Site Investigation and Characterisation of Soft Alluvium for Limerick Southern Ring Road - Phase II, Ireland

Transcription

1 Site Investigation and Characterisation of Soft Alluvium for Limerick Southern Ring Road - Phase II, Ireland Buggy, F., Roughan & O Donovan, Dublin, Ireland Peters, M, Faber Maunsell Limited, Dublin, Ireland, ABSTRACT: Limerick Southern Ring Road Phase II involves 1km of dual carriageway plus two toll plazas predominantly on embankments 3 to m high constructed on soft alluvium soils. Construction commenced in August and is expected to be complete in 1. The soft alluvium comprises mainly organic silt / clay to depths of up to 13m, being underlain by deposits of glacial tills and/or limestone. Alluvium contains isolated layers of highly organic soils or peat up to m thick. An extensive final design phase SI including PCPT, dissipation tests, dynamic penetration testing, cable percussion boreholes with in-situ vane tests and piston tube sampling, piezometers & rising head permeability tests etc. has been performed, complemented by extensive laboratory testing, including oedometer tests, UU and KoCU triaxial compression and extension tests, and direct simple shear tests on the alluvium. The paper describes and contrasts the various field investigatory and laboratory testing techniques employed with special emphasis on the following: a) comparative data for in situ undrained shear strength obtained from PCPT, in situ shear vane & UU triaxial testing on piston tube samples; b) comparative data for undrained strength ratio Cu / Po derived from PCPT, KoCU triaxial compression, extension and DSS tests; c) primary compressibility and coefficient of consolidation derived from laboratory oedometer tests and back analysis of nearby case history data; d) secondary compressibility and the effects of surcharge load cycle in laboratory tests; e) drained shear strength parameters for alluvium derived from KoCU triaxial tests. 1 INTRODUCTION The Limerick Southern Ring Road Phase II is generally located to the west of Limerick City. A consortium of private financing agencies and major construction companies, known as Direct Route JV, is constructing the project. Construction commenced in September and is anticipated to be completed in just under years by August 1. The facility will be tolled for users upon completion. The mainline comprises a dual two lane carriageway extending approximately 1km from the proposed N1 Interchange west of Cratloe Creek, north west of Limerick city, across and beneath the flood plain of the River Shannon, interchanging with the N9 Dock road and continuing south to the new Interchange at Rossbrien. The Clonmacken Link Road joins the mainline at Coonagh West to the existing N1 at Clonmacken. The project includes a major crossing of the River Shannon by means of a submersed tube tunnel. A location plan showing the extent of the project and associated major structures is given in Figure 1. This paper relates only to the areas of above ground highway earthworks north, including the Clonmacken Link, and south of an immersed tube tunnel to be constructed to carry the Ring Road beneath the River Shannon. The mainline chainage limits defining the earthworks within the scope of Roughan & O Donovan- Faber Maunsell Alliance Design brief, and discussed in this paper, are approximately Ch + to 5+m and Ch to 9+75m. A number of flood bunds are to be incorporated into the highway scheme to maintain continuity of existing flood protection system in the area and to protect the roadway. The flood plain of the River Shannon is underlain by extensive deposits of very soft soft alluvium comprising mainly organic silt / clay to depths of up to 13m. Areas of alluvium contain isolated layers, or pockets, of highly organic soils and peat. These layers of more organic soils are up to.5m thick.the alluvium layers are underlain by deposits of glacial tills and/or limestone. Areas of soft, highly compressible or organic ground may not be suitable as foundations for highway earthworks. In such circumstances consideration needs to be given to ground improvement measures such that a solution is developed in line with the design criteria stated in the project Construction Requirements, for the settlement and stability of earthworks.

2 Figure 1 Site Location Plan for Limerick Southern Ring Road Phase II

3 The method of ground improvement must also consider the construction programme, and the available time for completion or phasing of the works and the influence on secondary settlement rate. Principle methods adopted for earthwork embankment areas of the Limerick southern Ring Road Phase II Project include one of more of the following ground improvement solutions: Excavate and Replacement; Vertical Drainage Measures; Basal Reinforced Earthworks. Staged Construction Techniques & Limiting Rates of Construction Surcharging Generally where the extent and depth of soft or organic soils is limited to less than to 3 m deep, the most economic solution is excavation and replacement with acceptable fill, provided there is a readily accessible disposal area. The class of fill to be used will depend upon the location of the groundwater table. Excavation and replacement solutions may be feasible to greater depths but this will be dependent upon the prevailing ground and groundwater conditions, temporary support measures adopted, the proximity of existing infrastructure and available working space. Where the extent and depth of soft soils is deep, and where sufficient time exists in the construction programme, vertical drainage measures may be considered to accelerate consolidation settlement to within acceptable post construction limits. The coefficient of radial consolidation (C vh ) may be evaluated by several means including obtaining Cv from lab consolidation tests, deriving Cv from field tests such as CPT dissipation tests or field permeability tests and finally back calculation from instrumented field case histories in similar soils. All of these data sources were available for this project and are discussed in further sections of this paper.. GROUND INVESTIGATION Several phases of ground investigation have been carried out to investigate the ground and groundwater conditions along the route of the Limerick Southern Ring Road Phase II PPP Scheme. The ground investigation phases performed prior to PPP construction contract award to DirectRoute are summarised below: 1157), May August by Norwest Holst Soil Engineering Ltd; Supplementary Ground Investigation Limerick Southern Ring Road Phase II (Site Investigation Contract No F1), Feb - March by Norwest Holst Soil Engineering Ltd; Additional Supplementary Ground Investigation Limerick Southern Ring Road Phase II (Site Investigation Contract No F1), November by Norwest Holst Soil Engineering Ltd; Tender Phase Ground Investigation Limerick Southern Ring Road Phase II (Site Investigation Contract No KC195), by Geotech Specialist Limited. Prior to commencement of the detailed design, Roughan & O Donovan FaberMaunsell (ROD-FM) initiated a programme of further ground investigation fieldwork, sampling and laboratory testing to characterise specific properties of the soft alluvium deposits. The details of this ground investigation are summarised below. Detailed Design Phase Ground Investigation Limerick Southern Ring Road Phase II (Site Investigation Contract No 113--) April by Ground Investigations Ireland Ltd. The detailed ground investigation was aimed specifically at insitu testing using borehole shear vane and cone penetration testing in conjunction with the retrieval of good quality undisturbed samples using piston sampling techniques. The ground investigation was tailored for determination of the following; Comparative data for in situ undrained shear strength obtained from CPT, in situ shear vane & UU triaxial testing on piston tube samples. Comparative data for undrained strength ratio Cu / Po derived from PCPT, KoCU triaxial compression, extension and DSS tests. Primary compressibility and coefficient of consolidation derived from laboratory oedometer tests. Secondary compressibility and the effects of surcharge load cycle in laboratory oedometer tests Drained shear strength parameters for alluvium derived from KoCU triaxial tests. Each of these aspects will be examined through the various sections in this paper. Preliminary Site Investigation Limerick Southern Ring Road Phase II (Site Investigation Contract No

