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Updated Summary Geotechnical Interpretative Report CORRIB ONSHORE PIPELINE UPDATED SUMMARY GEOTECHNICAL INTERPRETATIVE REPORT FORESHORE GROUND INVESTIGATION SRUWADDACON BAY Prepared for: Shell E&P (Ireland) Ltd. October 2010 AGEC Ltd The Grainstore Singletons Lane Bagenalstown Co. Carlow Ireland Home page: www.agec.ie Page i agec
Updated Summary Geotechnical Interpretative Report DOCUMENT APPROVAL FORM Corrib Onshore Pipeline Document title: Updated Summary Geotechnical Interpretative Report Foreshore Ground Investigation ‐ Sruwaddacon Bay File reference Number: 1055_006 Document Revision No. 0 Note: File Reference Document Amendment/Comment Number Revision No. Task Nominated authority Approved (signature) Prepared by Authors: Paul Jennings Martin Lyttle Heather Gibbons Checked by Approved by Quality check Report Checker: Turlough Johnston Project Director: Turlough Johnston Quality Manager: Marion English This document has been prepared for the titled project and should not be relied upon or used for any other project without an independent check being carried out as to its suitability and prior written authority of AGEC being obtained. AGEC accepts no responsibility or liability for the consequences of this document being used for a purpose other than the purposes for which it was commissioned. Any person using or relying on the document for such other purposes agrees, and will by such use or reliance be taken to confirm his agreement to indemnify AGEC for all loss or damage resulting therefrom. AGEC accepts no responsibility or liability for this document to any party other than the person by whom it was commissioned. Page ii agec
Updated Summary Geotechnical Interpretative Report NON TECHNICAL SUMMARY This report provides an updated summary of the geotechnical interpretation for Sruwaddacon Bay using the results of the ground investigation carried out between July 2010 and October 2010 along the proposed gas pipeline route within Sruwaddacon Bay, North Mayo. This updated report supersedes the previously issued report entitled ‘Interim Summary Geotechnical Interpretative Report’ dated August 2010, which was included in the Environmental Impact Statement (EIS) Addendum Appendix E, 24 August 2010. The interim report was based on the ground investigation available at the time of writing in August 2010. The updated report includes a summary of the 2010 ground investigation data, geological sequence within the bay and the material properties of the geological strata. The 2010 ground investigation comprised the following works: (1) Twenty six (26) cable percussion (CP) boreholes (2) Thirty one (31) rotary core drill (CD) holes (3) Sixty one (61) static cone penetration tests (CPTs) (4) Insitu testing within a number of exploratory holes (5) Representative soil and rock samples recovered from exploratory holes (6) Laboratory testing was carried out on representative soil and rock samples The geological sequence in Sruwaddacon Bay as interpreted from the ground investigation confirmed the geology presented in the EIS. The geology within the bay comprised the following: (1) Fine to medium sand (estuarine deposit) (2) Organic silt and very localised peat (3) Sand and gravel (possible glacial soil) (4) Bedrock The dominant deposit within the bay along the line of the proposed tunnel is the estuarine deposit. The estuarine deposit through which the tunnel will be excavated is considered a relatively benign and stable deposit with respect to the proposed tunnel works comprising essentially medium to fine sand which is occasionally gravelly. Localised layers of less dense, gravelly soils and thin layers of silt and peat are also present at the tunnel depth; though these are considered not to represent any difficulties to the proposed tunnel works. The line of the proposed tunnel within the estuarine deposit provides a relatively uniform medium that can be readily excavated by a tunnel boring machine (TBM). The route minimises the extent of rock tunnelling, which can be abrasive on the TBM. Page iii agec
Updated Summary Geotechnical Interpretative Report TABLE OF CONTENTS Page No. i ii iii iv REPORT TITLE DOCUMENT APPROVAL FORM NON TECHNICAL SUMMARY TABLE OF CONTENTS 1 2 3 4 INTRODUCTION 1 1.1 1.2 1.3 1.4 1.5 1 1 1 2 3 SITE GEOLOGY 4 2.1 2.2 4 4 Bedrock Geology Soils/Subsoils in the Sruwaddacon Bay Area FIELDWORK AND LABORATORY TESTING 6 3.1 3.2 6 7 Fieldwork Laboratory Tests GROUND PROFILE 4.1 4.2 4.3 4.4 4.5 5 Background Scope of Report Geological Drawings Sruwaddacon Bay Geology in Updated Report ‐v‐ Interim Summary Report Ground Investigation Works 8 Fine to Medium Sand (Estuarine Deposit) 4.1.1 Soil Description 4.1.2 Spatial Variation 4.1.3 Material Properties Organic Silt and Peat 4.2.1 Soil Description 4.2.2 Spatial Variation 4.2.3 Material Properties Sand and Gravel (Possible Glacial Soil) 4.3.1 Soil Description 4.3.2 Spatial Variation 4.3.3 Material Properties Bedrock 4.4.1 Rock Description 4.4.2 Spatial Variation 4.4.3 Material Properties Groundwater Conditions within Sruwaddacon Bay Sediments REFERENCES 20 8 8 9 9 12 12 12 13 14 14 14 14 16 16 16 17 18 Page iv agec
