DYNAMIC BEHAVIOUR OF MICROPILES UNDER HORIZONTAL ACTIONS: EXPERIMENTAL EVIDENCES

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1 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 15 DYNAMIC BEHAVIOUR OF MICROPILES UNDER HORIZONTAL ACTIONS: EXPERIMENTAL EVIDENCES Maria Chiara Capatti Università Politecnica delle Marche Francesca Dezi Università della Repubblica di San Marino Abstract Micropiles are increasingl adopted for the seismic retrofit of foundations in eisting structures. Moreover, both vertical and inclined micropiles are often used as foundation sstem for new constructions, ground improvements and man other applications. In order to deepen the knowledge of the dnamic behaviour of those sstems under horizontal loading, an etensive eperimental stud was carried out in an alluvial silt soil deposit on two single vertical micropiles and on a group of four inclined micropiles connected at the head b a concrete cap. Several testing procedures are eploited, in order to investigate the dnamic behaviour of micropiles under different loading conditions and increasing force level, with special attention on the role of eecution techniques and foundation configuration. 1. Introduction Micropiles are small-diameter cast-in-situ bored pile formed b cement grout injection and equipped with steel reinforcement element. The are increasingl used as foundations of new constructions and to strengthen foundations of eisting structures in seismic zones, thanks to their simplicit of eecution. Despite their growing use, results from static and cclic lateral load tests on micropiles are limited (Abd Elaziz and El Naggar, 15), as well as dnamic test on small-scale prototpes (Shahrour and Juran, ). Concerning in-situ dnamic tests, most works focus on piles (i.e. Dezi et al., 1, 13 and 16), while field test on real scale micropiles are almost absent. Moreover, various phenomena need further investigations, such as the role of eecution stages and construction techniques that can be particularl significant in the case of micropiles. To investigate these aspects, an eperimental campaign was carried out, that includes both two single vertical micropiles and a group of inclined micropiles. In the present paper, results obtained from ambient vibration, lateral impact loading tests and free vibration tests are discussed.. Description of the site and instrumented micropiles The eperimental field campaign is performed in alluvial silt deposit on IRS (Injection Répétitive et Sélective) micropiles. The are small diameter cast-in-situ bored piles formed b cement grout injection, equipped with a hollow-core steel bar and finall completed b multi-step high pressure grouting at predetermined depths via valves a manchèttes placed along the bar. In this stud, the traditional eecution stages were suitabl modified to allow the preliminar instrumentation of the hollow-core steel bar of the micropiles, and to avoid damages of the sensors induced b mechanical stresses during the in-situ installation and the high pressure injection stages. Maria Chiara Capatti e Francesca Dezi

2 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 17 Micropiles are installed in an industrial zone near Ancona (central Ital). The geolog of the area, is characterized b clae and silt alluvial deposits. A specific geotechnical investigation campaign was carried out including continuous vertical boreholes to a depth of 15 m, CPTs to a depth of m (Fig. 1 a), and laborator tests on undisturbed soil samples (volumetric characteristics, Atterberg limits, unconfined compression test, direct shear test, oedometer test, unconfined undrained triaial test). Furthermore, the profile of the shear wave velocit Vs with depth and the fundamental period of the deposit have been evaluated from passive and active geophsical surve techniques (MASW, ESAC, HVSR). The micropiles shafts are embedded in a homogeneous alluvial silt-clae laer with poor mechanical properties (Table 1), with Vs = 18 m/s. The seismic bedrock is recognized at a depth of 75 m from the ground level. Micropiles reinforcement is constituted b 8 m long steel pipe bars, assembled trough the junction of elements (each element is m in length). The outer diameter of the circular cross section of each pipe is 76.1 mm, and 6 mm thick. From the head of the micropiles, the 3rd and th elements are equipped with four 5 