4 3. CASE HISTORY DATA In order to validate the characteristic behaviour of the alluvium deposits at Limerick Southern Ring Road Phase II full scale field behaviour data was sought from nearby construction projects. The emphasis of the field evaluation was focused primarily on settlement and rate of consolidation, as these were the parameters most readily monitored in construction projects. 3.1 Bunlicky WWTW & Corcanree Rising Main Pump Station. Settlement monitoring records were obtained from Roadbridge Ltd for the works at the Limerick Main Drainage Project, Limerick Drainage Scheme. Waste Water Treatment Plant (WWTP) Dock Rd, Bunlicky Contact 1. and Limerick Drainage Scheme Corcanree Rising Main and Pumping Station Contract.1. Both of these case history contracts were located within a few hundred metres of the Limerick Southern Ring Road Phase II project, just north of the N9 Dock Road (see Figure 1). Ground conditions encountered at the WWTW were generally as indicated in Table 1. Table 1 Generalised Ground Conditions at Bunlicky WWTW Soil Type Depth Range Firm Alluvial Crust 1. to 1.5 m Soft Alluvial 1. to to 7. m Silts/Clays/Peat Glacial and Fluvial 5. to 7. to >9. m Gravels Limestone Not Proven At the WWTP Bunlicky, Dock Road, vertical drains were installed in a triangular grid at centres of.75m beneath the footprint of the treatment plant. Fill was placed in stages to a height of 3m above existing ground and a further 1.5m of surcharge applied for up to 1 year. Fill was placed at varying rates within the surcharge area. Close to the perimeter, fill was placed relatively slowly and the presence of a berm aided stability. Towards the centre of the surcharge area fill was placed more rapidly, in some locations 3.m were placed in approximately 1.5 weeks. The settlement and fill elevations for three settlement plates, S1, S, and S3, in the central surcharge region are presented in Figure. The rate of consolidation observed at these monitoring locations appears to be around 1m /yr for this level of imposed loading. It should be noted that this is an average back calculated value for the full load increment due to placement of fill and surcharge combined. At the WWTP access roads vertical drains were installed in a triangular grid at centres of.9m. Surcharge fill was placed in stages to a height of.5m above existing ground, a transition zone was also constructed in which fill varied between.5 to.5m in height to match that placed at the treatment plant area. The surcharge was applied for up to 1 year. It can be interpreted from the monitoring observations at the access road that the rate of consolidation appears to close to 1.5m /yr, back calculated over the full load increment. Back analyses of the magnitudes of settlement for each of these areas reveal an average coefficient of compression (C c / 1 + e o ) of approximately.3. Insufficient data was available to assess whether secondary consolidation has commenced at each of these locations, or the relative magnitude of secondary settlement rates. Roadbridge Ltd also made settlement monitoring records available for the works undertaken at Corcanree Pumping Station, and the associated rising main crossing at Ballinacurra Creek. At this location vertical drains were installed on a 1m centre triangular grid and 3.m of surcharge fill was placed, remaining for at least 1 weeks. The settlement monitoring records for settlement plates, 1 and 1 located in the region of the rising main, north east of Ballinacurra Creek are presented in Figure 3. The ground conditions at each settlement plate are indicated in Table. Table Ground Conditions at Corcanree Rising Main, Soil Type Firm Alluvial Crust Soft Alluvial organic Silts/Clays Glacial and Fluvial Gravels Ballinacurra Creek. Settlement Plate 1 GL. m Settlement Plate 1 GL. m. 5. m. 11. m 5.. m m Limestone >. m >13. m Back analysis of the magnitudes of settlement for each of these areas reveal an average coefficient of consolidation ( C c / 1 + e o ) of approximately.5.

