Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay

*Herman Peiffer*

## **Abstract**

In this chapter, the origin and remediation of an important sliding in the overconsolidated Boom Clay in Kruibeke (Belgium) is discussed. Local and environmental factors caused an unstable slope about 30 m deep, A larger sensitivity to erosion resulted finally in the instability of the slope. Because of the formation of fine cracks in the soil there was a possibility for the water to penetrate in the clay close to the surface, resulting in the presence of higher water pressures. Also, the presence of the excavator on top of the slope during exploitation had an important impact on the stress state of the soil. Both an analytical and numerical approach were used to estimate the factor of safety. Because of the change of the soil characteristics, the factor of safety decreases, which can be estimated through a numerical analysis (using the Strength Reduction Method). This chapter also discussed the applied techniques for the remediation using numerical analysis. Also, the importance of the field test is discussed. An integrated approach, using numerical analysis and field tests in combination, is capable of predicting the instability. This approach can also be used to evaluate the stability of the slope after remediation.

**Keywords:** slope stability, monitoring, in situ testing, DMT (dilatometer), rehabilitation

## **1. Introduction**

A natural or an artificial slope in a clay deposit results in a change of the morphological properties and the stress state in the soil This evolution of the stress state can be measured by in situ tests considering the variations over time of the lateral stress at rest K0 [1].

K0 is an important parameter in the design and monitoring in many geotechnical applications (retaining walls, building foundations, and definitely the stability of slopes). For the evaluation of the stability of slopes, it can be estimated out of the results of field tests (Pressuremeter Tests [1], Vane Tests and Piezometer Cone Tests [2], Dilatometer Test [3–5] or numerical modeling, usually considering elastoviscoplastic models.

Although the analysis of the lateral stress, in case of slope stability, also can be made based on the results of laboratory tests [6–8], one of the main problems is that K0, due to the natural variations of horizontal and vertical stresses, at each location depends on the actual stress conditions.

Silvestri [9] performed a post slide analysis using the critical stress curve method for two failures that occurred in overconsolidated sensitive clay. The results of the analysis yielded the shear strength parameters. They found that the strength at the time of failure was due to the fissured nature of the clay, the spreading of the discontinuities caused by stress relief after excavation, and subsequent desiccation of the exposed clay mass.

Grefsheim [10] made a back analysis calculation from full scale landslides in overconsolidated clays using stress-strain plots based on the results of laboratory tests. The author compared the results of his research with those for similar overconsolidated clay soils available in the literature (London Clay and Panama Canal Clay). The correlation was made based on the relationship between shear strength and the plasticity index.

Besides the stress state of the soil, also the groundwater conditions and groundwater pressure plays an important role in the stability of slopes in clay.

The groundwater pressure also determines the stress state in the soil and plays an important role in the equilibrium of the slope.

Chandler [11] discussed twelve case records of failures of slopes in the Upper Lias Clay cutting slopes. The author evaluated that the groundwater behavior based on the results of piezometric measurements. The results of effective stress stability analyses revealed a similar pattern to that found earlier in London Clay (which has the same geological origin as the Boom Clay).

Dehandschutter [12] studied the effect of fracture-development in the Boom Clay on the fluid-flow changes (fracture permeability) and mechanical strength of the soil. This fracture-development is partially due to the soil relaxation after excavating the pit.

D'Elia et al. [13] (1996) and Totani et al. [2] used the Dilatometer Test for the evaluation of the soil conditions after a sliding occurred.

Slope stability is influenced by many factors, among which are subsoil structure, hydrologic conditions (e.g., soil moisture, groundwater position), climatic factors (e.g., rainfall), and the hydraulic boundaries. This chapter presents a multi-disciplinary approach for the remediation of the stability of slopes, which includes a numerical analysis based on the results of the in situ geotechnical monitoring, laboratory geotechnical tests (for the determination of the shear resistance and stiffness of the soil) and piezometric measurements. Thus, a reliable 2D model of the subsoil can be obtained, with well-defined boundaries on which it is possible to apply appropriate hydraulic conditions. This geotechnical model has been used for studying the remediation of the stability of the slope.

## **2. Methodology**

#### **2.1 General**

In this chapter, a comparative analysis is made considering the results of in-situ tests in order to evaluate the sensitivity of these testing methods for the evaluation of slope stability.

The research was conducted in Kruibeke (Belgium) (**Figure 1**.). This research site has overconsolidated Boom clay. In the period of 1963–2010, 30 meters deep pit was excavated in the Boom Clay. The clay was used for the fabrication of expanded clay granulates.

In 2010, the borders of the concession were reached, and the exploitation of this pit was stopped. One year later, in 2011, a first (limited) instability of the slope of this pit occurred (sliding). Apparently, it became clear that more stability problems

**163**

**Figure 2.**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

could be expected. In 2012 and 2013, additional (limited) slidings occurred. In the beginning of 2014, it was decided to set up an extensive monitoring program in

In addition to the monitoring of the settlements and the pore water pressures in the environment, in-situ-tests were carried out on a regular basis to evaluate the

• January 2014—DMT 1 to 5 (before the major sliding) (**Figure 2a**)

• March 2016—DMT1' (in the immediate neighborhood of DMTI)

• July 2014—DMT A to F (about 1 month after the sliding) (**Figure 2a**)

For the analysis, cone penetration tests (CPT) and dilatometer tests (DMT) were executed. A layout of the test locations is presented in **Figure 2a**. Four campaigns

• January 2015—DMT I to IV (after stabilisation) : only DMT 1 was conducted in

The Cone Penetration Test (CPT) is internationally one of the most widely used and accepted test methods used to determine geotechnical properties of soils and

*Photo view of the site with indication of the DMT test locations (a) and presentation of the magnitude of the* 

order to evaluate the risk of further instability of the slopes.

*Layout of the slide (2014) (as adapted from Peiffer (2016) [5]).*

the neighborhood of the sliding (**Figure 2a**)

**2.2 Site investigation: CPT-tests and DMT-tests**

*vertical movement (b) (as adapted from Peiffer (2016) [5]).*

*2.2.1 Cone penetration test (CPT)*

the soil strategy.

stability of the slope.

**Figure 1.**

have been organized:

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

**Figure 1.** *Layout of the slide (2014) (as adapted from Peiffer (2016) [5]).*

*Landslides - Investigation and Monitoring*

of the exposed clay mass.

and the plasticity index.

excavating the pit.

**2. Methodology**

of slope stability.

clay granulates.

