Technical Journal of Engineering and Applied Sciences Available online at www.tjeas.com 2013 TJEAS Journal-2013-3-13/1301-1305 ISSN 2051-0853 2013 TJEAS Two-dimensional finite element analysis of influence of plasticity on the seismic soil micropiles structure interaction Mahzad Esmaeili-Falak Young Researchers and Elites Club, East Azarbaijan Science and Research Branch, Islamic Azad University, Tabriz, Iran Corresponding author email: Mahzad.ef@Tabrizu.ac.ir ABSTRACT: This paper includes a finite element analysis of the influence of soil plasticity on the seismic response of micropiles. Analysis is carried out using a global two-dimensional modeling in the Plaxis code. The soil behavior is described using the Mohr Coulomb and liner elastic criterion. Both the micropiles and the superstructure are modeled as two-dimensional beam elements. Proper boundary conditions are used to ensure waves transmission through the lateral boundaries of the soil mass. Analyses are first conducted for harmonic loadings and then for real earthquake records. They show that plasticity could have a significant influence on the seismic response of the soil micropiles structure systems. This influence depends on the amplitude of the seismic loading and the dominant frequencies of both the input motion and the soil micropiles structure system. Key Words: Plasticity, Elastic, Micropiles, Seismic INTRODUCTION Micropiles were for the first time innovated in Italy in 1950 as a creative method to establish foundation for historic building and comments gradually damaged, especially during World War II. It was an efficient and secure foundation establishment system as a support for structural loadings with minimum required transmission which could be founded in limited places with difficult availability and it also caused existing structure to be tampered minimum. Micropiles are piles with less than 300 mm diameter which can be founded by digging bore, putting in and injecting grout (FHWA, 2000). Special digging and injection methods used in founding micripiles lead to a strong joint between grout and the soil and it causes an increase in loads handling capacity in contact lever of the earth and grout. The injected grout, transfers the applied loading from to the earth in joint point via friction (FHWA, 2000). Microplies are used as controlling factor over the foundation structures damaged because of seismic harms. According to experiences, micropiles present suitable behavior when pressed by seismic loading as a result of their high flexibility. However, sufficient studies, appropriate to real measurements have not been done yet and it is obvious that dynamic behavior of micropiles need to be studied. This situation has lead to numeric analysis such as limited elements in studying micropiles behavior when there is an earthquake. Limited elements software has been used in order to do dynamic analysis-numeric model shows that seismic loading causes reveal that non-liner state of soil must be considered in designing micropiles. The present research studies the influence of soil plasticity on the interaction of structure soil-micropiles under seismic loading in which two forms of loading including harmonic loading and real earth quake records have been used (Federal Highway Administration, 2005). NUMERICAL MODELING OF STRUCTURE-SOIL-MICROPILES INTERACTION Numerical modeling has been carried out for dynamic analysis using Plaxis 2D ver 8.2 which is able to do with the manner plain-strain. The soil behavior is described using a liner elastic model based on Mohr-coulomb criterion. Micropiles are modeled as elastic beam elements. Superstructure is modeled as a system with a linear one
