Free Range Broiler Farm, Arapohue: Geotechnical assessment

Transcription

1 Free Range Broiler Farm, Arapohue: Geotechnical assessment Prepared for Date September 2017 Job Number

2 Document Control Title: Date Version Description Prepared by: Reviewed by: Authorised by: 08/ Initial issue M Gosling P Malan J Simpson Distribution: Northland Regional Council Kaipara District Council (FILE) 1 copy 1 copy 1 copy 1 copy

3 iii Table of contents 1 Introduction 1 2 Scope of works 1 3 Site description 1 4 Development proposal 2 5 Site investigation Regional geology Site walkover Geotechnical investigations General Hand augers and scala Borehole Cone penetration test Geological model 6 6 Groundwater 7 7 Seismicity and liquefaction 7 8 Geotechnical considerations General Proposed chicken sheds Bearing capacity Settlement Suitability of local materials for reuse Suitability of high ground Bund construction 9 9 Conclusions Recommendations Applicability References 12 Appendix A : Appendix B : Appendix C : Appendix D : Appendix E : Appendix F : Appendix G : Appendix H : Tegel Farm Layout Site Walkover Observation Plan Investigation Plan Hand Auger Logs Borehole Logs Core Box Photographs CPT Data Geological Model

4 1 1 Introduction (Tegel) is proposing to develop a free range broiler chicken farm at 5763 State Highway 12 (SH12), Arapohue, Dargaville. (T+T) have been engaged to conduct a geotechnical assessment of the site to support a resource consent application. This report summarises and presents the geotechnical assessments and investigations at the proposed site. We conclude that in geotechnical terms, the site is suitable for the proposed poultry farm development provided the recommended additional work set out in Section 10 is undertaken to support detailed design. Based on our preliminary site investigations we have not identified any significant geotechnical barriers to developing the site into a poultry farm. 2 Scope of works The following scope of works has been undertaken in accordance with the geotechnical tasks detailed in our proposal dated 7 July The work comprised a preliminary review of published geological literature, site walkover by an engineering geologist and the ground investigations comprising: 14 Cone Penetration Tests (CPT) 8 Hand augers 5 Scala Penetrometers 4 Standpipe Piezometers pushed in by the CPT rig 2 Rotary Boreholes with nested piezometers (4 in total) We have also carried out an assessment of geotechnical risks associated with the proposed development, including the expected suitability of materials onsite for re-use, and the settlement, bearing capacity and liquefaction potential of the subsurface materials. 3 Site description The approximately 250 hectare site is shown in Figure 1 and is located approximately 12 km south of Dargaville on the eastern floodplain of the Wairoa River, opposite the town of Te Kopuru. The property is currently operated as a dairy farm, with the approximate property boundaries indicated with red in Figure 3.1. The landforms at the site can be divided into the flat area to the west, and the hilly area to the east. The western area of the site comprises an alluvial flood plain, is flat, and is the part of the site currently being operated as a dairy farm. The eastern part of the site comprises hills, and is therefore referred to as the hilly area. The flat area is highlighted yellow, and the hilly area is highlighted red, in Figure 1 below. The flat area has a number of water courses that carry water towards the Kaipara Harbour.

5 2 Figure 3.1: Site location with approximate property boundaries marked with a red line, the floodplain highlighted yellow and the hilly area highlighted red (Source: Northland Regional Council Local Maps, 2017). 4 Development proposal The proposed free range poultry farm will include the development of 32 sheds on the site, which will be clustered in four groups of eight (refer to Appendix A). Each shed s dimensions will be approximately 20 m long x 138 m wide x 4.5 m in height. Developing the proposed farm will require earthworks, the construction of the foundations and sheds, installation of infrastructure, including an energy centre building powered by litter combustion, stormwater management devices, wastewater treatment and disposal, rainwater storage tanks, installation of groundwater bores, water reticulation and a water treatment plant. In addition, the proposed poultry farm development will require construction of associated facilities, such as workers accommodation, utility sheds and vehicle tracks around the sheds connecting to a new access off SH12. To mitigate flooding from the Kaipara Harbour, it is proposed to surround the sheds at the site with a flood protection bund approximately 2.5 m high. The flooding assessment and bund design, along with other (non-geotechnical) aspects of the proposed development, are considered separately and fall outside the scope of this report. 5 Site investigation 5.1 Regional geology The site is dominated by two main geological units, comprising of unconsolidated to poorly consolidated alluvial deposits (Q1a) and the undifferentiated mélange of the Northland Allochthon

