Instrumented Load Tests Research Articles

Numerical analysis of piled rafts with short bored piles

Abstract Numerical analysis of the behavior of piled rafts seated on uniform soil with low bearing capacity has been important to support the project design. The calibration of the numerical model with instrumented load test allows usage of the results, in given conditions, in the foundation design of small and medium-sized buildings built on soils with the same mechanical properties analyzed in this paper. In this context, this paper evaluates numerical models, which allow to consider the influence of short bored piles connected to the raft. The models are also assessed using piled rafts concepts. From the results obtained, the load supported by the total shaft resistance is significant for the case of bored piles, and this load is from 14% to 30% higher in the case more flexible or thinner raft. Inserting piles in the rafts reduces settlement and increases overall stiffness. Such effects are amplified in piles with higher slenderness ratios (L/d).

Instrumented load tests and layered elastic theory analysis of a large-scale EPS block embankment

This paper presents the behavioral results of a large-scale experimental EPS block embankment under different test and load conditions. The experimental embankment was designed and constructed to be similar to a road structure of flexible pavement and 22 kg/m3 EPS geofoam blocks supported by a soft soil subgrade. The experimental embankment was subjected to the Benkelman beam test and a load test using a 50,000 kg six-axle tractor semitrailer, and was instrumented with total pressure cells (TPC) to verify the vertical stresses propagated from the surface to the depth. Complementary analyses of both tests were performed by the layered elastic theory (LET). The results of the tests and analyses in the LET showed that using a correctly designed flexible pavement over the EPS blocks generates a structure with adequate performance. As well as in some previous studies, the elastic parameters determined in small samples of the EPS blocks tested in laboratory did not correspond to the behavior observed in field, where large blocks submitted to reduced deformations levels are employed. The conclusion was that LET can be used simply and conveniently as an adequate tool for pavement analysis with the presence of EPS blocks in its structure.

Closure to “Development of Axial Pile Load Transfer Curves Based on Instrumented Load Tests” by Cécilia Bohn, Alexandre Lopes dos Santos, and Roger Frank

The critical reading and discussion by the discusser is appreciated. It shows the interest of the geotechnical community on this subject and gives the technical paper an additional perspective. One comment from the discusser relates to an obvious mistake having been made in the paper: the load test results shown in Figs. 7(a and b) in the paper “Main results of an instrumented load test with removable extensometer” do not match with those shown in Fig. 7(c). This is because they have not been taken from the same load test results, and the writers wish to apologize for any misunderstanding. In fact, the goal of Fig. 7 was merely to show the principle of an instrumented load test and the general shape of the load-displacement and load-depth curves. For sure, the displacement at the shaft on Fig. 7(c), if taken from the same load test results should be somewhere between the head displacement and the tip displacement.

Discussion of “Development of Axial Pile Load Transfer Curves Based on Instrumented Load Tests” by Cécilia Bohn, Alexandre Lopes dos Santos, and Roger Frank

Discussion of “Development of Axial Pile Load Transfer Curves Based on Instrumented Load Tests” by Cécilia Bohn, Alexandre Lopes dos Santos, and Roger Frank

The application of continuous surface wave testing for settlement analysis with reference to a full-scale load test for a bridge at Pont Melin, Wales, UK

The presence of a significant thickness of soft silts beneath the site of a proposed new road bridge caused concerns over the suitability of shallow foundations. Despite extensive conventional ground investigations, it was not possible to obtain reliable stiffness data, and so a full-scale, fully instrumented load test was undertaken to better determine ground stiffness. The data obtained was used in conjunction with sophisticated numerical analysis to demonstrate that the settlement of shallow foundations would be acceptable. Following completion of the bridge construction, Continuous Surface Wave testing was undertaken and a range of simple non-linear elastic settlement analyses conducted to model the behaviour of the load test. Through these analyses this paper demonstrates the benefit of using continuous surface wave ground stiffness testing in conjunction with simple analysis methods. It underlines the importance of obtaining high-quality stiffness data to accurately predict settlement of shallow foundations.

