Penetration and Pullout Capacity of Low-Skirted Suction Caissons

The bearing capacity of suction caissons is the key to the design of offshore structures. A new type of cross-shaped low-skirted suction caisson is invented to effectively improve the bearing capacity, considering inevitable “soil plug” phenomenon. The behaviors of penetration and pullout for new low-skirted suction caisson are investigated by performing model tests. A new formula for calculating the penetration resistance is suggested based on the limit equilibrium theory and the test data, which can consider the change of the lateral area of the suction caisson during penetration. The behaviors of low-skirted suction caisson under inclined loading are analyzed by carrying out finite element simulation. The effects of loading angles and loading positions on the ultimate bearing capacity and failure mechanism of low-skirted suction caissons are discussed. The research results can provide a reference for the design of suction bucket foundation for offshore structures.

MODEL TESTING OF THE "PILE-SOIL" INTERACTION UNDER AXIAL FORCE

Modern marine structures (berths, breakwaters, offshore platforms, etc.) often include steel tubular piles of essential length (80-100 m and more) that should provide high bearing capacity in case of external axial loads application. Interaction between elements of the system “piled structure – soil media” is not studied sufficiently yet. It relates also to the bearing capacity of the long steel tubular piles of large diameter. One of the interesting peculiarities of long tubular piles behavior is the formation of soil plug at the piles tip. There are a lot of suggestion and methods aimed to increase piles bearing capacity under static pressing load. One of them relates to use of the additional structural element, i.e., the internal diaphragm welded to the internal surface of the pile shaft. Such approach has been applied in some practical cases of marine construction and demonstrated its effectiveness. At the moment there are no researches focused on study of the peculiarities of internal diaphragm application. So proposed research aimed to study two connected processes during steel tubular pile driving: soil plug formation at the tip of the open-end pile and soil behavior under the internal diaphragm fixed inside the tubular pile shaft. To study mentioned processes we provided several series of laboratory experiments fulfilled at the Geotechnical laboratory of the Department “Sea, River Ports and Waterways” in Odessa National Maritime University. In these experiments the model of steel tubular pile has been driven (pressed) into fine sand by mechanical jack. The first series was devoted to determination of the conditions related to the soil plug formation at the pile tip. The next series were aimed to study the influence of the flat rigid diaphragm inside the pile shaft. Obtained experimental results allow to conclude that (a) in the fine sand the plug is formatted at the comparatively early stage of pile installation (in case of our modeling – at the penetration depth of some 4-5 pile diameter); (b) our empirical assessment of the conditions of soil plug formation corresponds to the approaches based on PLR and IFR characteristics; (c) formation of soil plug at the pile tip is followed by decreasing of soil level in the pile shaft relatively its initial value (on completing the plug formation the soil level in the shaft become stable); (d) regarding above mentioned, we may note that in case of use of internal diaphragm on the recommended depth (5-7 pile diameters) there may be no contact between diaphragm and the soil inside the pile (e) application of the diaphragm may lead to increasing of the pile’s bearing capacity. It was proposed (and checked by our tests) the technological improvement based on sand filling into space under the internal diaphragm to provide constant diaphragm-soil contact and related soil resistance.

Improved soil plug height calculation formula

This paper analyzes the soil plugging effect of the open pipe pile during the pile sinking process. The soil in the pipe pile is regarded as a continuous and uninterrupted multiple units, and the force analysis is carried out in the vertical direction, and the vertical balance equation of the soil in the pile is obtained. By establishing an equation, the expression of the plug height of the pipe pile during the pile sinking process is obtained. Comparing the theoretical calculation results with the actual project, it is concluded that the theoretical calculation results can reflect the overall change in the height of the soil plug. Therefore, the pile plug height obtained by calculation has certain guiding significance for the project.

