Consideration of impact assessment of vibrations for strength and compression in silty soils

Strength and deformation of soil are determined in laboratory tests through shear strength and compression tests. However they do not take into account the impact of dynamic vibration which can cause thixotropy or fluidization of soil. To allow for that, a method which modifies the course of laboratory tests has been proposed. It includes dynamic vibration into shear strength and compression tests of soil. It makes it possible to measure and classify the tested ground with respect to susceptibility to thixotropy caused by dynamic vibrations which lower strength parameters of water saturated silty soils. Earlier research of soil strength parameters tended to underestimate the impact of dynamic vibrations which sometimes constitute initiatory impulse for numerous landslides in rockmass, especially in flysch.


Introduction
Two physical and mechanical parameters decide about strength and deformation of soil: the first one being shear strength, the second one -soil compression.Shear strength is resistance of soil to static stress in the considered fragment.In order to determine shear strength, laboratory tests are held with intermediate shearing test apparatus or triaxial compression test apparatus, whereas soil compression is determined in oedometric tests.In the procedure of both the test types, a soil sample is made to reach its natural structure.The triaxial method reflects shearing conditions in the isotropic layer of high thickness, whereas the immediate shearing method additionally allows to reflect conditions found in narrow contact zones of soil layers of different characteristics, where landslides can be initiated.Results of the above mentioned tests are obtained with assumption of consistency or insignificant changes in the soil structure within the whole period of testing in a laboratory.They, above all, do not consider a possibility of changes in ground framework structure at the time of testing caused by dynamic impact.In site conditions the external dynamic impact can result from: a) vehicles travelling on soil surcharge, b) vibrations of machines and equipment, c) vicinity of belt conveyor flights ( for instance in open-pit mines), d) shooting or rockbust ( for instance in underground mining), e) natural factors such as earthquakes or thunders and others.
The above mentioned factors caused by dynamic impact can significantly change stress conditions and form strength parameters of soil.It mainly considers soils of considerable content of silt fraction (silty soils), especially when they are found in water saturated environment.Dynamic vibrations can have multiple impact on changes in soil strength: increase in strength caused by self-thickening of soil can be observed in case of unsaturated coarse soils, in case of water saturated cohesive silty soils a phenomenon called thixotropy can occur .It causes a significant decrease in particle friction resulting in lessening or loosing contact strength, which is caused by pore pressure and results in transformation into liquid state, in case of water saturated non-cohesive soils with no drainage, liquefaction of soil can occur as a result of dynamic vibrations of cyclic load resulting in increase in pore water pressure, and in extreme cases to the extent when effective stress reaches zero.
The above discussed issues have partially been reflected in numerous publications, for example [1][2][3][4][5][6][7].The test method described by Taslagy, Chan and Morgenstern [4 and 5] is equally worth consideration.They tested the effect of sand vibration [4].The analysis was based on innovatory method of laboratory tests published in [5] in 2015.The idea of the solution has been referred to and used in this article.

Characteristics of influence of dynamic impact on soil
Soil vibrations caused by dynamic impact can initiate thixotropy in cohesive (silty) soils and liquefaction of non-cohesive soils.In non-cohesive soils liquefaction can be the result of vibrations in high water saturation conditions and lack of drainage.It is manifested by reduction of particle friction causing their mutual relocation.In this case we speak about quicksand occurring in sands and silty sands.It can be stopped by lowering water pressure in ground.In the case of cohesive soils, the framework of which is made of small particles with dominance of silt, long lasting structure disturbances can occur leading to changing its state to liquid.However, to make it happen, not only water saturation is needed, but also an impulse.This role can be taken by all sorts of vibrations.The changes are hardly reversible, sometimes impossible to reverse, despite ceasing of their initial reason.
Thixotropy consists in solation (into a liquid body of sol) of colloid saturated soils (small particles) from gelatinised state (gelatinous body) under the influence of quakes and vibrations or increase of pore water pressure (Fig. 1).Soils containing small particles of clay of colloidal sizes (< 0.0002 mm) are characterised by thixotropy.It also occurs when the ground framework is partially composed of particles significantly bigger than colloids, for instance silt or fine sand.[7].Gelatinisation of soil is possible under the influence of mechanical stimuli when mineral particles in the solution get freedom of motion and become able to rotate.This phenomenon is partially reversible and the time of getting a stable state depends on the type of soil.Second mutual clinging of neighbouring particles with their sharp edges is caused by forces of molecular and electrostatic attraction between them (Fig. 1a).Special susceptibility to thixotropy causing transition from firm to soft or even very soft state is observed mainly in silt, silty clay, sandy silt, sandy clay, silty sand, unwashed rock-mantle of all types of rock.

