Underground effects of earthquakes

When an earthquake occurs in a tunnel, there are a variety of factors that can be studied to determine how the underground structure will respond. These factors include the surface and pseudo-static motion, ground motion along the tunnel axis, and liquefaction. It is important to understand how the ground moves so that you can protect yourself and your neighbors from the effects of an earthquake.

Liquefaction

Liquefaction is a secondary hazard that occurs during earthquakes. It is caused by shear stress reversals and has a wide range of effects on the ground. A liquefaction event can topple buildings, destroy roads, and cause fires. Luckily, scientists are working on ways to prevent liquefaction.

During an earthquake, the ground loses its bearing pressure and can no longer support the weight of buried structures. When this happens, things can float away, get stuck in new positions, and even become buoyant.

While the term "liquefaction" refers to soil that changes from solid to liquid, there are actually several forms of liquefaction, each with its own set of potential hazards. Some of the most common forms of liquefaction are due to the influx of water into the pore space of the soil. This can cause the ground to behave like quicksand.

Another form of liquefaction, known as lateral spreading, is caused by cyclic loading of the soil. Sediment is piled on top of each other as the ground shifts. After a cyclic load is lifted, the lateral displacement will stop.

Interestingly enough, the amount of cyclic loading required to trigger a liquefaction event is a lot less than the amount of shear stress generated by the initial jolt. The magnitude of the cyclic shear stress ratio (CSR) needed to initiate a liquefaction is a function of the initial effective confining pressure (ECP), the duration of the shaking, the density of the soil, the degree of saturation, and the strength of the triggering event.

Landslides

When earthquakes occur, they produce faults that influence the underground fluid flow. This fluid may resurface as springs or it can percolate further into the ground. It can also cause the formation of mud flows and liquefaction.

The underground effects of earthquakes and land slides are important to understand. They have the potential to be extremely damaging. Land slides have been known to take away hilltop homes and block roadways. These types of events can cause significant economic losses.

In the United States, landslides are responsible for more than 25 fatalities annually. In addition to causing damage to structures, the land can also present real dangers to humans. Some landslides can contain hot volcanic ash and toxic gases from deep within the Earth.

Landslides can be highly dangerous due to their erratic movement. They can be triggered by strong ground motions, such as tremors or heavy rainfall. If they reach a property, they can cause damage and may even bury people.

While earthquakes are the most common cause of landslides, other factors can also contribute to their formation. These include deforestation, stream erosion, land modification, and human activity.

Landslides are caused by the movement of debris, soil, and rock down a slope. Because of their rapid speed, they can destroy roads and houses. For this reason, it is essential to take precautions to protect your property from landslides.

Inertia

Inertia underground effects of earthquakes are often overlooked, but understanding them can help minimize your risks. For example, in a strong earthquake, liquefaction and ground deformations can lead to total collapse of structures.

The most notable underground effect of an earthquake is liquefaction. This is when damp soil loses its strength, causing the surrounding tunnel to shift. Liquefaction can also cause the soil to collapse.

However, it is not all bad. Aside from the hazard of liquefaction, earthquakes can also produce waves known as seismic waves. These waves are powerful and can damage and even destroy buildings.

One of the most important aspects of earthquakes is their direct shaking effect. This happens when the ground shakes randomly in all three directions. The ground tries to return to its original position after the building has started moving.

Another hazard is the tsunami. Tsunamis can cause destruction and landslides. Fortunately, they are often small, but they can cause serious disruptions.

Aside from liquefaction and tsunamis, there are other hazard-related underground effects of earthquakes. These include shearing and axially propagating waves. If these are not properly managed, you can end up with a huge mess.

There are several methods used to calculate the effects of inertia. These methods vary in complexity. Some of them are pseudo-static, and others involve time-history analysis.

Surface seismic motion

Seismic waves have the potential to impact all aspects of our lives. They can disrupt water systems, communication networks, transportation, and waste disposal. Their effects are sometimes very intense. When they reach far distances, damage may be severe enough to threaten built infrastructure.