degree-of-freedom composed of a column and concentrated mass which is rigidly connected to the cap of micropiles with 1.25m in height. The Mass superstructure equals to 40ton. The cap of Micropile which superstructure is connected also was modeled in form of a concrete foundation with one-meter thickness. Considered Damping is the use of Rayleigh method. In numeric model of the present research, vertical boundaries have been used in the sides of model and horizontal boundary in the bottom of model in order to separate the model from surrounding soil half-extreme environment. In dynamic analysis, absorbing boundaries have is in vertical ones and bed stone placed in the bottom of model. The minimum distance between model side boundaries and micropile is suggested to be 50 times as micropile diameter; as the model measurements have no effects on micropile behavior under side loading further than the mentioned distance. The distance between the bottom of model and micropile has also been suggested to be half length of micropile (Wong, 2004). Regarding to seismic boundary condition, model depth was selected to be 15 m and the length 40m. Spacing between micropiles is 1.20 m. Micropiles diameter 20 cm and their length are 10 m. Figure 1 shows the schematic form of micropile and Figure 2 shows the Two dimensional finite element meshes in Plaxis 2D ver 8.2 Code. RESPONSE IN CASE OF HARMONIC LOADING This analysis is performed on micropile group 2 2 embedded into soil layer in two-dimensional form. Micropiles are connected to a square-shaped cap (2.40 m wide and 1m thick) free from soil contact. And the cap mass are 24Gpa and 14.40ton respectively. Micropiles Spacing is six times as long as their diameter (S=6Dp) and fundamental frequency of soil layer equals to 2.12Hz which is considered equal to harmonic loading frequency. That is why harmonics loading is applied with approximate frequency of fundamental frequency of soil layer and the acceleration on foundation stone will have amplitude of 0.10 g. Figure 1. Schematic form of micropiles Figure 2. Two-dimensional finite element meshes in Plaxis 2D ver8.2 Code. Soils features have been summarized in table one which Reveals a soft soil with internal friction angle φ=300. Cohesion C=17 kpa, dilatancy angle ψ = 0 is the coefficient of the lateral earth pressure at rest unit weight γ=1700kg/m3 and the damping ratio x = 5%. Length of micropiles, diameter, axial stiffness and flexure stiffness are L=10 m, Dp=0.2 m, EAp=629 MN, EIp=434 MN.m2 respectively damping ratio is equal to 5% for the soil and 2% for both the structure and micropiles (Gazetas and Mylonakis, 1998). 1302
Table 1. Soil properties E (MPa) Υ γ (kg/m³) K 0 ξ (%) C (kpa) Φ (º) ψ (º) 21 0.45 1700 0.50 5 17 30 0 Table 2. Micropiles properties D (m) L (m) EA (MN) EI (MN.m 2 ) ξ (%) 0.20 10 629 1.57 2 Figure 3. Influence of plasticity on the lateral displacement of micropiles cap Figure 3 shows the Influence of plasticity lateral displacement of cap for elastic and elastoplastic models. It can be observed that elastic response is about 120 percent Higher than the Elastoplastic one. It is the result of the influence of plasticity on the Damping of the foundation structure system and the dissipation of energy because of plastic deformation; plasticity also leads to reduces the frequency of the system in comparison with the elastic system (In this form, loading frequency equals to fundamental frequency of elastic soil layer). Figure 4. Influence of plasticity on the axial force and bendingmoment of micropiles 1303
Figure 4 interprets curved of influence of plasticity axial force and bending moment of micropiles coursed by input motion. Figure 4 illustrates the envelopes of the axial force and bending moment in the micropiles due to the input motion. Maximum forces in micropiles are obtained in both cases; this form reveals the important role of internal force for the system. Similar observations have been found by Beslimand et al (Benslimane, 2000). It is also observed that soil plasticity results in micropiles internal force degrease. In fact, plasticity decreases the energy transmission towards cap and therefore it leads to a remarkable decrease in internal force which is shown in Tables 3 and 4. Tables 3 and 4 show the increase normalized shearing force and bending moment in normal shearing force and bending moment in piles as well as axial force decrease in elastoplastic form. In other words, for the same still forces, micropiles stand larger cutting force and anchor in plastic soil compared to elastic ones. Table 3. Influence of plasticity on the soil micropiles structure response Maximum acceleration Inertial shear force concentrated mass micropiles cap T a mass (m/s²) st (KN) a cap(m/s²) Elastic 32.58 7.83 834 1324 Elastoplastic 12.45 7.97 398 529 Table 4. Influence of plasticity on the internal force Micropiles Axial force in Shear force in Head micropiles Head micropiles N mass (KN) T