Near Surface Technology and Management
Dr. H.R.G.K. Hack, Dr. P.N.W. Verhoef
In the field of near surface technology and management various research activities have taken place during the year 2001. Research was executed towards the development of tools for a better characterization and quantification of geotechnical properties of subsurface geology and materials. The second line of research is the use of existing tools to improve characterization and describe the subsurface geology and to define improved constitutive models for the behaviour of subsurface materials. A third line of research is towards an optimum handling of subsurface data for application in civil engineering projects. Major research questions that have risen during the last years are the proper describing and quantification of the heterogeneity of subsurface materials and the related problem of the likelihood of models of subsurface geology and geotechnical property distributions. Both problems are addressed in the research done in Near Surface Technology and Management during the year 2001 and are far from being solved and research will be ongoing to these items in the coming years.
Tool development
Tool development for determination of rock and soil mass parameters has concentrated on the application of laser imaging (Lidar), on improvements of shallow shear wave seismics, and on Magnetic Resonance Sounding (MRS). Lidar is promising as it better and faster determines the orientation of discontinuity sets in soil and rock masses on surface exposures and, hence, probably can be used for describing the heterogeneity of soil and rock masses. The shear wave seismic and MRS technologies are promising as these are expected to improve describe the geometry and heterogeneity characterisation of soil and rock masses.
The new development of high-resolution laser scanning creates opportunities to characterize the discontinuity sets in a rock or soil mass by an automatic procedure and from a relatively large distance. Discontinuity sets in a rock or soil mass are one of the main features that determine the geotechnical behaviour of the rock or soil mass, such as block size and block form, permeability, failure criteria and deformation moduli. At present, the discontinuity sets are characterized by hand by measuring each discontinuity. Obviously, this is a tedious work, but more important it requires access to the rock or soil mass. Generally, this means that only a small part of the discontinuities in the rock or soil mass are measured because full access is nearly never possible. By using high resolution laser techniques the measurements can be done from a distance and access is not required. First trial results show that the method is promising and further results are expected in the coming years (Hack, Slob).
In shallow shear wave interpretation love waves form an obstacle for a proper interpretation of the shear wave reflections. Research is ongoing in order to develop algorithms that remove the love waves from the shear wave reflection data without having prior knowledge of the subsurface structures. For this an appropriate algorithm has been developed in former years, which gave good results if applied to artificial data. However, on real data the results are still disappointing. In order to understand why the developed algorithm does not work with real data the algorithm was tested further on synthetic data, in which distortions are introduced. The effects of three different types of distortions are examined: an erroneous estimation of the input parameters, effects not accounted for in the theory, such as inelastic attenuation, and an erroneous estimated wavelet. The results show that the method is particularly sensitive for an erroneous estimated source wavelet. Therefore, the research will continue with the development of a wavelet estimation procedure that decreases the inaccuracy of the already measured wavelet in the field. (Van Zanen)
The correlation between Dutch Cone Penetration Tests (DCPT's) and high-frequency shear wave reflection data has been under investigation for many years. Most investigations have been rather empirical in nature. It is thought that an understanding of the underlying physics will help to improve the correlation. Research has been focused on delineating lateral variations of the nature of a subsoil interface through the use of shear wave reflections. The scaling exponent at an interface extracted for the local shear wave velocity contrast function resembles closely the scaling exponent for the same interface as measured by a DCPT, and this conformity can thus be followed laterally. This result can lead to promising new applications. Secondly, using forward modelling, the sensitivity of an estimate of the friction angle of sand to uncertainties in horizontal stress and shear-wave velocity information has been quantified. (Ghose)
Magnetic Resonance Sounding (MRS) is a Nuclear Magnetic Resonance (NMR) implementation from the surface, using the earth's magnetic field for the static field and a large loop laid on the ground surface for the excitation/detection of in-situ groundwater. Similarly to NMR logging, it provides effective porosityand information on pore size, but in a totally non-invasive way (a borehole is not required). Among the noteworthy accomplishments for the reported year are the inversion of MRS data into three pore-size classes, the on-going adaptation of this procedure to more noisy data, and the updating of the in-house laboratory IP set-up to complement MRS information. Inversion in multiple pore-size classes is one of the 'foundation stones' for useful subsurface MRS imaging. (Roy).
