In rodding the sample, how deep should the rod penetrate each layer?

In rodding a sample, the method aims to ensure proper compaction and representation of the material throughout the layers. Penetrating to the full depth of the bottom layer allows for thorough mixing and proper density assessment at the most critical layer, which is typically the base. Following this, penetrating 1 inch into the second layer allows for sufficient interaction without disrupting the upper layer's integrity significantly. This approach balances thoroughness in evaluating the deeper layer while maintaining the structure and properties of the upper layer intact.

When assessing the other options, one might find that penetrating to half the depth of the layer fails to adequately mix or compact the full thickness, which is vital for accurate representation of the sample's characteristics. Penetrating 2 inches in each layer could disrupt the upper layer excessively and may also lead to inconsistent layering. Lastly, simply disturbing just the surface may not provide any meaningful data regarding the density and properties of the material beneath. Thus, the method described in the correct answer is designed to obtain reliable, representative data while ensuring the integrity of the sampled materials.