Annex I

(Informative)

# Field vane test (FVT)

## I.1 Examples of procedures to determine correction factors for the undrained shear strength

(1) Examples of procedures for the determination of correction factors of field vane test results to obtain the undrained shear strength (*c*_{u}) from the measured value (*c*_{fv}) of the field vane test are given in I.2 to I.5. These correction factors are based mainly on the back-analysis of embankment failures and load tests in soft clays. All procedures lead to a value of the correction factor (µ) which is used in the following equation for assessing the undrained shear strength.

*c*_{u} = µ × *c*_{fv}

where

*c*

_{fv}

(2) The procedure to be used should be based on local experiences in the actual type of clay. It should also be considered that the drained shear strength might be lower than the undrained shear strength.

NOTE For additional information see X.3.6.

## I.2 Example of the determination of the correction factor µ based on Atterberg limits

(1) For soft, normally-consolidated clays, the correction factor (µ) is linked to the limit of liquidity or to the plasticity index. A sample correction curve is presented in Figure I.1.

(2) A correction factor greater than 1,2 should not be used without support from supplementary investigations.

(3) In fissured clays, a correction factor as low as 0,3 can be necessary. In fissured clays, the undrained shear strength should be determined from other methods than from Field vane tests e.g. Plate load tests.

NOTE The Danish Geotechnical Institute (1959) gives examples of correction factors in fissured clays. For additional information see X.3.6.

Key

*w*

_{L}

*c*

_{fv}based on the liquid limit for normally consolidated clays

NOTE Figure I.1 was published by Larsson et al. (1984). For additional information see X.3.6.

*c*

_{fv}based on plasticity index and effective vertical stress (σ'

_{v0}) for over-consolidated clays

## I.3 Example of the determination of the correction factor µ on Atterberg limits and the state of consolidation

(1) This correction is linked to the plasticity index (*I*_{p}) and the effective vertical stress (σ'_{v0}) in the ground. Sample curves are presented in Figure I.2.

NOTE Figure I.2 was published by Aas (1979). For additional information see X.3.6.

## I.4 Example of the determination of the correction factor µ based on Atterberg limits and state of consolidation

(1) This procedure has been elaborated in order to take into account the effect of over consolidation.

(2) An estimate is first made of whether the clay is over-consolidated or not, using the relationship shown in Figure I.3 (relationship between the quotient of measured shear strength (*c*_{fv}) by the field vane test to the effective stress (σ'_{v0}) and the plasticity index (*I*_{p}) for clays). If the corresponding parameters fall between the curves for "young" and "aged", the clays are considered normally-consolidated (NC), whereas clays falling above the curve "Aged" are considered over-consolidated (OC).

(3) Normally-consolidated soils are then corrected according to the curve marked NC in Figure I.4 and over-consolidated soils are corrected according to the curve marked OC.

Key

- 1 curve of Fig. I.2
- 2 lower limit of young day
- 3 upper limit of young day; lower limit of aged day
- 4 range of normally consolidated days (NC)
- 5 range of over-consolidated days (OC)

NOTE This example was published by Aas et al. (1986). For additional information see X.3.6.

Key

- 1 normally consolidated (NC),
- 2 over-consolidated (OC)

NOTE For additional information see X.3.6.

## I.5 Example of the determination of the correction factor µ based on Atterberg limits and the state of consolidation

(1) This procedure has also been presented in order to take into account the effect of over-consolidation.

(2) The correction factor (µ) for normally-consolidated and slightly over-consolidated clays can be determined as

where

*w*

_{L}

(3) In clays with a higher over-consolidation ratio than 1,3, the following correction factor (µ) can be applied:

where

*R*

_{OC}

NOTE This equation stems from Larsson and Åhnberg, (2003). For additional information see X.3.6.

(4) If the over-consolidation ratio has not been determined, it can be estimated empirically from the relation *c*_{fv} = 0,45 × *w*_{L} × σ'_{p}. The correction factor (µ) then becomes:

NOTE The equation *c*_{fv} = 0,45 × *w*_{L} × σ'_{p} stems from Hansbo (1957). For additional information see X.3.6.