Abolfazl Eslami

Behavior of Piles under Different Installation Effects by Physical Modeling

AbstractMany factors influence the axial bearing capacity and performance of deep foundations embedded in sand deposits. Among these factors, the installation method is of great importance. The objective of this study was to investigate the construction effects on pile performance via physical modeling. The frustum confining vessel of Amirkabir University of Technology (FCV-AUT) was employed for this purpose. Six different installation methods were studied: jacking, drilling and grouting, driving, screwing, drilling and placing, and postgrouting. Up to 30 axial compressive and tensile load tests were carried out on different piles on Babolsar sand, from the northern coast of Iran, with relative densities of 45% to 50% within the FCV-AUT. Experimental results show that among different pile installation methods, jacked and precast-in-place piles had the greatest and lowest capacities, respectively. Moreover, the performance results of the different pile types in the FCV-AUT indicate that the axial bearing c...

Direct CPT and CPTu methods for determining bearing capacity of helical piles

ABSTRACT Helical piles are structural deep foundation elements, which can be categorized as torque-driven piles without any limitations to implement in marine situations. Different methods are used to predict the axial capacity of helical piles, such as static analysis, but have some limitation for this type of piles on marine conditions. In situ testing methods as supplement of static analysis have been rarely used for helical piles. In geotechnical engineering practice, the most common in situ tests particularly applicable for coastal or offshore site investigation are cone penetration test (CPT) and piezocone penetration test (CPTu). The CPT is simple, repeatable, and prepares the continuous records of soil layers. In this paper, a data bank has been compiled by collecting the results of static pile load tests on thirty-seven helical piles in ten different sites including CPT or CPTu data. Axial capacities of thirty-seven helical piles in different sites were predicted by direct CPT methods and static analysis. Accuracy estimation of ten direct CPT methods to predict the axial capacity of helical piles was investigated in this study. Comparisons have been made among predicted values and measured capacity from the pile load tests. Results indicated that the recently developed methods such as NGI-05 (2005), ICP-05 (2005), and UWA-05 (2005) predicted axial capacity of helical piles more accurately than the other methods such as Meyerhof (1983), Schmertmann (1978), Dutch (1979), LCPC (1982), or Unicone (1997). However, more investigations are required to establish better correlation between CPT data and axial capacity of helical piles.

Postgrouted helical piles behavior through physical modeling by FCV

ABSTRACT Piling procedure may disturb the surrounding soil, due to the installation particularly for cast-in-place piles. It causes a reduction in the soil strength parameters and, consequently, pile capacity. To overcome shortcomings and also for improving piles’ capacity, postgrouting as a compensation method is recognized and more developed in recent years. Helical piles, those are used widely in marine and land projects, although, are driven by torque implementation, but soil disturbance is noticed, where number of the helices become up to 3 and more. In this paper, an experimental study program is performed by frustum-confined vessel (FCV) to investigate bearing capacity of model helical piles and also postgrouted cases’ performance. FCV has been used because of its linear distribution of vertical and horizontal stresses from zero at top to maximum at bottom which simulates real field stress conditions. Through experimental study, small-scale helical model piles were made of 4-mm-thick steel plate and have been used with a length of 750 mm. The shaft and helix diameters of model piles have been 32 and 89 mm, respectively. So, the helix-to-shaft ratio (wing ratio) was about 2.8. The helical model piles installed in fine-grained sand as a surrounding soil and then axial loading tests before and after grouting were performed to achieve ultimate pile capacity. Results indicated postgrouting can improve both ratios of toe and frictional soil–pile interactions including upgrading β and Nt factors. In addition, the post grouting phenomena can change the pile geometry due to treated soil bond, resulting better functioning. Therefore, it is a proper method to improve helical piles performance and compensate installation effects in capacity mobilization.

