1 INTRODUCTION AND SCOPE OF WORK

Transcription

1 C & D Landfill 2 Landfill Rd Happy Valley Attentin: Christine Mrgan Dear Christine, RE: Review f Slpe Stability fr Resurce Cnsent, C & D Landfill, Happy Valley Our Reference: INTRODUCTION AND SCOPE OF WORK This letter reprt presents the results f ur assessment f recmmendatins prvided by Aurecn New Zealand Ltd (Aurecn) in regard t the future develpment f the C & D Landfill, Happy Valley, Wellingtn. The assessment was carried ut by Gescience Cnsulting (NZ) Ltd at the request f C & D Landfill Ltd as utlined in ur prpsal dated 23 Nvember 2012 (ref: P ) We understand that C & D Landfill are currently seeking cnsent fr the nging peratin f the landfill. The prpsed cnsent will allw fr filling up t a maximum level f 240 masl (metres abve sea level). The previus getechnical reprt prepared by Aurecn was cmpleted in 2009 essentially dealt with the prpsed lateral expansin f the landfill and did nt specifically cnsider increased heights much in excess f abut 205 masl. Additinally, as part f their prcessing f a Resurce Cnsent Applicatin by C & D Landfill, Wellingtn City Cuncil 1 requires cnfirmatin f the currency f the Aurecn reprt cnclusins, bth in regard t the increase in the prpsed fill height, as well as the ther recmmendatins (drainage requirements, fill slpe design etc). The main purpse f this reprt is t prvide a respnse t the request fr further infrmatin issued by Wellingtn City Cuncil. The scpe f wrk undertaken by Gescience fr this review has invlved the fllwing: Review f existing infrmatin prvided t us, including Aurecn s 2009 reprt; A reprt prepared by URS (2009)n the stability f the landfill n behalf f Wellingtn City Cuncil; Drawings and crss sectins f the prpsed landfill prvided by Opus; The Applicatin fr Land-use Cnsent dcument prepared by Opus (2012), and; WCC s RFI letter; 1 WCC letter dated 24 Octber 2012; Sectin 1 Getechnical. Gescience Cnsulting (NZ) Limited Level 1, Office A, 22 Panama St, Wellingtn 6011, New Zealand PO Bx , Wellingtn 6146 New Zealand T (+64) (4) F (+64) (4)

2 A half-day site assessment; this was undertaken n 4 December 2012 by a Principal Engineering Gelgist frm Gescience; Assessment f the landfill stability t the prpsed maximum fill height f 240masl; Review f the recmmendatins prvided in the Aurecn reprt and cnfirmatin f whether they are required t be adpted fr the n-ging develpment f the landfill; and Reprting (this letter). We nte that there are n plans available shwing the specific gemetry f the final structure fr which cnsent is currently being applied. As such, we cannt prvide definitive design drawings fr the getechnical aspects f the final cnsented structure. We have prvided sketch diagrams where apprpriate as Figures 1 t 3. A significantly greater level f engineering input will be required as part f detailed design f the landfill, shuld Resurce Cnsent be granted. 2 REVIEW COMMENTS Our review is fcused primarily with the Aurecn 2009 reprt which is referred t specifically in WCC s RFI letter. In general, we agree n a getechnical level with the findings f the Aurecn reprt. Our reprt is cnstructed t reference specific sectins f the Aurecn reprt and shuld be read in parallel with that reprt. Sectins 1 t 5 r the Aurecn reprt prvide backgrund infrmatin and describe fill cnstructin methdlgy practices which remain essentially unaltered at the time f ur site visit in December Our specific cmments in regard t Sectins 6.2 nwards in Aurecn s reprt are prvided belw: Sectin 6.2 Manmade Stability Sectin Fillings Aurecn cnsider that fill faces n the landfill shuld be cnstructed with the fllwing gemetry: Fill slpe f 2H:1V (26.5 ) 10m high batters, with 5m wide benches Under these cnditins, Aurecn have indicated that the faces will remain stable (Factr f Safety greater than 1.0) even under large earthquake events. Aurecn further nte that fill faces in 2009 were typically cnstructed at angles f arund 1.2H: 1V t 1.3H: 1V (38 t 39 ), and further nte that these face may underg lcalised failure under static (ie nn-seismic) cnditins. Whilst fr the majrity f the landfill, Aurecn did nt cnsider this t pse a significant level f risk, cncern was raised in regard the affect that this wuld have shuld failure ccur n the high face abve the culvert intake (Refer Figure 1, attached). Aurecn have identified (Sectin 7 f their letter reprt) that failure f the fill slpe abve the culvert may lead t blcking f intake and subsequent water pnding. Left uncntrlled, this culd lead t breach f the fill embankment dwnslpe and a subsequent debris flw int the ppulated valley belw. Aurecn nte that a very sizeable event [rainfall event] wuld be required t initiate such a failure and that C & D Landfill peratinal management plan has cntingency within it t deal with this situatin. We have nt reviewed this plan as part f ur brief. We agree with Aurecn s assessment blckage f the existing culvert is a ptential risk. Recmmendatins fr suitable risk mitigatin measures are prvided in ur cmments n Sectin 8 belw

