What is the name of the costly method recommended to investigate subsurface soil conditions?

The recommended method to investigate subsurface soil conditions is known as a Test Pit. This method involves excavating a deep pit in the ground to observe the soil layers and their characteristics directly. It allows for a detailed examination of the soil stratigraphy, including the types of soil present, their physical properties, and any groundwater levels. This method is particularly useful for understanding the geological conditions at a specific site, which is crucial for foundational design and construction planning.

The Test Pit method can be resource-intensive due to the labor involved in excavation and the equipment needed to safely dig and stabilize the walls of the pit. However, the comprehensive insights gained from this method make it valuable for projects where soil conditions can significantly influence the outcome, such as in construction or land development.

While other methods like Bore Holes, Soil Probes, and Auger Tests also contribute to understanding subsurface conditions, they may not provide the same level of detailed visual and tactile information regarding soil layers as a Test Pit does, which is why the Test Pit is favored despite its higher cost.