PRACTICE AND RESEARCH ON MICROPILE GROUPS AND NETWORKS Prof. François SCHLOSSER ENPC - CERMES 2 nd LIZZI lecture Tokyo IWM August 2004

PRACTICE AND RESEARCH ON MICROPILE GROUPS AND NETWORKS 1) Development in micropile construction 2) Examples of micropile groups under vertical loading 3) Behaviour and design of A-shape micropiles under horizontal loading 4) Micropiles in liquefiable soils

RECENT DEVELOPMENT IN MICROPILE CONSTRUCTION 1) Types of micropiles tested in the Forever project 2) Driven and grouted micropile 3) Self -drilling injected micropile

French Bored Micropile Classification

GROUTING WITH THE “TUBE A MANCHETTE” ( Type IV : Repeated and Selective Injection)

Main type of micropile tested at the FOREVER experimental site:  Fontainebleau sand (D r = 0.5)  Boring, Grouting by gravity (Type II ) Ii a : complementary grouting from the top ii b : complementary grouting from the bottom  Main parameter :  and q s (side friction stress)

R-SOL MICROPILE

Comparative Results

EXAMPLES OF MICROPILE GROUPS UNDER VERTICAL LOADING 1)Reinforced foundation of the Uljin nuclear power plant 2)Micropiled raft withstanding water uplift pressures at A86 urban highway in Rueil (near Paris) 3)Foundation reinforcement of the old Pierre bridge in Bordeaux

ULJIN NUCLEAR POWER PLANT

Micropile reinforced foundation in the fault zone

DESIGN METHODOLOGY: 1) Equivalent homogeneous material assumed for the plug of reinforced faulted rock. 2) Elastic isotropic FEM calculations for determining the required modulus E v. 3) Homogeneization of a group of micropiles in 1 D deformation using the results of load tests on isolated micropiles. DESIGN OF THE PILE GROUP

HOMOGENEIZATION METHODS 1-D method : (Blondeau et al., 1987) 2-D and 3-D methods : (de Buhan et al., )

RUEIL MICROPILED RAFT Traction load test Reinforcement : tube Ø = 89 mm e = 9.5 mm Borehole : Ø = 125 mm Soil : alluvium + chalk Total length : L = 19 m Free length : L f = 4 m

GOUPEG METHOD ( Hybrid model taking into account the micropile interaction ) 1) Load transfer functions model (GOUPIL - LCPC) : p-y, t-z, q-z ( Frank, 1983 – Degny and Romagny, 1987) 2) Mindlin’s equations for evaluating the micropile interaction (O’Neill, 1977)

Settlement (mm) Load (kN) Comparison between measured and calculated load - settlement curves of the micropiles of the group

FOUNDATION REINFORCEMENT OF THE PIERRE BRIDGE IN BORDEAUX

_______ Movement of the water level ( sea tide, river flow) _______ Wooden piles : B = 0,30 m s/B = 4 Micropiles : B = 0,22 m s/B = 10

MICROPILES : Bored micropiles Reinforced tube (178/154 mm) Type IV (injection with “tube à manchettes”) in the marl Type II ( global injection at low pressure ) in the masonry Measured load transfer 5 to 20 %

STABILIZATION OF THE SETTLEMENTS AFTER MICROPILES INSTALLATION

BEHAVIOUR AND DESIGN OF A-SHAPED MICROPILES UNDER HORIZONTAL LOADING 1) Results of the Forever full scale load tests 2) St Maurice anti-noise wall 2) Slope stabilization at the site of the Millau viaduct

FOREVER RESULTS THE THREE A-SHAPED NETWORKS

HORIZONTAL LOADING Bearing capacity of the networks largely exceeding bearing capacity of the group

Horizontal loading test of an A-shaped micropiles network

ST MAURICE ANTI-NOISE WALL

LATERAL LOADING TEST

Vertical Inclined ( traction) MICROPILE LOAD – DISPLACEMENT CURVES Displacement at the top (mm) Axial force at the top( kN )

ROTATION ( R ) VERSUS APPLIED LOAD ( F ) F (kN) R (rad) R : rotation of the footing F : (lateral) load applied to the wall

Applied load ( kN) Measurement Goupeg 1 Goupeg 2 Rotation (rad) COMPARISON BETWEEN MEASUREMENTS AND GOUPEG CALCULATIONS

SLOPE STABILIZATION AT THE MILLAU VIADUCT SITE

LANDSLIDE ON THE SOUTH WORKS TRACK (2001)

A – SHAPED MICROPILES Type 3 (global injection) Tube : Ø = 157/178 mm Inclination :  = 20° Borehole diameter :B = 0.30 m Spacing : s = 1.70 m Soil : alluvium, altered marl, marl Global safety factor against sliding : F = 1.13 with q = 46 kPa F 0 = 1.00 with q = 0

SOIL – MICROPILES INTERLOCKING EFFECT Present design methods do not take it into account ( C e  1 ) Energy equation : K.H 2 = 4k + G.V F = K.  M = k.  (    f )  = G.  (    f )  f <  f Plasticity of the soil without flow, then failure at  =  f  f =  f Failure  f >  f Plasticity of the micropiles without flow, then failure at  =  f

MICROPILES IN LIQUEFIABLE SOIL 1) Behaviour of single micropiles and micropile groups in liquefiable soil (FEM modelling) 2) Behaviour of A-shaped micopiles in liquefiable soil (centrifuge modeling)

F.E.M. MODELLING Shahrour and Ousta. (1997,1998) Shahrour, Sadek, Ousta. (2001)

SOIL LIQUEFACTION WITH AND WITHOUT MICROPILE

MICROPILE GROUPS IN LIQUEFIABLE SOIL (FEM)

CENTRIFUGE MODELLING ON A-SHAPED MICROPILE GROUPS Juran I. ( New York Polytechnic University)

a max = 0,4 g at the base MAX. ACCELERATION IN THE VICINITY OF THE A-SHAPED MICROPILES

EXCESS PORE PRESSURE IN THE VICINITY OF THE A-SHAPED MICROPILES r u =  u / σ’ v0 Max. value = 0,6 compared to 1 in free field.

CONCLUSIONS 1) The main parameter in micropile vertical bearing capacity is the side friction stress q s. Grouting with the “tube à manchettes” gives the best results. 2) Present design methods of micropile groups are conservative, leading to C e  1. They neglect the soil/micropiles interlocking. 3) A-shaped micropiles (elementary networks) quite well withstand lateral loading. In slope stabilization, interlocking has a beneficial effect on the global safety. 4) In seismic events A-shaped micropiles are well withstanding horizontal movements and thus prevent liquefaction of saturated loose sands.

THANK YOU FOR YOUR ATTENTION !

Micropile and nail launcher using compressed air ( Myles and Bridle, 1991 )  Metallic bar  Energy at the tip  Relatively large penetration  Apparently good lateral friction, but further research needed

RAILWAY EMBANKMENT STABILIZED BY MICROPILES : Additional settlement due to the installation of the micropiles

LATERAL LOADING TEST

AXIAL LOAD ON VERTICAL PILE Force (kN) Depth z (m) ( compression )

AXIAL LOAD ON INCLINED MICROPILE Depth z (m) Force (kN)( traction )