Updated Summary Geotechnical Interpretative Report TABLES (within report text) FIGURES (at end of report text) Figure 1 Moisture Content versus Depth Figure 2 SPT versus Depth Figure 3 Unconfined Compressive Strength versus Depth DRAWINGS (see Sections 3 and 4) SECTION 3 Drawing 1045_001 Not used Drawing 1045_002 Plan and Section Showing Sruwaddacon Bay Geology (Sheet 1 of 3) Drawing 1045_003 Plan and Section Showing Sruwaddacon Bay Geology (Sheet 2 of 3) Drawing 1045_004 Plan and Section Showing Sruwaddacon Bay Geology (Sheet 3 of 3) SECTION 4 Drawing 1045_002a Plan and Section Showing Sruwaddacon Bay Geology with CPT Data (Sheet 1 of 3) Drawing 1045_003b Plan and Section Showing Sruwaddacon Bay Geology with CPT Data (Sheet 2 of 3) Drawing 1045_004c Plan and Section Showing Sruwaddacon Bay Geology with CPT Data (Sheet 3 of 3) Page v agec
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INTRODUCTION 1.1
Background Updated Summary Geotechnical Interpretative Report Applied Ground Engineering Consultants Ltd (AGEC) was requested by Shell E&P Ireland Limited (SEPIL) to prepare an updated summary geotechnical interpretative report of ground conditions along the proposed gas pipeline route within Sruwaddacon Bay, North Mayo. The proposed pipeline between Glengad (ch. 83,910) and Aghoos (ch. 88,770) will be installed within a segment lined bored tunnel, which will be constructed using a Tunnel Boring Machine (TBM). The proposed tunnel alignment through Sruwaddacon Bay will be approximately 4.6km long and predominantly through superficial deposits. The 2010 investigation works within the bay comprises the latest investigation within the bay; previous investigations within the bay have included Osiris (2007) and Irish Drilling Ltd (IDL, 2008 and 2009). The purpose of the 2010 investigation was to confirm that the bay geology along the proposed pipeline route is consistent with findings of previous investigation works, and as presented in the Environmental Impact Statement (EIS) 2010. 1.2
Scope of Report This report describes the ground investigation work carried out between July 2010 and October 2010 along the proposed gas pipeline route within Sruwaddacon Bay, North Mayo. This report supersedes the previously issued report (AGEC ref. 1045_042) entitled ‘Interim Summary Geotechnical Interpretative Report’ dated August 2010, which was included in the EIS Addendum Appendix E, 24 August 2010. The interim report was based on the ground investigation available at the time of writing in August 2010. This updated report provides a revised summary interpretation of the geology and ground investigation data based on the substantially completed 2010 ground investigation in Sruwaddacon Bay. 1.3
Geological Drawings Completed geological long sections based on the 2010 ground investigation are provided on the long sections Drawings 1045_002, 1045_003 and 1045_004 (see Section 3). Page 1 agec
Updated Summary Geotechnical Interpretative Report In addition, the geological long sections with cone penetration test (CPT) data are shown on Drawings 1045_002a, 1045_003a and 1045_004 (see Section 4). The above drawings show the location of exploratory holes on plan, together with a geological section along the line of the proposed tunnel. It is noted that exploratory holes are off‐set from the line of the tunnel. At the location where the exploratory holes are located the sea bed level may be different from the ground level above the line of the proposed tunnel. This results in exploratory holes appearing either above or below the sea bed level on the geological section along the line of the proposed tunnel. 1.4
Sruwaddacon Bay Geology in Updated Report ‐v‐ Interim Summary Report The geological sequence and material properties presented in the interim report have remained substantively unchanged in this updated report. The spatial variation (location, depths and thicknesses) of the geology has essentially remained unchanged though locally the following have been updated to include: (1) Fine to medium sand (Estuarine Deposits). A greater thickness of this deposit extended further southeast within the bay. Gravel content within this deposit was found to be more common, particularly northwest of a promontory that extended into the bay at about Ch. 86,900m. (2) Organic Silt and Peat. The maximum thickness of this deposit was found to be 2.5m. The additional exploratory holes confirmed the organic silt and peat layer is generally discontinuous across the bay and forms the juncture between the estuarine deposit and the glacial soil. This organic deposit was found to be generally absent where the estuarine deposits are less thick, such as at Ch. 86,900m where a promontory extended into the bay. (3) Sand and Gravel Deposits (Possible Glacial Soil). The main difference was found to be an increase in thickness of this glacial soil in the vicinity of Ch. 86,900m. It appeared that glacial material formed the promontory into the bay. Otherwise, thicknesses and depths of these deposits were found to be broadly similar. (4) Bedrock. In the outer part of the bay, rockhead was found at relatively shallow depth in CD05. In the central part of the bay, previously weathered rock was interpreted to be at greater depth however CD33, CD36 and to a lesser extent CD37, encountered unweathered rock at shallower depths. Notwithstanding the above, in general rockhead levels were found to be broadly similar. Page 2 agec
1.5
