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- Compressibility and Consolidation
- What are the Compressibility and Consolidation of Soil?
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Chapter 5 Compressibility And Consolidation. Remember: Consolidation settlement is a function of effective stress and not the function of total stress. For cohesionless soil. Login New User.
Compressibility and Consolidation
Formation of varved clays is characterized by specific compressibility and consolidation features, which are difficult to assess. The construction of an expressway through the area of varved, glacilimnic sediments Vistula glacial period required careful analysis of the soil reaction to the increasing load caused by growing embankment.
The settlement analyses conducted in relation to the schedule of load increase during construction allowed to verify the deformability assessment of the compressible clays.
In order to quantify the compressibility and consolidation parameters of clays, an iterative calculation model was created. The observed delayed reaction of the soil to applied load allowed to assess the nature of consolidation.
Comparison of the parameters obtained from the model with the results of laboratory and field tests allowed to evaluate drainage characteristics during consolidation of varved clays as well as to introduce correlation coefficients for interpreting compressibility parameters on the basis of CPT tests.
The accurate deformability prognosis of soft soils under various objects such as highways and railways, dikes, housing building etc. The selection of deformability and consolidation parameters as well as calculation methods is of a great importance.
Very often, as a comparable experiences, the laboratory and field compressibility tests results must be verified with the results of behavior monitoring of existing facilities. It allows to increase the precision of forecasts, but at the same time it must be correlated with lithological types of the substrate. In the latest scientific literature, the interesting examples of back analysis application to predict corrected course of parameters on the basis of soft soils settlements may be found.
In reviewed papers the analysis was based on the course of settlement or pore distribution observed under load of embankments or buildings. Different methods of analysis based generally on exponential curve fitting methods [ 1 , 2 ] were discussed there, with the application of developed consolidation theory: elasto-viscoplastic constitutive model [ 3 ], coupled flow and deformation in unsaturated soils [ 4 ] and Burgers creep model [ 5 ].
The results of these analysis were expressed in different parameters and demanded adequate initial parameters from laboratory and field tests. The values of the obtained parameters reflect the geological and engineering characteristic of the analyzed soils. These were: estuarine, normally consolidated deposit from Brisbane in Australia [ 2 ], sludge and mucky soils from different province in China costal [ 1 ], alluvial soft clay in Japan [ 3 ] weathered Mud rock in China [ 5 ]. It is obvious that in soft soils [ 6 ] the solution of each engineering task must be strictly correlated with its location and geological and engineering conditions.
Evaluation of proper values of compressibility and consolidation parameters has a wider significance, for example in designing of subsoil reinforcement by preconsolidation [ 7 ]. Some investigations are dedicated to analysis of standard conditions [ 8 ] and specific behaviour of soils which depends of their origin [ 9 , 10 ].
The case presented in this article concerns the characteristics of the course of consolidation and further forecasts of the soil settlement under the rapid road traffic in northeastern vicinity of Warsaw Poland. The analyzed section of the road was built on a specific substrate - varved clays - with distinct characteristics in relation to the soft soil discussed commonly in the literature.
Compressibility and consolidation characteristic of varved clays is a complex issue due to structural sensitivity, specific character of post-sediment structural bonds, no consolidation in geological past and anisotropy of mechanical and filtration properties within thin several mm , alternate layers.
In comparison to clays of different origin, these soils show different nature of deformability, what has been confirmed by the results of laboratory and field tests presented in a number of publications [ 11 , 12 ].
However, these publications do not often take into account that the compressibility depends on stress and changes in the soil consistency. For accurate predictions of settlements, it is necessary to analyze in details the relationship between the compressibility characteristics obtained from:.
Comparison of above-mentioned three types of data allows not only to predict the settlements but also to assess factors conditioning the course of strain under load for such a specific formation as varved clays. The typical area of occurrence of varved clays, so-called Warsaw Lake Basin, was chosen for the study where sedimentation developed in the initial stages of the Vistula glacial period. Later, the erosion in the outer zone of the glacier removed some of the deposits, leaving structurally undisturbed several-meter-long clayey, varved sediments in the lower parts of the basin.
Their behaviour under loads transmitted by the embankment as well as by later expressway operation was a subject of further analysis of this study. The propose of the study was to present methodical experiments regarding the verification and forecasting consolidation behaviour of varved clays which are considered as geologically specific soil. Those analyzes were supposed to refer to optional methods of determining compressibility and consolidation parameters.
