Physical modeling of stone column installation in fine sand
and its effect on lateral to vertical stress ratio
GeoMontreal 2013
By Olivier Hurley1, Mathieu Nuth1, Mourad Karray1, Claire Mollereau2, Marc Lauzon2
1 Dept. Civil Eng., Université de Sherbrooke, Québec, Canada
2 Geopac Inc., Boucherville, Québec , Canada
Vibro-replacement is a soil improvement technique that incorporates coarse aggregates in a weak
soil in order to reinforce the soil structure by creating rigid columns. It is used to reduce soil
liquefaction behavior and settlement and to increase bearing capacity, by confining the weak soil and
rearranging the soil particles with vibration. The installation of stone columns changes the initial soil
state which is either considered at liquid state or not taken into account by most design methods.
The lateral to vertical stress ratio is a clear indication of the modification of the state and needs to be
quantified effectively.
The objective of the research is to verify the change in time of the lateral earth pressure due to
stone column installation. A physical model of a unit cell was built in our laboratory allowing us to
reproduce the insertion of stone in fine sand. The sand is confined within a PVC hydraulic tubing,
14.75 inch of inner diameter and two feet high, with three external annular braces keeping a rigid
boundary and a circular shape. The fine sand is deposited in a loose state to represent a weak
behavior. Similar to an oedometric device, a vertical confining stress is applied at the top to
represent different overburden pressure with a loading arm capable of applying up to 3.5 tons (>300
kPa). The plate is cut in its center to allow the insertion of stone without affecting the load.
The stone is inserted using the Frankie Pile method without vibration in which the aggregates are
rammed with a Proctor like hammer to simulate the expansion of a cylindrical cavity. Tekscan© total
stress flexible sensors are used (3X#5027-2X#6300) to measure lateral pressure in its circumference
and in its height. These paper-thin sensors made of piezoelectric ink conform to the shape of the PVC
cylinder and reduce the risk of noise and imprecision considering data acquisition. By varying the
replacement ratio controlled by the stone weight input, we are able to evaluate the lateral stress
increase and its variation in time radially and vertically. The vertical confining stress is changed to
express in situ conditions, which allows us to investigate the evolution of horizontal to vertical stress
ratio at the vicinity of a stone column. A better understanding of weak soil improvement with stone
columns can clearly reduce stone input designs and cost of contracts.