Developments in geotechnical site investigations in Christchurch following the Canterbury earthquake sequence

Transcription

1 Neeson, F.C. (2013) Developments in geotechnical site investigations in Christchurch following the Canterbury earthquake sequence Proc. 19 th NZGS Geotechnical Symposium. Ed. CY Chin, Queenstown Developments in geotechnical site investigations in Christchurch following the Canterbury earthquake sequence F C Neeson Opus International Consultants, Christchurch, NZ Frances.Neeson@opus.co.nz Keywords: Site Investigations, Sonic Drilling, Standard Penetration Tests (SPT), Cone Penetration Tests (CPT) ABSTRACT Following the Canterbury earthquake sequence beginning 4 th September 2010, site investigation techniques and practices have rapidly developed. Contractors locally and nationally have been quick to respond to the demand in Christchurch resulting in innovative applications of existing techniques sometimes not previously available in the New Zealand market. Updated design codes and better informed clients have led to more comprehensive site investigations and geotechnical assessments post 4 th September Guidelines have been revised and the Ministry of Business, Innovation and Employment (MBIE) (formerly the Department of Building and Housing) stresses the need to use calibrated Standard Penetration Test (SPT) hammers.. In the past liquefaction potential was assessed using SPT testing in boreholes often drilled by air flush rotary or direct push methods mainly due to uncertain ground conditions and limited budgets. At present liquefaction potential analysis commonly uses Cone Penetrometer Test (CPT) test results. However, Christchurch s interbedded alluvial geology often prevents CPT tests reaching sufficient depths and therefore boreholes with SPTs are still utilised below these depths. The availability and selection of a range of site investigation techniques, accuracy of the factual data gathered during site investigations and resulting geological model are critical to the accuracy of the resulting analysis and applicability of geotechnical models. This paper explores the development of site investigation techniques used in Christchurch since September 2010 and will compare the use of CPT and SPT N results for ground assessment. 1 INTRODUCTION Christchurch is New Zealand s second largest city and is located on the east coast of New Zealand s South Island. The city is situated near the coast, on alluvial outwash plains and on the western flanks of the Lyttleton volcano of the Banks Peninsula volcanic complex. The South Island of New Zealand accommodates the Pacific and Australian tectonic plate boundary by a system of faults, most notably, the Alpine Fault. At 4:35am 4th September 2010, the previously unknown Greendale Fault and at least two neighbouring blind faults ruptured inducing strong ground shaking felt across the South Island. This fault rupture has become known as the Darfield Earthquake Ms7.1. The epicentre was located 45km south of Christchurch s CBD. The earthquake caused severe effects including widespread land damage due to liquefaction in addition to shaking induced damage of buildings which in some cases resulted mainly in the collapse of unreinforced masonry buildings. Following this initial fault rupture there has been a well recorded aftershock sequence (Geonet,

