Download Proposed Plan Change 11 - Hot Water Developments Ltd

Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
Hot Developments Limited
Proposed Private Plan Change
at 790 Hot Water Beach Rd
ENGINEERING FEASIBILITY REPORT
FOR STRUCTURE PLAN PROPOSAL
26 April 2010
Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
PROPOSED PRIVATE PLAN CHANGE AT
790 HOT WATER BEACH RD
FOR HOT DEVOPMENTS LTD
ENGINEERING FEASIBILITY REPORT FOR STRUCTURE PLAN PROPOSAL
TABLE OF CONTENTS
1.0 BACKGROUND
1 2.0 SITE DESCRIPTION
1 3.0 SITE STABILITY ASSESSMENT
3.1 Geology
3.2 House Sites
1 1 2 4.0 BUILDING FOUNDATIONS
4 5.0 ROADING AND ACCESS
4 6.0 POTENTIAL FOR INUNDATION
5 7.0 WASTEWATER AND DISPOSAL
5 8.0 WATER SUPPLY
6 9.0 STORMWATER DISPOSAL
7 10.0 OTHER SERVICES
8 11.0
LIMITATIONS
8
APPENDIX:
APPENDIX 1
APPENDIX 2
APPENDIX 3
APPENDIX 4
Site Development Plans
Borelog Location Plan
Borelogs and Test Results
Stormwater Catchment Plans
Proposed Plan Change 11 - Hot Water Developments Ltd
1.0
Attachment 7 - Engineering Feasibility Report
BACKGROUND
We have investigated the engineering feasibility of carrying out the proposed development
consisting of:
•
a sixteen residential lifestyle lot development on the lower and flatter parts of the property as
shown CPG Preliminary Concept Plan 215385 C300
•
a further 4 bush lots, being formed within the regenerating bush areas on the upper part of
the property which are accessed by 2 higher driveways off the existing formed and metalled
ROW which runs around the northern and western boundaries of the existing property. The
lower 2 western sites are within an area previously used as a plant nursery. The 2 higher
sites are in clearings at the top eastern and western corners of the property.
The proposed house sites are shown on the Preliminary Concept Plan and on the DJ Scott
Development Overview dated October 2009.
The development is for residential dwellings. This feasibility report is prepared to support the
Structure Plan proposal.
2.0
SITE DESCRIPTION
The proposed development is to occur on Lot 1, DP 59176, a 34.76 ha block of land. The lower
one third of the property, where the majority of the proposed development is to take place is in
pasture with the remaining upper 2/3rds being in regenerating native bush and second growth
scrub which contains a number of clearings.
The land generally falls towards the north west dropping from an elevation of 160m at the upper
south eastern corner to around 14m adjacent to the existing northern metalled road that runs
along the southern edge of the Taiwawe Stream. This stream flows into the sea at Hot Water
Beach. Wave action and lateral drift often form a barrier of sand across the outlet of the stream
causing water to pond back up behind the beach.
Existing drainage depressions and the wetland area covering the lower northern parts of the
property discharge through two culverts under the northern ROW into the Taiwawe Stream. The
northern culvert is presently submerged by the water level in the stream which at times ponds
behind a sand bank that forms across the stream outlet on Hot Water Beach.
3.0
3.1
SITE STABILITY ASSESSMENT
Geology
In the northern ‘wetland area’ the IGNS Geological Map 17 maps the land as flood plain
alluvium. Within the remainder of the site the map defines the underlying formation as
Tapuaetahi Andesites (ap) which are coarsely phyric, two-pyroxene andesites of the Waiwawa
group. The underlying unweathered formations have very high strengths which can be reduced
by weathering and hydrothermal activity.
Soils of the site consist of relatively deeply weathered residual clayey silts and silty clays derived
from the underlying volcanic formations. Road cuttings along both Hot Water Beach Rd and the
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
existing peripheral northern road around Lot 1 stand at slopes of up to 30° (58%) to the
horizontal and to a height of about 3 metres. Long term steep cut batters are prone to
weathering by surface frittering together with shallow slumping, wedge and slab failures which
are common above road cuttings.
Water worn boulders are present in the bases of the watercourses within the deeper gullies
where there is continuous flow.
