Stiffness of fill material

Estimating the stiffness of a hydrauhc fill material is a different issue. In fact the fill material can be considered to be a very yoimg, non-aged soil deposit. In order to estimate the stiffness of a granular fill during the design phase, the following sequential procedure can be followed  

Although this approach is based on several empirical correlations and experience, a reliable estimation of the stiffness is possible. 

Once the fill is placed, testing can be performed to verify the initial assumptions. If necessary compaction needs to be performed in order to obtain the required density/stiffness. A CPT is the most common tool to test the fill mass over its entire thickness, however the derivation of stiffness is indirect and may require calibration (for instance Section 9.1.5.1, Cone Penetration and Standard Penetration testing in carbonate sands and see Appendix C). Additionally, with a seismic CPT, also Gq can be measured. 

Generally the fill is tested shortly after the reclamation has been completed. This means that the above described approaches neglect the ageing effects. Neglecting these effects is a safe but conservative approach. Observations demonstrate that the cone resistance in granular fill may increase in time after the filling works and compaction of the material. 

When using fine grained soil as a fill material, the initial stiffness of the fill is much more difficult to predict. The difficulty hes in the prediction of the initial void ratio and stress state. Reference is made to section 9.1, where the use of fine grained material is discussed. 

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