Landslide susceptibility mapping by a geotechnical approach. A case study

R. El Hamdouni, C. Irigaray, J. Chacón. Granada University, Spain. M.A. Fernández, A. Martínez. Vorsevi S.A., Spain.

WORKSHOP PROJECT. CGL2005-03332 Universities of Granada, Jaén and Porstmouth (Spain, UK)

5th-10th July 2007

1 – Introduction• During the last 25 years, urban areas of many small country villages were growing up and

the landslide incidence increased, giving place to its consideration in the new urban land-use planning.

• This research is part of a report required by the City Council of Villanueva de San Juan village (Seville) for a new land-use planning made in cooperation with the geotechnical consultant VORSEVI S.A.

The main objective was to made a 1:5.000 landslide susceptibility map. In this case a model of infinite slope with parallel planar failure was considered the most appropriated and objective, given the small extension to be covered and the availability of geotechnical data.

Initially the Municipality proposed eight research zones to be evaluated for future urban development. For the GIS analysis also the current village and surrounds were included as base map of the study area.

GEOLOGICAL SETTING

• Two tectonic domains are represented: the External (Sub-Betic) Zone and the Internal (Circum-Betic or flysch) Zone. Also several sedimentary units of Tertiary and Quaternary ages.

• Calcareous sandstones outcrop in the middle and upper sides of the hills overlying marl units, with saturation of the marl layers and reducing the stability at the foot of the hills.

Geology of the study area and distribution of the proposed zones for intensive geotechnical research.

1 – Introduction

1: Sub-Betic marls.2: Olistoliths. 3. Circum-Betic clay.4: Calcareous sandstones. 5: Slope colluvial. 6: Landslide deposits. 7: Alluvial. 8: Uncompacted fills.

Section AB (NS through the village). Section CD (Zone 5).Section EF (Zone 1). Section GH (Zone 2).

1 – Introduction

2 – Results Geotechnical features of the selected areas

Geotechnical data were obtained from: 6 rotary drills with inclinometers, 32 SPT, 29 undisturbed samples, 18 dynamic penetrations and 16 trenches. 33 temporal measuring of depth to the water table were maintained along 4 months. 120 laboratory tests were carried out to obtain geotechnical identification and strength parameters. Also data from several different previous geotechnical projects made in the village were supplied by the Municipality.

Detailed field survey and mapping of geological, geomorphic and geotechnical units were made at scales 1:5,000 to 1:10,000.

2 – Results Geotechnical features of the selected areas

Rebounds---undefinedOlistoliths. Clay, marl, sandstone and limestone

Unit 4B

cu= 85 kPaf=16º

cu= 94 kPaf = 15º

CU peak

c=50-70 kPaf = 19º–30º

c= 34 kPaf = 23º

c = 38 kPaf = 23 º

CD peak

Strength from laboratory tests

Hard

Hard

medium-hard

Soft –medium

Soft-medium

Consistence

Strength from in situ test

Rebounds

Rebounds

15-40

2-16

< 10

SPT N Values bpf

loose-medium

Density

CL

CL, MH, ML CH

CH

CH

CL, GC MH

USCS

undefinedSub-Betic. Brown greenish to pink marly sandy clays of middle plasticity

Unit 5.

f = 20º> 15Circum-Betic: Brown-greenish clay with middle to high plasticity

Unit 4A

f = 8-14º8-13Homogeneous brown-greenish clay

Unit 3

f = 8-12º3,5 - 8Brown-greenish clay with pebbles

Unit 2

1-5Heterogeneous fills and/or organic weathered soil

Unit 1

Residual undrained

Thickness (m)

DescriptionUnits

From the “in situ” and laboratory tests 5 different geotechnical units where recognised

Zones affected by slope creeping (58%)

Damages in walls around buildingsDeformations in boundaries of lots due to creeping processes.

2 – Results Landslides processes

Earth flow scarp damaging a swimming pools settled on Sub-Betic marly materials.

Earthflow in the right margin of the Las Mujeres stream

2 – Results

The slope rupture occurs along a plane surface

EARTH FLOWS (41 %)Landslides processes

Rock falls in calcareous sandstones on the upper part of the village.

Rock falls (1 %)

2 – Results Landslides processes

Susceptibility analysis using a model of infinite slope with parallel planar failure

www hP ⋅= γ

2 – Results Landslide susceptibility mapping

The landslide susceptibility map is obtained from this grid of safety factor values as a measure of how prone every cell is to develop a landslide.

( ) ( )iid

PidcPcFS wwnr

cossin'tancos''tan' 2

⋅⋅⋅⋅−⋅⋅+

=⋅−+

==γ

ϕγτ

ϕσττ

The safety factor under planar failure on infinite slope was calculated following to Taylor(1948):

The safety factor calculations were applied to a grid of 2x2 meters pixels

Optionally we can use seismic coefficient to obtain maps in dynamics conditions

2 – Results Landslide susceptibility mapping

The data for the calculations of the SF were: - i: angle slope of the ground (DEM by ArcGIS)- d: depth of rupture zone (Rotary drill with SPT, Inclinometer, Dynamic penetrometer)- Pw: Pore pressure (Rotary drill and existing wells) - γ, c’ and ϕ’: were determined from laboratory test.

Slope angle map

ENSAYO DE CORTE DIRECTOUNE 103 401 98

Identificación de la muestra.

