232 TRANSPORTATION RESEARCH RECORD Jl'JI

Shallow Refraction Surveys on a Low­Volume Road for Determining P-·w ave Velocity of Seasonal Thawing Soils

B. D. ALKIRE AND c. KELLER

Determination of soil strength during spring thaw is essential to developing mechanistically determined seasonal load restrictions. The work reported uses shallow refraction techniques to deter­mine P-wave velocities of the road subgrade at weekly intervals. From this information. the variation of P-wave properties with time is developed. From the relationship between P-wave velocity and time. it is shown that P-wave velocity starts at relatively high values before thaw commences. decreases to a minimum. and then increases. The minimum P-wave velocity occurs at about 3 weeks after the maximum degree-days for the freezing season. Results from the P-wave tests are shown to be comparable to results obtained using a Clegg impact test. However. the mini­mum Clegg value occurs at an earlier date than the minimum P­wave velocity. Overall, results indicate that shallow refraction surveys can be used to develop a curve of P-wave velocity versus time. This relationship. in turn, can be related to a soil strength value that wuulJ be usable in a mechanistic pavement design method.

The timing of road restrictions during spring breakup is one that has plagued many state. county, and city highway offi­cials. Presently, the decision of when to impose and lift load restrictions is left to the judgment of the road commission or county engineer. They base their decisions on past perfor­mance of the roadways during the spring thaw. visual eval­uation of the roadway's strength, and, possibly, measurement of some soil property. In most cases, the methods of deter­mining when to place and lift load restrictions is subjective; however, if the timing is not correct, there can be economic losse~ Lo the lnmspurlaliuu community.

In this study, the seismic refraction technique is used to determine the P-wave velocities of both frozen and unfrozen subgrade materials. Then, using methods from wave theory, the velocities are related to the soil's elastic properties (one of the physical properties that changes on freezing and thaw­ing). By repeating the test at regular intervals during spring breakup, the seismic method can be used to detect changes in the road strength. This, in turn, can be used to make rational decisions about when to apply and lift road restrictions.

The tests are from a series of shallow refraction surveys conducted at a test site near Houghton, Michigan. Tests were conducted on a regular basis throughout the spring thaw periods of 1986, 1987, and 1988. At the same time and site, Clegg impact tests were conducted to provide correlation with another indirect method of assessing soil stiffness. Because of the

B. D. Alkire, Michigan Technological University, Houghton, Mich . 49931. C. Keller, SME Consultants, Livonia, Mich.

nature of the test site, the results are appropriate to aggregate­surfaced roads only.

TEST SITE

The road chosen for this study was a low-volume aggregate­covered road south of Houghton, Michigan. The wearing sur­face of the road is a 5-in. (127-mm) layer of a local aggregate known as "stamp sand" with a USCS designation of SW-SM. The naturally occurring subgrade is a silty sand with 26 percent of the material finer than a #200 sieve and a liquid limit of 17. The soil has a frost susceptibility designation of F4. The subgrade soil is extremely frost susceptible and contains microscopic excess ice. During spring breakup. the road expe­riences severe distress in the form of rutting. frost boils. crack­ing, and potholes.

FIELD TEST

Seismic compressional wave (P-wave) velocities of the sub grade of the road were found using typical shallow seismic refraction techniques. For the tests reported, single-ended hammer seis­mic refraction surveys with a spread length of 50 ft ( 15 .2 m) were run using a Nimbus 55-125 single-channel. signal enhancement seismograph. For a typical test. the geophone was installed at the zero station and a tape was used to lay out the hammer blow locations at regular intervals along the survey line. Successive hammer impacts on a strike plate were repeated until a well-defined first-arrival wave was observed on the instrument's cathode ray tube. The travel time for the first arrival was noted, and the impact source was moved to the next station, where the process was repeated. The geo­phone was always set at the same location and spreads were run parallel and transverse to the centerline of the road. The short spread length, shallow depths probed, and nature of the soil layering justified use of a single-ended survey.

Early results indicated that the parallel to centerline profile was probably more typical of the assumed horizontal layer boundary condition because of the fact that thaw depth was nearly constant along any line parallel to the centerline. How­ever, across a transverse section, thaw depth was greater near the centerline and decreased with distance toward the shoul­der. As a consequence, only results from the centerline profile are used in the analysis.

