Infrastructure Development Code

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DS-5 - Appendix F.1   General

The disposal of stormwater from roofs, parking access and manoeuvring areas has historically been successfully undertaken by discharging the stormwater to drilled soakholes or soakpits on private property. However, such disposal methods have contributed to land instability and the creation of nuisance due to groundwater seepage in elevated urban areas on the edge of Tauranga Harbour.

The degree of success and long-term serviceability of such soakage systems is also dependent on the presence of permeable sub-soils (and their permeability), the construction of the soakage system and the amount of maintenance undertaken. The soils present in the Tauranga City area vary, with the more permeable soils likely to exist adjacent to Tauranga Harbour, on elevated ground at Te Puke, along the coastal margins where sand-derivative soils are present, at Pukehina, and on most elevated rural areas (generally east of Te Puke).

Some areas of Tauranga City are suitable for on-site disposal of stormwater by ground soakage.  Traditionally, building at Mount Maunganui and Papamoa has utilised soakpits constructed from porous concrete liners, while in elevated areas adjacent to Tauranga Harbour holes were augered down through the Rotoehu Ash (soakage) layer and into an underlying marker bed, the Hamilton Ash (“chocolate”) layer, to achieve adequate soakage. For Mount Maunganui and Papamoa the stormwater disposed of by soakage helps recharge the groundwater and prevent salt-water intrusion into the water table.  In Tauranga the stormwater is disposed of into a porous ash layer within the soil profile. As both communities developed stormwater infrastructure was sized, in general, to cater for stormwater generated on roads and not from residential lots.

With the intensification of development, the ability of the land to deal with the increasing volumes of stormwater generated has, in some cases, become compromised with the results affecting both the subject property and adjacent properties. 

With the intensification of development, the ability of the land to deal with the increasing volumes of stormwater generated has, in some cases, become compromised with the results affecting both the subject property and adjacent properties. In areas where onsite stormwater management is required, the stormwater management system for new roof or hardstand areas shall be designed by an SQE Professional when the total roof and hardstand areas are increased by more than 30m², singularly or cumulatively, since the original consented system was constructed.

In Mount Maunganui these off-site effects have mainly been overland flows into adjacent properties and in some cases inundation of buildings during significant rainfall. In Tauranga, off-site effects have resulted in elevated groundwater levels with surface “breakout” in some areas, overland flows when the soakhole capacity has been exceeded and, in some cases, slope failure due to erosion from overland flows and/or elevated pore pressures within the soil profile. Accordingly, the use of ground soakage for the disposal of stormwater is no longer appropriate in some areas.

DS-5 - Appendix F.2   Design Storm

The return period of the design storm depends on where excess stormwater will drain to if the capacity of the system is exceeded. The design storm duration will vary depending on the soakage characteristics of the site and the type of soakage system proposed. The critical storm duration is the storm that requires the largest system. 

Soakage systems shall be designed for the critical storm duration by undertaking calculations as summarised in Table 1: Design Storm Return Periods.

Table 1: Design Storm Return Periods

Site Features

Return Period

Further Requirements

Overland flowpath to road or Council reserve with no mapped ponding or flood areas that affect other properties downstream of the site.

10 Year ARI

Soakage systems must have capacity for the critical 10 year ARI event without overflow.

All other situations

50 Year ARI

Soakage systems must have capacity for the critical 10 year ARI event and demonstrate that the 50 Year ARI storm will not adversely affect adjacent property, through either an increase in runoff rate or volume from the pre-development characteristics of the site.

 

The designer shall ensure that all stormwater can enter the system and shall demonstrate that the entire capture and conveyance system from the surfaces to the soakage system (or alternative system) is adequately sized for the storm event.

 

Where onsite ponding is proposed, Council require details of the ponding depth and area affected. The design shall demonstrate that the floor levels will comply with the IDC requirements and shall not create a hazard for people entering a building.

 

The pre-development characteristics of the site shall be taken as the greenfield runoff and shall not assume that any existing soakage system was designed for or is functioning in a way that would imply compliance with current IDC requirements.