5 Figure. Settlement monitoring observations at Bunlicky WWTP plates 1 to 3. BUNLICKY WWTP : SETTLEMENT-PLATE RESPONSE TO FILLING Plates 1 and back analysis assessment (.75M DRAIN SPACING) FILL ELEVATION (m) PLATE SETTLEMENT *1(mm) ELAPSED TIME (days) SP1 Ground elev SP1 Settlement Ch = 1.m / yr (ROD-FM) Ch = 1.5m/yr BUNLICKY WWTP : SETTLEMENT-PLATE RESPONSE TO FILLING Plates and back analysis assessment (.75M DRAIN SPACING) FILL ELEVATION (m) PLATE SETTLEMENT *1(mm) ELAPSED TIME (days) SP Ground elev Ch = 1.m / yr SP Settlement Ch = m/yr. BUNLICKY WWTP: SETTLEMENT-PLATE RESPONSE TO FILLING Plates 3 and back analysis assessment FILL ELEVATION (m) PLATE SETTLEMENT *1(mm) ELAPSED TIME (days) SP3 Ground elev Ch = 1.m / yr SP3 Settlement Ch = m/yr

6 Figure 3 Settlement Monitoring Observations at Corcancree Pumping Station, Settlement Plates SP1 and SP1 RISING MAIN : SETTLEMENT-PLATE RESPONSE TO FILLING Plates 1 and back analysis assessment FILL ELEVATION (m) PLATE SETTLEMENT *1(mm) ELAPSED TIME (days) Rising main SP1 Elevation Ch = 1.m / yr Ch = m/yr Rising main SP1 Settlement RISING MAIN : SETTLEMENT-PLATE RESPONSE TO FILLING Plates 1 and back analysis assessment FILL ELEVATION (m) PLATE SETTLEMENT *1(mm) ELAPSED TIME (days) Rising main SP1 Elevation Ch = 1.m / yr Ch =.5 m/yr Rising main SP1 Settlement

7 3. N1 Bunratty Bypass & Northern Approach Road Mallow Street Bridge Limerick. These two projects were constructed during the mid 19 s and early 199 s north of the River Shannon and both incorporated vertical drains at spacings from.7 to 1.m and surcharge systems to improve soft organic silt alluvium soils. They were the subject of case history papers presented by Farrell, E. et al and Galbraith, R. at the IEI Seminar on Road Embankments on Soft Ground published in 199. Undrained shear strength ratios Cu/Po were reported as follows: a) Bunratty Bypass. to. (assessed from field vane test data).3 assumed in design. b) Mallow Street, Limerick..5 to.1;.3 assumed for design. Coefficients of Consolidation were reported also: a) Bunratty Bypass.35 to 1 m / year (from both lab tests and back calculated from field settlement data); b) Mallow Street, Limerick..5 to. m / year (derived from standpipe tests). Data was insufficient for back analysis but performance suggested that drainage occurred faster than expected. DESCRIPTION AND CLASSIFICATION OF ALLUVIUM DEPOSITS.1 General description and extent of alluvium deposits The uppermost 1m, approximately, of the fine grained alluvial material is often described as firm to stiff grey mottled brown, firm to occasionally stiff, with weaker soft to very soft fine grained alluvial deposits below. The stronger upper layer is known as the desiccated crust. Fine grained alluvial material comprises either of very soft to soft grey silty clay with organic material or uncompact grey silt with abundant organic material. The stratum occasionally contains bands of more sandy material or shell fragments but is generally free of distinct laminations, partings and other characteristics of sedimentary structure. Thickness varies between.3m and 1.7m at varying depths of between.1 and 13.5m below ground level. The alluvial fine grained strata is either absent or relatively thin and firm to stiff in the eastern portion of the alignment up to Ballinacurra Creek East. It then deepens towards Ballinacurra Creek West to a maximum depth of 5m and remains at 3 to m depth near to N9 Dock Road. Originally the alluvium deepened towards the southern bank of the River Shannon but a substantial thickness was removed and used in cement manufacture in the area that now form Bunlicky Lake. Alluvial soils are thickest in the vicinity of northern Tunnel Portal, U3 Underpass, and north of the mainline toll plaza south of Meelick Creek plus U Underpass and beneath the Clonmacken Link toll plaza. Peat and organic deposits are described in the previous site investigation phases, generally as very soft / soft dark brown slightly clayey slightly spongy slightly fibrous peat. The thickness varies between.5m and.5m and occurs at varying depths of between 1. m and m below ground level. The peat layer does not appear to be continuous throughout the section but rather occurs sporadically in isolated zones. The most significant peat layers are typically found at depth, in pockets, at the interface between the alluvial fine Clay and Silts and underlying glacial till deposits. In the most recent detailed design investigation discrete layers of fibrous peat were typically not observed but rather the organic material commonly appears as scattered decomposed wood and vegetation fragments. A photograph of a typical piston tube sample of slightly organic silt with occasional inclusions organic debris is shown in Figure For comparison a sample of highly organic silt (organic content = 7%) is shown in Figure 5. General Classification Indices of Alluvium For the purposes of clarity, only the classification test results for the detailed ground investigation samples are presented in this paper. The large amount of index test data collected during the previous phases exhibit similar results In order to appreciate an overall understanding of the characteristics of the soft alluvium soils at Limerick, samples obtained during the detailed ground investigation were subjected to classification index testing. The results of the classification testing programme are presented in Figures and 7. Although many soils are described as peat deposits in the previous GI phases there was limited organic content test results to confirm these soils as peat rather than highly organic alluvial soils. In the detailed ground investigation procured by DirectRoute, peat has been defined as soil having an organic content > 75%, whilst soils with organic content <5% are classified as slightly organic, soils with organic content between 5 % are described as organic and soils with organic content between to 75% are classified as highly organic. On the whole, the limited organic content data provided by the previous ground investigation phases would result in the peat soils being re-classified as slightly organic / organic soils rather than peat. Insufficient data exists to confidently reclassify all peat deposits

8 Figure Photograph of a cut piston sample showing slightly organic Silt with occasional inclusions organic debris Figure 5 Photograph of a cut piston sample showing highly organic Silt (organic content = 7%) 1 mm layer of grey organic Silt.

