Downdrag Forces Research Articles

Experimental evaluation of geosynthetic-modular block connection loads

The design of segmental geosynthetic mechanically stabilized walls with masonry block facing is often governed by the loads that develop at the connection between the facing and geosynthetic. Yet the current understanding of the mechanisms involved in the mobilization of such connection loads is, at best, incomplete. The testing apparatus developed as part of this study facilitates simulating the transference of stresses at the face and evaluating the facing connection loads in geosynthetic-reinforced soil walls. This study assesses the connection loads between a geogrid reinforcement connected frictionally to modular concrete blocks. A comprehensive instrumentation program was implemented to capture lateral earth pressures, geosynthetic strains and loads acting at the geogrid-block connection in a geosynthetic-reinforced unit cell subjected to incremental surcharge stages. Results indicate that conventional calculations, based on earth pressure theory, may underestimate the facing connection loads, mainly when the connection loads are triggered by the differential settlement of the backfill relative to the block facing. When this mechanism dominates the mobilization at the connection, reinforcement loads increase as the differential settlement increases, developing down-drag forces at the connection between the geogrid and modular blocks.

Analytical study of piles behavior for marine challenging substructures

Based on a new categorization, significancy and feasibility of vector-acting foundations for offshore challenging substructures are emphasized. A marine challenging status is typically related to loading, investigations, subsurface, environment, construction and sustainability requirements. The issue of the interaction of piles and complex subsoil is discussed in this paper. Three scenarios are studied comprising sensitive deposits at the full depth, the most impressive one, compressible deposits at the top layer and soft soil underlain by strong deposits. The role of CPTu records is addressed in recognition of problematic layers, geotechnical design and embedment depth designation. The effect of soil sensitivity on shaft and toe resistance is quantified through static analysis and reduction factors are suggested. Since bearing capacity, settlement and axial load are interactive, the neutral plane concept is applied, unifying the mentioned aspects. Through the neutral plane approach and considering the downdrag force due to the collapse of sensitive soil, primary design criteria incorporating sensitivity are discussed and quantified. Applying ground modification as a supplement for deep foundations is realized through two case studies, leading to enhancement in geotechnical performance. Overall, the mentioned phenomena must be discovered comprehensively in extraordinary conditions, aiming for a safe, reliable, knowledge-based and optimum design.

Experimental and Numerical Analysis of the performance of pile group subjected to downdrag force before loading

Experimental and Numerical Analysis of the performance of pile group subjected to downdrag force before loading

Measurement of pile uplift forces due to soil heave in expansive clays

Piled foundations are widely used to limit the movement of foundations in expansive clays. These piles are subjected to large uplift forces as the clay swells (and downdrag forces when it shrinks). The appropriate method to estimate these forces is not well understood, and estimates from various methods presented in literature result in large variations. The uplift force generated in piles by soil heave was derived from strain measurements in full-scale field tests where a pile was installed in a highly expansive soil profile and flooded for several months. The results were compared to available theoretical estimates to comment on the most appropriate procedure to estimate the generated tension in the pile. The results showed that the use of an elastic solution related to the expected soil heave combined with a limiting shaft friction estimated from total stress capacity methods provided the most appropriate match to the measured results both in terms of the magnitude of the developed tension in the pile and shape of the tension profile along the depth of the pile.

Analytical framework for prediction of facing connection loads in reinforced soil walls considering reinforcement downdrag

Several reinforced soil wall failures were reported to have occurred at facing connections with reinforcing layers. These failures were generally attributed to reinforcement downdrag, which accompanies differential settlements between facings and backfills in reinforced soil walls. Overlooking reinforcement downdrag forces in design can result in underestimation of reinforcement loads at facing connections, which are an evident cause of several wall failures due to inadequacy of design and/or construction protocols. This study presents an analytical framework developed based on an analogy between reinforcement downdrag effect on facings of reinforced soil walls and soil friction effect on interfaces of rigid retaining walls. Specifically, the developed framework provides predictions of soil stresses and reinforcement loads at facings of reinforced soil walls that can be used in design to account for reinforcement downdrag. It was concluded that model predictions of horizontal earth pressure magnitudes at the facings of three reinforced soil walls matched well with field measurements. Finally, a peripheral analytical method was developed to predict differential settlements between facings and backfills in reinforced soil walls.

