Raising of the Dam Wall
This long-overdue project, to raise a dam wall in South Africa, finally got underway in late 2019. The project involves the raising of the existing dam wall by building a new wall against the existing wall built in 1935 and extending the height of the wall by 13m. By raising the dam wall, it will increase the capacity of the dam to almost double. Allowing for more irrigation in the region by an estimated additional 5 500 hectares. Not only will this assist emerging farmers and agriculture in the region but also create additional work opportunities especially at the construction phase.
Due to the lack of geotechnical information on hand and the complexity of this design, Geomech Africa was appointed to assist with the Geotechnical Investigation for the raising of the dam wall. The requirement was to drill three 25m deep vertical N-sized boreholes on the Eastern bank of the dam adjacent to the current dam wall. An additional three 30m deep incline boreholes set at an angle of 60o were required at various intervals on the steep slopes of the Eastern bank with the last borehole being at the proposed 13m above the existing wall. Not only did we have to provide the client with the best possible quality core and recovery but also conduct In-situ testing such as SPT tests were conducted at 1m intervals in the vertical boreholes, Shelby samples where possible and core orientations to be done in all of the inclined boreholes.
There are many challenges when drilling on steep rocky terrains. Moving of machines and equipment, supply of water & a safe working area for the crews. With only narrow access roads and limited space to move took careful planning, organization, implementation, and teamwork from the entire crew. Accessing the three lower borehole locations were relatively easy as our YWE D90 drill rig is mounted on a set of rubber tracks which allows it to traverse over rough terrain. However, accessing the 3 higher borehole locations was not as simple, and given the gradient of the slopes, it was not a job for our rubber tracked undercarriage. Fortunately, Geomech Africa is equipped with MERC 4x4 Crane trucks which can access almost all hard-to-reach terrains. Our skilled driver negotiated the narrow tracks and tight turns with his 4x4 truck and was able to get close enough to place our YWE D90 drill machine to the borehole location. However, in some cases, a final short skid of the machine was needed.
Another challenge the crew faced was the anchoring of the drilling machine whilst drilling the inclined boreholes. Due to the angle, you no longer have the weight and direct downward pressure of the drilling machine to give you stability which required us to anchor the machine to prevent sideways movement. We had to make use of concrete and steel pins which were driven into the ground and allowed to set once the machine was in position. The steel pins would extend through the skids of the machine with a bolt securing it from the top. This gave us the stability to drill the inclined boreholes.
We quickly discovered that the formation we had to drill was extremely hard when preparing our circulation sumps at the first borehole location. In fact, it was so hard that we had to make use of a TLB to assist with breaking ground at some of the borehole locations. This got us thinking and wondering if the diamond-impregnated drill bits we had selected for this project were in fact the correct bits to tackle this extremely hard rock. What made matters worse is that this extremely hard rock was also highly fractured. In between these fractures was anything from tree roots to soft reddish clay making recovery and progress very difficult. The drill operator had to be very vigilant whilst drilling and monitoring his drill string for these fractured areas. It is not as simple as drilling with one type of core barrel and bit through these formations. Each time the formation changes, a different bit has to be exchanged or in some cases, an entire core barrel and bit needed changing. We found the best method was to drill with a NWD4 core barrel and appropriate drill bit through the fractured formations. To overcome the extremely hard rock formation and to prevent core loss and bit wear, we resorted to a TNW core barrel with a diamond impregnated bit specifically designed for extremely hard rock with the addition of biodegradable cutting oils. By using both these drilling methods we were able to provide the client not only with good core recovery and samples but all the information they needed for design purposes.
Drilling takes us to many wonderful and untouched places, but it is not often that you are privileged enough to work on locations such as these. Once you have overcome the challenges, survived the extreme heat and hardships and you look back on the positive results you have provided to a content and happy client, then you know it was all worth it!