**3.3 Results of a specific soil profile**

Site response analyses were conducted using DEEPSOIL [30] program version 6.1, and to be able to compare different approaches, five borings were excited at their base with three shakings with nonlinear and equivalent linear models for a total of 30 analyses. The results for the boring B1 shook by Duzce record, an example set, were presented in **Figures 10**–**12** showing the NL and EL model estimations for the surface acceleration, their corresponding spectral behavior on

**Figure 10.** *Base and surface acceleration observed from the nonlinear (NL) and equivalent (EL) linear models.*

**Figure 11.** *Spectral acceleration of the base and the surface calculated from the NL and EL models.*

#### *Estimation of Excess Pore Pressure Generation and Nonlinear Site Response of Liquefied Areas DOI: http://dx.doi.org/10.5772/intechopen.88682*

the ground and the peak spectral accelerations, and maximum shear strains throughout the soil profile, respectively.

Regarding the surface acceleration predicted by two different approaches, there is a big deviation in the peaks and the time they occur. NL model shows that soil totally de-amplified the energy exerted by the motion, and the peak ground acceleration was noted around 0.25 g, whereas the peak-based acceleration was 0.72 g, and the main peaks happened around the similar time interval (3–5 seconds). However, an entirely different case was observed for the EL model. The prediction of the peak surface acceleration happened to be around 10 seconds extending the peakbased motion to 5–6 seconds, and not much de-amplification was noticed. In order to evaluate this behavioral diversity, the spectral acceleration at the surface was presented below.

The peak-based spectral acceleration (around 2 g) stays in a wide range of period between 0.1 and 0.8 seconds as seen in the figure. The NL model has compressed the behavior in terms of the size of peak ground and peak spectral accelerations. The model lessens the spectral acceleration 2–2.5 times at any period, whereas the EL model lengthened the peak spectral range to 0.9–1.8 seconds with almost no change in the amplitude. Another point about the EL model is that the initial flat part lasts longer than the base and NL model prediction. The last figure of the example set is the variation of the peak spectral acceleration and maximum shear strains through the soil column, and it is presented in **Figure 12**.

The difference between the models to estimate two parameters is concrete as observed from the figure. The peak-based acceleration was amplified a bit between 10 and 20 m and then de-amplified some for the rest of the profile by the EL model, whereas it was weakened all over the soil profile by the NL model. The soil profile was separated into three divisions, and it is supported by the related maximum shear strains at the same depths. It is obvious that two definite soil failures happened at the 8 m and 14 m according to the NL model, whereas the soil failures

bottom of the figure, and these records were used as the base motions in the

*Geotechnical Engineering - Advances in Soil Mechanics and Foundation Engineering*

*Base and surface acceleration observed from the nonlinear (NL) and equivalent (EL) linear models.*

*Spectral acceleration of the base and the surface calculated from the NL and EL models.*

Site response analyses were conducted using DEEPSOIL [30] program version 6.1, and to be able to compare different approaches, five borings were excited at their base with three shakings with nonlinear and equivalent linear models for a total of 30 analyses. The results for the boring B1 shook by Duzce record, an example set, were presented in **Figures 10**–**12** showing the NL and EL model estimations for the surface acceleration, their corresponding spectral behavior on

analyses.

**Figure 10.**

**Figure 11.**

**134**

**3.3 Results of a specific soil profile**

seem to be predicted at the depth of 2 m and 8 m by the EL model. There are still other local failures between 9 and 13 m, but the deformation level is limited compared to the ones mentioned above. It can be concluded that not only the ground response predicted by two models is quite distinct, but also the seismic actions alter the seismic behavior extremely along the soil column.

#### **3.4 Interpretation of the analyses**

The loss of soil stiffness can be caused by either insufficient bearing capacity or liquefaction triggered by the excess pore pressure generated under cyclic loading. Thus, it is beneficial to see the pore pressure generation if it was the origin of excessive deformations during seismic excitations that only NL models are capable to investigate. Excess pore pressure ratio which is a value between 0 and 1 is one vital criterion in evaluation of the liquefaction. ru of 1 means that the soil layer is under risk to liquefy for such type of loading. **Figure 13** shows the excess pore pressure ratios projected by the NL models for all the logs.

One clear remark can be pointed out here that the liquefaction occurring at various depths in different logs considering all three base motions sets out the possible explanation of the unreasonable deformation indicated in the previous figure. The NL model did not anticipate liquefaction triggering for the shallow depths which include some fine content, whereas there are at least nine points that soil layers mainly composed by silty sands are expected to lose their strength due to liquefaction for such sequence of events. The ru value is greater than 0.8 for approximately 45 points meaning that even if there is no liquefaction initiated yet, soil would possibly lose its strength at these depths. Therefore, soil mitigation would be needed for possible construction in the area.