DS-5 - Appendix F.3   Soakage Testing 

In the past, standard soakhole guidelines were adopted for the Mount Maunganui and Papamoa areas. Soakage tests in these areas are displaying that variable soakage rates are present and the standard soakholes adopted a rate higher than what is generally shown by testing. For this reason soakage testing is required in all areas if soakage is proposed.

Soakage testing may be undertaken by either of the following methods:

  1. DS-5 - Appendix F.3.1 Falling Head Soakage Test
  2. DS-5 - Appendix F.3.2 Constant Head Soakage Test

The principles of DS-5 - Appendix F.3 Soakage Testing are in general accordance with the requirements of Verification Method E1/VM1 (New Zealand Building Code Compliance Document E1, Surface Water. Additional detail has been included and is required to ensure a consistent approach is adopted, considering the local geology.

Prior to undertaking a soakage test, the designer shall have established the winter groundwater level and the presence of any geological features that will require consideration during the design of the soakage system. This includes investigations to a suitable depth below the base of the intended soakage system. Investigations undertaken for foundation recommendations can be extended for this purpose.

For new subdivisions groundwater shall be monitored during bulk earthworks and the reduced level (RL) adjusted for the final ground surface.

Soakage testing shall be undertaken in separate test holes that are:

  1. Practical and safe.
  2. Typically 100-150mm diameter augured holes. 
Explanatory Note:
Most practitioners would typically use a borehole. Larger scale test holes may be used if more appropriate for the test situation. When opting for a larger test hole, practitioners shall be aware that the practicalities of a larger hole differ based on the soil characteristics, hole stability and the volume of water that is required to perform the testing. 
  1. Advanced to a depth that is the same as the intended depth of the soakage system itself. 
  2. 0.5m above the high winter groundwater table.
  3. Protected against collapse by inserting a slotted or perforated PVC sleeve that provides a reasonably snug fit. 
Explanatory Note:
This is to ensure that the test hole dimensions remain reasonably constant throughout the duration of the test.
Where silty or clayey soils are present, it may be necessary to scarify the sides of the hole to remove smearing caused by auger extraction.

The number of tests required shall be determined from the requirements of Table 2: Soakage Test Criteria.

Table 2: Soakage Test Criteria

Criteria

No. of Tests

Comments

Site-specific assessment.

 

Hardstand less than 300m²

1 test in the area of the soakage system     

 

Site-specific assessment.

 

Hardstand greater than 300m² and less than 600m².             

2 tests in the area of the soakage system.

 

 

Most conservative test to be used in design.

 

Site-specific assessment.

 

Hardstand greater than 600m².

Minimum of 3 tests for up to 1200m² of hardstand.

 

1 additional test for each additional 1200m² of hardstand.

Discard fastest rate and adopt average of remaining.

 

For roads, soakage rates may vary along alignment, requiring isolated systems designed for contributing catchments.

Residential subdivision completion reporting

1 test per 1000m² at the completion of earthworks.

 

Conservative rates to be adopted based on review of adjacent tests.

 

Different rates may be applicable across the development.

DS-5 - Appendix F.3.1   Falling Head Soakage Test

The soakage zone shall be pre-soaked by keeping the hole topped up with water for four (4) hours prior to undertaking the test. The exception to this is when the hole drains completely in less than 5 minutes. In this case the test hole shall be filled and drained three (3) times to saturate the soils surrounding it.

Falling Head Tests are generally terminated after either of the following:

  1. Once the hole is almost drained i.e. the lower water level of either standing water being <90% of hole depth or <200mm.  
  2. After a duration of four (4) hours. 

If the test is terminated without meeting these conditions, then a relatively constant rate of soakage must be demonstrated.

The water level recordings shall be frequent from the commencement of the test and can be further apart once the rate of drop within the test hole slows. Suitable time intervals and maximum distances between each reading will depend on the rate of water level drop within the hole.  

For slow draining tests (>5 minutes), there should be no issue in obtaining a suitable number of data points. For fast draining tests (<5 minutes) it may be necessary to repeat the test and record the drop at shorter time intervals.

