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Landslide phenomena in the area of Pomarico (Basilicata–Italy): methods for modelling and monitoring
Physics and Chemistry of the Earth 27 (2002) 1601–1607 www.elsevier.com/locate/pce
Landslide phenomena in the area of Pomarico (Basilicata–Italy): methods for modelling and monitoring F. Bozzano a, C. Cherubini
b,*
, M. Floris c, M. Lupo d, F. Paccapelo
e
a
b
Department of Earth Sciences, ‘‘La Sapienza’’ University, P.le A. Moro, 00185 Roma, Italy Department of Civil and Environmental Engineering, Technical University of Bari, Via Orabona 4, 70125 Bari, Italy c Engineering Geology Institute, University of Urbino, Localit a Crocicchio, 61029 Urbino, Italy d V.le Keenedy, Pomarico, Italy e Via M. Galiani 7/c, 70125 Bari, Italy Accepted 23 July 2002
Abstract This paper takes into consideration landslide phenomena in the clayey slopes facing the built-up area of Pomarico which is situated in the southern part of the ‘‘Fossa Bradanica’’, in Basilicata (Italy). Based on the great number of geologic, geomorphologic and historic informations a geotechnical model of the slope was built. Particular attention has been paid to define the geotechnical parameters of the soil and which mechanical models are to be used. The studies point out a correlation between the water level in the detritus cover and the stability condition of the slope showing that phenomena at first located at the foot of the slope spread quickly towards its summit as the piezometric height increases. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fossa Bradanica; Grey–blue clays; Cam-Clay; Debris mantle; Finite element analysis
1. Introduction
2. Main geological features
A geological and geomechanical analysis of the La Salsa landslide affecting the small village of Pomarico (Basilicata, Southern Italy) is exposed. The area, named Fossa Bradanica is well known as landslide and intense erosion prone area. Those phenomena involve mainly the Subapennine Clay Formation, also known as grey– blue clays. It consists of coarsening upward deposits from silty clays to silty and clayey sands; their physical and mechanical properties are variable but sufficiently characterised by many Authors (Cherubini et al., 1985; Genevois et al., 1984; Guerricchio and Melidoro, 1979; Valentini et al., 1979). Based on the results of the exposed analyses, a scheme of GPS monitoring system is proposed.
Pomarico village is located in the southernmost section of the Padano-Adriatic Foretrough (Fossa Bradanica). Since middle Pliocene this foretrough has represented a structural depression between the Apennine Mts. and the Apulian Foreland. From Lower Pleistocene this area is subjected to a general tectonic uplift. The most recent deposits that filled up this basin are ascribed to the Bradanic sedimentary cycle (LowerMiddle Pleistocene); they mainly consist of the grey– blue clays (Balduzzi et al., 1982) and coarse grained deposits (Monte Marano Sand and Irsina Conglomerate), in some sites overlaid by marine terraced deposits. Pomarico hill is characterised by the outcropping of Subapennine Clay Formation overlaid by yellow sand ascribed to Monte Marano Sand and locally by sandy marine terraced deposits. Clayey deposits are composed of stiff and jointed marine marl silty clay (lower Pleistocene) interbedded with silty and sandy levels whose frequency and thickness increases upward and with rare volcanic fall levels up to 1 m thick. Strata dip toward NE with a slope angle between 10 and 20; maximum
* Corresponding author. Tel.: +39-805-963-363; fax: +39-805-963675. E-mail address: [email protected] (C. Cherubini).
1474-7065/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 4 - 7 0 6 5 ( 0 2 ) 0 0 1 7 3 - 0
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F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
outcropping thickness is about 400 m. Faults with throw up to 10 m are present. Yellow sands consist of scarcely cemented sands interbedded with silty clay and arenaceous horizons up to 1 m thick. Maximum outcropping thickness is 50 m. Slopes surrounding Pomarico hill are characterised by the large outcropping of a debris mantle originated by erosion and landslides involving both formations. Debris bodies consist of yellowish sandy and clayey deposits with pebbles and conglomeratic blocks. Large volumes blocks constituted by well-stratified deposits dipping into the slope are present. They represent landslide bodies originated by rotational landslide frequently affecting the slopes. Field survey, soils macroscopic analysis and technical documentation consultation allowed us to recognise two types of debris deposits based on the grain size composition: sandy or clayey predominantly. As shown in
Fig. 1 sandy debris characterises the upper part of the slopes while the clayey one outcrops in the medium part of slopes and in topographic depression. These debris bodies are characterised by a varying thickness (Fig. 1), and at present, because of direct rainwater infiltration and multistrata groundwater flow, they are involved in slope movements. Pomarico hill geomorphologic features have been dealt with by many authors (Guerricchio and Valentini, 1979; Genevois et al., 1984; Cherubini et al., 1985; Lazzari, 1989). Most of the landslide phenomena affecting the slopes surrounding the hill are composite, retrogressive, translational––rotational slide according to Cruden and Varnes classification (1996). Landslide named ‘‘La Salsa’’, located in the NE slope of the Pomarico historical centre, is a representative example of that phenomena. It involves mainly debris up to 15 m thick.