cap(kn) Elastic 963 276 123 Elastoplastic 487 124 64 Inertial bending moment M st (KN.m) Bending moment in head micropiles M st(kn.m) 1. Response to real recorded earthquake motions In this modeling Seismic loading is used as time history of acceleration in the bottom of model analysis, studies are done one time history of acceleration USGS in Apland in 1990. In Figure 5, time history of Apland earthquake has been displayed (Alsaleh and Shahrour, 2006; PLAXIS ). Figure 5. Time historical Upland earthquake Figure 6 reveals the effect of plasticity on the speed spectrum of structure- soil micropile system. It is observed that plasticity leads to frequency of elastic system equals to 0.8 Hz where as it equals to 0.75 Hz in elastoplastic systems. So, entering dominant frequency equals to 0.9 Hz which frequency, and dynamic expansion decreases. The results obtained in the present article are according to Alsaleh et al studies (2009) (Alsaleh and Shahrour, 2009; Hunt, 2005). In table 5, the influence of plasticity on structure-soilmicropile response has been revealed. It is observed that elastoplastic acceleration of superstructure (amass) equals to 50 percent of elastic acceleration which shows that plasticity leads to a considerable decrease in system expansion in order to provide necessary resistance. Similar studies were carried out by Alsaleh et al (2009) using in a three-dimensional modeling in the time domain in which similar results were obtained (Alsaleh and Shahrour, 2009). 1304
Figure 6. Influence of plasticity on the velocity spectra of the superstructure (Kocaeli earthquake record, Vpeak = 0.40 m/s, E0s= 8 MPa, f1=0.67 Hz) Alsaleh et al (2009). Table 5. Influence of plasticity on the soil micropiles structure response concentrated mass a mass (m/s²) micropiles cap a cap(m/s²) Inertial shear force T st (KN) Elastic 20.71 11.59 694 894 Elastoplastic 10.53 8.34 386 478 CONCLUSIONS Inertial bending moment M st (KN.m) The present research is a complete two-dimensional analysis of plasticity influence on structure-soil-micropile system. Analyses were conducted for both harmonic loading and recorded earthquake motions. Results of The research have been done show that non-liner behavior of the soil has a considerable effect on seismic response of micropiles and the main effect is on the amplitude of seismic loading and structure-soil-micropile frequency and input motion frequency. It is observable that the response related to elastic is about 120 percent more than elastoplastic and it is a result of energy dissipation caused by plastic deformation; plasticity leads to system hardness decrease in soil micropiles structure system. Finally, soil plasticity results in energy transmission decrease to the cap. REFERENCES FHWA.2000. Micropile design and construction guidelines. US Department of Transportation, Federal Highway Administration,Priority Technologies Program, Implementational, manual, Publication No.FHWA - SA-97-070. Federal Highway Administration.2005. Seismic Behavior of Micropiles. Report No. FHWA WA- RD 6041, United States Department of Transportation. Wong JC.2004. Seismic behavior of micropiles. M.S. Thesis. Washington State University, Pullman, WA. Gazetas G, Mylonakis G.1998. Seismic soil-structure interaction: New evidence and emerging issues. ASCE Special Edition No.75 Geotechnical Earthquake Engineering and Soil Dynamic. Benslimane A.2000. Dynamic behavior of micropile systems Subjected to Sinusoidal Ground Motions,Centrifugal Studies. Thesis submitted in partial fulfillment of requirements for the degree of Doctor of philosophy at The Polytechnic University. Alsaleh H, Shahrour I.2006. Three dimensional nonlinear finite difference analysis for seismic soil-micropile structure interaction effects of nonlinearity of soil and micropile-soil interface. 4th International Flac Sympsium on Numerical in Geomechanics. Alsaleh H, Shahrour I.2009. Influence of soil properties on the seismic soil micropiles structure interaction. Soil Dynamics and Earthquake Engineering 574 578. Hunt RE. 2005. Geotechnical engineering investigation handbook. Second Edition. CRC Press Taylor & Francis Group, 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742. P.O. PLAXIS 2D Ver 8.2 Reference Manual. Plaxis by P.O. Box 5722600 AN Delft The Netherlands. www.plaxis.nl. 1305