Secondary Consolidation Settlement Research Articles

Secondary consolidation simulation of peat with vertical drains under time-dependent loading

Preloading with vertical drains is an effective ground improvement technique used to accelerate the consolidation process. Generally, embankments are constructed over a period, by increasing the fill height with time. Hence, it is not always realistic to analyse embankments as a single ramp loading or an instantaneous loading. Although many studies have been conducted to calculate the settlement variation under time-dependent loading, those methods are complex in practical applications and do not consider the secondary consolidation settlement, which is important for very soft organic soils. In this study, the original Barron consolidation theory is applied to calculate the variation of settlement, pore water pressure and degree of consolidation due to incremental step loading of embankments, constructed on compressible soft organic soil, stabilised with vertical drains (gravel compaction piles, sand compaction piles and prefabricated vertical drains). The settlement predictions are compared with the actual settlement monitoring data obtained during the construction of road embankments, and pore pressure variation is compared with the results obtained from a case study reported in the literature. The comparisons show that the results obtained from the proposed method match very well with the actual field measurements.

A Case Study of Assessing the Accuracy of Secondary Consolidation Prediction Using Qualitative Approach

Infrastructure constructed, especially over soft organic soil layers, can be subjected to excessive settlement within its life cycle due to the complex behaviour of the soft soil with time. However, with necessary actions prior to construction, the impacts from such soil layers can be mitigated to some extent. The case under consideration is a leisure resort in Matara, a 15-storey hotel resting on a raft foundation near the southern coastal line of Sri Lanka. With time, some cracks have formed within the building, and an investigation was done to identify the cause for the crack formation. It has been determined that cracks have appeared due to the excessive settlement of the subsurface. Furthermore, ground investigation results suggest that a peat layer is beneath the building within a depth of 15-24m. As per the survey report on the settlement of the building, secondary consolidation of the soft soil significantly impacts the excessive settlement. Therefore, three methods were used to predict the secondary consolidation settlement of the peat layer beneath the building. The methods are prediction using empirical correlations, laboratory experiment results that will follow the constant coefficient of secondary consolidation throughout time and the qualitative method, which assumes that the secondary consolidation coefficient varies with time. Based on the above techniques, predictions were made, and results suggest that the qualitative method has a significant accuracy compared to the actual settlements of the building. These observations provide some proof that the coefficient of secondary consolidation varies with time according to the qualitative approach and does not remain constant throughout the lifespan of the building as suggested by conventional methods. KEYWORDS: Coefficient of secondary consolidation, Crack formation, Settlement, Soft soil, Time-dependent settlement

Time Series InSAR Monitoring and Analysis of Spatiotemporal Evolution Characteristics of Land Subsidence in Yan'an New District

Objectives Yan'an New District has attracted much attention because of its large-scale mountain excavation and city construction project. Such mega-scale land creation, rapid urban construction and complex geological conditions has induced wide land subsidence in the region. Methods In this paper, the temporal and spatial evolution of land subsidence in Yan'an New District was assessed using small baseline subset synthetic aperture radar interferometry (SBAS-InSAR) to process the Sentinel-1A images covering the period from May 2016 to October 2019. Then, the mechanism and evolution trend of land subsidence in the new district was analyzed. Results Experimental results suggest that the spatial evolution pattern of land subsidence in Yan'an New District is highly relevant with the process of land creation project. The land subsidence area in the new district gradually expanded from the major urban area to the area of forest along with the land creation project, and its area decreased gradually with the passing of time. The temporal deformation evolution of land subsidence in the new district is related to the thickness of the fill, and all exhibit non-linear subsidence with various velocities. The temporal evolution of land subsidence undergoes three processes of rapid-slow-steady. The greater the thickness of the fill, the larger the amount of sedimentation, the longer it takes for the sedimentation to stabilize. Conclusions The main internal mechanism of land subsidence is the consolidation and compression deformation of the loess fill, which can be divided into three stages: Instantaneous settlement, consolidation settlement, and secondary consolidation settlement. Based on the characteristics of different stages combined with time series deformation, the land subsidence process and development trend can be analyzed. The research results provide a scientific reference for further monitoring and early warning, urban planning, and prevention and control of subsidence disasters in the new district.

