Shallow foundations are used where the soil has enough bearing capacity and shear strength (in other words strong enough) at a reasonably close depth. The Federal Highway Administration defines shallow foundations as, “a foundation that bears at a depth less than about two times the foundation width”.
Under soil mechanics, we have said that foundations must be checked for two things. Bearing capacity and settlement. Settlement is the main determining criteria because a foundation that can bear the load may stil settle above acceptable levels. So the settlement value must be under a certain acceptable level. There are different types of settlement, such as the immediate settlement, which is called elastic settlement, time dependent settlement which is called consolidation settlement which we have covered under soil mechanics. There can also be differential settlements, which means, different footings settling in different amounts, which can be very detrimental to a structure. We must keep in mind that totally preventing foundation settlement is just not possible. We can only minimize settlements. A settlement will always occur, even if very small, even when the strongest foundation is used in strongest soil or rock.
First the foundation location must be prepared. The soil below the foundation must be brought to a proper depth that is specified by the design, which depends on both the soil strength and the loading and settlement requirements. The proper depth can be achieved by either excavation or filling and then grading and fine grading. Under any shallow foundation or slab on grade, the foundation soil must be firmly compacted to a a level specified in project documents, such as the 90 or 95% of the maximum dry density value.
After the soil grade is brought to an acceptable level, depending on the project sand can be placed, and compacted, and on top of that a slab on grade, which is made of low grade, unreinforced concrete may also be poured. Up to this point we are at the bottom of the foundation level and prepared the earth to receive foundation on it.
At this stage, the foundation location is ready and reinforcing steel that will go into the foundation itself, can be placed. Note that during this stage, the plumbing and electrical cables and pipes will also be laid out and placed which will run under the foundation so coordination between these trades and the foundation work is needed. After all this work, or if the project is big enough, when sufficient area is completed by this work, the formwork of the foundation can be started, so that the reinforcing steel and formwork erection can also take place at the same time, where formwork follows reinforcing work. Larger shallow foundations, such as mat foundations, contain a lot of reinforcing steel. This can create problems during concrete pour such as rebar congestion or vibration difficulties during pour.
In the next post of this series, we will discuss ” Deep Foundations”