The average slope down the whole property is 1V: 6.5H, with slopes steepening to 1V: 2.8H
over the upper parts of the pastoral area. House sites below the toe of the steeper slopes are
generally less than 1V: 8H (7º).
3.2
House Sites
The proposed house sites on each lot have been inspected. No evidence of surface instability
was visible. All the houses are on northern facing slopes.
House sites are described below.
•
Lot 1 The house site is located on the lower slopes, below the proposed public road, on
top of a minor spur with ground slopes at about 9° (16%).
•
Lot 2 The house site is located on the lower slopes, below the road, with slopes at
about 9° (16%).
•
Lot 3 The house site is located on the lower slopes, below the road, with slopes at
about 5° (9%).
•
Lot s 4 and 5
These house sites are located on the lower slopes, below the road on top
of small raised ridges, which have ground slopes at around 7° (12%).
•
Lot 6 The house site is located on top of a small flat mound to the north of the wetland
areas against the northern boundary. Part of this site has been used as a helicopter
landing area. There are signs of recent filling having occurred on the eastern end of this
area. A site specific investigation will need to be carried out to determine the extent of this
filling and to determine foundation requirements.
Proposed treated effluent disposal areas are below the house sites on Lots 1 to 6.
•
Lot 7 The house site is located on the toe of the steeper slopes above the public road
where ground slopes have a grade of about 11° (19%). The effluent disposal area for Lot
7 is on the eastern side of the access ROW to Lot 10.
•
Lot 8 The house site is located on the toe of the steeper slopes above the public road
where ground slopes have a grade of about 8° (14%).
•
Lot 9 The house site is located on the toe of a steeper slope of a small spur above the
turning head at the head of the public road where ground slopes have a grade of about 6°
(11%).
A short ephemeral gully runs down the slope between house sites on Lots 8 and 9; and Lots 3
and 4.
Proposed treated effluent disposal areas are to the side of house sites on Lots 7 to 9.
•
Lot 10 The house site is on the eastern side of the property part way up the steeper
slope at the head of a short wide gulley. Ground slopes below the house site are around
24º (45%) and 20° (36.5%) across the building site. Three pine stumps from large trees
recently cut down are visible just below the building site. These stumps will decay over
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
time leaving large holes which could fill with water and lead to instability within the lower
slope.
This house site which has steep slopes on two of the lower sides should be founded on
specifically designed piles embedded into the underlying unweathered formation and
designed to resist possible creep acting over 3 times the pile diameter within the upper
2.0m of residual soils below existing ground levels.
The treated effluent disposal area located within the gully floor below will need to be built
up by placing permeable topsoil or sand in this area to lift finished surface levels to 1.0m
above the levels of the small watercourse running down the eastern boundary.
•
Lots 11 & 12 are on larger sites, towards the top of the steeper slopes, at the end of the
proposed new 2.75m wide access ROW. Ground slopes across the house sites range
between 10º to 13º. Steeper lower slopes range between 18º and 20º. Houses in these
areas should be set back 10m from the top of the steeper slopes and founded on piles
embedded into the underlying unweathered formation and designed to resist possible
creep acting over 3 times the pile diameter within the upper 2.0m of residual soils below
existing ground levels.
•
Lots 13 & 14 are on the top eastern side of a narrow ridge towards the upper end of the
5.5m wide ROW accessway that runs up the western side of the site. The grade of the
accessway in this area is 1V : 5H. House site ground slopes range between 9º down the
spine of the ridge to 22º into the small gully to the east. Houses in this area should be set
back 10m from the edge of the gully steeper slopes and founded on piles embedded into
the underlying unweathered formation and designed to resist possible creep acting over 3
times the pile diameter within the upper 2.0m of residual soils below existing ground
levels.
The treated effluent disposal areas for Lots13 and 14 are on the western side of the new ROW
access opposite the house sites.
•
Lots 15 & 16 are within regenerating native scrub on the western side of the 5.5m wide
accessway ROW that runs up the western side of the site. Ground slopes across the
house site range between 8º across Lot 16 to 13º across Lot 15. The wastewater disposal
areas for these lots, which are below the building sites, should be set back 10m from the
edge of the gullies steeper slopes.
