Piping effect has serious hazards to river embankment engineering. The understanding of piping is gradually deeper and deeper to people. Nowadays

people know more about the beginning conditions of piping occurred and the consequences of piping channel completely penetrated the levee. However

because of the failure and development mechanism of piping is very complex

the study of the emission of sand and the flux after the piping effect is not deep enough. Three groups of cohesionless soils

in the same soil features

are used to make a series of tests by changing the thickness of overlying clay layer

using a self-designed equipment. A comparative analysis is made from the aspects such as piezometric head

scope of piping channel

sand production and flux. Furthermore the changing laws of the porosity

coefficient of permeability and flux are studied. It is found by the tests that cumulative amount of sand changing with time satisfies the Boltzmann model

a kind of nonlinear model. By using the relationship between permeability and porosity and the relationship between the porosity and cumulative amount of sand

a theoretical formula

from the changing flux over time

can be deduced and verified. The formula has a very important significance for the understanding of piping

the effective controlling of piping development

and the reducing or avoiding disasters of embankment collapsing.