**2.1 General**

Silvestri [9] performed a post slide analysis using the critical stress curve method for two failures that occurred in overconsolidated sensitive clay. The results of the analysis yielded the shear strength parameters. They found that the strength at the time of failure was due to the fissured nature of the clay, the spreading of the discontinuities caused by stress relief after excavation, and subsequent desiccation

Grefsheim [10] made a back analysis calculation from full scale landslides in overconsolidated clays using stress-strain plots based on the results of laboratory tests. The author compared the results of his research with those for similar overconsolidated clay soils available in the literature (London Clay and Panama Canal Clay). The correlation was made based on the relationship between shear strength

Besides the stress state of the soil, also the groundwater conditions and ground-

The groundwater pressure also determines the stress state in the soil and plays an

Chandler [11] discussed twelve case records of failures of slopes in the Upper Lias Clay cutting slopes. The author evaluated that the groundwater behavior based on the results of piezometric measurements. The results of effective stress stability analyses revealed a similar pattern to that found earlier in London Clay (which has

Dehandschutter [12] studied the effect of fracture-development in the Boom Clay on the fluid-flow changes (fracture permeability) and mechanical strength of the soil. This fracture-development is partially due to the soil relaxation after

D'Elia et al. [13] (1996) and Totani et al. [2] used the Dilatometer Test for the

Slope stability is influenced by many factors, among which are subsoil structure, hydrologic conditions (e.g., soil moisture, groundwater position), climatic factors (e.g., rainfall), and the hydraulic boundaries. This chapter presents a multi-disciplinary approach for the remediation of the stability of slopes, which includes a numerical analysis based on the results of the in situ geotechnical monitoring, laboratory geotechnical tests (for the determination of the shear resistance and stiffness of the soil) and piezometric measurements. Thus, a reliable 2D model of the subsoil can be obtained, with well-defined boundaries on which it is possible to apply appropriate hydraulic conditions. This geotechnical model has

In this chapter, a comparative analysis is made considering the results of in-situ tests in order to evaluate the sensitivity of these testing methods for the evaluation

The research was conducted in Kruibeke (Belgium) (**Figure 1**.). This research site has overconsolidated Boom clay. In the period of 1963–2010, 30 meters deep pit was excavated in the Boom Clay. The clay was used for the fabrication of expanded

In 2010, the borders of the concession were reached, and the exploitation of this pit was stopped. One year later, in 2011, a first (limited) instability of the slope of this pit occurred (sliding). Apparently, it became clear that more stability problems

water pressure plays an important role in the stability of slopes in clay.

important role in the equilibrium of the slope.

the same geological origin as the Boom Clay).

evaluation of the soil conditions after a sliding occurred.

been used for studying the remediation of the stability of the slope.

**162**

could be expected. In 2012 and 2013, additional (limited) slidings occurred. In the beginning of 2014, it was decided to set up an extensive monitoring program in order to evaluate the risk of further instability of the slopes.

In addition to the monitoring of the settlements and the pore water pressures in the environment, in-situ-tests were carried out on a regular basis to evaluate the stability of the slope.

For the analysis, cone penetration tests (CPT) and dilatometer tests (DMT) were executed. A layout of the test locations is presented in **Figure 2a**. Four campaigns have been organized:


## **2.2 Site investigation: CPT-tests and DMT-tests**

## *2.2.1 Cone penetration test (CPT)*

The Cone Penetration Test (CPT) is internationally one of the most widely used and accepted test methods used to determine geotechnical properties of soils and the soil strategy.

#### **Figure 2.**

*Photo view of the site with indication of the DMT test locations (a) and presentation of the magnitude of the vertical movement (b) (as adapted from Peiffer (2016) [5]).*

## *Landslides - Investigation and Monitoring*

The test method consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate of 2 cm/s. The typical cone tip has a cross-sectional area of 10 cm2 , corresponding to a diameter of 36 mm (**Figure 3**).

During the CPT-test, the cone resistance qc is measured over the whole depth of the soil profile, generally up to more than 20 m below the surface or until the cone reaches a hard layer. A typical setup for the execution of a CPT-test is presented in **Figure 4**. The cone is pushed in the soil using a counterweight (in general, a heavy truck) that is positioned horizontally. The cone resistance is recorded using force sensors in the tip. These values allow for a good understanding of soil behavior. In particular, the strength of the soil and the relative density (compaction degree) can be determined.

**165**

**Figure 5.** *Dilatometer blade.*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

A typical application of the CPT-test is the evaluation of the effect of the change in stress state on the strength of the soil by doing a test before and after, for example,

The flat dilatometer (DMT) is a stainless steel blade with a thickness of about 15 mm, having a flat, circular steel membrane (diameter 60 mm, thickness 0.1 to

The blade is connected to a control unit on the ground surface by a pneumatic-

The pressure readings A and B must then be corrected by the values ΔA and ΔB, determined by calibration, in order to determine the soil stresses P0 (vertical position of the membrane) and P1 (expansion of 1.1 mm of the membrane). P0 is used for the determination of K0, (P1-P0) is related to the

The blade is pushed into the ground using common field equipment as for the CPT-test. The test starts by pushing the dilatometer into the ground. The blade is advanced into the ground of one depth increment (typically 20 cm), and the procedure for the A, B readings are repeated at each depth. A more detailed

The field of application of the DMT is very wide. The immediate determination of the stress state and the stiffness of the soil are important advantages of this test

electrical tube (transmitting gas pressure and electrical continuity) running through the insertion rods (same rods as for the CPT-test) (**Figure 6**). A gas tank, connected to the control unit by a pneumatic cable, supplies the gas pressure required to expand the membrane. At the beginning of the expansion, the horizontal pressure A can be measured. After an expansion of 1.1 mm at the center of the

flexible membrane, the horizontal pressure B can be measured.

description of the test procedure is discussed in [4].

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

0.2 mm) mounted on one side (**Figure 5**).

ground improvement works.

*2.2.2 Dilatometer test (DMT)*

stiffness of the soil.

technique.

**Figure 4.** *Execution of the CPT-test.*

**Figure 3.**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

A typical application of the CPT-test is the evaluation of the effect of the change in stress state on the strength of the soil by doing a test before and after, for example, ground improvement works.

