Large-scale pile tests in Mercia mudstone: Data analysis and evaluation of current design methods

Full-scale load tests were carried out on six instrumented large diameter bored, cast in-situ piles formed in Mercia mudstone, as part of the design of a new Viaduct in Cardiff, UK. In this paper, the results from six test piles and extensive data from 218 ground investigation boreholes are systematically processed in order to study the load transfer and resistance mechanisms in Mercia mudstone. Data from strain gauges embedded in each pile are first analysed to calibrate the load-deformation relationship of each pile as-built, taking into account (i) the non-linearity of concrete and (ii) the effect of partial steel encasement on pile stiffness at various levels. The shaft and base capacity of the piles are each predicted using 10 calculation methods belonging to the four basic categories: (i) Undrained analysis, (ii) Drained analysis, (iii) Mixed approach and (iv) Empirical correlation. It is found that the shaft capacity prediction methods are moderately consistent. The standard deviations of the ratio Q _sp/Q _sm of predicted to observed shaft capacity lies in the range 0.06–0.24. However, 8 of these methods are over-conservative, giving Q _sp/Q _sm values in the range 0.29–0.67. The remaining two methods yield Q _sp/Q _sm = 1.01 and 1.49. In contrast, the prediction methods for base capacity are found to be much less consistent. The ratio Q _bp/Q _bm of predicted to measured base capacity falls in the interval 0.52–1.93, with corresponding standard deviations of 0.16–0.82. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of analytical models to estimate the increase in pile capacity with time (pile setup) from soil properties

This paper presents the analyses of twelve prestressed concrete (PSC) instrumented test piles that were driven in different bridge construction projects of Louisiana in order to develop analytical models to estimate the increase in pile capacity with time or pile setup. The twelve test piles were driven mainly in cohesive soils. Detailed soil characterizations including laboratory and in situ tests were conducted to determine the different soil properties. The test piles were instrumented with vibrating wire strain gauges, piezometers, pressure cells that were monitored during the whole testing period. Several static load tests (SLTs) and dynamic load tests were conducted on each test pile at different times after end of driving (EOD) to quantify the magnitude and rate of setup. Measurements of load tests confirmed that pile capacity increases almost linearly with the logarithm of time elapsed after EOD. Case pile wave analysis program was performed on the restrikes data and was used along with the load distribution plots from the SLTs to evaluate the increase in skin friction capacity of individual soil layers along the length of the piles. The logarithmic linear setup parameter “A” for unit skin friction was calculated of the 70 individual clayey soil layers and was correlated with different soil properties such as undrained shear strength (S_u), plasticity index, vertical coefficient of consolidation (c_v), over consolidation ratio and sensitivity (S_t). Nonlinear multivariable regression analyses were performed, and three different empirical models are proposed to predict the pile setup parameter “A” as a function of soil properties. For verification, the subsurface soil conditions and setup information for additional 18 PSC piles collected from local database were used to compare the measured versus predicted “A” parameters from the proposed models, which showed good agreement.

     

Single piles under horizontal loads in sand: determination of <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">Y</Emphasis> curves from the prebored pressuremeter test

Lateral load-deflection behaviour of single piles is often analysed in practice on the basis of methods of load-transfer P–Y curves. The paper is aimed at presenting the results of the interpretation of five full-scale horizontal loading tests of single instrumented piles in two sandy soils, in order to define the parameters of P–Y curves, namely the initial lateral reaction modulus and the lateral soil resistance, in correlation with the pressuremeter test parameters. P–Y curve parameters were found varying as a power of lateral pile/soil stiffness, on the basis of which hyperbolic P–Y curves in sand were proposed. The predictive capabilities of the proposed P–Y curves were assessed by predicting the soil/pile response in full-scale tests as well as in centrifuge tests and a very good agreement was found between the computed deflections and bending moments, and the measured ones. Small-sized database of full-scale pile loading tests in sand was built and a comparative study of some commonly used P–Y curve methods was undertaken. Moreover, it was shown that the load-deflection curves of these test piles may be normalised in a practical form for an approximate evaluation of pile deflection in a preliminary stage of pile design. At last, a parametric study undertaken on the basis of the proposed P–Y curves showed the significant influence of the lateral pile/soil stiffness on the non-linear load-deflection response.     

