GEOTECHNICS

Foundations and settlement report

Foundations and settlement report

Answer 1

From the description given in the borehole 1, it can be said that at a depth of 0.8 meters, the soil is soft silty clay. As we go further deep, the soil changes characteristics and becomes firm to stiff grey silty clay. At a depth of more than 8 meters, however, weathered mudstone is encountered, implying that as we go deeper into the strata, the soil becomes tougher and in order to construct very tall buildings such as skyscrapers in an efficient and safe manner, foundations should be deep, since the strata deep down is more strong and sturdy as compared to the strata which is closer to the surface.

Answer 2

In this case, the size of the square footing needs to be determined. For the calculation of the size of the square footing, the formula that will be used is;

Required footing size=Column spacing x Loading x Module width/Bearing capacity of soil.

Answer 3

Accurate estimation of the foundation settlement under a structural load is very important during the design phase of a structure for comparison of the expected settlement against the limiting values according to the new Load and Resistance Factor Design (LRFD) method. The LRFD design of structural foundations requires satisfying two principal criteria: the ultimate limit state and the serviceability limit state. The foundations are checked against penetration or sliding in bearing capacity failures (ultimate limit state) and against tolerable limits of settlements (serviceability limit state).

Foundation settlements are restricted from the safety and the serviceability considerations of particular structures. LRFD codes limit the maximum differential settlements for the safety of structural elements (since differential settlements cause additional stresses) and the maximum total settlements for proper functioning of the structures. The differential settlements in practice are usually managed, however, by limiting the total settlement as opposed to taking into account the differential settlement. D'Appolonia et al., (1968) developed correlations between differential settlement and total settlement, based on experimental observations. The settlement (total) for shallow foundation is generally limited to 25 mm to control the differential settlements (which have to be less than 20 mm) and any subsequent distresses to the structure (Terzaghi et al. 1996). Thus the methods of settlement calculation are the subjects of considerable interest to the practicing engineers in applying the LRFD method for foundation design.

To this day, a number of methods are available for settlement calculation of shallow foundations in granular and cohesive soils. Douglas (1986) stated around 40 various methods of settlement calculation for foundations in soils which are granular. These methods were mostly empirical and/or semi-empirical based and rely on particular types of field and/or laboratory tests for calculation of the total settlement. For foundations in clay, two fold calculations are generally performed for vertical settlements. Settlements under undrained loading are first calculated using a three-dimensional elastic solution (e.g., Skempton and Bjerrum 1957; D'Appolonia et al. 1971; Foott and Ladd 1981). Next the one-dimensional consolidation theory is applied to calculate drained primary settlements ...