In New Zealand a common method of managing rain water is Soak Holes. In the following information we will be focusing on the city of Auckland which is the most populated and quickest growing region in New Zealand.
In Auckland managing rain or so called “Stormwater” correctly is critical for the cities growth and infrastructure. Auckland Council is constantly challenged by the ever changing weather patterns and population growth. As the population grows so must the public stormwater system, the problem councils have is that the population is:
- Outgrowing the public stormwater system
- The public Stormwater system can’t grow quick enough to keep up with the demand. This is a costly task that only can only be done during certain times of the year and only as it’s needed. As such, councils are always behind on growing the city’s public infrastructure.
Several years ago, council engineers came up with a quick fix that would help slow down the effects of population growth and the introduction of more water being collected from new roofs that would discharge into the public system.
Design regulations for new builds and subdivisions now included the requirement and/or option to;
- Allow a property to manage its own stormwater as much as much possibly practical by means of retention tanks, detention tanks (regulated overflow allowing the public system to keep up when there is a high demand) and soak holes.
In section, we will be focusing on the different types of soak holes and how they can help property owners manage rain water from roofs and surfaces successfully.
Three Different Types of Soak Holes in Auckland
Residential Soak holes
In the early days many areas around Auckland did not have a public storm water system to connect onto. If you had a public connection, you were considered very lucky! Year later in 2020, some areas still don’t offer public a infrastructure. Many homes, especially those built in the 60’s and 70’s continue to manage their roof and surface water within the boundary. This is typically done by means of an onsite ‘Soakhole”.
As drainlayers when we ask for a LIM to identify what
Its all about ground percolation and enabling a site or home to manage its own roof and/or surface water within the site itself. What better way is there than allowing nature to do its own thing? The concept of the Onehunga soakhole does exactly that. (Speak to Drainage NZ about Soakhole drilling, engineering and design.
A GEOTECH drilling rig is arrange to drill down into the volcanic bedrock where a pipe riser is inserted to enable water to overflow as far down as applicable design stipulates. At that point, water will disburse naturally underground. Geotechnical drilling services are available)
As you can see in the design below, the soakhole does have a safety mechanism in case the drilled bedrock hole blocks or should the incoming water exceed the soakhole capacity. This is the overflow point shown as the entry head.
At this point water may overflow into the public system or external soakage trench as shown below.
Trench Field Soakholes
When rock is not available the GEOtechnical engineer may explore the option to discharge stormwater into a scoria layer underground if the site is suitable. The principal is simple. If found scoria layers can act as a disbursement field by allowing water to travel downstream across a whole suburb below the ground, naturally finding a route to the ocean. Because percolation is limited.
Design Notes may Include:
ADDITIONAL SOAKAGE PIT NOTES:
- THE CONTRACTOR SHALL TAKE NOTE OF HOLD POINTS ON THE DRAWINGS AND FOLLOW INSTRUCTIONS AS NOTED AND ARRANGE FOR THE APPROPRIATE ENGINEERING REPRESENTATIVE TO BE PRESENT WHERE APPLICABLE AND/OR PASS ON TESTING INFORMATION AS REQUIRED PRIOR TO PROCEEDING WITH FURTHER WORKS
- HOLD POINT – THE CONTRACTOR SHALL CARRY OUT SOAKAGE TESTING IN ACCORDANCE WITH THE AUCKLAND SOAKAGE DESIGN MANUAL AT THE PROPOSED SOAKAGE PIT LOCATION WITH THE ENGINEER IN ATTENDANCE. FINAL SOAKAGE PIT DEPTH AND DIMENSIONS TO BE CONFIRMED BY THE ENGINEER PRIOR TO SOAKAGE PIT CONSTRUCTION.
- HOLD POINT – THE BASE OF THE SOAKAGE PIT SHALL BE INSPECTED BY THE GEOTECHNICAL ENGINEER FOR APPROVAL, THE BASE SHALL BE 500mm INTO THE BASE OF THE INSITU FREE DRAINING GRAVEL / SCORIA LAYER. DEPTHS SHOWN INDICATIVE BASED ON GEOTECHNICAL INVESTIGATION.
