Shore crossings construction and installation of structural foundations usually take place within the nearshore zone, which is typically between the shoreline and the toe of a coastal structure. In this region, there are generally multiple geotechnical challenges that can complicate or even halt the project.
These challenges can be due to sharp changes in bedding plane or structural discontinuity at the shoreline, soft alluvium overlying stiff clay or bedrock, damage to existing structures such as gas pipelines with high water table, etc.
One of the most promising techniques that address many of these challenges is the Horizontal Directional Drilling (HDD) method. The process involves precision placement of casings using drilling rigs. Casing is driven in open boreholes to depths well below the water table and without disturbing existing structures because there is no soil displacement or surface dissection.
The following are five important steps that must be considered while performing any project involving hdd crossing. These steps will ensure optimal performance of the HDD operation and mitigate risk to project schedules and budgets.
Specify System Requirements
Typically, foundation design engineer specifies HDD system requirements for a given crossing. A project manager should verify that the selected drill rig has capacity to meet these requirements within the time available for the installation phase of an HDD project.
Conduct Core Samples
Involve conducting core samples before allowing any drilling work to commence. This involves taking a soil sample from the seabed area to determine its strength and capability of supporting heavy loads.
If the drilling machine has a very high power output, it is important to check whether or not the soil can take all that load or if it will collapse under pressure.
Verify Drill Rig Capacity
The drill rig capacity must be verified by experienced drillers prior to the construction of hdd crossing as many rigs produce stated capacities that do not cover all possible contingencies for a given project.
For example, as per our experience, you can easily lose 20% or even more capacity if the soil contains boulders and cobbles as it is difficult for drill rigs to apply constant downward thrust.
Create Contingency Plan
Even after selecting a proper drill rig and verifying its capacity, the project manager must plan for contingencies. Soil can change during the drilling process and HDD casings wear out over time. These factors will require you to stop installation and resurface casing to replace worn out components such as drill bit, saw/reamer blades, etc.
You should also be prepared to end the day early if the rig hits rock during installation. This will impact your final output for the day and project schedule.
Set a Clear Communication System
Project managers must be proactive and communicate these contingencies to the construction team well in advance to avoid delays and cost overruns. No matter how well-planned the project is, if the instructions and other important specifications of it are not properly communicated within the team, the risk of errors and accidents from taking place is high.
Apart from the key measures mentioned above, it’s also important to have everything documented and verified before seabed drilling can begin. An independent third party should be hired to check that all work has been carried out to the highest standard, including environmental standards, before a final inspection is conducted. This gives some peace of mind to everyone involved in the project and ensures a strong base for a strong foundation on which future projects can be erected.