Engineering Measures for Slope Stabilization

Engineering measures for 

slope stabilization 

Giovanni Vaciago 

Studio Geotecnico Italiano , Milan, Italy

• Introduction 

CONTENTS 

• Measures to reduce the probability of landslides 

(stabilization s.s.) 

• Measure to intercept the run-out or protect elements at risk 

(control works) 

• Case histories 

2

2002 – 2012 

Congratulations and best wishes 

from the Italian Geotechnical Society 

to ICG on their tenth anniversary 

But first of all …. 

3

ACKNOWLEDGMENTS 

Much of the work presented here is based on Deliverable 5.1 of the 

SafeLand Project “Compendium of landslide risk mitigation measures”, 

(http://www.safeland-fp7.eu/results/Documents/D5.1.pdf) compiled by 

Studio Geotecnico Italiano with contributions from: 

• ICG of Norway, also responsible for quality assurance, 

• AMRA (a consortium of universities in Naples), Italy, 

• the University of Salerno, Italy, 

• Aristotle’s University of Thessaloniki, Greece, 

• Zurich Technical University , Switzerland 

• the University of Lausanne, Switzerland 

• the Geological Institute of Slovenia 

• the Geological Institutes of Romania 

The work was carried out as part of the EC 7th Framework 

Programme, under Grant Agreement No. 226479. 

4

Within a risk-based approach to landslide management, the classification of 

mitigation measures is usefully related to the terms of the “risk equation” 

R = (E)·(H·V) 

• Measures to reduce HAZARD 

• Measures to reduce VULNERABILITY to landslides 

• Measures to reduce the ELEMENTS at risk 

• Measure to SHARE RESIDUAL RISK 

INTRODUCTION 

“Stabilization” refers to the implementation of engineering works to reduce the 

probability of occurrence of landslides, i.e.: the Hazard. 

“Stabilization” sometimes (e.g.: Evangelista et al., 2008) used referring to the 

implementation of engineering works to intercept the run-out or to protect the 

elements at risk. Also referred to “Control works” (e.g.: Ambrozic et al., 2009). 

5

Slope stabilization measures operate by increasing the resisting forces 

and/or by decreasing the driving forces. 

They can be subdivided in relation to the physical process involved: 

• Surface protection and erosion control 

• Modifying the geometry and/or mass distribution 

• Modifying the surface water regime; surface drainage 

• Modifying the groundwater regime; deep drainage 

• Modifying the mechanical characteristics of the unstable mass 

• Transferring loads to more competent strata 

Retaining structures are described in the Compendium as an additional class 

of hazard mitigation measures, even though they do not address a specific 

physical process 

6

In so far as landslides are typically the result of several concurrent factors, 

it is often necessary to implement different stabilization measures. 

Appropriate measures will be selected taking into account: 

• factors which determine the Hazard: stratigraphy, mechanical characteristics 

of materials, surface and underground water regime, morphology, processes; 

• factors which affect the nature and quantification of Risk for a given hazard, 

such as the presence, vulnerability and value of elements at risk; 

• factors which affect the actual feasibility of specific mitigation measures: 

phase and rate of movement, accessibility, environmental constraints, preexisting 

structures and infrastructure, capital and operating cost, maintenance 

7

MEASURES TO 

REDUCE THE PROBABILITY OF LANDSLIDES 

(STABILIZATION S.S.) 

8

SURFACE PROTECTION AND EROSION CONTROL 

• Vegetation (hydroseeding, turfing, 

trees/bushes) 

• Fascines/brush 

• Geosynthetics 

• Substitution; drainage blanket 

• Beach replenishment; rip-rap 

• Dentition 

9

MODIFYING GEOMETRY AND/OR MASS DISTRIBUTION 

• Remove mass from area driving the landslide 

(possible substitution by lightweight fill). 

• Add mass to area maintaining stability, with 

or without gravity, cantilever, crib/cellular 

and/or reinforced soil walls. 

• Reduction of the slope angle. 

• Scaling (removal of loose/unstable 

blocks/boulders) 

10

• Vegetation 

• Impermeabilization 

• Sealing tension cracks 

MODIFYING SURFACE DRAINAGE 

• Surface drains (ditches, piping) to 

divert flows from the slide area 

• Diversion channels 

• Check dams 

11

MODIFYING THE GROUNDWATER REGIME 

• Shallow or deep trenches with free-draining 

geomaterials and geosynthetics 

• Sub-horizontal drains 

12

MODIFYING THE GROUNDWATER REGIME (CNTD.) 

