Installing a stormwater management tank is a complex task that requires careful planning and a deep understanding of site-specific challenges. For a drainlayer, asking the right questions before starting a project is crucial for ensuring a successful, compliant, and safe installation. This guide breaks down the essential questions into key categories.
Site Assessment & Environmental Factors
Before any digging begins, it’s vital to evaluate the site.
- Soil and Water Table: Ask about the soil conditions and the depth of the water table. Soil type affects excavation and backfill needs, while a high water table may require dewatering systems or anchoring to prevent the tank from floating once empty.
- Site Accessibility: Is the site accessible for heavy machinery like excavators, cranes, and delivery trucks? Access can dictate the type of equipment needed and may require additional planning for temporary road closures. The size of the tank often determines the required machinery; smaller tanks may only need a 3-5 tonne digger, while larger ones might require an 8-13 tonne machine.
- Underground Utilities: Have all existing underground utilities, such as gas, water, electrical, and fiber lines, been located and marked? Failing to do so can lead to dangerous and costly accidents. Always request a utility locate before you start.
- Land Slope: What is the slope of the land? The slope determines the required depth of the tank and the design of the inlet and outlet pipes to ensure proper gravity flow.
Tank Specifications & Design
Understanding the specifics of the tank itself is key to a proper installation.
- Tank Volume: What is the required storage capacity? A civil engineer determines this based on the site’s impervious area, rainfall data, and local regulations. In New Zealand, this is often measured in liters or cubic meters (e.g., 2000L or 2m³). Knowing the exact volume is paramount for selecting and installing the correct tank.
- Tank Material: What material is the tank made of—concrete, plastic, or fiberglass? Each has different properties that impact installation. For example, heavy concrete tanks require a crane, while lighter plastic tanks may need specific backfill compaction to prevent deformation.
- Pipe Sizes and Levels: What are the inlet and outlet pipe sizes and their invert levels? The invert level – the lowest internal point of the pipe – is critical for gravity flow. Inlet, outlet, and overflow pipe sizes depend on the number and size of incoming pipes. For instance, two 100mm inlet pipes would typically require a 150mm overflow to prevent flooding.
- Overflow System: Does the tank require an overflow or bypass system? An overflow is crucial for directing excess water during heavy rain to prevent the system from failing. The overflow pipe size should be equal to or larger than the main inlet pipe to prevent system backup.
Regulatory & Compliance
Proper documentation and adherence to regulations are non-negotiable.
- Permits: Are all necessary permits from the local council or regulatory body in place? Installing without a permit can lead to fines or forced removal and reinstallation.
- Inspections: Does the project require a pre-settlement inspection or a final sign-off by a council inspector? Knowing this helps you schedule your work and ensures you meet all regulatory requirements for a successful sign-off.
- Backfill Requirements: Are there specific backfill and compaction requirements? Many jurisdictions have strict rules on the type of backfill material (e.g., gravel, crushed rock) and the required compaction to ensure the tank’s structural integrity.
Installation & Logistics
Efficient project management depends on logistical clarity.
- Project Timeline: What is the planned timeline for excavation and installation? A clear timeline helps coordinate equipment rentals and material deliveries.
- Lifting Equipment: Will a crane or other heavy lifting equipment be needed? While larger commercial projects may require a crane, many standard installations can be handled by a specialized Hiab truck or even the site’s own digger, provided the tank isn’t excessively large.
- Anchoring: Are there special anchoring or ballast requirements? In areas with a high water table, an empty tank can float. Knowing if the design calls for a concrete slab or strapping to anchor the tank is vital. Today, anchoring is a standard practice to provide an extra layer of protection against uplift and ensure compliance.