9 Figure reveals an overall trend of decreasing moisture content and organic content with depth. Soils with the highest moisture content and organic content appear to dominate in the upper m and close to the base of the alluvium deposits at around 1m depth. These layers also characteristically have lower bulk density. At depth greater than m the organic content of the alluvial soils generally lies below 1%. Figure 7 demonstrates higher liquid limit and plasticity index in the upper deposits. In this upper m zone the alluvial soils exhibit liquid limit values of between 5 and 15%. Below m the soils show a tendency for liquid limit values to reduce with depth with an average value close to %. Some isolated samples with high moisture content, high organic content and liquid limit values in excess of 1% at depths of, and 1.5m demonstrate the pocket like nature of highly organic deposits within the alluvium. In order to adopt a suitable value for the Nk factor for this project a direct comparison was made between CPT tip end resistance and the results of quick undrained shear strength test on samples obtained by thin wall piston sampling in adjacent boreholes. Figure shows the comparison of the undrained shear strengths derived from CPT tests using an Nk value of 17, insitu vane tests and the laboratory quick undrained triaxial tests for two nearby locations. A single Nk value was used to derive the undrained shear strength from the CPT data. In hindsight it may have been more exact to adopt three distinct values of Nk, one for the dessicated crust, a second for the upper m zone and a third for the remainder of the alluvium deposits. However given the resultant very low shear strength values in the upper m there would have been no significant difference to the undrained shear strength values adopted for design had this alternative approach been adopted 5. Insitu Field Shear Vane Testing 5 UNDRAINED SHEAR STRENGTH TESTS Undrained strength of cohesive deposits may be determined from one of the following methods: Insitu Cone Pentration Testing ; Insitu borehole vane testing; Quick undrained triaxial tests in the laboratory; 5.1 Cone Penetration Testing The preliminary undrained shear strength (Su or Cu) of a Clay can be estimated from insitu CPT Testing using the following relationships: Cu = qc / Nk' where: qc = minimum CPT cone end resistance Nk'= 17-1 for weak normally consolidated Clays. Nk = for overconsolidated (o.c.) clays, e.g. Glacial Tills. A more detailed undrained shear strength profile can be obtained from: Cu = qc po / Nk where: qc = minimum CPT cone end resistance po = overburden pressure Nk'= 15-1 for weak normally consolidated Clays. Nk = 1-19 for overconsolidated Clays, e.g. Glacial Tills. Insitu shear vane measurements for undrained shear strength for soft Clays give higher values than undrained triaxial tests in the laboratory. The vane test values derived from the field are normally corrected in accordance with the work by Bjerrrum. The correction factor for the measured shear strength is dependent upon soil plasticity. The vane apparatus used throughout the detailed ground investigation was the Geonor H-1 Vane. The plasticity values used to determine the correction factors are based on the global plasticity for the strata depth, rather than the specific value at the position of the vane testing. Figure 9 presents the results of all the field vane tests recorded during the detailed ground investigation. These undrained shear strength values have been corrected for soil plasticity. 5.3 Quick Undrained Triaxial Testing A series of. quick undrained triaxial tests were performed on samples obtained by piston sampling techniques during the detailed ground investigation.. A range of sample depths were chosen in order to develop a relationship of initial undrained shear strength profile with depth. In addition the UU tests were used to establish a correlation with CPT data and also to establish a site specific relationship between undrained shear strength and effective overburden necessary for multi-stage embankment construction. (Refer Section ). Figure 1 presents the undrained shear strength v depth profile resulting from the quick undrained triaxial tests

10 Figure Moisture content, bulk density, specific gravity and organic content.of Alluvium samples from detailed ground investigation laboratory testing programme (carried out at NMTL, Carlow). Moisture content (%) Bulk density (Mg/m 3 ) Specific gravity Organic content (%) Depth (m) 1 Average basic/uu tests = 3.9% 1 Average basic/uu tests = 1.59 Mg/m 3 1 Average = Basics / UU CAUC CAUE Oed - stand. Oed - non-stand. DSS Figure 7 Liquid limit, plasticity index and liquidity index.of Alluvium samples from detailed ground investigation laboratory testing programme (carried out at NMTL, Carlow). Liquid limit (%) Plasticity index (%) Liquidity index (%) Depth (m) Average = 95% 1 1 Average = % 1 1 Average =.7%

11 Figure Undrained Shear strength v depth derived from CPT tests and compared to UU triaxial results from adjacent Boreholes Undrained Shear Strength (KPa) Depth (m) CPT53 CPT5.3 x P'o UU Triaxial BH51 UU Triaxial BH5 BH51 Corrected Insitu Vane

12 Figure 9 Undrained Shear strength, Corrected for Plasticity, derived from Geonor H1 Vane Testing. Vane Test (Corrected) Undrained Shear Strength (KPa) Depth (m) BH577 BH BH59 BH577 BH51 BH5 BH BH59 BH BH9 BH57 BH55 BH35 BH593 Cu / Po'=.3 Line

13 Figure 1 Undrained Shear strength measured in Quick Undrained Triaxial Testing. Undrained Shear Strength (Unconsolidated Undrained Triaxial Testing) (KPa) Depth (m) Cu =.3 Po'