Runoff Characteristics in Urban Drainage Systems Considering Secondary Consolidation Settlement

In general, underground facilities are designed accounting for the primary consolidation settlement and are buried in consideration of the allowable residual settlement. The subsequent secondary consolidation settlement is smaller than the first one and occurs during a longer period of time. However, in areas where deep soft grounds (coastal areas, etc.) are abundant, settlement of up to 50 cm may occur due to secondary consolidation, the hydraulic characteristics may change, and the facilities may operate under conditions that are different from the initial planned design. Therefore, this study establishes scenarios accounting for secondary consolidation settlement (long-term settlement) and various settlement degrees and compares the hydraulic characteristics of rainwater pipelines with those of the initial plan for storm-water drainage. An attempt is made to evaluate the uncertainties concerning the capabilities of the facilities. Keywords: Hydraulic Characteristic, Secondary Consolidation Settlement, Urban Flood

Experimental Evaluation of Consolidation Behavior of Double-Layer Soft Soil Ground

Abstract Double-layer grounds are characterized by one layer of soft soil on the top of another, which are frequently encountered in land reclamation projects in China. Consolidation behavior of the double-layer grounds is complicated compared with that of single-layer soil because of the complex drainage conditions that exist between two soil layers and the different consolidation properties of the two soils. The consolidation behavior of the double-layer ground becomes more complicated when it is subjected to a surcharge preloading. Therefore, it is necessary to understand the consolidation behavior of individual soils that make up the double-layer ground as well as the double-layer ground as a whole. In this article, double-layer specimens were prepared to simulate double-layer grounds, which include an upper layer clay with high plasticity and a lower layer clay with low plasticity. Consolidation tests were performed on both single-layer specimens and double-layer specimens using a modified consolidation test device. The influences of surcharge ratio and surcharge preloading duration on the consolidation behavior of double-layer ground, such as consolidation settlement and consolidation rates, were studied in addition to the influence of the thickness ratio (i.e., the ratio of the thickness of the upper layer of soil to the lower layer of soil). The experimental results show that a longer surcharge preloading duration could reduce the time needed to complete the rebound of the double-layer ground. The secondary consolidation settlement of double-layer ground can be reduced by increasing either the surcharge ratio or surcharge preloading duration. In addition, an increase in the thickness ratio could result in an increase in the primary consolidation settlement and a decrease in the consolidation rate. However, the effect of the thickness ratio on the consolidation settlement and the consolidation rate gradually decreased with the increase in the thickness ratio.

Study on Foundation Deformation of Buildings in Mining Subsidence Area and Surface Subsidence Prediction

The deformation damage of buildings above the subsidence area is not only caused by the unstable deformation of the mining residual cavities and fractures, the time effect of soil compression is also an essential consideration, at the same time, the additional stress of the new building group in the mining-induced subsidence area is often influenced by multiple building loads. The physical properties and thickness change of soil layer are studied by drilling and laboratory test, the corner method is used to divide the load of building group and calculate the additional stress of foundation, the magnetotelluric imaging technique is used to investigate the distribution of mining residual cavities and fractures in the mining-induced subsidence area of the Shandong blue ocean pilot e-commerce industrial park, and the superposition prediction method and additional stress analysis method are used respectively to evaluate the stability of the foundation. The results show that the secondary consolidation settlement of soil layer accounts for about 10% of the residual deformation of subsidence area, the additional stress obtained by the corner method is 0.1935P. The evaluation results show that the proposed building will be affected by the unstable deformation of the mining residual cavities and fractures in the subsidence area. The evaluation results of the two methods are similar, it reflects their respective effectiveness. The two methods should be combined to evaluate the stability of the foundation in the engineering application.