Fig. 1. Geologic and geomorphologic map of the ‘‘La Salsa’’ slope.
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
3. Configuration of the model The entire body of the landslide extends for approximately 800 m longitudinally and is characterised by an average width of around 150 m, thus reaching an extension equal to approximately 12 ha, a value that makes the ‘‘La Salsa’’ landslide the most imposing of those present in Pomarico. The maximum difference in level found is approximately equal to 160 m. A stability analysis has been carried out using the finite element method and referring to two calculation codes, SIGMA/W (1998) and SEEP/W (1998) (Geoslope International Ltd., 1998). The first one has been used to carry out the tensional and deformation analysis and the second one to analyse the circulation of water within the slope. The model used to carry out the analysis has been built referring to the section A–C (Fig. 2), elaborated using data relative to the boreholes S4, S9, S10 and S19. Proceeding from the base upwards, there are PlioPleistocenic clay deposits, above which there are Monte Marano Sands, in the higher part of the slope, and sandy-clayey detritus in the lower zones. The depth at which the passage between the strata of clay and detritus occurs varies from 6 to 20 m. The thickness of Monte Marano Sands is included between 12 and 18 m, plus approximately 10 m relative to the transition terms that mark the passage from sands to clays. The model has been built using a grill with a square mesh having sides of 6 m. This choice derives from a
1603
compromise between accuracy of the analysis and calculation time. The boundary conditions have been defined as follows: • To the nodes belonging to the vertical surfaces which close the section one simple tie has been applied, consenting movement in the only vertical direction; • To those nodes belonging to the horizontal surface which marks the lower boundary of the section delimit the base of the profile, a double tie was applied, impeding movement in all directions.
4. Physical and mechanical properties of the soils 4.1. Blue clays The physical-mechanical characteristics of this formation were deduced by referring to Valentini, 1979; Genevois et al., 1984; Cherubini et al., 1989; Cherubini and Lupo, 1998 and the values adopted in the construction of the model are summarised in Table 1. Starting from the available data it is necessary to determine the parameters required by the software to identify the behaviour of the soils in the context of the constitutive model chosen, the modified Cam-Clay. This constitutive model describes the behaviour of a soil by means of the following parameters: M, k, k, C. The parameter M is equivalent to the friction angle / and its value is obtained utilising the following equation:
Fig. 2. Section used to build up the finite element model.
Table 1 Physical and mechanical characteristics of the Blue Clays E (kPa)
m
LL (%)
PL (%)
cd (kN/m3 )
cn (kN/m3 )
/
c (kPa)
OCR
6.81Eþ08
0.25
47
21
16
20
24
29.4
5
1604
M¼
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
6 sin /0c ¼ 0:94 3 sin /0c
Table 3 Characteristic parameters assigned to sandy-clayey detritus
k is in strict correlation with the compression index cc , which can be determined using the well known Terzaghi and Peck (1967) equation:
E (kPa)
m
M
cs (kN/m3 )
k
k
C
/
c (kPa)
1.86Eþ05
0.2
0.92
19
0.12
0.02
2.33
26
2
cc ¼ 0:009ðLL 10Þ ¼ 0:33 The value of k can therefore be determined as: cc ¼ 0:14 k¼ 2:3 The value of the parameter k is determined by considering that for plastic clays the k=k ratio assumes values between 0.2 and 0.5 (Atkinson, 1993). Adopting a value equal to 0.2 gives k ¼ 0:03. Finally, C is evaluated by referring a study of Schofield and Wroth (Atkinson, 1993), in which it is demonstrated that the critical state lines of many soils pass through the same point in the compressibility plane, denoted X with co-ordinates: vX ffi 1:25; pX0 ¼ 10 MPa. Introducing these values into the equation v ¼ C k ln p0 gives: C ¼ 1:25 þ k ln 10; 000 ¼ 2:58
Table 4 Characteristic parameters assigned to Monte Marano Sands E (kPa)
m
cs (kN/m3 )
/
c (kPa)
4.00Eþ7
0.3
19
32
0.1
of pressure to which they are subjected is below the breaking point of these materials. On the basis of these considerations the constitutive model adopted to describe the behaviour of sands is the linear elastic one. The parameters utilised were deduced from a study conducted by Cherubini and Walsh (1982) and are reported in Table 4. 5. Stability analysis