Physical modeling of steel tubular piles installation into sandy soil

Modern marine structures (berths, breakwaters, offshore platforms, etc.) often include steel tubular piles of essential length (80-100 m and more) that should provide high bearing capacity in case of external axial loads application. Interaction between elements of the system “piled structure – soil media” is not yet studied sufficiently. It relates also to the bearing capacity of the long steel tubular piles of large diameter. One of the interesting peculiarities of long tubular piles’ behavior is the formation of soil plug at the piles’ tip. There are a lot of suggestion and methods aimed to increase piles bearing capacity under static pressing load. One of them relates to use of the additional structural element, i.e., the internal diaphragm welded to the internal surface of the pile’s shaft. Such approach has been applied in some practical cases of marine construction and demonstrated its effectiveness. At the moment there are no researches focused on study of the peculiarities of internal diaphragm application. So proposed research aimed to study two connected processes during steel tubular pile driving: soil plug formation at the tip of the open-end pile and soil behavior under the internal diaphragm fixed inside the tubular pile’s shaft. To study mentioned processes we provided several series of laboratory experiments fulfilled at the Geotechnical laboratory of the Department “Sea, River Ports and Waterways” in Odessa National Maritime University. In these experiments the model of steel tubular pile has been driven (pressed) into fine sand by mechanical jack. The first series was devoted to determination of the conditions related to the soil plug formation at the pile’s tip (results are presented in this paper). The next series were aimed to study the influence of the rigid diaphragm inside the pile’s shaft (to be presented in the further publications). Obtained experimental results allow to conclude that (a) in the fine sand the plug is formatted at the comparatively early stage of pile installation (in case of our modeling - at the penetration depth of some 4-5 pile’s diameter); (b) our empirical assessment of the conditions of soil plug formation corresponds to the approaches based on PLR and IFR characteristics; (c) formation of soil plug at the pile’s tip is followed by decreasing of soil level in the pile’s shaft relatively its initial value (on completing the plug formation the soil level in the shaft become stable); (d) regarding above mentioned, we may note that in case of use of internal diaphragm on the recommended depth (5-7 pile’s diameters) there may be no contact between diaphragm and the soil inside the pile and the diaphragm does not come up with the soil. So, for the next series of our experiments, it should be foreseen assured contact of the diaphragm’s surface with soil underneath. As proved by previous studies, one of the interesting features of the behavior of long tubular piles is the formation of a soil plug at the lower end of the pile. From this point of view, it is important to study the effect of soil plug not only on the bearing capacity at the lower end of the pile, but also on the behavior of the soil inside the pile. It is shown that in fine-sandy soils a plug is formed at a relatively early stage of pile immersion (in this case - at a depth of immersion of about 4-5 pile diameters). The process of forming a soil plug at the lower end of the tubular pile during its immersion is accompanied by a decrease in soil surface level in the pile trunk relative to its initial value (upon completion of plug formation the soil surface level in the pile trunk stabilizes).

Soil flow mechanisms of full-flow penetrometers in layered clays through particle image velocimetry analysis in centrifuge test

This paper reports the soil flow mechanisms observed in centrifuge tests around full-flow (T-bar and ball) penetrometers in layered clays. The layered clay samples consisted of soft–stiff, stiff–soft, soft–stiff–soft, and stiff–soft–stiff soil profiles. Particle image velocimetry (PIV), also known as digital image correlation (DIC), allowed accurate resolution of the flow mechanism around the faces of the T-bar and half-ball penetrated adjacent to a transparent window. For the T-bar, overall, a full symmetrical rotational flow around the T-bar dominated the behavior. A novel “trapped cavity mechanism” was revealed in stiff clay layers, with the evolution of the trapped cavity being tracked. No soil plug was trapped at the base of the advancing T-bar regardless of penetration from stiff to soft layer or the reverse. For the ball, two key features of the soil flow mechanism were identified, including (i) a combination of vertical flow, cavity expansion type flow, and rotational flow for a fully embedded ball and (ii) a stiff soil plug trapped at the base of the ball advancing in a stiff–soft clay deposit. For both penetrometers, a squeezing mechanism mobilized as they approached a soft–stiff layer interface.