Geotechnical characteristics and consolidation properties of Tianjin marine clay

Tianjin, which is located on the west shore of the Bohai Sea, is part of China\'s Circum-Bohai-Sea Region, where very weak clay is deposited. From the 1970s to the early 21st century, Tianjin marine clay deposits have been the subject of numerous geotechnical investigations. Because of these deposits\' geological complexity, great depositional thickness, high water content, large void ratio, excessive settlement, and low shear strength, the geotechnical properties of Tianjin marine clay need to be summarized and evaluated based on various in situ and laboratory tests so that Tianjin can safely and economically sustain more infrastructure in the coming decades. In this study, the properties of Tianjin marine clay, especially its consolidation properties, are summarized, evaluated and discussed. The focus is on establishing correlations between the geotechnical property indexes and mechanical parameters of Tianjin marine clay. These correlations include the correlations between the water content and the void ratio, the depth and the undrained shear strength, the liquid limit and the compression index, the tip resistance and the constrained modulus, the plasticity index and the ratio of undrained shear strength and the preconsolidation pressure. In addition, the primary consolidation properties of Tianjin marine clay, such as the intrinsic compression line (ICL), sedimentation compression line (SCL), compression index, Cc, coefficient of consolidation, Cv, and hydraulic conductivity change index, Ckv, are evaluated and discussed. A secondary consolidation property, i.e., the secondary compression index, Ca, is also investigated, and the results show that the ratio of Ca/Cc for Tianjin marine clay can be used to calculate Ca in secondary consolidation settlement predictions.

Analysis and discussion on post-construction settlement of soft soil foundation treatment test section of highway

Combined with the settlement observation data in soft soil foundation test section of highway projects, the final settlement of soft soil foundation by preloading was calculated by using three-point method, Asaoka method and hyperbola method, and the comparative analysis on the final settlement and post-construction settlement of soft soil foundation were conducted by various calculation methods, then the suitability of calculation methods for post-construction settlement of highway soft soil foundation was discussed. And the results are shown as follows: (1) The final settlement results calculated by three-point method and Asaoka method were less than hyperbolic method, due to only considering the primary consolidation settlement for the first two methods; the calculation results of three methods were close to each other when the secondary consolidation settlement was considered, and the secondary consolidation settlement should be considered when the post-construction settlement was calculated by the first two methods. (2) The post-construction settlement and differential settlement of soft soil foundation test section of highway calculated by three methods were all satisfied with the design requirements.

Experimental study on grouting in underconsolidated soil to control excessive settlement

The underconsolidated soils in reclaimed land have low strength and high compressibility, resulting in susceptibility to excessive settlement and associated negative effects, which can be controlled by grouting. However, there is little research on the mechanism of grouting in underconsolidated soil, compared with the normally consolidated or overconsolidated soil. In this study, we present a series of laboratory grouting tests to investigate the grouting mechanism and the effect of degree of consolidation, soil type, load increment ratio and initial overlying load on the grouting performance. The results showed that the influence of grouting on primary consolidation settlement for underconsolidated soil included the heave effect, the additional settlement due to the dissipation of excess pore water pressure induced by grouting and the reduced settlement due to the increased modulus induced by grouting. However, grouting in underconsolidated soil had no influence on the secondary consolidation settlement. Grouting in underconsolidated soil with low degree of consolidation could obtain a shorter time of primary consolidation and a higher final grouting efficiency in primary consolidation stage. The final grouting efficiency in primary consolidation stage increased linearly with increasing load increment ratio, but decreased with increasing initial overlying load. As the overconsolidation ratio increased, the final grouting efficiency in primary consolidation stage decreased for underconsolidated soil, dropping to the lowest value for normally consolidated soil and then increased for the overconsolidated soil, reaching a limit. Moreover, the peak grouting pressure was proportional to the vertical effective stress at the beginning of grouting.