## *2.2.2 Dilatometer test (DMT)*

*Landslides - Investigation and Monitoring*

cross-sectional area of 10 cm2

The test method consists of pushing an instrumented cone, with the tip facing down, into the ground at a controlled rate of 2 cm/s. The typical cone tip has a

During the CPT-test, the cone resistance qc is measured over the whole depth of the soil profile, generally up to more than 20 m below the surface or until the cone reaches a hard layer. A typical setup for the execution of a CPT-test is presented in **Figure 4**. The cone is pushed in the soil using a counterweight (in general, a heavy truck) that is positioned horizontally. The cone resistance is recorded using force sensors in the tip. These values allow for a good understanding of soil behavior. In particular, the strength of the soil and the relative density (compaction degree) can be determined.

, corresponding to a diameter of 36 mm (**Figure 3**).

**164**

**Figure 4.**

**Figure 3.** *The CPT-cone.*

*Execution of the CPT-test.*

The flat dilatometer (DMT) is a stainless steel blade with a thickness of about 15 mm, having a flat, circular steel membrane (diameter 60 mm, thickness 0.1 to 0.2 mm) mounted on one side (**Figure 5**).

The blade is connected to a control unit on the ground surface by a pneumaticelectrical tube (transmitting gas pressure and electrical continuity) running through the insertion rods (same rods as for the CPT-test) (**Figure 6**). A gas tank, connected to the control unit by a pneumatic cable, supplies the gas pressure required to expand the membrane. At the beginning of the expansion, the horizontal pressure A can be measured. After an expansion of 1.1 mm at the center of the flexible membrane, the horizontal pressure B can be measured.

The pressure readings A and B must then be corrected by the values ΔA and ΔB, determined by calibration, in order to determine the soil stresses P0 (vertical position of the membrane) and P1 (expansion of 1.1 mm of the membrane). P0 is used for the determination of K0, (P1-P0) is related to the stiffness of the soil.

The blade is pushed into the ground using common field equipment as for the CPT-test. The test starts by pushing the dilatometer into the ground. The blade is advanced into the ground of one depth increment (typically 20 cm), and the procedure for the A, B readings are repeated at each depth. A more detailed description of the test procedure is discussed in [4].

The field of application of the DMT is very wide. The immediate determination of the stress state and the stiffness of the soil are important advantages of this test technique.

**Figure 5.** *Dilatometer blade.*

**Figure 6.** *DMT – Test procedure.*

## **3. Instability of the studied slope**

#### **3.1 A general overview of the main sliding**

In the next figures, a general view of the main sliding is presented. After excavation of the clay pit till a depth of about 30 m, in the Northern part of the pit, instability (small slidings) of the walls started to appears after some years. The main reason for this instability was the lack of sufficient resistance against horizontal stability. During the period of excavation, the pit became the central dewatering point for the region. Unfortunately, the surface between the upper quaternary layer and the underlying Boom clay was slightly declining towards the pit. The upper part (quaternary layer), with a thickness of about 4 to 5 m, moved towards the pit, resulting in instability. Also, the underlying Boom Clay was affected by this slide.

**Figures 1** and **7** present a general view of the slides and the soil movement in 2014. As shown in these figures, a block of 100 m by 50 m in size started to move downward, resulting in a movement of about 9 m down at the corner of the pit.

In **Figure 2**, the extent of the sliding can be noticed. The legend in **Figure 2b** indicates the vertical displacement of the soil mass after the sliding.

After the first sliding in the northwestern part of the pit, a monitoring program was set up. The monitoring includes the execution of CPT-tests and DMT-test. Besides these tests, also the pore-water pressures were measured on a regular basis.

#### **3.2 First sliding – period 2011 - 2012**

The first slide occurred in 2011. The size of the slide was limited. In the second half of 2013, one more slide occurred in the neighborhood at point 5 shown in **Figure 2a**., A photo of the slide is presented in **Figure 8**.

#### **3.3 More pronounced instability of the slope (2014)**

After a more pronounced sliding in 2013, in January 2014, a monitoring program was established for different points at the top of the slope around the pit. Besides topographic surveys and the measurement of the phreatic water level,

**167**

1 and 2 (**Figure 2a**).

**Figure 7.**

**Figure 8.**

figures refer to measurement points.

for the evaluation of slope stability.

**4. Analysis based on the site investigation**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

Cone Penetration Tests (CPT) and Dilatometer-tests (DMT) were carried out. The

*Photo of the unstable zone after the instability of the slope (as adapted from Peiffer (2016) [5]).*

The extent of the affected zone can be seen in the **Figure 9**. This is a CADtranslation of the zone presented in **Figure 2a**. Two profiles (cross-sections) are also presented in **Figures 10** and **11**. The general Belgian reference system for the heights indicated as 'm TAW' refers to the height above sea level. The arrows on the

In July 2014, 1 month after the sliding, additional site investigation was done at the top of the slope in the area where the sliding occurred. Six additional points

As discussed in 2.2.2., The DMT test can be used for risk assessment concerning the stability of slopes [3]. In recent publications [5, 14], the results of CPT and DMT tests were interpreted in order to evaluate the sensitivity of these testing methods

were defined (points A to F) for the execution of CPT and DMT tests.

A lot of in situ tests were conducted. In this section, the results of the DMT tests at point 1, and point 2 (**Figure 2a**) were discussed, because the sliding occurred in this zone. For the general discussion of the results of the in situ tests can be referred to [5]. In June 2014, a big sliding occurred (size of about 100 m x 50 m) between points

in situ tests were executed in points 1 to 5 (**Figure 2a**).

*Detailed impression of the instable slope (as adapted from Peiffer (2016) [5]).*

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

#### **Figure 7.**

*Landslides - Investigation and Monitoring*

**3. Instability of the studied slope**

**Figure 6.**

*DMT – Test procedure.*

**3.2 First sliding – period 2011 - 2012**

**3.1 A general overview of the main sliding**

In the next figures, a general view of the main sliding is presented. After excavation of the clay pit till a depth of about 30 m, in the Northern part of the pit, instability (small slidings) of the walls started to appears after some years. The main reason for this instability was the lack of sufficient resistance against horizontal stability. During the period of excavation, the pit became the central dewatering point for the region. Unfortunately, the surface between the upper quaternary layer and the underlying Boom clay was slightly declining towards the pit. The upper part (quaternary layer), with a thickness of about 4 to 5 m, moved towards the pit, resulting in instability. Also, the underlying Boom Clay was affected by this slide. **Figures 1** and **7** present a general view of the slides and the soil movement in 2014. As shown in these figures, a block of 100 m by 50 m in size started to move downward, resulting in a movement of about 9 m down at the corner of the pit. In **Figure 2**, the extent of the sliding can be noticed. The legend in **Figure 2b**

After the first sliding in the northwestern part of the pit, a monitoring program

The first slide occurred in 2011. The size of the slide was limited. In the second half of 2013, one more slide occurred in the neighborhood at point 5 shown in

After a more pronounced sliding in 2013, in January 2014, a monitoring program was established for different points at the top of the slope around the pit. Besides topographic surveys and the measurement of the phreatic water level,

was set up. The monitoring includes the execution of CPT-tests and DMT-test. Besides these tests, also the pore-water pressures were measured on a regular basis.

indicates the vertical displacement of the soil mass after the sliding.

