__see citations by world experts [DA EVIDENZIARE]__

**SUMMARY**

Predicting settlements of shallow foundations is the No. 1 application of the DMT, especially in sands, where undisturbed samples cannot be retrieved. Many world experts consider DMT the best presently available tool for predicting settlements, notoriously not well predicted by conical probes (see citations).

Settlements are generally calculated, both in sands and in clays, by means of the one-dimensional formula (Fig. 8a) :

(1)

where D*s** _{v}* is calculated according to

The validity of the method has been confirmed by a large number of observed agreement between measured and DMT-predicted settlements. (see e.g. summaries of comparisons by Schmertmann 1986, Monaco et al. 2006, Failmezger 2015).

Settlements estimates are useful, for instance, when the designer has to choose between a shallow foundation and a piled foundation.

**POSSIBLE REASONS OF THE DMT’s ABILITY TO PREDICT SETTLEMENTS**

(a) Blades cause penetration distortions lower than axy-cylindrical probes.

(b) Modulus by a mini load test relates better to a modulus than a penetration resistance

(c) Availability of the parameter *K _{D}*, providing stress history information (note that DMT is a 2-parameter test. It is fundamental to have both: Ed and Kd). The availability of at least two measurements is indispensable, if deformation properties have to be investigated.

(d) The soil is loaded at a lower, more appropriate, strain level

Fig. 8b distortions caused by probes of different shape.

**IMPORTANCE OF THE STRESS HISTORY PARAMETER Kd TO PREDICT SETTLEMENTS**

The dilatometer modulus Ed should not to be used, as such, for predicting settlements, because Ed is only slightly sensitive to Stress History, while the operative modulus increases considerably with Stress History.* E _{D}* must first be corrected for Stress History :

*M _{DMT} = E_{D} *

where *E _{D}* is the dilatometer modulus and

Incorporating stress history information in the settlement calculation, using *K _{D}*, is not a refinement, but a necessity. The availability of the parameter

**NOTES**

- Not rarely
*M*estimated in situ are higher than*M*estimated in the laboratory, due to sample disturbance. E.g. Schmertmann (1988) compared*M*obtained by different methods at Sunshine Skyway Bridge, Tampa Bay in Florida. On the average,*M*estimated by DMT was 200 MPa, by laboratory oedometers 50 MPa, back calculated from observed settlements 240 MPa. The laboratory oedometers were in this case 4 times too soft, possibly due to sample disturbance and stress relief. On the other hand already__Ladd__(1971) and__Terzaghi__and Peck, in their 1967 book, had warned that even a good oedometer of OC clay may be 2 to 5 times softer than in situ. In sands in situ estimates of*M*are even more useful, due to the difficulty of recovering undisturbed samples in sand. - Eq. (1), based on linear elasticity, provides a settlement proportional to the load, and is unable to provide a non linear prediction. The predicted settlements is meant to be the
*settlement in working conditions* - Immediately after a DMT is completed, the predicted settlements may give an idea of the proper type of foundation. E.g. in case of buildings, very roughly, if the predicted settlement is < 3 cm (or possibly 4 or 5 cm), then a shallow foundation can be adopted, otherwise a deep foundation has to be adopted. In general the entity of settlements has a significant economical consequence. Accurate estimates may permit a more economical design.
*M*can also be predicted as*M*= a*Q*. The problem is that a depends on stress history or OCR, a missing information if only_{c}*Q*is available. The range of variation of a is quite wide (a = 2 to 20)._{c}

APPENDIX. CLARIFICATIONS OF THREE ITEMS

1.1 Legitimacy of using M as a constant

It is well known that oedometer moduli *M* are not constant, but vary with the applied vertical load. In particular the oedometer modulus increases up to the maximum past pressure *p _{c}*. At

Fig. 14. Schematic variation of the oedometer moduli with applied load. |

Fig. 14 shows schematically two typical e-log p oedometer curves, and the values of the moduli *M* at various applied vertical load *p*. In many natural soils, with the exception of highly structured clays, where the break is sharp, the variation of the modulus across *p _{c}* is moderate. Hence the error in assuming

It is reminded that *M _{DMT}* provides an estimate of the operative modulus during the consolidation. Hence the predicted settlement is the primary settlement, and does not include the secondary settlement.

1.2 Deriving M drained from an undrained test

In clay, the expansion of the membrane occurs in undrained conditions. Therefore the dilatometer modulus *E _{D}* is an undrained modulus. Thus, according to logic, the correlation to be investigated should be between

Obviously the final word goes to real world observations. Large number of case histories have generally proved the favorable comparisons between observed and DMT-predicted primary settlements, thereby supporting the use of *M _{DMT}* as operative constrained modulus.

Note also Lambe et al. (1977) : “Drained moduli of saturated clays are typically about one-third to one-fourth the undrained values”. Hence a broad connection drained-undrained stiffness has already been invoked in the past.

Settlements calculations are generally carried out using the 1-D elasticity formula in 1-D problems, say *large* rafts, or the 3-D elasticity formula in 3-D problems, say *small* isolated footings. The well known formulae are respectively:

(10)

(11)

However the general recommendation is to use in all cases the 1-D formula, for the following reasons (Marchetti, 1991):

- The 3-D formula (unlike the 1-D) involves Poisson’s ratio n (unknown) and makes use of the horizontal stresses s
_{h}that “may be grossly over-/underestimated by theory of elasticity”, while the vertical stresses “are surprisingly well predicted by simple elastic theory” - In most cases the 1-D formula gives settlements that are within 10% of the 3-D calculated settlements, because of the following compensation : M in Eq. 10 is higher than E in Eq. 11, but Eq. 11 contains a negative term
- Errors due to the formulae are small compared with errors in deformation parameters. As noted by Poulos : What is important is the modulus, not the formula.

Since the above two formulae give similar answers, it appears preferable to use the 1-D formula, simpler and avoiding elusive n or s_{h} .

© 2016 Studio Marchetti / Cookie & Privacy / Legal Terms