At least eight (8) readings per test shall be recorded for tests that have duration of 1 minute or more. For a test draining in 1 minute or less, there is no minimum number of readings however, there must be at least 4 readings at 5 second intervals and no readings that exceed 10 second intervals. The data shall be presented on a graph of depth vs time.

The soakage rate in units of litres/m²/hour shall be determined for each time step by considering the volume of water, the average soakage surface area and the time. The designer shall then interpret the data such that any outliers are discarded and an average of the remaining data can be used as the calculated soakage rate. The soakage rate shall be determined using the method DS-5 - Appendix E Soakage Rate Worked Example. The calculated soakage rate shall be divided by two (2) to obtain the design soakage rate.

Explanatory Note:
Litres/m2/hour can be reduced to mm/hour. Council requires a conversion to litres/m2/hour for clarity in subsequent calculations.

DS-5 - Appendix F.3.2 Constant Head Soakage Test

Fill the hole as much as possible using reticulated water supply or tanked water. Maintain the level for at least 4 hours to ensure the hole is adequately pre-soaked. Continue the test for a further 10 to 15 minutes and ensure that a constant rate is achieved.

Measure the flow rate using a calibrated flow meter or by timing how long it takes to fill a twenty (20) litre container at least two (2) times. The soakage rate is then calculated in accordance with method DS-5 - Appendix G Soakage Rate Worked Example. The calculated soakage rate shall be divided by two (2) to obtain the design soakage rate.

DS-5 - Appendix F.4   Soakage System Design

The base of the soakage system shall be located at least 0.5m above the established winter water table. The design shall consider the cover necessary for the proposed end use of the site.

Soakage systems shall be designed using the design soakage rate. During the design process, the engineer shall review the design soakage rate if the test is representative of the systems soakage zone. The designer shall consider how the soak hole will perform during filling and draining along with the variations in soil characteristics and long-term issues such as clogging to determine how the soakage rate is applied to the system. This shall be summarised within the design report. The soakage system shall provide sufficient soakage and storage such that the design storm will not overflow the system and will fully drain within 24 hours.

DS-5 - Appendix F.4.1   Soakage System Location

The soakage system shall be located within the site so that it is accessible for maintenance or replacement. In most instances, this will require that the soakage system is located at the front of the property unless it is demonstrated that there is sufficient space to permit the required machinery and excavations at other locations within the site.

To enable future works to be undertaken on soakage systems without damage to building foundations or adjacent properties, all soakage systems shall be located at least 1.5m away from property boundaries. The proximity to building foundations shall comply with DS-5.5.3.1 Close Proximity Rules.  

Soakage systems shall be entirely located below a 1V:2.5H projection from the toe of an adjacent down slope retaining wall and a 1V:5H projection from the toe of an unretained cut or fill batter up to 1.5m high. For higher cuts or slopes, a specific geotechnical assessment shall be undertaken by an appropriate Category 1 or 2 Geo-Professional (based on the slope characteristics).

DS-5 - Appendix F.4.2   Loading on Soakage Systems

Soakage systems shall be designed and detailed to prevent or mitigate against point loading on the system and the receiving environment.  

  1. For soakholes this will involve an interconnection system appropriate for the receiving soils.  
  2. For trenches this will typically involve distribution pipes with access near the top of the trench.  

Consideration shall be given to the system location and concentration of stormwater discharged to ground with respect to structures, topographic and geomorphic features.

DS-5 - Appendix F.4.3   Soakholes

Where soakholes are proposed, all of the soakholes shall be interconnected with multiple entry points to create a balanced system with a controlled overflow. The system shall be provided with an air release / entry vent to prevent issues resulting from air pressure and suction due to rapid changes in water volume within the soak holes. Building Code E1/AS1 requires soakholes to be wrapped in geotextile. Council considers this to be good practice. If geotextile wrap is not specified, the designer shall provide supporting information to show that it will not adversely affect the long-term operation of the system.

Soakhole design shall consider the wall thickness and the effect of any rocks placed in the base of the system e.g. it cannot be designed with 100% voids because the rocks reduce the storage capacity. 

