Stone Column Installation Research Articles

Soil compaction by vibro-replacement: a case study

During the earthquake of 23 November 1980, the historic centre of the village of San Michele di Serino in Southern Italy was almost completely destroyed. The site investigations carried out after the earthquake revealed the subsoil to consist fundamentally of compressible silty soils with interbedded gravelly layers. To improve the poor ground conditions prior to reconstruction, it was decided to use the vibro-compaction method with the installation of stone columns (vibro-replacement technique). In this study, the degree of soil improvement achieved at the site is assessed by comparing both the profiles of numerous penetration tests performed before and after the treatment, and the respective values of the foundation settlement calculated using the practical method developed by Van Impe and De Beer, which allows the reinforcing and soil compaction effects due to the installation of stone columns to be accounted for in a simple manner. The obtained results prove that remarkable soil compaction was achieved by the treatment, especially in the deep soil layers characterised initially by low penetration resistance. Moreover, it is shown that, if the soil compaction effect is ignored, the settlement of the composite system consisting of stone columns and soil subject to loading could be significantly overestimated. À la suite du tremblement de terre du 23 novembre 1980, le centre historique du village de San Michele di Serino, dans le sud de l'Italie, a été presque complètement détruit. Par la suite, les études menées sur le terrain ont révélé que le sous-sol consistait essentiellement de sol limoneux compressible avec des couches graveleuses intercalées. Pour améliorer les conditions de terrain peu favorables avant la reconstruction, il a été décidé d'utiliser la méthode de vibrocompaction avec l'installation de colonnes de pierre (technique de vibro-remplacement). Dans cette étude, le degré d'amélioration du sol atteint sur le site est é valué en comparant à la fois les profils de nombreux tests de pénétration réalisés avant et après traitement et les valeurs respectives de tassement de la fondation calculées par la méthode pratique développée par Van Impe et De Beer, qui permet de prendre en compte de manière simple les effets de renfort et de compaction du sol dus à l'installation de colonnes de pierre. Les résultats obtenus montrent que le traitement conduit à une excellente compaction du sol, en particulier au niveau des couches profondes caractérisées initialement par une faible résistance à la pénétration. Il est également démontré que, si l'on ignore l'effet de compaction du sol, le tassement du système composite constitué de colonnes de pierre et de sol soumis à la charge pourrait ê tre significativement surestimé.

Improved soft clay characteristics due to stone column installation

A method is proposed for evaluating the improvement of the Young modulus of soft clay in which a vibrocompacted stone column is installed. By considering a composite cell model a numerical analysis is carried out using Plaxis software to simulate the vibrocompaction technique, which leads to a form of primary consolidation of the soft clay. For the numerical simulation Mohr Coulomb perfect plastic behaviour is considered for the improved soil constituents. From numerical results the degree of improvement of the Young modulus soft clay has been estimated. Also, the zone of influence of the improved soft clay has been predicted.

Amperage and Treatment Time Monitoring to Evaluate Stone Column Effectiveness

During stone column installation for the seismic retrofit of the Mormon Island Auxiliary Dam, several important parameters of the construction process were continuously monitored using a data acquisition system. This system provided data on operating amperage, stone volume, and treatment time as a function of probe depth. In this study, treatment time and amperage values were analyzed in detail. In addition, equations combining amperage and time were developed to define the amount of energy applied to the soil during remediation. Applied energy, as measured by treatment time and amperage, was then compared to (N1)60 values from Becker penetration tests. This test was selected as the evaluation tool because of the presence of gravel and cobbles. Analysis of the data suggests that the maximum amperage at a given depth can be used to separate layers with high and low fines contents. In general, the maximum amperage was more a function of soil type than of treatment time. For soils with high fines content, the maximum amperage was consistently low relative to soils with low fines content and did not increase significantly with continued vibration. Comparisons between the applied energy and the posttreatment Becker penetration tests results showed that slight improvement is possible in soils with higher fines contents. However, increasing the vibration time in these soils did not increase the (N1)60 values. Therefore, additional time beyond that required to install the column was not useful. For soils with low fines contents, the posttreatment (N1)60 value did tend to increase with additional applied vibration energy, but the scatter in the results was substantial.

An instrumented trial of vibro ground treatment supporting strip foundations in a variable fill

Full-scale instrumented load tests have been carried out to study the installation and performance of vibro stone columns supporting a strip foundation in a variable fill and the performance of a similar strip foundation on untreated ground. The results from site investigation and laboratory testing are presented, and the vibro design principles—in particular the use of a modification to a standard formula that reflects the radial densification of soil around the stone column—are outlined. The treated foundation performance is compared with predicted behaviour; calculated settlements are in reasonable agreement with measured values, but stress measurements suggest that current design assumptions significantly overestimate the degree of stress concentration on the stone columns. The degree of radial densification of fill around the stone columns varied according to installation technique and soil type. The implications of the trial observations for existing design methods are considered.

