Techbrief Csds Web

8/6/2019 Techbrief Csds Web

1/712401 W. 49th Ave., Wheat Ridge, CO

CROSSHOLE SEISMIC (CS) investiga-

tions are performed to provide infor-mation on dynamic soil and rockproperties for earthquake designanalyses for structures, liquefactionpotential studies, site development,and dynamic machine foundationdesign. The investigation determinesshear and compressional wave depth

versus velocity profiles. Other param-eters, such as Poissons ratios andmoduli, can be easily determinedfrom the measured shear and com-pressional wave velocities. In addition,

the material damping can be deter-mined from CS tests. The CS method isa downhole method for the determination of material properties of soil and rock. Asource capable of generating shear and compressional waves is lowered in one of theboreholes, and a pair of matching three component geophone receivers are lowered tothe 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 in the figure above.The receivers are positioned on the side of the borehole casing to allow detection of thepassage of shear and compressional waves.

S T A N D A R D S

A P P L I C AT I O N

Experts in Nondestruct ive Evaluat ion

CR O S S H O L E/ D O W N H O L E SE I S M I C [C S / D S ]G E O P H Y S I C A L

CS-DS Method

DOWNHOLE SEISMIC (DS) investi-gations are similar to the CSmethod, but require only oneborehole to provide shear andcompressional velocity waveprofiles. The DS method uses aplank source at the surface togenerate shear and compres-sional waves. A pair of matchingthree component geophonereceivers are lowered downholespaced 5 to 10 feet apart tosense the P- and S- wave energy.

This method is performed in accordance with ASTM D4428-D4428M-00 Standard TestMethods for Crosshole Seismic Testing. CS is an acceptable investigation method forobtaining soil classification ratings as required by IBC/UBC. An example of this require-ment can be found atwww.ci.las-vegas.nv.us/Files/IBC Amendments 2003.pdf, which out-lines the City of Las Vegas amendments to the 2003 IBC Chapter 18 Section 1802. Theparticular section describing necessary investigations is 1802.4.2 Investigation.

See end of document for full references.

SOURCERECEIVER 1

Olson Freedom Data PC

RECEIVER 2

80033-1927 USA 303.423.1212


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OLSON ENGINEERING , INC . , 12401 W. 49th Ave., Wheat Ridge, CO 80033-1936 USA 303.423.1212

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CR O S S H O L E/ D O W N H O L E SE I S M I CCS /DS

F I E L D I N V E S T I G A T I O N

G E O

The CS investigation requires drilling of two or more(typically three) boreholes. The boreholes are typi-

cally 3-4 inches in diameter cased with PVC or slopeinclinometer casing, and grouted in accordance withASTM Standards to ensure good transmission of the

wave energy. The testing is simplified if inclinome-ter casing is used rather than normal PVC pipe.Typical distances between adjacent boreholes are onthe order of 10 feet. A field setup for CS measure-

ments is shown on the previous page The receiverboreholes are drilled to the total investigation depth.

For tests using the split spoon as a source, thesource borehole is advanced during testing at intervals equal to the measurement intervals required(2-5 feet). If a source containing an impactor thatcan be clamped to the borehole wall is used, thenthe source borehole can be drilled to the total inves-tigation depth prior to testing.

ACCESS

In a CS investigation, the source is lowered to themeasurement depth and is incrementally advanced

to the bottom of the borehole. One or two triaxialgeophone receivers are incrementally lowered to thesame depth in the other boreholes. Dummy incli-nometer probes are used to maintain correctreceiver orientation throughout the investigation asshown in the schematic above; if PVC casing isused, orientation rods must be attached to thesource and receiver. The source is triggered from thesurface to generate shear and compressional waveenergy at depth. In some instances where a splitspoon is used as the source, an instrumented hammerstrikes the rod to generate shear and compressional

wave energy. The source borehole is incrementallyadvanced for the split spoon source. The vertical

component of the receiver is used to capture the ver-tically polarized shear waves (SV). Both upward anddownward polarized energy is generated for duplic-ity of data and to measure shear arrival effectively.The radial component senses the propagating com-pressional waves (P) and the tangential componentsenses the horizontally propagating shear waves(SH). An Olson Instruments Freedom Data PCwiththe Crosshole or Downhole Seismic System (DS/CS-1and CS/CS-2) is used to record the P-SV sourceinput as well as the receiver outputs. (see below).

COLLECTION OF DATA

In a DS investigation, the source is typically ahammer hitting a plank at the surface andrequires only one borehole. The three compo-nent geophones are separated 10 feet andlowered together downhole. The hammersource generates both shear and compres-sional wave energy and which are recordedby the geophones. The vertical component ofthe receiver is used to capture the verticallypropagating compressional waves (P) andthe radial transverse component senses thehorizontally polarized shear wave (SH).


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OLSON ENGINEERING , INC., 12401 W. 49th Ave., W heat Ridge, CO

CR O S S H O L E/ D O W N H O L E SE I S M I CCS /DS G E O

[ Page 3

Data processing is performed usingseismic analysis software such as IXFoundation to pick arrival times of thesource and separate receiver compo-nents. The figure below shows a screenshot of the upper 25 feet of a profile.The split in polarization, as indicated,allows shear wave arrival times to bepicked (red hash). IX Foundationexports pick times into a standardspreadsheet to calculate velocities,

moduli, and Poissons ratios.

