ADVICE TO DESIGNERS

CYLINDRICAL-SURFACE SOIL TESTS IN DIRECT SHEAR DEVICES

A. N. Dranovskii and M. S. Vorob'ev UDC 624.131.37

The effect iveness of most soil models depends on the correct determination of the strength parameters. For example, at the present time the economy of shallow foundations fully depends on the accuracy of the Coulomb--Mohr parameters ~ and c.

The simplest test on a soil in a triaxial device isconsiderably more complex than in a shear device, for practically the same information content. Hence, it is necessary to develop sufficiently accurate shear devices applicable under construction conditions, on which the economy of massive design solutions depends to a significant extent.

For engineering tests, one=plane direct-shear devices are most widely used. The advantages and shortcomings of these devices depend on which of the two sections of the soil container (the upper or the lower) is moved in the horizontal direction [ I ].

The chief disadvantage of devices with an upper movable section lies in the possibility of its sticking to the lower under simultaneous jamming of the "sliding" lever system used for developing the vertical pressure. In this connection, the GOST 12248-78 Norms [I] recommend that for sand tests use be made of devices with lower movable sections.

In our country, wide use is made at engineering laboratories of VSV- I, VSV-25, and SPF-2 devices with lower movable sections, as well as of other similar devices.

It is considered [2, 3] that the basic inaccuracies of devices with lower movable sections are connected with incomplete transmission of the vertical force to the shear plane because of friction between the upper part of the lateral surface of the sample and the walls of the upper section of the soil container. For this reason, S. R. Meschyan [2] recommends the carrying out of a special calibration for determining the friction forces.

In [3], such a calibration was performed after subjecting the samples to shear tests. The vertical force at the shear plane level was measured by means of a Type DOSM dynamometer . It was found that if account is not taken of the actual value of the vertical force acting on the shear surface, the strength properties of the soil are significantly underestimated. For example, for sandy loams the values of ~ are underestimated by I0-26°/0, and the values of c by I0- 50%. On the other hand, in [4] it is a f f i rmed that for dense sands Type VSV devices lead to overestimation of ¢ by 28- 32% in comparison with the results obtained in a GGP-30 device with an upper movable section. In this connection it has been concluded that it is impermissible to use Type VSV devices for testing sands, despite the instructions of the

GOST 12248=78 Norms. As shown by the investigations described in [5, 6], both authors of works [3, 4] are right. In [6], investigations were carried out by means of an SPF=2 device equipped with a special system for measuring

the actual values of the vertical force acting on the lower movable section of the shear device. The vertical force is determined by three strain=gage resistor converters on which a platform with ball supports rests [7]. The disadvantage of this device lies in the complexi ty of the system for measuring the vertical force.

The cyl indrical-surface direct-shear test device [8] differs mainly f rom similar existing types because of the fact that the lower movable section 1 of the soil container (Fig. I) is secured to a suspender hinged to the force meter and has the possibility of moving like a pendulum with respect to the upper fixed section 2. The lower end of the upper section and the upper end of the lower are defined by coaxial cylindrical surfaces, the axis of which coincides with the horizontal axis of the hinge. The connection of the lower movable section to the suspender 3, joined by the hinge 4 to the force meter 5, makes it possible to omit the installation of the lower section on the ring with ball supports and to minimize the spurious friction which distorts the results of the shear force measurements. Support of the suspender on the force meter permits measuring the normal force transmitted directly to the shear surface rather than to the upper end of the sample. The data f rom measurement of the normal force on the shear surface permit determining the friction forces developed along the lateral surface of the soil sample within the limits of the upper section of the soil container.

For construction of the device and interpretation of the results, the following initial criteria were adopted: I ) the shearing stresses r n are uniformly distributed along the cylindrical surface of shear; 2) the horizontal resultant of the shear resistance forces is equal to rnS I, in which S I is the area of the plane transverse section of the soil sample; 3) the vertical resultant of the shear resistance forces is equal to zero. The last two criteria are demonstrated in an elementary

Kazan Civil Engineering Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp.

21-22, May-June , 1990.

0038-0741/90/2703-0121 $12.50 ©1990 Plenum Publishing Corporation 121

~ 5

ij/ ' Fig. 1. Schematic of device for cylindrical-surface direct

shear testing of soils.

f0

6 7 '

~

0~r r~ 6 ~ 2 0

'2

+

6

r o , MPa

[I__~ _ .._ [ . . . . P a

Fig. 2. Device construction alternative, in which the vertical compacting force is developed by means of a fixed lever system.

way. Shear along the cylindrical surface occurs by application of the horizontal shearing force T, equal to rnS 1. The rotational moment is equal to rnS1A, in which A is the arm of the horizontal force with respect to the axis of rotation, and the moment of the restraining forces is rnSR + Q A in which R is the radius of the shear surface; S is the area of the cylindrical shear surface; Q is the weight of the suspender; and A is the horizontal displacement of the center of gravity of the suspender.