Geotechnical Lessons Learned From Nineteen Railway Trenchless Crossings During Construction of a Transmission Pipeline

When new pipelines are constructed, they often cross existing major infrastructure, such as railways. To reduce potential service disruption, it is a common practice to complete these crossings using trenchless technologies. Without proper methods and oversight in planning and construction, there may be serious safety and financial implications to the operators of the railways and the public due to unacceptable settlement or heave. If movement tolerances are exceeded, the schedule and financial loss to the railway operators could be in the millions of dollars per day. Recent construction of a new pipeline across the Canadian prairies implemented ground movement monitoring plans at 19 trenchless railway crossings in order to reduce the potential for impact to the track and railway operations. The specifics of the plan varied for each site and were based on the expected ground conditions, as well as permit requirements from the various railway operators, but typically included ground movement monitoring surveys, observation of the cuttings, recommendations for a soil plug at the leading edge of the bore casing, and frequent communication with both the railway operators and the contractors. For all crossings, the expected soil and groundwater conditions were obtained from pre-construction boreholes and confirmed during excavation of the bore bays. Based on the expected ground conditions, appropriate soil plug lengths, if required, were recommended. In general, fine-grained clay/silt-dominated soils needed minimal to no soil plug in order to minimize the potential for ground heave, while coarser-grained sand-dominated soils needed a longer soil plug in order to reduce the potential for “flowing soil” which would cause over excavation along the bore path. Prior to boring, surface monitoring points were established along the tracks to monitor for changes in the ground surface elevation. Additional subsurface points were installed for crossings where the potential for over excavation was higher. These monitoring points were surveyed before, throughout, and following completion of construction, and the frequency of the surveys was increased when the movement was nearing or exceeding specified tolerances. The effort to monitor and reduce the potential for ground movement was a coordinated effort between the geotechnical engineers, railway operators, and construction contractors. The purpose of this paper is to present the lessons learned from the 19 trenchless railway crossings, including the challenges and successes. Recommendations for ground movement monitoring are also provided to help guide railway operators, design and geotechnical engineers, and contractors during the construction of future trenchless pipeline crossings of railway infrastructure.

Centrifuge modelling of uplift response of suction caisson groups in soft clay

This paper describes a program of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile, and type of footing (i.e., mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid, and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons groups, respectively, at which a turning point can be identified in the load–displacement curve. This was found to be attributed to the adhesion on the caisson–soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissons group exhibits much greater normalized uplift resistance than the mono-caisson group before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large-amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism.

Numerical Simulation of Cone Penetration Tests Inside Suction Caisson Foundations in Sand

Offshore wind is increasingly utilised as a renewable energy source. A growing number of bottom fixed wind turbines installed offshore are supported by suction caisson foundations. The suction-assisted installation remains a source of uncertainty towards the in-service performance due to the unknown post-installation soil plug state. Cone penetration tests within the suction caisson can help to improve the reliability. Therefore, cone penetration tests were employed in centrifuge tests to investigate the plug state in a previously installed suction caisson. However, the performance of a cone penetration test in a small-scale experiment is connected to uncertainties: A relatively large diameter device is required to conduct the cone penetration test — especially in a centrifuge test. Different finite element models are developed in order to visualise and investigate a cone penetration test inside a suction caisson. The numerical analysis results are validated through the back-calculation of centrifuge cone penetration tests. The results of the simulated cone penetration tests inside a suction caisson are evaluated and compared to the centrifuge experiments. This investigation reinforces the scope of the centrifuge experiments and emphasises a considerable effect of the pressure transferral through the caisson lid in the soil plug state. Hence, the results of this study reduce existing uncertainties regarding possible suction installation effects on the in-service performance of caisson foundations.

Analytical Solution for Estimating Bearing Capacity of a Closed Soil Plug: Verification Using An On-Site Static Pile Test

When the open-ended pile penetrates the soil layer, the resistance generated by the soil plug cannot be ignored. A pile with a full-size pressure sensor installed at pile tip can detect resistance more accurately than a microsensor when the pile penetrates into the soil. In this paper, the pile installed full-size pressure sensor was used for penetration test and the relationship between formation parameters and pile tip force is obtained. Using the solution of the Kelvin problem in infinite space and the plane stress distribution function, the analytical solution of the bearing capacity of the soil plug is derived under the condition that the displacements of the bottom of the pile and the soil plug are consistent. The results show that the ultimate stress of the soil plug is closely related to the pile diameter and pipe thickness. The bearing capacity of the soil plug is closely related to the properties of the soil layer. The analytical solution of the bearing capacity of the soil plug has a linear relationship with the formation parameters SPT and CPT. The analytical solution of the ultimate bearing capacity of the soil plug has been verified by field test data and has a good match with the geometric dimensions of the pile tip and the formation parameters.