**Figure 2a**., A photo of the slide is presented in **Figure 8**.

**3.3 More pronounced instability of the slope (2014)**

**166**

*Detailed impression of the instable slope (as adapted from Peiffer (2016) [5]).*

#### **Figure 8.**

*Photo of the unstable zone after the instability of the slope (as adapted from Peiffer (2016) [5]).*

Cone Penetration Tests (CPT) and Dilatometer-tests (DMT) were carried out. The in situ tests were executed in points 1 to 5 (**Figure 2a**).

A lot of in situ tests were conducted. In this section, the results of the DMT tests at point 1, and point 2 (**Figure 2a**) were discussed, because the sliding occurred in this zone. For the general discussion of the results of the in situ tests can be referred to [5].

In June 2014, a big sliding occurred (size of about 100 m x 50 m) between points 1 and 2 (**Figure 2a**).

The extent of the affected zone can be seen in the **Figure 9**. This is a CADtranslation of the zone presented in **Figure 2a**. Two profiles (cross-sections) are also presented in **Figures 10** and **11**. The general Belgian reference system for the heights indicated as 'm TAW' refers to the height above sea level. The arrows on the figures refer to measurement points.

In July 2014, 1 month after the sliding, additional site investigation was done at the top of the slope in the area where the sliding occurred. Six additional points were defined (points A to F) for the execution of CPT and DMT tests.

## **4. Analysis based on the site investigation**

As discussed in 2.2.2., The DMT test can be used for risk assessment concerning the stability of slopes [3]. In recent publications [5, 14], the results of CPT and DMT tests were interpreted in order to evaluate the sensitivity of these testing methods for the evaluation of slope stability.

**Figure 9.** *Print of the NE-corner.*

**Figure 10.** *Section A-A.*

#### **Figure 11.** *Section B-B.*

The consecutive DMT-measurements revealed an evolution towards instability. After a significant failure (sliding) in one zone in June 2014 (affecting an area of 100 m by 50 m), remedial works in the destabilized zone were undertaken (October – November 2014). After stabilization in 2015, further DMTmeasurements were taken in order to investigate the evolution of the stress state of the soil in the potential sliding surfaces.

## **5. Boom clay**

Based on geological investigations and data maps, the upper layer consists of sandy silt (quaternary deposit). Under this layer, the tertiary overconsolidated

**169**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

Boom Clay can be found. **Figure 9** shows the characteristics of this soil as measured

The tests allow for good identification of the soil. In the column on the left side, the results for the material index Id are presented. Based on Id, it is possible to make a reliable interpretation of the nature of the soil. One can identify in this graph a top layer of sand to silt and starting from a depth of about 4.5 m, a silty clay layer (the Boom Clay). In the second column, the horizontal stress index Kd is presented. A Kd-value between the two dashed lines (Kd = 1.8 and Kd = 2.4) refers to a normally consolidated soil. A Kd-value higher than 2.4 refers to an overconsolidated soil

Also, the stiffness (constrained modulus M) and the strength (undrained shear strength cu) can be determined out of the basic measurements of the DMT. Both an undrained shear strength of 100 to 150 kPa and a constrained modulus up to 35 MPa

The profile confirms the results of the undisturbed sampling of the soil. This section focused on the horizontal stress index Kd. This parameter reflects the ratio

The phreatic level is at a depth of about 1.5 m. The thickness of the clay at the bottom of the pit is about 10 m. This thickness is sufficient to resist the upward

During the period of excavation, the pit became the central drainage point for the region. Unfortunately, the surface between the upper quaternary layer and the

**6. DMT as a tool for the detection of slip surfaces – the DMT-Kd-method**

The principle of the DMT-Kd-method for the detection of slip surfaces have

In many OC clay landslides, the sequence of sliding, remolding, and reconsolidation leave the clay in the slip zone in a normally consolidated (NC) or nearly NC

Based on field data from different clay sites in various geographical areas, correlations between OCR and Kd could be established, as shown in **Figure 13** [2, 13, 15–18]. In genuinely NC clay (no structure, aging or cementation), the horizontal stress index Kd from the DMT is approximately equal to 2, while Kd values in OC clays are

For this reason, it is known that, if an OC clay slope contains clay layers with Kd ≈ 2, these layers are highly likely to be part of a slip surface (active or quiescent).

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

(OC), as can be seen for the Boom Clay.

been discussed by many authors [3, 13].

considerably higher (for the Boom Clay about 8).

of horizontal stress to vertical stress (**Figure 12**).

underlying Boom clay was declined slightly towards the pit.

state, with one or more loss of structure, aging or cementation.

refer to very stiff clay.

artesian water pressures.

by the DMT.

*DMT results in Kruibeke.*

**Figure 12.**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

**Figure 12.** *DMT results in Kruibeke.*

*Landslides - Investigation and Monitoring*

**168**

**5. Boom clay**

**Figure 11.** *Section B-B.*

**Figure 10.** *Section A-A.*

**Figure 9.**

*Print of the NE-corner.*

the soil in the potential sliding surfaces.

The consecutive DMT-measurements revealed an evolution towards instability. After a significant failure (sliding) in one zone in June 2014 (affecting an area of 100 m by 50 m), remedial works in the destabilized zone were undertaken (October – November 2014). After stabilization in 2015, further DMTmeasurements were taken in order to investigate the evolution of the stress state of

Based on geological investigations and data maps, the upper layer consists of sandy silt (quaternary deposit). Under this layer, the tertiary overconsolidated

Boom Clay can be found. **Figure 9** shows the characteristics of this soil as measured by the DMT.

The tests allow for good identification of the soil. In the column on the left side, the results for the material index Id are presented. Based on Id, it is possible to make a reliable interpretation of the nature of the soil. One can identify in this graph a top layer of sand to silt and starting from a depth of about 4.5 m, a silty clay layer (the Boom Clay). In the second column, the horizontal stress index Kd is presented. A Kd-value between the two dashed lines (Kd = 1.8 and Kd = 2.4) refers to a normally consolidated soil. A Kd-value higher than 2.4 refers to an overconsolidated soil (OC), as can be seen for the Boom Clay.

