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A series of numerical calculations have been performed to investigate the effect of soil improvement on seismic site response. Seismic site response analyses were also performed using data collected from a study area in Babol city. The improved site is a composite ground and has more or less different mechanical properties than the natural ground. In this research, the influence of the elastic modulus of the pile, the pile distance ratio, ground motion input, distance to fault rupture, and PGA of the earthquakes on seismic response characteristics are especially investigated. The results reveal that the values of the PGA and amplification factor on the surface of the natural and improved grounds depend strongly on the fundamental period of the site, the predominant period, and the intensity of the ground motion input. The acceleration response spectra also are affected by the characteristics of ground motion input and soil layers. Changing the pile distance ratio doesn’t have a significant effect on the seismic response of the site.

The influence of local site conditions on ground motions has been investigated since the early days of earthquake engineering. Observations from as early as the 1800s exist in the literature indicating the effects of local geology on ground motions [

In recent years, due to the flexibility of finite element method to deal with complex issues, people began to use numerical simulation to study this issue; Siegel et al. [

The soil consisted of one layer of soft soil underlain by a rock. Piles were used to increase the capacity of the soft ground. The bottom of the piles was at the top of the rock layer. A sand cushion, 0.5 m thick, was placed over the soft clay. The diameter of each pile wall was 1 m and the typical center-to-center spacing between two piles ranged from 2 to 3.0 m.

The characteristics of near-fault and far-fault earthquakes, which are used in this study, are presented in

Earthquake | Northridge | Loma prieta |
---|---|---|

Date | 1994/01/17 | 1989/10/18 |

Station | 24,207 Pacoima Dam (upper left) | 47,379 Gilroy Array |

Magnitude | 6.7 | 6.9 |

Direction | 194 | 090 |

PGA (g) | 1.285 | 0.473 |

Focal depth (Km) | 17.5 | 17.5 |

Closest to fault rupture (Km) | 8.0 | 11.2 |

Earthquake | Northridge | Loma prieta |
---|---|---|

Earthquake | Northridge | Loma prieta |

Date | 1994/01/17 | 1989/10/18 |

Station | 23,598 Rancho Cucama-Deer Can | 58,338 Piedmont Jr. High |

Magnitude | 6.7 | 6.9 |

Direction | 180 | 315 |

PGA (g) | 0.051 | 0.071 |

Focal depth (Km) | 17.5 | 17.5 |

Closest to fault | 8.0 | 77.2 |

The use of 3d numerical analyses in geotechnical earthquake engineering is very scarce. For practitioners such analyses are considered a luxury, since they are very both time-consuming and computational effort. In addition, the commercially available 3d codes for performing numerical analysis of geotechnical earthquake engineering problems are very few and usually these codes have a smaller potential than commercial 2d codes. For example, 2d codes offer the use of advanced constitutive models or element types that are not found in the libraries of 3d codes. Hence, the numerical research in geotechnical earthquake engineering has been historically based on the use of (1d and 2d) analyses [

In these analyses numerical modeling was performed using the computer program PLAXIS V8, a two-di- mensional finite element program developed specifically for the analysis of deformation and stability in geotechnical engineering applications. Dynamic analysis in PLAXIS can mainly be divided into two types of problems. The first one is related to single source vibrations and the second one is concerned to earthquake problems. In earthquake problems, the dynamic loading source is usually applied along the bottom of the model resulting to shear waves that propagate upwards. These types of problems are generally simulated using a plane strain model [

Many researchers have 2d numerical analyses for the 3d problem of the improved sites. Omine et al. [

Having examined different finite element meshes, a refined mesh was introduced to decrease the effect of mesh dependency on the finite element modeling. At the interface between the piles and soft clay, interface elements have been used. Finite element analyses were carried out using 15-noded triangular elements. The bottom boundary of the mesh was set at the top of the rock with zero displacements. The vertical boundaries of the model were constrained to have vertical movement only. Next the material modeling will be discussed.

The soft soil and sand cushion were represented by an elastic plastic model with the Mohr Coulomb failure criterion, while the piles were assumed to be linearly isotropic elastic. The material properties of the piles, the sand cushion, and the soil layer are presented in

In this section, first the authors concentrate on the influence of the elastic modulus of piles and second the effect of the pile distance ratio will be discussed. In order to investigate the effect of the elastic modulus of the piles on seismic site response, this parameter is considered to be equal to 100, 500, 1000 and 5000 MPa. The fundamental period of the natural ground (T) is calculated using the familiar formula: T = 4H/VS, in which H is the soil depth and VS is the effective shear wave velocity of the soil deposit which is equal to 1.12 sec. The fundamental periods (T) of improved grounds and the predominant periods of the earthquakes are presented in

