LIQUEFACTION

Definition

The condition, in which the soil has little or no shearing strength and will flow as a liquid is known as liquefaction.

Mechanism of liquefaction

strength of saturated cohesionless soils depends upon the effective stress acting between particles. When external forces cause the pore volume of a cohesionless soil to reduce the amount V, pore water pressures are increased during the time required to drain a volume V of water from the soil element. 

Consequently, pore pressure increases depend upon the time rate of change in pore volume and the drainage conditions (permeability and available drainage paths). When conditions permit the pore pressure, u, to build up to a value equal to the total stress, sn, on the failure plane, the shear strength is reduced to near zero and the mixture of soil grains and water behaves as a liquid.

Consequence of liquefaction

Liquefaction or flow failure of sands involves a substantial loss of shearing strength for a sufficient length of time that large deformations of soil masses occur by flow as a heavy liquid. Stucture sink into soil or suffer excessive settlement, resting on such soil without considering liquefaction.

Liquefaction due to seismic activity

Soil deposits that have a history of serious liquefaction problems during earthquakes include alluvial sand, aeolian sands and silts, beach sands, reclaimed land, and hydraulic fills. During initial field investigations, observations that suggest possible liquefaction problems in seismic areas include low penetration resistance; artesian heads or excess pore pressures; persistent inability to retain granular soils in sampling tubes; and any clean, fine, uniform sand below the groundwater table. The liquefaction potential of such soils for structures in seismic areas should be addressed unless they meet one of the criteria in Table below . In the event that none of the criteria is met and a more favorable site cannot be located, the material in question should be removed, remedial treatment applied , or a detailed study and analysis should be conducted to determine if liquefaction will occur.

Table - Criteria for Excluding Need for Detailed Liquefaction Analyses

1. CL, CH, SC, or GC soils.

2. GW or GP soils or materials consisting of cobbles, boulders, uniform rock fill, which have free-draining boundaries that are large enough to preclude the development of excess pore pressures.

3. SP, SW, or SM soils which have average relative density equal to or greater than 85 percent, provided that the minimum relative density is not less than 80 percent.

4. ML or SM soils in which the dry density is equal to or greater than 95 percent of the modified Proctor (CE 55) density.

5. Soils of pre-Holocene age, with natural overconsolidation ratio equal to or greater than 16 and with relative density greater than 70 percent.

6. Soils located above the highest potential groundwater table.

7. Sands in which the "N" value is greater than three times the depth in feet, or greater than 75; provided that 75

percent of the values meet this criterion, that the minimum "N" value is not less than one times the depth in feet, that there are no consistent patterns of low values in definable zones or layers, and that the maximum particle size is not greater than 1 in. Large gravel particles may affect "N" values so that the results of the SPT are not reliable.

8. Soils in which the shear wave velocity is equal to or greater than 2000 fps. Geophysical survey data and site geology should be reviewed in detail to verify that the possibility of included zones of low velocity is precluded.

9. Soils that, in undrained cyclic triaxial tests, under isotropically consolidated, stress-controlled conditions, and with cyclic stress ratios equal to or greater than 0.45, reach 50 cycles or more with peak-to-peak cyclic strains not greater than 5 percent; provided that methods of specimen preparation and testing conform to specified guidelines.

Fault Geometry in Relation to Earthquake Movement

Dear reader we will discuss about fault geometry to make smooth understanding topics of fault activity, fault movement and also moment magnitude scale formeasuring earthquake.

Specific geologic notationsare used to define orientation of fault with some approximation. A large fault have surface of much irregularity (usually) but are assumed that the surface is plane over small distance.

Two terms are used to define orientation of a fault plane

-Strike

-Dip

Strike:

This is a horizontal line that is generated by intersection of horizontal plane with fault plane.

Dip:

Dip angle is used to define slope of fault plane (obviously downward) and is defined as angle created by horizontal plane with fault plane which is measured normal to strike.

Azimuth of strike is used to define orientation of fault relative to north, say N 60° E.

When a fault have vertical surface the dip angle will be 90°.

What is Fault Move ment? what is Dipslip Movement?

Dear reader we have already discussed about movement of fault in our previous post. Here actually we will define type of fault movement. Fault movement can geologically reduced to two components i.e. in strike and dip direction. The movements are:

a. Dip slip movement

b. Strike slip movement

But some movements have components in both directions; however only one direction of movement is generally predominant.

 

In this post We will discuss about dip-slip fault movement. From the term dip-slip movement, we can easily realize that movement is primarily occurred along dip direction or can be said normal to strike. Different types of dip-slip movement also found depending on dip angle and direction of movement of respective fault. They are:

a. Normal faults

b.Reverse fault

c.Thrust faults

a. Normal faults:

These types of movement are happened when dip-slip movement have horizontal component of extensional type. in this regard we are introducing two term

-Hang wall

-Foot wall

Hang wall:

The materials remain above inclined fault plane is called hang wall as shown in figure below:

Foot wall:

The materials beneath fault plane is called foot wall as shown in figure above.

