The Auckland Unitary Plan (AUP) introduced new regulations for groundwater take and diversion in the Auckland Region. Whilst the authors of the rules unquestionably had the correct intentions, the interpretation and application of the regulations has been overly conservative and lacking in context. The unfortunate effect of this has been to add significant additional time, cost, and complexity to projects at a time when society requires the opposite from the engineering profession.
This article has been prepared to outline the key issues with the AUP (from a groundwater perspective) and propose minor revisions to the AUP standards for groundwater take (AUP Section E188.8.131.52). We have also proposed changes to the assessment of groundwater related effects during the consenting process which we believe would both simplify the regulatory requirements and increase certainty for all parties.
Auckland Unitary Plan Groundwater Drawdown and Diversion Standards
Section E7 of the AUP relates to taking, using, damming and diversion of water [and drilling]. The relevant permitted activity standards for groundwater diversion and take associated with excavations are as follows:
- Section E184.108.40.206 – Diversion of groundwater caused by any excavation (including trench) or tunnel.
- Section E220.127.116.11 – Groundwater Take for dewatering or groundwater level control associated with a groundwater diversion permitted under Standard E18.104.22.168.
A summary of the permitted activity standards for each is outlined below for context.
Section E22.214.171.124 – Diversion of Groundwater caused by any excavation.
Under E126.96.36.199 of the AUP, a consent is required unless the following permitted activity standards are met:
2) Any excavation that extends below natural groundwater level, must not exceed:
(a) 1ha in total area; and
(b) 6 m depth below the natural ground level.
3) The natural groundwater level must not be reduced by more than 2m on the boundary of any adjoining site.
4) Any structure, excluding sheet piling that remains in place for no more than 30 days, that physically impedes the flow of groundwater through the site must not:
(a) impede the flow of groundwater over a length of more than 20m; and
(b) extend more than 2 m below the natural groundwater level.
5) The distance to any existing building or structure on an adjoining site from the edge of any;
(a) trench or open excavation that extends below natural groundwater level must be at least equal to the depth of the excavation.
(b) tunnel or pipe with an external diameter of 0.2 – 1.5 m that extends below natural groundwater level must be 2m or greater; or
(c) a tunnel or pipe with an external diameter of up to 0.2 m that extends below natural groundwater level, has no separation requirement.
6) The distance from the edge of any excavation that extends below natural groundwater level, must not be less than;
(a) 50 m from the Wetland Management Areas Overlay:
(b) 10 m from a scheduled Historic Heritage Overlay; or
(c) 10 m from a lawful groundwater take.
Pile foundations, pipes, and trenching works (associated with linear infrastructure) are exempt from the above standards.
The above rules consider the risk and associated effects from potential groundwater drawdown, particularly where compressible soils are present (as they are in some areas of the Auckland Region) in combination with elevated groundwater levels. However, under the AUP, groundwater diversion associated with an excavation can only occur in combination with ‘Dewatering or Groundwater Level Control’. Therefore, even when an excavation meets permitted activity criteria under E188.8.131.52, the related Standard E184.108.40.206 for ‘Dewatering or Groundwater Level Control’ must also be considered, as outlined below.
Section E220.127.116.11: Dewatering or groundwater level control associated with a groundwater diversion (permitted under Standard E18.104.22.168)
Under E22.214.171.124 – Dewatering or Groundwater level control (associated with a groundwater diversion under E126.96.36.199) is a permitted activity only of it meets all of the following standards:
- The groundwater take is not geothermal water
- The groundwater take is only for a period of 10 days (in peat soil) or 30 days (in other soils or rock); and
- The groundwater take must only occur during construction.
Implications for the AUP Standards for Groundwater Take and Diversion
The implication of E188.8.131.52 is that, unless an excavation is wholly above the ‘natural groundwater level’ (i.e., there is no dewatering or groundwater level control associated with the excavation), then a consent is required unless the works can be completed within 10 days for peat or 30 days for other soils. In practice this means that a groundwater consent is required for any excavation below the ‘natural groundwater level’ for any basement or other substantial below ground structure as it is not practical to excavate and construct in less than 30 days.
In our view, the key issue with the above lies with the following:
- There is no clear definition of “natural groundwater level” in the AUP. In the absence of a clear definition, Council’s present interpretation is that any free water present within the soil matrix is “natural groundwater”. For the reasons we outline below, this is not incorrect for most Auckland sites with elevated topography;
- While E184.108.40.206 presents a rational, risk-based approach to determining whether an activity is permitted or not, E220.127.116.11 “overrides” this risk-based approach. Accordingly, the permitted activity criteria for dewatering or groundwater control (E18.104.22.168) effectively means that any excavation extending even nominally (say 100 mm) below ‘natural groundwater level’ (for the purposes of constructing a basement or any other substantial below ground structure) requires a consent, unless full groundwater cut off is provided to prevent drawdown. In addition, we have found that when a consent is required because this permitted activity standard is not met, Council reviewers require a full and detailed assessment of effects for all magnitude of groundwater diversion/drawdown, not just where it exceeds the limits in E22.214.171.124. The reason for this is not clear.