•
Lots 17 & 18 have house sites located within the old plant nursery area half way up the
western side of the property just off the ROW. Both building sites are nearly flat.
•
Lots 19 & 20 proposed house sites are at the top eastern and western corners of the
subdivision. Ground slopes across the building sites are around 8%.
The designated building site locations for Lots 17 to 20 within the upper regenerating bush areas
will require further investigation for specific foundation designs prior to construction.
It is our professional opinion, not to be construed as a guarantee that all the house sites will be
suitable with respect to slope stability for the founding of single dwellings, though to achieve this,
a number will require specifically designed foundations. This opinion is subject to the following
conditions:
i)
Concentrated stormwater discharge from impermeable surfaces shall be piped to the
existing natural runoff watercourses on each lot and discharged through energy
dissipating structures to avoid scour erosion at the discharge point and within the
watercourse itself.
ii)
The removal of vegetation on each of the house sites within the regenerating native
bush areas Lots 17 to 20 should be limited to the minimum necessary to establish the
new dwellings and access driveways.
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
4.0
Attachment 7 - Engineering Feasibility Report
iii)
Any cut batters proposed steeper than 1V to 3H and higher than 0.75 m vertical are to
be reviewed by a suitable qualified geotechnical engineer and a satisfactory retaining
system installed as considered necessary.
iv)
Any site filling outside of building sites is to be carried out under the direction of a
suitably qualified CPEng.
BUILDING FOUNDATIONS
House foundations shall be in cut only and not on fill. Cut material shall be removed from the
house sites and not placed in positions that could load the steeper slopes.
Shallow subsoils at the house sites consist of firm to stiff materials and have an available
Dependable Ultimate Limit State bearing capacity of 100 kPa after using a strength reduction
factor of Ø= 0.5. Below 3m depth the Dependable Ultimate Limit State bearing capacity
increases to 300 kPa.
This professional opinion does not remove the necessity for the normal site investigation for the
design of foundations and their subsequent inspection during construction of proposed dwellings
and other buildings.
5.0
ROADING AND ACCESS
i)
Lower Road No 1 to be vested in the Council. - The road into the site to Ch 320m is to
have a road reserve width of 15m with a formed asphaltic concrete surfaced
carriageway width of 6.0m to a 20m turning circle at the end. A 1.5m wide concrete
footpath will be constructed on the lower side against a kerb and channel with a grass
swale draining the upper side. The flexible carriageway will be designed using a
maximum subgrade CBR = 5%.
ii)
ROW Accessway No 2 - Up Slope from 330m to Ch 520m will have a formed 5.5m
concrete carriageway over the steeper 1V : 5H (22%) to 1V : 6H (16.7%) grades with in
a legal right-of-way width of 8.0m. The ROW will be drained by concrete lined side
drains with check dams to reduce water velocities on both sides.
iii)
ROW Accessway No 3 - from Ch 520m to Ch 630m with grades of less than 16.7% will
have a formed width of 2.7m with an asphaltic concrete surfacing inside a legal right-ofway width of 6.0m. The ROW will be drained by a grassed swale on the lower side.
iv)
ROW and private driveway accessway No 4 to proposed Lot 10 house site which has a
grade of more than 16.7% will have a formed 2.7m concrete surface inside a legal rightof-way width of 6.0m. The ROW will be drained by a concrete lined side drain on the
lower side.
v)
ROW and private driveway accessway No 5 to proposed Lot 20 house site will have a
formed 2.7m surface inside a legal right-of-way width of 6.0m. The ROW will be drained
by side drains.
During construction erosion and silt control will be in accordance with Environment Waikato
and TCDC standards. The various accessways have been designed and located to follow
existing contours and existing farm tracks to minimise unnecessary earthworks.
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
6.0
Attachment 7 - Engineering Feasibility Report
POTENTIAL FOR INUNDATION
All the proposed lower house sites are well elevated above the highest known Taiwawe Stream
flood level of around RL ~14.8. When the stream is in flood, the higher water levels can enter
the wetland by backing up through the culverts which normally drain the ephemeral wetlands
under the northern ROW. Flood levels in the past have topped the lowest sections (~RL = 14.3)
of both Hot Water Beach Rd and the northern ROW by around 0.5 metres. The Hot Water
Beach Rd bridge has a deck level of RL = 15.8m. In the case of the dwelling on proposed Lot 6
the minimum habitable Finished Floor Level shall be not less than 16.2m.