6 3 (KOm) (Edbrooke, 2009). The alluvium occurs on the floodplain in the western portion of the site, and the Northland Allochthon in the hills to the east. Additionally the area contains several basaltic, andesitic and dacitic intrusions as well as breccia pipes (MTs) - a notable occurrence is the nearby Maungaraho Rock. The high ground to the east of the site also contains areas of Mahurangi Limestsone (Omm), which is a pervasively shattered limestone, overlying the Northland Allochthon mélange (Edbrooke, 2009). The geological map with the approximate location of the site boundary is presented in Figure 5.1 below. The published geology is generally consistent with the results of the geotechnical site investigation. Figure 5.1: Geological Map with site marked in red (Edbrooke, 2009) 5.2 Site walkover Before drilling commenced on 13/07/2017, a walkover of the site was conducted by an engineering geologist and geotechnical engineer. Observations made during that walkover are set out here, and are linked to locations identified on a plan in Appendix B (i.e. Observation 1 refers to a location on the plan in Appendix B that is labelled 1 ). The walkover confirmed that the property functions as a dairy operation, with the main milking shed located in the middle of the floodplain area (Observation 1). The area surrounding the shed is also being used as a storage area for farm materials including piles of tyres (Observation 2). An older, disused milking shed is located 350 m west of this new milking operation, and is surrounded by various pieces of disused farm equipment (Observation 3).

7 4 The paddocks appear well pastured, and are soft underfoot in some areas. The farm manager prohibits any vehicle movement on the paddocks due to damage concerns. The walkover was conducted during a period of high precipitation, and there was significant standing water evident in the paddocks across the site. The drainage trenches around the eastern paddocks seemed to contain a substantial amount of water, whereas the western paddock drains were almost dry, despite standing water in the fields. The farm races have been constructed of locally sourced limestone, which is observed to be slippery when wet. The races are in mostly good condition, with several locally soft zones across the site. A four wheel drive is advised for any site vehicle movements. The base of the hill adjacent to CPT07 has andesitic rock fall debris (Observation 4, Figure 5.2), suggesting the presence of andesite in the hill. Figure 5.2: Rock fall at base of hill The road to the limestone quarry comprised limestone with trace andesite gravels, and the occasional cobble up to 100 mm diameter (Observation 6). Hummocky ground was observed in the hills to the east of the road, with volcanic breccia outcrops appearing higher up in the hills (Observation 7). An open excavation was encountered near the top of the road leading to the quarry where a small amount of limestone was encountered, surrounded by mélange soils (Observation 8, Figure 5.3).

8 5 Figure 5.3: Small amount of limestone in an open excavation The quarry is approximately 70 m by 50 m in plan and around 5 m in depth (Observation 9, Figure 5.4). The material being quarried is a sheared, chalky limestone, and is the same material as that gravelling the farm races. Figure 5.4: The site quarry looking southwest

