• Journal of the Korean Geotechnical Society
• /
• v.20 no.4
• /
• pp.15-27
• /
• 2004

In this study, skin friction evaluation methods developed f3r deep foundation system were investigated and a method that can properly evaluate the skin friction of large size pneumatic caisson was proposed. Especially, based on Hong Won-Pyo's method, new technique (Kn parameter method) was suggested for estimation of the skin friction. The $\lambda$ method used widely to pile foundation was also investigated fur the applicability of estimation of the skin friction of large size pneumatic caisson. To do this, the data measured from the pneumatic caissons installed as a substructure of main tower in the suspension bridge part of Youngjong Grand Bridge were utilized. The data show that the skin friction is proportional to the rate of sinking, and the skin friction distribution with depth is similar to parabolic type rather straight line, which is a type generally observed in pile foundation. The skin frictions predicted by the Kn and $\lambda$ methods were plotted with the measured data for comparisons. It is cleary shown that the skin frictions estimated by the proposed Kn parameter method are well matched with the measured data. That is, for the large size pneumatic caisson having wide base, the new technique developed from Hong Won-Pyo's method is more suitable for estimation of the skin friction rather than the $\lambda$ method.

• Journal of the Korean Geotechnical Society
• /
• v.21 no.1
• /
• pp.93-103
• /
• 2005

Both static pile load test with load transfer measuring system and the pile dynamic load test are performed to estimate the skin friction and behavior characteristics of a large drilled shaft. And the numerical modeling of large drilled shaft is performed by applying the FDM program. Since the magnitude of friction resistance depends on the relative displacement between soil and shaft, load and displacement at the arbitrary depth along the large drilled shaft are estimated to analyze the correlation. According to the measuring results of load transfer, unit skin friction along the large drilled shaft was fully mobilized at gravel layer in the middle of shaft and the frictional resistance transmitted to bedrock was relatively small. Also, even for the same drilled shaft, the results of PDA and static load test are different with each other and the difference is discussed.

• Geotechnical Engineering
• /
• v.14 no.1
• /
• pp.71-80
• /
• 1998

Three prestressed concrete(PC) piles were installed for research purpose at Seosan area of west sea of Korea, and also cone penetration tests (CPT) were performed near two pile locations in order to compute PC pile shaft resistance by using CPT data measured. Three common CPT prediction methods that ia, Schmertmann method, Tumay Sl Fakroo method and LCPC method in France were used to predict pile shaft resistance. The pile shaft resistance predicted by each method was compared with that obtained by full-scale loading test and pile driving analyzer to estimate reliability of each prediction method. The predicted resistances based on three CPT-based methods underestimated significantly the resistances obtained from by fullrcale loading test, performed at 25 days and 42 days text pile installtion. There were, however, good agreements of predicted shaft resistance of piles between three CPT-based methods and pile driving analyzer tested two weeks after pile installtion.

• Journal of the Korean Geotechnical Society
• /
• v.20 no.9
• /
• pp.115-124
• /
• 2004

According to Sagong and Paik (2003), the side resistance of rock socketed drilled shafts is affected by rock quality, types, uniaxial compressive strength, and confining stress. Their approach based upon the Hoek-Brown criterion provides reasonable predictions of the side resistance. In this study, we propose an equation to calculate the side resistance considering size effects of the shafts and investigate the influence of drilled shaft diameter on the side resistance. A new method employs the modified Hoek-Brown criterion together with an empirical size effect of rock core. From the previous field tests, 12 pile load test results were collected and compared with prediction calculated from the equation proposed in this study. In a given condition, similar results between measurement and estimate are observed. From the parametric study on the GSI, confining stress, uniaxial compressive of intact rock and pile size, it is shown that uniaxial compressive strength is the most influential parameter on the side resistance. Though pile size shows the least influence on the resistance, the size effect is apparent as rock quality increases.

• Journal of the Korean Geotechnical Society
• /
• v.19 no.1
• /
• pp.209-220
• /
• 2003

Modification of general Hoek and Brown criterion is carried out to estimate the shaft resistance of drilled shaft socketed into rock mass. Since the general Hoek-Brown criterion can consider the in-situ state of the rock mass, the proposed method, estimating the unit shaft resistance of drilled shafts based on the Hoek-Brown criterion, has increased flexibility compared to other methods exclusively considering uniaxial compressive strength of intact rocks. The proposed method can form the upper and lower bounds, and most culled data (from 21 pile load tests) from the literature can be found between these two bounds. A comparison between the estimated and observed unit shaft resistances shows quite a good correlation even with crude assumptions for the input parameters. The best-fit line drawn from this analysis shows that at the lower strength of intact rocks (up to 10MPa), Horvath and Kenney's equation shows a good correlation with the measured values, and fur strong rocks Rosenberg and Journeaux's equation provides a close estimation with colleted data. The results of parametric studies for GSI and confining stress show that the normalized unit shaft resistance increases with these two factors. In addition, coefficient of the equational form of the estimation can vary with GSI and confining stresses.