The arm of application of the horizontal force is

s 0 .-1 = - ,A. S, T

The second term in this equation is negligibly small. The value of S is determined from the equation

I

I I--" r )I I for / '>'~ --" A' I , ):

in which r is the internal radius of the soil container. In test samples of the device, r = 3.57 cm, R = 40 cm, and S = 1.0025~'r 2. Assuming R = 60 cm, we obtain S =

1.0009~rr 2. Thus, for constructionally advisable values R = (10-15)r, we have S = S 1. Figure 2 shows an alternative for construction of the device, in which the value of the vertical force acting on

the upper end of the sample is constant during the shearing process and is established only by the variation of the

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TABLE 1

.fi P =etersl on,

Apparent ] 21,1 I 45,2 } Actual 27,3 2i ,3

q~ll degl CI'kPa [ ~ll,,deg

18,8 I 20,4 19,B 26,1 1 10,6 26,6

I cII, kPa

I 30,4 14~9

Fig. 3. Test results for coarse-sand shear (mix of fractions as follows: 0.5- 1.0 mm, 70%; 0.05-0.1 mm, 30%, for Pd = 1.68 g/era s, and w = 0.07). Values of Oo: 1) 0.1; 2) 0.2; 3) 0.3; 4) 0.4; 5) 0.5; 6) 0.5; 7) 0.6; 8) 0.7; 9) 0.8; 10) 0.8 MPa.

vertical force transmitted to the shear surface. The test results for such a device are shown in Fig. 3. They are typical and illustrate the basic laws which are characteristic for tests in devices with lower movable sections.

Semi-rounded feldspar-quartzi t ic sands of alluvial origin were tested. The samples were 7.14 cm in diameter and 3.5 cm high. The tests were performed under a regime of constant shear deformation rate of 0.01 mm/sec. The maximum shearing displacement ~ was 8 mm. On the rn-O n loading path graphs, the residual values of the shearing resistance are denoted by asterisks. In what follows, use is made of the symbols o o and a n for the mean values of the normal stresses acting on the upper end of the sample and on the shear surface, respectively. From Fig. 3 it is seen that before shearing (r n = 0) the values of a n are always lower than those of a o. The experiments showed that the difference between o n and o 0 depend not only on the friction characteristics of the soil and of the soil container, and on the sample dimensions, which is well known f rom [2, 3], but also on the rigidity of the force meter. It is essential that during the shearing process the difference between o n and o o will vary and be able to experience changes in the sign. Within a range of small values of o o, the value of o n increases as r n increases for all soils, with the exception of the less dense soils. The value of o n may exceed that of o o by a factor of 1.2-1.5 for sample heights of 2.2-5 era.

For soils of medium density, the apparent graph of shearing (dashed line), constructed f rom the peak values ant and o0, and the actual graph of shearing rnf ffi f(o n) (dashed--dotted line) intersect. Under these conditions, the actual angle of internal friction is larger and the actual cohesion is smaller than the corresponding apparent values.

Analysis of the m - a n loading path graphs made it possible to conclude that at the time the peak strength is reached for di f ferent values of o o almost equal values of a n can be obtained, whereas equal values of o o in the tests yield different values of o n. Hence, large scatter of the values of rnf obtained in the tests for equal values of a o usually leads to the erroneous conclusion of significant variation of the strength properties of the soil and to the corresponding underestimation of the design values of # and c. However, the actual standard deviation of the shearing resistance rnf with respect to rnf = f(an), for example for tests of Fig. 3, amounts to 7.1 kPa, and is smaller by a factor exceeding 2 than the deviation of 17.9 kPa with respect to the relation rnt = f(oo). Consequently, the effect of random factors on the strength scatter is not so significant when the real values of o n are known. For the tests of Fig. 3, the normative and design values of the apparent and actual strength parameters for confidence probabilities ax = 0.95 and c~ n = 0.85 are

presented in Table 1. The above results are confirmed by tests performed using the device described in [6], in which a plane shear

surface is maintained. These and other experiments performed independently demonstrate that the existing Type VSV

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devices with lower movable sections and without measurement of the actual value of the normal force acting on the shear surface cannot be recommended for construction or, especially, scientific investigations.

CONCLUSIONS

1. At the present time, there are no reliable and accurate devices for massive engineering soil strength tests. 2. Direct-shear devices with upper movable surfaces frequently distort the test results. 3. Application makes it possible not only to substantially refine the values of the normative strength parameters

but also to increase their design values.

I . 2.

3.

4.

5.

6.

7.

8.

LITERATURE CITED

GOST 12248-78, Soils. Methods of Laboratory Determination of Shearing Resistance [in Russian]. S. R. Meschyan, Mechanical Properties of Soils and Laboratory Methods of Determining Them [in Russian], Nedra, Moscow (1974). A. F. Barkhatov, "Characteristics of determination of the shearing strength indices of soils in VSV devices," Vestn. Leningr. Univ., No. 6, 66-72, Leningrad (1975). Recommendations for Integrated Study and Evaluation of Construction Properties of Sandy Soils [in Russian], Stroiizdat, Moscow (1984). A. N. Dranovskii and M. S. Vorob'ev, "Correct determination of the strength characteristics of soils under one- plane shear," Sb. VNIIOSPa, 1, 45-47 (1987). I. Yu. Amsheyus, "Investigation of variation of normal force on shear plane in soil tests in one-plane shear devices with movable lower sections," Sb. VNIIOSPa. 1, 47-49 (1987). A. A. Alikonis, I. I. Amsheyus, and V. A. Kuleshyus, Inventor's Certificate No. 1217983 USSR, MKI s G 01 N 3/24; E 02 D1/02, "Device for shear testing of soils," Otkrytiya Izobret. No. 10, 2 (1986). A. N. Dranovskii and M. S. Vorob'ev, Inventor's Certificate No. 1332186 USSR, MKI s G 01 N 3/24, 33/24, "Device for shear testing of soils," Otkrytiya. Izobret. No. 31, 3 (1987).

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