Soak Pit Pamphlet

SOAK PIT GUIDE - Can I have a soak pit? - How big should it be?

BUILDING SECTION

USE OF SOAK PITS Storm water disposal is an important part of any building project. If all storm water from hard surface areas is not conveyed off site or to an approved soak pit ponding may develop on the property or neighbouring properties. If ponding does occur on a neighbouring property there will often be complaints and then Council will require you to remedy the problem. This is often more difficult and costly once drives and gardens have gone in, it is much easier to do it right the first time. This pamphlet has been produced to aid the ‘doing right’ and inform you in regard to storm water management and the use of soak pits. The best method of storm water disposal is to convey it off the property, either into a piped or open drain. Council requires you to deal with all hard surface water, that means all water from building roof areas (including sheds however big) but also water from concrete and other hard surface patio, driveway and parking areas. In some instances it is impractical to pipe water off the property and a soak pit may be the only viable alternative. Generally this will only apply where there is no Council service or the building is a small ancillary structure at the rear of a property. Soak pits will not be approved for houses unless there are exceptional circumstances. Soak pits will never be approved for hill slope properties or flat properties bordering the sea or a stream or river. The efficiency of a soak pit depends on the permeability of the ground and the size of the pit. Much of the Gisborne District (i.e. Kaiti) is underlain with impermeable clays and soak pits will be impractical. However on sandy, gravel, or pumous soils a soak pit will be acceptable if it is sized correctly.

Part E1- ‘Surface Water’ of the New Zealand Building Code shows a generic design for soak pits and includes a sizing methodology. That method is repeated in this brochure in a simplified form. Any building consent application where it is intended to use soak pits must contain calculations for soak pit size. E1 states that acceptance of soak pits is at the discretion of the Council and they will only be approved if there is no viable alternative. Where the collected surface water is to be discharged to a soak pit, the suitability of the natural ground to receive and dispose of the water without causing damage or nuisance to neighbouring property, shall be demonstrated to the satisfaction of the territorial authority.

19/07/2007 12:05:00 p.m. 1-7430-CS_Pamphlet_-_Soak_pit_brochure_(_WEB) printed 7-Mar-09

SIZING OF SOAK PITS 1. Carry out a Percolation test on site. 1. Drill a 100 diameter or 150 diameter hole to the expected depth of the soak-pit. If groundwater is encountered then this shall be taken as the maximum depth of the soak pit. 2. Fill the hole with water and maintain for at least 4 hours to pre-soak the ground. 3. Refill the hole with water to within 750mm of ground level with a measure rod inside. 4. Record the drop in water level at 1 minute intervals. 5. Plot the readings on a graph chart (see the attached example).

2. Determine the soakage rate in the ground 1. 2. 3. 4.

Take a selected range of readings from the graph chart Select a range of readings over a 5 minute duration period. These readings need to taken on the straight part of the graph line (not the curved part) Determine how much the water level has dropped during this 5 minute period

5. Soakage rate =

60 × water level drop in mm 5 minutes 2. ………………….…..mm/h

6. Soakage rate = - - - - - - - mm / hour

Example: Soakage rate If the graph result shows 100mm soakage over 5 minutes Hourly rate =

60 × 100mm 5 Min

= 1200 mm/hour

3. Assess the storm water catchment area (Rc) Measure all surface areas which collect rainwater in square metres, the calculation changes the area to part of a square hectare as required.. Include the roof area and also any decks, patios and paving which have a storm water sump. Calculate the volume per hour. Rc = 10 × C × I × A Where C = runoff coefficient (1 for hardsurface areas) I = local rain intensity = 70 for Gisborne city (see note below) A = catchment area in hectares Rc = 10 × 1.0 × 70 ×

area = 10,000

m

Example: Surface water catchment (Rc) If the total surface catchment area = 350 m2 350 Rc = 10 x 1.0 x 70 x = 24.5 m 3 10,000

3

3. ……………………..…..m3

Note

Local rain intensity is shown in appendix 1 of the New Zealand Building Code section E1 – ‘Surface Water’. For Gisborne this is 70, the rest of the coast north of Tatapouri has an intensity of 100. Inland areas are considerably higher. Please check with the Council if you are not in the city or Poverty Bay flats.

4. Determine the area of bottom of hole. It is preferable to have a wide soak-pit (say 2.4m diameter) rather than narrow. This prevents the need for very deep holes to be bored. 22 ⎞ ⎛ ⎜π = ⎟ 7 ⎠ ⎝

Base surface area = radius 2 x π

Example:

4. ...…………………..m2

Soak pit base area for a 2.4m diameter hole

Area = 1.2 x 1.2 x π = 4.5 m 2

5. Calculate soakage allowance into the bottom of hole. (This is not necessary if there was a zero result from the percolation test)

Soakage volume = base area x

rate per hour 1000

5. ……………….……m3

Example

Soakage volume = 4.5 (from box 4) x

1200 (from box 2) = 5.4 m 3 1000

6. Depth of soak-pit

Volume of catchment/ hr - volume of soakage/hr Base area

Example

Depth of soak pit =

24.5 m 3 (from box 3) - 5.4 m 3 (from box 5) = 4.2 m 4.5 m 2 (from box4)

Therefore the soak pit is to be 2.4 m in diameter and 4.2 m deep

The depth at 4.2 metres is obviously too deep. Soak pit diameter could be increased to lessen the depth or two soak pits in parallel constructed. This would bring the depth down to 2.1 metres.

7. Produce drawing details of the soak-pit Details need to include:• Location on site plan • A secondary flow path with sufficient fall (overflow) • Where discharging the overflow to (street or water course) • A sump on the inlet pipe to filter out leaves and debris • A removable lid Note that these calculations are for a “chamber” type soak-pit. If a “rock filled” soak-pit is to be used, additional allowance needs to be made for the voids between the rocks.