2 2013) including large aftershocks now referred to as the Boxing Day quake (26th December 2010), the Ms6.3 22nd February 2011 (Christchurch quake) where the epicentre was located just 8km from the CBD, June 13th quakes and December 23rd aftershock. Surface expression of liquefaction has been clearly evident following several large seismic events of the Canterbury Earthquake sequence. Flooding and ejected sand and silt were wide spread following the September and February events but also occurred during smaller seismic events in more localised areas, for example the suburb of Parklands was particularly hard hit following the December 23rd aftershock. Liquefaction had dramatic effects and led to bearing failures beneath buildings, floating of buried structures, differential settlement damage to infrastructure and buildings, sinkholes or voids, lateral spreading and surface deposits of silt and sand with associated flooding. 2 GEOLOGICAL SETTING 2.1 General Setting Christchurch is situated on Holocene sedimentary deposits and on the slopes of Miocene aged volcanic rock (Brown & Weeber, 1992). Christchurch is located near the east coast of the South Island on the Pacific Ocean, where large alluvial fans have been deposited from rivers and glaciers in the Southern Alps. The Waimakariri River, a large braided river has been the dominant source of alluvial materials. The geological setting of Christchurch is comprised of swamp deposits formed behind beach dune sand; estuaries and lagoons; gravel, sand and silt derived from river channel and overbank flood deposits of the Waimakariri River floodplain. Marine sand deposited with the last marine transgression incurs beneath the CBD and underlies the eastern half of the city. The Avon and Heathcote rivers originate from springs west of Christchurch and meander through the city to form the main drainage system. 2.2 Stratigraphy Typical stratigraphy in the Christchurch area consists of river channel and overbank silt, sand and gravel of the Springston Formation in the central and west of the city, much of the CBD is underlain by 3-6m of shallow gravel. Underlying the Springston Formation are marine and estuarine deposits of the Christchurch Formation which predominantly consist of silt and sand. Below this lies the Riccarton Gravel Formation, this is typically brown or blue gravel with some sand or silt. It is also the top most aquifer, and can demonstrate artesian pressures in wells and boreholes, historically this formation has been targeted as a water resource and the gravel is now often targeted for piled foundations. 3 GEOTECHNICAL INVESTIGATIONS PRE-QUAKE 3.1 General Practice Prior to the seismic activity beginning in September 2010, Geotechnical practitioners faced a hard sell in order to ensure clients spent money on geotechnical site investigations in Christchurch. Although widely known as a drained swamp, Christchurch had been relatively seismically quiet this century. As such, when clients commissioned geotechnical assessments and site investigations, budgets were kept tight restricting the scope of investigations and designers had to make do with minimal investigations. Cone penetration tests (CPTs) were only used occasionally, as they risked being limited by potential shallow refusal on gravel lenses. Boreholes with Standard Penetration Testing (SPTs) were favoured, as whatever the local ground conditions were, information in the form of material samples and SPT results would be

3 obtained. However, these compromises tended to lead to a one size fits all approach when designing site investigations and alternative investigation techniques were often not used, if at all considered. 3.2 Available Site Investigation Methods Prior to the Canterbury Earthquake sequence, site investigations in Christchurch predominantly consisted of hand auger, Scala Penetrometer testing and test pitting for shallow investigations and boreholes for deeper investigations. Boreholes were generally conducted by air flush rotary/ concentrix or cable tool drilling (downhole hammer) or direct push (top of hole hammer) drilling. Airflush/concentrix techniques produced a disturbed sample which was flushed from the borehole usually using compressed air. Samples are bagged for each run and coarse materials such as coarse gravel or cobbles are broken into smaller sized pieces. This technique is quick in coarse alluvial materials such as sand and gravel, but slower in cohesive soils. SPTs allowed samples with fewer disturbances to be collected and allow for a more accurate soil description. However, the energy efficiency of different hammers were not often calibrated. Direct Push drilling methods were more recently introduced which produce a disturbed sample by jacking the core barrel with an internal plastic liner. The core barrel is retrieved from each run and the plastic liner containing the soil samples removed and the soil samples are found in their relative positions but does not maintain their relatively density (see Figure 1 below). This method generally produces a consistent core sample, although gravel sampling is limited to the core barrel/ plastic liner size. This method is however, limited to shallower depths than downhole hammer techniques and until recently was not able to penetrate sufficiently into dense gravels found in the Springstons and Riccarton Formations. Conventional triple tube coring was also undertaken in Christchurch with varying success (Figure 1). However, it can tend to wash fines from the core samples and drilling in the Riccarton gravel formation proves tough on drill bits. Figure 1a: Example of Direct Push Core from Christchurch (Springston Formation) Figure 1b: HQ Rotary Core from Sydenham, (Christchurch Formation and Riccarton Gravel) CPTs were used relatively infrequently in Christchurch prior to the earthquake sequence as their success was not guaranteed across a site nor between sites. Concern for the CPT tests pulling up short due to shallow gravel led to the testing being considered costly to both time and budget (i.e. mobilisation costs) and with no guaranteed result. Where client s budgets for site investigations were restricted, a borehole with SPTs was favoured over a CPT which may or