cm spaced valves a manchèttes for high pressure injections. The th element is also provided with a bottom plug for the grout injection at the micropile tip. Valves a manchèttes are realized b means of small holes for each valve, covered b a rubber band that is fastened b steel rings welded at the valve end. Traditionall, elements are assembled in situ during the insertion of the hollow bar into the grouted borehole. In this eperimental stud, elements were assembled in lab to allow the proper installation of measuring devices along the pipe. Once the sensors were mounted and zealousl protected, the pipes were transported in situ for the installation. Firstl, soil borings were made with a diameter of 17 cm and a length of 7.5 m. Then, after the first grouting of each borehole, the instrumented pipes were carefull inserted. The upper 5 cm of the pipes were left above the ground level to allow the eecution of the lateral dnamic tests. After 8 hours from the first grouting, additional grout was injected via valves a manchèttes in one of the two vertical micropiles using a packer with double effect piston: when the packer was at the required depth, the grout was injected at a pressure of 6 8 MPa. The cement slurr used for both the first and the secondar (selective) grouting has a water cement ratio of.5. In the sequel, the injected micropile is referred to as P1, while the noninjected one as P. Micropiles are represented in Fig. 1 (b, c, d, e). The instrumentation included permanentl installed Strain Gages (SGs), to measure the longitudinal strains along the shafts during the dnamic loading. The choice of the position of the sensors was suggested b the results of a preliminar soil-pile interaction analsis performed with the 3D model for the inertial and kinematic interaction model presented in Dezi et al. (9, 16). Moreover, depending on the tpolog of test performed, micropiles and the cap were instrumented with suitable accelerometers and displacement transducers. qc (kg/cm ) 8 m.5 m Section 1 Section Water table P1 P Section 1 Section.75 m Section 1 Water table 15 Section 1.5 m 1.5 m (d) 1. m 1. m 1. m 1. m PDepth (m) 1 m Section 3 Section 3 (a) (b) (c) Micropile section Section 1 Section Section cm 17 cm 3 cm (e) Fig 1. (a) qc profile in the first soil laer; lateral view of (b) single micropiles and (c) group; (d) plan view of the field; (e) piles section

3 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 15 Table I. Geotechnical properties of superficial alluvial laer Unit weight (saturated) γ kn/m 3 Undrained cohesion c u - kpa Friction angle ϕ -6 Elastic modulus E -3 kpa 3. Eperimental tests Several testing procedures were adopted for the stud, to investigate the single micropiles and the group under different loading conditions. This work focuses on the results obtained from ambient vibration, impact, and snap back tests on the single vertical micropiles, and impact load tests on the group. 3.1 Ambient vibration tests Ambient vibration tests allow investigating the dnamic response of full scale structures in the elastic range, b acquiring the response of the sstem to natural vibrations. To the authors knowledge this procedure has never been adopted before to investigate the dnamic lateral behavior of a simple soil-pile sstem. Concerning the vertical micropiles, two accelerometers per micropile were positioned on the pile head (i.e. measuring along two orthogonal aes called and ), as shown in Fig. a. A time length of about 15 seconds and a sample frequenc of Hz are adopted for the acquisitions. 3. Impact load tests For impact load tests on the single vertical micropiles, a pipe etension was rigidl connected at each micropile head to facilitate the eecution of the impacts and the identification of the dnamic parameters. Two set of tests (Fig b) were performed: the first set (SET A) was characterized b a maimum force level of about N to avoid the saturation of the accelerometer (at 1 g), while the other (SET B) was characterized b a higher force level in order to acquire the signal of strain gauges embedded in the soil. The present paper focuses on the results of SET A, for which impulses were imparted along and orthogonal directions and the accelerometers were applied to the micropile so that the signal was acquired along the same direction of the impact. 