Settlements
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 one of the best presently available tool for predicting settlements, notoriously not well predicted by conical probes.
Settlements are generally calculated, both in sands and in clays, by means of the one-dimensional formula.
Where Dsv is calculated according to Boussinesq and MDMT is the constrained modulus by DMT. In clays the predicted settlement is the primary settlement (net of immediate and secondary).
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.
Settlements Calculation using “DMT Settlements”
A computer program for calculating settlements according to Eq. 1 is freely available. The DMT Settlements Software is freely downloadable in the section SOFTWARE.
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).
Settlements estimates are useful, for instance, when the designer has to choose between a shallow foundation and a piled foundation.
- (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 KD, 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.
- 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 Terzaghi and Peck, in their 1967 book, had warned that even a good oedometer of OC clay may 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 Qc. The problem is that a depends on stress history or OCR, a missing information if only Qc is available. The range of variation of a is quite wide (a = 2 to 20).
Citations by world experts on settlements
Case History
Cost savings by using DMT to redesign the foundation system
Roger A. Failmezger, P.E., F. ASCE, D GE – In-Situ Soil Testing, L.C., Lancaster, Virginia, U.S.A.