1S-129J. A. Jendrzej czykM. W. WambsganssDecember 1987

SURFACE MEASUREMENTS OF SHE WAVE VELOCITY AT THE 7-GeV APS SITE

1.0 INTRODUCTION .,-...

A knowledge of shear wave speeds as a function of site location and soil

depth is fundamental to the vibration study of the 7-GeV Experiment Hall

foundation supporting the storage ring magnets, insertion devices, and

experiments. Among other things, knowledge of the shear wave speed allows one

to calculate the shear modulus of elas tici ty of the soil using the

relationship

G2pVS 0)

where G is shear modulus of elasticity, p is soil density, and Vs is shear

wave speed. The shear modulus, in turn, is one of the mos t important

parameters in performing a dynamic analysis of the response of the foundation

to both external excitation (ground motion) and excitation sources internal to

the Experiment Hall.

Shear (S) wave speed, as well as compression (p) wave speed, is typically

determined from crosshole seismic testing (1 J . The procedure is to use an

energy source, receivers, and a recording system. The energy source and

recei vers are placed at the same elevation in drilled and cased boreholes,

which are spaced 10 to 15 feet apart. Measurements are made at prescribed

depth increments. Wave speeds are determined using the spacing between

receivers and the arrival times of the different waves as obtained from

response-time plots. As part of a geotechnical investigation of the 7-GeV APS

site, the APS Project has contracted to have crosshole seismic testing

performed. These tests are scheduled for early in calendar year 1988.

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To provide early insight into the dynamic characteristics of the soil, to

obtain information that will supplement the crosshole seismic test data, and

to develop experience and a reference for an internal review and

interpretation of the results to be obtained by the geotechnical contractor,

surface measurements of shear wave speeds were made at the 7-GeV APS site and

are reported herein.

2.0 MEASUREMENT METHOD

The measurement method is based on the utilization of an energy source

that is rich in the type of energy required to excite the particular wave of

interes t. For a shear wave, the source should transmit energy to the ground

primarily by directionalized distortion. The energy source should also be

repeatable and reversible (11. A weighted wooden timber impacted at the end

wi th a hammer blow provides such a source; the polari ty of the source is

reversed by impacting the opposite end. As will be shown later. the reverse

polarity of the source greatly facilitates the identification of the S-wave.

The timber used in this study has cross-sectional dimensions of

approximately 6-in. x IO-in. and a length of 7 feet. The timber was weighted

by driving the front wheel of a truck up onto it. An 8-lb hammer was used to

impact the ends.

Such an energy source excites horizontally polari zed shear waves that are

optimally sensed using receivers posi tioned on a line emanating from the

center of the timber and perpendicular to it; receivers are horizontally

oriented to detect motion in a direction parallel to the timber. A layout of

the source and receivers is given in Fig. 1. The receivers are piezoelectric

accelerometers mounted on 6-in. long, I-in. diameter steel stakes driven into

the ground (21. The accelerometers are spaced lS-ft apart with the first

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-4-

accelerometer 20-f t from the source. The accelerometers are connected to a

battery-powered tape recorder (3 J .

A vertically-oriented accelerometer is located halfway between the source','.~

and first receiver to serve as a trigger. The trigger senses the passage of

a wave and sends a signal to start the time analysis. For this application it

is important to note that the trigger was set at a relatively high level to

sense the passage of the Rayleigh wave, a large-amplitude, vertically-oriented

wave with a fast rise time.

The test procedure consists of generating reverse polarity shear waves,

first by impacting one end of the timber (~ 10 times in succession), and then

by impacting the other end (~10 times). Acceleration-time traces,

corresponding to each impact, are recorded on magnetic tape for subsequent

processing and analysis.

A typical set of time records is given in Fig. 2. There are three pai rs

of traces, one pair from each of the three receivers (accelerometers). Each

pair consis ts of reverse polari ty shear waves. The S-wave arrival is

determined where a 1800 polarity change is noted to have occurred. As can be

observed froin Fig. 2, this technique allows for positive identification of the

S-wave.

