PERMEATION OF POLYMER FLUIDS IN SOILS (POPFS)

When tunnels for railways or deep foundations to high rise buildings are built, the first step is to excavate a large hole in the ground. A key challenge is to prevent the excavated hole from collapsing before inserting the final, permenant structure. One way to do this is to pump a special liquid called a support fluid into the open excavated hole. Currently the fluid that is most often used is a suspension of bentonite clay. When this fluid flows into the soil around an excabayion the clay clogs the pore space in the soil at the open face, forming a layer called a filter cake, which prevents fluid and soil movement, and supports the excavation.

A newer technology has emerged that uses fluids that are polymer solutions rather than suspensions of small clay particles. These polymer fluids work in a very different way to the bentonite clay suspensions. It is the high viscosity of the fluid that prevents collapse of the hole; these fluids can keep the excavation supported and safe without the need to form a filter cake. Support systems that use polymer fluids are cheaper and have a lower environmental footprint than systems using bentonite suspensions. However the interaction of the polymer fluids and the soil is more complex than the interaction between the soil and the bentonite suspensions. It is therefore more difficult for engineers designing these support systems to predict exactly how they will work and this has slowed their uptake by the construction industry. Our overall aim is to provide the fundamental science needed to reduce any technical uncertainty and therefore enable wider use of these materials. This will have both environmental and economic benefits.

In this project engineers with experience of working with polymer-based fluids in the laboratory and on construction sites will team up with engineers who are experts at studying the detail of fluid flow in porous materials to get a much better understanding of how polymer-fluid based support systems work. Members of this newly formed team have backgrounds in civil engineering, mechanical engineering, and petroleum engineering and are based at Imperial College London (ICL), the University of Cambridge (UoC) and the University of Oxford (Oxf). To deliver the research we will link advanced numerical modelling (at ICL) with detailed experimental measurements (at UoC and Oxf ).

The planned research will be divided into 4 work packages (WPs). In WP1, researchers at ICL will simulate flow in the pore space using computer models that are created using high resolution 3D X-ray images of the actual pore space. These models will provide a lot of detailed information, but only small volumes can be considered as they use a lot of computer power. Therefore, in WP2 ICL will use a simpler type of model, called a pore network model, to run larger scale simulations to look at the migration of the polymer front in a model of the soil. In WP3, UoC will use a specially developed laboratory apparatus called a permeameter to study the flow of the polymer fluids in real samples of soils; different types of polymer fluids will be considered. In WP4, Oxf will develop and carry out special 2D flow experiments so that we can see the polymer fluid as it flows through the pores in the soil. We will use the experimental data to confirm the computer models work and the computer models will generate data that can't be measured in the laboratory, such as the flow profiles in the 3D voids and the forces on the soil grains.

The key questions we will answer for engineers designing excavations will include:
(1) How easy it is for the polymer fluid to move through the pores in the soil (we call this the conductivity of the polymer fluid in the soil)?
(2) How much stabilizing pressure is exerted on the soil grains as the very viscous polymer fluid flows into the soil?
(3) How do the polymer chains suspended in the fluid interact with the soil grains?