Assaf Klar

Theoretical Study on Pile Length Optimization of Pile Groups and Piled Rafts

Pile groups are frequently designed with equal or similar pile lengths. However, the significant interaction effects among equal-length piles imply that this may not be the optimized configuration. This paper presents the optimization analyses of piled rafts and freestanding pile groups, where pile lengths are varied across the group to optimize the overall foundation performance. The results of the analyses are applicable in cases where the piles derive a majority of the capacity from the frictional resistance. It is demonstrated that, with the same amount of total pile material, an optimized pile length configuration can both increase the overall stiffness of the foundation and reduce the differential settlements that may cause distortion and cracking of the superstructure. The benefits of the optimization can be translated to economic and environmental savings as less material is required to attain the required level of foundation performances. The reliability of the optimization benefits in relation to construction-induced variability is also discussed.

Explicitly Coupled Thermal Flow Mechanical Formulation for Gas-Hydrate Sediments

This paper presents an explicit time-marching formulation for the solution of the coupled thermal flow mechanical behavior of gas- hydrate sediment. The formulation considers the soil skeleton as a deformable elastoplastic continuum, with an emphasis on the effect of hydrate (and its dissociation) on the stress-strain behavior of the soil. In the formulation, the hydrate is assumed to deform with the soil and may dissociate into gas and water. The formulation is explicitly coupled, such that the changes in temperature because of energy How and hydrate dissociation affect the skeleton stresses and fluid (water and gas) pressures. This, in return, affects the mechanical behavior. A simulation of a vertical well within a layered soil is presented. It is shown that the heterogeneity of hydrate saturation causes different rates of dissociation in the layers. The difference alters the overall gas production and also the mechanical-deformation pattern, which leads to loading/ unloading shearing along the interfaces between the layers. Copyright

Distributed optical fiber strain sensing in a secant piled wall

An optical fiber strain sensing technique, based on Brillouin Optical Time Domain Reflectometry (BOTDR), was used to obtain the full deformation profile of a secant pile wall during construction of an adjacent basement in London. Details of the installation of sensors as well as data processing are described. By installing optical fiber down opposite sides of the pile, the distributed strain profiles obtained can be used to give both the axial and lateral movements along the pile . Measurements obtained from the BOTDR were found in good agreement with inclinometer data from the adjacent piles. The rela tive merits of the two different techniques are discussed.

Role of Linear Elasticity in Pile Group Analysis and Load Test Interpretation

This paper compares linear-elastic and nonlinear pile group analysis methods through settlement analyses of hypothetical scenarios and real case studies, and elaborates on the implications for interpretation of pile load test data. Comparisons between linear-elastic and nonlinear methods justify the proposition that pile-to-pile interaction is dominated by linear elasticity, characterized by the small-strain soil stiffness. As the size of a pile group increases, nonlinearity in individual pile behavior becomes overwhelmed by the interaction effects. In such cases, similar estimates will be achieved by both linear and nonlinear methods if the soil modulus is derived from the initial tangent, rather than some secant stiffness, assessed from the load test data. The study clarifies the capabilities and limitations of linear elasticity in pile group analysis and provides guidance on pile test interpretation for analysis of pile group response.

The use of fibre optic sensors to monitor pipeline response to tunnelling

This paper describes the use of fibre optic sensing with Brillouin Optical Time-Domain Reflectometry (BOTDR) for near-continuous (distributed) strain monitoring of a large diameter pipeline, buried in predominantly granular material, subjected to a pipe jack tunnelling operation in London Clay. The pipeline, buried at shallow depth, comprises 4.6 m long sections connected with standard bell and spigot type joints, which connect to a continuous steel pipeline. In this paper the suitability of fibre optic sensing with BOTDR for monitoring pipeline behaviour is illustrated. The ability of the fibre optic sensor to detect local strain changes at joints and the subsequent impact on the overall strain profile is shown. The BOTDR strain profile was also used to infer pipe settlement through a process of double-integration and was compared to pipe settlement measurements. The close approximation of the measured pipe settlement provides further confidence in fibre optic strain sensing with BOTDR to investigate the intricacies of pipeline behaviour, pipe-soil interaction and interaction between pipe sections when subjected to ground movement.

Coupled Soil Deformation-Flow-Thermal Analysis of Methane Production in Layered Methane Hydrate Soils

