantageously, before the phase b) , a phase of ground-sampling is foreseen at progressive depth to operate the calculation at the various sampling heights of said coefficient K of the ground.

Advantageously, said determination of the coefficient K includes a phase of determination of volumetric compressibility (iOv) resorting to an adequate number of draining and oedometric compressibility tests. Advantageously, if a deformation is detected, a further phase d) is foreseen, which comprises the making of a drainage in such a way as to maintain the level of the water column below the relative value measured by the piezometric device in correspondence of said deformation.

Advantageously, each column (40) remains arranged inside its drilling hole for all the monitoring time in such a way as to allow a continuous measuring cycle in real time.

Brief description of drawings

Further characteristics and advantages of the present method, according to the invention, will seem clearer with the following description of some embodiments, made by way of an example and not as a limitation, in reference to the drawings attached, wherein :

- Figure 1 represents a succession of formation of cracks in a band of the substratum in which the order of magnitude of the parameter K is 10^~7 cm/s;

- Figure 2 shows in the form of a scheme the flow of the interstitial pressures highlighting the maximum point of pressure in the point of nucleation of the cracks of pre- rupture of figure 1 and thus at the depth of the ground in which K is comprised in an a range of orders of magnitude between 1CT⁶ cm/s and 10^~7 cm/s, and preferably to the point where K is in the order of magnitude of 10^~7 cm/s.

- Figure 3 shows in a schematic way the insertion of a column 40 provided with an inclinometric drill 45 and a piezometric device 50 into a drilling hole 30;

- Figure 4 shows in a schematic way a slope on which said monitoring is made and highlights the making of more drillings 30 in the ground in selected points; the same figure 4 emphasizes a graph of the movements measured by the inclinometer (45);.

- Figure 5 shows structurally a column 40 wrapped around a revolving support;

- Figures 6 to 10 represent tables and data pertaining to an example of calculation of parameter K;

- Figure 11 shows in a schematic way a controlling station 100 placed in surface and communicating, for example wireless or electrically connected, with the sensor 45 and the piezometric device 50;

- Figure 12 shows a graphic example of a movement measured by the inclinometric sensor;

- The succession of figures 13 and 14 shows a sequence of formation of a crack and its relative detection through the sensor 45 of the column 40.

Description of some prefered embodiments

In cohesive grounds the landslide process is essentially due to the hydric infiltration of the water in the substratum that produces a variation of the conditions of effective tension of the ground. At the base of the piezometric column a value of horizontal thrust (a_ho) is generated, which is higher than the resistance value of the saturate ground. Here the deformations of pre-rupture appear that, gradually in the course of time, expand until the collapse of the slope. Such deformations are therefore prodromal of a landslide.

Figure 1 shows a sequence numbered from 1 to 5 that shows a progression in the development of the pre-rupture fractures, as introduced above.

Such cracks widen progressively under the tractive tension (a_ho) in the course of time because the natural slope is subject to the variation of the regime of the interstitial pressures for the oscillations of the piezometric level. The succession from 3 to 5 of figure 1 shows in fact the effect of enlargement of the crack until the state 5 of rupture and developement of the final sliding surface.

In particular the width dx of the interested band increases until it reaches a limit value (a_ho = σ_νο + 2 c_u) for K₀ = 1 which corresponds physically to the plastic- fluid state of the ground at the depth in which the grounds presents a specific value of K cm/sec, better specified consecutively. It is well-known that the parameter K₀ is defined by the following ratio:

K₀-—r-

Therefore it results that when the piezometric surface is at the plane of site the horizontal tension has already exceeded the state of the limit balance of a value highest of 2 c_u. The cause of the plastic-rotational and translational landslide movements is to be attributed to a condition of pressure of the interstitial water developed to the contour of the potential landslide surface in saturation state.

In such principle the amplitudes of the piezometric level produce deformations and movements of the ground, especially if the additional effect of the time on the mechanical decay of the ground itself is also considered.

The proposed method foresees therefore a continuous monitoring of the ground exctly at the height of trigger of the pre-cracking state (height showed in figure 1) thus in order to control whether such transversal deformations occur and so to allow a corrective intervention in time. The development of the transversal deformation at the height of trigger of the pre-rupture cracks has in fact very long times of propagation, even years, allowing thus a corrective intervention of prevention.

According to said method it has been surprisingly found, experimentally in situ, that the genesis of the ruptures of. extension, as per figure 1, occur in the subsurface in the band dx (delimited by horizontal lines traced in Fig. 1) placed at a depht where the ground presents a value of K comprised between K = E-10^'6 cm/sec and K = E-10^'7 cm/sec and preferably K = E-10^"7 cm/sec. The parameter "E" is a decimal number and so, excluding such parameter, the trigger occurs where K presents orders of magnitude comprised in a range between 10^~6 e 10^~7. It results, therefore, that the maximum value of the piezometric pressure is exerted exactly at such depht as it is shown in figure 2.