The characteristic parameters assigned to the Sub Apennine Clays are reported schematically in Table 2. 4.2. Sandy-clayey detritus As for the clays, the behaviour of this formation was also identified using a modified Cam-Clay model. Thus the determination of the characteristic parameters was carried out using analogous methodology to that described above. In order to characterise this formation in a homogeneous way, the characteristic parameters are assigned in such a way as to take into consideration the fact that the variability of detritus can be even greater than that of the Blue Clays (Table 3). 4.3. Monte Marano sands The soils that make up this formation are mainly sandy, and as such their mechanical behaviour can no longer be described using a constitutive model such as the modified Cam-Clay, which was elaborated to describe the behaviour of a prevalently clay material (Atkinson, 1993). Considering that the Monte Marano Sands are outcropping on the slope analysed and with a thickness that does not exceed 18 m, it can be supposed that the state
The stability analysis, conducted with the aid of the finite elements method, was carried out considering different flux conditions within the slope, in order to identify the limit situations for the stability, in terms of tensional and deformation aspects. When the flux is absent there are no yield zones and significative displacements and the slope is in a stable situation. So the analysis have been carried out considering different water levels, in order to define the influence of pore water pressure on the slope stability. There are five wells and several piezometers on the ‘‘La Salsa’’ slope (Fig. 1), thanks to which it has been possible to reconstruct the trend of the water level in recent years. From the analysis of the data reported in Table 5 you can note that, in the period from November 1997 to October 1998, the detritus cover was constantly effected by circulation of water. On the basis of these considerations four different flux conditions have been analysed, always keeping the circulation of water within the detritus cover and varying the value of the hydraulic load along the left vertical boundary surface. Proceeding in this way four different water levels have been obtained, as shown in Fig. 3. The flux models obtained in this way have been analysed, as previously specified, by the SEEP/W finite
Table 2 Characteristic parameters assigned to Blue Clays E (kPa)
m
M
cs (kN/m3 )
k
k
C
/
c (kPa)
OCR
6.81Eþ05
0.25
0.94
20
0.14
0.03
2.58
24
29.4
5
0.9 3.8 8.1 4.7 4.4 1.2 4.2 9.0 5.0 5.6 1.2 3.5 8.4 5.4 3.4 0.0 closed 9.0 5.2 3.2 2.4 3.5 7.7 5.5 3.6 0.5 2.6 6.9 4.7 3.0 0.5 2.2 7.0 4.5 2.7 0.5 1.7 7.9 4.2 2.2 0.5 1.4 7.2 4.4 2.2
0.0 1.6 7.1 4.3 2.3
February January
0.9 2.2 7.3 4.7 2.7 4 6 13 5 6 384 378 362 358 348
0.6 3.4 7.3 4.3 3.2
December November
Depth (m)
Water level depth (m)
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elements calculation code, which allows the system to reach the flux without taking into account the mechanical response. Once the flux conditions have been defined for each of the models considered, the final values of pore water pressure are reported, as initial conditions, in the analysis of the model carried out using the SIGMA/ W calculation code. Considering the flux condition represented by the water level which is the nearest to the top of blue clay, yield zones are localised only at the foot of the slope. So there are localised phenomena that, as confirmed by the small size of the displacements (maximum 3 cm), coexist in a situation of substantial stability (Fig. 4). Considering higher phreatic levels you can note a progressive increase in instability phenomena until the last flux condition is reached, which is represented by a water level which in some points coincides with the countryside level (Figs. 3 and 5). In particular it is important to notice that, in the last flux model analysed, the water circulation involves Monte Marano Sands too. This situation has a great influence on the behaviour of the slope, leading to new instability mechanisms: all the detritus cover is in yielding condition and the displacements, which maximum value is 10 cm, concern the full extension of the slope. Analysing the obtained results you can observe that phenomena at first located at the foot of the slope spread quickly towards its summit as the water level increases. This confirms the hypothesis, supported by experimental data, of a strikingly retrogressive landslide mechanism. Finally, the first analysed flux conditions represents a limit situation for the slope stability, so the water level and the displacements relative to this flux model may be assumed to be threshold values.
6. Monitoring
Height on the s.l. (m)
The stability analysis has made it possible to define very well the behaviour of the slope and the most important elements which influence the slope stability. But it is important to notice that the reported results have been obtained analysing a model of the slope. So there are some uncertainties which are difficult to be defined and quantified. For this reason, a complete and exhaustive study of a slope has to take into account the monitoring phase, which is very important to check the results obtained by the modelling and to fit the model. In particular a monitoring system for a landslide has to control the following quantities: • Pore water pressure and water level; • Superficial and deep displacements.