So when hang wall moves down ward with respect to foot wall, normal fault movement is occurred which is usually observed in conjunction with tensile stress within the earth crust producing lengthening of crust in horizontal direction.

b.Reverse faulting:

When the movement have compressional horizontal movement, reverse faults are said to be occurred. In this movement hang wall move upward with respect to foot wall and horizontal shortening (crust) is observed.

c. Thrust faulting:

In this movement also hang wall move upward with respect to foot wall but difference is small angle of dip of fault plane. The example is European Alps which is thrust structure indicating potential of producing large movement.

Basic of Earthquake Loading on Structure

According to theory of plate tectonics, a constant motion of earth’s crust produces collisions between floating plates. In this colliding boundary, the movement is temporary prevented by frictional resistance.

The stresses built up in this way are released when sudden slippage is occurred under elastic rebound; sometimes rock may get fractured. The strain energy released in this way may produce fractures in the upper crust and in certain direction fault may occur.

Some fraction of energy (as shockwave) is propagated in all directions. The wave motion stated above is calledearthquake. It was found that the faults that was formed due to past earthquakes are vulnerable to suffer future disturbance. Now we will learn some information about recording of earthquake data.

Within United States monitoring grids were created across seismic active regions by installing stations equipped with automatic sensing equipment like tiltmeters and seismographs. The equipment can record seismic ground motion which facilitates us to locate focus and epicenter of earthquake. They also can record point of highest intensity of seismic wave.

An interesting matter is that, these peak intensities are found at or near epicenter and in most cases found around nearby fault. Now as structural engineer we need that portion of motions that are affect structures. A strong motion accelerograph can record such intensities of the ground movement.

This can record components of ground motion (acceleration) in three direction, two in horizontal and on in vertical direction i.e. North-South, East-West and vertical as usual. The acceleration is expressed as a percentage of gravity G.

Depending on seismic data collected in this way, structural design is done. The objective is to design a building that can survive earthquake loading without collapse but not to make earthquake proof building. According to uniform building code probability of earthquake hazard of any region is calculated.

Seismic ground motion propagates through earth crust and contact point between structures and earth is foundation, so ground motion acts on structure by moving foundation back-forth. Inertia forces generated by mass structure resist earthquake motion throughout whole structure. Lateral inertial on person traveling in a vehicle under deceleration is the similar example of this phenomenon. Inertia forces in vertical direction are often ignored as structure is designed for static vertical load.

What is Response Spectrum in Earthquake Analysis?

We know earthquake forces shake building foundation back-forth which is finally resisted by inertia forces produced throughout a structure. The vertical inertia forces are ignored as buildings are usually designed for vertical static loading. Vertical forces may critical when building is located very close to point of fault rupture.

So we will consider lateral forces here. Horizontal inertial forces depends on

-Building mass

-Ground acceleration

-Nature/type of structure

Now the ground acceleration transmitted to structure through foundation. But does full ground acceleration is passed through foundation? Actually the both acceleration are never found same, as building structure is not rigid i.e. the structure and its foundation are not rigid. If the rigidity criteria is satisfied both would have same acceleration i.e. Newton’s Law (F=M X A) is Valid.

Where

F=Horizontal inertia force

M=Mass of building

A=Ground acceleration.

All structures have flexibility to some extent. When a structure suffers less deformation (i.e. it absorb some energy), the inertia force would be less than MXA (product of mass and ground acceleration).

When a structure is very flexible (has natural period/frequency near ground waves), it would be subjected to more force than MA under repeated ground motions.

Thus we have learnt that lateral force acting on a structure not only depend on acceleration of ground motion but also type of building structure i.e. response of the building and foundation too.

The relationship between structure and ground acceleration is expressed as response spectrum. To understand response spectrum let us consider oscillators of different periods. Oscillators of cantilever pendulums of different heights are taken which are mounted from a base that can be shaken back-forth (cyclic motion) to simulate seismic waves recorded in previous earthquakes (probable earthquake to be happened).

Here buildings are represented by these oscillators. For simplification single degree of freedom is considered and various periods represent for different structural type. Under cyclic motion maximum response Vs natural period of vibration is plotted as shown in above figure.

Maximum response under oscillation can be represented as a function of

-Acceleration

-Velocity

-Deflection

-Force action

Again every structural system contains inherently its damping mechanisms. This damping obviously reduce large amount of response especially when repetitive motion continues.

Now we made an assumption to consider actual building as oscillator. The response curve found in this way can be applied depending on how nearly behavior of these simple oscillators represents complex behavior of a building.

The response spectrum is the simple way to determine maximum response under seismic ground motion. The uniform building code provides us to use seismic co-efficient that responds nearly in shape to response spectra produced from some/probable earthquake and also reflects the consideration of having longer period of taller building (as taller pendulum) and shorter period for short building. Thus shorter and stiffer building will be subjected to greater inertia forces than taller building.