What does this mean for consenting excavations and basement structures in the Auckland Region?
The implications of the AUP rules under E126.96.36.199 and E188.8.131.52 are that almost all projects in Auckland which have basement levels or often even just nominal excavations to form building platforms, require a consent. Unfortunately, it is also our experience, that both the interpretation of the permitted activity standards, as well as Council’s requirements for assessing the effects of groundwater drawdown, are becoming more onerous with time. These requirements are usually disproportionate to the potential risks or effects associated with groundwater drawdown. The outcome of this is:
- Significantly increased costs associated with additional geotechnical investigation and installation of expensive groundwater monitoring instrumentation which is arguably not needed for shallow excavations.
- Increased professional services (consulting) fees, consent application, and Council review fees.
- Delays in obtaining consent for applicants, particularly given the scope for interpretation of the AUP rules and groundwater conditions at a site.
- Higher compliance/monitoring costs during construction as relatively rigid, standardised monitoring regimes are imposed by default as a condition of consent.
- Potential construction delays due to ‘alert level’ triggers in groundwater levels.
However, one of more concerning outcomes of the AUP groundwater consenting requirements is that, in our experience, geotechnical engineering professionals are implementing overly conservative design and construction solutions to mitigate the ‘apparent’ risk of groundwater drawdown. In extreme cases, the authors of this article have observed the detailing of secant piled walls for single level basements, often for sites positioned on or near to ridgelines where elevated groundwater levels and soil types that could lead to consequential settlement effects do not exist. We can only surmise that such decisions have been made in the interest of programme, consultant budgetary constraints or because of protracted debates of technical issues with Council reviewers. Regardless, the cost implications of such decisions can extend into the hundreds of thousands of dollars, and potentially upwards of $1M in extreme cases.
While these outcomes are clearly in part the responsibility of the geotechnical professional, it is not surprising that such outcomes are occurring on projects where geotechnical engineers, unfamiliar with the unique regulatory environment in Auckland and the “unwritten rules” regarding groundwater consenting, are defaulting to “easier to consent”, expensive solutions. These sorts of outcomes are highly undesirable and contribute to increased construction costs at a time when the industry needs to reduce the cost of construction.
Before we can propose how the AUP rules could be modified (or clarified), it is important to consider the geology and hydrogeology of the Auckland Region.
Auckland Hydrogeological and Geological Conditions
With Auckland located at the isthmus of the Waitemata and Manukau Harbours, much of the land is proximal to coastal regions or other bodies of water (streams, rivers, swamps, lakes etc). Near surface ‘natural groundwater levels’ persist near the coastline, around lakes and streams and near to wetland areas. Furthermore, it is in these areas that compressible soils tend to be prevalent (e.g. Marine Sediments, Peats and other Holocene Age alluvial deposits). The primary risk associated with groundwater drawdown in these areas is almost always the potential for ground settlement which can affect surrounding buildings, land, and infrastructure. Where near surface, permanent, hydrostatic groundwater levels are present in combination with soft and compressible soils, this is a significant risk that needs to be addressed carefully. The permitted activity criteria for groundwater diversion (E184.108.40.206) is therefore appropriate for these sites. It is also accepted that it is not possible to accurately delineate (by way of GIS overlays) the areas of Auckland where compressible soils are present. Therefore, it is appropriate that the E220.127.116.11 permitted activity standards apply to the wider Auckland Region. This minimises the risk of groundwater drawdown effects (principally settlement) on land, property, and infrastructure.
However, Auckland is also a region with significant topographic highs. In these areas, permanent hydrostatic groundwater levels are generally not present. Such conditions persist across vast swathes of high intensity development areas in the Auckland Region – the CBD, city periphery areas (Parnell, Ponsonby, Newton, the Great North Road ridgeline) and many other urban areas of the city (Remuera, Grey Lynn, St Marys Bay, the Eastern Bays, and much of the North Shore to name just a few). In these areas, where ground surface levels are elevated well above sea level, and where sites are remote from lakes, streams, creeks and other bodies of water, permanent hydrostatic groundwater levels are often well below ground level and below the influence of most developments. In addition, ground conditions in these areas usually consist of residual soils, sometimes capped with volcanic ash & tuff or stiff, old, over-consolidated Puketoka Formation deposits – i.e. soils which are not known to be significantly impacted by groundwater drawdown. Historical investigations on the Auckland Isthmus have indicated that “regional” groundwater levels within the East Coast Bays Formation soils and rock of the Auckland Region have a gradient of between approximately 2 and 5% toward the coastline or other bodies of static water. Figure 1 presents the typical groundwater conditions of an elevated site in Auckland (i.e., located above the coastal zone and/or other bodies of water).