Updated Summary Geotechnical Interpretative Report Ground Investigation Works The 2010 ground investigation works within Sruwaddacon Bay have been carried out using cable percussion (CP) and rotary core drilling (CD) together with cone penetration testing (CPT). The ground investigation works have been carried out from two jack‐up barges within the bay and one ‘Meercat’ barge for CPT works. The completed ground investigation has an average spacing of less than 100m. This coverage satisfies the spacing requirements of 20m to 200m for pipelines and tunnels as set out in IS EN 1997‐2: 1997 (Eurocode 7), Annex B. Page 3 agec
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SITE GEOLOGY 2.1
Bedrock Geology Updated Summary Geotechnical Interpretative Report According to the bedrock Geology of Mayo, Sheet No. 6 (Geological Survey of Ireland, 1992), the proposed route is underlain by Dalradian Rocks, laid down over 600 million years ago. The Dalradian Rocks were subjected to massive compression and faulting over an initial 200 million years due to several periods of continental convergence. As a result, the Dalradian Rocks consist of metamorphic and faulted sedimentary sandstones, which have been altered to quartzites, psammitic schists and pelitic schists with some marbles also present. The Dalradian rocks, having undergone a number of deformation events, are also characterised by extensive multiple folding. Ground investigations along the terrestrial section of the route by Geotechnical and Environmental Services (GES, 2007) indicated psammite rock with bands of pelitic schist and pelite with rockhead between 1.4m and over 21.0m below ground level. Ground investigations in Sruwaddacon Bay (IDL, 2008) indicated psammite rock with bands of semi pelite, quartz muscovite schist, semi‐pelitic schist and psammitic schist. Rockhead was encountered between 3.3m and 24.8m below seabed level within the bay. The recovered rock cores were generally highly fractured. Rock strength from Point Load and Unconfined Compressive Strength (UCS) testing varied from very weak to extremely strong. Cerchar abrasivity testing on samples from the bedrock indicated that the rock can be classified as very abrasive. Geophysical surveying in Sruwaddacon Bay (Osiris, 2007) indicated rockhead between about 1m and 25m below seabed level. Rockhead was shallower towards the edges of the bay with rockhead deeper towards the centre of the bay. 2.2
Soils/Subsoils in the Sruwaddacon Bay Area A small amount of soil derived from windblown sands is present at the northwestern edge of Sruwaddacon Bay. Alluvial deposits comprising mixed granular material (sand to boulder sized particles) are present along river channels in the area. At the Corrib offshore pipeline landfall location at Gleann an Ghad (Glengad), topsoil is underlain by aeolian sands and gravel over colluvium or weathered rock over weathered to fresh rock. Clayey sands are visible in Sruwaddacon Bay at low tide. On the foreshore cliffs surrounding Sruwaddacon Bay there are exposures of peat underlain by sandy gravely clay. Excavations undertaken by SEPIL in the area of the Landfall Valve Installation site at Gleann an Ghad (Glengad), as part of the offshore pipeline landfall works, exposed sand and gravels with cobbles to a depth of between about 2.5m and 3.0m. Page 4 agec
Updated Summary Geotechnical Interpretative Report Ground investigations carried out in Sruwaddacon Bay (AGEC 2004, IDL 2008), which comprised boreholes and some trial pits, indicated sediments that comprised sand and gravel with some cobbles. These sediments where encountered in boreholes to a depth of about 25m below seabed level within the narrow northwest (outer) part of the bay. In the southeast (inner) part of the bay, boreholes encountered occasional thin clay/silt layers and some thin peat layers. The clay/silt layers become more prominent towards the southwest shoreline of the bay. Geophysical investigation survey results for Sruwaddacon Bay (Osiris, 2007) indicated sediments of dominantly granular material to a depth of about 25m below river/seabed level in the northwest (outer) part of the bay and to a depth of about 12m below river/seabed level in the southeast (inner) part of the bay. These sediments become shallower towards the margins of the bay. The sediments comprised a mixture of reworked fine to medium sand through the central part of the bay and mixed gravel sediments, derived from glacial tills and weathered bedrock, at the bay margins and in areas of stronger current flow. The mixed gravel sediments are incised by the Glenamoy and Muingnabo River channels, which enter the bay as one channel at its extreme southeast point. Page 5 agec
Updated Summary Geotechnical Interpretative Report 3
FIELDWORK AND LABORATORY TESTING 3.1
Fieldwork The current ground investigation work commenced in July 2010 and was substantially completed by October 2010. The ground investigation comprised the following exploratory holes. Twenty six (26) cable percussion (CP) boreholes. These exploratory holes have been sunk between depths of 5.0m (CP62) and 21.95m (CP09). These exploratory holes are as follows: •
CP05, CP07, CP08, CP09, CP13, CP14, CP15, CP16, CP17, CP18, CP26, CP30, CP33, CP36, CP37, CP40, CP42, CP43, CP45, CP47, CP49, CP53, CP57, CP59, CP62 and CP65. Thirty one (31) rotary core drill (CD) holes. These exploratory holes have been drilled between depths of 13.1m (CD06) and 41.2m (CD42). These exploratory holes are as follows; •