14 5. Comparison of Results. The undrained shear strength derived from the above three methods have been plotted on Figure. The plotted results are derived from boreholes and CPT tests in close proximity to each other in one area of Clonmacken Link. Figure demonstrates a good comparison of undrained shear strength profile derived from UU triaxial compression tests and PCPT profiles for this particular location with the field vane tests being noticeably higher. However in other locations the comparison of results between the different methods was not so good. In particular the field vane values showed a high degree of variability. In such cases the insitu field vane generally appeared to overestimate the shear strength in comparison to other methods. UNDRAINED SHEAR STRENGTH RATIO CU/P O The ratio of undrained shear strength to effective vertical overburden stress Cu / Po has a controlling influence upon the short term stability of multi-stage embankments constructed upon soft alluvium foundation soils. For normally consolidated clays the ratio is frequently found to be constant and related to Plasticity or Liquidity Index. The ratio may be derived from many methods including direct measurement in the laboratory via triaxial tests or direct simple shear tests as well as field vane test measurements and back calculations of failures. For the same soil the ratio will vary with test method. For the Limerick Southern Ring Road Phase II project the ratio for organic silt alluvium can be interpreted from different laboratory test methods including the following: UU triaxial compression tests; KoCU triaxial compression tests; KoCU triaxial extension tests; Direct Simple Shear (DSS) tests. Figure 1. presented in the previous section plots undrained shear strength measured in UU triaxial compression against depth. Making a global assumption that the density of alluvium is constant at its mean measured value of 17 kn/m 3 and a constant water table prevails at a depth of 1m, a line may be developed on this graph representing any chosen value for Cu / Po ratio. For comparative purposes the assumed line for Cu / Po =.3 is shown on Figure 1 and it can be seen to represent a reasonable lower bound to the data. A number of data points lie well above the line in the upper 1.5m depth reflecting the fact that the surficial dessicated crust is heavily overconsolidated. The fact that much of the data lies to the right of the line suggests that either the value of.3 is conservative for the mean ratio or that the soil in its in-situ condition may be slightly overconsolidated. Evidence supporting the latter conclusion was derived from laboratory oedometer tests as shown in Figure 1 (Refer section 7.). Although there is much scatter, the mean trend for OCR in the alluvium is between 1. & 1.5 and approximately constant below 1.5 m depth. KoCU triaxial testing was performed in by NMTL Tullow on 17, 1mm nominal diameter, piston tube samples of soft alluvium recovered in the recent detailed design GI from throughout the route. Samples of highly organic silt (over %) were not selected for KoCU triaxial testing primarily due to concerns about sample disturbance. In addition 1mm nominal diameter piston tube samples obtained in the Tender Phase GI in from the vicinity of the tunnel were tested by Surrey Geotechnical Services. These nine samples exhibited somewhat lower natural moisture ( to 5%) and lower PI (13 to 1) than the typical range for alluvium described previously for the project route as a whole. Both sets of samples were prepared in a similar manner. Following an initial isotropic stress consolidation phase to approximately half of the estimated value of horizontal effective stress, the final vertical effective stress at the end of the anisotropic consolidation phase was set approximately equal to the estimated insitu vertical effective stress and a Ko ratio of.5 was maintained. Finally the samples were sheared at a rate of.mm / hour. The results of both triaxial compression and extension tests plotted as measured shear strength to initial vertical effective stress prior to shearing is shown on Figure 11. Direct Simple Shear (DSS) testing was performed in by UCD using a Geonor H1 apparatus on test specimens, each mm diameter by 19 mm high, obtained from 1 piston tube samples. Samples were consolidated in four stages with saturation applied at the end of the second stage. Finally samples were sheared at a strain rate of about % / hour. Results are shown on Figure 11. Note that a number of samples were identified as highly disturbed and the results have been omitted. Also some specimens were inadvertedly tested at too low a vertical stress below the estimated insitu stress condition. These tests gave rise to untypically high ratios of Cu / Po, in effect because the samples were overconsolidated, and these results have similarly been omitted.

15 Figure 11 Ratio of Undrained Shear Strength (Cu) to Effective Overburden Stress (Po ) C u Vs Po' (Anisotropic CU Triaxial Test) 5 Cu (kpa) Initial Eff. Vertical Pressure, Po'(Kpa) KoCUC Tender Triaxial Comp KoCUC Detailed GI Triaxial Comp KoCUE Detailed GI Triaxial Ext Cu/Po'ratio.3 Direct Simple Shear Detailed GI Figure 11 indicates that as expected the KoCU triaxial compression tests consistently give the highest values of measured undrained shear strength and the 9 samples obtained from the tunnel area are somewhat higher in strength than those obtained along the entire route. An average ratio for this latter subset of KoCUC tests would be around.3 (ignoring the 9 tunnel samples). Next are the DSS tests which exhibit a mean ratio of about.3 and finally KoCUE triaxial tests with a ratio of.. Some authors suggest that a mean of all three tests is close to the operational conditions prevailing along the potential failure plane beneath a typical embankment and this would produce a value of.9. It is comforting to reflect how close this mean ratio is to the design values selected by others on nearby case histories in similar soils as described in Section 3.. samples have been divided into two groups dependant upon their stress history. Figure 1 shows samples consolidated to their existing insitu stress condition which exhibit storng dilational behaviour consistent with high silt content and sample disturbance. Figure 13 shows samples consolidated to higher stress levels and which exhibit the stress paths expected of normally consolidated clays. Figure 1 shows the drained friction angle measured in various tests from different phases of site investigation plotted against Plasticity Index. A conservative design value of o adopted for the project as well as the correlation with PI developed by Kenny are both shown. DRAINED SHEAR STRENGTH PARAMETER The KoCU triaxial tests described in the previous section were performed with pore pressure measurement so that effective stress conditions could be ascertained at any time during the shearing stage of the test. Stress paths obtained from these tests are shown on Figure 1 and 13 where the