Figure 1: Typical Groundwater Conditions of elevated Auckland sites
In general, the groundwater conditions of most sites can be defined with the following ‘zones’:
– The Soil-Water Zone – often in the upper 1-2 m below ground surface levels where trees and other vegetation extract water from the pores of the soil matrix and/or where the ground surface is sealed with pavements and other impermeable surfaces preventing recharge by rainfall and stormwater runoff.
– The “Unsaturated Zone” (also referred to as the ‘Vadose Zone’) where ‘wetting fronts’ are present. Groundwater is present within this zone – occurring as water within the pores of the soil matrix – however, the soil is not fully saturated and porewater pressures are less than atmospheric pressure.
– The “Capillary Zone” where soils are fully saturated but capillary forces causes a negative pore water pressure component, i.e., the porewater pressure is less than atmospheric pressure.
– The “Saturated Zone” where the pores of soil are permanently and fully occupied by water, the porewater pressure is greater than atmospheric pressure and a hydrostatic groundwater profile exists. The surface of the “Saturated Zone” is often referred to as the ‘phreatic surface’ or more commonly just the groundwater level. This surface will fluctuate throughout the year – varying due to rainfall conditions, being elevated in winter and depressed in summer. The ‘groundwater level’ often follows the surface topography but with smaller relief.
Figure 2 presents the various zones of groundwater beneath most sites.
Figure 2: Groundwater Zones
‘Natural Groundwater Level’
In the absence of any clear definition of the ‘natural groundwater level’ within the AUP, it is reasonable to assume that Council intended it to relate to a ‘groundwater level’ which, if reduced by dewatering, would cause ground settlement. It therefore follows, that the ‘natural groundwater level’ must be the ‘phreatic surface’ or the level at which porewater pressure (pw) is equivalent to atmospheric pressure (pa) as presented on Figure 2 and is permanently saturated. In addition, within the context of groundwater drawdown induced settlements, this should be a “permanent” groundwater level which has been present since the deposition of the soil/rock unit.
In our experience groundwater conditions above the phreatic surface (i.e. above the permanent, hydrostatic groundwater level) are complex in the Auckland Region. Standpipe piezometers on sites with elevated relief (e.g. on ridgelines or cliff tops) in Auckland often collect groundwater within the ‘Vadose Zone’, above the permanent, hydrostatic groundwater level. In our view, the measured groundwater levels in shallow piezometers relates to the collection of downwardly percolating groundwater. Standpipe piezometers act as a high permeability zone within the soil profile, effectively providing a path of least resistance and a “sump” that intercepts downward groundwater seepage flowing toward a lower permanent, hydrostatic groundwater level. This is similar to conditions often observed behind shallow retaining walls, where outlet drains are blocked – the drainage aggregate behind the walls is highly permeable and preferentially collects groundwater seepage. In some cases, groundwater pressures (head) can build up within the drainage; however, such water levels do not have any relationship to permanent, hydrostatic groundwater levels.
The above interpretation is also consistent with our observations of seepage within excavations at these sites during construction; inflows are often limited to a trickle at most. These minor seepages are most likely vertically and horizontally percolating groundwater migrating through the ‘Vadose Zone’ toward the ‘Saturated Zone’ or the permanent, hydrostatic groundwater level.
The type of situation described above is often attributed to a “perched” groundwater level by Council (and/or their reviewers), which is subsequently interpreted as the ‘natural groundwater level’. In our opinion, this is a misinterpretation in the context of the Unitary Plan. While a true “perched” groundwater level could represent a permanent “natural groundwater level” in specific hydrogeological settings (i.e. an aquifer overlying a low permeability aquiclude, which in turn sits above a deeper groundwater level), this does not appear to be a relevant model for the typically low permeability soils in Auckland.
Measuring the ‘natural groundwater level’ for analysis and consenting purposes
The reality is that there is no “one size fits” all approach to investigating and assessing groundwater conditions and hydrogeology for all sites. In areas of low-lying relief – in proximity to the coastline or lakes, rivers, streams and other bodies of water – determining the groundwater level will not be difficult. This requires little more than a common-sense approach. However, in areas of higher relief, one or more of the following methods could be considered:
- Installation of a series of nested vibrating wire piezometers installed over the depth range of the excavation and down toward the anticipated permanent, hydrostatic groundwater level” or at least one deep piezometer that extends into the permanent hydrostatic groundwater level. Vibrating wire piezometers, grouted into place, should be used. In this situation the hydrostatic groundwater level would be adopted as the ‘natural groundwater level’ at the site, unless there was evidence to suggest a true perched groundwater level is present.