CD03, CD04, CD05, CD06, CD07, CD08, CD09, CD12, CD13, CD14, CD15, CD16, CD17, CD18, CD26, CD30, CD33, CD36, CD37, CD40, CD42, CD43, CD45, CD47, CD49, CD53, CD57, CD59, CD62, CD63 and CD65. Sixty one (61) static cone penetration tests (CPTs). These tests have been taken to depths between 0.28m (CPT17) and 21.43m (CPT08A). The completed CPTs are as follows: •
CPT05, CPT06, CPT06A, CPT06B, CPT06C, CPT07, CPT08, CPT08A, CPT09, CPT10, CPT10A, CPT14, CPT16, CPT16A, CPT17(Note 1), CPT17A(Note 1), CPT17B(Note 1), CPT17C(Note 1), CPT17D, CPT18, CPT18A, CPT18B, CPT18C, CPT19, CPT20, CPT22, CPT24, CPT26, CPT28, CPT30, CPT31, CPT31A, CPT32, CPT32A, CPT33, CPT34, CPT34A, CPT35, CPT36, CPT37, CPT37A, CPT38, CPT38A, CPT39, CPT40, CPT40A, CPT43, CPT44, CPT45, CPT46, CPT49, CPT53, CPT57, CPT59, CPT62, CPT100, CPT101, CPT102, CPT104, CPT106, CPT107. (Note 1: CPT terminated at shallow depth due to obstruction or penetration difficulties) Insitu testing was carried out in a number of exploratory holes. This included; Standard Penetration Tests (SPT‐N) and falling head permeability (FHP) tests. Representative soil and rock samples were recovered from exploratory holes for detailed logging and laboratory testing. Laboratory testing was carried out on representative soil and rock samples from the exploratory holes to determine soil classification and geotechnical properties. Some chemical testing was also carried out on samples. Page 6 agec
Updated Summary Geotechnical Interpretative Report 3.2
Laboratory Tests The following soil laboratory tests have been carried out: •
Natural Moisture Content (NMC) •
Atterberg Limits •
Particle Size Distribution (PSD) •
Sedimentation •
Compaction •
Chemical (pH, Sulphate and Chloride) •
Organic Content •
Triaxial •
Bulk Density •
Permeability • Specific Gravity The following rock laboratory tests have been carried out: •
Point Load Test (PLT) •
Uniaxial Compressive Strength (UCS) •
Cerchar Abrasivity •
Porosity •
Brazilian Tensile Strength • Petrographic Analysis The following water laboratory tests have been carried out: •
Total Dissolved Solids •
Chemical (pH, Sulphate and Chloride) •
Iron content •
Total Soluble Salts Page 7 agec
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Updated Summary Geotechnical Interpretative Report GROUND PROFILE The fieldwork has revealed ground conditions in Sruwaddacon Bay as follows: •
Fine to medium sand (estuarine deposit) •
Organic silt and very localised peat •
Sand and gravel (possible glacial soil) •
Bedrock TABLE 1: GROUND PROFILE Strata Depth to Top of Strata (m bsl) (Note 1) Maximum Thickness of Strata (m) (Note 2) Fine to medium sand (estuarine deposit) Seabed level 20.4 Organic silt and very localised peat 7.5 to 19.6 2.5 Sand and gravel (possible glacial soil) 2.0 to 20.4 0.1 to 14.5 Bedrock 4.7 to 24.8 ‐ Notes (1) Depth given as metres below seabed level (m bsl) (2) Maximum recorded strata thickness is based on the current ground investigation data. The dominant deposit within the bay along the line of the proposed tunnel is the estuarine deposit. The estuarine deposit through which the tunnel will be excavated is comprises essentially medium to fine sand which is occasionally gravelly. Localised layers of less dense, gravelly soils and thin layers of silt and peat are also present at the tunnel depth; though these would not be considered to represent any difficulties to the proposed tunnel works. 4.1
Fine to Medium Sand (Estuarine Deposit) 4.1.1
Soil Description The estuarine deposit generally comprised very loose to dense light brown, greyish brown and grey medium to fine SAND. This deposit is generally uniformly graded with little to no fines content (particles less than 63 microns) and occasionally slightly gravelly. In several boreholes (CD12, CP13, CP15, CP16, CP17 & CP18) a gravelly layer Page 8 agec
Updated Summary Geotechnical Interpretative Report with some cobbles was encountered. Shells and shell fragments occurred throughout the deposit to varying degrees. A gravelly deposit was encountered from 6.00m to 11.4m below seabed level in CP33 and CP36. A consistent firm to occasionally stiff silt/clay layer (thickness range from 0.2 to 0.8m) was indicated in CPTs at approximately 10m depth (CPT19, CPT20, CPT22, CPT24, CPT26, CPT28, CPT30, CPT31, CPT31A, CPT32 and CPT32A). Where this layer was encountered in CP30 (between 11m and 11.4m below seabed level) it was found to be predominantly organic silt. 4.1.2
Spatial Variation The estuarine deposit was thickest in the mouth of the bay, that is at the narrowing between the northwest (outer) part of Sruwaddacon Bay and the southeast (inner) part of the bay, with recorded thicknesses of up to about 20.4m (CD08) and 19.60m (CP09). Locally the deposit thickness decreased to 12.40m (CD18) where the bay widened. In the majority of the southeast (inner) part of the bay the deposit thickness gradually decreased southwestward from 17.5m (CPT22) to 7.50m (CD49). A promontory extended into the bay from the southwest shore at Ch. 86,900m where the estuarine deposit reached a depth of 5.00m (CP37). A gravelly layer appeared to occur at the base of the estuarine deposit at this location (CP33 and CP36). In the northwest (outer) part of the bay and the main channel that passes through the bay, the estuarine deposit tended to comprise medium sand to a depth of typically 6m to 8m below seabed level before becoming fine to medium sand with depth. In the southeast (inner) part of the bay, the deposit comprised fine to medium sand becoming predominantly fine sand with depth. The deposit in the southeast (inner) end of the bay tended to be thinner (2m to 5m) grey and slightly organic. 4.1.3
