Mitigation measures: stabilization works

Intensive Course in Quantitative Landslide Risk Assessment and Risk management Mitigation measures: stabilization works E.E. Alonso Department of Geotechnical Engineering and Geosciences UPC, Barcelona September 3rd, 2008. Barcelona

A NOTE ON SAFETY FACTOR The discussion on safety factor is largely based on the existing knowledge of relevant strength parameters The better and more reliable the knowledge of shear strength parameters (and other relevant aspects: geometry; hydrologic regime), the lower can be the adopted SF SF= 1.5 (= τ rot /τ eq ) is a reference value for geotechnical projects for design conditions (limited knowledge of real conditions) Substantially lower SF s are acceptable in some occasions (magnitude of the landslide, probability of occurrence of extreme conditions and others

GLOBAL SAFETY FACTORS FOR SLIDE MITIGATION Case Safety factor, F Slides of relatively small volume whose failure is likely to produce: light damage 1.25 medium damage 1.35 high damage 1.5 Slides of relatively large volume whose failure is likely to produce: light damage 1.1 medium damage 1.15 high damage 1.2 An alternative to global safety factors is to work with partial safety factors. Typical values are: Fc = 1.5-1.6 for cohesion (or undrained strength) Ftanφ = 1.1-1.2 for friction (tanφ ) These values are similar to those proposed in table 2.1, hypothesis C, of the Eurocode 7.

STABILIZATION METHODS 1.CHANGES IN GEOMETRY 2.RETAINING STRUCTURES 3.DRAINAGE 4.PASSIVE PILES 5.SOIL NAILING 6.OTHER

1.CHANGES IN GEOMETRY

Two-block slide. Equilibrium for a purely frictional contact

Change in block weight ( w). Removed from upper part (w 1 ) or added to lower part (w 2 ) Best strategy: the fastest increase in R

Cortes landslide Location of the slide: Left margin of Júcar River. Valencia province Immediately upstream of Cortes dam: a 100 m high concrete arch dam The left margin of Júcar river according to the geological report of the dam project:

CORTES SLOPE AT THE BEGINNING OF DAM CONSTRUCTION

Plan of the site

Silos and auxiliary installations in upper part of landslide

QUARRY EXCAVATION. POSITION OF INCLINOMETERS

Measured deformations of inclinometer P-2-2 Displacements of top of inclinometer P-6-1 and measured rainfall intensities for individual storms in January 1998 Note: - Rigid body motion of upper 65 m - Insensitivity to relatively heavy rainfall

A representative profile of the slide along Profile P-6 Dip of marl layer of upper region: 25º. 10-30 m of cover in this area Intermediate zone: marl layer buried by cracked limestone (70 m thick). Uniform dip of marl layer: 16º- 17º Narrow lower zone. Horizontal marl layer

The marl: Low porosity (n= 0.25), low-plasticity clay (w l = 20%-28%; w p = 13%-14%) High consistency (w = 16%) Results of drained direct shear tests. Two complete strain reversal cycles to reach residual conditions CU triaxial tests performed on core specimens (σ 3 = 0.2; 0.45; 0.7 MPa) c = 0; φ = 20º-21º

BACK ANALYSIS The slide was assumed to be a reactivated slide (induced by quarry excavations) Therefore the outcome of the analysis should be the residual friction angle No indications of water table in the cracked limestone layer The marl layer was saturated Limit equilibrium analysis: Carter s method: equivalent to simplified Bishop If phreatic surface on top of marl layer: φ = 17.7º If zero water pressure in marl layer: φ = 16º These values were accepted as more reliable than the values derived from tests (despite the coincidence of direct and triaxial shear tests).

Influence line resulting from 10 MN load travelling Profile P-6 A convenient stabilizing method is to displace weight from the upper section to the lower one. 700.000 m3 initially planned

Profile P-6 after stabilization Safety factors achieved: 1.3 (average of the analysis of seven cross-sections) 1.19 for rapid drawdown conditions assuming a rapid reduction of water level from elevation 326m to elevation 320m Considered low by dam owners!