During times of flood, runoff from above the subdivision is stored in the wetlands until the stream
levels drop.
The two existing 450Ø culverts under the northern ROW and the low ROW formation levels over
a length of 70 metres will ensure that there is no potential for inundation of dwellings from runoff
from the 47.5 Ha catchment above the wetland.
During periods of low stream flow wave action and lateral drift often on Hot Water Beach form a
sand barrier across the outlet of the stream causing water to pond back up behind the beach
which at times submerges the lower western culvert.
Flows from the small eastern gully, which has a 7.25Ha catchment area, are culverted under the
existing Lot 1 farm entrance and the northern ROW road. Peak flows will need to be controlled
to prevent water from spreading out across the waste disposal area below House Site 10 as
described in Section 7 below. Non peak discharges are minor. The existing 373Ø culvert should
be replaced with a 600Ø.
Ephemeral flows from the short gully, with an existing catchment of 5Ha, between Lots 3 & 4
and 9 & 10 will need to be channelled with in a shallow swale between the houses and culverted
under the new road to control the water that may flow between the waste disposal areas and
house sites. Runoff from the upper part of this catchment is likely to be diverted by the drainage
swales along side the upper ROW to Lots 11 & 12.
7.0
WASTEWATER AND DISPOSAL
Each of the proposed lots is of substantial area and in excess of 3500m². Gross Lot Area to
Discharge Volume ratios for 4 bedroom dwellings are greater than 3. The subsoils of the site can
be classified as “Imperfectly Drained” (Category 4) in terms of AS/NZS 1547:2000.
Because of the relatively steep and elevated nature of the upper lots it can be expected that
seasonal fluctuations will not raise the groundwater table closer to the surface than 1.5 m in
these areas.
Lower sites, which are at the toe of the steeper slopes with more undulated surfaces, may
experience isolated groundwater seepage areas or even stormwater sheet surface flows during
and after heavy precipitation. Cut off surface swales and subsoil drains would mitigate the
adverse affects from these flows.
A satisfactory wastewater treatment and disposal system serving a single dwelling meeting the
requirements of AS/NZS 1547; 2000 could be provided on each lot as follows:
i.
Cleared Upper Lots 11 to 12
•
First stage septic tank – 2700 litres.
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
ii.
•
Second stage septic tank fitted with 150 mm diameter OSI Biotube filter or
similar – 1800 litres.
•
Effluent disposal in a trench system in accordance with AS/NZS 1547:2000
at a design loading rate of 10 mm/day.
Regenerating Bush Disposal Areas Lots 13 to 20
•
iii.
Attachment 7 - Engineering Feasibility Report
Treated wastewater from the second stage tank may be disposed of by
way of a pump fitted within an Innoflow screen filter to an aerial irrigation
disposal system within the bush humus layer at an application rate of
5mm/day. Wastewater disposal areas should have a 15m clearance from
watercourses, including ephemeral, especially in the area of the spring on
the western side of Lot 14
Lower Lots 1 to 10
Wastewater disposal areas on the lower Lots, 1 to 6 and 7 to 10 inclusive, which
may have intermittent higher ground water levels or be closer to the edge of the
wetlands than 15 metres:
•
Secondary package treatment systems such as:
•
Innoflow Technologies Advantex Treatment System or,
•
Clearwater package treatment system or
•
TCDC approved similar.
The secondary treated wastewater discharged from these systems have very low BOD5 levels
which may be disposed of by way of aerial irrigation within planted areas or by subsurface drip
irrigation at a loading rate of 5mm/day.
Where treated effluent is to be disposed of within 15 metres of the edges of wetland ponds,
watercourses (including ephemeral) or where winter ground water levels area are less than 1.0m
from the surface the wastewater should be disinfected by a UV disinfection unit prior to disposal.
The effluent disposal area below the house site on Lot 10, within the floor of the small eastern
gully is likely to need to be raised by the importation of permeable topsoil or sand to at least
1.0m above the level of the water level of the existing small stream.