Also, the stiffness (constrained modulus M) and the strength (undrained shear strength cu) can be determined out of the basic measurements of the DMT. Both an undrained shear strength of 100 to 150 kPa and a constrained modulus up to 35 MPa refer to very stiff clay.

The profile confirms the results of the undisturbed sampling of the soil. This section focused on the horizontal stress index Kd. This parameter reflects the ratio of horizontal stress to vertical stress (**Figure 12**).

The phreatic level is at a depth of about 1.5 m. The thickness of the clay at the bottom of the pit is about 10 m. This thickness is sufficient to resist the upward artesian water pressures.

During the period of excavation, the pit became the central drainage point for the region. Unfortunately, the surface between the upper quaternary layer and the underlying Boom clay was declined slightly towards the pit.

## **6. DMT as a tool for the detection of slip surfaces – the DMT-Kd-method**

The principle of the DMT-Kd-method for the detection of slip surfaces have been discussed by many authors [3, 13].

In many OC clay landslides, the sequence of sliding, remolding, and reconsolidation leave the clay in the slip zone in a normally consolidated (NC) or nearly NC state, with one or more loss of structure, aging or cementation.

Based on field data from different clay sites in various geographical areas, correlations between OCR and Kd could be established, as shown in **Figure 13** [2, 13, 15–18].

In genuinely NC clay (no structure, aging or cementation), the horizontal stress index Kd from the DMT is approximately equal to 2, while Kd values in OC clays are considerably higher (for the Boom Clay about 8).

For this reason, it is known that, if an OC clay slope contains clay layers with Kd ≈ 2, these layers are highly likely to be part of a slip surface (active or quiescent).

#### **Figure 13.**

*Correlation Kd-OCR for cohesive soils [2, 13, 15–18] (as adapted from Peiffer (2016) [5]).*

The DMT-Kd-method consists of identifying zones of NC-clay in a slope, using Kd ≈ 2 as the identifier of the NC zones.

## **7. Evaluation of the test results**

The detailed evaluation of the test results of profiles with depth is discussed in [14, 19].

In **Figure 14**, the results are presented for the tests before (January 2014 – DMT 1), immediately after (July 2014 – DMT E) and about 2 years after the sliding (March 2016 – DMT 1′). In the test diagrams, the presence of the upper quaternary sandy silt layer can clearly be distinguished. Between January and July 2014, Kd decreased from about 3 to 5 t0, about 1.2 to 1.8 between 3.5 m and 4.5 m of depth. One can conclude that at this depth, the soil modified from an overconsolidated soil to an unstable soil (horizontal stress state lower than that for a normally consolidated soil). Above and below this strongly altered zone, the Kd-value decreased also, but less pronounced. Also, the stiffness, in particular the constrained modulus M, reduced sharply. The CPT diagram also shows a reduction of the cone resistance at a depth of about 3 to 4 m, but not that pronounced as in the results of the DMT, especially not at the top of the overconsolidated clay.

Out of the results of the DMT-tests, one can observe an unstable layer with a thickness of about 1.5 m where the soil is unstable where the slip was developed, but above and below this zone, the stress state is also affected by the sliding. Although cone resistance is decreased by a factor about 2, and a significant decrease of qc between 2 and 4 m, it can be seen in this layer that the qc-profile does not give a clear reflection of the sliding. It is also important to note that the Kd-values below the sliding zone decreased to a depth of about 5 m below the sliding surface.

It is worth noting how and to what extent the stresses were changed after the sliding. In **Figure 14c**, the results are presented for the measurements carried out in March 2016, after remedial works had been carried out in October–November 2014. The qc-value did not change significantly. The Kd-diagram shows clearly that the Kd-values exceeded again 1.8 (the lower limit for normally consolidated soils). For the clayey layer, the reading actually reflects a normally consolidated soil. For the intermediate sandy silt layer, one can see a slightly overconsolidated soil condition.

**171**

**8. Stability analysis**

*(DMT 1′) (as adapted from Peiffer (2016) [5]).*

**Figure 14.**

pressures on the horizontal equilibrium.

appropriate improvement of horizontal stability.

increased, and also the flow gradient increased.

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

Based on the field measurements, the origin and the propagation of the instable zone where the sliding surface was initiated and developed could be clearly investigated. A numerical analysis was performed, considering the geometry of section AA' (**Figure 10**) and the soil characteristics as they were measured in situ (before the sliding) and in laboratory tests. In this analysis, variable piezometric boundary conditions were considered to evaluate the impact on stability. Further investigations and additional calculations showed the importance of the groundwater

*Comparison CPT-DMT results – (a)January 2014 (DMT 1); (b) July 2014 (DMT E) and (c)march 2016* 

Additional CPT-tests and laboratory tests were executed in order to make a detailed stability analysis. Based on an analytic and numerical analysis (PLAXIScalculation), it was possible to prove the cause of the instability, and to design an

Apparently, during the period of exploitation, the environment around the pit gradually became a central dewatering point. The flow of water towards the pit

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

**Figure 14.**

*Landslides - Investigation and Monitoring*

Kd ≈ 2 as the identifier of the NC zones.

**7. Evaluation of the test results**

in [14, 19].

**Figure 13.**

overconsolidated clay.

sliding surface.

The DMT-Kd-method consists of identifying zones of NC-clay in a slope, using

*Correlation Kd-OCR for cohesive soils [2, 13, 15–18] (as adapted from Peiffer (2016) [5]).*

The detailed evaluation of the test results of profiles with depth is discussed

immediately after (July 2014 – DMT E) and about 2 years after the sliding (March 2016 – DMT 1′). In the test diagrams, the presence of the upper quaternary sandy silt layer can clearly be distinguished. Between January and July 2014, Kd decreased from about 3 to 5 t0, about 1.2 to 1.8 between 3.5 m and 4.5 m of depth. One can conclude that at this depth, the soil modified from an overconsolidated soil to an unstable soil (horizontal stress state lower than that for a normally consolidated soil). Above and below this strongly altered zone, the Kd-value decreased also, but less pronounced. Also, the stiffness, in particular the constrained modulus M, reduced sharply. The CPT diagram also shows a reduction of the cone resistance at a depth of about 3 to 4 m, but not that pronounced as in the results of the DMT, especially not at the top of the

