Premature refusal can pose a threat to driven pile installation. One contingency measure that has been applied is pre-drilling ahead of the pile tip to ease driving. While some practitioners may assume that carefully conducted drilling will not give any adverse effect, little evidence is currently available to assess the possible impact on long-term axial capacity. This paper describes a standard gravity (1g) small-scale model laboratory study in which 18 small closed-ended piles were jacked into constrained 300mm cubes of stiff high overconsolidation ratio (OCR) natural London clay, with and without pre-drilling. Four control piles were also installed without any boring. Pile loading tests performed after appropriate equalisation periods showed that pile capacity reduced systematically as the ratio of the drill hole-to-pile solid area ratio increased. The model experiments showed comparable reductions in capacity to those in earlier model and field studies on soft Mexico City clays. An effective stress analysis is suggested to help the application of the model tests to other soils types and pile geometries. Capacity reductions are concluded to be likely when pre-drilling is performed in highly overconsolidated offshore clays. The results have implications for both new piling operations in exceptionally hard clays and the re-analysis of existing installations in a broader spread of soil types, where refusal may have resulted from pile hammer capacity limitations.
1.1 Use of pre-drilling to ease pile installation
Driving refusal has been encountered frequently during offshore foundation pile installation. One field contingency measure applied to achieve target penetration with tubular piles has been pre-drilling ahead of the tip to ease driving. The development of more powerful hammers and improved driveability predictions has reduced the incidence of refusal events, but consideration still has to be given to this possibility when dealing with unusually hard or dense materials.
Two soil investigations were performed in soft, lightly overconsolidated clays in 1300m of water depth. This paper reviews and compares the results of these investigations and the subsequent onshore laboratory testing. It focuses in particular on the quality of the samples and the performance and efficiency of the seafloor based drilling system that was utilised. The laboratory testing results clearly demonstrate that the lower plasticity clays encountered at Site L of a Norwegian gas field are more susceptible to sample disturbance, compared to the high plasticity clays from Site C offshore West Africa. Previous soil investigations were also conducted in the vicinity of both these fields, and they utilised conventional vessel based drilling techniques. Additional geotechnical information was available from seafloor based systems operated in non-drilling mode in similar ground conditions. The two sites are both deepwater lightly overconsolidated soft clay sites and are geotechnically homogeneous. Given the similarity in water depth, they offer a unique opportunity for comparison of performance and sample quality.
Two deepwater soil investigations were performed in 2009 and in 2010 utilising a remotely operated seafloor based drilling system – the Benthic Geotech Portable Remotely Operated Drill 1 (PROD 1) (Kelleher and Hull, 2008). The first campaign was at a Norwegian gas field (Site L), and the second was for a development offshore West Africa (Site C). Both sites are remote from shore and from existing field installations, located in ~1300m of water where the soil conditions consist of soft, lightly overconsolidated clays. The detailed field programme at these two sites was aimed at resolving all early requirements for regional and local seabed data for both geohazards and foundation studies. Samples were taken to depths in excess of 40m and complemented with in situ testing.
The results from 1D consolidation tests on gassy clay samples, containing uniformly distributed methane gas bubbles, indicate that their compressibility is ‘delayed’ compared to samples of the same clay without gas. This behaviour is believed to be due to the interaction of the gas bubbles with both the clay platelets and the pore water, which generates menisci representing zones of high pressure and partial load sharing between the gas bubbles to the soil structure as the clay particles confine the methane gas. As time passes, the gas becomes part of the structure, and a new structure is formed consisting of a combination of the pore water, gas and clay particles. In this new structure, the gas is confined and behaves as a spring, which temporarily increases the rigidity of the structure until the applied load exceeds the confining stress provided by the intermolecular forces developed by the clay particles, at which time conventional consolidation occurs.