Experimental study on the performance of compensation grouting in structured soil

Most laboratory test research has focused on grouting efficiency in homogeneous reconstituted soft clay. However, the natural sedimentary soils generally behave differently from reconstituted soils due to the effect of soil structure. A series of laboratory grouting tests were conducted to research the effect of soil structure on the performance of compensation grouting. The effects of grouting volume, overlying load and grouting location on the performance of compensation grouting under different soil structures were also studied. Reconstituted soil was altered with added cement to simulate artificial structured soil. The results showed that the final grouting efficiency was positive and significantly increased with the increase of stress ratio within a certain range when grouting in normally consolidated structured clay. However, in the same low yield stress situation, the artificial structured soil had a lower final grouting efficiency than the overconsolidated reconstituted soil. The larger of normalized grouting volume could increase the final grouting efficiency for both reconstituted and artificial structured soils. Whereas, the effect of the overlying load on final grouting efficiencies was unfavourable, and was independent of the stress ratio. As for the layered soil specimens, grouting in the artificial structured soil layer was the most efficient. In addition, the peak grouting pressure was affected by the stress ratio and the overlying load, and it could be predicted with an empirical equation when the overlying load was less than the yield stress. The end time of primary consolidation and the proportion of secondary consolidation settlement varied with the different soil structures, grouting volumes, overlying loads and grouting locations.

Investigation of the long-term settlement of a cut-and-cover metro tunnel in a soft deposit

This study investigates the long-term settlement of Nanjing Metro Line 10 in China. A maximum settlement of 240mm was measured within 5.75years after the track system was constructed. Four settlement troughs were observed along the longitudinal direction of the railway line. The settlement of the tunnel developed rapidly in the first two years and then the rate of settlement gradually decreased. The excessive settlement and large differential settlement of the tunnel were primarily caused by the deep and non-uniform distribution of the underlying soft soil. The long-term tunnel settlement mainly consists of two components, including recompression settlement following construction and secondary consolidation settlement of the foundation. Each component of settlement was analyzed and estimated in this study. The computed results correspond well with the measured data, and the recompression deformation and the secondary consolidation deformation represents approximately 78.0–100% and 0–22.0% of the total tunnel settlement, respectively. The measured data and calculated results revealed that the lengths of the cement mixing piles used to improve the tunnel foundation are insufficient to reduce the long-term settlement. Piles entirely penetrating the soft silty clay layer are recommended.

Arresting Settlement of Peaty Soil in Built-Up Area

In Singapore, ground settlement is one of the major problems encountered in built-up area in housing, private and industrial estates. While the structure is not affected by the settlement as they are supported by piles. Some amount of settlement may occur on the ground surrounding the structures, thus affecting the service road, car park area and other underground services. This ground settlement, mainly secondary consolidation settlement will continue for long time due to presence of thick layer of soft peaty soil. Conventional ground improvement methods may not be suitable in solving such problem especially in built-up area. Hence, in present research, a new method called Liquefied Soil-cement Mix (LSM) method is developed to treat peaty soil and thus arrest the ground settlement in built-up area. This method involves injecting cement-clay slurry into peaty soil mass to fill the voids and fissures of peaty soil. The appropriate injection pressure is carefully designed to minimize ground disturbance during and after LSM treatment. In this study, a field trial was executed to treat the peaty soil at depth of 7 to 10 m by using this method. The field trial and laboratory tests were conducted to evaluate the efficiency of LSM method. The test results indicated that settlement was significantly reduced with LSM treatment.

