Framework for Managing Dewatering and Accidental Artesian Aquifer Interception

Transcription

1 Northern Arterial Extension and Associated Stormwater Works - Resource Consents Application Appendix 12 Framework for Managing Dewatering and Accidental Artesian Aquifer Interception Beca // 13 August // NZ // page 118

2 Northern Arterial Extension and Associated Stormwater Works - Resource Consents Application Beca // 13 August // NZ // page 119

3 Report Northern Arterial Extension Consent Application Dewatering and Accidental Artesian Aquifer Interception Framework Prepared for Christchurch City Council (Client) By Beca Ltd (Beca) 19 August 2014 Beca 2014 (unless Beca has expressly agreed otherwise with the Client in writing). This report has been prepared by Beca on the specific instructions of our Client. It is solely for our Client s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which Beca has not given its prior written consent, is at that person's own risk.

4 Revision History Revision Nº Prepared By Description Date A Mike Thorley Document Acceptance Action Name Signed Date Prepared by Mike Thorley Reviewed by Sian France Approved by Paul Whyte on behalf of Beca Ltd Beca // 19 August 2014

5 Table of Contents Introduction...1 Mitigation of Environmental Effects Dewatering Consents... 2 Discharge to the Environment... 2 Dewatering Discharge Quality... 2 Methods of Discharge Treatment... 3 Effects of Groundwater Drawdown on Surface Water Bodies... 5 Avoidance of Ground Settlement... 5 Dewatering Practice Method Selection... 7 Accidental Interception of Artesian Aquifers or Large Inflows Excavations Pile Holes Piling Current Estimate of Dewatering Required Across Christchurch Northern Arterial Extension...12 Current Estimate of Spring Monitoring Across Christchurch Northern Arterial Extension...12 References...12 Beca // 19 August 2014 // Page ii

6 1 Introduction The purpose of this framework is to set out the key issues associated with dewatering and the accidental interception of artesian conditions. Generally, the following items should be considered when managing dewatering activities: 1 Purpose of dewatering (i.e., an explanation of why dewatering is necessary) 2 Dewatering technique (i.e., well point, deep well, open hole, etc.) 3 Anticipated dewatering flow rate and total dewatering duration 4 Controls (i.e., settling tank, turbidity curtain, etc.) and method of discharge 5 Measures and techniques to manage noise, vibration and odour issues 6 Measures and techniques to manage geotechnical stability issues 7 Contingency plan in case of any emergency situation 8 A monitoring programme to ensure that discharge water will comply with applicable water quality standards 9 Baseline assessment of the existing environment (e.g. fauna, water quality) that will receive the discharge 10 A strategy for monitoring and managing effects during the life and after the closure of the project 11 The point of discharge to the stormwater system and to any waterway or water body. Dewatering across the Christchurch Northern Arterial Extension is not anticipated to be extensive. In particular some minor dewatering maybe required during the placement of culverts. The philosophy is to build on top of existing ground level and avoid excavations as much as possible. The following framework has largely been adapted from the Stronger Christchurch Infrastructure Recovery Team (SCIRT) DEWATERING GUIDELINE: Guideline for Dewatering Practice for Christchurch (SCIRT, 2013). Beca // 19 August 2014 // Page 1

7 2 Mitigation of Environmental Effects 2.1 Dewatering Consents Environment Canterbury consents are required for extracting groundwater for dewatering purposes and disposal of the discharge into water bodies or the stormwater system. The dewatering management plan needs to fully consider the conditions and ensure the required dewatering activities are conducted appropriately. Compliance with consent conditions will be demonstrated though the requirements of the dewatering management plan and associated QA records. 2.2 Discharge to the Environment Specific factors that need to be addressed are the siting of the discharge, the effects of the discharge on the discharge location and the ability of the discharge environment to accept the volume of discharge. The following mitigation options should be considered in the dewatering management plan: Determine the quality of the discharge water; Ensure appropriate treatment of the discharge prior to discharge; Minimise erosion and scour at the discharge point(s); Utilise the existing Christchurch City Council drainage network where possible; Ensure an appropriate mixing zone in the water body; Ensure the discharge does not have a detrimental impact on flora and fauna downstream of the discharge point; and Minimise the volume of dewatering water discharged to surface water bodies through reuse and land-based detention and soakage. 2.3 Dewatering Discharge Quality Suspended Solids Dewatering water shall pass through a sediment removal device prior to discharge. The TSS can be checked through a visual inspection of the water being released into the environment. Standard samples will be used for comparison to allow a rough instant field assessment of discharge quality (Figure 1). Beca // 19 August 2014 // Page 2

