Nuclear density also called radiation method is one of the fastest means of determining the insitu dry density of soils. The method is gaining more popularity but it has the more risk due to radiation. Generally, when the equipment is set up people are instructed to move far away from it till the reading is completed and it is returned to safe mode by the operator.
Other methods of determining insitu dry density of soils are:
The water displacement method
The submerged density method
The core cutter method
The sand replacement method and
The water ballon method
In the nuclear density method, there are two methods: the direct transmission method and the back scatter method. In the direct transmission method, the meter consists of two probes, one containing radio isotope source and the other a gamma detector.
Procedure:
- Place the meter on a cleaned and levelled surface.
- Create a pilot hole and extend the probe to a maximum depth of 200 mm to 300 mm into the ground.
- The detector records the amount of radiation which passes through the soil from the probe attached to the meter when inserted into the ground.
- The denser the soil, the greater is the absorption of gamma rays, and the lesser will be the gamma rays energy at the detector.
In the back scatter method, the same procedure applies. However, the source and the detector is contained in one probe and it does not require pilot hole. The detector records the radiations which had been reflected by the soil. The bulk mass density of the soil is determined from the radiation count over a fixed time period. The mass density obtained is for the top 40 to 50 mm. The advantage of the back scatter method over the direct transmission method is that it is simpler but it requires greater source strength.
Weather Limitations
The nuclear density gauge shall not be operated when:
1. the surface is frozen
2. the surface is wet
3. the temperature is below -5°C
4. the weather conditions are unfavourable, or are likely to become unfavourable
Reference
Arora, K.R. (2008). Soil Mechanics and Foundation Engineering. 7th edition. Standard Publishers Distributors, New Delhi.
