Testing methods

There are many techniques available in-situ or in laboratory for measurement of dynamic soil properties. Each of them has its own advantages and limitations with respect to different engineering problems. As mentioned above, soil behaviors under dynamic loading are mainly affected by the magnitude of shear strains. The Fig. 2-3 shows the ranges of shear strain amplitude of some laboratory and in-situ tests compared to those induced by earthquakes. A broad introduction on laboratory and in-situ tests will be present in the following sections in which tests are categorized by induced strain levels.

Fig. 2-3 Ranges of variability of cyclic shear strain amplitude in laboratory and in-situ tests (Ishihara, 1996)

Under a well-controlled circumstance, laboratory tests definitely have advantages on precise measurement, repeatability and controlled boundary condition. However, original conditions of soil are usually altered during sampling tasks and it may be difficult for some kinds of soil to be sampled. Laboratory tests on samples are also time consuming and cost effective.

The main advantage of in-situ tests is no need for sampling. This avoids degrees of disturbance of soil specimen brought by sampling. Another nice feature of in-situ tests is more spatially representative since in-situ tests measure the responses of a relatively large volume of soil. Furthermore many in-situ tests induce the deformations similar to problems of interest, especially seismic tests. However, for most of in-situ tests, field operations lack of standard procedures and data interpretation is generally more difficult. That makes in-situ tests not user friendly. The comparison between in-situ and laboratory tests is summarized on Table 2.1.

Table 2.1 Measurement of low-Strain dynamic properties of soils comparison between in-situ and laboratory techniques

(Lai et al, 1998)

2.1.2.1 Laboratory tests

For laboratory tests, as shown in Table 2.2, only few of them provide ability to measure the soil dynamic properties at low strain levels. At high-strain level, the volume of soil usually has irrecoverable change. Under drained condition it is easily to be observed from changes in volumetric strain. When under undrained condition the tendency of volume change in volume results in the change in porewater pressure and effective stress. So methods in this category should have capability to control the porewater drainage and measure the change in volume and stress.

Table 2.2 Laboratory tests for measuring dynamic properties of soil

Test strain

level Note

Resonant column test Low

1. Most common used in laboartory 2. Well control testing condition

3. Both stiffness and damping of soil measured

Ultrasonic pulse test and Piezo- electric bender element test

Low

1. Useful for very soft materials

2. Incorporated into conventional cubical txiaxial devices and other

Cyclic triaxial test

Cyclic direct simple shear test Cyclic torsional shear test

High

1. Most commonly used at high strain levels

2. Control the porewater drainage and measure the change in volume and stress

2.1.2.2 Field tests

Field tests for measuring dynamic properties of soil, as shown in Table 2.3, induce different levels of strain. When selecting an appropriate test for purposes, the representatives of the soil behaviors shall be considered. Another important concern is the necessity of invasive tasks. Some of the tests need drilling of boreholes or penetration of testing devices, while some can be performed on the ground surface non-destructively. Tests performed on the ground surface are usually more efficient and cost effective. They are practically useful for geomaterials in which drilling and penetration are difficult. But the information gained from borehole tests is more direct than tests on ground surface.

Most of the high-strain tests aim to measure characteristics like soil strength at high-strain level and can be correlated to the low-strain behaviors. Several high-strain tests are common used in geotechnical engineering of different purposes. Basically the strains induced by low-strain tests are usually small enough for assuming linear stress- strain behavior of soil, in other words, the strain is smaller than the linear cyclic threshold shear

strain γ^tl. Most of them are based on the theory of wave propagation in linear materials. The measured body or surface wave velocity and frequencies or wavelengths can all be directly related to the low-strain mechanic modulus of a soil.

Seismic tests involve generating a transient or steady stress wave and interpreting soil dynamic properties via measurements made at one or more different locations. Stress waves may be generated by various seismic sources ranging from sledgehammers to buried explosive charges. The measurement can be the distance and traveling time of waves or a digitized wavefield recorded by array receivers. For those needing boreholes, results (mostly wave velocities verse depth profile) can be easily obtained from simple computation of traveling time and distance of wave. But for those performed on the ground surface, the characteristic properties (such as traveling time- distance relation, dispersion relation) involve the signal processing of raw data (recorded digitized wavefield) and final results need inversion based on certain theories and hypothesis. The more complicated the interpretation process, the more uncertainty of results and expertise of testers required.

Non-invasive methods for measuring shear wave velocity include shear wave refraction survey and surface wave methods. Refraction techniques for near surface survey are traditionally based on head-wave methods. Recent developments in refraction tomography have enhanced the spatial resolution of the refraction survey. However, the results are subject to limitation that velocity must increase with depth. Furthermore, S-wave refraction survey may not provide the true S-wave velocity because of wave-type conversion in an area of non-horizontal layers. Surface wave methods do not suffer from aforementioned problems associated with refraction survey, hence are considered of special interest for the site surveys of geotechnical problems.

Table 2.3 Field tests for measuring dynamic properties of soil

Test strain

level

Borehole

required Remarks

Seismic reflection test Low No

Reflected signals are recorded typically using common midpoint arrays. Velocity between reflectors may be estimated during normal move out correction.

Seismic refraction test Low No

Velocity profile is deduced from recording the first arrival times versus source-to-receiver dstance.

Seismic tests using surface wave Low No

Steady State Rayleigh Wave method (SSRW) Spectral Analysis of Surface Waves Method (SASW)

Multi-station Analysis of Surface Waves method (MASW)

Suspension logging test Low Yes

Frequency components are much higher than those of interest in geotechnical earthquake engineering

Seismic cross-hole test Low Yes At least two boreholes required Seismic down-hole (up-hole) test Low Yes Only one borehole required Standard Penetration Test (SPT)

Cone Penetration Test (CPT) Dilatometer Test (DMT) Pressuremeter Test (PMT)

High Yes Penetration or borehole required