Liquefaction Mitigation Using Stone Columns Around Deep Foundations: Full-Scale Test Results

The results presented were developed as part of a larger project analyzing the behavior of full-scale laterally loaded piles in liquefied soil, the first full-scale testing of its kind. Presented here are the results of a series of full-scale tests performed on deep foundations in liquefiable sand, both before and after ground improvement, in which controlled blasting was used to liquefy the soil surrounding the foundations. Data were collected showing the behavior of laterally loaded piles before and after liquefaction. After the installation of stone columns, the tests were repeated. From the results of these tests, it can be concluded that the installation of stone columns can significantly increase the density of the improved ground as indicated by the cone penetration test. Furthermore, it was found that the stone column installation limited the excess pore pressure increase from the controlled blasting and substantially increased the rate of excess pore pressure dissipation. Finally, the stone columns were found to significantly increase the stiffness of the foundation system by more than 2.5 to 3.5 times that in the liquefied soil. This study provides some of the first full-scale quantitative results on the improvement of foundation performance due to stone columns in a liquefiable deposit.

Field performance of the port of Sepetiba test fills

The design of the industrial port of Sepetiba, 50 km south of Rio de Janeiro, Brazil, required a detailed evaluation of the underlying soft soil deposits. Initially, on the basis of laboratory tests, it was proposed to remove approximately 3.7 × 106 m3 of the very soft deposits in the stockpile area by dredging and substitute with hydraulic sand fill. Subsequently, in view of the cost of such a measure, a large program of field investigations was initiated to study the in situ characteristics of the soft clay to evaluate whether replacement of this material and (or) ground improvement was necessary. As part of this investigation, two large identically instrumented test fills (test fills B and D), each 65 m2 in plan and 5 m high, with 3:1 slopes were constructed. Test fill B was constructed over natural ground, whereas the subsoil beneath test fill D was treated with stone columns. The instrumentation for each test fill consisted of piezometers, deep settlement plates, surface settlement plates, and inclinometers. This paper provides a description of the field investigations, observations on installation of stone columns, analysis of instrumentation, a comparison of the behaviour of the two test fills, and a discussion on load tests on individual stone columns. Key words : case history, embankment, ground improvement, instrumentation, soft clay, stone columns.

Design and installation of stone columns at Naval Air Station : Hayden, R F; Welch, C M In: Deep Foundation Improvements: Design, Construction, and Testing (Papers to a Symposium presented at Las Vegas, 25 January 1990)P172–184. Publ Philadelphia: ASTM, 1991 (ASTM Special Technical Publication N1089)

Design and installation of stone columns at Naval Air Station : Hayden, R F; Welch, C M In: Deep Foundation Improvements: Design, Construction, and Testing (Papers to a Symposium presented at Las Vegas, 25 January 1990)P172–184. Publ Philadelphia: ASTM, 1991 (ASTM Special Technical Publication N1089)

Field testing to evaluate stone column performance in a seismic area

For construction of a 16 mgd sewage treatment plant on predominantly deep, soft, cohesive soils in an area of highest seismic susceptibility, soil improvement with stone columns was considered as one foundation solution. While the deformation behaviour of subsoil improved with stone columns, under static vertical load, has been documented, little if any information is available on the performance of such improved soils under seismic loading conditions. For this reason, large scale field tests were performed to demonstrate that (a) in the process of stone column installation, sand lenses in the predominantly cohesive subsoil are sufficiently densified with respect to liquefaction potential; (b) the combined mass of stone columns and native, intervening soil develops suficient shear strength to resist safely horizontal forces resulting from a ground acceleration of 0·25 g; (c) the stone column pattern which satisfied the shear and density requirements also provides an adequate load-settlement relationship. Pour construire une usine de traitement de résidus (ayant une capacitéde 16 millions de gallonspar jour) sur des sols cohérents mous sur une grande profondeur dans une zone à grande séismicité on a envisagé d'améliorer le sol avec des colonnes de Pierre. Alors que l'on connaît bien le comportement sous une charge statique et verticale d'un sol amélioré par des colonnes de Pierre, il y a peu d'information concernant le comportement de tels sols sous les actions sismiques. Pour cette raison, on a réalisé des essais de chantier, importants, afin de démontrer que (a) par la méthode d'installation de colonnes depierre, les lentilles de sable dans les sous-sols essentiellement cohésifs, sont sufisamment densifiées vis à vis de la possibilité de liquéfaction; (b) les colonnes de Pierre et le sol originel intervenant ensemble, peuvent développer sufisamment de résistance de cisaillement pour résister en sécurité aux forces horizontales résultant d'une accélération terrestre de 0·25 g; (c) le modèle de colonne en Pierre satisfaisant aux exigences de densité et de cisaillement présente aussi la relation charge–tassement correcte.