CROSSHOLE/ DOWNHOLE SEISMIC TEST - Welcome to …igsdelhichapter.com/handout.pdf · · 2017-08-21Bore logs provide soil samples for soil type classification and laboratory testing

C e n g r s G e o t e c h n i c a P v t L t d

A ‐ 1 0 0 , S e c t o r 6 3 , N o i d a , U P ‐ 2 0 1 3 0 9

T e l : + 0 1 2 0 4 2 0 6 7 7 1

F a x : + 0 1 2 0 4 2 0 6 7 7 5

1 1 / 2 3 / 2 0 1 3

By Mr.Sorabh GuptaWorkshop cum Demo Session

CROSSHOLE/ DOWNHOLE SEISMIC TEST

C R O S S H O L E / D O W N H O L E S E I S M I C

WELCOME NOTE

On behalf of IGS Delhi Chapter and CENGRS, we have great pleasure in welcoming you to our Workshop on the use of latest seismic techniques in in‐situ ground characterization.

The Indian Geotechnical Society (IGS), Delhi Chapter is an active association of academicians and professions interested in, or involved with, Geotechnical Engineering. One of the main objectives of the Society is to promote healthy technical‐social interaction between the members, catalyze information exchange, and contribute to the growth of Geotechnical Engineering in the country. For those of us who are not yet members of IGS Delhi Chapter, we urge you to sign up for the same by downloading the membership form from the following website: http://igsdelhichapter.com/

The current elected Executive Committee, led by Dr. A.K. Nanda, has decided to conduct a series of Practical Workshops / Demonstrations in the term 2013‐14, aimed at encouraging hands‐on engineering and fun interactions. We are proud to be associated with the first such activity.

The Workshop is being conducted by the team at CENGRS, which is a leading consultancy firm in the field of Geotechnical Engineering, based in Delhi NCR. CENGRS has vast experience in the field of Geotechnical Engineering, with a repertoire of more than 4000 projects successfully executed across India and abroad since 1990. The team at CENGRS has conducted over 150 cross‐hole / down‐hole tests at various project locations up to a maximum depth of 100 m.

We hope that you shall enjoy the Workshop. Please feel free to ask questions and initiate discussions during the course of the presentations.

In case you require any further technical clarifications on the subject even after the Workshop, you may contact the undersigned at the contact details given below.

Warm Regards, Sorabh Gupta Sr. Project Engineer, CENGRS Executive Committee Member (2013‐14), IGS Delhi Chapter Cengrs House, A‐100, Sector‐63, Noida (U.P.)‐201309 t: +91 120 420 6771 | f: +91 120 420 6775 | m: +91 99108 61118 | [email protected]


TECHNICAL NOTE ON DOWN‐HOLE SEISMIC TESTING (DST)

1.0 Introduction

Construction of foundation systems for civil structures often requires detailed information of the site soil properties. Bore logs provide soil samples for soil type classification and laboratory testing to determine strength and consolidation parameters (among other properties) with respect to depth. A number of soil‐boring related in‐situ tests have also been correlated with soil strength (e.g. standard penetration test, cone penetration test), etc. However, in the interest of accuracy, it is certainly advantageous to measure an in‐situ soil property directly related to soil modulus. Shear wave velocity (Vs) has become the standard property from which in‐situ soil modulus is determined, due to its direct relationship with modulus via the soil mass density (which can be assumed with little error or easily measured from soil samples), as well as its relative ease of measurement, due to the advancement of seismic techniques.

A number of in‐situ test methods have been developed to measure Vs with respect to depth; such as Cross‐hole Seismic (CS), Down‐hole Seismic (DS), Spectral Analysis of Surface Waves (SASW), Multiple Impact of Surface Waves (MISW), etc. Traditionally, CS testing has been considered the most accurate method in determining Vs, because it is a direct measurement of the wave speed. SASW and MISW however, can be employed much more rapidly and economically because the methods are performed on the ground surface (unlike CS where at least two boreholes are required to perform the testing).

2.0 Benefits of DST in Geotechnical Engineering

The utilization of DST in estimating in‐situ wave velocities and the corresponding elastic soil parameters is of considerable benefit to the Geotechnical Engineer.

Some of the important geotechnical design problems which require the input of the elastic constants and absorption properties are:

• Static and dynamic soil analysis

• Pile and Footing Foundation Design for Vibrating Loads

o Calculate Constrained Modulus (M), Shear Modulus (G), and Poisson’s Ratio from local seismic velocities

o Calculate dynamic spring constants

• Liquefaction assessment

• Input for near‐surface seismological models

• Evaluation of soil improvement from blasting

• Assessment of the regulatory requirements such as those included in the Uniform Building Code.