Parameter | Name | Unit | Pile [ | Cushion [ | Clay [ |
---|---|---|---|---|---|

Material model | Model | - | Linear elastic | Mohr coulomb | Mohr coulomb |

Type of material behavior | Type | - | Non-porous | Drained | Drained |

Soil unit weight | Unsat | kN/m^{3} | 22 | 21.3 | 17 |

Young’s modulus | E | kN/m^{2} | 260,000 | 50,000 | |

Poisson’ ratio | 0.2 | 0.3 | 0.25 | ||

Cohesion | c | kN/m^{2} | 1.5 | 5 | |

Friction angle | 0 | 30.5 | 21 | ||

Dilatancy angle | 0 | 0 | 0 | ||

Interface strength | Rinter | 0.9 | 0.67 |

Elastic modulus of the piles (MPa) | Ts (sec) |
---|---|

100 | 1.03 |

500 | 0.63 |

1000 | 0.47 |

5000 | 0,22 |

Earthquakes | Tp (sec) | |
---|---|---|

Loma Prieta | Near-fault | 0.372 |

Far-fault | 1.241 | |

Northridge | Near-fault | 0.65 |

Far-fault | 0.539 |

Next, the amplification factor for the near-fault earthquakes will be discussed, followed by that of the far-fault earthquakes. The amplification factor is defined as the ratio of the PGA of the improved ground to the PGA of the bedrock.

At low levels of input motion the maximum surface accelerations are greater than the maximum base accelerations. It means that the soil has a linear elastic behavior and amplifies the earthquakes [

It is seen from

On the other hand, under near-fault Northridge earthquake, the improved grounds act in a different way with respect to near-fault Loma Prieta earthquake. The amplification factor of the improved ground with Epile = 100 MPa is slightly less than the amplification factor of the natural ground. In Epile =500 MPa, resonance happens and the amplification factor increases. Then as the elastic modulus of the piles increases, the amplification factor decreases.

In order to investigate the effect of the PGA, the accelerograms have also been normalized to 0.7 g. The PGA and amplification factor on the surface of the natural ground under Loma Prieta and Northridge earthquakes with the PGAs equal to 0.7 g are presented in

From

The PGA and amplification factor on the surface of the natural ground under far-fault earthquakes with PGAs = 0.2 g are presented in

Earthquakes | PGA (g) | Amplification factor |
---|---|---|

Loma Prieta | 0.445 | 2.23 |

Northridge | 0.396 | 1.98 |

Earthquakes | PGA (g) | Amplification factor |
---|---|---|

Loma Prieta | 0.550 | 0.786 |

Northridge | 0.531 | 0.758 |

Earthquakes | PGA (g) | Amplification factor |
---|---|---|

Loma Prieta | 0.529 | 2.645 |

Northridge | 0.380 | 1.900 |

Earthquakes | PGA (g) | Amplificatin factor |
---|---|---|

Loma Prieta | 0.529 | 2.645 |

Northridge | 0.380 | 1.900 |

modulus of piles increases, the amplification factor decreases. Under far-fault Northridge earthquake with PGA = 0.2 g, the trend of the effect of the elastic modulus of piles on the amplification factor, considering the fundamental periods of improved grounds and the predominant period of the earthquake, is acceptable.

The PGA and amplification factor on the surface of the natural ground under far-fault earthquakes with PGAs = 0.7 g are presented in

The results of this section graphically show the acceleration response spectra (Sa for 5% damping), amplification spectra and the effect of improvement on Sa for the input motion and at the ground surface for both the natural ground and improved ground. The amplification spectrum is defined as the ratio of the acceleration response spectrum of the improved ground to the pertinent spectrum of the bedrock. The effect of improvement on Sa provides insight to the effect of the improvement on the ground surface response and is defined as the ratio of the amplification spectrum of the improved ground to the pertinent spectrum of the natural ground, which is of primary interest to civil engineering works.

The results of the site response analysis under near-fault Northridge earthquake predict that the improved ground will respond as a stiffer profile than the original ground. As it can be seen in

Therefore, Figures 3-6 show that the acceleration response spectra depend strongly on the ground motion input. Under Loma Prieta earthquake, the acceleration response spectra have also two peaks, but in contrast with Northridge earthquake, the acceleration response spectra of the natural ground are approximately smaller than the acceleration response spectra of the improved ground at the two peaks.

As it can be seen in

response spectra for improved ground with Epile = 5000 MPa is less than the acceleration response spectra of the natural ground for periods greater than 0.54 sec.

As it can be seen in

In

Therefore, from the figures of the effect of improvement on Sa, it can be understood that under near-fault Loma Prieta earthquake, the improved ground with Epile = 5000 MPa de-amplifies the motion at all periods except for short periods and other improvements de-amplify the motion at short periods.