Development of Axial Pile Load Transfer Curves Based on Instrumented Load Tests

The load transfer method is an efficient and practical tool for the analysis of single piles and combined systems such as pile-raft foundations and rigid inclusions. The existing transfer curves rely either on the measurement of the pressuremeter modulus, which is not widespread at the international level, or they have been developed only for specific pile and ground types. In order to develop new transfer curves valid for all pile and ground types without the need of pressuremeter tests, a database of 50 instrumented pile load tests was examined. The existing curves were first compared to the measurements at the pile shaft and at the tip. Then the parameters of the most appropriate curve types, the cubic root and the hyperbolic curve, were calibrated in order to best match the measurements. A single set of parameters for each proposed curve type is suitable enough for most pile and ground types. The good agreement of the proposed curves was confirmed by applying them to assess the overall load-settlement curve of 72 other pile load tests.

Field load tests on plastic tube cast-in-place concrete piles

An instrumented axial load test programme was conducted to study the load transfer behaviour of plastic tube cast-in-place concrete piles (TC piles), which are increasingly being used for support of embankments over soft ground in China. Additionally, in order to enhance the bearing properties of TC piles, the effects of shaft pre-grouting and increasing the diameter of the pile were investigated through load tests. Grouting was found to increase the capacity by 8–20%, while settlement decreased by 20–36%. Additionally, increasing the diameter from 16 to 20 cm resulted in a 10–17% decrease in capacity, due to the accompanying decrease in pile length to accommodate the capabilities of the available construction equipment. Also, increasing the diameter increased settlement by 14–33%. The effect of the pile set-up was investigated, and the capacity was increased by 20%, 8% and 11% for TC piles, gravity-grouted piles and enlarged-diameter piles for axial load tests conducted after 105 d relative to tests conducted after 24 d. In addition, three calculation models of single-pile settlement were evaluated with respect to predictions made of the bearing capacity and settlement.

Erratum for “Discussion of ‘Instrumented Static Load Test on Rock-Socketed Micropile’ by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado”

Erratum for “Discussion of ‘Instrumented Static Load Test on Rock-Socketed Micropile’ by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado”

Piled raft with hollow auger piles founded in a Brazilian granular deposit

Geotechnical projects typically achieve load transfer to the ground using shallow or deep foundations. The conventional design approach does not provide for the combination of these two types of foundation. The piled raft philosophy allows the association of the soil elements, raft, and piles to obtain technical and economic advantages over conventional design. The city of João Pessoa, in northeastern Brazil, has developed foundation practices with hollow auger piles in piled raft design. The coastal area of the city has topsoil layers with favorable conditions for using such a technique. This paper addresses the results of a research project with instrumented load tests on foundation systems of hollow auger piles and a piled raft. The analysis is based on the load–settlement curve through extrapolation criteria. The Poulos–Davis–Randolph (PDR) method is applied according to a trilinear and hyperbolic approach to simulate the load–settlement curve of piled rafts. The results indicate that the raft absorbs most of the load, and the raft–soil contact significantly increases the load capacity of the foundation. The PDR hyperbolic method could apply to practical use in the foundations of the region, as it allows a more detailed assessment of the behavior of the foundation and can forecast the behavior of the (locally nontraditional) piled raft foundation system.

Composite Properties from Instrumented Load Tests on Mini-Piers Reinforced With Geotextiles

Abstract Four pairs of large-scale instrumented geosynthetic reinforced soil (GRS) square columns were load tested to study the effects of varying reinforcement strength to spacing ratio, to discern the lateral pressures during construction and during load testing, and to derive shear strength parameters of the GRS composite. Each pair was identical in every respect, except one was loaded with a dry-stacked concrete masonry unit (CMU) facing in place and the other without. Lateral pressures during construction were found to be small for the facing type used in this study. Also, based on the derived GRS composite shear strength parameters, it was found that (1) the GRS composite Mohr–Coulomb envelopes are not parallel to those for the unreinforced soil; (2) the reinforcement increased the composite cohesion compared to the unreinforced soil (cohesion increases with decreasing spacing and increasing reinforcement strength); (3) the composite friction angle is less than that of the unreinforced soil (friction angle increases with decreasing reinforcement strength and increasing spacing); (4) as the composite friction angle increases, the active lateral earth pressure coefficient decreases; and (5) the benefits of reinforcing a soil become increasingly significant as the reinforcement spacing decreases.