3.0 Choice between CS and DS Seismic Testing

Crosshole Method Downhole Method

Constant Travel Paths Negligible Borehole Effects Receivers Properly Aligned for SV‐Waves High Signal‐to‐Noise Ratio at All Depths Detailed Profile Workable in Limited Space Accuracy Independent on the Measurement Depth

Two or More Boreholes Simple Borehole Source Predominantly P‐ and SV‐ Waves, but SH‐waves Also Possible Reversible Source Measure Borehole Verticality Detect Low‐Velocity Layers Possible Refraction Problems

Useable in Noisy Areas More Expensive

One Borehole No Verticality Measurements Simple Surface Source Minimum Refraction Problems Less Expensive

Generate P‐ and SH‐Waves Reversible Source Travel Path Increases with Depth

Possible Borehole Effects Control of Receiver Alignment Preferable Signal‐to‐Noise Ratio Decreases with Depth Detect Low‐Velocity Layers

More Average Profile Useable in Noisy Areas Workable in Limited Space Accuracy dependent on the measurement depth

4.0 Calculation of Dynamic Soil Parameters

The calculations of dynamic soil parameters are based on the relationships given in IS: 5249‐1992.

The Poisson’s Ratio is determined directly from the compression (P) wave and shear (S) wave data. It is expressed by the ratio of transverse strain to longitudinal strain.

Young’s Modulus E is the uniaxial stress‐strain ratio. Its dynamic value is expressed by the

following equation:

μμμ

ρ−

−+=

1)21()1(2VpE

where: ρ = mass density of soil = (γ/g) γ = bulk density of soil Vp = P‐wave velocity

μ = Poisson’s ratio


The shear modulus G is the stress‐strain ratio for simple shear. Its dynamic value is

obtained by the following:

2

)1(2VsEG ρ

μ=

+=

Coefficients of elastic uniform compression (cu), elastic uniform shear (cτ), elastic non‐

uniform compression (cφ) and the coefficient of elastic non‐uniform shear (cΨ) are given by the following relationships:

cu = 1.13 A1

1E

2 ×μ−

[A = Standard foundation area, taken as 10 m2]

cτ = 0.67 to 0.5 cu (for design purpose, cτ may be taken equal to 0.6 cu) cφ = 3.46 cτ cΨ = 1.5 cτ

5.0 Selection of Dynamic Parameters for Design

Since the cross‐hole seismic tests completed on site are low‐strain methods, the dynamic soil parameters computed here correspond to very low strains. However, actual design strains on the site are usually much higher (often in the range of 2~3%); particularly for earthquake conditions. Hence, the design dynamic parameters should be selected carefully as per the anticipated strain levels(1).

The selection of dynamic parameters must be done based on the project specifications, as well as the general guideline given in IS 5249:1992.

As per IS 5249:1992 (Clause 9.0), the value of dynamic shear modulus, G, is affected by

a number of parameters; out of which confining pressure, shear strain amplitude, and relative density are most important. In the range of strains associated with properly designed machine foundations, the effect of variation in strain on shear modulus is small and the values of G for design purposes may be determined from the in‐situ test values using the relation given below:

G1G

= (σ 01σ 0

)m

(1) Steven L. Kramer (1996), “Geotechnical Earthquake Engineering”, Pearson Education, Inc.,

Section 6.4, pp. 232-238.


where: G1 and G = Dynamic shear modulus for the prototype and from field test, respectively

σ01 and σ0 = Mean effective confining pressure, associated with the prototype foundation and

the in‐situ test, respectively, and m = Constant depending upon the type of soil, shape of grains, etc. Their value has

been found to vary from 0.3 to 0.7 and may on the average be taken as 0.5.

IS: 5249 states that in situations where high strain levels are associated (as in the case of

analysis for earthquake conditions), the effect of strain level shall be considered along with that of confining pressure. In such a case, the values of G from different field tests may first be reduced to the same confining pressure (expected below the footing) and their variation with strain levels may be studied to arrive at an appropriate value corresponding to the expected strain level.

The four parameters (Cu, Cτ , Cφ and CΨ) are highly dependent on strain levels. Keeping this in view, we suggest that a range of ± 20 percent of the above values be used for design. The higher values of these coefficients may be used for machines having an operating frequency higher than that of the machine‐foundation‐soil system. Similarly, the lower values of the coefficients may be used for machines operating at frequency that is lower than that of the system.


D O W N H O L E S E I S M I C T E S T

APPL I C AT I O N

The DOWNHOLE SEISMIC (DS) investigations are similar to CS investigations, but require only one borehole to provide shear and compressional velocity wave profiles. The DS method uses a hammer source at the surface to impact a wood plank and generate shear and compressional waves. This is typically accomplished by coupling a plank to the ground near the borehole and then impacting the plank in the vertical and horizontal directions. The energy from these impacts is then received by a pair of matching three component geophone receivers, which have been lowered downhole and are spaced 5 to 10 ft (1.5 to 3 m) apart.