1 impacts for each direction were imposed, to get a reliable averaged response. Concerning impact load tests on the group, seven uniaial accelerometers (measuring along, or z ais) were used to allow for a proper recognition of the rocking motion coupled with the translational modes of vibration, and for an eventual roto-traslational mode in the horizontal plane (Fig. c). For both single micropiles and the group, a sampling frequenc of 8 Hz was chosen to get high resolution, and an acquisition duration of s was considered. 3.3 Free vibration tests Free vibration test allows identifing natural frequencies and damping of the soil-micropile sstem, alongside with its fleural deformed shapes, b studing the decrement of the vibrations. The horizontal load was applied on the vertical micropiles using a double acting hdraulic jack, which has a capacit in tension of t. Releases were carried out on both P1 (along direction) and P (along direction), as shown in (Fig d). The quick release of the load was achieved thanks to calibrated steel pins (Fig e), consisting of a steel element, the cross section of which was opportunel determined so that it failed once a predetermined load was reached. After the pin failure, the micropile underwent a number of steadil decreasing horizontal oscillation around its equilibrium position. The Maria Chiara Capatti e Francesca Dezi

4 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 17 instrumentation comprises SGs, displacement transducers on the pile head, and two accelerometers (placed at the same height of the displacement transducers, but diametricall opposed in cross section). Releases induced b four force levels were investigated: kn; for each force level, tests were carried out, for a total amount of 8 shear pins per pile. In Fig f a view of the mechanical scheme adopted for the realization of the test is provided.. Results Some of the results obtained from tests performed on single vertical micropiles and micropiles group are presented in the following. In particular, results relevant for single vertical micropiles are presented in terms of ambient vibration, impact load and snap back tests, while those relevant for the inclined micropiles group are presented in terms of impact load tests..1 Single micropiles Results of ambient vibration tests are here presented in terms of Power Spectral Densit (PSD) function for P1 and P along and ais. From the superposition of and PSD functions for P1 (Fig. 3a) it appears clearl the dnamic behavior along the two orthogonal directions is substantiall different, especiall in terms of fundamental frequencies. On the other hand, results for P are ver similar in the two directions (Fig. 3b). This fact can be attributed, at least partiall, to the directionalit of the high-pressure injections eecuted on P1. Results of impact load tests for SET 1 are here presented in terms of modulus of Frequenc Response Function (FRF) of the registered acceleration at P1 and P. Results refer to the averaged values obtained from 1 impacts. (a) Test on P1/ Uniaial accelerometer (b) Test on P1/: SET A SET B pipe etension coupling joint Uniaial accelerometer Strain Gauge (main alignement) (c) z 1 1 Test on the group 1z 1 3 Uniaial (horizontal) accelerometer Uniaial (vertical) accelerometer 3z (d) 1.75 m Test on P Concrete contrast pin P Hdraulic jack P Test on P1 Hdraulic jack 1.5 m Test on P1/ D1 pin D Displacement transducer Uniaial accelerometer Strain Gauge (main alignement) pin P1 A1 A (e) (f) Fig. Test setup for (a) ambient vibration tests, (b) impact load tests on single micropiles, in SET A and B, (c) snap back tests (d) impact load tests on group; (e) calibrated pins; (f) scheme for free vibration tests

5 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 15 Coherentl with results of the previous tests, the comparison of averaged FRFs obtained considering impacts along both directions (Fig. 3c, d) shows that for the injected micropile a marked difference eists between the behavior along the two direction (the pile seems stiffer along the direction), especiall for the second mode, while the non-injected micropile doesn t show significant differences between the behavior along and that along ais. To stud the dnamic properties of the single vertical micropiles in the non-linear field, free vibration tests were performed on P1 and P. Given the high level of the forces, the traditional frequenc-domain