Additional stability procedures analyzed Drainage of marl layer: Slight effect and uncertainties Anchoring: Very expensive due to the extremely long anchor length Additional weigth transfer Final total excavated volume: 800.000 m 3 RESULTS

FINAL EXCAVATION DEFINED IN PROFILE P-6

Displacement records of inclinometers P-22-1, P-2-1 and P-2-2

Aerial view of the landslide after stabilization

2. RETAINING STRUCTURES

Stabilizing a simple planar landslide The stabilizing force increases linearly with the length of the slide Note the high stabilizing forces for an increment of 0.1 of SF

Multiple walls

Concrete reinforced walls Gravity wall Wall anchored to firm stratum Counterfort wall eventually anchored

Anchored pile wall

Case study: Multiple wall failure in Les Costes, Andorra Upper wall Traza aproximada de la sección AA Intermediate wall Lower wall Local instability? (just wall overturning) Global instability (controlled also by the natural slope)?

Les Costes, Andorra 200 saturada: 4 µm/min (una probeta multi-etapas) seca: 25 µm/min (dos probetas) The walls before the failure Tensión tangencial, τ (kpa) 150 100 50 0 φ' = 40 o φ' = 35 o 0 50 100 150 200 6m 3.5 m 3m 2m 0.7 m Tensión normal, σ n (kpa) 7m Conjunto de viviendas Houses Fase 1 3m 1 1 3m 2m 3m 0.5 m 2m 3m 0.7 m

Les Costes, Andorra Intermediate wall after overturning

Les Costes, Andorra. The natural soil γ d = 21 kn/m3 c = 13,1 kpa φ = 38º

Les Costes, Andorra Simulation of the construction process. Drained analysis Stability of natural slope. Incremental shear strains. SF = 1.35 Stability of natural slope after first excavations. Incremental shear strains. SF = 1.02

Les Costes, Andorra Simulation of the construction process Calculated deformations (x 200) after construction of houses and lower wall Failure mechanism in a (c,φ) reduction calculation. SF = 1.15

Les Costes, Andorra Simulation of the construction process Calculated deformations (x 200) after construction of intermediate wall Plastified areas. Note active states against walls

Les Costes, Andorra Critical failure mechanism after construction of intermediate wall. SF = 1.18

Les Costes, Andorra Simulation of the construction process Calculated deformations (x 200) after construction of upper wall Plastified areas. Note active states against walls

Les Costes, Andorra Simulation of the construction process Critical failure mechanism after construction of upper wall. SF = 1.17

Les Costes, Andorra The failure mechanism

Les Costes, Andorra Stabilization works. Anchored micropile wall. SF = 1.35 (long term)

Flow of soil under the footing of intermediate wall

Case study: Multiple wall failure in Les Costes, Andorra CONCLUSIONS The lower wall was probably the first to fail Failure of the lower wall can be a attributed to a general slope failure The intermediate micropile foundation was not enough to limit the earth pressures against the lower wall The triggering mechanism was probably some excess pore pressures, induced by rain, associated with the wall barrier effect

3. DRAINAGE

Drainage trenches FILL

Californian drains. Charts for design (Desnouveaux et al, 1990)

Deep drainage (Collota et al, 1988)

Drilling rig Courtesy of Rodio Spain

Case study: slopes in Guadalquivir blue/brown clays. El Carambolo. Sevilla 10 m spacing; D = 1.5 m

Case study: slopes in Guadalquivir blue/brown clays. El Carambolo. Sevilla Botanic garden

Case study: slopes in Guadalquivir blue/brown clays. El Carambolo. Sevilla Terraces built in upper slope to remove weight

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut.

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut. Stabilization design

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut.