Uplift capacity of single piles: predictions and performance

The paper pertains to the development of a simple semi-empirical model for predicting the uplift capacity of piles embedded in sand. Various pile and soil parameters such as length (L), diameter (d) of the pile and angle of friction (ϕ), soil–pile friction angle (δ) and unit weight (γ) of the soil which have direct influence on the uplift capacity of the pile are incorporated in the analysis. A comparative assessment of the ultimate uplift capacity of piles predicted by using the proposed theory and some of the available theories are made with respect to each other and with reference to the measured values obtained from model tests in the laboratory. For this purpose experimental data have been collected from the literature and also from model tests conducted as a part of the present investigation. The study shows the proposed model has an excellent potential in predicting the uplift capacity of piles embedded in sand that are consistent with model pile test results.     

Field study on the behavior of pre-bored grouted planted pile with enlarged grout base

This paper presents the results of field tests performed to investigate the compressive bearing capacity of pre-bored grouted planted (PGP) pile with enlarged grout base focusing on its base bearing capacity. The bi-directional O-cell load test was conducted to evaluate the behavior of full scale PGP piles. The test results show that the pile head displacements needed to fully mobilize the shaft resistance were 5.9% and 6.4% D (D is pile diameter), respectively, of two test piles, owing to the large elastic shortening of pile shaft. Furthermore, the results demonstrated that the PHC nodular pile base and grout body at the enlarged base could act as a unit in the loading process, and the enlarged grout base could effectively promote the base bearing capacity of PGP pile through increasing the base area. The normalized base resistances (unit base resistance/average cone base resistance) of two test piles were 0.17 and 0.19, respectively, when the base displacement reached 5% D_b (D_b is pile base diameter). The permeation of grout into the silty sand layer under pile base increased the elastic modulus of silty sand, which could help to decrease pile head displacement under working load.

     

On the Study of Pile Driving Formula Based on Blow Counts

Accuracy of predicting pile capacities by pile driving formulas have been investigated. Five test piles were driven up to a depth of about 9 m of clay deposit and the penetrations due to final blows were recorded. The pile bearing capacity of each pile was predicted using 6 different pile driving formulas and the predicted pile capacity was compared with measured pile capacity from the pull up tests. Hiley formula, Modified Engineering News Record (ENR) formula, Janbu formula, Dutch formula, Danish formula, and Gates formula were used. The performance and accuracy of each formula was evaluated and the correlation coefficient of each pile driving formula was determined for a more accurate pile capacity prediction. Methods used to evaluate the performance of each formula were; (1) the best fit line for Q _p versus Q _m (2) cumulative probability for Q _p/Q _m and (3) the arithmetic mean and standard deviation for Q _p/Q _m. From the study, it was found that using Dutch formula provided the most accurate pile capacity estimate compared to the other formulas with an average of 7% deviation from value obtained from the field pull up test. It was followed by the Danish formula, Janbu formula, Hiley formula, Modified ENR formula, and Gates formula. The ability to predict the accuracy of estimating pile capacity using an appropriate method is very important and valuable to contractors, developers, geotechnical engineers, and manufacturers.     

Response of rigid piles during passive dragging

This paper develops a three‐layer model and elastic solutions to capture nonlinear response of rigid, passive piles in sliding soil. Elastic solutions are obtained for an equivalent force per unit length p_s of the soil movement. They are repeated for a series of linearly increasing p_s (with depth) to yield the nonlinear response. The parameters underpinning the model are determined against pertinent numerical solutions and model tests on passive free‐head and capped piles. The solutions are presented in non‐dimensional charts and elaborated through three examples. The study reveals the following:

• On‐pile pressure in rotationally restrained, sliding layer reduces by a factor α, which resembles the p‐multiplier for a laterally loaded, capped pile, but for its increase with vertical loading (embankment surcharge), and stiffness of underlying stiff layer: α = 0.25 and 0.6 for a shallow, translating and rotating piles, respectively; α = 0.33–0.5 and 0.8–1.3 for a slide overlying a stiff layer concerning a uniform and a linearly increasing pressure, respectively; and α = 0.5–0.72 for moving clay under embankment loading.
• Ultimate state is well defined using the ratio of passive earth pressure coefficient over that of active earth pressure. The subgrade modulus for a large soil movement may be scaled from model tests.
• The normalised rotational stiffness is equal to 0.1–0.15 for the capped piles, which increases the pile displacement with depth.