A potential of methane hydrate to be an alternative energy resource has triggered numerous researches to investigate the physical, chemical and mechanical properties of methane hydrate bearing soils. Among various issues, geotechnical characterization of methane-hydrate soil is particularly important. For example, depressurization process used for methane gas extraction significantly changes the stress state and the mechanical properties of hydrate soils, which may lead to potential geotechnical hazards such as submarine landslide or wellbore collapse. At present, most of the geotechnical analyses associated with methane gas extraction assume that depressurization of methane hydrate happens in a homogeneous hydrate soil layer. This paper presents results of the analyses of methane hydrate extraction in alternating layers of clay and hydratebearing sand, which are encountered in the Nankai Trough. Using a coupled deformation-flow-thermal code implemented in FLAC, the methane gas extraction process in such soil conditions is simulated. Results show that heat flows from the clay layer to the sand layer leads to faster rate of hydrate dissociation in the hydrate region near the clay/sand boundary than at the center of the hydrate-sand layer, which influences stress distribution around the wellbore. As the soil relaxes toward the wellbore, arching effect in the vertical plane can be seen in the sand layer in addition to the usual increase in the circumferential stress. This is due to the force transfer from the casing to the clay layer, which deforms greater than the sand layer during depressurization. The thickness of the layered soils is also varied and it is demonstrated that the heat flow from the interface has a greater effect in the case of a smaller thickness of the clay-sand layer. Consequently, the arching effect and vertical compression are more prominent inside such case. The above findings highlight the importance of understanding geotechnical characteristics of methane hydrate soil in production. Introduction Methane hydrate is an ice like solid material composed of a gas molecule caged in a structure made of water molecules, and its natural form only exists under high pressure and low temperature condition, i.e. permafrost regions or beneath deep seabed. It is formed inside soil pores and it will dissociate into methane gas and water if the stable temperature-pressure equilibrium is breached. Sites with high concentration of methane hydrate are considered to be a possible energy resource for future years. This has triggered numerous researches to investigate the physical, chemical and mechanical properties of methane hydrate soils. Two methods are generally accepted to be feasible for dissociating methane hydrate and hence extracting methane gas in situ: (1) the use of depressurized wells, which reduce the pressure in the soil around the wells, and (2) the use of thermal injection wells, which increase the temperature in the surrounding soil by injecting hot fluid. Since methane hydrate is likely to behave as a cohesive material to sedimentary unconsolidated sandy soils (Soga et al., 2006), the dissociation reduces its bonding effect and hence the methane hydrate soil loses its apparent cohesion associated with the hydrate. The mechanical properties of hydrate soils are significantly changed and excessive ground deformation is possible. Potential geotechnical hazards such as submarine landslide and collapse of wellbore wall may occur. Therefore, the stability of a gas-hydrate extraction well during depressurization process has been studied and a homogeneous hydrate sand layer was assumed for simplicity in the work by Klar and Soga (2005) and many others. However, in sites such as Nankai Trough and Mallik Mackenzie Delta, a soil profile of alternating layers of sands and clays is encountered (e.g. Soga et al, 2006). In the Nankai Trough, the soil underneath the 950 m deep of water is firstly 40 m of calcareous silt and clay with volcanic ash layers. Then, 90 m of calcareous silt and clay with volcanic ash and thin sand layers are encountered. Underneath this is about 70 m hydrate-bearing stratum, which has alternating layers of weakly consolidated calcareous silt/clay and thick,

Feasibility study of the automated detection and localization of underground tunnel excavation using Brillouin optical time domain reflectometer

Cross-borders smuggling tunnels enable unmonitored movement of people, drugs and weapons and pose a very serious threat to homeland security. Recent advances in strain measurements using optical fibers allow the development of smart underground security fences that could detect the excavation of smuggling tunnels. This paper presents the first stages in the development of such a fence using Brillouin Optical Time Domain Reflectometry (BOTDR). In the simulation study, two different ground displacement models are used in order to evaluate the robustness of the system against imperfect modeling. In both cases, soil-fiber interaction is considered. Measurement errors, and surface disturbances (obtained from a field test) are also included in the calibration and validation stages of the system. The proposed detection system is based on wavelet decomposition of the BOTDR signal, followed by a neural network that is trained to recognize the tunnel signature in the wavelet coefficients. The results indicate that the proposed system is capable of detecting even small tunnel (0.5m diameter) as deep as 20 meter.

Simple Energy-Based Method for Nonlinear Analysis of Incompressible Pile Groups in Clays

This note presents a method for predicting nonlinear response of pile groups in clays, subjected to vertical loads. The method is based on mobilizable strength design (MSD) concepts, in which the mobilized strength is associated with the shear strains developed in the soil. The suggested procedure is incremental, and requires evaluation of a displacement field. A simple procedure of superposition of pattern functions is suggested for the construction of a complete displacement field. The incremental procedure allows for the variation of the displacement field throughout the loading process, according to principles of minimum energy and compatibility requirements among the piles. Essentially, the procedure allows consideration of a nonlinear continuum between the piles. The pattern functions are an adaptive form of the logarithmic function suggested by Randolph and Wroth in 1979. Under small load levels, when the soil is essentially elastic, the procedure yields values comparable to those from the elastic solution of Randolph and Wroth. At larger strain levels, nonlinear pile group response is simulated based on the soil constitutive models specified by the practitioner. The method is applicable to cases where shaft loading does not induce volume changes in the soil. The method is compared with three dimensional finite difference simulation of undrained loading of pile groups with a nonlinear soil constitutive model. Fair agreement is observed.

Detection of Sinkhole Formation by Strain Profile Measurements Using BOTDR: Simulation Study

AbstractThe sudden collapse of sinkholes in the Dead Sea area represents a serious threat to infrastructure in the area. The formation of these sinkholes has been shown to be directly correlated with the drop in the Dead Sea water level which is accompanied by a corresponding lowering of the groundwater level and permits the penetration of low-salinity groundwater into coastal areas. This water causes dissolution of the salt layers which results in the formation of subsurface voids that develop into collapse sinkholes. Various tools and measurement methods have been investigated in order to attempt to detect the formation of sinkholes, but to date, there is no method capable of providing early warning of possible collapse. This paper investigates the use of fiber-optic Brillouin optical time-domain reflectometry (BOTDR) or Brillouin optical time-domain analysis (BOTDA) for such detection. Brillouin optical time-domain reflectometry or analysis (BOTDR/A) is an optical measurement technique that provides di...

Detection of tunnel excavation using fiber optic reflectometry: experimental validation

Cross-border smuggling tunnels enable unmonitored movement of people and goods, and pose a severe threat to homeland security. In recent years, we have been working on the development of a system based on fiber- optic Brillouin time domain reflectometry (BOTDR) for detecting tunnel excavation. In two previous SPIE publications we have reported the initial development of the system as well as its validation using small-scale experiments. This paper reports, for the first time, results of full-scale experiments and discusses the system performance. The results confirm that distributed measurement of strain profiles in fiber cables buried at shallow depth enable detection of tunnel excavation, and by proper data processing, these measurements enable precise localization of the tunnel, as well as reasonable estimation of its depth.