8 Figure 1: Example of comparative samples to visually assess approximate TSS within the discharge Because primary settlement tanks remove the solids that settle quickly, it is only particles with a long settling time that are usually discharged from the primary treatment. Therefore samples of discharge water that meet the consent conditions can be prepared in a laboratory based on the typical particle size expected to be discharged from the primary tanks. These can be compared with samples taken on site to allow approximation of the TSS value of the discharge. 2.4 Methods of Discharge Treatment Several options exist to treat dewatering water prior to it discharging to the environment. One or more of the following options will be used Sediment Control Tanks Examples of TSS <150 g/m 3 Sediment control tanks are the most common method of treatment of discharge water. The tank will be sized appropriately for the quantity of the discharge so that the flow velocity is lowered to promote sedimentation and that the retention time is sufficient. If there are concerns about settlement times, tanks can be used in series to increase discharge water withholding times and increase settlement of sediment. The overflow from the tank to the ground will be controlled through the use of a pipe or hose. This will avoid erosion and overland transport of entrained sediment resulting from the overflow. It will also allow the flow to be directed to the closest suitable discharge point. Regular (visual) monitoring of the clarity of the water exiting the sediment tank, especially during excavation, is likely to be required. Advantages Good gross settlement traps Sediment can be removed with suction trucks Allows incorporation of additional discharge water treatments, such as oil separators Ability of water to be accessed and used for other purposes (trench compaction, dust suppression) Can be used in conjunction with other methods of sediment capture. Disadvantages Not generally large enough to allow settlement times for clay particles Safety risk of water depth in public environment Large item to place in the road environment. Beca // 19 August 2014 // Page 3

9 2.4.2 Filtering Discharge Through Vegetation Filtering dewatering discharge through vegetation generally comprises the surface application of the discharge onto vegetated land, allowing water to soak through the soil and recharge the groundwater table. In general this requires a large area or an area of heavy/rank grass growth to capture the fines. Swales and coffer dams can also be used to collect the sediment and allow the water to drain away. Remediation of the land following its use in this way will be carried out by regrassing or aeration to avoid permanent clogging of the soil structure by the deposited fines. Filtering discharge though vegetation will only be used as a secondary treatment after the discharge has been passed through a sedimentation tank to remove larger particles. Advantages Discharge seeps into ground and not directly into a waterway Vegetation provides extended flow paths and captures sediment which is bound by grasses Not in road environment Disadvantages Needs grassed flow path Constant water flow can compromise grass health Pores in ground can become clogged and require remediation Some maintenance required if coffer dams in place Rain events can mobilise discharged site sediment Collection with sediment control bag / Flocculent impregnated sock A sediment control bag or flocculent impregnated sock comprises a geotextile bag attached to the pump outlet, filtering the larger sediments from the discharge water. There are a number of types of proprietary treatment devices available ranging from pore size sieving to flocculent impregnated fabric. These systems vary in size, efficiency and cost, but will be investigated and considered where appropriate. These systems may be used in series with other sediment treatment methods, such as filtering through vegetation. Advantages Small in size Gross silt contained in small area Easy to dispose of silt Disadvantages Unable to cope with large discharges or high pressure discharges Silt must be disposed of with the bag Fines/ silt weep out of the bag Connection of pump discharge hose to bag is a common point of failure ECan approval of the flocculent planned for use will be sought prior to implementation Opportunities for use of Dewatering discharge Opportunities to utilise dewatering water on site will be investigated. These include: Use for dust suppression Use for establishment of planting/berm areas Use for compaction or piling requirements Beca // 19 August 2014 // Page 4