16 Figure 1 KoCU Triaxial Stress Paths. Tests Consolidated at In-situ Effective Vertical Stress Level Shear stress = (σ`a - σ`r) / (kpa) φ` = 3 deg., c` = 3 kpa Mean stress = (σ`a + σ`r) / (kpa) Figure 13 KoCU Triaxial Stress Paths. Tests Consolidated above In-situ Effective Vertical Stress Level Shear stress = (σ`a - σ`r) / (kpa) φ` = 35 deg., c` = kpa 1 1 Mean stress = (σ`a + σ`r) / (kpa) Figure 1 Drained Friction Angle of Alluvium plotted versus Plasticity Index (PI). Plasticity Index v Drained Shear Strength Friction Angle 5 Drained Shear Strength Friction Angle Plasticity Index (%) Supplementary GI Data Detailed Ground Investigation - KoCUE Tests Detailed Ground Investigation - KoCU Tests Friction Angle v PI (Kenny, 1955) Design Line

17 7 PRIMARY SETTLEMENT AND RATE OF CONSOLIDATION FROM OEDOMETER TESTS For the very soft, normally consolidated or lightly overconsolidated fine grained alluvial deposits, the magnitude of primary settlement due to embankment loading is likely to be large. At locations where the highest loadings occur coexistent with the deepest alluvial deposits, settlements may be in excess of 1.5m. Settlement rate will depend on the rate at which consolidation occurs and whether improved drainage conditions are engineered by adoption of ground improvement techniques such as vertical wick drains. Without improved drainage conditions consolidation settlement of highly compressible and organic soils may continue for some years following completion of construction. The length of time is dependent upon strata thickness and drainage characteristics. 7.1 Laboratory Testing Programme. A specific programme of Oedometer testing was undertaken on thin walled piston samples recovered from the detailed ground investigation to examine primary compressibility characteristics and rate of consolidation for the fine grained alluvial deposits. Standard consolidation testing was performed in accordance with BS1377: Part 5. The loading sequence for the standard oedometer was 1.5, 5, 5, 1,, kpa, and then unloaded to and 1.5 kpa. Each load and unload stage was maintained for a period of hrs, except for the loads at kpa, which was maintained for the next log cycle to evaluate creep and determine Cα. Therefore any calculated, or assessed, values of permeability, magnitude and rate of settlement, must be treated with caution as the natural ground may vary considerably, both laterally and vertically. In particular thick lenses or layers of peat, or highly organic soils, may locally result in larger longterm settlements and variation in consolidation rate. Long term, drained settlement of cohesive soils can essentially be divided into two distinct phases; primary and secondary settlements. Primary settlements occur due to the dissipation of excess porewater pressures caused by changes in loading. Assessment of the magnitude of primary settlement can be calculated using the classical theory of onedimensional consolidation. The degree of pre-consolidation that a soil layer has undergone in the past will affect the magnitude of settlement calculated. Pre-consolidation pressures and field corrected compression ratios have been derived from laboratory oedometer testing using the Casagrande principles. A number of empirical relationships have been developed to assess the consolidation parameters of soft soils. Most of these relationships endeavour to relate consolidation characteristics to soil index properties such as moisture content or liquid limit Figure 15 presents the primary virgin settlement compression ratio, Cc / (1 + eo), derived from the consolidation tests undertaken during the ground investigations. The data collated from the Limerick Southern Ring Road ground investigations is compared to the empirical relationships derived from testing of soft soils. (Ref: Simons, 197 and Eide, 197). The trendline for the Limerick GI data fits within the two empirical relationships 7. Primary Compressibility and Rate of Primary Consolidation The permeability and consolidation characteristics of soil strata are extremely difficult to assess accurately due to the large variations in soil constituents, dominant grain size, void ratio, compressibility, soil structure, ground chemistry, stress history and insitu conditions, prevailing in a particular layer within each stratum. Figure 1 presents the OCR values determined from the laboratory test programme. Note that below 1.5 m depth the measured OCR typically varies between 1. and 1.5. Figure 17 presents the results of the standard oedometer test for the rate of consolidation, Cv plotted against the mean effective stress range for each test. Ground investigation only examines a small proportion of the ground to be affected by a scheme such as the LSRR. Soil stratum constituents may vary significantly a short distance away from an exploratory hole, or soil sample, which may result in a significant effect on ground behaviour, both in the short and longer terms.

