Category Archives: Subsoil Drainage

Subsoil Solutions For Aucklanders

Since the deluge in 2017, Auckland’s problem with its ageing stormwater drainage pipes came to light. The city’s population growth, coupled with the rise of multi-unit blocks of flats, is putting too much strain on stormwater facilities.

While the Council is already building a multi-million dollar central interceptor to address Auckland’s water woes, many can be done on the private property owners’ part to help the situation. This includes improving their home’s drainage system to minimise runoff to the streets and prevent debris from funnelling into stormwater inlets. Silt and debris blockage was one of the major contributing factors to the massive flooding and damage in New Lynn during the storm.

Installing structures such as subsoil drainage can help to maintain flow conveyance and remove litter and excess sediments that block hotspot inlets during heavy rainfall events.

The problem with old pipes and clay

About 16,000 households or 2 per cent of Auckland’s wastewater network use combined stormwater and sewage pipes. Many of the pipes in older parts of the isthmus were installed a century ago and have not been replaced or “separated.” In heavy downpour, stormwater can easily overload the pipes and backflow into the sewage system.

The stormwater dilutes the wastewater. And when it overflows on private property, it does not only cause a smelly problem but also poses a bigger issue from a public safety perspective. This usually happens on parts of the network that weren’t separated. In some areas, like Hauraki Gulf Islands, there is no formal stormwater system and natural land drainage isn’t sufficient to deal with stormwater.

The problem is exacerbated by Auckland’s clay-dense soils formed under a warm and moist climate. Clay accumulations are particularly high in western suburbs and the inner city areas have soils.

Clay particles accumulate in the layer of the earth below the surface soil, hence called the subsoil. This layer is made up primarily of minerals and leached compounds, and lacks most of the organic elements and the rich soil fauna and flora found in the top soil. Clay has low-porosity and impedes the land’s natural ability to absorb and drain away stormwater.

So when the rain pours, stormwater tends to pond on low-lying areas around homes or run off to the roads, overloading the public wastewater infrastructure. If the flooding reaches the foundation of a house, it can compromise its structural integrity and safety.

This video shows the usual drainage problems that Aucklanders are facing.

What can be done?

The Auckland Council is in charge of stormwater management, but property owners are responsible for maintaining private stormwater assets that run to their property. These private stormwater assets include drains, catchpits, grates, pipes, roof gutters, rain gardens, permeable paving, ponds and soakage pits.

Soak hole repairs and inspections

Stormwater is typically directed to a soak hole rather than led directly to the stormwater network. In areas with no stormwater reticulation facilities, soak holes allow water to percolate to the ground.

Soak holes are basically manholes with boreholes ranging from 5 to 20 metres deep. Over time, soak holes get blocked by the silt, vegetation and litter they trap, and this can cause flooding. These devices require annual cleaning and maintenance, which can include flushing the bores, disposing the waste properly, and checking the inlet pipes.

Soak hole inspections may also be done at any time to ensure the device is functioning as it should and complies with the required health and safety standards.

Subsoil drainage

If your existing stormwater assets are not sufficient, consider incorporating a subsoil drainage system. It can be as simple as digging a trench and filling it with gravel or rock (called a French drain or weeping tile). Modern subsoil drainage uses a specialised perforated pipe covered with geotextile material, which is laid around the foundation or in low-lying areas within the property.

As well as minimising flood risk, installing an effective drainage system can give your property’s market value a boost. In fact, part of the process of property valuation is a thorough inspection from little things such as materials quality and efficiency wiring and plumbing, to general details like the location, the lot and the house itself.

Nobody wants to buy a house that gets flooded every time it rains. So things like upgrading old plumbing fixtures or installing an efficient drainage system are good investments with greater potential reward. Even if you don’t plan to sell, these projects can increase your quality of life and protect your home from costly water damage.


Hiring qualified and experienced drainlayers

Below are some of the important benefits of hiring an authorised drainlayer for the job:

  • Compliance. Drainlaying is generally a restricted work because it involves sanitary fixtures that concern public heath safety. There are also minimum requirements, specifications and standards to keep in mind when building subsoil drainage, especially for subdivision construction. For instance, the subsoil drainage must have a cesspit or holding pit before overflowing into an approved point of discharge. The pipes must also be laid deep enough into the subsoil, otherwise it will simply drain away the surface water and flooding will persist.
  • Real value for money. The right people will provide the right solution for your property. You get quality results that prevent expensive problems from recurring.
  • Professional insurance reports. A standard home insurance typically includes water damage cover. If you want to make a claim due to sewer backup, overflow and discharge, or flood, a licensed drainlayer can provide you with a detailed and professional insurance report to serve as proof. If you are taking out an insurance policy, a professionally written report for your new drainage system will increase your chances of getting coverage for drainage emergencies.

To learn more about subsoil drainage solutions in Auckland, talk to us.

NZ’s Wet Season has begun

Is your basement, garage, underground flat and / or storage area ready for the upcoming wet season by being protected against flooding?

Spring has started with a wet bang in New Zealand, especially in Auckland. In 2017 an additional 5% (L/sqm) in rain fall is expected when compared with 2016 stats. Storms are more frequent, heavy and short resulting in “flash floods”.

How does the ground respond to flash floods and what solutions are available?

The earth’s condition prior to rainfall is very important to determine the outcome when a flash storm hits. Has it been raining slowly for a few days prior or was the ground dry?

For example in Auckland many suburbs have been developed on clay (predominantly West Auckland and Northern areas) and still don’t have access to the public storm water system. Such properties rely on the stormwater management system within the boundary which is often in form of a soakhole or if lucky a curb discharge.

Unlike areas such as Mt Wellington or Onehunga that have access to underground volcanic cavities, or the public system, homes that manage their own stormwater rely on soil percolation. In simple terms “how quick can my ground soak the water?” The higher the rate, the higher the soakage reducing risk of flooding.

Lower percolation values are common and when they apply, alternative methods need to be used to manage stormwater. If nothing is done, a high level of surface water will result in flooding and over a longer period of time the ground will be charged with so much water it will expanding to a point where underground streams may result finding the least path of resistance into your basement area if no protection is available.

Site conditions such as the following play a critical role:

  • Level of elevation
  • Ground type
  •  Percolation values (soakage tests can be done)
  • Depth of areas affected
  • Current stormwater management system

In the following pictures (Birkenhead Auckland) you can see some of the processes we use to tackle ground water (not in order of works). Are you affected? Call us for a ground water solution.




Mouldy Walls During Winter Days?

Are you finding that your walls are getting wet and mouldy during the winter season? Many properties in Auckland were built with a habitable layer below ground level, and this can cause drainage problems if not constructed correctly leading to wet and mouldy walls. If you find yourself with the problem, what do you do?

mouldy_wallsWet walls are a result of improper subsoil drainage around the building, which means that the drainage needs to be either repaired or replaced with a proper system for it to work correctly. Most often the subsoil system needs to be excavated, removed and replaced. If there is no drainage system, a new one will have to be installed.

Drainage NZ are specialists in basement flooding problems and offer affordable subsoil drainage solutions to ensure that you do not have to live with wet and mouldy walls. Talk to us today about getting rid of your mould!