**2.3 Shear banding table test results**

**Figure 8** shows a building model placed on a shear banding table. When the left side of the shear banding table was lifted up obliquely relative to the right, the failure pattern of the building model was similar to that of the Guangfu Junior High School building in Taichung, Taiwan during the 921 Jiji earthquake (**Figure 4b**). Therefore, it is known that in the failure process of the building model during the earthquake, the building model first changed to have the seismic conditions that resulted in structure failure caused by shear banding. With the increase in the amount of shear banding, the degree of inability of the building model increases, and finally fails.

**Figure 9a** shows a bridge model placed on a shear banding table. When the left side of the shear banding table was tilted and uplifted relative to the right side, the bridge model also underwent tilting and uplift. The failure pattern was similar to that of the Wuxi Bridge in Taichung, Taiwan due to the 921 Jiji earthquake as shown in **Figure 9b**. Therefore, it is known that during tectonic earthquakes, when a bridge is deformed by tilting and uplift, the seismic conditions shift from those that do not result in failure to those that do result in failure due to shear banding.

**Figure 8.** *The shear banding table test results of a building model [10].*

*Seismic Conditions Required to Cause Structural Failures in Tectonic Earthquakes DOI: http://dx.doi.org/10.5772/intechopen.108719*

**Figure 9.**

*Comparison of the tilting uplift failure of a model bridge and a real bridge [10]: (a) model bridge after the shear banding table test; (b) Wuxi Bridge in Taichung, Taiwan after the 921 Jiji earthquake.*

**Figure 10** shows another example of bridge failure by further uplift and tilting due to shear banding. **Figure 10a** shows a bridge model placed on the shear banding table. When the right side of the shear banding table was further tilted and uplifted relative to the left side, the failure pattern of the falling bridge model was similar to that of the Shiwei Bridge in Taichung, Taiwan during the 921 Jiji earthquake as shown in **Figure 10b**. Therefore, before the bridge fell during the tectonic earthquake, the initial seismic conditions that would not have resulted in failure are

#### *Natural Hazards – New Insights*

#### **Figure 10.**

*Comparison of falling bridge phenomena [10]: (a) model bridge after a shear banding test; (b) the Shiwei Bridge in Taichung, Taiwan after the 921 Jiji earthquake.*

changed to those that would result in failure due to shear banding. Ultimately, the bridge collapsed after the amount of shear banding had increased significantly.

**Figure 11a** shows a model of a weir placed on a shear banding table. When the right side of the shear banding table was tilted and uplifted relative to the left side, the failure pattern of the weir model was similar to that of the Shigang weir in Taichung, Taiwan during the 921 Jiji earthquake shown in **Figure 11b**. Therefore, before the weir failed during the tectonic earthquake, the initial seismic conditions that would not have resulted in weir failure were changed to those that would result in failure because of shear banding. Ultimately, the weir failed as the amount of shear banding was greatly increased.

*Seismic Conditions Required to Cause Structural Failures in Tectonic Earthquakes DOI: http://dx.doi.org/10.5772/intechopen.108719*

**Figure 11.**

*Comparison of the collapse of a model weir and a real weir [10]: (a) the model weir after the shear banding table test; (b) the weir in Shigang, Taichung, Taiwan after the 921 Jiji earthquake.*

### **3. Seismic failure of a caisson pier**

Since the traditional soil liquefaction safety factor, *FSL*, is defined such that the ratio of the cyclic stress ratio of liquefaction resistance (CSRRL) to the period stress ratio of an earthquake (CSRE) be less than 1.0 [11], from the point of view of plastic mechanics this actually indicates soil yield, and it is thus quite easy to misjudge the cause of soil yield as soil liquefaction. Taking the 921 Jiji earthquake as an example, the rear line region of the caisson pier in Taichung Port, Taiwan (**Figure 12**) showed three different types of failure: subsidence, a pothole, and the spouting of silt, sand, and gravel.

#### **Figure 12.** *Different types of seismic failures at the rear line region of the caisson pier of Taichung port [12].*

After the 921 Jiji earthquake, most traditional scholars believed that all three types of failure were caused by soil liquefaction. The reason they provided was that under the ground vibration of a tectonic earthquake and the densification process of the saturated loose sand below the groundwater table, the effective stress would have been reduced to zero or less than zero with the increase in the excess pore water pressure; therefore, soil liquefaction, by definition, would have occurred. Thus, it is believed by traditional scholars that the significant subsidence, the large pothole, and the ejection of silt, sand, and gravel shown in **Figure 12** were all caused by soil liquefaction.

Only after identifying the major causes of the three different types of failures that occurred at Taichung Port during the 921 Jiji earthquake can we develop effective suppression methods to prevent such failures from taking place in the rear line region of other caisson piers.

In 2017, Hsu *et al*. [13] presented evidence that soil liquefaction occurs only in shear banding or shear texturing zones induced by the plastic strain softening of dense soil, but not in the yield failure of loose soil. In addition, the compacted soil of the rear line region of the caisson pier had already passed the field density quality test. According to the contract of the entrusted project, the degree of compaction was greater than 90%, and the corresponding relative density was greater than 70%. Therefore, the compacted soil was dense; however, the compacted soil of the rear line region of the caisson pier was misjudged as loose by conventional scholars in Taiwan.