18 Figure 15 Virgin Compression Ratio (Cc / (1+eo) v Moisture Content..7. Virgin Compression Ratio ( Cc / 1 + eo) Moisture Content (%) Consolidation Test Results (Supplementary GI) Consolidation Test Results (Detailed GI) Empirical Mean Trend Line (Ref :Simons, 197) Empirical Trend Line (Ref: Eide, 197) Log. (Trend line (all data)) Figure 1 Overconsolidation Ratio, OCR v Depth Depth V OCR graph OCR (Pc'/Po') Depth (m bgl) GI Acceptable Oedometer Test Results

19 Figure 17 Rate of Consolidation Cv v Mean vertical Stress Consolidation Rate derived from Oedometer Test Results 5 Mean Rate of Consolidation, Cv, (m /yr) from Oedometer Testing Mean Vertical Stress (Range from Initial Stress to Maximum Embankment Loading (KPa)) Detailed GI Test Data Back analysed Rate from Bunlicky WWTW

20 7.3 Comparison between Case history and Laboratory Test Results. Figure 1 Effect of Surcharge on Secondary Compression Ladd (199) The numerical mean value of Cc / (1 + eo ) derived from the laboratory test data at Limerick is.5 based upon the results available, with a standard deviation of.11. This laboratory mean value of the virgin compression ratio, Cc / (1 + eo), agrees very well with the back calculated value from the field data at Bunlicky WWTW and Corcanree RM projects. (Refer Section 3 The range of rates of consolidation Cv derived from laboratory testing and presented on Figure 17 appear in good agreement with the average field rates indicate from the case histories presented in Section 3. The rate of consolidation does not appear to be strongly correlated with vertical stress for the range of stresses from 7 to 15 kpa. SECONDARY COMPRESSIBILITY AND EFFECTS OF SURCHARGE LOADING. Highly organic soils and peats are also likely to be susceptible to significant secondary creep settlement and the spatial distribution of these highly organic soils within the embankment foundation is expected to greatly influence the magnitude and timing of differential settlements. Figure 19 Reduction in Rate of Secondary Compression due to Surcharge for Cohesive Soils A specific programme of Oedometer testing was undertaken on thin walled piston samples recovered from the detailed ground investigation to examine the secondary compressibility rates for the fine grained alluvial deposits..1 Effects of Surcharge Loading Surcharge has multiple benefits concerning the deformation performance of embankments constructed on soft foundations soils. Firstly surcharge, if applied for a sufficient length of time, can increase the effective stresses within the foundation soil and thereby increase the amount of consolidation drainage at a given time. Following surcharge removal, after sufficient time has elapsed, the amount of remaining primary consolidation under the permanent load is greatly reduced and possibly eliminated. The degree to which this occurs can readily be evaluated by standard consolidation theory in the case of surcharge used in combination with vertical drains, radial drainage theory is assumed as described above. The second benefit derives from the reduction in magnitude of the rate of secondary compression, which occurs following the cessation of drainage and effective stress changes in the soil. Additionally there is a lag, or delay, in the onset of secondary compression due to surcharge removal.

21 These benefits have been studied and quantified by reference to several well documented case histories by Ladd, C.C. (199 and ), Ng, N.S.Y. (199) and Mesri, G. (197). A summary of the findings is indicated in Figures 1 and 19. Figure presents the secondary compression index derived from oedometer testing carried out during the detailed ground investigation. The data can be compared with the empirical relationship postulated by Simons below. The findings relate reductions in the rate of secondary compression C α (following surcharging) to the normally consolidated rate of secondary compression Cα (NC) (without surcharge). The degree of reduction depends on the degree of over-consolidation achieved by use of a surcharge. In addition to the reduction in rate the observations indicate a delay in the time to commencement of secondary compression. The ratio of C α to Cα (NC) is therefore related to a parameter called Adjusted Amount of Surcharge (AAOS). AAOS = (P s P f) / P f (expressed as a percentage) where P s = maximum effective stress during surcharge fill placement P f = final effective stress following surcharge removal. Laboratory Testing Programme. The validity of the effects of surcharging on Irish alluvial soils at Limerick was examined by the performance of consolidation testing during the detailed design phase ground investigation. In addition to the standard consolidation testing outlined in Section 7.1, a series of oedometer tests were performed with non-standard load increments adopted to simulate field conditions including embankment construction and surcharge removal after completion of primary consolidation. A non-standard loading procedure mimicking placement of the embankment and surcharge load followed by surcharge removal and pavement construction was adopted for oedometer testing. The rates of secondary compression measured at peak load and following reloading after surcharge removal were measured and used to provide a comparison of the relative benefit of using surcharge along with standard load consolidation test results..3 Discussion of Results The secondary compression index is measured at maximum loading and is referred to as the normally consolidated secondary compression index, Cα (NC). Following derivation of Cα (NC), loading is removed to mimic removal of surcharge loading from the embankment and then reloaded a small amount, approx Kpa, to simulate pavement construction. The secondary compression index on reloading is designated as Cα. Cα(NC) =.1 x mc (%) It would appear from examination of the data presented in Figure that the empirical relationship derived by Simons will provide a conservative estimate of the secondary compression ratio, Cα(NC) for all moisture contents. Figure 1 presents the available data from the detailed ground investigation testing programme to examine the influence of surcharge loading on secondary compression. The results seem to validate the relationship presented by Ladd et al at values of AAOS from 33 to 5 percent. However it is acknowledged that the tests were not maintained for sufficient time to evaluate the time lag effect since this would be impractical. Hence the final secondary creep compression index following load cycles mimicking surcharge removal may be higher than reported from the laboratory oedometer tests. 9 CONSTRUCTION INSTRUMENTATION AND MONITORING PROGRAMME An instrumentation installation strategy and monitoring programme has been developed to observe full scale field behaviour during the construction processes. The general objectives for the instrumentation and monitoring programme may be summarised below: Monitoring of total and differential settlement performance of embankments during construction to evaluate their conformance with design assumptions; Monitoring of deformations of single and multistage embankments during construction to confirm that design assumptions are being met and to provide advance notification of the development of unstable slopes; Monitoring of pore pressures within the soft alluvial foundation soils to confirm that excess pore pressures are fully dissipated prior to surcharge load removal; Monitoring of pore pressures within critical layers of the soft alluvial foundation soils near critical failure planes under slopes to provide advance notification of the development of unstable slopes; Monitoring of extension strain in geosynthetic basal reinforcement layers to ensure that design assumptions are being met;