9 10 11 12 14
Well number
Table 5 Static water levels in the wells in the period November 1997–October 1998
March
April
May
June
July
August
September
October
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
As reported before, on the analysed slope there are five wells and several piezometers, thanks to which the
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F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120
392 386 380 374
0
100
200
300
400
500
600
700
800
900
1000
Fig. 3. Table waters obtained for the four flux conditions analysed.
height
Monte Marano Sands 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 –0.1
Sandy-clayey detritus and transition sand/clay Blue clays
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
di stanc e (x 1000)
Fig. 4. Displacement situation obtained considering the first flux condition.
height
Monte Marano Sands 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 –0.1
Sandy-clayey detritus and transition sand/clay Blue clays
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
distance (x 1000)
Fig. 5. Displacement situation obtained considering the fourth flux condition.
modalities of the circulation of water within the slope have been reconstructed. With regard to displacements, a monitoring system based on GPS technique has been planned. This technique makes it possible to reach the same precision of the classical techniques with shorter surveys. (Paccapelo et al., 1999). For monitoring displacements on the analysed slope a network consisting of 11 points located on the landslide zone and two control points located in the Pomarico built-up area, has been planned. The adopted network scheme is formed by 34 baselines, leading to a factor of redundancy equal to 2.8, a value which makes it possible to reach a great precision.
7. Conclusions The analysis carried out has made it possible to define the phenomenon and study it on the basis of greater geological and geomechanical data leading to the following conclusions: (a) The phenomenon effects a soil thickness that never exceeds 20 m, therefore consisting mainly of sandy-clayey detritus; (b) The circulation of water plays a fundamental role. In fact, considering the filtration parallel to the slope and the water level close to the surface, the yield zones involve all the detritus cover.
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
(c) Correspondingly, displacements have been calculated, making it possible to plan a monitoring system based on GPS technique and piezometers, able to verify the behaviour of the slope and to compare it with the build up model. References Atkinson, J., 1993. An Introduction to the Mechanics of Soils and Foundations. McGraw-Hill International. Balduzzi, A., Casnedi, R., Crescenti, U., Mostardini, F., Tonna, M., 1982. Il Plio-Pleistocene del sottosuolo del bacino lucano (avanfossa appenninica). Geologica Romana 21, 89–111. Cherubini, C., Genevois, R., Guadagno, F.M., Prestininzi, A., Valentini, G., 1985. Sulle correlazioni geotecniche spaziali, lÕerosione e la stabilit a dei pendii dei depositi argillosi pleistocenici della fossa bradanica. Geol. Appl. e Idrogeol. 22, 671–690. Cherubini, C., Giasi, C.I., Guadagno, F.M., 1989. Il coefficiente di spinta a riposo delle argille azzurre subappennine di Matera. Rivista Italiana di Geotecnica, 186–192. Cherubini, C., Lupo, M., 1998. Physico-mechanical properties of Matera Blue Clays and correlations among them. Eighth International IAEG Congress. Balkema, Rotterdam, pp. 283–288. Cherubini, C., Walsh, N., 1982. Caratteristiche geolitologiche e geotecniche delle sabbie di Monte Marano (dintorni di Gravina). Geol. Appl. e Idrogeol. 17, 202–216.
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Cruden, D.M., Varnes, D.J., 1996. Landslide type and processes. In: Keith Turner, A., Schuster, R.L. (Eds.), Landslide Investigation and Mitigation. Transportation Research Board, Special Report 247, pp. 36–75. Genevois, R., Prestininzi, A., Valentini, G., 1984. Caratteristiche e correlazioni geotecniche dei depositi argillosi bradanici affioranti a NE della fossa. Geol. Appl. e Idrogeol. 19, 173–212. Guerricchio, A., Melidoro, G., 1979. Fenomeni franosi e neotettonici nelle argille grigio-azzurre calabriane di Pisticci (Lucania) con saggio di cartografia. Geol. Appl. e Idrogeol. 14 (1), 105–138. Guerricchio, A., Valentini, G., 1979. Un modello matematico per la valutazione dellÕerosione tratto dallÕesame dei pendii calanchivi nelle argille azzurre lucane. Geol. Appl. e Idrogeol. 10 (1), 241–275. Lazzari, S., 1989. Strutture profonde passive drenanti a protezione di alcuni abitati della Basilicata. Proceedings of 17 Convegno Nazionale di Geotecnica, Taormina, vol. I, pp. 154–161. Paccapelo, F., Rutigliano, P., Vespe, F., 1999. Controllo di movimenti franosi mediante lÕutilizzo della tecnica GPS. Atti della 3^ Conferenza Nazionale ASITA, Napoli, vol. II, pp. 1133–1137. SEEP/W, 1998. UserÕs guide. Geo-slope International Ltd. SIGMA/W, 1998. UserÕs guide. Geo-slope International Ltd. Terzaghi, K., Peck, R.B., 1967. Soil Mechanics in Engineering Practice. John Wiley and Sons, NY. Valentini, G., 1979. I fenomeni di erosione e di frana nei depositi argillosi della fossa bradanica. Geol. Appl. e Idrogeol. 14, 126–141. Valentini, G., Cherubini, C., Guadagno, F.M., 1979. Caratteristiche geotecniche dei sedimenti argillosi pleistocenici tra Pisticci ed il mare. Geol. Appl. e Idrogeol. 14 (3), 569–611.