Material Properties 4.1.3.1 Soil Classification Particle size distribution tests were carried out on samples and show fines content (particles less than 63 microns) that ranged between 0.7% (CP16 at 6m depth) and 11.7% (CP49 at 10m to 10.95m depth). The typical fines content in this layer was in the order of 3%. Following ‘Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification’ (EN ISO14688‐2: 2004, Figure B.1), the estuarine material can be generally classified as SAND with localised zones of gravelly Page 9 agec
Updated Summary Geotechnical Interpretative Report SAND (CP08, CP15 and CP 33), GRAVEL (CP16) and sandy GRAVEL (CP09, CP18, CP33 and CP65). This classification corresponds with borehole log descriptions. There was a number of natural moisture content tests (Figure 1) carried out for this deposit with values reported between 6.2% (CP42 at 4m depth) and 57.2% (CP09 at 19.6m depth). The higher moisture content values may be anomalous for sand with nominal fines, and are attributed to samples containing excess water due to sampling below sea level. 4.1.3.2 Shear Strength SPT’s were performed in the estuarine deposit at various depths within most CP boreholes and some CD drillholes (CP5, CP8, CP9, CD9, CP13, CP14, CP15, CP16, CP17, CD17, CP18, CP26, CP30, CP33, CP36, CP37, CP40, CP42, CP43, CP45, CP47, CP49, CP53, CP57, CP59, CP62 and CP65). The SPT N values ranged from 0 to 81 with a typical value of 23 (Figure 2). These SPT N values indicate densities from very loose to very dense. Similarly, CPT results varied from very loose to dense. Generally, density increases with depth (Figure 2) with the upper layers being loose to medium dense and the lower layers medium dense to dense. There were three shear box tests performed on samples taken from this deposit. The result gave a range of friction angles (phi’) between 34 and 37 degrees. This friction angle corresponds to medium dense to dense granular material. At the depth of the proposed tunnel the estuarine deposit comprised essentially a medium dense dominantly sandy soil. Localised layers of less dense and more gravelly soils are also present at the tunnel depth; though these do not represent any difficulties to the proposed tunnel works. 4.1.3.3 Stiffness SPT N values can be used to derive stiffness values (E’). The E’ value can be determined following Burland and Burbridge (1985), see CIRIA Report 143, Table 11 (Clayton, 1995). The E’ value at the proposed tunnel alignment depth ranged from about 3.2MPa to 370MPa based on the range of SPT N values. 4.1.3.4 Permeability Falling head permeability tests were carried out insitu in a number of boreholes at depths between 4m and 11.35m below seabed level. Permeability values (k) ranged Page 10 agec
Updated Summary Geotechnical Interpretative Report between 1.27 x 10‐7 and 9.77 x 10‐6 m/s. These k values indicate low to very low permeability. Two laboratory permeability tests were carried out on samples from this deposit with results ranging from 1.25 x 10‐5 and 7.25 x 10‐7 m/s. These k values indicate low to medium permeability. These k values indicate sand, very fine sand/silty sands and silt and interlaminated silt/sand/clays which correspond with the descriptions of the sediments in the bay. 4.1.3.5 Chemical Results Chemical testing was carried out on soils with pH values ranging between 7.87 and 8.98. Water soluble sulphate in 2:1 extract (SO3) testing was performed on samples with values ranging from 0.192 to 0.391 (g/l). There was water soluble sulphate tests performed in soil (SO3) with values ranging from 0.038 to 0.078%. One loss on ignition test was performed giving a result of 11.1% (CP09). 4.1.3.6 Earthworks Classification It is proposed to use the tunnel arisings where appropriate in construction of the stone road for the onshore section of the pipeline in the non‐sensitive peat areas. The estuarine deposit has been classified based on borehole records and in accordance with the National Roads Authority’s Specification for Road Works (NRA SRW), Volume 1. The material is typically classified as a Class 1A (well graded granular material) and Class 1B (uniformly graded granular material). There are localised zones of Class 6A and Class 6C (selected well graded granular material) as recorded in CP16 and CP18. Based on the classification test results, the estuarine deposit would be suitable for reuse within the proposed stone road. A number of laboratory compaction tests were performed to determine the maximum dry density (MDD) of the material. These MDD’s ranged from 1.57Mg/m3 to 1.91Mg/m3 at optimum moisture contents (OMC) ranging 15% to 20%. A California Bearing Ratio (CBR) test was performed on samples to determine performance of this material. The CBR values recorded ranged from 4% to 23% with a typical value of 13%. This typical value would render the deposit suitable for reuse in the proposed stone road. Page 11 agec
4.2
Organic Silt and Peat 4.2.1
Soil Description Updated Summary Geotechnical Interpretative Report The organic silt and peat deposit comprised the following: (1) Brown and greyish brown slightly sandy (fine, micaceous) organic SILT with some shells fragments (generally sand and fine gravel sized), and (2) Localised dark brown highly decomposed amorphous PEAT with many woody fragments.