The three‐layer model solutions well predict nonlinear response of capped piles subjected to passive loading, which may be used for pertinent design. Copyright © 2016 John Wiley & Sons, Ltd.     

Assessment of direct CPT and CPTU methods for predicting the ultimate bearing capacity of single piles

Twelve methods to determine axial pile capacity directly based on cone penetration test (CPT) and piezocone penetration test (CPTU) data are presented, compared and evaluated. Analyses and evaluation were conducted on three types of piles of different size and length. All the tested piles have failed at the end of static load test. Both the CPT methods and the CPTU methods were used to estimate the load bearing capacities of the investigated piles (Q_p). The static load test was performed to determine the measured load bearing capacities (Q_m). The pile capacities determined through different methods were compared with the measured values obtained from the static load tests. Two criteria were selected as bases of evaluation: the best fit line for Q_p versus Q_m and the arithmetic mean and standard deviation for the ratio Q_p/Q_m. Results of the analyses showed that the best methods for determining pile capacity are the two CPTU methods. Furthermore, the CPTU method is simple, easy to apply, and not influenced by the subjective judgements of operating staff. Therefore, it is quite suitable for the application in pile engineering practice.     

Load-settlement behaviour of socketed piles in sandstone

Nine Static vertical pile loading tests were carried out in Abu-Dhabi, the United Arab Emirates in order to assess the end bearing capacity and settlement of piles bored in slightly silty sands overlying soft rocky horizons like sandstone and calcarenite. The paper is aimed at presenting the results of interpretation of the pile tests in order to estimate the end bearing capacity in correlation with the unconfined compressive strength R _c and the rock quality designation (RQD). Load-settlement behaviour is also studied through an analysis by the elastic methods commonly used in pile design.     

Experimental investigation of the behavior of pile groups in sand under different loading rates

The behavior of pile groups in sand under different loading rates is investigated. A total of 60 tests were conducted in the laboratory using model steel piles embedded in a medium dense sand. The model piles have an outside diameter of 25 mm and embedment length of 500 mm. Five different configurations of pile groups (2 × 1, 3 × 1, 2 × 2, 2 × 3, 3 × 3) with center to center spacing between the piles of 3d, 6d and 9d (d is the pile diameter) were tested. The piles were subjected to axial compressive loads under four different loading rates: 1.0, 0.5, 0.1 and 0.05 mm/min. Test results indicated that the axial compressive capacity of pile group increases with the loading rate such that the pile capacity versus logarithm of loading rate data plot approximately along a straight line. The slope of this line increases as the number of piles in a group increases and it decreases by increasing the spacing between piles in a group.     

黏土中打入桩竖向承载力计算方法效果评价

准确预测打入桩的承载特性是一个重要的工程课题,特别是在近海工程中,如大型跨江海桥梁、风力涡轮机、港口和近海工程结构物平台等,这些重要结构物都将打入桩作为主要的基础形式,然而通过现场静载试验确定这类桩的承载力却十分有限。目前,国内外存在众多关于黏土中打入桩的竖向承载力计算方法,其中较为常用的有Furgo-96、NGI-99、ICP-05和UWA-13这4种设计方法。但将这些设计方法直接应用于工程设计和实践却仍然较少,其主要原因在于缺乏高质量的静载试验数据来评价这些设计方法的可靠性。因此,有必要搜集一个具有广泛代表性的桩基承载力静载试验数据库来客观评价上述设计方法的可靠性,用以帮助工程设计人员选择最适合的设计方法。基于这个目的,本研究在国际上一些常用数据库的基础上进行扩充整理,最终得到一个包含89根高质量打入桩静载荷试验数据的新数据库,称之为ZJU-ICL Clay数据库。采用上述4种设计方法预测ZJU-ICL Clay数据库的桩基承载力,并将预测结果与实测数据进行对比,应用统计方法,客观评价这4种方法的优缺点及可靠性。     

Effect of Arching on Uplift Capacity of Single Piles

An analytical method has been proposed to predict the ultimate uplift capacity of single vertical piles embedded in sand considering arching effect. The present analysis takes into consideration of various pile and soil parameters such as length (L), diameter (d) of the pile, angle of internal friction of soil (ϕ), soil pile friction angle (δ) and unit weight of soil (γ). A modified value of coefficient of lateral earth pressure in uplift has been developed considering the arching effect of soil. A comparative assessment of the uplift capacity of piles predicted by using proposed theory and the existing available theories is made with the existing field and model test results. It has been observed that the present model considering the arching effect predicts the results closer.     