• Wells; self draining or drained by siphoning pumps 

13


• Wells drained by base conductors 

14


• Drainage tunnels, galleries, adits, 

with or without secondary drains 

15

MODIFYING THE MECHANICAL CHARACTERISTICS 

OF THE UNSTABLE MASS 

• Substitution 

• Compaction 

• Deep mixing with lime and/or cement 

• Permeation or pressure grouting 

with cementitiuous or chemical binders 

• Jet grouting 

• Modification of the groundwater 

chemistry 

16

TRANSFERRING LOAD TO COMPETENT STRATA 

• Shear keys: counterforts, 

piles; barrettes (diaphragm 

walls); caissons 

17

TRANSFERRING LOAD TO COMPETENT STRATA (CNTD.) 

• Pretensioned 

multistrand anchors 

(with facing consisting 

of plates or beams) 

18


• Passive anchors: 

soil nails; dowels, 

rock bolts; (with or 

without facing 

consisting of plates, 

nets, reinforced 

shotcrete) 

19


• Anchored walls 

(combination of anchors 

and shear keys) 

20

MEASURES TO 

INTERCEPT THE RUN-OUT OR 

PROTECT ELEMENTS AT RISK 

(CONTROL WORKS) 

21

MEASURE TO INTERCEPT THE RUN OUT OF 

FLOWSLIDES AND DEBRIS FLOWS 

• Diversion channels; 

• (Selective) Check dams, baffles; 

• Guide barriers/walls 

• Terminal barriers, basins 

22

MEASURE TO INTERCEPT THE RUN OUT OF ROCKFALLS 

• Vegetation on the slope 

• Catch trenches 

• Rockfall barriers 

• Rockfall nets (or drapery) 

• Rockfall sheds 

23

CASE HISTORIES 

24

A1 MILAN – NAPLES M/WAY – KM 315+000 

In the 1960s, following repeated landsliding on the southern flank of the 

original cutting the alignment was abandoned and the whole slope reprofiled. 

Following completion of the works, movement was observed in the N-bound 

c/way, requiring construction of stabilization measures in the 1990s. 

A1 to Florence 

N 

Alignment of original design 

Limit of reprofiling, 1960s 

Stabilization works, 1990s 

A1 to Rome 

25


N 

26


The embankment immediately to the SE of the underbridge failed during 

construction in the 1960s, possibly mobilizing in part pre-existing shear 

surfaces. The stabilization required the construction of an extensive toe fill. 

In the 1990s movement was observed to thr NW of the underbridge, requiring 

stabilization by Tee shaped barrets anchored at the top. 


Limit of reprofiling, 1960s 

Stabilization works, 1990s 

High speed railway 

Roma-Florence 

A1 to Rome 

27


28


The Valdilago landslide occurred in 1966. Gabions were constructed along the 

local road uphill of the m/way. 

In the 1970s the newly constructed m/way started to show signs of distress. 

Following accumulation of over 40 cm of movement, the toe of the retaining wall 

was consolidated in the early 1980s by 20 m long, 1 m diameter stone columns. 

This slowed, but did not stop the movement 


High speed railway 

Roma-Florence 

Stone columns, 1980s 

Barrettes & anchors, 2006 

A1 to Rome 

29


The right bank of the Arno River was protected with drainage and rock armor 

after ongoing erosion of the bed and bank (1 m and 10 m respectively, in the 

period 1963 to 1984) was identified as the main cause of the slide. The river bed 

deepened by a further 1.4 to 1.5 m between 1984 and 2002. 

In fact, the area is located just downstream of the Levane dam, built in 1957 for 

the production of electricity 

30


The results of extensive inclinometer monitoring indicated that the landslide and 

the movement of the wall are not directly linked; the downhill portion moves at a 

much higher rate on a basal shear surface up to 20 m deep. Back-analyses 

confirmed fluvial erosion as the main “driver”. 

31


The rate of movement indicated a possible progressive acceleration and 

broadening of the movement, with potential repercussions on the m/way. The 

area was therefore earmarked for additional stabilization and a final design was 

Indagine Integrativa 1a e 2a fase 

prepared in 2004 – 2005. 

s (mm) 

50.00 

40.00 

30.00 

20.00 

10.00 

0.00 

29/11/2001 29/03/2002 27/07/2002 24/11/2002 24/03/2003 22/07/2003 19/11/2003 18/03/2004 

data 

IG 

ID 

IE 

IF 

I1N 

I2N 

I4N 

32


Based on a careful joint examination of the results of stability analyses and of the 

geotechnical monitoring it was possible to subdivide the area in two parts having 

significantly different rates of movement. 

Where the movement had progressed most (approximately 270 m), stabilization 

works consisted of a continuous 20 m deep free-draining diaphragm wall (porous 

concrete) supported by 30 m deep, 3x1m rectangular barrettes at 3 m spacing 

with a connecting slab and beam at the top, anchored by 40 m long strand 

anchors at 1.75 m spacing. 