4.2.2
Spatial Variation The organic silt and peat layer recorded in the exploratory holes comprised a discontinuous and thin layer of variable thickness encountered between the estuarine and glacial soil deposits. The greatest thickness of organic silt and peat was encountered in CP43 (2.5m). This comprised a layer 1.10m thick (at 11m depth) of stiff dark grey slightly sandy SILT with many shells and occasional plant and root fibres and a layer 1.4m thick of dark brown amorphous PEAT with plant fibres, rootlets and wood fragments. In CP40, a layer 2.0m thick (at 12m depth) of firm brown to greyish brown slightly sandy organic SILT with thin peaty laminae (generally less than 0.01m thick) and decayed plant fragments was recorded. At the base of the organic silt layer, a layer of soft to firm woody PEAT was recorded some 0.5m thick. Substantial thicknesses of organic SILT and PEAT (>1.0m) were also found in CP30, CP47 and CP49. Traces of organic material were recorded at various depths usually immediately above the glacial soil in boreholes (CP09, CD14, CP15, CD16, CP17, CD26 and CD42) and consisted of a variety of deposits ranging from an isolated fragment of wood (CP16) and traces of organic silt and peat. The organic material recorded in the exploratory holes indicated the presence of a discontinuous organic layer of variable thickness that either formed in situ or possibly of wash‐in origin. This layer has been recorded across the bay and lies between the estuarine deposit and glacial soil deposit. The organic layer comprised: (1) organic silt up to a thickness of 1.5m with (2) basal peat up to a thickness up to 1.4m. This organic layer was identified in the previous ground investigation (IDL, 2008). Silt was also encountered in CP30 (between 11m and 11.4m below seabed level) within the estuarine deposit. Page 12 agec
4.2.3
Updated Summary Geotechnical Interpretative Report Material Properties 4.2.3.1 Soil Classification Natural moisture content testing (Figure 1) was carried out for the organic silt with values ranging from 33.2% (CP49 at 7.75m bsl) and 215.4% (CP40 at 14m bsl). Particle size distribution tests were carried out on samples from CP40 at 14m bsl and CP30 at 15.4m bsl and showed fines content (particles less than 63 microns) of 68% and 39% respectively. This classification typically corresponds with borehole log descriptions. 4.2.3.2 Shear Strength SPT’s were performed in the organic deposit in a limited number of boreholes (for example CP 40, CP43, CP45, CP47 and CP 49). The SPT N values results ranged from 9 to 78 (Figure 2). These SPT N values indicated likely undrained strength in the range of 50kN/m2 to greater than 150KN/m2. The relatively high undrained strength in the organic silt indicated a degree of over‐
consolidation. 4.2.3.3 Stiffness SPT N values were used to derive stiffness values (Eu). The Eu value can be determined following Butler (1975) using Eu/N = 1.2, see CIRIA Report 143 (Clayton, 1995). The Eu for the organic silt and peat ranged from 11MPa to 94MPa. The relatively higher stiffness of the silt and peat reflects the relatively higher strength of this material, see above. 4.2.3.4 Chemical Results Chemical testing was carried out on a number of samples. A pH value of 5.75 was recorded. Water soluble sulphate in 2:1 extract (SO3) testing was performed on a sample with a value of 1.633 (g/l). A water soluble sulphate test (SO3) was performed with a resulting value of 0.327%. Loss on ignition tests were performed showing results between 4.1% and 23.52% (CP40). 4.2.3.5 Earthworks Classification Due to the organic nature of this material it would not be suitable for reuse as an earthworks material and is therefore not classified for earthworks engineering purposes. Page 13 agec
Updated Summary Geotechnical Interpretative Report 4.3
Sand and Gravel (Possible Glacial Soil) 4.3.1
Soil Description This sand and gravel deposit generally consisted of medium dense to very dense grey silty sandy GRAVEL/silty gravelly SAND/sandy gravelly SILT with cobbles. The gravel and cobble constituents of the deposit were recorded as sub‐angular to sub‐rounded and comprised a mixture of mainly metamorphic rocks. This well graded material indicated that its origin was likely to have been glacial. 4.3.2
Spatial Variation The sand and gravel deposit was sampled to a limited extent in a number of cable percussion boreholes (for example CP07, CP09, CP30, CP40, CP43, CP47, CP49, CP53, CP57, CP59, CP62 and CP65). Sampling was limited due to the difficulty of retrieving samples at depth within the marine environment and the cobble content of the deposit. Medium to coarse gravel and some cobbles of mixed lithologies were recovered from most of the rotary boreholes. The finer component (silt and sand) of the glacial deposit may have been washed out during the drilling process. An exception was CD37, where approximately 3.0m of very stiff brown slightly sandy gravelly SILT was recovered during coring. These glacial soils are likely to be present across the whole bay and have been recorded to range from 0.1m (CD15) to 14.5m (CD37) in thickness at depths from 2.0m (CP65) to 20.4 (CD08). 4.3.3
Material Properties 4.3.3.1 Soil Classification Particle size distribution (PSD) tests were carried out on a number of samples. The PSD results showed fines content (particles less than 63 microns) ranging between 6% (CP49 at 10m to 10.95m depth) and 29% (CP53 at 9m depth). The typical fines content in this layer was in the order of 12%. Following EN ISO14688‐2: 2004, Figure B.1 the material has a mixed classification of SAND (CP62), sandy GRAVEL (CP62) and GRAVEL (CP49). This classification typically corresponded with borehole log descriptions. There was a number of natural moisture content tests (Figure 1) carried out in this stratum with values reported between 5.1% (CP53 at 7m depth) and 38.1% (CP62 at 3m depth). The higher moisture content values are anomalous for a granular deposit with nominal fines, and are attributed to samples containing excess water due to sampling below sea level. 4.3.3.2 Shear Strength SPT’s were performed in the deposit at various depths within a number of boreholes (for example CP16, CP18, CP30, CP43, CP47, CP53, CP57, CP59, CP62 and CP65). The SPT Page 14 agec