In soil mechanics, it is customary to consider that marine soils are completely saturated. However, several studies have identified that numerous types of gas exist in marine sediments, with methane gas being the most common (Claypool and Kaplan, 1974; Schubel, 1974; Christian and Cranston, 1977; Esrig and Kirby, 1977; Bhasin and Leland, 1978; Whelan et al., 1978). In the case of clays, gas is prevented from escaping from the soil into the water column because of the low permeability of clay soils. Over time a gaseous soil or a soil forms containing thin layers of methane gas, which can lead to the formation of gas hydrates under appropriate temperature and pressure conditions. Trying to measure and understand the effects of methane gas on the compressibility behaviour of clay soils using natural samples from the field is confronted by various challenges.
Interface shear tests undertaken with a Cam-shear device have been conducted on very soft clay core samples from the Gulf of Guinea. These tests have been completed at normal effective stresses applicable to pipelines and suction caissons. Pipeline tests were conducted on rusted and coated steel, and on smooth and rough polypropylene interface material at speeds of 0.050mm/s and 0.001mm/s. Tests relevant to suction caisson installation and extraction were conducted on a rusted steel plate at shear rates of 1mm/s and 5mm/s. It is shown that the interface friction value is strongly influenced by both the shearing rate and the interface roughness, leading to the following conclusions. Firstly, rough and smooth interfaces produce comparable peak interface friction values. However, shearing on rough interfaces can produce lower residual friction values than on smooth interfaces. Secondly, slow shearing can produce residual interface friction values that are significantly higher than obtained during fast shearing. Thirdly, the apparent roughness of an interface is likely to be a function of the existing natural soil structure, which includes robust faecal pellets ranging in proportion from 20% to 50% by dry mass. The paper concludes by highlighting relevant implications for the design of pipelines and suction caissons located in similar sediments.
Very soft offshore sediments within the Gulf of Guinea present sizeable challenges for the design of pipelines and suction caissons that are to be installed into these sediments. Undrained shear strength profiles of these sediments typically exhibit high values at shallow depths, corresponding to what is commonly referred to as a ‘crust’. During operation, thermally induced cycling of the pipelines due to hot product flowing through initially cold pipelines causes pipe walking and ratcheting in the axial direction, and pipe buckling in the lateral direction.
With recent developments and application of geographical information system (GIS) capabilities, geohazard assessments can benefit from detailed data integration. This facilitates the merging of different datasets, such as 3D exploration seismic; high-resolution seismic (e.g. HR2D, sub-bottom profiler); geotechnical (e.g. boreholes, cone penetration tests (CPT)); or environmental (e.g. sampling, visual inspection) data. The result of this integration is a conceptual geological model to be used as a support for the geohazard assessment, and which is provided to the engineering team in charge of a facility design. The engineering team, however, expects predictions (time, place, magnitude and probability of an event), while most geoscientists can only offer an improved forecast (general statement of future possibilities). To reduce this subjectivity, the interpreter should conduct an evaluation of the reliability of the model, taking into account the uncertainties related to each dataset (e.g. accuracy, resolution). The quantification of uncertainties may be carried out for each dataset, but that of the conceptual geological model as a whole is not easy to determine. This paper reviews best practice in terms of data integration by means of a GIS and details uncertainties that should be taken into account. It also addresses considerations related to geostatistics and probabilities, in order to provide a reliable conceptual geological model for geohazard assessment.
2. Geographical Information System: A PowerfulTool for Data Integration
At that time, geotechnical engineering was still regarded as part of civil engineering, relying upon the principles of mechanics and hydraulics. Today, geotechnical engineering is recognised as an obvious bridge between geology and civil engineering, and in many areas it requires an integrated and multidisciplinary approach. Such an approach linking geosciences and geotechnical engineering should be considered in terms of common concerns and requirements, such as obtaining, organising, validating, displaying and interpreting surface and subsurface data.
To investigate whether chemical additives could be effective at accelerating the strength regain of a clay slurry, a series of laboratory tests were undertaken at the University of Glasgow using a range of additives. Bothkennar clay was used in the test programme, with a small addition of bentonite to replicate the plasticity of the Witch Ground clays found in an area of the North Sea. To produce the slurry, water was added to the samples to increase the water content to 1.5 and 2.0 times the liquid limit. The results of the test programme found that calcium hydroxide can significantly improve the strength of the soil over 1-day, 3-day, 7-day, 30- day and 1-year time periods, for a range of dosing concentrations of 1–5%. This paper presents the laboratory test results and highlights the potential use of calcium hydroxide offshore.
