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Hydraulic fracturing is employed as a stimulation treatment by the oil and gas industry to enhance the hydro-carbon recoveries. The rationale is that by creating fractures from the wellbore into the surrounding formations, the conductivity between the well and reservoir is significantly increased and the hydro-carbon flow is therefore stimulated. The hydraulic fracture is initiated and driven by pressurizing a bore-hole section via fluid injection. Despite its half century’s practice, hydraulic fracturing treatments sometimes fail to increase the well productivity. One prominent reason is that there are pre-existing (natural) fractures in connection with the wellbore or in the way of the hydraulic fracture propagation. If the natural fracture is opened by the injection fluid, the borehole pressure will decrease as if a hydraulic fracture break-down has taken place. If a hydraulic fracture is intercepted by natural fractures that layer the formation, its dimension could be restricted in only one of the layers. In both of the cases, the fluid losses via the natural fractures could mislead the interpretation of the bore-hole measurements. Laboratory tests were design to investigate the interaction between the hydraulic fracturing process and natural fractures’ fluid infiltration. To characterize the natural fractures’ mechanical and hydraulic properties under normal confining and shear, we performed shear and flow tests with rock samples cleaved into layers. To characterize the hydro-natural fracture interaction, we performed the injection fracturing tests on the layered samples. We interpreted the test results by correlating the possibility of the hydraulic fracture crossing-over natural fractures with the test conditions. We concluded from the test results that if the natural fracture is closed by the normal stress significantly higher than the minimum in-situ stress, it is then likely to be crossed by the hydraulic fracture. Tests were facilitated by a tri-axial set-up in combination with an acoustic monitoring system. The tri-axial loadings maintained the samples’ in-situ stresses which determined the hydraulic fracture orientations. The monitoring system tracked the fracture propagation by converting the diffraction signal arrival times to the fracture tip locations via a velocity model. Comparisons between the recovered fractures from the acoustic monitoring and sample postmortems showed agreement. In modeling the fracture initiation and propagation, we introduced a quasi-static numerical algorithm that coupled the 2D fluid domain and 3D matrix domain via boundary conditions. The model was extended to include a layer interface (natural fracture) transverse to the bore-hole. As a result of the fluid infiltration into the interface, the coupling between the layers was weakened. The displacement loss across the interface was related to the weakening process when the hydraulic fracture met the interface. The model results showed discontinuities across the interface in the fracture width and injection fluid distribution, which indicate the impact of the natural fracture infiltration to the hydraulic fracture development. In conclusion, the model was a novel approach to simulate such a complicated process and the comparison between the modeled and test results showed agreements.

Pile testing, which plays an importance role in the field of deep foundation design, is performed by static and non-static methods to provide information about the following issues: (Poulos, 1998) - The ultimate capacity of a single pile. - The load-displacement behavior of a pile. - The performance of a pile during the test conditions. - The integrity of a pile (pile integrity test). For the purposes of verification the design axial capacity and the static load – settlement behavior of piles, the static pile load test has long been considered as the most reliable method but because of its high cost and time consuming, non – static pile load tests are looked as efficient substitutions. The two non – static testing methods, i.e. dynamic and quasi – static pile load test are objects of this report. The non – static pile load tests are performed by means of exerting an impact force on the pile head while measuring and recording the responses of the pile, from which the test results are determined. Duration of the impact force (T), longitudinal wave velocity of tested pile (c) and pile length (L) are used as key factors to classify the testing methods.

Pile load tests are commonly used by engineers to determine its bearing capacity. At present, there are three methods of pile load tests: the static, the dynamic and the quasi-static test. The static pile load test is done by applying an axial load on the pile with a long duration. The dynamic and quasi-static tests are done with an impact load on pile head of very short duration. However, the required force pulse in the quasi-static test is longer than in the dynamic test. This research focuses on the comparison between quasi-static and static tests. An important aspect in order to verify the results of quasi-static application with respect to more widely used static loading. The results of quasi-static tests have both static and dynamic components. Then, in order to convert the results of a quasi-static test to static pile bearing capacity, the dynamic component (inertial and damping effects) in the soil responses have to be understood. The effect of generates pore water pressure and its dissipation during pile penetration are unclear and can limit the interpretation of the results of a quasi-static test.

In order to improve the interpretation of the quasi-static (e.g. Statnamic) pile load tests, a research project has been started to investigate effects of the loading rate on the bearing capacity of a pile in sand. A series of laboratory tests has been carried out. The testing program consists of a series of triaxial tests for sand and a series of load tests on a model pile embedded in sand in a large calibration chamber. The research pointed at answering two fundamental questions: - The effect of loading rate on the strength of sand and on the bearing resistance of a pile in sand; - The characteristics of excess pore pressure in sand and in the sand near the pile toe during a quasi-static load test. The results of the triaxial tests are: - In dry sand, a higher loading rate gives higher shear strength. In the range of applied loading rates, the angle of internal friction of the sand increases up to 2 degrees (strength increases 5-10%). - During high speed tests on dry sand an excess of pore air pressure is observed. So the dry sand is not in fully air drained condition during these tests. - The effects of loading rate in dry sand increase with the increase of relative density. - In saturated sand, the shear strength increases about 5% due to the rate effect. But, the true rate effect may be obscured by cavitation which occurs during the test. - Before cavitation occurs, the excess pore water pressure is independent of the loading rate. It depends on the relative density of the sand.