10 2.5 Effects of Groundwater Drawdown on Surface Water Bodies There are no surface water bodies of significance in the Cranford Basin area. Management of dewatering across the Christchurch Northern Arterial Extension will consider the effects on the various drains that cross the alignment. Potential effects will depend on the depth of drawdown relative to the level of the watercourse, the period of drawdown, the rate of dewatering, the distance from the water body, and the permeability of the soil between the site and the watercourse. Measures which could be implemented on site to limit effects of groundwater drawdown on surface water bodies include: Recharge to the surface water body Installation of temporary cut-offs to lengthen flow paths (e.g. sheet pile wall, grout curtain, groundwater recharge) Modification of dewatering methodology to reduce influence beyond site. 2.6 Avoidance of Ground Settlement Dewatering activities can result in settlement of surrounding ground because: Drawdown of groundwater increases effective stress leading to both elastic (immediate) and consolidation settlement of soils. Settlement magnitude is dependent on the magnitude of groundwater drawdown, the soil profile and material properties at the site High flow velocities through the soil to dewatering wells and/or ineffective filters result in mobilisation and loss of fines, which can lead to local ground settlement Settlement of adjacent ground associated with instability of excavation sides, insufficient reduction of pore water pressures or seepage which compromises stability. Sands and gravels are comparatively permeable and stiff, so the settlements which result from changes in pore water pressure and effective stress occur rapidly during the period of construction and effects on the ground surface are generally small. However, softer and less permeable soils such as clay, silt and organic soils are prone to consolidation. Settlements in these materials may take some time to develop and have adverse effects on adjacent property Settlement Mitigation The dewatering management plan will identify methodologies to limit adverse dewatering settlement. The following points are key considerations in dewatering settlement: Settlement associated with loss of fines can be mitigated though appropriate design of the dewatering system to control flow velocity and provide screens and/ or filters matched to the grading of the in-situ soils. Entrainment of fines must be monitored during construction; actions are expected to include analysis of TSS in discharge water and/ or monitoring of accumulation of sediment in sedimentation tanks. Drawdown-induced ground settlement will be mitigated though estimation of groundwater drawdown and settlement coefficients to identify risk at the detailed design stage prior to drawing the groundwater down. Water level monitoring in monitoring boreholes and/or nearby springs will occur to check that larger drawdowns than anticipated at distance from the excavation are not occurring. Differential settlement is most problematic; this will be controlled by managing the rate of drawdown and understanding where clear changes in soil type occur. Should the potential for drawdown that could result in damaging settlement be indicated, this will be mitigated by installing groundwater cut-offs to stem or restrict groundwater flow and limit drawdown beyond the site. Beca // 19 August 2014 // Page 5

11 Provide sufficient temporary support to excavations to maintain stability, where seeps might otherwise induce progressive collapse of sides of the excavation. During dewatering implement staged drawdown (where appropriate) and monitor field settlement and water level changes beyond the immediate site, comparing against theoretical settlements and water levels to allow warning of potential dewatering settlement issues Assessment of Immediate and Consolidation Settlement Prior to commencing assessment of potential ground settlement associated with dewatering at a site, the following will be determined: sensitivity of adjacent structures to total and differential settlement, soil profile and material properties at the site (affecting magnitude of total and differential settlement and time for consolidation), groundwater conditions at the site and proposed drawdown, details of the dewatering methodologies being implemented, and temporary works design for the excavation Settlement Monitoring Details of proposed settlement monitoring will be set out in the dewatering management plan for sections of the project where there is a risk of settlement causing damage to surrounding structures or infrastructure. Prior to commencing dewatering, condition surveys of adjacent properties that could potentially be affected by dewatering will be carried out. The scope of settlement monitoring will be confirmed following detailed design, considering the risks and potential consequence of adverse settlement at the site or on adjacent property. It is anticipated that a settlement monitoring system will comprise a series of piezometers and settlement markers sited at various distances beyond and at the site, within the zone of influence of groundwater drawdown. Alert and Action settlement thresholds will be selected by theoretical assessment of anticipated settlements and review of sensitivity of adjacent structures and infrastructure. Beca // 19 August 2014 // Page 6