장기압밀시험에 의한 광양항 점성토의 응력이력 특성 연구

본 연구에서는 재성형한 광양항 점성토를 이용하여 응력이력과 OCR이 연약 점성토의 장기압밀특성에 미치는 영향을 규명하기 위해 표준압밀시험기를 개조하여 장기압밀 시험을 수행하였다. 과압밀상태의 점토 시료에 대해, OCR이 1.5를 초과한 경우, 이차압밀 및 최종침하량은 OCR이 2.0에서 3.0으로 증가함에도 큰 차이를 보이지 않았으며, 이로부터 이차압밀과 최종침하량을 저감하기 위해 현장적용 OCR은 1.5로 나타났다. 더욱이 선행하중 재하기간과 장기압밀 거동 특성사이의 관계를 알아보기 위해 광양항 재성형 점토를 이용한 실험결과로부터 압밀도 70~80%를 초과하여 재하하중을 제거하여도 장기압밀에 미치는 영향은 크지 않은 것으로 나타났다. In this present study, the long-term consolidation tests were performed using the remolded Kwang-Yang port clayey soil to clarify the effect of stress history and over-consolidation ratio (OCR) on the long-term consolidation characteristics of the soft clayey soil. For the over-consolidated state clayey soils, in case OCR exceeds 1.5, there are no great differences of secondary consolidation settlement and final settlement even if OCR increases from 2.0 to 3.0. Therefore, it has been understood that the value of OCR applied on the field site to reduce the secondary consolidation settlement and the final settlement is about 1.5. In addition, in order to investigate the relationship between the pre-loading period and the characteristics of long-term consolidation behavior obtained from the test results using the remolded Kwang-Yang port clayey soils, the influence on long-term consolidation behavior was not large though the pre-load was unloaded with the consolidation degree 70~80% exceeded.

Actual Example Analysis of Settlement Prediction during Soft Ground Treatment

It is difficult to investigate the geological condition entirely, and the physical and mechanical indexes are inaccurate. All these uncertain factors make both the theoretical and numerical calculation results of foundation settlement different with actual situation. Based on summarizing research results about settlement prediction methods for soft subgrade, actual example analysis is provided with using the measured settlement data to predict the settlement of the soft ground treated by the preloading drainage consolidation method after construction. The analysis method of soft ground settlement during secondary consolidation stage is discussed, and a new calculation model for secondary consolidation settlement is proposed. Considering the different characteristics of the settlement curves during primary and secondary consolidation stages, an improved method is provided to calculate settlement by piecewise curve fitting, and the rationality of the method is verified by analyzing the monitoring data of typical settlement plate in Shenzhen Western Corridor Project. To sum up, in actual engineering practice the settlement developing trends and final settlement can be well predicted with using part of the measured settlement data obtained. Comparing to other methods, settlement prediction with using actual measured data is relatively accurate and reliable. It does not need geological data and the calculation process is simple for engineering and technical personnel to grasp and accept.

Preloading and prefabricated vertical drains design for foreshore land reclamation projects: a case study

The Changi East Reclamation Project in Singapore comprises land reclamation in foreshore conditions. As the foreshore area was underlain by a thick layer of soft marine clay, a high magnitude of primary and secondary consolidation settlement was expected, owing to fill and future live load. An average fill thickness of 10 m was predicted to contribute to a magnitude of settlement greater than 2 m over a period of several decades. In order to minimise this expected future settlement, it was necessary to accelerate the consolidation process to complete the majority of the settlement during construction stage. A combination of prefabricated vertical drains (PVDs) with preloading ground improvement was successfully applied in this project. The technique comprises the installation of prefabricated vertical drains and the subsequent placement of surcharge to accelerate the consolidation of the underlying marine clay. This paper discusses the theories of radial drainage and the preloading technique, and considerations and design methodologies for the ground treatment of marine clay with prefabricated vertical drains in such foreshore land reclamation projects. In addition, predictions of magnitude, time rate of settlement and performance assessment of prefabricated vertical drains are also discussed. The design predictions at a case study site were compared with the field instrumentation results in order to verify the design approach used. It is found that PVDs with preloading are the most effective method for improving soft clay under land reclamation, based on the soil conditions present at the case study site. Le projet Changi East Reclamation à Singapour porte sur la réhabilitation de terrain dans les conditions d'estran. Comme la zone d'estran repose sur une couche épaisse d'argile marine molle, un tassement des consolidations primaire et secondaire de grande amplitude était présumé, en raison de remplissage et de charge utile future. On estimait qu'une épaisseur de remplissage de 10 m pourrait contribuer à une amplitude de tassement de plus de 2 m sur une période de plusieurs décennies. Afin de minimiser l' étendue du tassement prévue, il était nécessaire d'accélérer le processus de consolidation pour que la majorité du tassement se termine pendant la phase de construction. Ce projet combine avec succès l'utilisation de drains verticaux préfabriqués (DVP) avec amélioration des sols par préchargement. Le technique adoptée consiste à installer des drains verticaux préfabriqués puis à placer une surcharge pour accélérer la consolidation de la couche d'argile marine sous-jacente. Cet article présente une discussion sur les théories de drainage radial et la technique de préchargement, ainsi que des considérations et des méthodologies de conception géotechnique pour la consolidation de l'argile marine dans le cadre d'un tel projet de réhabilitation de terrain dans les conditions d'un estran. Il examine également les prédictions concernant l'amplitude et le taux de tassement en fonction du temps, et les performances des drains verticaux préfabriqués. Les prévisions de conception sur un site d'étude de cas sont comparées avec les résultats d'expérimentation de terrain afin de vérifier l'adéquation de l'approche technique utilisée. En conclusion, la méthode de PVD avec préchargement se montre la plus efficace pour améliorer l'argile molle dans le cadre de réhabilitation de terrain, lorsque l'on se base sur les conditions de sol présentes sur le site d'étude de cas.