Recent subsidence rates for Barataria Basin, Louisiana

Subsidence in south Louisiana is a fundamental process and major source of uncertainty influencing restoration planning and design and wetland habitat resilience. The primary goal of this study was to conduct an analysis of recent subsidence rates within the Barataria Basin. High-resolution geodetic GPS elevation measurements at 19 benchmarks were used to determine recent subsidence rates for the Barataria Basin. Net elevation changes at all stations equaled or exceeded predicted measurement uncertainties based on session duration. Water elevation change from two gauges in the southern part of the basin supplemented survey data, resulting in a range of subsidence velocities at 21 locations from about 2 to 7 mm/year. These data record short-term subsidence trends (6- to 16-year time series) that are expected to be representative of conditions at proposed restoration sites over the next 20 to 50 years. Foundation/rod depths for benchmarks ranged from near surface to about 30-m deep. None of these foundations/rods were sleeved or anchored and thus were not isolated from surrounding consolidating sediment and associated downdrag forces. Therefore, subsidence rates at these sites were considered representative for the entire sediment column. Subsidence rate comparisons for nearby deep and shallow rod benchmarks support this conclusion. Spatial variability in subsidence indicates a compelling relationship between subsidence rates and the age, composition, and thickness of Holocene deltaic deposits. The Coastal Protection and Restoration Authority of Louisiana intends to use this same approach to continue refining subsidence rates throughout coastal Louisiana to support coastal restoration planning and design.

Detection of Differential Settlement of Man-Made Structures Coupled with Urban Development by Using Persistent Scatterer Interferometry (PSI)

Many cities are prone to land subsidence, particularly due to the overuse of ground water. However, because man-made structures are normally built upon foundations that are stiffer than the surrounding ground, such structures react to land subsidence to a lesser extent. This settlement mismatch between ground and buildings, also known as differential settlement (DS), may cause serious problems in urban management, such as foundation overloading due to down-drag forces and damage to underground pipelines. Here, we present a technique for determining DS from multi-temporal satellite synthetic aperture radar (SAR) images. Permanent scatterers originating from ground and man-made structures are extracted using the differential interferometric SAR (DInSAR) technique, whereupon the DS is obtained by subtracting the settlement of the former from that of the latter. For validation purposes, we demonstrate that the estimated DS in Bangkok is consistent with field observations.

Static and Dynamic Stability of a Multi-stepped Timoshenko Column Including Self-weight

A simplified and easy to implement approach for the solution of the static and dynamic stability of a Timoshenko column with multiple partitions is derived in a classic and condensed manner. The proposed methodology includes a non-uniformly distributed axial load along the structural member which can represent the self-weight of columns, chimneys, tall buildings or the axial load on piles induced by downdrag forces. The proposed model includes the effects of shear deformations along the member and the second-order shear force induced by the applied axial load as the member deforms. The effects of the self-weight on the structural member stability are studied using the proposed approach and conclusions regarding the contribution of the self-weight and discrepancies by neglecting it are presented. Three comprehensive examples including columns with weakened sections, distributed axial loads arising from the self-weight, symmetrical tapered sections, and stepped cross sectional members are included to validate and show the applicability of the proposed formulation.

Discussion of “Shielding Piles from Downdrag in Consolidating Ground” by S. Y. Lam, C. W. W. Ng, and H. G. Poulos

Discussion of “Shielding Piles from Downdrag in Consolidating Ground” by S. Y. Lam, C. W. W. Ng, and H. G. Poulos