After liquefaction assessment of the soil profiles, the next step is to evaluate the site response in terms of nonlinear and equivalent linear approaches. Fifteen different analyses were conducted for each, and the peak spectral accelerations on the surface are presented in **Figures 14** and **15**. As mentioned before, building codes

rule out the possible designs with seismic maps and some regulations. In order to see if they are good representatives of the site, corresponding response spectrum envelopes were evaluated using the new Turkish Building Code (TBC2018), Eurocode8 (EC8), and International Building Code-ASCE 7.05 (IBC), and they are shown in figures along with the ground response spectra gathered from the analyses.

*Ground responses evaluated from the EL analysis and building codes' design spectra.*

*Ground responses evaluated from the NL analysis and building codes' design spectra.*

*Estimation of Excess Pore Pressure Generation and Nonlinear Site Response of Liquefied Areas*

*DOI: http://dx.doi.org/10.5772/intechopen.88682*

Considering the PGAs, the variation is almost 100% from one log to another (0.20–0.40 g) in such a small area. The trend of the spectral accelerations is similar for increasing period; however the amplitudes differ a lot especially the change in peak spectral accelerations between 0.1 and 1 seconds. The lowest is reported as 0.3 g, whereas the highest is around 1.2 g with one outlier with the red dots. It is a bit off compared to the rest of the data; therefore it can be excluded to represent the

general picture of the area.

**Figure 14.**

**Figure 15.**

**137**

**Figure 13.** *Excess pore pressure ratios estimated from 15 NL analysis.*

*Estimation of Excess Pore Pressure Generation and Nonlinear Site Response of Liquefied Areas DOI: http://dx.doi.org/10.5772/intechopen.88682*

**Figure 14.** *Ground responses evaluated from the NL analysis and building codes' design spectra.*

**Figure 15.** *Ground responses evaluated from the EL analysis and building codes' design spectra.*

rule out the possible designs with seismic maps and some regulations. In order to see if they are good representatives of the site, corresponding response spectrum envelopes were evaluated using the new Turkish Building Code (TBC2018), Eurocode8 (EC8), and International Building Code-ASCE 7.05 (IBC), and they are shown in figures along with the ground response spectra gathered from the analyses.

Considering the PGAs, the variation is almost 100% from one log to another (0.20–0.40 g) in such a small area. The trend of the spectral accelerations is similar for increasing period; however the amplitudes differ a lot especially the change in peak spectral accelerations between 0.1 and 1 seconds. The lowest is reported as 0.3 g, whereas the highest is around 1.2 g with one outlier with the red dots. It is a bit off compared to the rest of the data; therefore it can be excluded to represent the general picture of the area.

seem to be predicted at the depth of 2 m and 8 m by the EL model. There are still other local failures between 9 and 13 m, but the deformation level is limited compared to the ones mentioned above. It can be concluded that not only the ground response predicted by two models is quite distinct, but also the seismic actions alter

*Geotechnical Engineering - Advances in Soil Mechanics and Foundation Engineering*

The loss of soil stiffness can be caused by either insufficient bearing capacity or liquefaction triggered by the excess pore pressure generated under cyclic loading. Thus, it is beneficial to see the pore pressure generation if it was the origin of excessive deformations during seismic excitations that only NL models are capable to investigate. Excess pore pressure ratio which is a value between 0 and 1 is one vital criterion in evaluation of the liquefaction. ru of 1 means that the soil layer is under risk to liquefy for such type of loading. **Figure 13** shows the excess pore

One clear remark can be pointed out here that the liquefaction occurring at various depths in different logs considering all three base motions sets out the possible explanation of the unreasonable deformation indicated in the previous figure. The NL model did not anticipate liquefaction triggering for the shallow depths which include some fine content, whereas there are at least nine points that soil layers mainly composed by silty sands are expected to lose their strength due to

liquefaction for such sequence of events. The ru value is greater than 0.8 for approximately 45 points meaning that even if there is no liquefaction initiated yet, soil would possibly lose its strength at these depths. Therefore, soil mitigation would

After liquefaction assessment of the soil profiles, the next step is to evaluate the site response in terms of nonlinear and equivalent linear approaches. Fifteen different analyses were conducted for each, and the peak spectral accelerations on the surface are presented in **Figures 14** and **15**. As mentioned before, building codes

the seismic behavior extremely along the soil column.

pressure ratios projected by the NL models for all the logs.

be needed for possible construction in the area.

*Excess pore pressure ratios estimated from 15 NL analysis.*

**Figure 13.**

**136**

**3.4 Interpretation of the analyses**

In terms of the performance of the building codes with regard to expressing the site specifics, all of them are on the safer side (EC8 is the most conservative one and the IBC is the least) following the similar flow of spectral acceleration of the observed data from the nonlinear analyses. IBC's spectra would be considered as the most economical one compared to the others with missing some longer period content, and it would possibly be a better representation if the flat part of the spectra shifted on the right (by using larger corner periods). About the TEC2018, the spectra at higher period were estimated better, but the content from small to medium periods would not be cost-effective in the seismic design. As seen in the figure, EC8 envelope is not practical at all, and such design would be the most expensive one among three building codes.

