At the start of a project, if weak soils are encountered on site, one solution is to make deep foundations. Another remedy in this case is to improve the properties of that soil so that it will become denser, stronger and more stable.
There are many ground improvement methods available, but the first thing done is usually is to excavate a little deeper and replace the weak soil with stronger material if possible, such as replacing clay with compacted sand. We try to reach deeper as the soil in greater depths were subjected to more loads in the past and hence have greater bearing capacity.
There are many different ground improvement techniques, which depend on the ground conditions and the project requirements.
The soils, as mentioned before, from largest to smallest grain size can be classified as:
Gravel Sand Silt Clay
Each of these soil types or mixture of these, combined with different structure size and load requirements, available time, budget and work space, and different underground or even seepage conditions, would necessitate different ground improvement technique.
Using these methods add to the cost but they often enable us to build shallow foundations instead of piles, which are far less expensive with faster construction schedules than deep foundations, if we had not improved the ground. The cost benefit alternatives as far as time and money of improving ground vs. building deeper foundations should be carefully considered before moving on, by experienced geotechnical engineering consultants. Sometimes, regardless of the budget and schedule and the type of foundation, the ground must be improved anyway.
Through ground improvement, soil’s engineering properties are modified and improved such as bearing capacity, shear strength, improved stress distribution in soil, decreased elastic or consolidation settlement, slope stability, liquefaction resistance, frost resistance, control of shrinkage and swell of soil, groundwater and seepage control (waterproofing and improved runoff), improvement of substandard materials. These methods can also be used to increase capacity of existing foundations.
So usually, ground improvement and foundation repair subjects are inseparable, and for the most methods we will discuss, remember that they can be used for both purposes. Some ground improvement techniques are also used for groundwater and seepage control especially during construction, when there are open excavations in the ground and water needs to be evacuated or prevented to fill in the excavated area in order for construction work to proceed.
Based on extensive field application experience and research, today a lot of effective ground improvement methods are at engineer’s disposal. These methods include but not limited to the methods we will discuss in the following posts under this title.
The methods below are about improving soil that will carry structures on them. For improving the condition of slopes and retained earth, see Retaining Structures, Excavations section later.
In the next post of this series, we will discuss “Grouting Methods”