Cliente: DIPUTACION DE SEVILLATrabajo: VILLANUEVA DE SAN JUANEnsayo: SI-2 Cota: 3.00-3.60

Dimensiones de las probetas. Párametros Físicos de las Probetas.I II III

Diámetro (mm): 50 Humedad inicial (%): 26,28 24,57 24,82Altura (mm): 25 Humedad final (%): 29,62 28,2 26,14Área (cm2): 19,63 Densidad seca (g/cm3): 1,6 1,57 1,62Volúmen (cm3): 49,08

Párámetros del Ensayo. Tensiones aplicadas a la probeta.I II III

Velocidad del Ensayo (mm/min): 0,5 T. Normal (kp/cm2): 1,25 2,25 3,25Tipo de Ensayo: Residual T. Tangencial (kp/cm2): 0,34 0,58 0,83

0,03

14,04

COHESIÓN (kp/cm2): 0,03ÁNGULO DE ROZAMIENTO INTERNO (º): 14,04

VºBº VªBª

JEFE DPTO. MECÁNICA DE SUELOS

Entidad acreditada en el "Area de Ensayos de Laboratorio de Mecánicade suelos" (SE), con el nº11014SE91 (BOE 25/3/93 y BOJA 05/01/93)

DIRECTOR DE LABORATORIO

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0 1 2 3 4 5 6 7 8 9 10Desplazamiento (mm)

Def

orm

ació

n Ve

rtic

al

Probeta 1Probeta 2Probeta 3

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 1 2 3 4 5 6 7 8 9 10Desplazamiento (mm)

Tens

ione

s Ta

ngen

cial

(kp/

cm2 )

Probeta1Probeta2Probeta3

y = 0,2464x + 0,0286

0,00

1,00

2,00

3,00

4,00

5,00

0,00 1,00 2,00 3,00 4,00 5,00

Tensiones Normales (kp/cm2)

Tens

ión

Tang

enci

al (k

p/cm

2 )

PETICIONARIO: DIPUTACIÓN DE SEVILLA OBRA: P.G.O.U. DE VILLANUEVA DE SAN JUAN

SONDEO SI-2

ENTORNO ARQUETA METÁLICA

SONDEO SI-2 CAJA 1/5 DE 0.00 A 3.00 m SONDEO SI-2 CAJA 2/5

DE 3.00 A 6.40 m

SONDEO SI-2 CAJA 3/5 DE 6.40 A 9.60 m SONDEO SI-2 CAJA 4/5

DE 9.60 A 12.80 m

Proyecto: Dato inicial Dato ActualCliente: Fichero: Villa-1 Villa-1Sondeo: Cota: 100,0 m Fecha: 09-nov-04 15-dic-04Azimut: 0 º Sonda/torpedo:Ángulo Computado de Azimut: 0 º Observaciones:

GK-603SI-1

DIPUTACION DE SEVILLAVILLANUEVA DE SAN JUAN

GRÁFICA DE DESVIACIÓN DEL EJE B.

Desviación Eje B0

2

4

6

8

10

12

14

-30,00 -20,00 -10,00 0,00 10,00 20,00 30,00

Desviación acumulada (mm)

Prof

undi

dad

(m)

19/11/200429/11/200415/12/2004

25% of the zone shows high susceptibility, 31% low susceptibility and 44% moderate susceptibility

Landslide susceptibility map of Villanueva de San Juan (Seville, Spain).

2 – Results Landslide susceptibility mapping

Susc SF

High <1

Mod 1-1,2

Low >1,2

Landslide susceptibility mapping

From aerial photo-interpretation and field survey an inventory of landslides was prepared. The classification of Varnes (1978) was applied.

Landslides inventory in Villanueva de San Juan (Seville, Spain).

2 – Results

• Crossed analysis between the safety factor map (reclassified into three classes) and the landslide inventory map. Both set of data were fully independent.

• The degree of adjustment (GA) was calculated as follow:

∑=

SiZiSiZiGA/

/ Where: Zi: surface of the rupture in the i susceptibility classSi: surface of the i susceptibility class

9,95,4Low (>1,2)43,925,2Moderate (1-1,2)46,169,1High (<1)

Creeping zonesEarth flow rupture zone Susceptibility (SF values)

2 – Results Landslide susceptibility mapping

High coincidence between earth flows ruptures and susceptibility zones

For creeping processes there is a low correlation as it is shown by the GA values

3 –Limitations

• The GIS analysis calculates SF values in cells of 2x2 m although the SF is a concept dealing with slopes or profiles.

• Development of earth flows involves plastic strain during the failure which is not considered by the plane failure model. Also swelling and shrinkage processes are very frequent in these soils mainly because of changes in moisture content.

• Possibility of negative pore pressures were not considered in this study because the lack of data about capillary of the water table.

Despite all these limitations, the fairly good correlation between independent SF values and the landslide inventory suggest that these landslides are mainly earth slide flows in

which the early stage of rupture may be modelled by the proposed method.

4 – Conclusions

• The most susceptible units are “Circum-Betic brown-greenish clay with middle to high plasticity” and “Olistoliths, Clay, marl, sandstone and limestone”.

• The results of Safety Factor fit fairly well to the observed distribution of previous landslides. A much better assessment for earth flow than for creeping zones is obtained.

• About a 25% of the study area is affected by high susceptibility, from which more than 7% is on the village.

• For the future urban areas: zones number 1, 4 and 5 are little affected by stability problems while zones 2 North,2 East and 2 West are the most unstable. Zone 2 South and 3 show moderate susceptibility.

• The recommended zones for a new land-use planning are zones 1, 4 and 5, first and then Zones 2 South and 3, although these only after further geotechnical investigations and avoiding shallow foundations.