12 3 Dewatering Practice Whilst dewatering across the Christchurch Northern Arterial is not expected to be extensive, selection of a dewatering method will be made considering the factors set out in Figure 2. Table 1 summarises the common dewatering techniques used for different soils and identifies the key issues that will be considered in the management plan. 2.7 Method Selection Key factors that will control selection of a dewatering method are: Soil profile and soil type; permeability of each layer Extent of the area to be dewatered (excavation dimensions and depth) Existing depth to the groundwater table and the level to which it has to be lowered below this Proposed method of excavation and ground support Proximity of existing structures, water courses etc Conditions Favouring Sump Pumping Well-graded sandy gravel, clean gravel, firm to stiff clay Unconfined aquifer Modest drawdown required and there is no immediate source of recharge (e.g. no stream nearby) Shallow excavation slopes or deep driven sheet piles Excavation by backhoe or dragline Light foundation loads Low risk of contamination of discharge water Conditions Favouring Well-pointing Sandy or interlayered soils including sands (k = 10-3 to 10-5 m/s) Unconfined aquifer Drawdown of 5m or less required, or up to 10m where a large excavation area is available Conditions Favouring Wells Ground conditions too permeable for well-points to be viable Silty soils where correct filter grading design is needed Drawdown of more than 8m required or drawdown over a wide area for a long period Good understanding of ground conditions in and around the site and ideally, pumping test data Access to the excavation and top of batter slopes needed or congested sites (wells can be located away from working areas). Beca // 19 August 2014 // Page 7

13 Figure 2: Example dewatering methodology decision tree (from SCIRT, 2013) Beca // 19 August 2014 // Page 8

14 Table 1: Summary of the soils, possible issues and commonly used dewatering techniques Soil Type Grain Size (mm) GW Flow Rate Possible Issues Dewatering Methodology Gravels/ Cobbles >2 High Large flows of groundwater requiring wells if the excavation is to be deep and likely trench sumps if excavation is just into the water table wells and sumps Sand 0.06 to 2 Low to medium Trench stability low if sand allowed to run into excavation well pointing Silt to 0.06 low Clay <0.002 Very low Stability variable and yields could be low requiring close spacing of well points; localised perching possible Minimal trench stability issues; localised perching possible well pointing and sumps well pointing and sumps Peat variable Variable (low to high) Specialist requirements as dewatering peat can result in compression of the layers causing settlement and damage to surrounding land and infrastructure specialist advice required Mixed soils variable Variable (low to high) A majority of the conditions will be mixed and therefore the methodology is generally based on the predominant soil type depends on hydrogeology and highest yielding unit Beca // 19 August 2014 // Page 9