Study on pile ultimate capacity criteria and CPT-based direct methods

Due to the variety of current Cone Penetration Test (CPT)-based methods of estimating the pile bearing capacity, for optimum design, it is necessary to evaluate the performance of such methods in various geotechnical conditions. Geotechnical databases including piling and in situ testing records have been recognised as useful tools for analysis, design and economical construction. In order to evaluate current CPT-based pile bearing capacity methods, AUT-CPT and Pile database has been compiled including 450 full scale pile load tests and CPT sounding records. This database consists of different pile types with a relatively wide range of geometries and various soil conditions. Forty-three records of piles driven in sand deposits were then employed to evaluate effects of ultimate capacity interpretation criteria from load displacement diagrams. The Brinch Hansen 80% criterion and the load at the displacement of 10% of the pile diameter were compared to estimated capacities from 10 CPT-based design methods currently used in practice. The Brinch Hansen 80% criterion and the load at the displacement of 10% of the pile diameter lead to reasonable results, the Brinch Hansen 80% criterion showed less scatter. For evaluating the accuracy and the precision of CPT-based methods, the results were compared to estimated capacities. Methods with the best performance are introduced. Generally, comparisons indicate that the CPT-based methods mainly predict the pile capacity with reasonable accuracy.

Non-stationary realisation of CPT data: considering lithological and inherent heterogeneity

ABSTRACT The main sources of uncertainty in the soil specification and mechanical behaviour consist of the lithological and heterogeneous randomness of soil deposits. It is quite obvious that the cone penetration testing (CPT) data and the variation of soil characteristics are not stationary. Hence, this paper investigates a new approach to realise a CPT data, taking both sources of uncertainty into consideration. In this regard, the first part of this study illustrates a simple approach to stratify the CPT data, using the Eslami–Fellenius chart of classification. In the second part, the non-stationary algorithm of generating random field is introduced to generate a multi-layer random field. This algorithm takes account of each layer’s statistical properties (i.e. standard deviation, mean, and the trend value), separately. To validate the proposed approach, 41 case histories from different worldwide sites, have been regenerated by considering both the stationary and non-stationary algorithms. The correlation coefficient between real and realised CPT data has been employed to show that the proposed non-stationary algorithm can simulate the CPT data more accurately in comparison with the stationary algorithm.

Experimental study on the settlement of marine deposits of Anzali under cyclic loading by laminar box apparatus

ABSTRACT The central Alborz mountain range, located in northern Iran, neighboring the Caspian Sea and where the two Persia and Eurasia plates meet, is known as a seismologically active area. In this regard, investigation of the behavior of saturated sand deposits located in this area may be of particular interest. Saturated sand deposits are subjected to instabilities owing to liquefaction or volume change due to earthquake shakings. A particular type of saturated sand deposits is Anzali sand which is abundant in Anzali port and other cities located in this area in northern Iran. This type of sand is a representative for most sands found in this area, i.e., the southern coastal line of Caspian Sea. This research is solely focused on the volume change behavior of marine deposits of Anzali area, often characterized as Anzali sand, in terms of the settlement of a model footing located on the surface of the sand by the aid of a transparent laminar shear box apparatus. Effects of a number of factors such as the frequency of the cyclic loading, the initial density of the sand, and the sample preparation method have been investigated. Observations indicated that the density index and the frequency of loading which are proportional to the energy of an earthquake have direct effects on the accumulation and amount of the final settlement of Anzali sand.

Bearing capacity of semi-deep skirted foundations on clay using stress characteristics and finite element analyses

ABSTRACT Semi-deep skirted foundations are now considered to be a viable foundation option for a variety of onshore and offshore applications. The capacity under combined vertical, horizontal, and moment loadings must be found to ensure their capability and stability. In this study, undrained bearing capacity subjected to vertical loading, as part of combined loading is determined through stress characteristics and finite element analyses. Circular skirted foundations with different soil strength and geometries considering embedment depth effects have been studied. Stress field, kinematic mechanism accompanying failure, and bearing capacity factors for various embedment ratios are investigated. Acquired vertical failure mechanism has demonstrated the transition from a general shear to a punch shear failure. Comparisons with different research works including conventional methods, upper and lower bound, finite element analyses, physical modeling, experimental, and centrifuge tests have indicated the underestimation of conventional approaches and accuracy of proposed methods in determining bearing capacity. Furthermore, differences between predicted bearing capacities and the results of this study increased with D/B ratio due to ignoring the significant role of skin friction in larger embedment circumference.