• Journal of the Korean Geotechnical Society
• /
• v.39 no.1
• /
• pp.5-14
• /
• 2023

The pile driving process may lead to ground heaving, causing additional positive skin friction to act on the piles, compromising their stability. This study proposes a new pile foundation type that can reduce positive skin friction. This was investigated by designing and constructing a pile with a hydraulic cylinder which actively responds to ground deformation. The newly proposed pile design was compared against traditional piles in multiple model tests where ground heaving was simulated. In the tests, base load and total shaft resistance were measured during ground heaving and with expansion of the hydraulic cylinder. As a result of the tests, a very small amount of expansion of the hydraulic cylinder member completely reduced the positive skin friction and increased the base load. Excessive expansion of the hydraulic cylinder, however, generates negative skin friction beyond the zero skin friction state. Therefore, it is necessary to estimate the appropriate level of hydraulic cylinder expansion, taking into account the amount of ground heaving and the allowable displacement of the pile.

• Journal of the Korean GEO-environmental Society
• /
• v.14 no.10
• /
• pp.11-16
• /
• 2013

Success or failure of the bi-directional pile load test for drilled shaft depends on point selection for loading cells, that is balanced location both uplift force and downward force. Methods to evaluate the ultimate unit side resistance in rockmass layer in both domestic and foreign are based on the uniaxial compression strength of rock core, which can hardly be obtained in domestic rockmass layers which are weathered rockmass layer and soft rockmass layer with very low RQD. Therefore, this study suggested the relation charts between the revised SPT N values and developed unit side resistance of each different layers, which were obtained from bi-directional pile load tests in various domestic sites. To evaluate the appropriateness of the relation charts, the developed unit side resistances from the relation charts were used to select the loading cell position and compared with the measured unit side resistances from field pile load test. Results showed that the developed side resistance from relation charts and the measured side resistance of weathered soil layer and weathered rock layer were very close. Average developed side resistance($1,325kN/m^2$), which are average of upper soft rock layer of loading device($1,151kN/m^2$) and lower($1,500kN/m^2$), was similar with the estimated value ($1,250kN/m^2$).

• Journal of the Korean Geotechnical Society
• /
• v.28 no.6
• /
• pp.19-29
• /
• 2012

Pack micropiles were recently developed to improve pile capacity of general micropiles. Pack micropiles were made by warping thread bar or steel pipe of general micropile by geotexlile pack and grouting inside the pack with pressure. According to the pressure, the boring hole could be enlarged. A series of pile uplift tests were performed on three micropiles. Two out of the three piles were the pack micropiles and the other was the general micropile, in which a thread bar was used in the boring hole. According to the pressure applied to the pack micropiles, the diameter of boring hole was enlarged from 152 mm to 220 mm. Unit skin friction mobilized on side surfaces of micropiles increased with displacement of pile head and reached on a constant value, which represents that the relative displacement between piles (or thread bar) and soils was reached on critical state. And the uplift resistance of pack micropile was higher than that of general micropile. Two reasons can be considered: One is that the frictional surface increases due to enlarging diameter of boring holes and the other is that the unit skin friction could increase due to compressing effect of surrounding soils by soil displacement as much as the enlarging volume of boring hole. The compression effect appeared at deeper layer rather than surface layer. The unit skin friction mobilized on micropiles with small diameter was higher than the ones on large bored piles.

• Journal of the Korean GEO-environmental Society
• /
• v.6 no.1
• /
• pp.45-51
• /
• 2005

The distribution of shaft resistance is measured by the static load test with the strain gauge or stress gauge, so that the long-term load distribution must be considered for the pile design. However, the measurement by strain gauge generally assumes the 'zero reading', which is the reading taken at 'zero time' with 'zero' load and the residual stress, which is the negative skin friction(or the negative shaft resistance) caused by the pile construction, is neglected. Therefore, the measured value by strain gauge is different from the true load-distribution because residual stresses were neglected. In this study, the three drilled shafts were constructed, and the strain measurements were carried out just after shaft construction. As a result of this study, it is shown that the true load-distribution of drilled shaft is quite different with known load distribution and the true load-distribution of drilled shaft changed from the negative skin friction to the positive skin according to the load increment.

• Journal of the Korean Geotechnical Society
• /
• v.29 no.1
• /
• pp.161-170
• /
• 2013

In this paper, numerical pile segment analysis is conducted with an advanced soil elastoplastic model to investigate the diameter effects on skin friction behaviour of a drilled shaft in sand. Ultimate skin friction and 't-z' behavior from the pile segment analyses for drilled shafts show good agreement with those from design methods. Higher ultimate skin friction for the smaller diameter pile is related to the greater increase in the effective radial stress at the interface due to the localized dilation at and near the pile interface. Stiffer t-z curve for the smaller diameter pile is related to the early occurrence of three shear stages (early, dilation, constant volume shear stages). The diameter effects on ultimate skin friction of drilled shafts are more prominent for denser sand and lower confining pressure.