- INLET CONNECTIONS INTO MANHOLES TO COMPLY WITH COUNCIL STANDARDS
- CONCRETE DRIVEWAY SLAB TO EXTEND A MINIMUM OF 500mm OVER THE ROCKFILL FOOTPRINT ON ALL SIDES
- GEOTEXTILE TO BE BIDIM A29 OR APPROVED EQUIVALENT AND SHALL EXTEND DOWN TO GRAVEL / SCORIA LAYER ON ALL SIDES, BRING GEOTEXTILE 500mm UP SIDE OF MANHOLE RISER AND TIE OFF WITH DENSO TAPE. (HOLD POINT – INSPECTION PRIOR TO BACKFILL)
For more detailed information about soakholes and soakhole regulation, please download the following guide called Auckland Soakhole Design Manual.
The purpose of this Manual is to provide guidance in the design of stormwater soakage devices for residential and commercial properties in soakage areas of Auckland City. Soakage areas have limited stormwater reticulation, and are primarily located in parts of Ellerslie, Penrose, Onehunga, Mt Eden, Epsom, Mt Roskill and Mt Albert. Public soakage devices are provided for runoff from roadways in these areas, but individual property owners must construct and maintain their own soakage devices for runoff from private properties. The soakage devices allow stormwater to percolate into the ground, and generally consist of either boreholes into fractured rock or large holes filled with scoria.
Can I make my own Soakhole?
The building code does allow a certain extent of “Garden Drainage” without involving a specialist, designer or even a registered drainlayer. We highly recommend you familiarise yourself with the rules before you attempt a DIY. Getting wrong could cause severe your or others property.
The quick answer to the question above is no. Soakhole need to be designed based on a series of things, including:
- Location (certain criteria needs to be met)
- Incoming water capacity
- Geotech conditions
- Size and depth
- Environmental effects.
The law is a more flexible around existing soakholes that need repair or redrilling. A repair may bypass all the consent and engineering requirements.
Not only should clients be aware of what the engineering notes are but so should the contractor to ensure a realistic scope is covered which will allow the client to budget the project accordingly.
Some notes may include:
- THE CONTRACTOR SHALL LOCATE ALL EXISTING SERVICES WITHIN THE VICINITY OF THE PROJECT SCOPE OF WORKS AREA PRIOR TO THE COMMENCEMENT OF CONSTRUCTION AND CONFIRM EXISTING LEVELS AND ALIGNMENTS.
- THESE NOTES SHALL BE READ IN CONJUNCTION WITH THE NOTES ON DRAWING C001.
- ALL WORKS AND MATERIALS TO COMPLY WITH AUCKLAND COUNCIL STANDARDS AND WATERCARE STANDARDS. ANY AMBIGUITY BETWEEN THESE DRAWINGS AND COUNCIL STANDARDS SHALL BE REFERRED TO THE ENGINEER FOR CLARIFICATION.
- FOR PIPE BEDDING DETAILS, REFER TO STANDARD DETAIL SHEETS UNLESS OTHERWISE NOTED.
- WASTEWATER PIPES SHALL BE 150mm dia. PVC SN16 UNLESS NOTED OTHERWISE. (LOT CONNECTIONS SHALL BE 100mm AS PER WATERCARE STANDARDS). NOTES ON THE DRAWINGS SHALL TAKE PRECEDENCE, REFER DISCREPANCIES TO THE ENGINEER FOR CLARIFICATION.
- ALL PRIVATE DRAINAGE TO BE CONSTRUCTED IN ACCORDANCE WITH THE APPROVED CONSENTS.
- BACKFILL MATERIAL SHALL BE FREE FROM ORGANIC MATERIAL.
- ALL PIPE CROSSINGS UNDER ROADS AND DRIVEWAYS TO BE HARDFILL BACKFILLED.
- ALL MANHOLES ARE TO BE 1050mm DIA UNLESS NOTED OTHERWISE.
- ALL SW AND WW 100mm DIA. LOT CONNECTIONS (RAMPED RISERS) SHALL BE EXTENDED INTO THE FUTURE LOT BOUNDARY AND BE CAPPED AT THE LEVELS SHOWN IN THE DRAWINGS, BELOW THE FINISHED GROUND SURFACE. THE CONTRACTOR SHALL PROVIDE AN AS-BUILT SURVEY OF NEWLY INSTALLED CONNECTIONS UPON COMPLETION OF THE WORKS.
- IT IS THE CONTRACTORS RESPONSIBILITY TO PEG OUT MANHOLES AND CHECK FINISHED EARTHWORKS LEVELS (MH LID LEVELS) PRIOR TO ORDERING MANHOLES. DISCREPANCIES SHALL BE REFERRED TO THE ENGINEER FOR REVIEW.
- PIPE GRADES SHOWN ON THE DRAINAGE LONG-SECTIONS ARE CALCULATED FROM MANHOLE CENTRE TO MANHOLE CENTRE.