3 Sectin 7 Discussin The first part f this sectin f Aurecn s reprt reiterates and expands n the risks assciated with flding f the culvert intake. We have prvided cmment n this abve and additinally in ur cmments n Sectin 8, belw. The secnd part f Sectin 7 makes reference t the ptential fr fill height t prceed up t 250masl ( r higher ) and suggests that this is feasible frm a getechnical pint f view. See Sectin 8 belw fr further discussin. Sectin 8 Future Develpment Engineering Cnsideratins The first part f Aurecn s Sectin 8 utlines aspects that were cnsidered by Aurecn as requiring t be incrprated int future develpment f the landfill. Specific recmmendatins are prvided in a numbered list (i) t (x). Table 1 belw prvides an utline f Aurecn s recmmendatins and ur cnsideratins f the requirements fr adptin f each recmmendatin. Direct qutatins frm Aurecn s reprt are shwn in italics in Table 1. Table 1. General Recmmendatins fr Landfill Develpment Aurecn Recmmendatin Requirement fr Adptin Gescience Cmments i) Remediate existing fill slpes plus new fills t meet: (a) maximum fill height between benches f 10m (b) maximum fill slpe 2H:1V (c) benches t be 5m minimum width ii) and vii) Incrprate a secndary flw path; preferably n natural dwn ne margin f the fill and lngitudinally slpe the intermediate bench dwn twards the new drainage curse iii) Permanent cut batters t be cnstructed n steeper than 70, increments f nt higher than 10m between benches f 5m width. These recmmendatins shuld be adpted fr final design slpes f the landfill and t the final height f 250masl. Temprary batter slpes can be cnstructed at up existing slpe angles (38 t 39 ) These recmmendatins shuld be adpted using the design drainage gemetry utlined in vii) f Sectin 8 f Aurecn s reprt and shwn f Figure 3. (ie 3m wide x 1m deep channels excavated at grund surface). Sediment cntrl measures (silt traps r similar; Refer Figure 3) shuld be incrprated at the dwnstream edge f each bench prir t discharge frm the surface f the landfill nt natural grund. These recmmendatins shuld be adpted fr final design slpes f the landfill. Aurecn nte that the recmmended final design gemetry will result in a Factr f Safety f 1.0 under large earthquake events. We cnsider that this is a cnservative apprach. We nte that in the RFI letter, WCC mentin under Pint 2 the requirement fr a strmwater management system that wuld place n r limited reliance n the existing culvert. We d nt cnsider this t be feasible given land bundary issues n the eastern side f the current landfill. Aurecn nte sme variatin t this verall gemetry culd be undertaken subject t specific engineering design. We agree with this psitin

4 iv) Bth cut and fills shuld be cnstructed t their finished face slpe and revegetated at cmpletin f their individual filling peratin v) Extensin t main culvert up t the West Valley shuld have branch lines extending ff the main at the junctin with secndary valleys Each entry lcatin t have a prper permanent intake structure vi). Cnstructin f a separate debris/trash screen sme 10m upstream acrss the base f the gully t catch larger material viii) Access tracks t the west valley may be achieved by use f the higher level fill benches. ix) This recmmendatin relates t infilling frm the base f fill at the level f the culvert intake (132.5masl) t the 150m level. The recmmendatin discusses the requirement fr a vertical manhle abve the exiting culvert intake x) This recmmendatin relates t the lack f subsil drainage within the existing fill and recmmends that any extensin f the main culvert up the valley shuld be accmpanied by subsil drains The remainder f the recmmendatin deals with design f an infiltratin structure abve the intake t the existing culvert This recmmendatin shuld be adpted. This recmmendatin shuld be adpted. This recmmendatin shuld be adpted as shwn n Figure 3 N cmment we cnsider this t be a design cnsideratin; rather than a getechnical cnsideratin We cnsider that filling shuld be undertaken up t a minimum level f 150masl as a matter f pririty, and the culvert intake mved up valley t suit (Figure 2). We d nt cnsider that a vertical manhle is required prvided that adequate strmwater cntrl is prvided. Subsil drainage fr new fillings shuld be cnsidered as shwn n Figure 3. We d nt cnsider that an infiltratin structure is required; rather apprpriate sedimentatin cntrl measures (silt traps r similar) shuld be incrprated int the strmwater management design. Infilling in the base f the valley and relcatin f the culvert intake significantly reduces the risk f blckage due t slpe failure within the area shwn n Figure 1. 3 RECOMMENDATIONS FOR DETAILED DESIGN 3.1 Overall Landfill Stability Based n the stability analysis undertaken by bth URS and Aurecn, we cnsider that filling f the landfill t a height f 240masl under the current prpsed fill design is achievable. Aurecn indicate that they cnsider that with the prpsed design, a Factr Safety (FOS) f apprximately 1.0 (ie a state f marginal stability) is attained nly under large scale seismic events