4 may not reach the target depth. Predrilling was rarely used to enable CPTs to be advanced below shallow gravels. Hand augers, Scalas and test pits have and continue to provide valuable information on surficial soils up to 3.0 metres for a small expense. However, in the past hand auger and Scala results in some cases tended to have been heavily relied upon, sometimes without additional deeper site investigations. 3.3 Liquefaction Hazard Liquefaction was a well-known hazard prior to the Canterbury earthquake sequence. The Canterbury Region Council, Environment Canterbury (ECan) have the role of monitoring natural hazards in the region. ECan produced a Liquefaction Hazard Map for Christchurch in 2006 which used information gained from ground investigations to predict where damaging liquefaction may occur in a future seismic event. Prior to this the University of Canterbury had conducted many research programmes and produced a series of papers on liquefaction and developed a CPT rig in the 1990 s. 4 GEOTECHNICAL INVESTIGATIONS POST-QUAKE 4.1 Post-Quake Developments Following the Canterbury Earthquake sequence, there has been a rapid increase in Site Investigation methods available in Christchurch. CPTs have become common place with numerous rigs now available from both local and national contractors and consultants. Significant investments have been made by local and national contractors to adapt, and purchase new testing equipment in response to the demand for site investigations following the earthquakes. New methods now available in Christchurch include Sonic drilling, seismic CPTs, Dilatometer testing (DMT), Geophysical techniques including Multi-Channel Analysis Surface Waves (MASW), Gel-Push sampling, and SPT calibration. Two of these new methods are discussed in further detail below. 4.2 Sonic Drilling Drilling using Sonic methods is a new technique in the Christchurch market and is relatively new to New Zealand. Numerous drilling companies now offer variations of sonic drilling. Advantages of sonic drilling include the timeliness of the investigation, a 25m sonic drilled borehole can be achieved in one to one and a half days; a disturbed but well recovered core sample is collected (see Figure 2), recoveries of near 90 to 100% are usual; sampling appears representative, with both fine and coarse material recovered (ie fines within the Riccarton Gravel are recovered, as opposed to rotary coring where they can be washed away); and sonic drilling is capable of reaching significant depths, similar to rotary and much greater than direct push methods. Figure 2: Example of Sonic Core from Christchurch CBD (Riccarton Gravel Formation)

5 Some limitations of sonic drilling have been and continue to be discussed amongst the industry including concern regarding sonic disturbance of the insitu soil sediments and its effect on SPT results. Two main types of sonic drilling techniques are available in Christchurch these are dual sonic and bottom out sonic. The dual sonic method consists of the casing and sampler advanced together using sonic methods with little need for water. Bottom out methods advance the sampler to the bottom of the run first then the casing is advanced to the same depth. If this method is used, water may need to be used to flush the hole to prevent material jamming between the sampler and the casing. There are concerns that this flushing may disturb the in-situ materials below the toe of the sampler. These effects have yet to be quantified. This possible disturbance however, is only one variable of many that may affect SPT results. Idriss and Boulanger (2008) assert that the most important variable of the SPT test is the amount of energy delivered by each drop of the hammer to the drill rods which can vary by 30-90%. In addition the seating drive or first 150mm of the SPT test is designed to accommodate some drilling induced disturbance. 4.3 Cone Penetration Tests (CPT) Following the Canterbury earthquakes CPTu s have become the prevalent site investigation testing method for liquefaction analysis in Christchurch. This is mostly likely due to the results being continuous, testing being relatively quick and affordable (compared to boreholes), and comprehensive geotechnical investigations becoming more prevalent. CPT testing has developed over time from mechanical CPTs to electric piezocones (CPTu) and more recently to seismic CPTs where cones have a built in geophone. Rigs vary from truck to track mounted, some have drilling equipment integrated so that testing is not limited by course fill or shallow gravel and now it is also possible to use rigs with sampling capacity. 5 CPT AND SPT COMPARISONS IN CHRISTCHURCH CPTs are now the preferred site investigation method for liquefaction analysis in preference to SPT test results that were widely used in the past. SPT testing must be conducted with a drill rig and tests are most commonly conducted at 1 to 1.5m intervals. SPT tests are undertaken over 450mm rather than continuous results gained as the CPT probe is advanced. However, there are still situations where SPT testing is required either in addition to or without CPTs. The presence of gravel horizons or lenses within Christchurch s alluvial sediments can lead to shallow refusal of CPTs and therefore SPTs are still used to provide soil parameters for liquefaction at depths below CPT termination. A comparison of SPT results and adjacent (within 2-3m) CPT tests has been conducted at three different sites across Christchurch. Drilling has been undertaken by Sonic, Direct Push and Conventional rotary coring methods. SPT testing was conducted at each site as per NZS4402 Test where a standard 63.5kg hammer with automatic release was dropped on the anvil to drive the split spoon 450mm or to a refusal value of N= 60+. All hammers however have different energy efficiencies which refer to the amount of energy delivered through the SPT rods. Robertson et al. (1983) suggest this is the most significant factor affecting the measured N value and Idriss and Boulanger (2008) assert that the actual delivered energy can vary by 30-90% of the theoretical maximum. This variance becomes important when measured (N M ) raw SPT N values are near the boundary of liquefiable or non-liquefiable conditions. Measured SPTs for all three sites have been corrected using Idriss and Boulanger s (2008) following equation: Where C E is the measured or empirical energy ratio correction factor, C B is the borehole diameter correction factor, C R corrects for SPT rod length, C S provides correction for the spoon