The condition of the road at the test site was also measured using a Clegg impact test. This apparatus was developed in

Alkire and Keller

Australia in the mid-1970s as an in-place stiffness test (J) . The instrument used in this research project had a 10-lb (4.54-kg) hammer and was dropped 18 in. (457 mm). The Clegg impact value (CIV) is the reading obtained on the recording meter after the fourth drop.

RESULTS

For each field test, the distance from the source to the geo­phone and the time for the P-wave first arrival was obtained. A typical data sheet (for the test on March 12. 1986) is shown in Figure 1. On this sheet are the measured distance from impact source to geophone, the observed first-wave travel time to the geophone . and the CIV value of the soil taken at a position adjacent to the strike plate . Also shown on the figure are the time, date, and weather conditions at the site, as well as the calculated average CIV values and standard deviations . Data from tests were used to plot graphs of dis­tance from geophone versus travel time of first arrival. Figures 2-4 show typical results obtained at the same location on three different days.

Figure 2 is typical of the results obtained for an unfrozen soil without a well-defined layer to refract the P-wave . The test was conducted on April 28, 1988, and the velocity asso­ciated with this curve is approximately 2,000 ft/sec (610 ml sec), typical of the P-wave velocity in a thawed silty sand with a relatively low water content. The actual measured water content 3 ft (0.91 m) below the surface on this date was 10.5 percent.

When there is a low-velocity layer over a higher-velocity layer, the normal relation between time and distance occurs as shown in Figure 3. In this case, the top layer is the thawed subgrade soil with a high water content and low velocity of

Apr 28, 88

+ 250 +

{/)

E 200

a.) 15.0

E F 10.0

+

0 5.0

> ·s:::: t '-<t 0 .0

00 10.0 200 30.0 400 50.0

Dist From Geophone, FT FIGURE 2 Typical arrival time versus distance for unfrozen subgrade soil.

233

1,773 ft/sec (540 m/sec) and is over the frozen subgrade soil with the higher velocity of 4,433 ft/sec (1351 m/sec) . The relatively short distance to the break in the curve is caused by the shallow depth of the top layer. For example, if the break in Figure 3 is at 6 ft (1.83 m) and the time intercept is at 2 msec, the thickness of the thawed layer can be calculated using the formula (2):

(1)

where T1 = thickness of Layer 1, V, velocity of P-wave in Layer 1, V~ = velocity of P-wave in Layer 2. and f; = time intercept. The calculated value using the values obtained from

Massie Rd. Project Data: 3-12-86; Weather: T • 35F, Sunny

Note: Thaw about 3" into road surface, heavy rutting

Profile Distance Time CIV

Centerline 0.00 0.00 3.00 0. 20 48 6.00 0.40 55 9.00 0.60 48

12.00 0. 70 45 15 . 00 1.20 51 18.00 1.90 41 21.00 2.50 49

Avg. CIV 48 Std. Dev. 4.1

Transverse o.oo 0.00 34 4.00 0.10 33 6.00 0.30 35 8.00 0.50 34

10.00 0.80 32 14.00 l.30 65 18.00 I. 70 21.00 2.50

Avg . CIV 39 Std . Dev . 11. 7

l ft. = 304.8 mm

FIGURE 1 Typical data sheet for test program.

234

4o.o (j)

E :mo

Q)

E 20.0

F 10.0

0 > ·s:::: ~

<t 0 .0

00

Dist 10.0 200

From

+

Apr 13, 88

+

30.0 40.0 50.0

Geophone, FT FIGURE 3 Typical arrival time versus distance for partially thawed suhgrade soil.

Figure 3 is 23 in. (584 mm) . This value is close to the observed 20-in. (508-mm) depth of thawed soil.

The most interesting and hardest relationship to explain is shown in Figure 4. The apparent velocity reversals shown here should not be obtained from a normal refraction survey. How­ever , Irving (3) explains that reverse breaks of this type result from rapid attenuation of first arrivals and are common in frozen ground . The fact that the signals were being attenuated rapidly manifested itself in this test series as the amplifier gain had to be increased to nearly its maximum value when the energy source was at 50 ft (15.2 m) . Another factor that contributes to this behavior is the change in Poisson 's ratio that occurs as a soil goes from frozen to thawed. The velocity of the frozen soil is determined from the steepest slope and for this case is approximately 4,500 ft/sec (1371 m/sec).