Closure to “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Closure to “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Discussion of “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Fig. 9 of Seo et al. (2013) shows load reaching at the tip of the micropile, but from the text, it appears that loads were calculated using readings of strain gauge (SG1) located 177 mm above the tip. The 177-mm-long segment of the pile above the tip also develops side frictional resistance; if that is deducted from the measured load at SG1, then it can be shown that the tip load is almost zero at all load increments. At the final load increment, the test load is about 3,600 kN, which is equal to 78% of the pile structural capacity of 4,600 kN. The easiest way of determining load at the tip is to extend the load transfer lines between strain gauges SG2 and SG1 to the tip of the pile. For brevity, the strain gauge located at Location 1 in Table 5 of the original article has been termed as SG1 and Location 2 as SG2, and so on. For example, Osterberg Cell is placed on a 150–300-mm-thick concrete bearing pad, which rests on the base of the hole. The side friction developed around the bearing pad is deducted from O-cell readings to determine tip load (Schmertmann et al. 2001). Nearly zero tip loads for all the load increments when accompanied with a significant amount of tip settlement shall indicate that the tip resistance is negligible because of the presence of soft sediments between the tip and the base of the hole.

Discussion of “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Discussion of “Instrumented Static Load Test on Rock-Socketed Micropile” by Hoyoung Seo, Monica Prezzi, and Rodrigo Salgado

Determination of soil properties from standard penetration test complemented by torque measurement (SPT-T)

The major problem on geotechnical work is to ensure that no settlements occur during the life cycle of the construction. This involves proper design of foundations and their bearing capacity. The Brazilian standard for design and execution of foundations, ABNT (2010) NBR 6122 imposes the utilization of field tests when designing building foundations. The Standard Penetration Test, SPT, ABNT (2001) NBR 6484, is still the most common in-situ test for those purposes. Ranzini (1988) suggested supplementing the conventional SPT with the measurement of the torque (SPT-T) required to turn the split spoon after driving, in order to provide a 'static' component to a 'dynamic' test. The adhesion between the soil and the sampler, obtained by the torque measurement, could be used to calculate the lateral skin friction of piles. This paper describes the SPT-T procedure including both a supplementary equipment and practical aspects. Also it presents an accurate torque measurement, a prediction method to calculate the bearing capacity of piles used in building foundations using the SPT-T test and a comparison between the estimated bearing capacities of building foundations with instrumented load tests in order to validate the method.

Discussion of an instrumented screwpile load test and connected pile-group load-settlement behaviour. - Authors' reply to the discussion by Fellenius, B.H.

Authors wish to thank the Discusser for his significant effort in analysing the case history and providing a very clear review of the paper. Authors agree with the analysis provided in the review but wish to clarify some points. Authors agree that a pile capacity of 3000 kN would have allowed for a significant reduction of the number of piles in the system. However, the mentioned pile capacity of 3000 kN is linked to the CPT result at the location of the test pile. The actual foundation design was based on a number of CPT results per tank foundation. According to the Belgian NAD of the Eurocode 7, pile capacity is calculated for all available (relevant) CPT results. Within this group of results, the determining value is the lower of the average value and the minimum value, each corrected with their own specific correlation factor. In this specific case, pile capacity was determined by the minimum value (linked to the most critical CPT), which lead to the service load of 780 kN. The mentioned drag force of 180 kN was also based on the relevant CPT result. Settlement calculations were done based on the stiffness parameters as mentioned in table 1 of the paper. The unconstrained modulus of the underlying OC clay was taken as 17 MPa + 1.87 MPa/m (starting at 24 m depth). The resulting deformations are slightly lower than those obtained by the Discusser, leading to a settlement difference between the tank center and perimeter of about 30 to 40 mm. As inspection of the tanks after the hydrotest has indicated that no significant local bulging is to be expected in the bottom plate, authors believe that the edge-to-center distortion for the tanks is well within allowable limits. Authors indeed have planned to perform long-term monitoring, consisting of settlement measurements along the edge of the tanks. Additionally, deep CPTu’s with dissipation tests will be performed in order to better determine the relevant stiffnessand consolidation parameters of the underlying clay layer, which will allow a re-assessment of the long term deformations.