Updated Summary Geotechnical Interpretative Report N values ranged from 2 to 68 (Figure 2). These SPT N values indicated densities from very loose to very dense. Similarly, CPT results varied considerably from very loose to dense. Given the limited number of SPT N results and the difficulties of testing in such a heterogeneous material in the marine environment, the lower range of SPT N values are not considered representative. A shear box test was performed on a sample taken from CP62 at 3m below seabed level. The result gave a friction angle (phi’) of 32 degrees. This friction angle corresponds to medium dense granular material. 4.3.3.3 Stiffness SPT N values can be used to derive stiffness values (E’). The E’ value can be determined following Burland and Burbridge (1985) assuming a granular soil. The E’ for the deposit ranged from 3MPa to about 305MPa. Given the limited number of SPT N results and the difficulties of testing in such a heterogeneous material in the marine environment the lower range of stiffness values are not considered representative. 4.3.3.4 Permeability Falling head permeability tests were carried out insitu in a number of boreholes at depths between 5m and 19.9m below seabed level. Permeability values (k) ranged between 1.27 x 10‐7 and 9.77 x 10‐6 m/s. These k values indicate low to very low permeability. These k values indicate very fine sand/silty sand which corresponds, which would suggest that there was a higher fines content within the deposit than observed in samples retrieved from the boreholes. 4.3.3.5 Chemical Results Chemical testing was carried out on a number of samples. The test results showed a pH value of 7.78. Water soluble sulphate in 2:1 extract (SO3) testing was performed on a sample with a value of 2.278 (g/l). A water soluble sulphate was test performed in soil (SO3) resulting in a value of 0.456%. 4.3.3.6 Earthworks Classification It is proposed to use the tunnel arisings where appropriate in construction of the stone road for the onshore section of the pipeline in the non‐sensitive peat areas. The sand and gravel deposit has been classified in accordance to the NRA SRW, Volume 1. The material is typically classified as a Class 1A (well graded granular material) and Page 15 agec
Updated Summary Geotechnical Interpretative Report Class 1B (uniformly graded granular material) in CP49 and CP62. The material in CP49 between 10m to 10.95m is also classified as Class 1C (coarse granular material) and Class 6C (selected well graded granular material). Based on the classification test results the deposit would be suitable for reuse within the proposed stone road. 4.4
Bedrock 4.4.1
Rock Description The rock encountered consisted of metamorphic lithologies of varying composition and strength. These rocks ranged from strong to extremely strong light grey fine to medium grained PSAMMITE with medium to closely spaced discontinuities, to extremely weak highly to completely weathered green and orange fine to medium grained MICA/SEMI‐
PELITIC SCHIST with extremely closely spaced discontinuities. The dip of the rock discontinuities ranged from vertical/sub‐vertical (70 to 80 degrees) to horizontal and was often aligned parallel to the dip of the foliation of the rock. In the upper weathered zones the discontinuities were often open and infilled with sandy clay/silt. 4.4.2
Spatial Variation In the northwest (outer) part of Sruwaddacon Bay the underlying rock (encountered in CD04, CD05, CD06, CD08, CD09, CD15, CD12, CD16, CD17 and CD18) generally comprised extremely strong to strong medium fresh light grey fine to medium grained PSAMMITE with medium to closely spaced discontinuities (low Fracture Index (FI)). Within northwest (outer) part of the bay there was a minor weathered zone at rockhead (typically up to 2.6m thick) where the rock was highly fractured (FI >20) and was recovered as gravel fragments in boreholes. In the main southeast (inner) part of the bay the rock was found to be weathered to considerable depths (23.3m to greater than 40.1m) in CD26, CD30, CD40, CD43, CD45, CD47, CD49 and CD53 These rocks were found to be generally moderately to completely weathered and comprised residual soil (recovered as sandy gravel/gravelly sand), extremely weak to weak, grey, green and orange brown fine to medium grained QUARTZ MICA SCHIST, SEMI‐PELITIC SCHIST and PSAMMITE with extremely closely spaced discontinuities (FI >20). The depth to rockhead (weathered/un‐weathered) encountered in the rotary borehole ranged from 4.7m (CD04) to 24.8m (CD08) below seabed level. On drawings 1045_002 to 004, rock is shown as weathered rock and bedrock. Weathered rock is defined as Fracture Index (FI) greater than 20 or no index (NI). Page 16 agec
4.4.3
Updated Summary Geotechnical Interpretative Report Material Properties 4.4.3.1 Petrographic Analysis Thin section petrographic analysis was used to provide a detailed mineralogical description of the rock. Petrographic analysis was performed on four (4) rock samples taken from boreholes CD08, CD17, CD18 and CD62. The analysis was based on ISRM suggested methods (Ulusay and Brown, 2007) and BS 5930. The results are shown in Table 2. TABLE 2: PETROGRAPHIC ANALYSIS RESULTS Borehole No. Depth (m bsl) Rock Type BH CD08 32.4 ‐ 32.55 Psammite BH CD17 23.8 ‐ 23.9 Psammite BH CD18 30.4 ‐ 30.5 Psammite BH CD26 30.65 – 30.84 Weathered Psammite BH CD62 13.73 ‐ 13.9 Psammite/Semi‐Pelite 4.4.3.2 Rock Density Bulk density tests were performed on eight (8) rock samples taken from holes CD08 CD12, CD17, CD17, CD40, CD42 and CD63. The bulk density tests were carried out in accordance with ASTM D7012‐07. The results varied from 2.4 Mg/m3 to 2.74 Mg/m3. Dry density tests were performed on rock samples. The dry density tests were carried out in accordance with ISRM 2007 Part 2 (Ulusay and Brown, 2007). The results ranged from varied from 2340 to 2740 kg/m3. Porosity tests were performed on seven (7) rock samples taken from rotary holes. The porosity tests were carried out in accordance with ISRM 2007 Part 2. The results varied from 0.7% to 12.5%. 4.4.3.3 Strength The Brazilian Tensile Strength (BTS) test was used to evaluate the tensile strength of the rock encountered. The BTS test was performed on a number of samples and was carried out in accordance with ISRM 2007 Part 2 (Ulusay and Brown, 2007). The results varied from 0.9 to 15.23MPa with an average of 6.85MPa Page 17 agec
Updated Summary Geotechnical Interpretative Report The Unconfined Compressive Strength (UCS) test was performed on rock samples taken from exploratory holes. The UCS test was carried out in accordance with ASTM D7012‐
