After finding stresses in soil, we can calculate settlement amount.
Settlement of soils in general has three stages:
- The immediate settlement as the load is applied, which is elastic settlement, (for all soils)
- the gradual, slow consolidation settlement while water is escaping through soil pores, (for cohesive soils only)
- and the secondary compression due to rearrangement of soil skeleton structure only (for cohesive soils only)
In this section, we will cover the first one, elastic settlement, which applies to all soils. For sands, this is the only settlement that matters, and the second and third one are only for clay soils. So when we study elastic settlement, we also study sand settlement. Because for sandy soils which have coarse grains, permeability is large enough, so that water can be assumed to dissipate immediately when the extra load is applied, so second and third items do not apply. The settlement of sand soils due to applied loads from above occurs mainly because of compression of soil skeleton, and it is called elastic settlement. Elastic settlement requires more force than consolidation settlement, for the same amount of settlement, and therefore sands are stronger than clays, as we also have seen before.
Other things that facilitate settlement in sand is vibration or submerging or soaking in water, together with applied loads from above. The vibrating motion can naturally be produced by earthquakes too. This is why loose sandy soils are vulnerable to earthquakes, especially if they are loose, and water is present close to surface, as in liquefaction that we described previously in this book .
So whenever you see construction work where a vibrating equipment is used to compact soil, you can understand that it is sandy soil, and you will probably notice that some water also has been poured into it, to make compaction easier.
In addition to particle rearrangement as described above as a result of loading, sandy soils can also further settle due to crushing of particles, if extremely heavy loads are applied on them as we described before, such as at the bottom parts of earth embankment dams.
The more angular the sand particles, the stronger the soil due to interlocking of particles and higher internal friction. And needless to say, denser soils are also stronger than loose soils.
In some situations, there can be a stronger sandy soil on top, but under that a weaker soil of loose sand or even clay can exist. So a settlement analysis of the top soil layer is not enough in that case. Lower layers must also be analyzed for the given loads.
In practice, to calculate settlements from dead loads for most ordinary shallow foundation systems, Meyerhof’s bearing capacity method, and Terzaghi’s works usually form the basis of designs. International Building Code Chapter 18 also provides more information on this subject.
As far as earthquake effects, seismic load duration is very small in relation to gravity loads, and it reverses a number of times in a short time, so for dense soils, seismic loads are not expected to contribute a lot to settlement. Regardless of that they must be accounted for and this can be done by including them in design load combinations by increasing soil bearing capacity requirement by a certain factor (i.e. %30).
The equations for calculating settlement amounts will not be given here as it goes beyond an introductory level text, and as always, interested readers are guided to countless of engineering texts that are available in the books or on internet. But we can at least list the following about calculating elastic settlement:
- Elastic settlement is calculated by using soil’s modulus of elasticity, E, (which can vary with depth), Poisson’s ratio (ratio of how a material converts vertical stress to horizontal and vice versa), dimension of the foundation, the stresses in soil (which is calculated using principles given in previous section).
- The settlement amount of rigid versus flexible foundation assumptions make a difference and they can be converted to each other using equations.
- Elastic settlement on clays and sands are calculated differently.
- Settlement on sand can also be estimated from the results of a commonly used tests, called Standard Penetration Test (SPT) and Pressuremeter Test (PMT), as the data are correlated with extensive research. We will talk briefly about these tests in the following sections.
In the next post of this series, we will discuss “Consolidation Settlement”