Cross Hole Test - Handout.pdf

   

CROSSHOLE/  DOWNHOLE  SEISMIC TEST 

Cengrs Geotechnica Pvt  Ltd A‐100, Sector 63, Noida,  UP‐201309

By Mr.Sorabh Gupta Workshop cum Demo Session   

Tel:+01204206771 Fax:+01204206775 11/23/2013

   

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]       

 

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

 

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.  

 

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

  3.0

Choice between CS and DS Seismic Testing 

Crosshole Method  

Downhole Method 

y Constant Travel Paths  y  Negligible Borehole Effects  y  Receivers Properly Aligned for SV‐Waves  y  High Signal‐to‐Noise Ratio at All Depths  y Detailed Profile  y Workable in Limited Space  y Accuracy Independent on the  Measurement Depth  y Two or More Boreholes  y Simple Borehole Source  y Predominantly P‐ and SV‐ Waves, but SH‐waves  Also Possible  y Reversible Source   y Measure Borehole Verticality  y Detect Low‐Velocity Layers  y Possible Refraction Problems  y  Useable in Noisy Areas  y  More Expensive 

y One Borehole  y No Verticality Measurements  y Simple Surface Source  y Minimum Refraction Problems  y Less Expensive  y Generate P‐ and SH‐Waves   y Reversible Source  y Travel Path Increases with Depth  y  Possible Borehole Effects   y Control of Receiver Alignment  Preferable  y Signal‐to‐Noise Ratio Decreases with  Depth  y Detect Low‐Velocity Layers  y  More Average Profile   y  Useable in Noisy Areas  y  Workable in Limited Space  y  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:  (1+ μ ) (1− 2μ )   E = ρ Vp 2 1− μ where: ρ  γ  Vp  μ  

=  =  =  = 

mass density of soil  bulk density of soil P‐wave velocity  Poisson’s ratio 

= (γ/g) 

 

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

   

The shear modulus G is the stress‐strain ratio for simple shear. Its dynamic value is  obtained by the following:    G =

E = ρVs 2   2 (1 + 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:     E 1   [A = Standard foundation area, taken as 10 m2]  cu =  1.13  × 2 A 1− μ = 0.67 to 0.5 cu (for design purpose, cτ may be taken equal to 0.6 cu) = 3.46 cτ = 1.5  cτ   Selection of Dynamic Parameters for Design 

cτ cφ cΨ 5.0

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:      

G1 σ 01 m =( )   G σ0

                                                                  (1)

Steven L. Kramer (1996), “Geotechnical Earthquake Engineering”, Pearson Education, Inc., Section 6.4, pp. 232-238.

 

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

    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, C τ , C φ 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.   

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

DOW NHOLE SEISMIC TEST    

A P P L I C AT I O N

 

 

The D OWNHOLE S EISMIC (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. 

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

CROSSHOLE SEISMIC T EST    

A P P L I C AT I O N

 

CROSSHOLE S EISMIC (CS) A  customized  P‐SV  investigations are performed source  provides  the  user  to provide information on with  the  most  accurate  dynamic soil and rock and  rapid  method  of  properties for earthquake generating impacts.   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 ROSSHOLE / D OWNHOLE S EISMIC

CASE

STUDY

PROPOSED M ALL At Noida OF

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

Test

UTM Co-ordinates, m (Zone-43 R) Easting

Cross hole seismic test Pressuremeter test

737883 737900

Northing 3166886 3166848

A satellite image indicating the site location is presented below:

SITE LOCATION

Test Depth Interval, m

Maximum Test Depth, m

1.5 3.0

30 30

C ROSSHOLE / D OWNHOLE S EISMIC TEST RESULTS AND D E S I G N P R O F I L E S

C ROSSHOLE / D OWNHOLE S EISMIC

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 (S D & Sc) to the maximum explored depth of 30 m.

b.

The measured shear wave velocity (V s) 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 Rock Very Dense Soil and Soft Rock Stiff Soil Very Soft Soil

>1500 760 – 1500 360 – 760 180 – 360 <180

SA SB SC SD SE

C ROSSHOLE / D OWNHOLE S EISMIC

VS-N CORRELATIONS REPORTED IN LITERATURE VS TEST DATA

CORRELATION OF SHEAR WAVE VELOCITY