Landslide phenomena in the area of Pomarico (Basilicata–Italy): methods for modelling and monitoring F. Bozzano a, C. Cherubini
b,*
, M. Floris c, M. Lupo d, F. Paccapelo
e
a
b
Department of Earth Sciences, ‘‘La Sapienza’’ University, P.le A. Moro, 00185 Roma, Italy Department of Civil and Environmental Engineering, Technical University of Bari, Via Orabona 4, 70125 Bari, Italy c Engineering Geology Institute, University of Urbino, Localit a Crocicchio, 61029 Urbino, Italy d V.le Keenedy, Pomarico, Italy e Via M. Galiani 7/c, 70125 Bari, Italy Accepted 23 July 2002
Abstract This paper takes into consideration landslide phenomena in the clayey slopes facing the built-up area of Pomarico which is situated in the southern part of the ‘‘Fossa Bradanica’’, in Basilicata (Italy). Based on the great number of geologic, geomorphologic and historic informations a geotechnical model of the slope was built. Particular attention has been paid to define the geotechnical parameters of the soil and which mechanical models are to be used. The studies point out a correlation between the water level in the detritus cover and the stability condition of the slope showing that phenomena at first located at the foot of the slope spread quickly towards its summit as the piezometric height increases. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fossa Bradanica; Grey–blue clays; Cam-Clay; Debris mantle; Finite element analysis
1. Introduction
2. Main geological features
A geological and geomechanical analysis of the La Salsa landslide affecting the small village of Pomarico (Basilicata, Southern Italy) is exposed. The area, named Fossa Bradanica is well known as landslide and intense erosion prone area. Those phenomena involve mainly the Subapennine Clay Formation, also known as grey– blue clays. It consists of coarsening upward deposits from silty clays to silty and clayey sands; their physical and mechanical properties are variable but sufficiently characterised by many Authors (Cherubini et al., 1985; Genevois et al., 1984; Guerricchio and Melidoro, 1979; Valentini et al., 1979). Based on the results of the exposed analyses, a scheme of GPS monitoring system is proposed.
Pomarico village is located in the southernmost section of the Padano-Adriatic Foretrough (Fossa Bradanica). Since middle Pliocene this foretrough has represented a structural depression between the Apennine Mts. and the Apulian Foreland. From Lower Pleistocene this area is subjected to a general tectonic uplift. The most recent deposits that filled up this basin are ascribed to the Bradanic sedimentary cycle (LowerMiddle Pleistocene); they mainly consist of the grey– blue clays (Balduzzi et al., 1982) and coarse grained deposits (Monte Marano Sand and Irsina Conglomerate), in some sites overlaid by marine terraced deposits. Pomarico hill is characterised by the outcropping of Subapennine Clay Formation overlaid by yellow sand ascribed to Monte Marano Sand and locally by sandy marine terraced deposits. Clayey deposits are composed of stiff and jointed marine marl silty clay (lower Pleistocene) interbedded with silty and sandy levels whose frequency and thickness increases upward and with rare volcanic fall levels up to 1 m thick. Strata dip toward NE with a slope angle between 10 and 20; maximum
* Corresponding author. Tel.: +39-805-963-363; fax: +39-805-963675. E-mail address: [email protected] (C. Cherubini).
1474-7065/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 4 - 7 0 6 5 ( 0 2 ) 0 0 1 7 3 - 0
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F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
outcropping thickness is about 400 m. Faults with throw up to 10 m are present. Yellow sands consist of scarcely cemented sands interbedded with silty clay and arenaceous horizons up to 1 m thick. Maximum outcropping thickness is 50 m. Slopes surrounding Pomarico hill are characterised by the large outcropping of a debris mantle originated by erosion and landslides involving both formations. Debris bodies consist of yellowish sandy and clayey deposits with pebbles and conglomeratic blocks. Large volumes blocks constituted by well-stratified deposits dipping into the slope are present. They represent landslide bodies originated by rotational landslide frequently affecting the slopes. Field survey, soils macroscopic analysis and technical documentation consultation allowed us to recognise two types of debris deposits based on the grain size composition: sandy or clayey predominantly. As shown in
Fig. 1 sandy debris characterises the upper part of the slopes while the clayey one outcrops in the medium part of slopes and in topographic depression. These debris bodies are characterised by a varying thickness (Fig. 1), and at present, because of direct rainwater infiltration and multistrata groundwater flow, they are involved in slope movements. Pomarico hill geomorphologic features have been dealt with by many authors (Guerricchio and Valentini, 1979; Genevois et al., 1984; Cherubini et al., 1985; Lazzari, 1989). Most of the landslide phenomena affecting the slopes surrounding the hill are composite, retrogressive, translational––rotational slide according to Cruden and Varnes classification (1996). Landslide named ‘‘La Salsa’’, located in the NE slope of the Pomarico historical centre, is a representative example of that phenomena. It involves mainly debris up to 15 m thick.