5 3.2 Requirements fr Detailed Design We nte in WCC s RFI letter their request fr relevant drawings, plans t supprt the recmmendatins abve. We have attempted t d s fr aspects f the design in Figures 1 t 3; hwever, it is difficult t prvide cmprehensive drawings at this stage, particularly in terms f strmwater design, as the final gemetry f the landfill is uncertain. Assuming that Resurce Cnsent is granted, the Aurecn recmmendatins utlined in Table 1 abve shuld incrprated int a engineered design fr the finalised landfill scheme. 4 LIMITATIONS i. We have prepared this reprt in accrdance with the brief as prvided. This reprt has been prepared fr the use f ur client, C & D Landfill, their prfessinal advisers and the relevant Territrial Authrities in relatin t the specified prject brief described in this reprt. N liability is accepted fr the use f any part f the reprt fr any ther purpse r by any ther persn r entity. ii. iii. iv. The recmmendatins in this reprt are based n the grund cnditins indicated frm published surces and site inspectins described in this reprt based n accepted nrmal methds f site investigatins. Only a limited amunt f infrmatin has been cllected t meet the specific financial and technical requirements f the Client s brief and this reprt des nt purprt t cmpletely describe all the site characteristics and prperties. The nature and cntinuity f the grund between test lcatins has been inferred using experience and judgment and it must be appreciated that actual cnditins culd vary frm the assumed mdel. This Limitatin shuld be read in cnjunctin with the IPENZ/ACENZ Standard Terms f Engagement. This reprt is nt t be reprduced either whlly r in part withut ur prir written permissin. Yurs sincerely Fr and n behalf f Gescience Cnsulting (NZ) Limited Richard Justice Principal Engineering Gelgist Reviewed by Guy Cassidy Assciate Engineering Gelgist

6 5 REFERENCES Aurecn New Zealand Ltd (2009) Getechnical Engineering Reprt Stability C & D Landfill, Landfill Rd, Happy Valley, Wellingtn. Letter reprt prepared fr Burrell Demlitin Ltd, 9 July 2009 ref 42373/001 URS New Zealand Ltd (2009) Getechnical Assessment, Burrell s C & D Landfill, Happy Valley, Wellingtn. Reprt prepared fr Greater Wellingtn Reginal Cuncil and Wellingtn City Cuncil, 27 July Ref /R001/E

7 lcatin f current Pipe Inlet (apprximate nly) Fill Face inclined at 38 ; up t apprximately 50m height Baseplan Surce: Opus Internatinal Cnsultants: Applicatin fr Land use Cnsent Date 14/12/12 Client C & D Landfill Drawn by RJ Prject Review f Landfill Slpe Stability fr Resurce Cnsent Apprved by GC Descriptin Plan View Scale NTS Figure N. 1 Prject Number

8 Apprximate lcatin f culvert inlet after filling t 150m level (refer Figure 3 fr intake details). Intake level t be prgressively raised upvalley as filling prceeds Fill placed t 150m level (minimum). Fill t be placed prgessively abve this height t remai at a lwer relative level than the culvert intake. Baseplan Surce: Opus Internatinal Cnsultants: Applicatin fr Land use Cnsent Date 14/12/12 Client C & D Landfill Drawn by RJ Prject Review f Landfill Slpe Stability fr Resurce Cnsent Apprved by GC Descriptin Requirements fr Fill Placement t 150masl Scale NTS Figure N. 2 Prject Number

9 Primary trash rack cmprising driven rail irns (r similar) spaced at 2m centres acrss full extent f valley 2m Precast cncrete headwall t suit pipe diameter 1 2 General Landfill General Landfill Material Culvert piping (900mm diameter assumed) Pipe bedding/cmpacted backfill Subsil Drain (refer detail 3) Rck surface at valley flr Free draining backfill Culvert Piping (assumed t be 900mm dia) NOT TO SCALE Natural grund surface (stripped f tpsil) Filter fabric (Bidim A29 r similar) Secndary Trash Rack (refer detail 2) Punched Nvacil 160mm dia (r (1) CROSS SECTION SHOWING CONCEPTUAL WATER INTAKE (3) SUBSOIL DRAINAGE DETAIL (VALLEY FLOORS) Precast cncrete Galvanised steel n 100mm grid, securely fastened t cncrete headwall 3m 1m (4) DRAINAGE CHANNEL DETAILS Lw permeability material (Clay liner, HDPE, Bentfix r similar within landfill material (nt required within natural grund) (2) SECONDARY TRASH SCREEN DETAIL (ISOMETRIC VIEW) (5) BENCH DRAINAGE DETAILS 2 1 5m bench General Landfill Material Sediment trap at dwnstream edge f fill batter prir t discharge t natural slpes r draiange channels within natural material Fill bench t be cnstructed with 1:100 back slpe t direct strmwater runff t te f fill slpe Date 14/12/12 Client C & D Landfill Drawn by RJ Prject Review f Landfill Slpe Stability fr Resurce Cnsent Apprved by GC Descriptin Cnceptual Strmwater Management Details Scale NTS Figure N. 3 Prject Number