9 6 5.3 Geotechnical investigations General This section summarises the results of the investigations. It includes hand augered boreholes, Scala penetrometers, machine drilled boreholes, and cone penetration tests. The investigation locations are shown on the Figure attached in Appendix C Hand augers and scala Eight 50 mm hand augers to a maximum depth of 5.0 m have been carried out with down hole shear vanes. The hand augers were carried out by Geotechnics Ltd and logged in general accordance with NZGS Guidelines (2005). Five Scala Penetrometers were carried out adjacent to selected hand augers. Copies of the Hand Auger and Scala logs are presented in Appendix D Borehole Two rotary boreholes were carried out by McMillan Drilling Ltd to investigate the ground conditions at depth. The boreholes were of HQ size, and included in barrel shear vanes. Nested standpipe piezometers which installed in the holes following drilling. The holes were logged by a T+T Engineering Geologist in general accordance with NZGS guidelines (2005). Copies of the borehole logs are presented in Appendix E and the core box photographs in Appendix F Cone penetration test Fourteen Cone Penetration Tests (CPT) were carried out across the site, with the intention of inferring the geology (with comparison with the rotary boreholes logs) and assessment of geotechnical parameters. The CPT investigation was carried out by Ground Investigation Ltd. using a 10cm 3 Compression Piezocone. The results of the CPT investigation are presented in Appendix G. 5.4 Geological model The site is divided into a floodplain area (the flat area away from the hills), and the hilly area to the east of the site. The focus of this stage of the investigation has been on the flat floodplain area, with the hills to be further investigated in the future. The flat floodplain area comprises topsoil extending approximately 0.1 to 0.2 m below ground level (BGL), which is underlain alluvial deposits. These materials have a stiff to very stiff crust of orange brown clay down to a depth of 0.6 to 0.7 m BGL. This is then is underlain by very soft, normally consolidated alluvium, consisting of a very soft to soft clayey silt. Towards the west of the site, the alluvium overlies beach sands, comprising loose sand with fragments of shell material, changing to medium dense with depth. BH1 penetrated through the beach sands and terminated in stiff silty alluvial soil To the eastern part of the flat area, the alluvium is underlain by the same materials that comprise the hilly area to the east. This is Mangakahia Complex dominated undifferentiated mélange of the Northland Allocthon, which is the basement rock of the area and typically comprises a shattered, weak siltstone. A geological section along A-A (location of the section line is presented in Appendix C) and a Northland Allochthon Contour Map is presented in Appendix H, interpreting these observations. Two areas fell outside this generalised geological scheme, the area investigated by BH2/CPT14 in the south eastern flood plain area, and HA1 at the northern extent of the site. BH2 encountered volcanic ash of the Hukatere Subgroup underlain by approximately 3 m of slightly weathered Andesite emplaced by local volcanism. The andesite was underlain by Mangakahia Complex

10 7 dominated Northland Allochthon material at 5.5 m BGL. HA1 refused at 4.7 m on a very hard white silt, which is likely to be the Mahurangi Limestone. Table 5.1 below presents the properties of each material encountered: Table 5.1: Lithologies encountered and their general engineering properties Geological unit Shear Vane (KPa) Scala Penetrometer (Blows) Parameters SPT N Value Tip Resistance Qt (MPa) Topsoil n/a 1-2 n/a Alluvium - Crust n/a Alluvium Normally consolidated Beach Sands (loose) Beach Sands (medium dense) Northland Allochthon 6 Groundwater n/a n/a n/a n/a n/a n/a n/a n/a n/a During the investigation in the flat area, groundwater observations were carried out in the boreholes and in the exposed channels at the site. Groundwater levels were typically within a metre of the ground surface, and generally between 600 and 900 mm below the ground surface. 7 Seismicity and liquefaction In terms of NZS (2004), the site is classified as predominantly Class D. This is due to the depth of rock (and hence thickness of overlying soils) rapidly exceeding 30 m. For the purposes of design, the flat area of the site is classified as Class D, noting that during detailed design individual groups of sheds towards the east of the site could be individually classified as Class C based on their location and the depth to rock. Designs in the hilly area will require specific assessment, and may comprise Class C or Class B depending on the depth of overburden. Based on the material properties identified in the CPT, the risk of liquefaction occurring in the alluvial deposits at the site under 500 year return period seismic loading is assessed as negligible. This is due to the fine grained and plastic nature of the subsurface materials, which are assessed as unlikely to be capable of developing significant excess pore pressures and liquefy under cyclic loading. Given this, and the relatively low seismic loading in Northland, the risk of substantial cyclic softening is assessed as negligible. The underlying sands may have lenses of material that could experience some liquefaction effects, but given the thickness of overlying material we do not consider there is any appreciable risk of consequential liquefaction effects. The Allochthon materials at the site are not considered to be at risk of liquefaction related effects.