Fig. 1. Geologic and geomorphologic map of the ‘‘La Salsa’’ slope.
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
3. Configuration of the model The entire body of the landslide extends for approximately 800 m longitudinally and is characterised by an average width of around 150 m, thus reaching an extension equal to approximately 12 ha, a value that makes the ‘‘La Salsa’’ landslide the most imposing of those present in Pomarico. The maximum difference in level found is approximately equal to 160 m. A stability analysis has been carried out using the finite element method and referring to two calculation codes, SIGMA/W (1998) and SEEP/W (1998) (Geoslope International Ltd., 1998). The first one has been used to carry out the tensional and deformation analysis and the second one to analyse the circulation of water within the slope. The model used to carry out the analysis has been built referring to the section A–C (Fig. 2), elaborated using data relative to the boreholes S4, S9, S10 and S19. Proceeding from the base upwards, there are PlioPleistocenic clay deposits, above which there are Monte Marano Sands, in the higher part of the slope, and sandy-clayey detritus in the lower zones. The depth at which the passage between the strata of clay and detritus occurs varies from 6 to 20 m. The thickness of Monte Marano Sands is included between 12 and 18 m, plus approximately 10 m relative to the transition terms that mark the passage from sands to clays. The model has been built using a grill with a square mesh having sides of 6 m. This choice derives from a
1603
compromise between accuracy of the analysis and calculation time. The boundary conditions have been defined as follows: • To the nodes belonging to the vertical surfaces which close the section one simple tie has been applied, consenting movement in the only vertical direction; • To those nodes belonging to the horizontal surface which marks the lower boundary of the section delimit the base of the profile, a double tie was applied, impeding movement in all directions.
4. Physical and mechanical properties of the soils 4.1. Blue clays The physical-mechanical characteristics of this formation were deduced by referring to Valentini, 1979; Genevois et al., 1984; Cherubini et al., 1989; Cherubini and Lupo, 1998 and the values adopted in the construction of the model are summarised in Table 1. Starting from the available data it is necessary to determine the parameters required by the software to identify the behaviour of the soils in the context of the constitutive model chosen, the modified Cam-Clay. This constitutive model describes the behaviour of a soil by means of the following parameters: M, k, k, C. The parameter M is equivalent to the friction angle / and its value is obtained utilising the following equation:
Fig. 2. Section used to build up the finite element model.
Table 1 Physical and mechanical characteristics of the Blue Clays E (kPa)
m
LL (%)
PL (%)
cd (kN/m3 )
cn (kN/m3 )
/
c (kPa)
OCR
6.81Eþ08
0.25
47
21
16
20
24
29.4
5
1604
M¼
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
6 sin /0c ¼ 0:94 3 sin /0c
Table 3 Characteristic parameters assigned to sandy-clayey detritus
k is in strict correlation with the compression index cc , which can be determined using the well known Terzaghi and Peck (1967) equation:
E (kPa)
m
M
cs (kN/m3 )
k
k
C
/
c (kPa)
1.86Eþ05
0.2
0.92
19
0.12
0.02
2.33
26
2
cc ¼ 0:009ðLL 10Þ ¼ 0:33 The value of k can therefore be determined as: cc ¼ 0:14 k¼ 2:3 The value of the parameter k is determined by considering that for plastic clays the k=k ratio assumes values between 0.2 and 0.5 (Atkinson, 1993). Adopting a value equal to 0.2 gives k ¼ 0:03. Finally, C is evaluated by referring a study of Schofield and Wroth (Atkinson, 1993), in which it is demonstrated that the critical state lines of many soils pass through the same point in the compressibility plane, denoted X with co-ordinates: vX ffi 1:25; pX0 ¼ 10 MPa. Introducing these values into the equation v ¼ C k ln p0 gives: C ¼ 1:25 þ k ln 10; 000 ¼ 2:58
Table 4 Characteristic parameters assigned to Monte Marano Sands E (kPa)
m
cs (kN/m3 )
/
c (kPa)
4.00Eþ7
0.3
19
32
0.1
of pressure to which they are subjected is below the breaking point of these materials. On the basis of these considerations the constitutive model adopted to describe the behaviour of sands is the linear elastic one. The parameters utilised were deduced from a study conducted by Cherubini and Walsh (1982) and are reported in Table 4. 5. Stability analysis
