Which method is not an approved method for soil density testing?

The method that is not an approved method for soil density testing is the balloon method. This technique involves using a balloon to measure the volume of a soil sample, but it lacks the rigor and accuracy of more established methods.

In soil testing, widely accepted methods include the sand cone method, which involves using a specific weight of sand to determine the density of the soil; and the nuclear density gauge, which uses radioactive isotopes to measure moisture and density levels in the soil. Core sampling is also a valid method, as it allows for the extraction of a soil sample, which can then be used to evaluate density among other properties. These methods are standardized and widely recognized for their reliability and accuracy in determining soil density, making them preferable for engineering and construction applications.

In contrast, the balloon method does not have a strong footing in conventional practice and lacks standardization, which is critical in ensuring consistent and reliable results in soil testing. This is why it is not recognized as an approved method for soil density testing.