11 8 8 Geotechnical considerations 8.1 General The sections below address geotechnical aspects of the proposed development for the flat area of the site. Development on the hill area is addressed in Section 8.4 below. The proposed chicken sheds and associated hard stand areas are the critical design elements as they tie into the site drainage and roading networks. Controlling groundwater and flooding are important elements of the design that must be consistent with the geotechnical assumptions around settlement. Recommendations and opinions in this report are based on data from Boreholes, Hand Augers and CPT. The nature and continuity of subsoil away from these investigation locations is inferred but it must be appreciated that actual conditions could vary from the assumed model. 8.2 Proposed chicken sheds Bearing capacity The flat area of the site comprises normally consolidated, very soft to soft materials, with an upper crust of stiffer and better material. The upper crust of material is suitable to support shallow pad foundations, provided the settlements can be accommodated. Accordingly, rather than remove the crust during development, and following discussions with Tegel, the crust will be left in place with the grass and organic material removed. This introduces a minor settlement risk by leaving some slightly organic material (topsoil) in place, but in the context of the expected settlements at the site and the design of the sheds to accommodate this, the risk of consequential effects is considered acceptably low. For design purposes, for strip footings up to 0.6 m wide, and pad footings up to 1 m square, the following bearing pressures can be adopted: Geotechnical ultimate capacity ULS capacity Allowable bearing capacity 200 kpa 100 kpa 65 kpa Settlement The soils that underlie the site are weak and compressible, and may contain organic materials that cause ongoing creep settlements. Accordingly, any load imposed on the ground by structures or ground raising (and bunds) will lead to primary consolidation settlements. All parts of the site, and particularly structures, roads and services, are at risk of experiencing ongoing secondary consolidation settlements in addition to the primary consolidation. Based on the CPT and borehole data we have assessed the settlement risks for raising ground sheds. Our analysis indicates that: 150 mm of ground raising (with 100 mm slab on top) could lead to mm of settlement 300 mm of ground raising (with 100 mm slab on top) could lead to mm of settlement The settlements presented above are likely to occur in the centre of the shed, and will reduce towards the shed perimeter. This will introduce differential settlements that must be considered. Settlements of this magnitude are reaching the limit of what can readily be accommodated during shed operations, and so pre-loading of the shed footprints may be carried out to reduce the postconstruction settlements. This preloading would comprise filling above ground level with soil (probably site won), and leaving it for the settlements to occur. This preloading will be monitored,

12 9 and when the post-construction settlements are assessed to be at an acceptable level, the fill can be removed. The settlement magnitude and duration depend on the bulk behaviour of the underlying alluvial deposits, and especially the presence (or otherwise) of sand lenses. The total settlement magnitude and rate are therefore difficult to accurately assess based on spot investigation points, and settlement trials can be carried out at the site using a pad of placed fill. This simulates loading of the site, and provides a full scale, site specific response to loading. Finally, it is important to note that varying water levels at the site are likely to have preloaded the soils when they were lower than today. Accordingly, it is possible that the loadings imposed by the sheds may lead to lower settlements than calculated. This is not relied on in the design, but is noted here in advance of the construction and to support an assessment of the operational risks. 8.3 Suitability of local materials for reuse The geotechnical investigation has identified two types of rock material in the geological Allochthon units in the hills; limestone and andesite. The limestone is being used at the site (and around Northland in general) as roading material. It is generally able to be excavated using a digger, and crushes under moderate to heavy compaction. We expect that it will be suitable for both bund construction and as a subbase for the roads around the site. When used for roading, it gets slippery when wet, and so a running course may be required to keep the roads accessible. The andesite is significantly harder, and will be more difficult to remove from the ground, and may require ripping. While it is a good quality material, it is likely to require crushing to an acceptable size before being used. Crushing the andesite can be expensive and timely. However, it would provide a consistent product for use at the site. We have assessed the potential volume of limestone at the site, based on the site walkover, exposures visible and the drilling results. Our assessment is that there is in excess of 100,000m 3 of limestone material available to be quarried at the site. This preliminary conclusion will be confirmed by site specific investigations and volumetric modelling during the detailed design stage. 8.4 Suitability of high ground The hills are formed of weathered allochthon rock. As such, they are generally suitable for founding water storage tanks or similar structures. Specific geotechnical investigation and design is likely to be required for any structures. The slopes are generally stable, but will require specific assessment during development. 8.5 Bund construction Stormwater bunds are proposed around the site. The limestone at the site is expected to be suitable for temporary water retaining structures when crushed, noting that this will be validated during site trials. In principle, the bunds will be around 2.5 high, and have batter slopes around 3H:1V. We note that significant settlements (potentially 300 to 600 mm for the bund loading) could occur after construction, and the crest of the bund should allow for either future raising, or for settlements to take place. The site settlement trials will allow a more detailed assessment of settlements to be made and more clearly define the likely bund settlement performance.