Discussion: A practical design approach for piles with negative friction

The author is grateful to Dr Bourne-Webb for his thoughtful and constructive discussion. He suggests that an alternative design criterion for control of the settlement of piles subjected to negative friction is to adopt a factor of safety with respect to shaft friction in the stable zone, rather than an overall factor of safety against combined shaft friction and end bearing. A value of 0·9 on the shaft friction within the stable zone is suggested by the discusser. He quite correctly notes that the pile shaft requires relatively small displacements for full mobilisation, thus implying that, once this friction is mobilised, the ‘softer' pile base may experience a rapid rate of increase of settlement with any increase in either applied load or induced downdrag force. The use of the discusser's criterion may lead to slightly longer piles, but this may be prudent if there is concern about the base conditions, for example with bored piles constructed under bentonite. It is difficult to argue against Dr Bourne-Webb's concept, as it is inherently more conservative than that proposed by the author. However, three points may perhaps be relevant here. (a) Suggesting a factor of safety for the shaft alone of less than unity may give rise to concern by checkers or reviewers of the design, especially in some parts of the world where there is a pervasive conservatism in the pile design process.(b) As acknowledged by the discusser, the approach may not be practical if the pile is driven to a hard stratum, with little or no penetration of the pile tip into the stratum.(c) There is often a tendency for end-bearing resistance to be underestimated, so that the real factor of safety of the pile within the stable zone may well be in excess of that used for design. A possible compromise approach may be to consider both the proposed criteria—that is, to seek a design that satisfies both of the following requirements. (a) A factor of safety against both shaft friction and end bearing in the stable zone of 1·25.(b) A factor of safety against shaft friction alone of 0·9 or greater. If a problem is encountered with respect to satisfying these criteria for truly end-bearing piles, then a settlement analysis can be carried out readily in this case. Finally, it must be borne in mind that the criteria suggested above are still a ‘shortcut' to avoid a more rigorous assessment of the requirements of serviceability. In the event of any uncertainty regarding the most appropriate shortcut approach, a proper settlement analysis should be undertaken in which both the applied load and the ground settlements are taken into account in an appropriate manner. Several of the references cited in the paper describe various approaches to such an analysis.

Treatment of soft ground by Fibredrain and highenergy impact in highway embankment construction

The economic growth of a country depends significantly on its transportation system. The highways that form part of this system sometimes pass over compressible soft soil deposits. Two major geotechnical problems associated with the construction of road embankments on these soil deposits are stability during construction and long-term settlement. If the latter problem is not addressed adequately, excessive post-construction settlement will have serious consequences for the maintenance of road pavement, in addition to the unwarranted downdrag force (negative skin friction) and lateral pressure on the piles supporting the bridge abutments and ancillary structures. To mitigate these adverse effects, deep-seated saturated clays are pre-consolidated with Fibredrain, a vertical drain manufactured with biodegradable jute and coir fibre, and surcharge, and surface deposits of granular, clayey or peaty materials are treated by high-energy impact (HEI) application to enforce settlement and improve their bearing capacity. If post-construction secondary compression is significant it can be reduced by placing a higher surcharge than required for primary consolidation. Because of their cost-effectiveness these techniques have been and are being used in several soil improvement projects in the Southeast Asian region. Details of the treatment, the properties of and design procedure for Fibredrains, and the application of HEI are discussed. L'expansion économique d'un pays est tributaire, dans une grande mesure, de son système de transport. Les grands axes qui font partie de ce système passent parfois sur des dépôts de sols boulants et compressibles. Les deux grands problèmes de nature géotechnique que comporte la construction de remblais de route sur ces dépôts de sol sont la stabilité au cours de la construction et le tassement à long terme. Si l'on ne se penche pas de façon adéquate sur ce dernier problème, un tassement excessif risque de se produire après la construction, ce qui comporterait des conséquences graves pour l'entretien de la chaussée, en plus d'une force d'entraînement vers le bas (frottement superficiel négatif) et de la pression latérale sur les piles de culée des ponts et structures auxiliaires. Pour pallier à ces phénomènes négatifs, on procède à la pré-consolidation d'argiles saturées profondes avec du Fibredrain, un drain vertical réalisé avec des fibres biodégradables de jute et de coir, et une surépaisseur, d'autre part des dépôts superficiels de matières granulaires, argileuses ou tourbeuses sont traités avec l'application d'un impact à haute énergie (HEI) pour assurer le tassement et renforcer leur résistance de portée. Si la compression secondaire post-construction est significative, il est possible de la réduire en appliquant une surépaisseur plus élevée que celle qui est nécessaire, pour la consolidation primaire. En raison de leur rentabilité, on a utilisé, et on utilise toujours, ces techniques dans des projets d'optimisation du terrain dans la région du sudest asiatique. On discute également de renseignements détaillés sur le traitement, les propriétés et la procédure de conception des Fibredrains, et l'application des impacts à haute énergie.