The characteristic parameters assigned to the Sub Apennine Clays are reported schematically in Table 2. 4.2. Sandy-clayey detritus As for the clays, the behaviour of this formation was also identified using a modified Cam-Clay model. Thus the determination of the characteristic parameters was carried out using analogous methodology to that described above. In order to characterise this formation in a homogeneous way, the characteristic parameters are assigned in such a way as to take into consideration the fact that the variability of detritus can be even greater than that of the Blue Clays (Table 3). 4.3. Monte Marano sands The soils that make up this formation are mainly sandy, and as such their mechanical behaviour can no longer be described using a constitutive model such as the modified Cam-Clay, which was elaborated to describe the behaviour of a prevalently clay material (Atkinson, 1993). Considering that the Monte Marano Sands are outcropping on the slope analysed and with a thickness that does not exceed 18 m, it can be supposed that the state
The stability analysis, conducted with the aid of the finite elements method, was carried out considering different flux conditions within the slope, in order to identify the limit situations for the stability, in terms of tensional and deformation aspects. When the flux is absent there are no yield zones and significative displacements and the slope is in a stable situation. So the analysis have been carried out considering different water levels, in order to define the influence of pore water pressure on the slope stability. There are five wells and several piezometers on the ‘‘La Salsa’’ slope (Fig. 1), thanks to which it has been possible to reconstruct the trend of the water level in recent years. From the analysis of the data reported in Table 5 you can note that, in the period from November 1997 to October 1998, the detritus cover was constantly effected by circulation of water. On the basis of these considerations four different flux conditions have been analysed, always keeping the circulation of water within the detritus cover and varying the value of the hydraulic load along the left vertical boundary surface. Proceeding in this way four different water levels have been obtained, as shown in Fig. 3. The flux models obtained in this way have been analysed, as previously specified, by the SEEP/W finite
Table 2 Characteristic parameters assigned to Blue Clays E (kPa)
m
M
cs (kN/m3 )
k
k
C
/
c (kPa)
OCR
6.81Eþ05
0.25
0.94
20
0.14
0.03
2.58
24
29.4
5
0.9 3.8 8.1 4.7 4.4 1.2 4.2 9.0 5.0 5.6 1.2 3.5 8.4 5.4 3.4 0.0 closed 9.0 5.2 3.2 2.4 3.5 7.7 5.5 3.6 0.5 2.6 6.9 4.7 3.0 0.5 2.2 7.0 4.5 2.7 0.5 1.7 7.9 4.2 2.2 0.5 1.4 7.2 4.4 2.2
0.0 1.6 7.1 4.3 2.3
February January
0.9 2.2 7.3 4.7 2.7 4 6 13 5 6 384 378 362 358 348
0.6 3.4 7.3 4.3 3.2
December November
Depth (m)
Water level depth (m)
1605
elements calculation code, which allows the system to reach the flux without taking into account the mechanical response. Once the flux conditions have been defined for each of the models considered, the final values of pore water pressure are reported, as initial conditions, in the analysis of the model carried out using the SIGMA/ W calculation code. Considering the flux condition represented by the water level which is the nearest to the top of blue clay, yield zones are localised only at the foot of the slope. So there are localised phenomena that, as confirmed by the small size of the displacements (maximum 3 cm), coexist in a situation of substantial stability (Fig. 4). Considering higher phreatic levels you can note a progressive increase in instability phenomena until the last flux condition is reached, which is represented by a water level which in some points coincides with the countryside level (Figs. 3 and 5). In particular it is important to notice that, in the last flux model analysed, the water circulation involves Monte Marano Sands too. This situation has a great influence on the behaviour of the slope, leading to new instability mechanisms: all the detritus cover is in yielding condition and the displacements, which maximum value is 10 cm, concern the full extension of the slope. Analysing the obtained results you can observe that phenomena at first located at the foot of the slope spread quickly towards its summit as the water level increases. This confirms the hypothesis, supported by experimental data, of a strikingly retrogressive landslide mechanism. Finally, the first analysed flux conditions represents a limit situation for the slope stability, so the water level and the displacements relative to this flux model may be assumed to be threshold values.
6. Monitoring
Height on the s.l. (m)
The stability analysis has made it possible to define very well the behaviour of the slope and the most important elements which influence the slope stability. But it is important to notice that the reported results have been obtained analysing a model of the slope. So there are some uncertainties which are difficult to be defined and quantified. For this reason, a complete and exhaustive study of a slope has to take into account the monitoring phase, which is very important to check the results obtained by the modelling and to fit the model. In particular a monitoring system for a landslide has to control the following quantities: • Pore water pressure and water level; • Superficial and deep displacements.