Where movement was still in the initial stages and the shear strength of the clay 

had not deteriorated to the same extent (approximately 140 m), the existing 

retaining wall was anchored using two levels of strand anchors on a 2 x 1.5 m grid, 

applying the load through a new reinforced concrete “skin” wall. 

The embankment, which over the years had been loosened by continued 

movement, was consolidated by systematic jet-grouting. 

33


The worksite is clearly visible in the aerial photograph. To the left, the excavated 

soil in temporary storage before final disposal. 

34


Construction of the porous concrete diaphragm wall 

35


Construction of reinforced concrete barrettes and connecting slab 

36


Construction of ground anchors and new structural facing 

37

CHAMPLONG, NR. COGNE (AO) 

Unusually intense and prolonged rain 

on 13 to 16 October 2000 caused 

significant distress in all of the Italian 

NW Alps, including the Val d’Aosta 

region, resulting in widespread 

landsliding and flooding throughout 

the region. 

In Cogne, south of Aosta, the total 

rainfall during these 3 days amounted 

to approximately 65% of the average 

annual rainfall. Furthermore, it 

occurred after prolonged, albeit less 

intense rain in the previous month 

and was accompanied by rapid snow 

melting. 

Champlong 

Landslide 

38


A large, mainly roto-translational landslide (approx. 100.000 m3, 20 m deep) 

occurred near Champlong and Lillaz, approximately 2 km from Cogne. 

This type of landsliding is rather unusual in the valley, where rockfall, 

flowslides and avalanches are the common forms of landsliding. 

Water jets were observed just before the slide, as confirmed by sand boils. 

39


The landslide moved as a series of 

“rafts”, tilting trees, disturbing the 

natural drainage of the area and 

obstructing the Urtier river, pushing it 

to erode the right bank. 

After the event, the area remained 

severely disrupted and unstable. 

40


Considering that the area lies within 

Italy’s oldest and most prominent 

National Park, the remediation had to 

strike a balance between: 

• returning the area to previous agropastoral 

and general amenity use; 

• preserving the new land forms; 

• ensuring the stabilization of the 

area to prevent reactivations which 

could jeopardize the safety of the 

only motor road to Lillaz. 

Remediation and stabilization relied 

on reprofiling, surface drainage, 

erosion protection at the toe and 

reestablishment of vegetation. 

41


Ten years after the event the 

remediation is complete: 

realignement of the road on the right 

bank, erosion control works on the 

left bank, regrading, drainage and 

vegetation of the slide body. 

42

DEEP ROCKSLIDES IN THE ALPS 

Major rockslides on a catastrophic 

scale are quite common in the Alps. 

In 1987 the Coppetto (or Val Pola) 

landslide destroyed the villages of 

Sant’Antonio, Morignone and 

Aquilone, causing several casualties. 

The debris filled the valley bottom, 

blocking the main highway and the 

Adda River, which formed a landslide 

lake which threatened even greater 

devastation downstream. 

Eventually both river and road were 

diverted into tunnels to bypass the 

landslide. 

43


The Ruinon landslide further up the 

same valley poses a similar hazard. It 

could form a landslide dam on the 

Frodolfo stream and block the road 

which is the only full year access to 

Santa Caterina Valfurva, but it does 

not threaten any village. 

44


The La Saxe landslide in Val d’Aosta 

poses a similar hazard. However, its 

run-out would cover most of the 

village of Entreves and it would block 

the access road to the Italian Portal of 

the Mont Blanc tunnel. 

by kind permission Prof. GB Crosta, Milan 

45


While for the Ruinon landslide it would be sufficient to prearrange a road 

and hydraulic bypass, (and indeed this is what is currently planned), this 

would provide only marginal benefit in the case of the La Saxe landslides, 

due to the much greater potential impact on a built-up area and on 

critical infrastructure. 

The La Saxe landslide is also a good example of how the geological, 

hydrogeological and geotechnical detail influences the selection of 

stabilization measures. 

The landslide accelerates significantly in response to snow melt, when the 

piezometric levels rise above the failure surface (at around 70 m depth). 

This would seem to suggest that a drainage gallery could be highly 

beneficial for the stability of the landslide mass. However, moments 

continue, albeit slower, even in periods of the year when the piezometric 

levels lie below the failure surface, indicating that drainage alone would 

be insufficient to stabilize the slide. 

46

AND IF NOTHING ELSE CAN BE DONE ABOUT IT….. 

DANGER 

rock fall and 

landslides for 1.2 km 

in case of rain 

and heavy 

snowfall 

(By Order n.27 of 12/11/97) 

47

THANK YOU FOR YOUR ATTENTION 

! 

chestnut fall 

in autumn 

48

THANK YOU FOR YOUR ATTENTION 

49

Engineering Measures for Slope Stabilization

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