Multiple-step Loading Triaxial Compression Test (ML-TCT) method is a useful tool to evaluate strength parametersfromasinglespecimen. However, because of accumulated damage in the specimenwithrepeatedcyclesofaxialloading/unloading,theshearstrengthispronetobe underestimated. Therefore, a Multiple-step Loading Damage (MLD) model was proposed to simulate ML-TCTswithvariousstresspaths. Two series of ML-TCTs were carried out on a sedimentary soft rock of mudstone. The first series was to determine the geotechnical parameters todescribethe MLD model,andthesecondserieswastoverifythe MLD model. The results demonstrated thatthe proposed MLD model was applicableforsimulation of ML-TCTs onthe mudstoneandcorrectiontheresultsofcarriedouttests. This model allows prediction of peak deviator stress and the relevant excess pore water pressure due to previous plastic deformations in the specimen for various types of rock.


Multiple-step Loading Triaxial Compression Test (ML- TCT), which was proposed by Kovari & Tisa in 1975, permits determinationofafailureenvelopebytestinga singlerockspecimeninaseriesofconsolidationand shearing stages. In appropriate rocks, this technique allows determinationofstrengthparametersfromfewer specimens thandoesconventionaltriaxialtesting, which requires threeor morespecimenstodetermineafailure envelope. Additionally, it may allow a more accurate representation ofthespatialvariabilityofrockstrength, because allbetween-samplevariationcanbefully reflected,ratherthanpartiallyhiddenbydeducing strength envelopesfromseparateconventionaltriaxial tests. Specially, in jointed and inhomogeneous rock masses,itishighlyimprobablethattwoidentical specimens canbeobtainedfromthesamelevel,unless several borings are made atthe samesite, which is very expensive. In these circumstances, it is necessary to have alargequantityof informationfromeachsingle specimen.

ML-TCT is recommended by Goodman (1976) to determine shear strength of joints in triaxial compression test. In1983itwassuggestedbyISRM(Kovarietal. 1983)as a standard loading method in triaxial compression tests. In addition, ML-TCT has been used in soil materialsforseveraldecadesandvariousauthors have alreadyshownthepossibilityofcarryingoutsuch tests to define undrained shear strength (Anderson 1974) or effectiveshearstrengthbothinunsaturated(DeBeer 1950)and saturated soils (Kenney & Watson 1961). Moreover, several attempts have been made to study stress path and testing procedurein multiple-steptesting in soil material (Schoenemann 1988; Sorenzo 1988).

Akai et al.(1981)conducted multiple-step loading triaxial compressionteststostudysiltstoneandtuffand compared theresultswith those byconvent

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