9 10 11 12 14
Well number
Table 5 Static water levels in the wells in the period November 1997–October 1998
March
April
May
June
July
August
September
October
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
As reported before, on the analysed slope there are five wells and several piezometers, thanks to which the
1606
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120
392 386 380 374
0
100
200
300
400
500
600
700
800
900
1000
Fig. 3. Table waters obtained for the four flux conditions analysed.
height
Monte Marano Sands 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 –0.1
Sandy-clayey detritus and transition sand/clay Blue clays
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
di stanc e (x 1000)
Fig. 4. Displacement situation obtained considering the first flux condition.
height
Monte Marano Sands 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 –0.1
Sandy-clayey detritus and transition sand/clay Blue clays
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
distance (x 1000)
Fig. 5. Displacement situation obtained considering the fourth flux condition.
modalities of the circulation of water within the slope have been reconstructed. With regard to displacements, a monitoring system based on GPS technique has been planned. This technique makes it possible to reach the same precision of the classical techniques with shorter surveys. (Paccapelo et al., 1999). For monitoring displacements on the analysed slope a network consisting of 11 points located on the landslide zone and two control points located in the Pomarico built-up area, has been planned. The adopted network scheme is formed by 34 baselines, leading to a factor of redundancy equal to 2.8, a value which makes it possible to reach a great precision.
7. Conclusions The analysis carried out has made it possible to define the phenomenon and study it on the basis of greater geological and geomechanical data leading to the following conclusions: (a) The phenomenon effects a soil thickness that never exceeds 20 m, therefore consisting mainly of sandy-clayey detritus; (b) The circulation of water plays a fundamental role. In fact, considering the filtration parallel to the slope and the water level close to the surface, the yield zones involve all the detritus cover.
F. Bozzano et al. / Physics and Chemistry of the Earth 27 (2002) 1601–1607
(c) Correspondingly, displacements have been calculated, making it possible to plan a monitoring system based on GPS technique and piezometers, able to verify the behaviour of the slope and to compare it with the build up model. References Atkinson, J., 1993. An Introduction to the Mechanics of Soils and Foundations. McGraw-Hill International. Balduzzi, A., Casnedi, R., Crescenti, U., Mostardini, F., Tonna, M., 1982. Il Plio-Pleistocene del sottosuolo del bacino lucano (avanfossa appenninica). Geologica Romana 21, 89–111. Cherubini, C., Genevois, R., Guadagno, F.M., Prestininzi, A., Valentini, G., 1985. Sulle correlazioni geotecniche spaziali, lÕerosione e la stabilit a dei pendii dei depositi argillosi pleistocenici della fossa bradanica. Geol. Appl. e Idrogeol. 22, 671–690. Cherubini, C., Giasi, C.I., Guadagno, F.M., 1989. Il coefficiente di spinta a riposo delle argille azzurre subappennine di Matera. Rivista Italiana di Geotecnica, 186–192. Cherubini, C., Lupo, M., 1998. Physico-mechanical properties of Matera Blue Clays and correlations among them. Eighth International IAEG Congress. Balkema, Rotterdam, pp. 283–288. Cherubini, C., Walsh, N., 1982. Caratteristiche geolitologiche e geotecniche delle sabbie di Monte Marano (dintorni di Gravina). Geol. Appl. e Idrogeol. 17, 202–216.
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Cruden, D.M., Varnes, D.J., 1996. Landslide type and processes. In: Keith Turner, A., Schuster, R.L. (Eds.), Landslide Investigation and Mitigation. Transportation Research Board, Special Report 247, pp. 36–75. Genevois, R., Prestininzi, A., Valentini, G., 1984. Caratteristiche e correlazioni geotecniche dei depositi argillosi bradanici affioranti a NE della fossa. Geol. Appl. e Idrogeol. 19, 173–212. Guerricchio, A., Melidoro, G., 1979. Fenomeni franosi e neotettonici nelle argille grigio-azzurre calabriane di Pisticci (Lucania) con saggio di cartografia. Geol. Appl. e Idrogeol. 14 (1), 105–138. Guerricchio, A., Valentini, G., 1979. Un modello matematico per la valutazione dellÕerosione tratto dallÕesame dei pendii calanchivi nelle argille azzurre lucane. Geol. Appl. e Idrogeol. 10 (1), 241–275. Lazzari, S., 1989. Strutture profonde passive drenanti a protezione di alcuni abitati della Basilicata. Proceedings of 17 Convegno Nazionale di Geotecnica, Taormina, vol. I, pp. 154–161. Paccapelo, F., Rutigliano, P., Vespe, F., 1999. Controllo di movimenti franosi mediante lÕutilizzo della tecnica GPS. Atti della 3^ Conferenza Nazionale ASITA, Napoli, vol. II, pp. 1133–1137. SEEP/W, 1998. UserÕs guide. Geo-slope International Ltd. SIGMA/W, 1998. UserÕs guide. Geo-slope International Ltd. Terzaghi, K., Peck, R.B., 1967. Soil Mechanics in Engineering Practice. John Wiley and Sons, NY. Valentini, G., 1979. I fenomeni di erosione e di frana nei depositi argillosi della fossa bradanica. Geol. Appl. e Idrogeol. 14, 126–141. Valentini, G., Cherubini, C., Guadagno, F.M., 1979. Caratteristiche geotecniche dei sedimenti argillosi pleistocenici tra Pisticci ed il mare. Geol. Appl. e Idrogeol. 14 (3), 569–611.