22 Figure Secondary Compression Ratio v Moisture Content..35 Secondary Compression Ratio (Cα) Moisture Content (%) Empirical Relationship ( C alpha =.1 mc) (Simons 197) Detailed GI Test Data C alpha from Supplementary GI Test Data Trendline for All LSRR GI Data Figure 1 Effects of Surcharge (AAOS) on Secondary settlement ratio ( Cα / Cα (NC)) Effects of Surcharge Loading on Secondary Compression Index Cα'. Reload Secondary Settlement Ratio Ca' / Ca (NC) Adjusted Amount of Surcharge (AAOS) (%) Mean Relationship Line, Ladd C.C. et al (19) Data from LSRR Detailed GI

23 It is anticipated that monitoring results will be presented in a subsequent paper once field performance data is available. 1 SUMMARY AND CONCLUSIONS Following an extensive site investigation performed in phases over several years, engineering strength and compressibility properties were developed for highly variable organic silt that forms the foundation soil for extensive earthen road embankments. The primary conclusions of these studies are as follows: Nearby well documented case histories of similar projects proved very valuable during preliminary design and evaluation stages, accurately predicting the likely range of critical strength and compressibility parameters for the project; Previous stages of site investigation indicating the presence of layers of peat soils proved to be somewhat misleading and were not validated by organic content testing. Highly organic silt layers identified during the detailed design SI proved to have organic contents of between and 5% and moisture contents of 1 to 5%. An organic content of between 5 and 15 % was representative of most of the alluvium. UU triaxial compression tests on piston tube samples generally correlated well with undrained shear strength profiles derived from corrected tip resistance assuming a global Nk correction factor of 17. The lower bound of this data was formed by a strength ratio Cu / Po =.3. Field vane testing provided far more scatter of shear strength data above and below this line. A mean trend value for Cu / Po ratio of approximately.3 was derived for the alluvium from the results of KoCUC, KoCUE triaxial tests and DSS testing on piston tube samples. Stress paths measured during KoCU triaxial testing showed dilatant behaviour (evidence of high silt content and sample disturbance) for those samples tested at in-situ vertical consolidation stress, while samples tested at higher stress levels resulted in contractant behaviour expected of normally consolidated clays. Friction angles in the range of 3 to 35 degrees were commonly observed and a conservative value of degrees adopted for design. The trend line for Compression Index Cc/(1+e ) against moisture content for the Shannon Alluvium falls between previous published correlations by Simons and Eide. Note that the potential variation of the index for any given value of moisture content is +/- 5%. Secondary compression ratio is also well correlated with moisture content and the relationship proposed by Simons Cα(NC) =.1 x mc (%) appears to represent a reasonable upper bound fit to data from Limerick. The effects of surcharge on reducing secondary compression rate and delaying its onset have been studied by Ladd, Ng and Mesri. Laboratory consolidation tests mimicking the loading sequence anticipated at Limerick appear to confirm that secondary compression rates are significantly reduced for AAOS of 35 to 5%. The tests could not be sustained long enough to fully validate this however. 11 ACKNOWLEDGEMENTS The authors would like to acknowledge the National Roads Authority and DirectRoute (Limerick) Limited for their kind permission to publish the data contained within this Paper. Billa Chana of NMTL Tullow provided detailed commentary on the interpretation of index tests and KoCU triaxial test results and was responsible for the majority of specialist soils laboratory testing described in the paper. Noel Boylan of UCD performed the DSS testing under the direction of Dr. Mike Long.. The views expressed in this paper are the sole views of the authors and do not represent the views of the National Roads Authority, DirectRoute (Limerick) Limited, FaberMaunsell Limited or Roughan & O Donovan. 1 REFERENCES Code of Practice for Earthworks. BS 31 : 191, British Standards Institute. Code of Practice for Site Investigations. BS 593 : 1999, British Standards Institute. British Standard methods of test for Soils for Civil Engineering Purposes BS 1377: 199, Parts 1 to 9,British Standards Institute. Kenney, T.C. (1959). Discussion: ASCE, Vol 5, SM3, pp.7-9. A Short Course in Foundation Engineering ( nd Edition), N. Simon and B. Menzies, Thomas Telford Limited, 1999.

24 Ng, N.S.Y. (199) Characterization of consolidation and creep properties of Salt Lake City clays, M. Sc.Thesis, MIT, Cambridge Ma. USA Mesri, G and Castro, A.(197). Cα / Cc concept and Ko during secondary compression Journal Geotech Engng Division ASCE, 113(3), p 3 9. Charles ladd papers. Unpublished class notes for 1.3, Soil behaviour Ladd C.C. Department of Civil and Environmental Engineering, MIT, Cambridge, M.A. USA. (199). Eide O & Holmberg S Test Fills to Failure on Soft Bangkok Clay. Proc ASCE Specialty Conf on Performance of Earth & Earth Supported Structures, Vol 1. West Lafayette. 197 pp Farrell, Davitt and Connolly, Bunratty Emanbankment Overview of Performance. IEI Proc. Seminar on Road Embankments on Soft Ground, Dublin (199). Galbraith, R. The Northern Approach Road to Mallow Street Bridge, Limerick, IEI Proc. Seminar on Road Embankments on Soft Ground, Dublin (199).