seismic behavior in liquefied zones. The in situ data was taken from Duzce area which was liquefied in 1999 in Adapazari earthquake, and the nonlinear analyses helped reproduce the liquefaction triggering at 45 points in different depths for varying earthquake scenarios. Since the equivalent linear approach cannot perform the liquefaction triggering, it alters the transmission of the shear waves affecting the spectral accelerations and the maximum shear strains throughout the soil layers. It was noted that the PSA was estimated as three times larger at shallow depths with corresponding shear strains as an example. Consequently, it should not be used for ground response in liquefaction-prone areas not to misinterpret the dynamic behavior of soils. Compared to three building codes to project the site response, the International Building Code is the most effective one to match the nonlinear analy-

*Estimation of Excess Pore Pressure Generation and Nonlinear Site Response of Liquefied Areas*

*DOI: http://dx.doi.org/10.5772/intechopen.88682*

Finally, it can be summarized that frequency content is highly effective on excess pore pressure buildup; the variation of its impact at different stress levels should not be disregarded in dynamic triaxial testing; and granting all this complexity of the nonlinear site-specific analyses, designers must be encouraged to run nonlinear analyses to model soil behavior better for safer superstructures, and the building regulations should be improved in terms of liquefaction susceptibility.

sis results.

**Author details**

**139**

Kamil Bekir Afacan

Eskisehir Osmangazi University, Eskisehir, Turkey

\*Address all correspondence to: kafacan@ogu.edu.tr

provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Equivalent linear analyses are easy to perform with less parameter needed to constitute a site and widely used in the practice and literature. However, one should be cautious to use this approach in such cases. Since this frequency-based analytical approach is not suited for liquefaction analyses, the results may canalize the engineer to an inappropriate design. Although the details of the results obtained from the equivalent linear analyses will not be covered here, it is important to call attention to the differences. The spectral acceleration of 15 analyses is presented in **Figure 15** with their corresponding envelopes suggested by building codes.

As seen in the figure, the soil amplified the base motion and lengthened it to the higher periods for these earthquake scenarios. None of the building codes were capable of expressing the data very well, neither the peak spectral acceleration nor the corner frequencies. However, it should be underlined here that it is not the performance of the building codes, but instead it is the analytical approach of the equivalent linear analysis lacking to model the excess pore pressure generation and the damping behavior of the soil. Thus, nonlinear site-specific response in liquefied areas is crucial in the design, and the concept of liquefaction susceptibility should be explained more clearly in the building codes.

#### **4. Conclusions**

Pore pressure generation under cyclic loading is an important phenomenon for liquefaction triggering, and the number of cycles and/or duration to liquefy soil is affected a lot by the frequency content of the loading. Three different stress levels were tested at varying loading patterns, and it can be concluded that harmonic loading to estimate the liquefaction initiation is not that reliable in order to evaluate the dynamic behavior of sands. Regarding the number of cycles, it took almost 1848 cycles to liquefy the sand samples which is at least two times later than Type 5 loading, and this value goes up to 5.5 times at most for Type 6 at CSR = 0.15. The same comparison at CSR level of 0.20 shows that the shortest time to trigger liquefaction is 130 seconds, whereas it lasted 1110 seconds for harmonic loading being almost nine times later. Even among the irregular loading types, the difference was calculated as 2.5–3 times in terms of the number of cycles for soil to lose its stability at different stress levels. Therefore, frequency content and its link with the CSR level are of great importance on excess pore pressure buildup. With reference to the stress-based models in the literature to estimate the recorded data in the laboratory, the mentioned models were not that distinct to predict the data, and it was noted that the best prediction was noticed at the medium stress level (CSR = 0.2). Moreover, the model proposed by Baziar [16] had a better job presenting the test data than the other two.

Though nonlinear site response is a bit complicated to gather enough information to simulate the ground response, it is the most appropriate way to model the

### *Estimation of Excess Pore Pressure Generation and Nonlinear Site Response of Liquefied Areas DOI: http://dx.doi.org/10.5772/intechopen.88682*

seismic behavior in liquefied zones. The in situ data was taken from Duzce area which was liquefied in 1999 in Adapazari earthquake, and the nonlinear analyses helped reproduce the liquefaction triggering at 45 points in different depths for varying earthquake scenarios. Since the equivalent linear approach cannot perform the liquefaction triggering, it alters the transmission of the shear waves affecting the spectral accelerations and the maximum shear strains throughout the soil layers. It was noted that the PSA was estimated as three times larger at shallow depths with corresponding shear strains as an example. Consequently, it should not be used for ground response in liquefaction-prone areas not to misinterpret the dynamic behavior of soils. Compared to three building codes to project the site response, the International Building Code is the most effective one to match the nonlinear analysis results.

Finally, it can be summarized that frequency content is highly effective on excess pore pressure buildup; the variation of its impact at different stress levels should not be disregarded in dynamic triaxial testing; and granting all this complexity of the nonlinear site-specific analyses, designers must be encouraged to run nonlinear analyses to model soil behavior better for safer superstructures, and the building regulations should be improved in terms of liquefaction susceptibility.