Modern secondary treatment wastewater package plants, when operated and maintained
correctly, provide consistent reliable treatment even during peak flow conditions. The treated
effluent water is so clean that it can be used for drip or subsoil irrigation or discharged to shallow
inconspicuous trenches. This will ensure that it will have no adverse effects on the down stream
wetlands, the Taiwawe Stream or on Hot Water Beach itself. When considering the existing land
use for the site is for grazing stock the development is expected to improve the downstream
water quality and the stormwater runoff discharging from the property.
8.0
WATER SUPPLY
There is no existing public reticulated water supply at Hot Water Beach. The potable water
supply for each house will be by way of roof water collected in individual water tanks located on
the individual lots.
i.
Potable Water & Fire Fighting
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
At the time a dwelling is constructed on each lot, domestic water and fire fighting storage is to be
provided. A minimum of 20,000 litres shall be maintained at all times as a static fire fighting
reserve within a 30,000 litre tank.
Alternatively an 11,000 litre fire fighting reserve is to be made available for any dwelling in
association with a domestic sprinkler system installed to an approved standard.
ii.
9.0
Alternate water sources for purposes of fire fighting could be drawn from any of the
following:
•
The wetland ponds.
•
The Taiwawe Stream.
•
The sea which is less than 500m distance.
•
Special fire fighting storage tanks filled by a 30 to 40mm PE water pipe
from the continually running all year round stream on the western side of
the subdivision above the level of the water tank.
•
Special fire fighting storage tanks filled from stormwater runoff collected
from upper ROW carriageways.
STORMWATER DISPOSAL
Stormwater runoff from most developed roofed areas will be initially collected by water tanks.
Overflows from the domestic water tanks and paved areas on Lots 1 to 6 below the level of the
proposed new road can be directly piped to the wetlands so as not to flow over or saturate
wastewater disposal areas.
Runoff from higher paved areas should be collected by catchpits with a minimum 450mm deep
sump to prevent silt passing into the wetlands.
Surface water runoff from the new road carriageways and ROWS will be collected by swales
and channels to catchpits spaced at 90m maximum centres and reticulated to the wetland.
Forebays constructed in accordance to TP10, below the outfall pipes at the head of the wetland,
would reduce silt build up within the wetland. Swales are and effective road-wash stormwater
treatment technique that acts to trap contaminants, allows infiltration, reduces flow velocities and
provides some water storage capacity. All water courses (including ephemeral) should be piped
under the proposed new roads and access right-of-ways.
The stormwater design should result in road runoff achieving an acceptable level of water quality
prior to its eventual discharge
Springs and seepage areas found on or below the toes of the steeper slopes should be drained
to either a piped system or to the wetland.
The primary piped system will be designed according to the TCDC Code of Practice for
Subdivision and Development (engineering Standards) for a 20% AEP storm event. Culverts will
designed for a 20% storm event unless habitable floors could be affected in which case
satisfactory overland flow paths clear of the dwellings shall be provided.
Minimum public stormwater pipe diameters are not to be less than 300Ø with 225Ø to public
catchpits.
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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Proposed Plan Change 11 - Hot Water Developments Ltd
Attachment 7 - Engineering Feasibility Report
Outlet pipes into the wetland are to be fitted with energy dissipators.
Swales and ephemeral water courses close to buildings are to be designed to carry the capacity
of a 2% AEP storm event with adjacent habitable floor levels to comply with TCDC’s minimum
clearances.
Surface water channels on roads and Rows’ with gradients steeper than 1V:12H are to be
concrete lined.
10.0
OTHER SERVICES
Telephone and power services can be provided to the site.
We understand that TCDC are likely to require a street light at the entrance intersection and at
the head of the turning circle.
An alternative to this is to install low bollard lighting in critical positions to mitigate the potential
effects of light spill.
11.0
LIMITATIONS
The above professional opinion is furnished to the TCDC and Hot Developments Ltd for their purposes
alone on the expressed condition that it will not be relied upon by any other person and does not remove
the necessity for further inspection during the course of the works.
SITE SUITABILITY REPORT FOR LOT 1, DP59176
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