Characterization
Research into the characterization of shallow subsurface materials concentrated on the heterogeneity of subsurface geology and geotechnical properties in space and time. Heterogeneity of subsurface materials is a major restriction to properly model the flow and transport of materials through subsurface materials. Also modelling engineering characteristics such as strength and deformation is hampered by the heterogeneity in space and time of these subsurface materials. Therefore, the research concentrates on defining procedures and constitutive models that incorporate the heterogeneity of the subsurface. Research studies have also been carried out to define constitutive models to the response of irregular topography during earthquakes, the consolidation of clay in relation to its geological history, and to define procedures in order to reduce clogging of clay in hydraulic excavation techniques.
Flow and transport processes in natural formations are complex due to the spatial scale effects caused by the heterogeneity of the subsurface materials and due to the time-scale effects caused by the geochemical reactions and biochemical transformations. The results of the research in 2001 showed that delineation of the geometrical configuration of aquifer heterogeneity is the key to obtain accurate prediction of plume evolution in heterogeneous formations. The Coupled Markov Chain model (CMC) developed in former years is a powerful tool to characterize formation heterogeneity and this model is reliable for predicting plume behaviour in a highly heterogeneous subsurface. (Elfeki)
The heterogeneity of soil and rock-masses is a major problem in many engineering projects. Satisfactory definitions describing heterogeneity for geotechnical property distributions do not exist. Errors due to heterogeneity are assumed to be taken care of by a standard safety factor (e.g. over-design) in engineering projects. The consequence is that for many projects the costs are higher than necessary. It would therefore be useful if heterogeneity could be expressed in mathematical relations that describe the expected heterogeneity in a particular rock or soil mass. Research has started that will investigate options to formulate relations defining heterogeneity. It is expected that this may lead to new constitutive models that incorporate expressions for heterogeneity. Data input is expected to come from Lidar applications. (Hack, S. Slob). The relations found may be applied in the dredging industry. (Verhoef)
Research towards developing methodologies and empirical relations between maximum acceleration, frequency components in earthquake tremors, and dimensions of irregular surface topography continued during 2001. (Hack, S. Slob). The engineering behaviour of clayey soils received further attention. The consolidation characteristics of the natural clay sediments and relation with the geological history have been investigated. Also, a study was done to the application of electro-osmosis to reduce clogging during hydraulic excavation. (Verhoef)
Management
The management of shallow subsurface data is still an area under investigation. In the last years enormous improvements in information technology allowed for large databases for data storage and for realistic models and visualization of the subsurface and the interactions between the subsurface and civil engineering constructions. However, the newly achieved IT products have also resulted in new research questions being risen. A new research question has become the reliability or likelihood of subsurface models. Other research questions concern the automatic interpretation of subsurface data to reduce (man-induced) subjectivity in data interpretation.
The research towards the optimisation of the use of three-dimensional geographical information system and expert systems in surface engineering and engineering geology was concluded in 2001 with an integrated system for tunnelling, linkage of finite- and distinct element programs to 3D-GIS programs, and a DSS for input data and analyses of calculation results. (Hack, Ozmutlu). Various projects were executed for database design and storage of marine offshore data, and to visualize these results in three-dimensions. (Maurenbrecher). Automatic interpretation and modelling from DCPT (Dutch Cone Penetration tests) have been undertaken by incorporating pattern recognition, such as transition analysis on processed CPT’s to detect preferential soil sequences in the vertical direction. (Ngan-Tillard). Together with NITG-TNO a study is performed to study the benefits of incorporating more detailed information on the geology of the subsurface into the planning of a suburban development. (Verhoef)