07. UCS results were also derived from point load tests. The point load test was carried out in accordance with ISRM Methods. The USC results for rock ranged between 0.71 to 282MPa with an average value of 78MPa. Figure 3 shows the UCS results plotted against the depth below seabed level at which the samples were taken within the exploratory hole. The range in rock strength reflects the variable weathering encountered at rockhead. 4.4.3.4 Abrasivity Abrasivity is the property of a material to remove matter when scratched or ground against another material. The Cerchar abrasivity test is used to calculate the abrasivity of rock. The Cerchar abrasivity test was carried out on samples taken from rotary drillholes. The Cerchar abrasivity mean number varied from 0.23 (not very abrasive) to 4.49 (extremely abrasive). The typical Cerchar abrasivity value was recorded as 2.45 (very abrasive). The line of the proposed tunnel is essentially within the estuarine deposit in the bay and limits hard rock tunnelling. As such, the potential for excessive wear on the tunnel boring machine (TBM) due to tunnelling in hard rock within the bay, which is shown to have a relatively high Cerchar abrasivity, will be similarly minimised. 4.4.3.5 Earthworks Classification It is proposed to use the tunnel arisings where appropriate in construction of the stone road for the onshore section of the pipeline in the non‐sensitive peat areas. The rock material would typically be Class 1 or Class 6 following appropriate processing. Class 1 and/or Class 6 material would be suitable for reuse in the stone road. 4.5
Groundwater Conditions within Sruwaddacon Bay Sediments The 2010 ground investigation was carried out below sea level and in tidal conditions. From a review of the CPT porewater pressure (u) readings this showed generally hydrostatic water conditions consistent with tidal sea level within the sediments in Sruwaddacon Bay. During the ground investigation works, difficult boring conditions were encountered at a number of exploratory hole locations. These difficult boring conditions were as a result of ‘blowing’ sands (water entrained with sand) entering the borehole casing. The ‘blowing’ sands are considered to be attributed to the operational difficulty of maintaining equilibrium water levels within the boreholes casing in combination with the tidal marine environment. Page 18 agec
Updated Summary Geotechnical Interpretative Report Water samples were retrieved for testing from boreholes CP17 (6m) and CP62 (4.5m). Water test results were as follows: • Total dissolved solids 29,500 mg/l in CP62 • Sulphate content 2290 mg/l (CP17) and 2310 mg/l (CP62) • Chloride content 17,400 mg/l in CP17 • pH values of 7.87 (CP17) and 7.78 (CP62) Page 19 agec
5
Updated Summary Geotechnical Interpretative Report REFERENCES Applied Ground Engineering Consultants (2004). Final Report on Onshore Gas Pipeline – Glenamoy River Estuary Crossing – Site Investigation Factual Report. September 2004. American Society for Testing and Materials (2007). ASTM D7012‐07, 2007. Standard Test Method for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures. BS 5930:1999 Code of practice for site investigations. British Standards Institution, 1999. Burland, J.B. and Burbridge, M.C. (1985). Settlement of foundations on sand and gravel, Proc, ICE, Part 1, 78, pp 1325‐71. Butler, F.G. (1975). Heavily overconsolidated clays General State of the Art review for Session 3. Proc. Conf. on Settlement of Structures, Cambridge, Pentech Press, London. Clayton, C. (1995). The Standard Penetration Test: Methods of Use, CIRIA Report 143. EN ISO14688‐2: 2004. Geotechnical investigation and testing — Identification and classification of soil — Part 2: Principles for a classification. Geological Survey of Ireland (1992). Geology of North Mayo. Sheet 6. Geotechnical and Environmental Services (2007). Corrib Onshore Pipeline Onshore pipeline Route Selection Geotechnical Ground Investigation Factual Report (See EIS Appendix M1) Irish Drilling Limited (2008). Corrib Foreshore Pipeline Site Investigation Factual Report (See EIS Appendix M1) Irish Drilling Limited (2009). Corrib Onshore Pipeline Boreholes at Aghoos and Glengad. Site Investigation Factual Report. November 2009. IS EN 1997‐2. Eurocode 7 ‐ Geotechnical Design ‐ Part 2: Ground Investigation and Testing Ulusay, R. & Brown, E. T. (2007). The Complete ISRM Suggested methods for Rock Characterization, Testing and Monitoring: 1974‐2006. ISRM. Ankara National Roads Authority (2000). Specification for Road Works: Manual of Contract Documents for Road Works, Volume 1. Osiris Projects (2007). Corrib Gas Pipeline Landfall. Sruwaddacon Bay. Report No. C7009. November 2007. Page 20 agec
Updated Summary Geotechnical Interpretative Report FIGURES agec
Updated Summary Geotechnical Interpretative Report Moisture Content (%)
0
20
40
60
80
100
120
140
160
180
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220
0
1
Sand & Gravel (Possible Glacial Soil)
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Fine to medium sand (Estuarine Deposits)
Organic Silt
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8
Depth (m bsl)
9
10
11
12
13
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Figure 1: Moisture Content versus Depth 240
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Updated Summary Geotechnical Interpretative Report SPT 'N' Value
0
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20
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40
0
50
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Sand & Gravel (Possible Glacial Soil)
‐1
Fine to medium sand (Estuarine Deposits)
‐2
Organic Silt
‐3
‐4
‐5
Depth, (m OD)
‐6
‐7
‐8
Typical extent of SPT N values for Estuarine Deposit ‐9
‐10
‐11
‐12
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‐20
Figure 2: SPT versus Depth 90
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Updated Summary Geotechnical Interpretative Report Unconfined Compressive Strength (MPa)
0
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40
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100 120 140 160 180 200 220 240 260 280 300
0
UCS Derived from PLT UCS
5
Depth (m bsl)
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Figure 3: Unconfined Compressive Strength versus Depth