In **Figure 14**, the results are presented for the tests before (January 2014 – DMT 1),

Out of the results of the DMT-tests, one can observe an unstable layer with a thickness of about 1.5 m where the soil is unstable where the slip was developed, but above and below this zone, the stress state is also affected by the sliding. Although cone resistance is decreased by a factor about 2, and a significant decrease of qc between 2 and 4 m, it can be seen in this layer that the qc-profile does not give a clear reflection of the sliding. It is also important to note that the Kd-values below the sliding zone decreased to a depth of about 5 m below the

It is worth noting how and to what extent the stresses were changed after the sliding. In **Figure 14c**, the results are presented for the measurements carried out in March 2016, after remedial works had been carried out in October–November 2014. The qc-value did not change significantly. The Kd-diagram shows clearly that the Kd-values exceeded again 1.8 (the lower limit for normally consolidated soils). For the clayey layer, the reading actually reflects a normally consolidated soil. For the intermediate sandy silt layer, one can see a slightly overconsolidated soil

**170**

condition.

*Comparison CPT-DMT results – (a)January 2014 (DMT 1); (b) July 2014 (DMT E) and (c)march 2016 (DMT 1′) (as adapted from Peiffer (2016) [5]).*

## **8. Stability analysis**

Based on the field measurements, the origin and the propagation of the instable zone where the sliding surface was initiated and developed could be clearly investigated.

A numerical analysis was performed, considering the geometry of section AA' (**Figure 10**) and the soil characteristics as they were measured in situ (before the sliding) and in laboratory tests. In this analysis, variable piezometric boundary conditions were considered to evaluate the impact on stability. Further investigations and additional calculations showed the importance of the groundwater pressures on the horizontal equilibrium.

Additional CPT-tests and laboratory tests were executed in order to make a detailed stability analysis. Based on an analytic and numerical analysis (PLAXIScalculation), it was possible to prove the cause of the instability, and to design an appropriate improvement of horizontal stability.

Apparently, during the period of exploitation, the environment around the pit gradually became a central dewatering point. The flow of water towards the pit increased, and also the flow gradient increased.

**Figure 15.** *PLAXIS-analysis and measurement of the slope after sliding (as adapted from Peiffer (2016) [5]).*

**Figure 15** [19] presents the calculated critical horizontal disequilibrium taking into account the presence of the groundwater and the piezometric site conditions. The red dots are points where the soil becomes plastic. The higher the density of the red dots, the more plastic the soil will behave, finally resulting in failure when there is no equilibrium of the soil stresses anymore. At the failure (the dark red line in **Figure 15**) zone, one can clearly detect the position of the sliding plane.

Out of these theoretical results, it became clear that the installation of drains would play an important role in rehabilitation.

## **9. Remediation**

The slope was stabilized using three techniques:


The PLAXIS-analysis resulted in a design where a deep drain has to be installed at a depth of 5 m., as shown in **Figures 16** and **17**. This drain was installed at the beginning of November 2015. The red line in **Figure 15** is the position of the drain in the plan view.

After the installation of this drain, there was an immediate effect on the depth of the groundwater. The original depth of 1.5 m increased to a depth of about 4.5 m. The safety factor of the slope stability increased from 0.9 to 1.31.

The original angle of the slope (before sliding) was 27° After the sliding, the clay was remolded.

Before the rehabilitation works of the slope, an angle of the slope of 18° was realized. These works were realized in order to reduce the risk of additional slidings. The result of this work is presented in **Figure 18.**

Although the slope angle was reduced, the soil mass remained moving to the center of the pit. It was not possible to remove the soil because of the bad soil conditions.

In order to stop the movement of the sliding mass, a piled barrier (6 rows of piles) was installed (**Figure 19**).

Between the piled barrier and the original slope, vertical drains were installed for the evacuation of water during consolidation.

**173**

**Figure 20**.

**Figure 18.**

*Smoothening of the slope.*

**Figure 16.**

**Figure 17.**

*Position of the drain in plan view (red line).*

*Installation of the deep drainage.*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

Since the beginning of 2019, the rehabilitation of the slope has been started. These works are executed in successive phases in order to guarantee the stability of the site during the works. Each phase has a height of 3 to 4 m, as can been seen on

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

**Figure 16.** *Installation of the deep drainage.*

*Landslides - Investigation and Monitoring*

**Figure 15** [19] presents the calculated critical horizontal disequilibrium taking into account the presence of the groundwater and the piezometric site conditions. The red dots are points where the soil becomes plastic. The higher the density of the red dots, the more plastic the soil will behave, finally resulting in failure when there is no equilibrium of the soil stresses anymore. At the failure (the dark red line in

Out of these theoretical results, it became clear that the installation of drains

The PLAXIS-analysis resulted in a design where a deep drain has to be installed at a depth of 5 m., as shown in **Figures 16** and **17**. This drain was installed at the beginning of November 2015. The red line in **Figure 15** is the position of the drain in

After the installation of this drain, there was an immediate effect on the depth of the groundwater. The original depth of 1.5 m increased to a depth of about 4.5 m.

The original angle of the slope (before sliding) was 27° After the sliding, the clay

Before the rehabilitation works of the slope, an angle of the slope of 18° was realized. These works were realized in order to reduce the risk of additional slidings.

Although the slope angle was reduced, the soil mass remained moving to the center of the pit. It was not possible to remove the soil because of the bad soil

In order to stop the movement of the sliding mass, a piled barrier (6 rows of

Between the piled barrier and the original slope, vertical drains were installed

**Figure 15**) zone, one can clearly detect the position of the sliding plane.

*PLAXIS-analysis and measurement of the slope after sliding (as adapted from Peiffer (2016) [5]).*

• by installing deep drains in order to reduce the water stresses

The safety factor of the slope stability increased from 0.9 to 1.31.

would play an important role in rehabilitation.

The slope was stabilized using three techniques:

**9. Remediation**

**Figure 15.**

the plan view.

was remolded.

conditions.

• by smoothening the slope

• by the construction of a piled barrier

The result of this work is presented in **Figure 18.**

for the evacuation of water during consolidation.

piles) was installed (**Figure 19**).

**172**

**Figure 17.** *Position of the drain in plan view (red line).*

**Figure 18.** *Smoothening of the slope.*

Since the beginning of 2019, the rehabilitation of the slope has been started. These works are executed in successive phases in order to guarantee the stability of the site during the works. Each phase has a height of 3 to 4 m, as can been seen on **Figure 20**.