Soakholes shall be located no less than 1.5m from other soak holes, measured from the outside edge.

Where placed within (below) drivable areas, soakholes shall have accessible lids that are rated for traffic loading.

DS-5 - Appendix F.4.4   Soakage Trenches

Soakage trenches shall use void ratios appropriate to the trench material. It can be anticipated that some settlement will occur within rock-filled trenches. Hardstand areas above rock-filled trenches shall incorporate a specific design for the slab (or a buried slab) that protects the trench from vehicle loads and protects the driveway from settlement induced damage. 

Soakage trenches shall be located parallel to the contour of any slope that required consideration to maximise load dispersal and prevent piping.

Where proprietary systems are specified then the detailed design shall reflect the requirements of the system.

DS-5 - Appendix F.4.5   Additional Requirements for Subdivision Designs

Subdivision completion reports often provide soakage recommendations for the new lots. These are typically provided as a ratio of hardstand area that can be serviced by each soakhole. With smaller lot sizes and a higher percentage of hardstand being created than in the past, it is more difficult to fit the required number of soakholes within the lot. In areas where soakage is proposed, it is necessary that this be addressed during the Resource Consent process. 

It has been identified that high levels of compaction are resulting in dense sands with lower soakage rates than that of natural undisturbed soils. To address the issues created by higher levels of urban densification and soil compaction, Developers and designers shall consider the following:

  1. At the time of Resource Consent, demonstrate that lot sizes will be suitable for the types of buildings and site coverage that is likely to occur. Post construction soakage rates shall be conservatively estimated, from site testing and experience with changes as a result of development.
  2. As part of the final sign off (s224) requirement: 
    1. Soakage testing shall be undertaken in accordance with the IDC and recommendations for soakage sizing shall be provided. These recommendations shall provide the number of soak holes required for the hardstand area and a method of determining the requirements for a rock filled trench or a high void trench.  
    2. The soakage design report shall comment on the amount of hardstand that can be reasonably serviced by soakage on the lots. 
    3. The soakage report shall provide the maximum depth of the soakage system as a reduced level (RL) to NZVD2016 Datum.

DS-5 - Appendix F.4.6   Alternative On-Site Stormwater Management Methods

Information relating to the design, detail, operation and maintenance requirements for alternative stormwater management systems can be found using:

  1. Council's Stormwater Management Guidelines
  2. Best practice guidelines such as the On-Site Stormwater Management Guideline 2004 from the New Zealand Water Environment Research Foundation.

Alternative stormwater management systems shall be designed and constructed in accordance with current best practice. 

DS-5 - Appendix F.4.7   Products and Materials

Where particular products are proposed to be used to achieve the design outcomes, design calculation information shall demonstrate that the product is appropriate to the design outcomes and any other requirements of the IDC and the New Zealand Building Code

DS-5 - Appendix F.4.7.1   Attenuation Tanks

If attenuation tanks are proposed, the calculations supplied to Council shall be based on the particular tank (brand and model) that is specified to be used in the design. Tanks come in a variety of sizes and the height of the tank controls the maximum and average outflow velocities. 

Tanks shall be designed with a minimum of 100mm dead storage below the control orifice to allow for sedimentation. Tanks shall be located so that:

  1. There is no adverse effect on slopes, retaining walls or building foundations as a result of the weight of the water within the tank.  
  2. They are supported by stable ground that is not affected by a building restriction line.

DS-5 - Appendix F.4.7.2   Other Ground Infiltration Methods

Other methods such as permeable paving, rain gardens or the like, will be considered on a case by case basis. It is important to recognise that these systems rely on ground infiltration. Permeable paving has increased runoff in comparison to undeveloped land due to both the paving materials and the underlying compacted basecourse it is placed upon.

In every case, all methods shall be designed and constructed in accordance with current best practice


Definitions in this section

Council

Design

Designer

Developer

Ground

Infrastructure

Lot

Resource consent

Rural area

S224

Stormwater

SQE professional

Urban area