Features:

■ DS method is cheaper than CS, since only one borehole is required for testing. ■ Real-time waveform display while testing ■ Thin layers, which are often invisible to surface methods, can be detected with CS/DS

investigations ■ Accuracy and resolution for CS/DS methods are constant for all test depths, whereas the

accuracy and resolution of the surface methods decreases with depth ■ Acquisition and processing software are easy to use yielding fast and accurate results ■ Triaxial geophones (receivers) can be oriented with inclinometer casing dummy probes

STANDARDS (1) IS: 13372 (Part 1): 1992, “Seismic Testing of Rock Mass‐ Code of Practice‐ Part 1: Within A Borehole”,

Bureau of Indian Standards, Delhi. (2) ASTM D7400‐ 08, “Standard Test Methods for Downhole Seismic Testing,” American Society for

Testing and Materials.

A customized P‐SV source provides the user with the most accurate and rapid method of generating impacts.


C R O S S H O L E S E I S M I C T E S T

APPL I C AT I O N CROSSHOLE SEISMIC (CS)

investigations are performed to provide information on dynamic soil and rock properties for earthquake design analyses for structures, liquefaction potential studies, site development, and dynamic machine foundation design. The investigation determines shear and compressional wave depth versus velocity profiles. Other parameters, such as Poisson's ratios and moduli, can be easily determined from the measured shear and compressional wave velocities. In addition, the material damping can be deter-mined from CS tests. The CS method is a downhole method for the determination of material properties of soil and rock. A source capable of generating shear and compressional waves is lowered in one of the boreholes, and a pair of matching three component geophone receivers are lowered to the same depth in two additional boreholes set at evenly spaced increments (typically10 and 20 feet from the source borehole) in a line, as shown in the figure above. The receivers are positioned on the side of the borehole casing to allow detection of the passage of shear and compressional waves.

Features: ■ CS method is the most accurate method for determining material properties of rock and soil

sites ■ Real-time waveform display while testing ■ P-SV source used in CS tests can impact in the vertical, transverse, and radial directions ■ Thin layers, which are often invisible to surface methods, can be detected with CS/DS

investigations ■ Accuracy and resolution for these methods are constant for all test depths, whereas the

accuracy and resolution of the surface methods decreases with depth ■ Correlation between CS and Spectral Analysis of Surface Waves (SASW) tests on soil sites

showed that the values from both tests typically compare within a 10-15% difference ■ Acquisition and processing software are easy to use yielding fast and accurate results ■ Sources and receivers can be oriented with inclinometer casing dummy probes

STANDARDS (1) IS: 13372 (Part 2): 1992, “Seismic Testing of Rock Mass‐ Code of Practice‐ Part 1: Between the

Boreholes”, Bureau of Indian Standards, Delhi. (2) American Society for Testing and Materials, “Standard Test Methods for Cross‐hole Seismic Testing,”

ASTM D4428‐D4428M‐00.


C A S E S T U D Y O F P R O P O S E D M A L L

At Noida SCOPE OF WORK

Details of the tests completed on site are summarized and tabulated below:

UTM Co-ordinates, m

(Zone-43 R) Test Easting Northing

Test Depth Interval, m

Maximum Test Depth, m

Cross hole seismic test 737883 3166886 1.5 30 Pressuremeter test 737900 3166848 3.0 30

A satellite image indicating the site location is presented below:

SITE LOCATION


TEST RESULTS AND D E S I G N P R O F I L E S

C R O S S H O L E / D O W N H O L E S E I S M I C INTERPRETATION BASED ON CROSS HOLE SEISMIC TEST We have the following observations;

a. The strata at the site classifies as very soft soil (SE) to about 1.5 m depth, as per the Uniform

Building Code (1997). Below this, the strata typically classifies as stiff to very dense soil (SD & Sc) to the maximum explored depth of 30 m.

b. The measured shear wave velocity (Vs) at the test location generally ranges from 217-350 m/s (i.e.

SD: stiff soil) to about 28.5 m depth and 380 m/s (i.e Sc.: very dense soil) at final tested depth of 30 m. However topmost layer of 1.5 m shows lower velocity of 152 m/s (i.e. SE: Very soft soil).

c. There is no significant variation in the velocity of shear waves with depth to the maximum tested

depth of 30 m.

d. The measured compression wave velocities (Vp) below about 1.5 m depth are generally in the range of 1764-1875 m/s (in the range of fluid wave velocity, possibly due to the saturation of strata owing to the shallow groundwater table at the site .

UNIFORM BUILDING CODE (1997):CLASSIFICATION SYSTEM

Based on the measured shear wave velocity, the strata may be classified into different categories as per the UBC Code (1997):

Type of Formation Average Shear Wave Velocities (Vs), (m/s) Classification

Hard Rock >1500 SA Rock 760 – 1500 SB

Very Dense Soil and Soft Rock 360 – 760 SC Stiff Soil 180 – 360 SD

Very Soft Soil <180 SE

C R O S S H O L E / D O W N H O L E S E I S M I C VS-N CORRELATIONS REPORTED IN LITERATURE VS TEST DATA

CORRELATION OF SHEAR WAVE VELOCITY

Documents

CROSSHOLE/ DOWNHOLE SEISMIC TEST - Welcome to …igsdelhichapter.com/handout.pdf · · 2017-08-21Bore logs provide soil samples for soil type classification and laboratory testing