The shear wave speed is readily computed by dividing the distance between

two pairs of receivers by the time for the signal to travel from the one

receiver to the next. Travel times can be computed using the start of the

S-wave, or any corresponding prominent feature on the time signals (e.g., zero

crossing or peak), as the reference. As an example, using the traces given in

Fig. 2 with the first zero crossing of the S-wave as the reference, the shear

wave speed is calculated as follows:

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-6-

(V S)(,~X) 1 15 ft

857 ft/ s(I: T) 1 x 10-31 17.5 sec

(V S)(I: X) 2

30 ft 896 ft/ s(I: T) 2 10-32 33.5 x sec

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Taking the average obtains, for this example,

Vs 876 ft/s .

3.0 PRELIMINARY INESTIGATION

Evaluation of the measurement method was accomplished by performing a

preliminary investigation. A single set of measurements was made on August 5,

1987. The source-trigger-receivers system was set up on the west side of

Building 335, between the building and the parking lot, as designated on the

proj ect area plan of Fig. 3 as location P.

The tests were performed following the procedure outlined in Section 2.0:

shear waves were generated by impacting the end of the timber; reverse

polarity waves were obtained by impacting the opposite end; the procedure was

repeated ~ 10 times in each direction; with accelerometer 7 as a trigger,

acceleration-time traces from receivers 2, 4, and 6 were obtained; random

pairs of time traces, from pairs of reverse polarity waves, were plotted. The

sample trace given in Fig. 2 is from the preliminary inves tigation.

The test results were independently analyzed by three evaluators. The

average value of shear wave speed as determined by each of the threeevaluators is given in Table 1. The computed values ranged from 866 to

945 ft/s, with a variation of (-4%, +5%) from the mean value of 897 ft/s.

Among other things, the preliminary inves tigation served to verify the

use of reverse polari ty waves as an excellent technique for posi ti ve

-7-

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Fig. 3. Project area plan showing the measurement locations for the preliminaryinvestigation (Location) and APS site investigation (Locations I-IV)

-8-

Table shear wave speed (ft/s)

Evaluator* Measurement Locationp I II III iv

FER 945 1131 1097 1244 990

JAJ 866 l082 1092 1206 908

MWW 880 1210 1070 1175 925

Avg 897 1141 1086 1208 941

*FER - F. E. Richart, Jr. (Consultant)JAJ - J. A. Jendrzej czyk (ANL)MWW - M. W. i'lambsganss (ANL)

-9-

identification of the shear wave from a time trace that includes compression

and Rayleigh waves as well. Once the shear wave is identified, coinputation of

wave speeds is straightforward using relative travel times. The study also

showed that the time traces from different impacts are very repeatable, with

subtle features repeating themselves in the different traces.

4.0 SURVEY OF APS SITE

4.1 Measurement Location

Having successfully demonstrated the measurement method, including

data processing and interpretation, via the preliminary investigation

described in Section 3, four measurement locations in the vicinity of the APS

site were selected for a follow-on study. The four measurement locations,

relative to the location of the APS Experiment Hall, are indicated on the

project area plan given in Fig. 3. A brief description of the soil type at

each of the measurement locations and the rationale for choosing that location

are given below:

i: Location I - Dry, hard, yellow clay filL. This location was a

dump site for fill. The fill was well packed. This si te was

selected since a segment of the 7-GeV APS Experiment Hall basemat

may be constructed on fill of this type.

@ Location II - Very moist, soft, black soil. This location isjust off Bluff Road, the East-West diameter of the storage

ring. The soil type and condition is typical of that on which

much of the Experiment Hall foundation will be constructed.

~ Location III - Miscellaneous fill site, including gravel, rock,

and concrete - reasonably well packed. While outside of the

ci rcumference of the Experiment Hall, this location was chosen to

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provide insight and a reference relative to the type of results

one obtains from this contrasting soil type.

~ Location iv - Very dark, very soft, extremely moist black soil.',¿-~

This location was selected because it represents essentially

undisturbed soil that is typical of much of the soil in the

vicini ty of the base soil upon which the Experiment Hall

foundation will be laid.

4..2 Results

The layout and relative orientation of the source-trigger-receivers

system is the same as that used in the preliminary investigation and shown

schematically in Fig. 1. The measurements were performed on August 24, 1987,

following the procedure discussed in Section 2 and applied in carrying out the

preliminary investigation described in Section 3. Typical sets of

acceleration-time traces, one from each of four measurement locations. are

given in Figs. 4-7 for Locations I-iV, respectively. As in the preliminary

investigation. the shear wave is readily identified by the reverse polarity of

the wave form.