- Obtaining a series of core samples over the depth range of the excavation and completing laboratory (oedometer) tests to confirm where the soils are fully saturated, or
- Carrying out cross-hole P-wave velocity testing.
Regardless of which method is adopted; the permanent hydrostatic groundwater level should meet the following criteria:
a) The porewater pressure is greater than atmospheric pressure; and
b) There is a permanent hydrostatic pressure profile present below the inferred groundwater level (temporarily higher transient/seasonal groundwater levels would not be captured by this criteria).
Confirming that the soils are fully saturated would also be beneficial, but this is implicit in the results of a) and b) above anyway.
Recommended Changes to AUP Groundwater Rules
To minimise unnecessary cost, time, and complexity, we recommend the following changes be considered for the AUP rules for E18.104.22.168.
E22.214.171.124. Dewatering or groundwater level control associated with a groundwater diversion permitted under Standard E126.96.36.199, all of the following must
- The water take must not be geothermal water;
- The water take must not be for a period of more than 10 days where it occurs in peat soils,
or 30 days in other types of soil or rock; and The water take must only occur during construction.
In our opinion, Standard E188.8.131.52 (2) is unnecessary in non-peat soils as the risk based rules in E184.108.40.206 are already sufficient to pick up situations where consequential groundwater induced settlement could occur.
In addition, we consider Standard E220.127.116.11 (3) to be redundant, since by the end of construction (likely to be 6- least 12 months for typical basements), most groundwater drawdown related effects would be complete, and the potential effects of groundwater drawdown have already been considered under E18.104.22.168 (2), (3) and (5).
Secondly, the ‘natural groundwater level’ must be clearly defined. We recommend the following definition:
‘Natural Groundwater Level’
The phreatic surface, where the pore pressure in the soil is equal to or greater atmospheric pressure, and below which a hydrostatic pressure profile exists with depth. This includes ‘Perched Groundwater Levels’ where the geological setting permits the presence of such.
Recommended Changes to Council Processing
In addition to the proposed changes and clarifications of permitted activity standards E22.214.171.124, we recommend Council, and their reviewers consider the following when processing consent applications for groundwater take/diversion:
- Assessment of the ‘natural groundwater level’ at a site should include consideration of multiple information sources including, but not just limited to piezometer measurements, but also an assessment of the topography, geology, and hydrogeology in the vicinity of the site.
- For determining if a consent is required and/or assessing settlement effects, the ‘natural groundwater level’ should be taken as the lowest measured groundwater level over the monitoring period. Consolidation settlement occurs where the stress in the soils exceeds the historical stress history of the soil, i.e. when the groundwater level has been lowest. Therefore, the “summer low” of the ‘natural groundwater level’ is the relevant level that should be used for assessing settlement effects.
- Permissible settlement magnitudes from groundwater drawdown combined with the mechanical deformation effects of the excavation itself (usually retaining wall deformation) should relate to the ‘tolerances or sensitivity’ of neighbouring buildings, infrastructure, and property, not what the engineer has calculated from his/her unique design solution.
Council should consider publishing permissible total and vertical settlement magnitudes for roads, footpaths, buried services etc. Maximum permissible total and differential settlements for adjoining buildings should be assessed and confirmed by structural engineers based on a review of the structure type and foundation systems of each building. These could in turn be reviewed by Council or their appointed reviewers.
An effects-based assessment would enable the geotechnical professional to detail and design retaining systems (including drainage or tanking systems for control of groundwater levels) at detailed design stage, rather than at preliminary design (for resource consent) when other design elements are not sufficiently advanced to support the level of detail required.
Alert and Alarm level settlement magnitudes, and the required monitoring regime could be agreed at resource consent, effectively locking in the design criteria which the geotechnical professional subsequently refers to for detailed design.
In addition, consideration could be given to allowing publication by the industry of actual settlement monitoring results on dewatered sites so that effects assessments can be better calibrated and unnecessary conservatism avoided.
The above approach would significantly streamline the groundwater consenting process, without compromising the process or increasing the risk to Council and to neighbouring landowners or asset owners.
Feedback and Next Steps
We are very interested in receiving feedback on the above article and invite you to share your own views with us. Please email either Nick or Matt at the email addresses below.
Following receipt of industry feedback, and once we are satisfied that there is consensus, we propose to submit an opinion to Auckland Council for consideration and discussion.