In particular, it concerned the relationship between back analysis results and the results of laboratory tests carried out under incremental loading IL and continuous loading CL with adaptation to changes in the load. The correlation of cone resistance obtained from CPTu tests with the values of compressibility modulus M determined during laboratory tests was determined. Such analyzes were focused on the possibility of applying the results of soundings in the spatial assessment of variability of the deformation parameters of varved clays.
The typical area of occurrence of varved clays, so-called Warsaw Lake Basin [ 13 ] Figure 1 , was chosen for the study where sedimentation developed in the initial stages of the Vistula glacial period [ 15 , 16 ].
Later, the erosion in the outer zone of the glacier removed some of the deposits, leaving structurally undisturbed several-meter-long clayey, varved sediments in the lower parts of the basin [ 16 , 17 , 18 ]. The analysed research area is located within Warsaw Basin which is located on so-called Radzymin terrace Radzymin-Marki level , which has been created during the Vistula glaciation period [ 15 , 16 ].
Its core is made of varved clays which origin is connected directly with sedimentation stagnation [ 14 ]. This terrace is now a flat and extensive plain with an accumulative and erosive character.
At the end of Pleistocene, it has been remodelled by aeolian processes,which caused the formation of irregular dunes and dune banks [ 16 ]. In the upper part or the profile, about meters thick river sands remodelled by aeolian processes may be observed. The lower part of the profile is represented by clays, silty clays, clayey sands and clayey silts Figure 2.
The geological profile has been recognized down to the depth of the impact of loads generated by the expressway. It is represented by horizontally oriented, quite uniform, non-cohesive river deposits and varved clays of glacilimnic origin [ 14 , 17 ].
The first layer is represented by medium-compacted, yellow-gray fine or locally medium sand. Density index I D varies between 0. The thickness of these sediments. Below, m layer of cohesive lake soils is observed. Their structure is an effect of periodical sediment supply, clearly segregated during the sedimentation process. As a result of these specific conditions, the varved texture is observed along almost the entire profile.
The main type of soil that forms this complex is horizontally laminated, silty clay - so-called varved clays. The structural conditions play a very important role in consolidation process. This influence is expressed in the clear anisotropy of the filtration parameters that determine the dissipation conditions of the excess pore pressure caused by the load increase.
The way, the direction as well as the velocity of the pore water outflow determines the rate and conditions of consolidation. Varved clays are characterized by significantly higher water permeability in the horizontal direction than in the vertical one.
In the perpendicular direction, the water flow rate is significantly lower. Silty clays and silts are less common in the profile and occur in the upper part of clayey formation, where varved texture has been lost or has not been developed. Sometimes some thin lamellas of sands are present in this part of the profile.
Such complex formation under the expressway embankment demands much more care during the earthworks. Below clayey complex, again river deposits is observed in the profile, represented by medium and fine sands. Previous studies have shown the presence of 2 aquifers in the direct substrate of the studied investment.
This aquifer significantly impedes the construction of the first layers of road embankment. The operation of vibratory plates and vibratory rollers in moist or saturated zones leads to soil liquefaction and becomes ineffective.
The second aquifer occurs at a depth of approximately m and is characterized by confined groundwater level that stabilizes at a depth of about meters below the surface. Due to the isolation by impermeable clays the groundwater pressure is about kPa. This aquifer occurs to a depth of m.
The hydrogeological situation piezometric levels, saturation, water permeability of layers, groundwater table system affects the way and direction of pore water outflow during consolidation. The basic parameter giving some information about the mechanical properties of clays is liquidity index I L.
The obtained I L values indicated that investigated clays are mainly stiff Figure 3. Locally, in the upper part of the profile, down to about 1 m clays may be plastic. The clays in the study area are normally consolidated as they had not been subjected to any geological loads apart from the current load of the overburden deposits.
This aspect is very important in compressibility assessment of these soils and their consolidation. The formation is subjected to new loads caused by the embankment and is compressed in the elastoplastic manner with a clear evidence of creeping during the consolidation process. It is a manifestation of the reconstruction of the structure in confrontation with the new impact, which these lands were not subjected to in their history. The other components are kaolinite, quartz and carbonates which are characterized by low activity and hydrophilicity.
Natural moisture conditions ensure structural stability of varved clay in terms of their potential expansive features. The studies of the expressway substrate indicate that the varved clays are generally stiff.
In parts with more permeable silts, they can become plastic, which is indicated by higher values of water content. The starting point for the calculations was the analysis of the variability of these parameters presented in the scientific literature, standards as well as based on the results of laboratory verification tests conducted for the purposes of this study.