As shown accordingly in figure 1 and in figure 2 the individuation of the band dx of pre-rupture is indicative of the area where the sliding surface will develop and along which the collapse of the slope will happen. The rupture occurs in variable range of time even of years between the apparition of the fractures of extension and the definitive development of the sliding surface. The knowledge of such kinematic effect is fundamental for the accomplishment of the present method.

The proposed method foresees therefore a continuous monitoring of such band of the ground in order to control whether transversal deformations occur that are indicative of a pre-cracking and to allow consequently a corrective intervention in due time.

As shown in the schematic drawing of figure 3 and figure 4, the method foresees therefore the making of one or more drilling holes 30 in the ground (namely drilling) to be monitored in order to be able to insert inside each of them some columns 40 of (DMS2D/3D) type. The holes are naturally distributed in the area at hydrologic risk previously individuated and the number and the distribution of the drillings are evaluated according to the real dimensions of the area to be explored. Structurally figure 5 shows a column 40 that is in fact shaped as a sole cable 40 flexible-type which is normally unwound by a revolving support inside the dug hole. The column has on the inside a sensor 45 to detect the movement of the ground, preferably the inclinometric type, and a piezometric device 50 that detects the overhanging water column. Said devices, both piezometric and inclinometric, are able to measure continuously the values of the piezometric surface and those of the deformation of pre-rupture in the same place of insertion. In such a way, as better detailed further below, the value of the interstitial hydric pressure corresponding to the apparition of the fractures of extension can be known.

Figure 3 shows better in the form of a scheme the revolving support from which the column 40 unwinds and that is inserted in the hole until the reaching of a predetermined depth corresponding to the value of K = 10^~7 cm/s . Always with the intent to be clearer figure 3 extrapolates in an adjacent way one to the other the piezometric device 50 and the sensor 45 belonging to the column 40. The sensor 45, preferably the inclinometric type, is able to detect movements of the grounds even of the order of milimetre.

According to the method, therefore, subsequently to the phase of the making of the drilling holes 30 the sampling of undisturbed specimens is made at progressive depht, for the determination of the coefficient of volumetric compressibility (m_v) resorting to an adeguate number of draining and oedometric compressibility tests, useful for the calculation of the permeability coefficient (K = cm/sec) in situ and better detailed further below.

During the drilling is furthermore made the analysis of the progressive dissipations of the interstitial hydric pressures (neutral pressures) in the course of time that are generated by a penetrometric drill C.P.T.U and registered in surface by a measuring station connected to a pressure transducer.

Afterwards, having calculated the parameter K at the various dephts, the column 40 is inserted in the holes, as per figure 3, for the monitoring in a continuous cycle.

Although this kind of drills and the piezometric devices are well-known in the state of the art and have been used since long, the innovative method consists in the making of monitoring holes ad such a depth that said sensors are placed in order to monitor the ground exactly into a band in which the subsurface presents a value of the parameter K variable in a range of orders of magnitude between 1CT⁶ cm/s and 10^~7 cm/s and preferably of 10^~7 cm/s.. According to the present method it is therefore possible to monitor exactly the band of the substratum within which the micro-fractures will develop that anticipate the sliding surface of the landslide.

The operation of the calculation of the value of K is known and can for instance be made as follows.

The above-mentioned analysis of the progressive dissipations of the neutral pressures in the course of time enables to reach, through the relation developed by Torstensson (1975) and Baligh & Lavadoux (1980), the value of the coefficient of horizontal consolidation Ch through the relation:

C h = - R²

t where:

τ = time factor at a determined degree of consolidation;

t = time relating to the degree of consolidation considered;

R = radius equivalent to the instrument (1,78 cm). To evaluate the permeability coefficient the following expression has been used: K = m_v -Ch-Y_w

mv = volumetric compressibility coefficient;

yw = volume weight of the water.

The values of m_v have been determined resorting to the results of 9 oedometric compressibility tests (Fig. 6) made on the samples taken into the drilling holes related in Table of figure 7.

Such values have been then evaluated by increments of tension of 1-2 kg/cm², from the geostatic tension and associated to the corresponding values of C_h obtained, at the same depth of tests C.P.T.U.. In particular, the values of the calculation of m_v in situ appear in fig. 8.

The dissipation tests in site have been conducted along 3 verticals named C.P.T.U. 2, 4, 5 corresponding to the drillings S2, S4, S5.

The value of Ch have been evaluated by consolidation degrees of 50% using the interpretative method of Torstensson B.A. (1975) and the following values of the rigidity ratio E/C_u (figure 9) .

The undrained module E has been determined making load and unload cycles in compressibility tests C.U.

The results reported in figure 10 have been obtained this way and from said figure it is possible to evaluate the local variation of the permeability coefficient K with the depth relative to two lithic horizons of the slimy- argilleous ground.