At the same location used for the hammer blows for the P-wave test, Clegg impact tests were also conducted. It has been shown that the Clegg values follow a thaw recovery curve ( 4). For the tests conducted as part of this work, the Clegg value was obtained at several locations along the survey line,

t 25

(j) 2.0

E Mar 12, 86

1.5 Q)

E I-

0 .5 0 > ~

00 ~

<t o.o 50 10.0 15.0 20.0 25.0

Dist From Geophone, FT FIGURE 4 Typical arrival time versus distance for frozen subgrade soil.

TRANSPORTATION RESEA RCH RECORD 1191

and the average value for a particular test was calculated as shown in Figure 1.

A summary of all test data is presented in Table 1. This table lists the date of each test , P-wave velocity, and the average CIV value for the centerline profile on that particular date . Also presented in this table is the number of days ( + or - ) that the test date was when compared to the day when the maximum value of freezing Fahrenheit degree-days occurred (this is assumed to be the day when spring-thaw begins) . Note that both the P-wave velocity value and the CIV values start high, decrease to a minimum, and then increase . In general. the minimum P-wave velocity and CIV value occurred at some time after the day of maximum freezing degree-days .

DISCUSSION OF RESULTS

Results from the study indicate that it is possible to use the velocity of the direct P-waves to monitor the roadway con­dition through spring breakup. Figure 5 shows a plot of direct P-wave velocity versus number of days since the start of thaw for the three seasons of data. It can be observed that the curves have similar shapes but are offset slightly from year to year.

An empirical equation relating P-wave velocity and days since the beginning of thaw is also plotted on Figure 5 and was developed from the following equation:

V = 2784.0 - 152.4T + 3.0T2 (2)

where V = P-wave velocity in ft/sec and T = number of days since thawing weather began. The correlation coefficient for the equation is 0.85, and indicates that time since thaw began is a good predictor of P-wave velocity.

Figure 5 and Equation 2 demonstrate that soil velocity var­ies substantially. As would be expected, 1 to·2 weeks before the start of thaw the soil is frozen and the velocity can be greater than 6,000 ft/sec (1824 m/sec). As the average daily temperatures increase, the velocity decreases to a minimum around 20 days after the start of thaw . Finally, as thawing degree-days continue to accumulate, the velocities increase until they are again at relatively high values 30 to 50 days after the beginning of thaw. If Equation 2 is differentiated with respect to time, the minimum value of the velocity is found to occur at 22 days after thaw begins. The resulting value of the velocity obtained using Equation 2 is 883 ft/sec (269 m/sec).

From the known (2) relationship between the compression wave velocity and the modulus of elasticity, it is possible to calculate the modulus of elasticity:

E = v2p(l + µ)(1 - 2 µ)/(l - µ) (3)

where E = modulus of elasticity, V = P-wave velocity, p = mass density of the soil, andµ = Poisson's ratio. Using Equa­tion 3 and a compression wave velocity of 1,000 ft/sec (304 m/sec) results in a modulus of approximately 6,800 psi (46.9 MPa) if it is assumed that Poisson's ratio is 0.45 (typical for frozen soil near 32°F (5)] and the unit weight of the soil is 120 lb/ft3 (18.2 kN/m3).

Alkire and Keller

TABLE I SUMMARY OF TEST RESULTS

Date P-Wave Velocity

(fps)

Average C1 egg Impact Value

(CIV)

Days Before ( - ) or After (+) day

of Maximum Freezing Fahrenheit Degree­

dav !days!

March 18, 1986 25

April 1, 1986 7 15 22 30

May 6, 1986

March 16, 1987 20 24

April 6, 1987 19 30

March 17, 1988 24 31

April 5, 1988 7 13 28

fps • .304 mps

5822 NA

1272 1385 982

1041 1261 1705

5199 1477 1134 799

1685 3000

5118 4623 3019

998 962

1773 2239

The modulus obtained using Equations 2 and 3 could be used in any road design technique that requires this param­eter. For example, a simple rut depth formula in the form of a uniaxial stress-strain relationship might be the following:

R = KWIE (4)

where R = rut depth, W = wheel load, E = modulus of elasticity, and K = a constant. By assuming a limiting rut depth , the minimum modulus and related times when they occur could be calculated and the dates (±days after begin­ning of thaw) the load limits should be applied and lifted

6000

~4000 "(} 0

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-3 2000

I n..

