lecture22 dynamic soil properties part2

1

Dynamic Soil PropertiesPart - II

Lecture-22

2

Important Field tests for measuring dynamic properties of soils

Low strain testsSeismic Reflection TestSeismic Refraction TestSuspension Logging TestSteady-State-Vibration TestSpectral Analysis of Surface Waves (SASW) TestCross-Hole TestDown-/Up-hole Test

Large Strain TestsSeismic Cone Penetration TestStandard Penetration TestDilatometer TestPressuremeter Test

3

Suspension Logging Test

Source: wikipedia

4


The suspension PS logging is a recently developed tool for measurement of seismic wave velocity profiles.

A seismic source and two receivers are built in a single borehole probe. Compression (P) and shear (S) waves are generated by a seismic source that involves the use of a solenoid hammer. The solenoid hammer produces a pressure wave in the borehole fluid. This pressure wave converts into seismic body waves (P and S) at the borehole wall. The waves travel in a radial direction from the borehole wall. Receivers contain two-component geophones, one vertical to record P-waves, and one horizontal for recording of S-waves. The body waves are converted back to pressure waves in the borehole fluid and detected by the geophones. The source and the two receivers are connected with rubber-filter tubes to isolate vibration between them.

5


The Spacing between two receivers is usually one meter.

Advantages of the suspension PS logging are that it is not necessary to clamp the probe against the borehole wall, and because the wavelength of excited shear waves is much greater than the borehole diameter, shear excitation is almost independent of the borehole fluid. As such, geophones in the probe can record the behavior of the borehole wall without clamping the probe.

The other advantage of the suspension logging is accurate measurement of the shear wave velocity values and because the frequency of the shear wave generated by the source is generally higher than the other methods, wavelengths are shorter and propagation time measurements are more accurate.

6

Seismic Cross Hole Test

Source Receiver

7

Seismic Cross Hole Test

In this test, a source of seismic energy (mainly S-waves) is generated in or at the bottom of one borehole and the time for that energy to travel to another borehole through the soil layer is measured.

From the borehole spacing and travel time, the velocity of the seismic wave is computed. Both body waves P-waves, and S-waves can be utilized in this test.

At least two boreholes are required, one for the impulse and one or more for receivers.

The shear wave velocity is then used to compute the soil's shear modulus

8

Seismic Cross Hole Test: Things to remember

(1) Although a minimum of two boreholes is sufficient to perform the test, three or more boreholes improve the capabilities of the crosshole method.

(2) The energy source should be rich in shearing energy (S-waves) and poor in compressional energy (P-waves) such that the arrival of S-waves can be detected easily.

(3) Geophones in the receiver boreholes should have proper frequency response and be oriented in the direction of particle motion and the geophones should be in contact with soil.

(4) Travel time measurement of shear waves should be accurate.

9

Seismic Down hole/ Up hole Test

Source Receiver

Receiver Source

Up-hole test Down-hole test

10

DownholeTesting

Oscilloscope

Cased Borehole

x

TestDepthInterval

HorizontalVelocity

Transducers(GeophoneReceivers)

packer

PumpHorizontal Plank

with normal load

Shear Wave Velocity:Vs = R/t

z1z2

t

R12 = z1

2 + x2

R22 = z2

2 + x2

x

Hammer

Seismic Down holeTest

Source: http://geosystems.ce.gatech.edu/Faculty/Mayne/Research/misc/Downhole.jpg

11

Seismic Down hole/ Up hole Test

The wave pattern is measured twice, using a horizontally directed sledge hammer blow on a firmly embedded post, which is struck in a direction parallel to the ground surface at first, then struck 180 degrees out of phase a second time (in the opposite direction).

Reversing the direction of the energy blow, allows for the shear wave pattern to be recorded in the reverse direction while the compression wave pattern is essentially unchanged. In this manner, the shear wave patterns are distinguished from compression wave patterns.

However, in the up-hole test, it is more difficult to generate selected shear waves. P-waves tend to be predominant within the source generated as investigated by several researchers.

12

Surface Wave Tests

13

Rayleigh waves are recognized to be really useful in identification problems due to some remarkable aspects of this kind of waves:

the propagation involves only a limited depth of soil equal to about one wavelength;

two third of the total energy released by a point source is transmitted through Rayleigh waves;

they attenuate with distance less than other waves;

in layered sites Rayleigh waves present a dispersive behavior, i.e. their phase velocity is a function of frequency.

Features of Rayleigh waves

14

Steady state vibration test

Source: Kramer (1996)

15


The steady state surface wave technique does not require boreholes.

It is used to measure the shear modulus (G) of all types of soils.

In this test, an electromagnetic oscillator at high frequency (30 to 1000 cycles/second, cps) or a rotating mass type oscillator to produce low frequency vibrations (less than 30 cps) are used.

These surface vibrators generate Rayleigh R-waves, which at low strains have nearly the same velocity as the shear waves. The ground surface gets deformed due to the passage of surface waves.

16


The vibrator, working at frequency f, is placed on ground surface and one vertical receiver is also placed on ground surface. The receiver is moved away from the vibrator until they are in phase. The distance between any two adjacent receiver positions is assumed to be the wavelength at that particular frequency. From wavelength, phase velocity of surface waves is determined.