Influence of reinforcement parameters on the seismic response of reduced-scale reinforced soil retaining walls

The focus of the paper is on the influence of reinforcement design parameters (i.e. stiffness, length and vertical spacing) on the simulated earthquake response of reinforced soil retaining walls using reduced-scale model shaking table tests. A series of instrumented 1/6-scale model walls was constructed and dynamically loaded using a stepped-amplitude harmonic base acceleration record. The reinforcement design parameters investigated were found to have a significant effect on model response. Experimental results showed that the magnitude of accumulated facing lateral displacement under base excitation decreased with increasing reinforcement length, greater number of reinforcement layers and larger reinforcement stiffness. Significant down-drag forces at the back of the rigid facing were measured for all model configurations. However, the measured vertical load at the footing was not significantly influenced by changes in reinforcement parameters. The horizontal restrained (hinged) toe at the bottom of the rigid facing was shown to attract 30–60% of the total horizontal earth force depending on the reinforcement configuration. The magnitude and distribution of reinforcement connection loads was significantly affected by the reinforcement length, vertical spacing and stiffness. Measured footing loads, reinforcement connection loads and amplification factors are compared to values calculated using current practice in North America for geosynthetic reinforced soil walls and discrepancies between experimental results and design methods are identified. Implications of the physical test results to level of conservatism in current design practice are noted.

Protective effect of overlying geosynthetic on geomembrane liner observed from landfill field tests and inclined board laboratory experiments

Geosynthetic liner systems are generally installed in landfill sites to prevent toxic leachate from escaping into the adjoining environment by utilizing their impervious characteristics. Therefore, it is important to protect the geomembrane from being damaged or destroyed during all phases of landfilling, namely landfill construction, waste tipping and landfill closure. This paper presents firstly the observed performance of a geomembrane liner from a landfill site where the geomembrane liner was installed on the slopes of a Korean landfill; and secondly the results of an inclined board laboratory test. Two types of experiments were conducted to identify the protecting effect of the overlaying geosynthetic on the geomembrane liners. At a testing landfill site, the slope consisted of three different sub-inclines and two 2-m-wide intermediate levels. The sub-inclines were each 8 m in vertical height and their angle of inclination was 1: 1.5 (vertical: horizontal). The reported observations were made for a time period of approximately 1 year, until the landfill was filled with wastes to the top of the uppermost slope. In addition, inclined board laboratory tests were carried out. During the inclined board test, a base table is inclined slowly and steadily until the block located on the base table starts to slide, when the tension and displacements of two geosynthetics, namely the geomembrane liner and protecting geotextile, are measured. In conclusion, test results showed that the down-drag force generated by waste accumulation and sliding of upper material was to a large extent dissipated through the elongation of the protecting geosynthetic overlying the geomembrane and thus was not transferred to the geomembrane. Unless the protecting geosynthetic undergoes structural failure, this stress relaxation phenomenon continues to occur so that the magnitude of tensile force to be applied on the geomembrane remains marginal.

A Variational Solution for Downdrag Force Analysis of Pile Groups

This article presents a variational approach for the analysis of downdrag forces of pile groups undergoing soil downward movement. The theoretical load-transfer curves are employed to describe the soil load-displacement relationship. A soil with stiffness varying with depth and with stiffer end-bearing stratum at the pile toe can be easily treated using the present method. The validity of the present method is demonstrated through comparison with existing theoretical solutions and field measurements.

Field Test of Geomembranes Downdrag Force on Side Slope

If the settlement of the waste occurs in a landfill, the geomembrane will be pulled down along a side slope. Evaluation of this downdrag force should be very important for the anchorage design. Authors conducted the large scale field test and five types of geotextiles on a geomembrane, and six types of geomembranes were tested. The test results indicat geotextiles have the cooling effect of geomembrane and decreasing thermal stress. The downdrag force depends on a type of geomembrane.

Numerical modelling of negative skin friction on pile groups

Simplified methods for the analysis of socketed pile groups subject to negative skin friction are reported. In these methods, the piles are modelled using discrete elements with an axial mode of deformation. The soil behaviour is modelled using a hybrid approach in which the soil response at the individual piles is modelled using the subgrade reaction method while pile-soil-pile interaction is determined using elastic theory. The main difference between the methods lies in the manner in which pile-soil-pile interaction is determined. The accuracy of the methods is assessed by comprehensive comparisons with more rigorous theoretical solutions available. The study shows that: (a) the solutions obtained by the continuum model using Chan et al.'s solution are in close agreement with the rigorous solutions, (b) for practical pile groups, the layer model can give reasonable solutions provided the socketing is not too deep and (c) the continuum model using Mindlin's solution predicts reasonable downdrag forces but seriously under predict the pile head settlements.

