Case Study: Sumas River Direct Pipe Void Fill
Challenge
Create and implement a grout-like bentonite slurry which could:
- Provide sufficient compressive strength to support the overlying formations and infrastructure
- Be pumpable, easily mixed and injected
- Resist groundwater incursions to mitigate dilution
- Stabilize and heal the local geologies
Description
A 42” diameter casing was to be removed after a 24” pipe was installed within it. The void created by the removal of the casing would eventually lead to instability in the overlying formation and infrastructure. To further exacerbate these problems the local geology was mostly gravel, sand and silt. A typical grout system would prove too costly and difficult to inject.
| Grout System | Star-Plex System | |
|---|---|---|
| Mass (kg) | 39766 | 9247 |
| Price | $27,591.14 | $7,828.15 |
| Void Volume | 192.7 m3 | |
Solution
A novel thixotropic fluid system was developed based on a Star-Plex bentonite system typically used in HDD and tunneling to address highly unconsolidated formations. Star-Plex systems exhibit very high and flat gel strengths while having a comparably low plastic viscosity. Star-Plex, an MMS type rheological modifier, boosts the properties of conventional bentonite systems allowing low concentrations systems to mimic high viscosity systems. The enhanced thixotropy of the fluid allows for the fluid to maintain its pumpability while at the same time still being able to readily gel. While in its high shear state, Star-Plex systems exhibit a low relative viscosity and can enter into porous formations. Once the fluid enters into the low shear environment of the formation it rapidly Viscosifies, thereby consolidating the formation. This characteristic would prove helpful in further stabilizing the bore until the fluid could finish pumping.
As the casing was removed the fluid was pumped into the casing and resulting void which formed. The fluid would begin to gel and support the overlying formation. This process of inching out the casing while pumping the fluid in continued until the casing was fully removed. The resultant gelled fluid was topped off to ensure all spaces were filled.
Result
No settlement of the overlying infrastructure was reported and the casing was removed without incident. The ultra-high viscosity Star-Plex void fill fluid provided the necessary compressive strength needed to hold the formation while still being pumpable.