D A T A RE D U C T I O N

Once shear velocity is calculated,a graph can be assembled toshow a velocity profile. Thischart shows the sensitivity of CSmethod in identifying layers ofparticular strata.

P R O C E S S I N G T E C H N I Q U E S

D

EPTH

(FT

.)

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If one receiver borehole is used, the travel time from

source to receiver is measured. This is referred to asdirect travel time measurements. If two receiverboreholes are used, the travel time between thereceivers is measured. This is referred to as intervaltravel time measurements. The wave velocities atthe measurement depth are simply calculated bydividing the travel distances by the measured traveltime. The travel distances are determined after the

verticality of the boreholes is evaluated (inclinome-ters are typically used). Note that interval traveltimes are normally more accurate than direct traveltimes, and thus the three hole test configuration is

preferred.

The Poissons ratio, as well as shear and con-strained moduli can be determined from the shearand compressional wave velocities using thefollowing equations:

G = rVS2

M = rVP2

r = [0.5 r (VP/VS)2 - 1] / [VP/VS)

2 - 1]

where G is the shear modulus, r is the mass density,VS is the shear wave velocity, M is the constrained

modulus, VP is the compressional wave velocity, r is

the mass density, and V is Poissons ratio.

OLSON ENGINEERING , INC., 12401 W. 49th Ave., Wheat Ridge, CO

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D A T A RE D U C T I O N c o n t .

I N T E R P R E T A T I O N O F DATA

E F F E C T I V E N E S S

As compared to surface methods, the CS method isthe most accurate method for determining materialproperties of rock and soil sites. Thin low-velocitylayers lying between high-velocity layers can bedetected with the method, like-wise, thin high-

velocity layers between low-velocity layers can bedetected, which may not be possible with surfacemethods such as Spectral Analysis ofSurface Waves (SASW) or RefractionSurvey tests. Often, when boringlogs indicate particularstrata of rock and soil ofinterest, such as ashallow, high-velocitylayer, the CS equipmentcan be set to that deptheasily and the overallstiffness of a particular

site can be improved. Inaddition, the accuracy andresolution of the CS method isconstant for all test depths, whereas the

accuracy and resolution of the surface methodsdecreases with depth. Olson Engineering hasperformed the CS and/or DS method in conjunction

with SASW for comparison purposes and forrepeatability of results. For CS investigations, two or

preferably three boreholes are required to performthe test. In circumstances where two or three

boreholes are noteconomical or per-haps limited space is

available the DSmethod can be utilized

with only one borehole.In rock site investiga-tions, the boreholes may

be uncased, but for mostof the soil site investiga-

tions, the borehole shouldbe cased (preferably with

inclinometer casing)and grouted.

[ Triaxial Geophone][ P-SV Source for CS Tests ]

80033-1927 USA 303.423.1212


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OLSON ENGINEERING , INC., 12401 W. 49th Ave., Wheat Ridge, CO

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To illustrate the concepts of the CS method, example results from CS tests on a soil site are presented below. Therecords shown are from CS measurements at a depth of 116 feet. The figure below shows a screen shot from anOlson Instruments Freedom Data PC showing the aforementioned record. Channel 5 is the vertical component of thethree component geophone, which is measuring the vertically polarized shear wave energy. Channels 6 and 7 arethe radial and transverse components, respectively, and they measure the compressional wave energy. Typically, theradial component is aligned with the source and is thus used to measure the arrival of the compressional wave moreaccurately. Channel 8 is the trigger component from the P-SV source for timing.

C S M E T H O D - S O I L S I T E T E S T

E X A M P L E RE S U LT S

80033-1927 USA 303.423.1212


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OLSON ENGINEERING , INC . , 12401 W. 49th Ave., Wheat Ridge, CO 80033-1936 USA 303.423.1212

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In determining a velocity profile, comparison between boring logs and calculated shear wave velocities are routine-ly done to confirm data clarity and accurate representation of the subsurface material. The figures below correspondwith each and show the CS chart as well as the boring log from a particular site. The example exemplifies the accu-racy of the CS method in identifying a change in soil strata. The red circle on the chart shows the area where a veloc-ity increase occurred; a similar change in soil strata is indicated at 100 ft. in the boring logs as a change from sandyclays to cemented sands and gravels.

C S M E T H O D - S O I L S I T E VE L O C I T Y P R O F I L E

E X A M P L E RE S U LT S c o n t .

Velocity increase occured

Change from sandy clays to cemented sands and gravels


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RE F E R E N C E S

S t a n d a r d s a n dG o v e r n m e n t a l R e p o r t s

ASTM D4428/D4428M-00, Standard TestMethods for Crosshole Seismic Testing,ASTM International.

City of Las Vegas: Amendments to 2003International Building Code, Chapter 18Section 1802 Soils and Foundations,2003 International Building Code .

O LSON ENGINEER ING , INC .

Phone: 303.423.1212Fax: 303.423.6071I www.olsonengineering.comI www.olsoninstruments.comI [email protected]

Wheat Ridge, CO 80033 USA12401 W. 49th Ave

Corporate Office:

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