Reliability analysis for bearing capacity of surface strip footing using fuzzy finite element method

ABSTRACT

This paper presents the reliability analysis on the basis of the foundation failure against bearing capacity using the concept of fuzzy set theory. A surface strip footing is considered for the analysis and the bearing capacity is estimated using the conventional Finite Element Method (FEM). The spatial variability of the variables is taken into consideration to capture the physical randomness of the soil parameters for an isotropic field. A variation of the probability of failure (P_f) against a varying limiting applied pressure (q) is presented for different Coefficient of Variation (COV) of the variables and different scale of fluctuation (θ). The results reveal that the friction angle of soil (?) is the most influencing parameter among the other variables. Further, the influence of the scale of fluctuation (θ) on the probability of failure (P_f) is also examined. It is observed that for a particular COV of ?, higher value of θ predicts higher P_f whereas, P_f increases as COV of ? increases for a particular θ value. Later, a comparison study is accomplished to verify the viability of the present method and it can be noticed that the present method compares well with the other reliability method (First Order Reliability Method) to a reasonably good extent.     

Computerised processing of ground investigation data and use in predicting the load-settlement behaviour of piled foundations

A new computer program “PILESET” is developed for use in predicting the bearing capacity and load-settlement behaviour of axially loaded single piles. The program can analyse almost any soil profile and accommodates (a) displacement piles (b) replacement (c) friction piles, (d) end-bearing piles, (e) under-reamed piles and (f) partially sleeved piles. A variety of soil input data can be used, including: (i) standard penetration tests, (ii) cone/piezo-cone tests, (iii) pressure-meter tests and (iv) laboratory tests. The above data types can be combined, if desired, for pile analysis by PILESET. The program calculates the shaft and base capacities of a pile based on 23 methods published in design guides in over 10 European countries. PILESET also predicts the pile load-settlement curve using five published methods, which include two modified load transfer (t-z) approaches formulated by the author. To demonstrate the capabilities of the program, analysis is carried out for case study involving seven full-scale screw piles formed in sand and tested to failure. In each case, the load-settlement curve computed using the author’s modified method in PILESET is found to be in excellent agreement with the actual pile test results.     

Statistical Evaluation of CPT and CPTu Based Methods for Prediction of Axial Bearing Capacity of Piles

Piles are structural members made of steel, concrete, or wood installed into the ground to transfer superstructure loads to the soil. Nowadays, many structures are built on poor lands, and therefore piles have crucial roles in such structures. Performing in-situ tests such as cone penetration (CPT) and piezocone penetration tests (CPTu) have always been of great importance in designing piles. These tests have a brilliant consistency with reality, and as a result, the outcome data can be used in order to achieve reliable pile designing models and reduce uncertainty in this regard. In this paper, the capability of various CPT and CPTu based methods developed from 1961 to 2016 has been investigated using four statistical methods. Such CPT and CPTu based methods are adopted for direct prediction of axial bearing capacity of piles using CPT and CPTu field data. For this purpose, 61 sets of field data prepared from CPT and CPTu have been collected. The data sets were utilized in order to calculate the axial bearing capacity of piles (Q_E) through 25 different methods. In addition, the measured axial pile capacities (Q_M) have been collected, recorded and prepared from field static load tests, respectively. Then, four different statistical approaches have been applied to assess the accuracy of these methods. Finally, the most reliable and accurate methods are presented.