4..3 Interpretation of Results

Again. the test results were independently analyzed by three

evaluators. Average values of shear wave speed, as determined by each of the

three evaluators, are given in Table 1 for each of the four measurement

locations at the APS site.

In studying the data, it was observed. as can be noted on the

acceleration-time traces given in Figs. 2. 4-7. that there appears to be a

characteris tic frequency associated wi th the acceleration-time signals. As

noted in Section 2, the timber was weighted by running the front wheel of a

truck up onto it. It was suggested that.... .the elastic system of soil-timber

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-15-

restraint) and the 'ringing' of the tire-spring-mass system of the front end

of the truck could create these relatively high frequency vibrations" (4).

To gain further insight into this phenomenon, the acceleration-time

signals were processed on a fast Fourier transform analyzer to obtain

frequency response information. Frequency response plots for each of the four

measurement locations are given in Fig. 8. In general) the observed

frequencies can be related to the soil type: the harder the soil, the higher

the frequency. This supports the hypothesis that the timber-soil-spring

system is involved in determining the observed characteristic frequencies.

However, the variation in principal frequency with receiver position

for the Location iv measurements (see Fig. 8c) indicates that the dynamic

characteristics of the soil-receiver assembly are also a factor in determining

the characteristic frequency of the signal. In this case the 6-in. receiver

stakes were located in medium-hard, soft, and hard fill, and the principal

frequencies were on the order of 95, 75, and 146 Hz, respectively. The

correlation of principal frequency with soil type is clearly demonstrated by

this set of measurements. At each of the other measurement locations the soil

type is essentially uniform where the receiver stakes were set, and the

principal frequencies associated with the various receivers are approximately

the same at a given measurement location.

5.0 DISCUSSION

Comparisons of the shear wave speeds as determined by the three different

evaluators can be made using the results given in Table 1. The ASTM Standard

(1) sugges ts) wi th regard to precision and bias in interpretation of such

data, that ..... within-laboratory repeatability of 5 :t 2.5% can be expected.

The greatest contribution to variance is likely to be the operator's

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-18-

interpretation of arrival times." The maximum variance of the results

presented in Table 1 (-5.2%, +6.0%) is within this range.

In Table 1, if we neglect the results of the preliminary investigation

and the data from Location III, which is not typical of the APS site, we

obtain the result that the shear wave speed, based on surface measurements, is

in the range 940 to 1140 ft/ s. It is known from a preliminary geotechnical

inves tigation of the APS site that the average dry uni t weight of the soil is

117 Ib/ft3. Substituting this value, and the measured values of shear wave

speeds, into Eq. (1) gives the result that the shear modulus of the soil is in

the range 2.2 x 104 lb/in.2 to 3.3 x 104 Ib/in.2.

I t has been demonstrated that, using an appropriate exci tation/

measurement method, shear waves can be readily identified on acceleration-time

traces and shear wave speeds calculated. Surface measurements can be

performed quickly and inexpensively with a minimal investment in equipment.

The results provide early insight into the dynamic characteristics of the

soil, provide the basis for a preliminary investigation of the vibration

characteristics of the Experiment Hall foundation, and will supplement the

data to be obtained from the planned crosshole seismic testing.

Acknowledgment

The authors gratefully acknowledge R. K. Smith for his assistance in the

conducting of the measurement program and F. E. Richart, Jr., for general

consultation and specific assistance in the analysis and interpretation of the

data.

-19-

References

1. ASTM Standard D-4428/D-4428M-84, "Standard TestSeismic Testing." American Society for Testing885-898.

Methods forMaterials.

Crosshole1984, pp.

2. Jendrzejczyk, J. A., and Smith, R. K., "Ground Motion Measurements in theVicini ty of Building 335," Argonne National Laboratory TechnicalMemorandum, MCT/MV1037, November 1986.

3. Jendrzejczyk, J. A., and Smith, R. K., "Evaluation of Amplitude andFrequency Response Characteristics of the TEAC Model MR30 Tape Recorder,"Argonne National Laboratory Technical Memorandum, LS-81, January 1987.

4. Letter, F. E. Richart, Jr., to M. W. Wambsganss, dated September 21,1987.