~ I I

)( i \ ... ,.. \ d~ \

'I ' \~ '. ' ' \

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I \ . ':1

0i986·-·- · ~1987 X·--·---·--· 1988

0 • - - • - - - -- · -Cole

O+-~-.-~--r~~..-~...,-~....,...~--,~--,

-20 "10 0 10 20 "30 40

Days After Thaw Begins

FIGURE S Summary P-wave velocity versus days after thaw begins.

50

26 34 13 18 10 30 36 43

37 42 28 40 40 47

190 28 33 12 19 39 48

- 7 0 7

+13 +21 +28 +36 +42

- 2 2 6

19 32 43

- 16 - 9 • 2

3 5

11 16

would be determined. Obviously, once the relationship between modulus and time is known, it would be relatively easy to set a load restriction based on even more sophisticated techniques such as those proposed by the U.S . Forest Service (6) and AASHTO (7).

In order to correlate the results for P-wave velocity with another test, there was an attempt to do a regression on the CIV value versus days after thaw begins. The results are plot­ted in Figure 6 and the regression equation developed from the data presented in Table 1 is

CIV = 29.0 - 0.48T + 0.020P

> 0

60

40

20

0----------Calc

01986

xj997 -- --L- --·· -- ·-- · -- ·----1988

O+-~~~--+~~..--~~~-..-~-..-~~

-20 -10 0 10 20 30 40

Days After Thaw Begins

FIGURE 6 Summary CIV versus days after thaw begins.

50

(5)

236

with a correlation coefficient of 0.52. The low correlation coefficient for this equation indicated that Equation 2 is a better pn;dictor of P-wave velocity than Equation 5 is of CIV. This should be expected because the Clegg device provides a measure of the near-surface characteristics of the soil and thus is more susceptible to small changes in the surface of the road caused by daily changes in the ambient temperature.

By differentiating Equation 5, the minimum value of CIV is determined to occur at + 7 days after thaw begins, which is almost 2 weeks before the minimum date predicted by the P-wave velocity equation.

Because the regression coefficient for Equation 2 is higher than for Equation 5, the P-wave velocity may be a more reliable predictor of soil strength. Thus, the seismic refraction technique may provide a more reliable way of developing a thaw strength recovery curve and predicting when load restric­tions should be applied on low-volume roads.

CONCLUSION

It is possible to obtain an indication of the thaw recovery curve by conducting shallow seismic refraction tests on a reg­ular basis. On the basis of the results, the following obser­vations can be made:

1. P-wave direct arrival velocity decreases as the soil goes from the frozen to thawed state.

2. The minimum value of P-wave velocity will occur at some time well after the day when the freezing degree-day curve has its maximum value.

TRANSPORTATION RESEARCH RECORD ]]9/

3. Clegg impact values are more sensitive to minor changes in ambient temperature conditions than results from a seismic refraction test. Although the CIV values decrease to a min­imum and then increase in much the same way as the P-wave velocity, individual reading are more likely to have large var­iabilities.

4. P-wave velocity can be used to circulate the modulus of elasticity of the surface layer and could be used as a direct indication of the soils stiffness.

REFERENCES

1. B. Clegg. An Impact Testing Device for In-Situ Base Course Eval­uation. Proc., 8th Conference of the Australian Road Research Board, Perth, 1976.

2. E. R. Irving. Seismic Surveying Methods Equipment and Costs in New York State, In Highway Research Record 81, HRB, National Research Council, 1965, pp. 2-8.

3. B. D. Alkire and L. Winters. Soil Strength Recovery Using a Clegg Impact Device. Proc., 4th International Conference on Cold Regions Engineering, ASCE, 1986, pp. 155-166.

4. M. B. Dobrin. Introduction to Geophysical Prospecting, McGraw­Hill, New York, 1960.

5. 0. B. Andersland and D. M. Anderson. Mechanical Properties of Frozen Ground. Geotechnical Engineering for Cold Regions. McGraw-Hill, New York, 1978, 261 pp.

6. U.S. Forest Service. Surfacing Handbook (Draft). U.S. Depart­ment of Agriculture, Washington, D.C .. 1983.

7. AASHTO Guide for Design of Pavement Structures, AASHTO, Washington, D.C., 1986.