Varved clays are characterized by variable compressibility depending mostly on their structure and particle size distribution as well as applied load [ 18 , 19 ]. The values of compressibility modules adopted in the literature and standards for non-consolidated cohesive soils deposited outside moraines are usually higher than the results for varved clays.
Primary compression of soils from Zielonka the site next to analysed one is at a constant level of about 15 MPa in the range of stresses up to 0.
Compressibility varved clay from Zielonka evaluated by oedometer tests data after [ 8 ]. Strengthening the structure obtained during the initial load is observed at a stress of 0. This indicates high structural sensitivity of analysed clays and several times less compressibility of dark layers with a higher content of clay fractions in relation to light, silty layers.
Consolidationfeatures of varved clays are not widely recognized due to specific drainage conditions thin, repeated, silty laminae cause the faster pore pressure dissipation and a complex creep mechanism. For the analysis of the soil deformation, 6 road profiles were selected, in representative regions where, according to geological and engineering data, higher settlements could be observed.
Schedules of the load stages during the embankments formation were individual for each profile. The last layer was formed after a longer period of 60 to days. Settlement observations at such load schedule allowed to model the contribution of quasi-immediate compressibility and the effects of primary and secondary consolidation.
In the stress state modelling, trapezoidal strip load was applied to the subsoil, where the width of the embankment crown was At the maximum embankment height of 7 m and slope of The restrictive guarantee requirements for line investments impose restrictions on the acceptable settlements in a 5 year period counting from the date of expressway commissioning.
This increases the awareness of contractors, and thus the need to monitor the settlements already at the construction stage. The benchmarks consist of a rigid steel plate with the column of measuring tubes placed on it in the friction reducing cover. Then, the embankment layers are constructed and a vertical column of measuring tube is installed on the plate. As the next layers of embankment are formed, the measuring tubes are extended by further modules.
Knowing the elevation of the top of the measuring tube in connection with the number of column extension modules allows precise positioning of the plate, and thus the determination of settlements. It should be noted that such measurement does not take into account the settlement of the embankment, but only the settlement of the subsoil substrate.
What are the Compressibility and Consolidation of Soil?
Soil consolidation refers to the mechanical process by which soil changes volume gradually in response to a change in pressure. This happens because soil is a two-phase material, comprising soil grains and pore fluid, usually groundwater. When soil saturated with water is subjected to an increase in pressure, the high volumetric stiffness of water compared to the soil matrix means that the water initially absorbs all the change in pressure without changing volume, creating excess pore water pressure. As water diffuses away from regions of high pressure due to seepage , the soil matrix gradually takes up the pressure change and shrinks in volume. The theoretical framework of consolidation is therefore closely related to the diffusion equation , the concept of effective stress , and hydraulic conductivity.
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Soil is a multi-phase medium made up of mineral grains which enclose voids that may be filled with gas, liquid or a combination of both. When stress is applied to a soil sample its volume decreases. Such a change in volume may be due to: a a compression of the solid soil particles, b a compression of water and air within the voids, or c an escape of water and air from the voids. The solid particles and the pore water are relatively incompressible and therefore under the loads usually encountered in geotechnical engineering, they will not undergo appreciable volume changes. Therefore the decrease in volume of a saturated soil mass when subjected to stress increase is due almost entirely to an escape of water from the voids.
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Formation of varved clays is characterized by specific compressibility and consolidation features, which are difficult to assess. The construction of an expressway through the area of varved, glacilimnic sediments Vistula glacial period required careful analysis of the soil reaction to the increasing load caused by growing embankment. The settlement analyses conducted in relation to the schedule of load increase during construction allowed to verify the deformability assessment of the compressible clays. In order to quantify the compressibility and consolidation parameters of clays, an iterative calculation model was created. The observed delayed reaction of the soil to applied load allowed to assess the nature of consolidation. Comparison of the parameters obtained from the model with the results of laboratory and field tests allowed to evaluate drainage characteristics during consolidation of varved clays as well as to introduce correlation coefficients for interpreting compressibility parameters on the basis of CPT tests.
Metrics details. The compression index and recompression index are one of the important compressibility parameters to determine the settlement calculation for fine-grained soil layers. These parameters can be determined by carrying out laboratory oedometer test on undisturbed samples; however, the test is quite time-consuming and expensive. Therefore, many empirical formulas based on regression analysis have been presented to estimate the compressibility parameters using soil index properties. In this paper, an artificial neural network ANN model is suggested for prediction of compressibility parameters from basic soil properties. For this purpose, the input parameters are selected as the natural water content, initial void ratio, liquid limit and plasticity index.