The table of figure 10 shows typical values of K at the various depths and shows how the band dx between about six (6 m) and eight metres from the plane of site results to be in this example the band of interest (K in the order of magnitude between 10^-6 cm/s and 10^~7 excluding the constant E) and beyond which the cracking does not occur.

In such example, therefore, the sensor and the piezometric device are arranged within said band. Distinct points of measurement of K in the area to be monitored can give distinct heights where K presents the requested value for which it is possible that the sensors, depending on the point of penetration, can each one reach a height different from the other.

As shown in figure 11, the inclinometer 45 and the piezometer 50 arranged in the cable 40 are placed in communication with a controlling station 100 placed in surface. The controlling station 100 is further fed by solar panels 110 whereas it is impossible to bring the electric current.

The controlling station memorizes in continuous cycle all the data and, through an Internet connection, makes them accessible on an apposite web page meant for the responsible personnel that control them in real time also from remote. Figure 12 shows as an example what has been measured and monitored in continuous by the inclinometers .

In such a way the operators are able to control in continuous whether any movements of the ground occur that are indicative of an initial state of cracking.

If a state of cracking is detected it is foreseen to intervene immediately at the relative height making a well-known drainage which maintains the piezometric level well below the value registered by the piezometric device associated to the moment of detection of the crack.

Figure 13 and figure 14 shows better in a schematic way a sequence of detection of a state of instability. In particular figure 13 represents a portion of substratum where is placed the sensor 45 and the piezometer 50 that are contained inside the column 40. The column 40 (the D S type) is, as said above, a flexible gummy cable that follows very well the deformations of the ground. So as shown in a schematic way in the subsequent figure 14, in correspondence of a deformation (for instance a translation of the ground) the tube flexes itself causing a rotation of the sensor 45 of the inclinometer type contained inside said tube. The sensor detects therefore a rotation which is indicative of a generic movement of the ground. Contextually the piezometric device 50 send the datum pertaining to the overhanging water column detected which is thus cause of such cracking.

At this moment it is possible to intervene for instance making draining holes all around the drill. In such a way whether the piezometric device has given for example a measure of 2 metres, the draining holes will be made in order to prevent the water column from exceeding such value which would cause a progression of the cracking and therefore subsequently the landslide.

Claims

1. A monitoring method of a site (1) for the prevention of landslide events comprising the phases of:

- a) Making of one or more drilling holes (30) in the ground;

and characterized in that said one or more holes (30) are dug at such a depth so that the sensor device (45, 50) results placed at a depth in which the ground presents a permeability coefficient value (K) , excluding its constant E, variable in a range between 10^"6 cm/s and 10^~7 cm/s .

2. A monitoring method, according to claim 1, wherein the selected sensor device (45, 50) foresees a sensor (45) for the detection of the deformation and a piezometric device to measure the overhanging water column which causes said deformation.

3. A monitoring device, according to claim 1 o 2, wherein the placement is foreseen in the hole (30) of the sensor (45) for the detection of the deformation and, in the same hole or in an adjacent hole, of the piezometric device (50) to measure the overhanging water column which causes said deformation.

4. A monitoring device, according to one or more claims from 1 to 3, wherein a further phase c) is foreseen of monitoring of the site through the sending of the data of deformation and/or water column measured to an electronic processor (100) placed in communication with the sensor device.

A monitoring method, according to claim 4, wherein said phase c) further includes the operation of sending through the Internet the data measured and loaded on the processor (100) in such a way that the said data can be downloaded and/or consulted from remote .

6. A monitoring method, according to one or more previous claims, wherein the depth to which is arranged said sensor device (45, 50) is that in which the permeability coefficient value (K) is, excluding its constant E, in the order of magnitude of 10^"7 cm/s.

7. A monitoring method, according to one or more previous claims from 1 to 6, wherein the insertion in the same hole (30) of a column (40) comprising both the sensor (45) and the piezometric device (50) is foreseen.

8. A monitoring method, according to one or more previous claims from 1 to 6, wherein the sensor (45) and the piezometric device (50) are coupled into two distinct holes substantially adjacent one to the other.

9. A monitoring method, according to one or more previous claims, wherein the sensor (45) used in phase b) is an inclinometer (45) able to incline itself in response to a movement of the surrounding ground.

A monitoring method, according to one or more previous claims, wherein previously to phase b) a phase of ground sampling at progressive depths is foreseen to operate the calculation at the various sampling heights of said coefficient K of the ground.

11. A monitoring method, according to claim 10, wherein said determination of the coefficient K includes a phase of determination of volumetric compressibility (τθ_ν) resorting to an adequate number of draining and oedometric compressibility tests.

12. A monitoring method, according to one or more previous claims, wherein in case of detection of a deformation the further phase d) is foreseen which comprises the making of a drainage in such a way as to maintain the level of the water column below the relative value measured by the piezometric device in correspondence of said deformation.

13. A monitoring method, according to one or more previous claims, wherein each column (40) remains arranged inside its drilling hole for all the monitoring time in such a way as to allow a continuous measuring cycle in real time.