Clay slurries can loosely be defined as cohesive soils with moisture contents in excess of their liquid limit (LL). The use of additives to increase the strength of such weak slurries has received relatively little attention, although there is an increasing commercial imperative, associated with offshore developments, for pursuing this ground-strengthening technique. For example, clay slurry is produced offshore during jet trenching in clay soils where near rectangular trenches are formed, which generally are partially filled with slurry. The slurry provides little resistance to the uplift movement of trenched pipelines and is typically removed and replaced with either rock dump, or more competent in situ material obtained by destabilising the side walls of the trench. This paper presents the results from laboratory tests undertaken at the University of Glasgow to investigate the influence of a range of chemical additives on accelerating the strength improvement of clay slurries.
In the Norwegian Sea, sea-bottom temperatures can be as low as –1.9°C in water depths greater than 1000m. In other deepwater areas, such as the Gulf of Guinea and the Gulf of Mexico, sea-bottom temperatures can be as low as 5°C. However, the standard practice up to now has been to carry out laboratory tests at room temperature (i.e. 20°C). Previous studies have indicated that testing at room temperature can result in laboratory measured strengths 10–20% lower than tests at in situ temperatures. Results from extensive parallel laboratory testing (at room temperature and at in situ temperature) on eight different types of soft clay are presented here, covering intact and remoulded specimens with the range of plasticity of 16–120%. This study quantifies the temperature effects on testing and storing of deepwater samples. The investigated soil parameters concentrate on the undrained shear strength (su) and preconsolidation stress (p’c). All the parallel tests showed that su increased on average of 2% to 40% when tested at cold temperature. With one exception, where a similar increase in measured p’c of 9% to 38% was observed. Recommendations are given for procedures for testing, transporting and storing deepwater samples to arrive at the closest possible representative soil design parameters for in situ conditions.
In the Norwegian Sea, sea-bottom temperatures can be as low as –1.9°C in water depths greater than 1000m. Due to the salt content of the pore water (typically about 30g/l), the soil does not freeze. In other deepwater areas, such as the Gulf of Guinea and the Gulf of Mexico, sea-bottom temperatures can be as low as 5°C. In connection with field developments in deep water, the use of low values of su and p’c can lead to foundation solutions that are unnecessarily conservative and costly.
The capacity and keying behaviour of strip anchors in dense silica sand is examined in this paper through a series of centrifuge tests conducted at 30g. Tests were conducted adjacent to the Perspex side panel of the centrifuge strongbox to facilitate optical observation and measurements of the keying response. Image analysis shows the failure mechanism to transition from a deep localised rotational mechanism to a shallow block mechanism extending to the soil surface. The onset of this transition coincides with the peak uplift resistance of the plate, which occurs at approximately 65°. The uplift resistance of the plate as it becomes horizontal is in good agreement with a limit equilibrium solution that neglects the normality condition and assumes a failure mechanism that is broadly similar to the eventual failure mechanism of the plate after keying.
Much work has been conducted in recent years on the capacity and keying behaviour of plate anchors in clay, with notable contributions from Gaudin et al. (2006a), Song et al. (2009) and Yang et al. (2012). There have been very few corresponding studies in sand, as clay is the dominant soil type in the deep-water environment in which plate anchors are currently used. However, the installation of floating wave energy converters and wind turbines in water depths of typically less than 100m will require anchoring systems that are suitable for sand deposits. Prior studies on the performance of plate anchors in sand have either considered vertically loaded horizontal anchors (Figure 1a; e.g. Ovesen, 1981; Murray and Geddes, 1987; Dickin, 1994), or horizontally loaded vertical anchors (Figure 1b; e.g. Das et al., 1977, Rowe and Davis, 1982; Merifield and Sloan, 2006). To the authors’ knowledge, no studies have addressed vertically loaded vertical anchors (Figure 1c).