In 2019, work was completed on the infill of four shafts in Essex with high density concrete. The concrete was designed to act as a plug to resist hydrostatic pressure uplift caused by the close proximity of the River Thames.
MagnaDense the optimum Solution
At the beginning of the project Fairhurst Structural Engineers conducted a preliminary assessment on the shafts, to determine the best option for plugging the shaft using the safest and most cost-effective method.
There were considerations to be taken into account when developing a solution;
- Due to the shafts’ proximity to the River Thames, additional weight was required to overcome hydrostatic pressure uplift from the water below.
- Due to the size and depth of the shaft, the use of standard density concrete would require people to enter the shaft and drill holes to enable flooding of the interlinking galleries, at a depth of around 20 metres. This would cause an issue with safety and was therefore not considered.
Five options were identified and considered and the option of High Density Concrete was subsequently adopted for the plugging of the shafts.
Using MagnaDense concrete would not only prevent hydrostatic uplift due to its density, but would also negate the need for people in the shaft, overcoming a substantial Health and Safety concern for the Engineers.
The agreed solution involved a combination of stone fill, high density concrete and normal concrete with hydrophilic water bars. These bars are waterstops which are designed to swell in moisture, effectively sealing concrete construction joints. These water bars would prevent future leakage along the top section of the plug.
Mixing high density concrete
A concrete with a density of 3.8t/m3 was developed by Capital Concrete containing a mix of MagnaDense 20s, 8s & 2. The MagnaDense concrete was placed in layers and pumped directly down 200 metres into the shaft, to prevent potential sliding due to the weight of the concrete. Following the final layer of concrete, the shaft was pumped out removing any remaining water and a waterbar ring was fixed to the perimeter. The shaft was then finished with normal concrete.
Jack Sindhu, Technical Manager of Capital Concrete commented, ”being involved with designing heavyweight concrete for bespoke applications from inception, coupled with the extensive experience of supplying MagnaDense concrete, we were able to provide a solution which met the expectations of all parties including our Partners, LKAB and Camfaud Concrete Pumps”.