Influence of thixotropy and fluidization on rock mass stability
The phenomenon of thixotropy has major influence of ground parameters and, as a result, stability of rock mass.Decrease in ground strength can result in landslides initiated by vibrations, lasting longer than the vibrations themselves.It causes hazard of the rock mass stability loss in slopes and scarps or in ground under buildings [6].However, to let that happen, the following favourable circumstances are necessary: a) full water saturation of the potential slide layer of ground as a result of precipitation or rise of ground water level, b) Simultaneous presence of dynamic stimuli, especially harmonic vibrations of given frequency and amplitude and duration, which make thixotropy or fluidization possible in the slide layer of ground, c) Possible additional external load, for example with trees, wind, vehicles, etc. causing lack of friction among particles in the ground framework.Both the coarse soils (Fig. 2a and 2b) and the fine ones (Fig. 2c and 2d) can, in such conditions, become fluidised or undergo thixotropy caused by vibrations.Sandy alluvia composed of smoothed grains, especially with addition of silt and clay are predestined to decrease in internal friction in their ground framework.This is caused by possibility of mutual rotation of individual grains (Fig. 2b).Silty soils of cellular structure (Fig. 2c) lose their apparent coherence to reach liquid state as a result of quakes Fig. 2d.Therefore it seems advisable to undertake laboratory tests of soils of similar structure to determine changes in their strength under the impact of dynamic vibrations.
Until now it was common to neglect these issues in laboratory tests despite their significance.Observations and analyses prove that the phenomenon appears to have considerable influence on stability and safety of numerous structures, such as scarps and slopes in open pit mines.

Methods applied to determine soil strength 2.1 Immediate shearing test apparatus
Coulomb interaction constitutes basis for determination of soil shear strength [7], however it does not take into consideration the impact of dynamic stimuli.As it results from this interaction, the amount of static stress constituting soil shear strength depends on friction resistance and cohesion: where: V'effective normal stress to the shear surface (V' V u), Vnormal stress to the shear surface, u water pressure in soil pores, W'fresistance of the ground against static stress (when 'u=0), I'proper (effective) internal friction angle (when 'u=0), c'effective cohesion (when 'u=0).

Oedometer
To determine the rate of ground subsidence the module of ground compression M is used, determined from the following interaction: where: Mo, M -oedometric module of primary compression, secondary,

Proposed modified method of testing
Modified testing equipment can ensure consideration of vibration impact on soil strength parameters.It should take into account transition of vibrations from the equipment to the tested soil sample.Such equipment was described in [4] and tested with success [5] with respect to sand, proving impact of vibrations on its strength.The range of amplitudes and frequency of vibrations applied at tests should reflect their parameters determined by measurements or expected for natural conditions.The same approach could be used to consider increase in subsidence caused by vibrations.

Modified tests with immediate shearing test apparatus
When there is a possibility for dynamic vibration to occur, one can determine their influence upon soil strength with use of methodology described in 3 with the following modifications to interaction (1): where: W'tresistance of the soil against static stress under dynamic vibrations V'effective normal stress, I'teffective internal friction angle with consideration of the effect of dynamic vibrations, c'teffective cohesion with consideration of the effect of dynamic vibrations,.
It is therefore possible to determine minimal values I't min and c't min for a number of tests with forced vibrations of various amplitudes and frequencies.When the obtained results are compared to soil strength parameters I' and c', determined for soil not subjected to vibrations, one can reach a coefficient to correct the obtained results in a typical way (with no vibrations used).A coefficient of vibration impact X can be a measure of potential influence of these vibrations on ground and its strength, and it is determined from the following interaction: The therefrom determined coefficient X used for the shear strength determined in tests, allows to consider its decrease as a result of vibrations initiating thixotropy (I' and c' are multiplied by coefficient X .It can be assumed that, as the author suggests, soils with relation to vibrations remain insensitive ( X =1), slightly sensitive (1 > t X 0.8), sensitive (0.8 t !X 0.5) and very sensitive (X < 0.5).

Modified oedometric test of soil compression
As a result of vibrations in the vicinity of a structure, additional deformations related to soil thickening in response to them can be observed.It is possible to take the phenomenon into account in laboratory tests.To do so one needs to apply an apparatus modelling close to natural dynamic vibrations in soil compression oedometer.The following interaction is then obtained: where: Moedometric compression module determined with no vibrations used, a) gelatinised state b) liquefacted sol 1water 2solid particles 1-flysch 2-rock-mantle 3-soil grains -movable 4-water 5-slide zone limit 6-pores (water and air) 7-stable soil grains 8-silty soil particles

Fig. 2
shows characteristic forms of soil structure sensitive to vibrations lessening or a) Coarse groundstable state, b) Coarse groundunder vibrations, c) Fine silty soilstable state d) Fine silty soilunder vibrations.
ground sample, hsample height before tests , 'hsubsidence of the sample as a result of stress change by 'Vi.
min , c't minminimum values of effective friction angle and cohesion, from all tests, I', c'effective internal friction angle and cohesion characteristic of soil not subjected to dynamic vibrations, max ' Vmaximum value of normal effective stress used in test.