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut. Evolution of slide displacement of a surface point

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut. Flow rates measured in drainage system of Sort slide

Case study: slide in Sort, Lleida. Ancient landslide reactivated by road cut. Anchoring the slope toe

Case study: Guardia de Tremp, Lleida. Unstable Garumniense formation reactivated by road embankments

4. PASSIVE PILES

Deep circular diaphragm wall Protection of deep foundations. Bridge for the High Speed train MAD-BCN. S. Sadurní

S. Sadurní slide. Barcelona Slide direction Plan view of sliding area and two railway bridges crossing it

S. Sadurní slide. Barcelona. Cross section

S. Sadurní slide. Barcelona Well drainage at the slide head

S. Sadurní slide. Barcelona RENFE railway bridge piles. Some piles are protected by a wall enclosure

S. Sadurní slide. Barcelona Anchored walls protecting piles 5 and 6 of RENFE bridge

S. Sadurní slide. Barcelona 200 150 100 Pilas RENFE sin proteger FS=1.05 50 0 50 100 150 200 250 300 350 400 Initial Landslide. Unprotected piles of RENFE railway bridge Backanalysis: residual friction angle of lower marly clays (Miocene). φ res = 20º

Forces against row of piles in a moving soil (Ito and Matsui, 1975) SECTION E 2 = σ αφ 1 αφ ( ', D1, d) = N φ F( z) '( z) ( ', D, d) 2 π φ ' N φ = tan ( + ) 4 2 (Drained solution) N 2 φ tan( φ ) + Nφ 1 D 1 A= D1 D 2 1 Ae D D2 = D1 d

S. Sadurní slide. Barcelona Summary of calculations Φ Fill and gravel: Φ clay: D 1 ; distance among pile axis (m) D 2 ; internal distance (m) d; diameter of protection structure (m) D 1 /d Total force against protection structure (KN) Average stabilizing force on the landslide (KN/m) 35º 20º 31.5 21.5 10 3.15 94939 3014

S. Sadurní slide. Barcelona Safety Factor of Landslide. Protected piles of RENFE railway bridge 200 150 100 Pilas RENFE con protección FS=1.208 50 0 50 100 150 200 250 300 350 400 SF = 1.208

Force against a pile in a row. Pile diameter : 2m. Effect of distance among pile axis F 20000 F 15000 10000 Empuje medio en talud (kn/m) Carga por pilote (kn) 5000 0 3 4 5 6 7 8 9 10 11 Separación entre ejes (m)

S. Sadurní slide. Barcelona Safety Factor of Landslide. Protected piles on RENFE and AVE railway bridges 200 150 100 Pilas RENFE y AVE con protección FS=1.33 50 0 50 100 150 200 250 300 350 400 SF = 1.33

S. Sadurní slide. Barcelona Protected shaft 16 Suggested distribution of protection piles 14 12 Rigid cap on piles 10 d 25 m long piles. D = 2m Eje y (m) 8 6 4 c c Sliding surface at 14 m depth Winkler model for the embedment depth (11m) 2 0 a b a -2-4 -2 0 2 4 6 8 10 12 14 16 Eje x (m) Landslide displacement

Structure of protection piles Calculated bending moments of protection piles

Calculated axial forces on protection piles Piles in extension (max: 7.5 MN) Pile in compression(16 MN)

S. Sadurní slide. Barcelona Final design of stabilizing structures at the foot of the landslide Slide direction

Final remarks 1. There are no single rules to select the appropriate stabilization method. The following aspects should be considered: Type of slide Importance of facilities affected Volume of the slide and intensity of stabilization forces Time available to design and implement the comforting action Cost

Final remarks 2. Stabilizing slopes remains an art with strong empirical basis In rotational slides, unloading the upper part is probably the most effective method. Check instability upwards In purely translational slides changes in geometry are not effective. Deep or surface drainage is effective and suitable in almost any case. It requires acceptable soil permeability and the possibility of gravity drainage. Passive piles are an attractive solution in surface to medium depth slides when bending moments induced on piles are moderate. They are also useful in slides controlled by thin plastic layers difficult to locate and drain. Anchored solutions provides large active retaining forces. Tend to be expensive. Limited information on its long term behavior

Final remarks 3. Very often mixed procedures are designed for a given landslide. They may benefit from a complementary action among them. Thank you for your attention!