**Figure 19.** *Piled barrier.*

**Figure 20.** *New profile of the slope.*

For each phase first a dike is constructed with well bearing sand. Between the dike and the actual slope, a backfill is made with sandy soil.

Today, about 50% of the total work has been completed.

## **10. Conclusions**

In this chapter, DMT is applied as a tool for the monitoring of the stress state in a slope before and after the sliding.

From an analysis of the sliding observed in the overconsolidated Boom Clay, it can be concluded that it is possible to measure the effect of sliding on soil conditions and to evaluate the stability of the soil. In addition, it is possible to detect the position of a slip surface and the zone, which is influenced by the sliding.

For both applications, the results of a CPT-test in this type of soil are, as could be expected, less sensitive to changes in a stress state.

**175**

**Author details**

Herman Peiffer

Ghent University, Zwijnaarde, Belgium

provided the original work is properly cited.

\*Address all correspondence to: herman.peiffer@ugent.be

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

This research was possible with the funding by Alpha-Engineering.

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

**Acknowledgements**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

## **Acknowledgements**

*Landslides - Investigation and Monitoring*

**174**

**Figure 20.**

**Figure 19.** *Piled barrier.*

*New profile of the slope.*

**10. Conclusions**

slope before and after the sliding.

expected, less sensitive to changes in a stress state.

For each phase first a dike is constructed with well bearing sand. Between the

In this chapter, DMT is applied as a tool for the monitoring of the stress state in a

For both applications, the results of a CPT-test in this type of soil are, as could be

From an analysis of the sliding observed in the overconsolidated Boom Clay, it can be concluded that it is possible to measure the effect of sliding on soil conditions and to evaluate the stability of the soil. In addition, it is possible to detect the

position of a slip surface and the zone, which is influenced by the sliding.

dike and the actual slope, a backfill is made with sandy soil. Today, about 50% of the total work has been completed. This research was possible with the funding by Alpha-Engineering.

## **Author details**

Herman Peiffer Ghent University, Zwijnaarde, Belgium

\*Address all correspondence to: herman.peiffer@ugent.be

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Bozzanoa, F., Bretschneider, A., Martinoa, S., Prestininzia,A. Time variations of the K0 coefficient in overconsolidated clays due to morphological evolution of slopes. Engineering Geology,Vol. 169; 2014; p.69-79.

[2] Chang, M.F. Interpretation of overconsolidation ratio from in situ tests in recent clay deposits in Singapore and Malaysia. Canadian Geotechnical Journal, Vol. 28 n°; 1991. p. 210-225

[3] Totani, G., Calabrese, M., Marchetti, S., Monaco, P. Use of the in-situ flat dilatometer (DMT) for ground characterization in the stability of slopes. XV ICSMFE; Hamburg; Vol. 1; 1997. p. 607-610.

[4] Marchetti, S., Monaco, P., Totani, G., Calabrese, M. The Flat Dilatometer Test (DMT) in soil investigations. A Report by the ISSMGE Committee TC16, International Conference On In Situ Measurement of Soil Properties; Bali (Indonesia); 2001.

[5] Peiffer, H. A comparative analysis of the stability of a slope in an overconsolidated clay pit based on CPT and DMT measurements, Fifth International Conference on Geotechnical and Geophysical Site Characterisation, Australian Geomechanics Society, Sydney, Vol.2; Queensland, Australia; 2016; p. 1359-1364.

[6] Doran, I.G., Sivakumar, V., Graham, J, Johnson, A. Estimation of in situ stresses using anisotropic measurements and suction measurements. Geotechnique, 50 (2); 2000; p.189-196.

[7] Sivakumar, V., Doran, I.G., Graham, J., Navaneethan, T.Relationship between K0and overconsolidation ratio: a theoretical approach. Geotechnique, 52 (3); 2001; p.225-230.

[8] Sivakumar, V., Navaneethan, T., Hughes, D., Gallagher, G. An assessment of the earth pressure coefficient in overconsolidated clays. Geotechnique, 59 (10); 2009; p. 825-838.

[9] Silvestri, V. Long-term stability of a cutting slope in an overconsolidated sensitive clay. Canadian Geotechnical Journal, Vol.17, N3; 1980; p. 337-351.

[10] Grefsheim, F.D. Laboratory testing for slope stability design parameters in overconsolidated clay. Proc 5th International Symposium on Landslides, Lausanne (Publ. A.A. Balkema); 1988; p. 169-174.

[11] Chandler, R.J. The longterm stability of cutting slopes. Geotechnique, 24; 1974; p.21-38.

[12] Dehandschutter, B. ; Vandycke, S., Sintubin, M.,Vandenberghe, N. Wouters, L. Brittle fractures and ductile shear bands in argillaceous sediments: inferences from Oligocene Boom Clay (Belgium). Journal of Structural Geology, Vol. 27; 2005; p. 1095-1112.

[13] D'Elia, B. Esu,F. Marchetti,S. and Totani,G. In situ DMT testing and observations in landslide areas in overconsolidated clays. In:Proc. 7th ISL; 17-21 June 1996; Trondheim, Norway. p. 1607-1612.

[14] Peiffer, H. The use of a DMT to monitor the stability of a clay exploitation pit in the Boom Clay in Belgium. 3rd International Conference on the flat Dilatometer; Rome, Italy; 2015; p. 127-133.

[15] Marchetti, S. In Situ Tests by Flat Dilatometer. J. Geotech.Eng. Div., Vol. 106, No. 3; 1980; pp. 299-321.

[16] Kamei, T., Iwasaki, K. Evaluation of undrained shear strength of cohesive soils using a flat dilatometer. Soils and Foundations 35 (2); 1995; p. 111-116.

**177**

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay*

*DOI: http://dx.doi.org/10.5772/intechopen.93859*

Calibration of dilatometer correlations",

Preconsolidation Stress and Penetration Pore Pressure from DMT Contact Pressures. Geotech. Test. J., Vol. 10,

[19] Peiffer, H. Studierapport stabiliteit kleigroeve te Kruibeke. Project Report;

[17] Lacasse, S. and Lunne, T.

[18] Mayne, P. Determining

No. 3; 1987; pp. 146-150.

Antwerpen; Belgium; 2015.

Proc. Isopt-1 1; 1988; p. 348-359.

*Techniques to Evaluate and Remediate the Slope Stability in Overconsolidated Clay DOI: http://dx.doi.org/10.5772/intechopen.93859*

[17] Lacasse, S. and Lunne, T. Calibration of dilatometer correlations", Proc. Isopt-1 1; 1988; p. 348-359.