6 sampler and N M is the measured SPT N value. Where measured energy efficiencies are not available empirical estimates have been suggested by Idriss and Boulanger (2008). The empirical estimates contain considerable uncertainties and Idriss and Bolanger (2008) have suggested ranges to allow for the underlying uncertainties as shown in Table 1 below. Also shown is the measured C E for the Automatic trip hammer as provided by the contractor for one of our case study sites. Table 1: C E estimated ranges (from Idriss and Boulanger 2008) and measured values Site Hammer type C E Empirical estimate Measured C E Heathcote Doughnut hammer Estuary Safety hammer CBD - Avon Automatic trip hammer For each site, a CPT derived (N 1 60) plot has had measured SPT N values (N M ) overlain for comparison. Three different types of SPT hammers namely a Safety Automatic, Doughnut and Safety Hammer were used for the investigations conducted. The N M SPT results are plotted against the CPT derived N 1 60 continuous plot and in addition the upper and lower C E values suggested by Idriss and Boulanger (2008) have also been shown in Figure 3. Figure 3: SPT N values and CPT derived N 1 60 for three different SPT hammers. As illustrated each hammer has variable energy efficiency (C E ) and the ranges suggested by Idriss and Boulanger (2008) result in significant possible variables. Energy corrected N60 (C E 1.0) for the Doughnut Hammer shows a good correlation with the CPT results up to 20m depth, however below this point an N60 (C E 0.5) appears to have a better correlation with the CPT results. The Safety Hammer and Safety Automatic Trip Hammer also show a poor correlation at depth. The Safety Automatic Trip Hammer has been calibrated for energy efficiency and it compares well to the CPT plot at this location to approximately 15m depth, where it appears a smaller C E value would provide a better correlation with the CPT plot. The poor correlation at depths greater than 15-20m could be due to several possibilities not accounted for in this analysis, such as overburden effects or fines content adjustments. Furthermore, it could be due to a poor correlation between CPT outputs and the derived N60 at depths greater than 15m.