There are two main types of seismic waves that travel through the ground: compressional (P) waves and body waves. P-waves are the first recorded by seismographs, and they travel at a high speed. During a P-wave, the ground may break open and liquids and gases can percolate into the ground.

Surface waves, on the other hand, are more complex in motion. These waves radiate from the epicenter and cause vertical movement at the surface. The wave's direction depends on the site conditions and the characteristics of the source.

In addition, lateral spreads can occur. This occurs when the ground shakes in response to the earthquake and it usually forms scarps or fissures. Lateral spreads can move as much as 100 feet.

Research on these effects has centered on the wave and rupture dynamics of the earthquake. Understanding these processes can help us to recognize when and how these phenomena happen. It will also identify the unique failure patterns that can be associated with an earthquake.

Ground motion along the tunnel axis

Underground tunnels experience ground motion during earthquakes. This motion can be destructive. In particular, it can lead to direct losses and extensive downtime. Therefore, a proper seismic design of underground tunnels must consider the effects of ground motions.

Various studies have investigated the effects of seismic motion on the performance of tunnels. Some studies used numerical analyses and others employed a probabilistic approach.

Seismic response of underground tunnels is determined by three factors: the three-dimensional geometries of the structure, the interaction of the structure with the surrounding soil, and the seismic loading conditions. For example, a highway tunnel or a pipeline may be more susceptible to flotation, while a railway tunnel is less prone to damage.

A simplified model, which extends the model developed for buildings, is introduced. It uses a pair of coupled Bernoulli and shear beams and Winkler-type springs. The model has been validated for two different earthquakes.

The results from this study show that the ground motions along the tunnel axis are asynchronous. While synchronous shaking produces large amplitudes of ground deformations, asynchronous shaking has smaller amplitudes. Compared with the synchronous ground motion, the asynchronous ground motion can lead to higher internal forces.

Pseudo-static analysis

Pseudostatic analysis is a method to evaluate the seismic response of underground structures. The study is performed by averaging free-field deformations of the ground. Its purpose is to provide an estimate of the anticipated deformations of the structure. However, the method is not perfect as it does not account for the effects of the surrounding ground and the structural supports.

Free-field displacement time histories are first computed at selected locations along the tunnel length. They must account for the effects of incoherence and wave passage phase shift. These time histories can be synthetic or based on real recordings of similar earthquakes.

Another method is to use a probabilistic approach. This is particularly useful in areas with active faults, where the ground motions may be amplified by wavelengths between one and four times the tunnel diameter.

A probabilistic approach has been used in many recent transportation tunnel projects. For example, the Boston Central Artery Third Harbor Tunnels used this method. The Seattle Metro has also adopted this method.

A second, more sophisticated method is to carry out advanced dynamic analyses. This is done through the use of a computer program. In this case, the LINOS software is used.

Dynamic analysis

Underground facilities play a key role in modern society. They are used for a wide variety of applications. These facilities are subject to strong earthquake motions. The design and operation of these facilities is based on their ability to withstand the effects of seismic loading. Historically, they have experienced lower damage rates than surface structures. However, they have also experienced significant damage during recent large earthquakes.

Dynamic analysis refers to the process of calculating the structural response of an underground structure to ground motion. A structural model is subjected to ground motion records and the results are used to determine the failure characteristics of the structure.

In order to perform a dynamic analysis, a number of methods are available. Among these, a probabilistic approach is commonly used. It incorporates uncertainties in the source-to-site distance, the rate of recurrence and the magnitude of the earthquake. This approach has been used in many transportation tunnel projects.

Another method is the incremental dynamic analysis (IDA). IDA is a parametric analysis technique. With IDA, a structural model is subjected to ground motion data and the amplitude of the ground motion records is adjusted. IDA produces curves that represent the structural response parameterized versus intensity level.