Reply to the discussion by Fellenius on “Load tests on full-scale bored pile groups”

Reply to the discussion by Fellenius on “Load tests on full-scale bored pile groups”

Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses

This paper is the second of a two-part series describing an investigation of the mechanical behavior of rammed aggregate piers in supporting rigid square footings. In this paper, the performances of two pier-supported footings and three isolated piers during compressive load tests were simulated using an axisymmetric finite element model and compared to experimental data. A hardening-soil constitutive model with parameters estimated from in situ and laboratory tests was used to characterize the constitutive behaviors of the pier material and the matrix soil. Pier groups were modeled as unit cells with the tributary area determined from the center-to-center spacing. Cavity expansion modeling was used to simulate the pier installation process. Verifications of the numerical model were carried out by comparing the numerical results with the data obtained from full-scale, instrumented load tests. Interpretation of the numerical results focused on the load-deformation behavior, group effect, stress concentration ratio, and the development of stresses in the matrix soil. The distributions of vertical stress underneath the pier-supported footings are also characterized.

Support Mechanisms of Rammed Aggregate Piers. I: Experimental Results

This paper is the first of a two-part series investigating the mechanical behavior of rammed aggregate pier (RAP) groups supporting isolated rigid footings. The first paper presents the experimental test results from instrumented load tests performed on two different 2.3 m square reinforced concrete footings supported by four 0.76 m diameter RAPs of two different pier lengths—2.8 and 5.1 m . Comparisons are made to load tests performed on three isolated RAPs of the same diameter and lengths. Instrumentation consisted of total stress cells, inclinometers, and tell-tale reference plates. Soil conditions at the test site were evaluated using various in situ testing techniques and consist of relatively uniform soft alluvial clay overlain by a 1-m-thick desiccated layer. Interpretations of the test results focused on load-deformation behaviors of the isolated piers and pier groups, group efficiencies in terms of settlement and bearing capacity, stress concentrations as a function of applied load at the top of ...

Case History: Capacity of a Drilled Shaft in the Atlantic Coastal Plain

This paper presents a single case history of a drilled shaft constructed in the Atlantic Coastal Plain deposits for a bridge foundation that was subjected to axial loading. The predicted nominal axial capacity is estimated based on state of practice empirically derived methods specified in the current AASHTO LRFD Bridge Design Specifications. Predictions are compared to observed soil resistance derived from a static load test conducted on a full-size instrumented test shaft using the Osterberg Cell method. The results suggest that the AASHTO specified prediction methods should be applied cautiously for drilled shafts in the Atlantic Coastal Plain, incorporating an appropriate in situ testing program for evaluating soil design parameters, considering variations from the specific geologic environment and construction methodology used to develop the specified prediction methods, accounting for the load-deformation behavior of the shaft, and providing for instrumented static load testing to measure the actual behavior of the drilled shafts.

Instrumented load tests in mudstone: pile capacity and settlement prediction

The present work stems from the design of a viaduct in South Wales, U.K., where full-scale pile testing was carried out to assess whether the proposed design methods would meet the required load capacity and settlement criteria for the working piles. Five fully instrumented large diameter bored cast in situ piles, up to 30 m deep, were installed in weathered mudstone and tested under vertical loading. A sixth pile, which had no shaft instrumentation, was formed with a voided toe. In conjunction with vast soil data from 218 site investigation boreholes, the extensive data produced from the load tests were analyzed to quantify the key parameters considered to influence load transfer and settlement behaviour. Each pile was first calibrated using four methods to establish the as-built stiffness, taking into account the nonlinearity of concrete and the effect of partial steel encasement. It is demonstrated that the current national norms for bored pile design in cohesive soil – soft rock are overconservative for South Wales ground conditions. To ameliorate this, alternative methods are proposed, which lead to improved reliability and accuracy in shaft and base capacity assessment. In addition, a numerical model is developed that can be used to predict the complete load-settlement variation up to the ultimate state. The model is sufficiently expounded to allow its immediate application in pile design by geotechnical engineers.Key words: piled foundations, load tests, bearing capacity and settlement, Mercia mudstone.