Study of soil disturbance effect on bearing capacity of helical pile by experimental modelling in FCV

Helical piles as deep foundations are a traditional solution for supporting structures such as residential construction, communication tower installations, static or seismic structural retrofitting and reconstruction. Two mechanisms of failure during helical pile loading occurred depending on pile geometry and soil strength as individual or cylindrical failure. Commonly, soil disturbance effects during pile driving by torque mechanism are not considered in conventional static analysis methods for determine bearing capacity of the helical pile toe or helices plates, while this disturbance can influence the bearing resistance of the pile. Present study considered the behaviour of three models of instrumented helical piles in two densities of Babolsar Sand by the frustum confining vessel developed at Amirkabir University of Technology (FCV-AUT) as the physical modelling apparatus. Bearing capacity of the pile toe and helices plates in addition to axial capacity of the pile are measured and controlled during pile compression load test. Two static analysis methods are used in order to verify the experimental results and deriving the coefficient of bearing capacity for promotion of the conventional static analysis to predict axial capacity of helical piles with more accuracy. The axial capacities of 21 full-scale helical piles are predicted by the new approach and compared with measured capacity from static pile load test. The results of comparison have shown that using the new approach errors in helical pile capacity estimation have been lowered almost 43%. Therefore, it can be realised as an alternative in static analysis of helical pile design.

Drained soil shear strength parameters from CPTu data for marine deposits by analytical model

Soil shear strength parameters, i.e. cohesion (C) and friction angle (ϕ) are typically determined using laboratory and in situ tests, although some limitations are involved in laboratory tests, such as the need for considering size effects and the use of undisturbed sampling. Cone penetration testing (CPT) has been recognised as a rapid and versatile procedure to provide continuous soil records, particularly in marine environment. In this study, an analytical approach is utilised to calculate drained soil strength parameters using piezocone penetration test (CPTu) records, i.e. qt (corrected point resistance) and fs (sleeve friction) and the results are compared with those obtained from laboratory tests. Current methods for obtaining shear strength parameters using CPT data are based on bearing capacity and cavity expansion theories and are able to estimate only ϕ in sands, and undrained shear strength (Su) in cohesive soils. In this paper, by combining bearing capacity theories and direct shear modes of failure at CPTu tip and sleeve resistances, and considering the pore water pressure at the shoulder of the piezocone (u2), a set of equations is derived. By inputting CPTu data including qt, fs and u2 at a certain depth, soil shear strength parameters can be calculated simultaneously. Finally results obtained from this method are compared with measured soil shear strength parameters, using a data bank consisting of 50 sets of CPTu sounding carried out in marine deposits at various locations around the world. The comparison between predicted and measured C and ϕ values indicates good consistency and low scatter for the results obtain from the proposed method. This demonstrates that the proposed method is able to predict soil shear strength parameters in difficult marine environments with acceptable accuracy.

Assessment of the likelihood of suffusion in alluvial soils: case history

AbstractSuffusion is a process of soil particle transfer in the soil body due to the effect of seepage flow on it. Although up to now internal instability has been considered in hydraulic structures such as dikes, dams and canals, todays, occurring of this phenomenon in arid regions and non-hydraulic constructions was seen. Shahriyar County in Iran, located on alluvial soil deposits, is an area where the non-hydraulic structures built there have been damaged by erosion. Even though there is no surface water flow, a seepage force has been generated by water leakage from buried pipes and sewage disposal wells. In this article, we tried to determine whether suffusion occurs in Shahriyar. Because of the limited number of available methods for the assessment of suffusion, in this article, a new approach is presented using statistical analysis to assess the internal instability of alluvial soils. This approach was developed and verified using a relatively large database of unstable soils. The performance of the presented approach was compared with those of three commonly used methods. It is showed this approach can predict the likelihood of internal instability with a relatively high accuracy rate.