15 3 Accidental Interception of Artesian Aquifers or Large Inflows While it is not anticipated that artesian conditions will be intercepted in most dewatering activities carried out across Northern Arterial Extension, there is nevertheless the possibility of interception of artesian or high flow conditions which can rapidly lead to failure of embankments and founding layers if not responded to quickly and appropriately. This part of the framework sets out a plan for preparing for and responding to artesian or high inflow conditions. 3.1 Excavations In the case of uncontrolled aquifer inflows to excavations bound by sheet piles or similar, the following steps shall be taken: Assess the situation. Determine if the flow is constant or increasing. Determine if the turbidity is constant or increasing. Determine if the flow is confined to the well-point/ pumping well or excavation Notify project engineer and/or project manager. Be able to describe in detail the conditions and events prior to encountering artesian flow photographs and/or video, in real-time if possible. Consult with the Engineer and determine primary strategy and contingency plan should the primary strategy be insufficient to arrest the artesian flow Measure check any nearby groundwater levels in piezometers and drain/spring levels Notify. Inform project representative of the situation and planned action items Emergency actions may include: Backfill the excavation until the depth of backfill is sufficient to control material transport in the inflow; Extend pipe sections to allow measurement of the artesian pressure; Control any discharge of water by established site erosion and sediment control measures Refill the excavation with water to the original level; Reconsider the design of the excavation keeping in mind the level of artesian pressures; (consider caisson construction and tremie concreting a gravity compensated base slab); Design will need to be altered to allow a casing to be spun into the green concrete to allow control of the artesian pressures; If the aquifer pressures are modest and not above surrounding ground level, place a thickness of graded crushed aggregate on top to act as a controlled filtered exit. Design the thickness of the aggregate to avoid piping or heaving depending on the difference in height between the aquifer level and the depth to be excavated. 3.2 Pile Holes This section outlines steps that can be taken to control, stop, and seal groundwater flow during the construction of piles. 3.3 Piling Avoidance of interception of artesian aquifers in piling is desirable to: Avoid floating of piles Beca // 19 August 2014 // Page 10

16 Avoid depressurisation of the aquifer Avoid the potential for mixing of water from the Springston Formation with that found in the Riccarton Gravels if depressurisation of the aquifer occurs Driven Piles It is anticipated that most piles will be driven through the Springston Formation to the top of the Riccarton Gravels. Water pressure may rise around the pile for a time and mitigation measures may be required to remove any water that might emerge at land surface. In many areas of the Christchurch Northern Arterial the pressure will be transferred to more permeable layers within the Springston Formation and no remedial action would be needed Bored Piles In the event that bored piles are required, the management plan should consider following steps: Install a temporary or permanent casing around the pile; Complete the drilling of each pile hole using a high viscosity, dense polymer Once full depth is reached, complete the pile with concrete tremmied from the base up Recover and re-use the displaced polymer which will be pumped from the hole as the tremmie proceeds Have any remaining neutralized polymer removed from site and disposed of by an appropriate hazard substances carrier. The density of the polymer must be sufficient to exceed the artesian pressure of the Riccarton Gravels aquifer. The viscosity of the polymer will allow the hole to remain open until the tremmie is complete and avoid penetration of the concrete into the aquifer. The concrete will seal against the formation and/or the permanent casing. Beca // 19 August 2014 // Page 11

17 4 Current Estimate of Dewatering Required Across Christchurch Northern Arterial Extension Given the project is currently at Specimen Design phase, temporary works and dewatering methodologies that will be implemented during construction have not yet been determined. For the purposes of identifying potential effects for the resource consent application, the likely need for dewatering has been identified for the major structures across the project. Temporary dewatering is likely to be required mostly for the excavation and replacement of shallow materials for embankment construction at the following locations: Northern and Southern wetlands; Croziers Drain Culvert; and the Southern Culvert. Initial indications are that excavations will be at or just below the water table, and therefore only minor dewatering is expected. The quality of shallow groundwater should be monitored in relation to the soil contamination found in the vicinity of the alignment. If dewatering is undertaken, a sampling plan is prepared to verify the existing baseline groundwater quality and ensure dewatering activities do not significantly increase contaminant loads into the drain(s). This will ensure that the quality of the discharge water from dewatering will be similar to background levels. If groundwater quality is poorer than background levels, then mitigation measures will need to be employed to better treat the water, conduct further assessment on the impact of discharging the water and/or discharge the dewatering water to an alternative location such as the CCC sewer. 5 Current Estimate of Spring Monitoring Across Christchurch Northern Arterial Extension In order to monitor the potential effects on springs, the management plan should consider monitoring of three springs M35/8139, M35/8138 and M35/8136. Visual inspections and the level of the spring are considered sufficient to determine any potential change in spring condition. 6 References SCIRT, DEWATERING GUIDELINE: Guideline for Dewatering Practice for Christchurch. Prepared by Beca Ltd. Beca // 19 August 2014 // Page 12