External Dynamics and Sediments; Thixotropy in Transported Fine-Grained Soils

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External Dynamics and Sediments; Thixotropy in Transported Fine- Grained Soils Ackermann, E.

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Publisher’s version / Version de l'éditeur: https://doi.org/10.4224/20331585 Technical Translation (National Research Council of Canada), 1953

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Technical Translation TT-379

Title: External Dynamics and Sediments ; Thioxotropy in Trans~ortedFine-grained Soils. (xussere Dynamik und Sedimente; Thixotropie bei Urnlagerungen feink6rniger Sedimente. )

Author: Dr. Ernst kckermann Reference: Geologische Rundsc1:au -37: 100-101, 1949.

Translated by: 14.11. G. Nathan External Dynamics and Sedinents

Thixotropy in Transported Fine-grained Soils

Thixotropy affects the sliding and flowing movements of moist cohesive soils. However, a distinction must be made as to whether the temporary viscosity change known as thixotropy finally results in liquefaction, as in the case of soils in the state of thixotropic consistency (e.g. gl~icksoils),or whether it only results in a (partly) reversible decrease in strength as in plastic soils.

The group of materials affected by thixot,ropic treak- down of the structure and thixotropic movements extends from fat clay to sand with low clay content and also comprises deposits of calcareous rnilrl as well as Tine--rained soils containing organic matter. Layers of quick clay with inherent florv dancer can be detected before and after the movement of the soil ky determining the Atterberg limits, from ?,he excess of natural

?.later conterit above the liquid limit, and the negative values of the relative consis5encp factor, xith tho aid of conventi:~n&l soil-mechanics e::perirnental methods.

Althout';h the quick zones embedtied in recent loose soils are not very thick, they provide a better sliding surface

?or flow slides than soft-plastic clays. This is due to tlie fact that the internal friction of quick clays in the ?.iriuicl state -2- approaches the value of zero. The paths of such movements are curved. The configuration is similar to that of typical wound ruptures or land slides. If thicker de~ositsof ?\lick soil become liquid and break through the enveloping layers they may flow for miles along the floors of valleys o? basins formed by sedi- mentation, before they settle and stif'fen. In 1p93 annroxi- mately $0 rnillion cubic meters of soil flowed seven kilometres down the Vaerdal in two hours. In such rapid flowing movements the dry crust is broken into blocks, which float separat,el;r in the clayey matrix. Frequently, f?-owing and sliding mo~rements are

combined. The dry crust is faulted by displacements, but the coherence between the tangentially sliced blocks is maintained. During the landslide at Lade (near Trondhjem) in 1944 the liquefied quick clay flowed downwards under the dry c:-ilst, which nds sinking down in the upper region of the slide. This res~ilteain the dry crust being raised and

bul!?ed up in the lower region of the slide. Process and conf iqrration of tlie flowing movements of the soil are determined chiefly by the thickness of the layers involved and the viscosity of the quick clay. The liquefaction occurs irrespective of the water content. The

movement is caused by vibra+,ions due to earthquakes, volcanic explosions, blasts, etc., as well as by hvdrostatic pressure -3- changes in the pore ;vster during spring tides, floods, etc.

Inclinations of nppro:timat,el.y 2 per cent are sufficient for t?.e rnasses of soil to flow off..

If a cohssive soil is in the state of plastic consistency, the thixotropic consistency change does not result in liquefaction b~itin a spontaneous ternporbry deer-case in strength. There'ore, slitlini; rnovements, as well as nlrl stic flow under pressilre occur 1,;illen tke internal llr-iction sr.lddenlv decreases. The thilcotropy facilitates the release and the progress of the r~ovement.

In subayueous slidings an influence of :,hixotrogy on the rnovernent can t:e assumed only z"or plastic clayev I-ayers and. perhaps also for ci.~lc:~reouslayers. The fine s5rat;if ication in !.jav:T zon.r?s shows that the cohr?rence of the rim\-ine layers has been i:,aintaineb. This is incompat;ibLe with thixotropic ILique- faction.

In contrast to the rapid flowing of liquefied quick clays, creep slides move slowly and only when soaked excessively.

Their movement is arrested by loss of water and is only re~rived by its recovery. These creeping movements are related to solifluction phenomena as l,!ell as mud flows and similar trans- portatinn of cletrital material ;:ith hi,n;h T;~atcrcontent.

P.lany land slides ma:. be considered model examples for the study of macrostri~ctures. This applies particularly to the behaviour of rigid layers on top of a plastically displsced -4- substratum. Erosion processes are accelerated by the sudden movements of large masses so that special morphologic structures arise. Normal sequences of strata are affected by the fact that fine-grained sediments precipitated near the coast are thixo- tropically liquefied by the load of recent deposits or by sliding of oversteep declivities and transported as mud flows to regions further remote from the coast. In this process, unstratified masses, possibly containing fragments of non- liquefied sediment s which have been dragged along, become em- bedded in the fine-layered sediments of the normal sequence of strata.