     

Interpretation of load tests on bored piles in the city of Matera

This paper briefly reviews several calculation methods to evaluate the bearing capacity expressed in terms of undrained strength (c_u) of piles bored in clay for their entire length and of piles whose tip is embedded into weak rock. The scope of the paper is to compare the results obtained with those from full-scale pile tests. These tests were carried out within the city of Matera which is well studied from a geotechnical point of view and for which there are statistically significant data on the geomechanical properties of the Subapennine Blue clays and the underlying Gravina Calcarenites. This represents the first attempt to show, on the basis of laboratory and field data, the influence of variability of the above mentioned soils and rocks on the real behaviour of deep foundations.For piles completely bored into Matera clay, the calculation in terms of total stress are able to interpret sufficiently well the bearing capacity of the piles. For piles having their toe embedded in calcarenite, the variability of the strength of the weak rock presents greater uncertainties in the calculation of base and soft resistance.     

Seismic Response Analysis of Pile Foundations

A quasi-3D continuum method is presented for the dynamic nonlinear effective stress analysis of pile foundation under earthquake excitation. The method was validated using data from centrifuge tests on single piles and pile groups in liquefiable soils conducted at the University of California at Davis. Some results from this validation studies are presented. The API approach to pile response using p–y curves was evaluated using the quasi-3D method and the results from simulated earthquake tests on a model pile in a centrifuge. The recommended API stiffnesses appear to be much too high for seismic response analysis under strong shaking, but give very good estimates of elastic response.     

Nonlinear Response of Single Piles in Sand Subjected to Lateral Loads using k hmax Approach

This paper presents results of analysis of full-scale pile load test data of 14 piles embedded in either loose or medium dense sands. The analysis was performed using two methods, p–y curve approach and a more recently developed k_hmax approach. Comparison of the results obtained using both the methods is also presented. A step-by-step analysis procedure is presented for predicting lateral load deflection response of single piles in sand using the k_hmax approach. The results presented show that the k_hmax approach has promise over the p–y curve approach because of its simplicity and the fact that it provides upper- and lower-bound curves, which are valuable guides to making engineering decisions. For loose sands, a new range of k_hmax values is recommended to better predict the lateral load–deflection response of single piles.     

Piles shaft capacity from CPT and CPTu data by polynomial neural networks and genetic algorithms

Cone penetration test (CPT) is one of the most common in situ tests which is used for pile design because it can be realized as a model pile. The measured cone resistance (q_c) and sleeve friction (f_s) usually are employed for estimation of pile unit toe and shaft resistances, respectively. Thirty three pile case histories have been compiled including static loading tests performed in uplift, or in push with separation of shaft and toe resistances at sites which comprise CPT or CPTu sounding. Group method of data handling (GMDH) type neural networks optimized using genetic algorithms (GAs) are used to model the effects of effective cone point resistance (q_E) and cone sleeve friction (f_s) as input parameters on pile unit shaft resistance, applying some experimentally obtained training and test data. Sensitivity analysis of the obtained model has been carried out to study the influence of input parameters on model output. Some graphs have been derived from sensitivity analysis to estimate pile unit shaft resistance based on q_E and f_s. The performance of the proposed method has been compared with the other CPT and CPTu direct methods and referenced to measured piles shaft capacity. The results demonstrate that appreciable improvement in prediction of pile shaft capacity has been achieved.     

An experimental study on static and dynamic bending behaviour of piles in soft clay

Static and dynamic lateral load tests were carried out on model aluminium single piles embedded in soft clay to study its bending behaviour. Model aluminium piles with length to diameter ratios of 10, 20, 30 and 40 were used. Static lateral load tests were conducted on piles by rope and pulley arrangement upto failure and load–deflection curves were obtained. Dynamic lateral load tests were carried out for different magnitudes of load ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a Data Acquisition System. Safe static lateral load capacity for all piles is interpreted from load–deflection curves. Dynamic characteristics of the soil–pile system were arrived from the acquired experimental data. The soil–pile system behaves predominantly in nonlinear fashion even at low frequency under dynamic load. The displacement amplitude under dynamic load is magnified by 4.5–6.5 times the static deflection for all piles embedded in soft clay. But, the peak magnification factor reduces with an increase in the magnitude of lateral load mainly because of increase of hysteretic damping at very soft consistency. The maximum BM occurs at the fundamental frequency of the soil–pile system. Even the lower part of the pile affects the pile head response to the inertial load applied at the pile head. The maximum dynamic BM is magnified by about 1.5 times the maximum static BM for model piles in tested consistency of clay. The maximum dynamic BM occurs at a depth of about 1.5 times the depth of maximum static BM for model piles, which indicates an increase of active pile length under dynamic load.