Geotechnical foundation conditions of Mesologion swimming pool center

Although geotechnical tests are standard and commonly used in determining the foundation type to be applied, in some cases the use ofin-situ tests are essential in order to obtain the correct values needed in the geotechnical calculations. In the case of the foundation of Mesologi Swimming-pool Center, an attempt was made to propose a foundation type, suitable from geotechnical point of view, for sediments that lie very close to the sea! The selection of the most suitable foundation type encloses the examination of several parameters. The subsoil there, consists of soft, fine grained, low plasticity materials, CL to SL with alternations of high plasticity clay layers CH. Additionally, the groundwater level is 0,8 meters under the soil surface, demonstrating constant saturation conditions. The combination of soft subsoil layers quality and the high level of groundwater leads to difficult foundation conditions in relation to immediate and secondary consolidation settlement. Geotechnical calculations and calculations of settlement were performed and the most suitable foundation type was semi-compensated mat foundation.

Secondary compression of sabkha ‘saline’ soils : Engng Geol V30, N2, April 1991, P155–169

Abstract Sabkha soils are coastal and inland saline deposits of arid climates consisting mainly of loosely cemented sandy silt to silty clay particles. Invariably the soils contain an appreciable amount of organic material and therefore are characterized by being highly compressible. A significant part of the soil settlement takes place as secondary compression. Prediction of field settlement is very important since one of the most feasible methods of soil improvement is the preloading technique. In this study settlements were predicted using the conventional theory of consolidation and the Lo rheological model utilizing long-term consolidation test parameters. The prediction was compared with the field settlement of the compressible sabkha layer loaded by an instrumented test embankment. It was found that the model provides a better approximation with regard to the rate of settlement, which was underestimated by Terzaghi's theory. The magnitudes of primary and secondary consolidation settlement were adequately estimated by the model. Furthermore, almost identical values of the measured and predicted combined settlements were obtained at the end of the nine week period of observation.

Secondary compression of sabkha “saline” soils

Sabkha soils are coastal and inland saline deposits of arid climates consisting mainly of loosely cemented sandy silt to silty clay particles. Invariably the soils contain an appreciable amount of organic material and therefore are characterized by being highly compressible. A significant part of the soil settlement takes place as secondary compression. Prediction of field settlement is very important since one of the most feasible methods of soil improvement is the preloading technique. In this study settlements were predicted using the conventional theory of consolidation and the Lo rheological model utilizing long-term consolidation test parameters. The prediction was compared with the field settlement of the compressible sabkha layer loaded by an instrumented test embankment. It was found that the model provides a better approximation with regard to the rate of settlement, which was underestimated by Terzaghi's theory. The magnitudes of primary and secondary consolidation settlement were adequately estimated by the model. Furthermore, almost identical values of the measured and predicted combined settlements were obtained at the end of the nine week period of observation.