But under Northridge earthquake the improved ground with Epile = 5000 MPa de-amplifies the motion at short periods and other improvements de-amplify the motion for intermediate periods (between 0.5 and 1.2 sec).

Next the effect of the pile distance ratio on seismic site response will be discussed. The pile distance ratio (s/d) is defined as the ratio of the center-to-center spacing (s) of piles to the diameter (d) of each pile. Each pile was 1 m thick and the typical center-to-center spacing between two piles ranged from 2 to 3 m. calculating the seismic response of site under Loma Prieta earthquake indicates that changing the pile distance ratio doesn’t have any significant effect on seismic response of the site, so the analyses were not repeated for Northridge earthquake.

In order to investigate the effect of pile distance ratio, the parameter of elastic modulus is kept constant and equal to 1000 MPa. The amplification factors for these analyses are presented in

s/b | Near-Fault | Far-Fault | PGA = 0.2 g | PGA = 0.7 g |
---|---|---|---|---|

PGA = 0.2 g | PGA = 0.7 g | |||

2 | 2.09 | 1.14 | 2.05 | 1.11 |

2.5 | 1.97 | 1.13 | 2.14 | 1.09 |

3 | 2.00 | 1.11 | 2.16 | 1.13 |

seen, changing the pile distance ratio doesn’t have any significant effect on the seismic response of the site.

Babol, a city in the Mazandaran province in the northern part of Iran, is our study area. The city is located approximately 20 kilometers south of the Caspian Sea, on the west bank of Babolrood River [

The PGA and amplification factor on the surface of the ground in Babol city ground under Loma Prieta earthquake with PGAs = 0.2 g & 0.7 g are presented in

In this section, the acceleration response spectra (Sa for 5% damping), amplification spectra and the effect of improvement on Sa for the input motion at the ground surface for both natural ground and the improved ground in the Babol city are graphically shown. From

Earthquake | Ground | PGA (g) | Amplification factor |
---|---|---|---|

Near-fault | Natural ground | 0.28 | 1.4 |

Improved ground | 0.33 | 1.7 | |

far-fault | Natural ground | 0.26 | 1.3 |

Improved ground | 0.38 | 1.9 |

Earthquake | Ground | PGA (g) | Amplification factor |
---|---|---|---|

Near-fault | Natural ground | 0.48 | 0.7 |

Improved ground | 1.11 | 1.6 | |

far-fault | Natural ground | 0.33 | 0.5 |

Improved ground | 1.23 | 1.8 |

for the periods between 0.06 and 0.26 sec, which can better be seen in

From

From

Using the finite element method to study the effect of improvement with piles on the seismic site response led to some important conclusions:

1) The values of PGA and amplification factor on the surface of the natural ground and improved grounds

depend strongly on the fundamental period of the site, the predominant period and the intensity of the ground motion input.

2) The results of the site response analysis show that the acceleration response spectra depend on the ground motion input. Under Northridge earthquake, the acceleration response spectra have two peaks and the improved grounds exhibit greater peak spectral accelerations at shorter periods and the peak spectral acceleration of the improved ground at larger periods is smaller than the peak spectral acceleration of the natural ground. Under Loma Prieta earthquake, the acceleration response spectra have also two peaks, but in contrast with Northridge earthquake; the acceleration response spectra of the natural ground are approximately smaller than the acceleration response spectra of the improved ground at two peaks.

3) From the figures of the effect of improvement on Sa, it can be understood that the improvement de-amplifies the motion for intermediate periods (between 0.5 and 1.5 sec). Moreover, under the near-fault earthquakes with PGA = 0.2 g, the improved grounds with Epile = 5000 MPa also de-amplify the motion at periods larger than 1.5 sec.

4) Calculating the seismic response of the site under the Loma Prieta earthquake indicates that changing the pile distance ratio doesn’t have any significant effect on the seismic response of the site.

5) Under Loma Prieta earthquake with PGA = 0.2 g, the natural ground in Babol city shows linear behavior; and under Loma Prieta earthquake with PGA = 0.7 g, it shows non-linear behavior de-amplifying the motion. Improvement makes the site stiffer, so strains become smaller and the improved ground shows elastic behavior. As it can be seen from this section, the seismic response of the natural ground in Babol city is different from the seismic response of a 30-m-thick clay layer. The amplification factors of the improved grounds are greater than the amplification factors of the natural grounds in Babol and improvement in this area de-amplifies the motion for large periods. So it can be concluded that the acceleration response spectra also depend on soil layers and its characteristics.

Asskar Janalizadechoobbasti,Mehran Naghizaderokni,Aida Talebi, (2016) A Study of the Effect of Soil Improvement Based on the Numerical Site Response Analysis of Natural Ground in Babol City. Open Journal of Civil Engineering,06,163-178. doi: 10.4236/ojce.2016.62015