**Figure 13** shows a schematic profile of a caisson pier. It can be seen that the caisson pier is placed directly on the cobble base of the seabed. At low tide and in the absence of embedded depth, *Df* when depth *h*2 from the seawater table to the seabed is less than depth *h*1 from the groundwater table of the rear line region to the seabed and the hydraulic gradient *i* of the water exit point E (shown in **Figure 13**) is greater than the critical hydraulic gradient *ic*, then some sandy gravel backfill in the rear line region of the caisson pier flows out to the seabed below the seawater table due to piping failure (as shown schematically in **Figure 14**), and then large piping holes appear such as those shown in **Figures 12** and **14**, respectively.

*Seismic Conditions Required to Cause Structural Failures in Tectonic Earthquakes DOI: http://dx.doi.org/10.5772/intechopen.108719*

**Figure 13.** *Schematic diagram of the caisson pier profile [14].*

**Figure 14.**

*Schematic diagram of the propagation of the arching effect and the appearance of a piping hole in the rear line region of a caisson pier (reproduced from [12]).*

Furthermore, **Figure 15a** shows the test model of the caisson pier with very dense backfill in its rear line region. **Figure 15b** shows that when the caisson pier is turned seawards, its rear line region appears to have shear textures with different strikes, resulting in substantial subsidence. The shear textures shown in **Figure 15c** include the principal shear D, thrust shear P, and Riedel shear R. During tectonic earthquakes, when shear textures of different strikes are dislocated, high excess pore water pressure will locally appear in the shear textures, and so the groundwater in the shear

#### **Figure 15.**

*Test results of shear textures and subsidence induced by the inclined caisson pier [12]: (a) before test; (b) after test; (c) the induced shear textures and uneven subsidence.*

textures will entrain the brittle fractured silt, sand, and gravel and eject upward (see **Figure 12**) along the outlet tunnel formed by the pore space of the shear texture.

The test results in **Figure 15** show that the subsidence shown in **Figure 12** was caused by the caisson pier turning seawards, rather than by soil liquefaction as claimed in the past.

Hsu *et al*. [13] proposed the three constituent elements that must exist in order to induce soil liquefaction based on real-world data: (1) local shear banding or shear texturing during tectonic earthquakes; (2) high excess pore water pressure localized in the shear band or in the shear texture; (3) groundwater entrained silt, sand, and/or gravel ejection upward along the outlet tunnel formed by the pore space of the brittle fractured shear band or shear texture. For the ejected silt, sand, and gravel shown in **Figure 12**, the cause was soil liquefaction since the three elements required for soil liquefaction clearly existed.

### **4. Methods of earthquake-resistance and building reinforcement**

Traditional building earthquake-resistance and reinforcement methods include the installation of vibration isolation pads, the installation of dampers, and increasing the stiffness of superstructural elements such as columns, beams, panels, and walls,

#### *Seismic Conditions Required to Cause Structural Failures in Tectonic Earthquakes DOI: http://dx.doi.org/10.5772/intechopen.108719*

thereby inducing vibration isolation, vibration reduction, and vibration resistance. Therefore, traditional earthquake-resistance and reinforcement methods only improve the vibration fortification level of a building under the seismic conditions required to prevent structural failure, but do not have an impact on the conditions in which structural failure does in fact occur due to shear banding during tectonic earthquakes. Therefore, buildings will still fail under the shear banding effect in future tectonic earthquakes.

The school building earthquake-resistance and reinforcement methods provided by the National Center for Research on Earthquake Engineering of Taiwan after the 921 Jiji earthquake included the reinforced concrete (RC) jacketing retrofit method, the RC wing wall retrofit method, the shear wall retrofit method, and the composite column retrofit method. It turns out that these methods can only improve the fortification level of a building against seismic vibration conditions required to prevent structural failure, but do not protect the building from failing under the shear banding effect of future tectonic earthquakes.

Since the failure of buildings during tectonic earthquakes is mainly caused by the shear band or shear texture extending into the area surrounding the shear failure plane induced in the foundation soil under the ultimate load, when the safety factor of the earthquake bearing capacity of a foundation is less than 1.0, earthquake subsidence will be induced; after the foundation loses its stability, an asymmetrical general shear failure plane will be generated, which will lead to the failure of the foundation and building [15]. Therefore, in order to effectively prevent the collapse of buildings during tectonic earthquakes, earthquake-resistance and reinforcement methods need to restrain the shear band or the shear texture that is induced by the ultimate load of a tectonic earthquake from extending into the area surrounding the shear failure plane in the foundation soil (see **Figure 16**). Only in this way can it be ensured that buildings complying with the seismic design specifications for ground vibration fortification will not be affected by shear banding or shear texturing, thereby ensuring the stability and safety of buildings during tectonic earthquakes.

#### **Figure 16.**

*Schematic diagram of effective structural earthquake-resistance and reinforcement methods for restraining the propagation of shear banding or shear texturing [16].*