The stiffness of soil is linked to phase velocity. This simple inversion procedure works quite well for sites where the stiffness of soil increases gradually with depth, but it can lead to serious mistakes if the upper layers are stiffer than the deeper ones

17


The shear wave velocity is computed from the Rayleigh wave-length measured with receivers placed along the ground surface, and the frequency of vibration at the source using the following equation

VR = f lR

Where,

VR = Rayleigh wave phase velocity

f = Frequency of vibration

lR = Rayleigh wave length (measured in test)

Shear wave velocity is then estimated as:

VS ˜ 1.09 VR

18

Spectral analysis of surface waves (SASW) test

The purpose of the SASW test is to determine a detailed shear wave velocity profile working entirely from the ground surface.

The method involves using a series of successively longer source-receiver arrays to measure the propagation of Rayleigh waves over a wide range in wavelengths.

A vertical impact is applied at the ground surface generating transient Rayleigh R-waves. Two or more receivers placed at the surface, at known distances apart monitor the passage of these waves

The receivers or vibration transducers produce signals that are digitized and recorded by a dynamic signal analyzer, and each recorded time signal is transformed to the frequency domain using a Fast Fourier Transform (FFT) algorithm.

19


Source: http://www.geovision.com/sasw.php

20


The distance between the source and the first receiver is usually equal to that between the two receivers. Several records are made for each test configuration in order to obtain statistical data, and the test is performed once again with the receivers at the same place but reversing the source position, to eliminate the equipment internal phase and the influence of bedding inclination.

21


By changing receivers spacing and using different sources, characterised by the frequency range of the impulse, a broad range of thickness of soil can be explored. Short spacing (0.5-5 m) and weak impulse sources (such as a small hammer ) are used for shallow layers, while long distance (up to 60 m) and heavy impulse sources) are suitable to characterise soil at great depth.

Source: http://www.geovision.com/sasw.php

22


The calculations of VR and lR are performed for each applied frequency, and the results plotted in the form of a dispersion curve.

The dispersion curve is the characteristic or “signature” of a site. Using forward modeling or “inversion” analysis, the dispersion curves are used to determine the shear wave velocity profile of the site.

23


The phase difference (f(f)) between two signals is then determined for each frequency, and the travel time (t(f)) between receivers is obtained for each frequency as follows:

t(f) = f(f) / 2pf where,

f(f) = phase difference for a given frequency in radians

f = frequency in cycles per seconds (cps)

The velocity of R-waves is determined as:

VR = Dd / t(f) = lR f

where

Dd = distance between receivers.

lR = surface wave-length

24

SASW Test: Inversion of Dispersion curve

25

SASW Test: Advantages

The SASW method offers significant advantages.

In contrast to borehole measurements which are point estimates, SASW testing is a global measurement, that is, a much larger volume of the subsurface is sampled. The resulting profile is representative of the subsurface properties averaged over distances of up to several hundred feet.

The resolution in the near surface (top 25 ft) is typically greater than with other methods.

Because the SASW method is non-invasive and nondestructive, it is relatively easy to obtain the necessary permits for testing. At sites that are favorable for surface wave propagation, the SASW method allows appreciable cost and time savings.

26

Multichannel Analysis of Surface Waves: MASW

Source: http://www.masw.com

27


MASW a seismic method for near-surface (< 30 m) characterization of shear-wave velocity (Vs).

It utilizes the Rayleigh-type surface waves recorded by multiple receivers (geophones) placed at equal spacing and connected to a common recording device (seismograph).

Surface waves recorded as they propagate along the receiver line are analyzed through powerful and diverse multichannel processing techniques similar to a pattern-recognition approach.

28

Body waves and surface waves: MASW

Source: http://www.kgs.ku.edu/software/surfseis/masw.html

29


The main advantage of the multichannel approach is its capability to distinguish all of the noise waves from the signal wave (the fundamental mode of Rayleigh waves) through filtering.

Dispersion properties of all types of waves (both body and surface waves) are imaged through a wavefield-transformation method that directly converts the multichannel record into an image where a dispersion pattern is recognized in the transformed energy distribution.

Then, the necessary dispersion property of surface waves is extracted from a specific pattern. All other reflected/scattered waves and ambient noise are automatically removed during the transformation.

30



31


The entire procedure for MASW usually consists of three steps:

1. Acquiring multichannel records (or shot gathers),

2. Extracting the fundamental-mode dispersion curves (one curve from each record), and

3. Inverting these curves to obtain 1-D (depth) Vs profiles (one profile from one curve).

4. Constructing a 2-D (surface and depth) Vs map by placing each 1-D Vs profile at a surface location corresponding to the middle of the receiver line.

32

MASW: 3 Steps


33

MASW: Step4


34

Kramer (1996) Geotechnical Earthquake Engineering, Prentice Hall.

Braja M. Das, Ramana G.V. (2010) Principles of soil dynamics, C L Engineering.

Prakash, S. (1981) Soil Dynamics, McGraw-Hill.

Kearey P., Brooks, M. Hill I. (2002) An Introduction to Geophysical Exploration,

Wiley-Blackwell.

Burger H.R, Sheehan A.F., Jones, C.H. (2006)Introduction to Applied Geophysics:

Exploring the Shallow Subsurface, W. W. Norton & Company.

http://civil.iisc.ernet.in/~madhavi/ce202/lecture3.pdf

References

http://civil.iisc.ernet.in/~madhavi/ce202/lecture3.pdf

lecture22 dynamic soil properties part2

Documents

shear s waves

body waves pwaves

waves travel

pressure waves

seismic body waves p

borehole wall

shearing energy swaves

borehole fluid