[18] Mayne, P. Determining Preconsolidation Stress and Penetration Pore Pressure from DMT Contact Pressures. Geotech. Test. J., Vol. 10, No. 3; 1987; pp. 146-150.

[19] Peiffer, H. Studierapport stabiliteit kleigroeve te Kruibeke. Project Report; Antwerpen; Belgium; 2015.

**176**

*Landslides - Investigation and Monitoring*

[1] Bozzanoa, F., Bretschneider, A., Martinoa, S., Prestininzia,A. Time variations of the K0 coefficient in overconsolidated clays due to morphological evolution of slopes. Engineering Geology,Vol. 169; 2014; [8] Sivakumar, V., Navaneethan, T., Hughes, D., Gallagher, G. An assessment of the earth pressure coefficient in overconsolidated clays. Geotechnique,

[9] Silvestri, V. Long-term stability of a cutting slope in an overconsolidated sensitive clay. Canadian Geotechnical Journal, Vol.17, N3; 1980; p. 337-351.

[10] Grefsheim, F.D. Laboratory testing for slope stability design parameters in overconsolidated clay. Proc 5th International Symposium on Landslides, Lausanne (Publ. A.A. Balkema); 1988;

59 (10); 2009; p. 825-838.

[11] Chandler, R.J. The longterm stability of cutting slopes. Geotechnique, 24; 1974; p.21-38.

[12] Dehandschutter, B. ; Vandycke, S., Sintubin, M.,Vandenberghe, N. Wouters, L. Brittle fractures and ductile shear bands in argillaceous sediments: inferences from Oligocene Boom Clay (Belgium). Journal of Structural Geology, Vol. 27; 2005; p. 1095-1112.

[13] D'Elia, B. Esu,F. Marchetti,S. and Totani,G. In situ DMT testing and observations in landslide areas in overconsolidated clays. In:Proc. 7th ISL; 17-21 June 1996; Trondheim, Norway.

[14] Peiffer, H. The use of a DMT to monitor the stability of a clay exploitation pit in the Boom Clay in Belgium. 3rd International Conference on the flat Dilatometer; Rome, Italy;

[15] Marchetti, S. In Situ Tests by Flat Dilatometer. J. Geotech.Eng. Div., Vol. 106, No. 3; 1980; pp. 299-321.

[16] Kamei, T., Iwasaki, K. Evaluation of undrained shear strength of cohesive soils using a flat dilatometer. Soils and Foundations 35 (2); 1995; p. 111-116.

p. 169-174.

p. 1607-1612.

2015; p. 127-133.

[2] Chang, M.F. Interpretation of overconsolidation ratio from in situ tests in recent clay deposits in Singapore and Malaysia. Canadian Geotechnical Journal, Vol. 28 n°; 1991. p. 210-225

S., Monaco, P. Use of the in-situ flat dilatometer (DMT) for ground characterization in the stability of slopes. XV ICSMFE; Hamburg; Vol. 1;

1997. p. 607-610.

(Indonesia); 2001.

[3] Totani, G., Calabrese, M., Marchetti,

[4] Marchetti, S., Monaco, P., Totani, G., Calabrese, M. The Flat Dilatometer Test (DMT) in soil investigations. A Report by the ISSMGE Committee TC16, International Conference On In Situ Measurement of Soil Properties; Bali

[5] Peiffer, H. A comparative analysis of the stability of a slope in an overconsolidated clay pit based on CPT and DMT measurements, Fifth International Conference on Geotechnical and Geophysical Site Characterisation, Australian Geomechanics Society, Sydney, Vol.2;

Queensland, Australia; 2016;

and suction measurements.

[6] Doran, I.G., Sivakumar, V., Graham, J, Johnson, A. Estimation of in situ stresses using anisotropic measurements

Geotechnique, 50 (2); 2000; p.189-196.

[7] Sivakumar, V., Doran, I.G., Graham, J., Navaneethan, T.Relationship between

K0and overconsolidation ratio: a theoretical approach. Geotechnique, 52

(3); 2001; p.225-230.

p. 1359-1364.

p.69-79.

**References**

**Chapter 9**

**Abstract**

altogether.

**1. Introduction**

serious issue in Taiwan.

**179**

*and Meng-Hsun Hsieh*

Landslide Potential Evaluation

The geological environment of Taiwan mainly contains steep topography and geologically fragile ground surface. Therefore, the vulnerable environmental conditions are prone to landslides during torrential rainfalls and typhoons. The rainfallinduced shallow landslide has become more common in Taiwan due to the extreme weathers in recent years. To evaluate the potential of landslide and its impacts, an evaluation method using the historical rainfall data (the hazard factor) and the temporal characteristics of landslide fragility curve (LFC, the vulnerability factor) was developed and described in this chapter. The LFC model was based on the geomorphological and vegetation factors using landslides at the Chen-Yu-Lan watershed in Taiwan, during events of Typhoon Sinlaku (September 2009) and Typhoon Morakot (August 2009). The critical hazard potential (*Hc*) and critical fragility potential (*Fc*) were introduced to express the probability of exceeding a damage state of landslides under certain conditions of rainfall intensity and accumulated rainfall. Case studies at Shenmu village in Taiwan were applied to illustrate the proposed method of landslide potential assessment and the landslide warning in practice. Finally, the proposed risk assessment for landslides can be implemented in the disaster response system and be extended to take debris flows into consideration

**Keywords:** landslide, fragility, landslide potential, probabilistic model

Taiwan is on the path of western Pacific typhoon path and on the circum-Pacific earthquake belt, indicating that Taiwan suffered from two or more natural disasters, which was the highest in the world [1]. Besides, most of the land in Taiwan, about 70% of total area, is hillside. Given the conditions of increasing impacts of climate change and extreme weathers, the rainfall-induced landslide has become a

Most landslide researches used the landslide susceptibility analysis (LSA) to develop landslide evaluation model [2]. The LSA models basically use factors and observed data to construct the description of landslides. The factors include rainfall intensity, accumulated rainfall, slope degree, vegetation, etc. The common models developed for landslide hazard or landslide evaluation are usually deterministic analysis, including the traditional slope stability analysis [2]. Recently, a novel concept of applying probability to landslide evaluation had been proposed.

Using Fragility Curve Model

*Yi-Min Huang,Tsu-Chiang Lei, Bing-Jean Lee*

## **Chapter 9**