13 10 9 Conclusions Key conclusions based on geotechnical investigations of the site are set out as follows: From a geotechnical perspective, the site is suitable for the proposed poultry farm development provided the recommended additional work set out in Section 10 is undertaken to support detailed design. The site is located on a flat floodplain underlain by soft alluvial soils with groundwater typically less than 1 m below ground level, and basement rock increasing in depth from east to west. All parts of the site are at risk of primary and secondary consolidation settlement, and a settlement trial pad can provide site specific settlement data (refer to Section 10 below). Locally sourced limestone is currently being used for roading in the area. Limestone gets slippery when wet so a running course may be required. Bunds for stormwater are expected to be able to be formed out of limestone material obtained from the site. We recommend validating the use of the limestone during site trials (refer to Section 10 below). Settlements in the order of 300 to 600 mm could occur. It is estimated that there is in excess of 100,000 m 3 of limestone material available to be quarried at the site. There are pockets of andesite located in the hills that could be used as part of construction. To use this material the andesite would need to be crushed. The high ground to the east of the site is generally suitable for building on, though specific geotechnical investigation and design will be required for any structure. Further work is required to develop the detailed design (refer to Section 10 below). 10 Recommendations We recommend undertaking the following additional investigations to support detailed design of the poultry farm development: Confirm in situ strengths around shed locations during construction; Refine settlement estimates by carrying out a full scale trial pad; Confirm the extent and available volume of limestone and andesite in the hills by carrying out additional geotechnical investigations; Carry out specific assessments of possible sites in the hills for tanks, including slope stability and foundation requirements; and Confirm the limestone is sufficiently impermeable to support the bund construction once it is excavated and compacted

14 11 11 Applicability This report has been prepared for the exclusive use of our client, with respect to our letter of engagement dated 7 July 2017 and it may not be relied upon in other contexts or for any other purpose, or by any person other than our client, without our prior written agreement. Report prepared by: Authorised for by: Pierre Malan Jenny Simpson Senior Geotechnical Engineer Project Director Report prepared in conjunction with Matt Gosling. MGOS p:\ \workingmaterial\planning\volume 2 - tech reports (pd review)\technical report a - geotechnical assessment\geotechnical assessment.docx

15 12 12 References Edbrooke, S.W. (compiler) (2009): Geology of the Whangarei area: scale 1:250,000. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences 1:250,000 geological map p. + 1 folded map. New Zealand Geotechnical Society (2005): Field description of soil and rock; guideline for the field classification and description of soil and rock for engineering purposes. NZ Geotechnical Society Inc, 38 p. ( NZGS Guidelines ) Standards New Zealand (2004): NZS1170.5:2004 Structural Design Actions Earthquake Actions. Standards New Zealand.

16 Appendix A : Tegel Farm Layout

17 L:\ \WorkingMaterial\CAD\DWG\ dwg, 020, 19/09/2017 9:29:46 a.m., caduser ORIGINAL IN COLOUR DRAWING STATUS: 105 Carlton Gore Road, Newmarket, Auckland Tel. (09) Fax. (09) RESOURCE CONSENT

18 Appendix B : Site Walkover Observation Plan

19 Site Walkover Observation Plan 6. Andesite in road 9. Limestone quarry 8. Open Excavation 7. Andesite outcrop and hummocky ground 2. Storage area/piles of tyres 5. Offal/rubbish pit 3. Disused milking shed 1. Milking shed 4. Rock fall debris

20 Appendix C : Investigation Plan

21 Investigation Plan HA5 CPT 05 HA1 CPT 01 SC3 CPT 03 CPT 06 HA2 CPT 02 HA4 CPT 04 HA7 CPT 13 A BH1 A' CPT 07 CPT 08 HA8 Legend CPT 09 HA9 CPT 10 Cone Penetration Test Borehole Hand Auger/Scala CPT 11 BH2 CPT 14 CPT 12 HA12

22 Appendix D: Hand Auger Logs