7 The discrepancies between corrected SPT N values (N60) and CPT derived N 1 60 values should be considered when undertaking liquefaction and lateral spreading analyses. By conducting at least one CPT next to a borehole during site investigations, comparisons between the two in-situ testing methods and results can be made. SPTs allow for samples to be collected which can then be used for soils description and laboratory analysis methods such as particle size and behaviour tests. CPT tests produce cone resistance (qc), sleeve friction (fs) and dynamic pore pressure (u2) outputs from which numerous other empirical results are derived, such as equivalent N60 values and soil types. The comparisons of SPTs allow checks to be conducted on empirical CPT outputs. In-situ test repeatability is another aspect to consider, while SPTs are conducted to a standard (NZS4402) there may be inherit differences when conducted by different operators. Robertson et al (1983) highlighted this issue before automatic trip SPT hammers were common place. This suggests there is value in comparing calibrated SPT hammers to adjacent CPT results, as there may be slight operator induced differences and it is a quick verification method. 6 CONCLUSIONS Site Investigation methods and budgets have developed following the Canterbury Earthquake sequence. Prior to the seismic activity, Geotechnical practitioners often faced a somewhat hard sell to ensure clients invested in geotechnical site investigations. As such, investigations were tailored to provide maximum information gained at minimal expense. This lead to CPTs being underutilised as Christchurch s highly variable ground conditions could not guarantee their success due to the presence of gravel lenses at shallow depths across the city. Drilling contractors have been quick to respond to the demand for site investigations in Christchurch and this had led to the emergence of Sonic drilling capability in Christchurch and subsequently the New Zealand market. Sonic drilling provides various advantages in Christchurch s variable ground conditions including arguably the best core recovery of any drilling method currently present in Canterbury, quick sampling and sufficient depth and penetration into the dense Riccarton Gravels, which are a significant formation often targeted for piled foundations. Various opinions exist regarding sonic disturbance of the in-situ soils and its effect on SPT results however, this is just one of the many possible variables that can affect SPT results and Robertson et al (1983) and Idriss and Boulanger (2008) assert that the most important varible affecting SPT results is the amount of engery delivered to the drill rods, known as the energy efficiency of the hammer. CPT tests have become the most favoured method for deriving site specific soil characteristics and conducting liquefaction potential analysis. CPTs are relatively quick and affordable in comparison to traditional drilling methods which used SPTs to determine liquefaction analysis parameters. Value can be added to both boreholes and CPT test results when they are conducted side by side and the results are compared. CPT results measure a continuous output of cone resistance, sleeve friction and dynamic pore pressure, however boreholes and SPT results allow for description of the soil sample by an engineering geologist, provide samples for laboratory testing and allow groundwater conditions to be monitored or further analysed. By comparing both methods empirical results derived from CPT measurements can be compared to retrieved samples and laboratory analysis. Measured SPTs need to be corrected for energy efficiency which can vary from 30-90% of the theoretical energy (Idriss and Boulanger, 2008). The MBIE now advocates for the use of calibrated SPT hammers and contractors are responding to this requirement. Other variables that need to be included when deriving a SPT N60 value include rod length, borehole diameter and SPT sampler. Comparisons to CPTs show that other considerations may also need to be incorporated at depth, due to the poor correlation between corrected SPTs and CPT derived N60

8 plot at depth. Further work needs to be conducted to determine if fines content corrections, overburden affects or drilling method have an effect on this poor correlation at depth. In addition the reliability of the CPT derived N 1 60 correction needs to be looked at as this could also be a contributor to the poor correlation from 15m or deeper. The comparison of CPT and SPT data would suggest that neither CPT nor SPT data should be used in isolation when considering liquefaction potential at depth. By comparing CPT and SPT results, the designer is able to build a better geological model of the sub-surface soil characteristics and properties at each site. If the differences between CPT and SPT results are considered, the designer has more understanding of the site characteristics and limitations of both in-situ test methods when opting to use CPT or SPT or both methods for liquefaction and lateral spreading analysis. ACKNOWLEGEMENTS I wish to thank Opus International Consultants and my colleagues, in particular Greg Saul, Christine Parkes and Kelly Walker, who have reviewed my abstract and paper. REFERENCES Brown, L & Weeber, J (1992) Geology of the Christchurch urban area. Scale 1:25,000. Institute of Geological & Nuclear Sciences geological map 1. 1 sheet + 104p Institute of Geological and Nuclear Sciences Limited, Lower Hutt, New Zealand. Environment Canterbury (2006) The Solid Facts on Christchurch Liquefaction Geonet (2013) Canterbury Quakes Idriss I.M and Boulanger R.W (2008) Soil Liquefaction During Earthquakes, Earthquake Engineering Research Institute Monograph pp70-83 Robertson, P.K; Campanella, R.G; M ASCE; Wightman, A (1983) SPT-CPT Correlations, Journal of Geotechnical Engineering, Vol. 109, No. 11, pp