Compaction is reducing the soil volume, to make it denser and stronger and also less permeable. Compaction of soils can be done by heavy construction equipment, that use different methods to compact the soil. One method is to keep dropping a heavy weight from a certain height, over the entire intended foundation area. So to do this for example, a crane-like equipment with a heavy iron weight attached to its hook, keeps elevating and dropping the weight, and does it by traveling over the entire area. Another method is placing a heavy vibrating equipment that compacts the soil while traveling over it. As mentioned under soil mechanics section, vibration is for compacting granular soils such as sand and gravel. Few of the most frequently used compaction methods are as below:
- Deep Dynamic Compaction: dropping a heavy weight from a high distance repeatedly, to improve the soil’s engineering properties. The high energy of the dropped weight, also causes deeper soils to get compacted as well. The dropping is performed on a grid pattern.
- Rapid Impact Compaction: this is done to densify shallow depth granular soils, by using a hydraulic hammer which repeatedly hits a plate that impacts the surface of the soil.
- (Vibratory) Roller Compaction: a construction equipment, called roller, with large cylindrical heavy wheels (called drum) travels over the soil, in order to compact it. Especially used for roadwork projects. The drum may or may not vibrate, depending on the compacted soil type.
- Vibro Compaction: A large vibrator is hung from a crane and lowered into the soil for compacting it (just like using a vibrator to remove air bubbles during pouring concrete of a structure, which we will cover later under concrete section). This method is most suitable for clean granular soils. To compensate for decrease in soil volume, new clean sand is added into the soil by puring it from the ground to the hole, while the vibrator is pulled back, unless a decrease in elevation of the final soil grade is purposefully desired.
- Explosive Compaction: this method involves setting explosives inside the soil in a controlled manner to create local, small scale artificial earthquake effects, to densify loose, saturated granular soils at deep levels, (which carry high risk for liquefaction as we discussed), to improve their density, strength and liquefaction resistance. Basically it is purposefully creating liquefaction beforehand, to densify the soil, rather than it happening later during a real earthquake when there is structure above.
In the next post of this series, we will discuss “Various other ground and foundation improvement methods”