Dynamic penetration is one of wide applied in-situ test techniques. The rod length during operation could be as long as hundreds of meters

calling for length modification in practical applications. Two major methods are proposed in most present specifications for the modification of dynamic penetration rod length. These two methods are founded differently on Newton elastic collision theory and elastic rod waving theory

and derived totally different modified coefficients. How to apply rod length modifications to the results of dynamic penetration test has currently become major issues that limit their application. Indoor model tests are put forward for the modification issues

altogether with varies of heavy dynamic penetration model being conducted on sand and gravel soil with series of rod lengths (2.0

8.9

16.4

23.4

30.0

36.0 m). The modification coefficient of heavy dynamic penetration rod length is obtained after the tests. It is analyzed and concluded that the modification to heavy dynamic penetration rod length followed the description of Newton elastic collision theory

and is independent of the material properties.