Analysis of downdrag on pile groups

This Paper describes a new iterative procedure for the analysis of the downdrag forces in end-bearing pile groups. Special emphasis is placed on the rate of increase of downdrag forces with free field soil settlement. Pile–soil–pile interaction is modelled using Mindlin's elastic solution for a point load in an elastic half-space. Pile–soil slip is taken into account by limiting the shaft friction at the interface. Parametric studies were conducted to elucidate the parameters that govern the relationship between free field soil settlement and downdrag forces induced in a pile group. Results indicated that interior piles of a large pile group experience relatively small downdrag forces, especially when the pile spacing is small. The validity of the proposed method is demonstrated by back-analysing two model studies of negative skin friction on pile groups. The proposed method is also validated against the field test results of a 38 pile group. The computed results show an encouraging degree of agreement with the experimental data. L'article décrit une nouvelle méthode itérative permettant d'analyser les forces d'enforcement induit s'exerçant sur un groupe de pieux travaillant en pointe. On s'intéresse plus particuliérement à la vitesse d'accroissement de ces forces lors du tassement libre du sol. L'interaction pieu–sol–pieu est modélisée à l'aide de la solution élastique de Mindlin pour un chargement ponctuel dans un demi espace élastique. Des études paramétriques ont été réalisées pour déterminer les paramétres qui régissent la relation existant entre le tassement libre du sol et les forces d'enfoncement créées sur un groupe de pieux. Il apparaît que les pieux internes d'un groupe de pieux de grande dimension subissent des forces d'enfoncement relativement faibles, surtout lorsque les pieux sont rapprochés. La méthode proposée a été validée par une analyse en méthode inverse de deux modéles en frottement négatif, ainsi que par des essais réalisés in-situ sur un groupe de 38 pieux. Les valeurs obtenues par les calculs sont encourageantes car assez proches des données expérimentales.

Negative skin friction and the neutral plane

Current views indicate that negative skin friction on piles can be mobilized at small relative deformations and should be considered in all designs, primarily for serviceability conditions. An elastic-plastic closed-form solution is presented that permits an estimate of down-drag forces and the location of the neutral plane. It is shown that the conventional rigid-plastic solution may overestimate down-drag forces by as much as 50% and may also overestimate the depth of the neutral plane. Key words : piles, negative skin friction, neutral plane, capacity.

Simplified approach for prediction of downdrag forces on piles in layered inhomogeneous profiles : Mohamad, R Proc 2nd International Conference on Foundations and Tunnels, London, 19–21 September 1989V1, P155–159. Publ Edinburgh: Engineering Technics Press, 1989

Simplified approach for prediction of downdrag forces on piles in layered inhomogeneous profiles : Mohamad, R Proc 2nd International Conference on Foundations and Tunnels, London, 19–21 September 1989V1, P155–159. Publ Edinburgh: Engineering Technics Press, 1989

Downdrag and axial load on piles : Kog, Y C Ground EngngV23, N3, April 1990, P24–30

This paper establishes the governing criteria to account for downdrag in the design of piled foundations based on detailed investigation of a case study of a series of piles placed in layered soil for a single storey terrace of factories in Singapore. Failure of some piles was reported to be due to the combined action of surcharge and axial load. The interaction of downdrag and axial load as well as the combined effect on the pile face distribution along the pile shaft is discussed. Downdrag forces on individual piles within a group are likely to be smaller than on a single pile. Pile settlement can be where the axial load and the surcharge is larger than the corresponding soil settlement causing positive friction along its complete length of the pile. However, negative skin friction can be induced under light loads. The investigation showed that failure will take place where the ratio of the axial load to downdrag is less than 0.6 for safety factors of two or three. It is concluded that downdrag must be accounted for in the design of piles when the axial load to downdrag ratio is less than unity. When this ratio is larger than unity no provision need be made for negative skin friction for the economical design of piled foundations.