representations based on Fourier Transform are not suitable. In fact, as shown in Fig. 3e for a specific input, time histories of acceleration change its features throughout time, and therefore, their characteristics in the frequenc domain should be investigated point b point. This can be done via Stockwell Transform (Stockwell, 1996), in Fig. 3f, that allow to see that the first resonance frequenc decreases as the time passes; this can be attributed to the widening and deepening of a gap at the interface between the micropile and the soil, as a result of the oscillation of the pile, as well as to the degradation of properties of grout and surrounding soil in the shallower portion of the soil pile sstem (a) P (b) P PSD [m /(s Hz)] FRF [m/(s kn)] f I =6. f I =7. (c) direction direction direction direction Freq [Hz] f II =13 f II =19 P1 PSD [m /(s Hz)] FRF [m/(s kn)] f I =5.5 f I =6. (d) direction direction direction direction Freq [Hz] f II =16 f II =131 P (e) (f) Acceleration (g) - Frequenc (g) T 1 T T 3 A.5 Time (s).65.5e- (g).5e- (h) FRF (g/n) FRF (g/n) H- H- 1 Accelerometer Hammer fi, = 38.5 Hz 3 z fi, = 5.5 Hz z 1z z 3 1 z z Frequenc (Hz) Fig 3. PSD of acceleration during ambient vibration tests for P1 (a) and P (b); FRF of acceleration during impact tests for P1 (c) and P (d); (e) Time histor and (f) Stockwell Transform of acceleration on the head of pile P during one of the Snap Back test (force level: 3 kn); FRF of acceleration on the group, acquired (g) along and aes Maria Chiara Capatti e Francesca Dezi

6 Incontro Annuale dei Ricercatori di Geotecnica 17- IARG 17 Matera, 5-7 Luglio 17. Micropiles group In Fig. 3g, h results of impact load tests on the group are shown, in terms of FRF of acceleration recorded along the direction of impact and in the vertical direction. It can be observed that along the direction in which micropiles are inclined the fundamental frequenc of the group is higher. Moreover, a greater damping is observed: with the half power bandwidth method, a damping of 13% can be estimated in direction, while it is about 1% in direction. Comparable results can be found in the centrifugal eperiments described in Escoffier et al. (8). Of particular interest is the different behaviour in terms of roto-translational coupling that can be recognized along the two horizontal directions, in terms of different amplitude of FRF: this ma be related to the fact that the rocking motion due to horizontal loading on foundations with inclined piles is in antiphase with the rotation of the structure, while these two contributions are in phase in the direction along which the micropiles behave as vertical. 5. Conclusive remarks First results of an eperimental stud carried out on two single vertical micropiles and on a group of four inclined micropiles embedded in alluvial silt soil are presented. Results of ambient vibration tests, horizontal impact load tests and snap back tests one single micropiles and inclined micropiles group are presented pointing out the influence on the micropiles dnamic behaviour of high pressure injections, load level and inclination. Acknowledgements The authors would like to epress their deep gratitude to Alseo srl for providing the micropiles and the technical support throughout all the eperimental stud. References Abd Elaziz A. Y., El Naggar M. H. (15), Group Behaviour of Hollow-Bar Micropiles in Cohesive Soils, Can. Geotech. J. 51: Dezi F., Carbonari S., Leoni G. (9), A model for the 3D kinematic interaction analsis of pile groups in laered soils, Earthq. Eng. Struct. Dn., 38: Dezi F., Carbonari S., Leoni G. (16), A Numerical Model For The Dnamic Analsis Of Inclined Pile Groups, Earthq. Eng. Struct. Dn., 5: Dezi F., Gara F., Roia D. (1), Dnamic response of a near-shore pile to lateral impact load, Soil Dn Earthq Eng, : 3 7. Dezi F., Gara F., Roia D. (13), Eperimental stud of near-shore pile-to-pile interaction, Soil Dn Earthq Eng, 18: Dezi F., Gara F., Roia D. (16), Linear and Nonlinear Dnamic Response of Piles in Soft Marine Cla, J. Geotech. Geoenviron. Eng., 13: Escoffier S., Chazelas J. L., Garnier J. (8), Centrifuge modelling of raked piles, Bull. Earthquake Eng., 6: Shahrour I., Juran I. (), Seismic behaviour of micropile sstems, Ground Improvement 8: Stockwell R.G. (1996), Localization of the comple spectrum: the S-transform, IEEE Transactions on Signal Processing, :