Full Answer
What is the stress-strain behaviour of overconsolidated clays?
(1) The stress-strain behaviour of overconsolidated clays is highly non-linear, such that the selection of appropriate 'linear elastic'moduli for calculations of settlement or heave is very difficult.
What is consolidation settlement of clay?
Some time is required for excess pore water pressed to dissipate and increase the stress to be transferred to the soil skeleton. This gradual increase in the effective stress in the clay layer will cause settlement over a period of time and is referred to as consolidation settlement.
Is overconsolidated clay more elastic than normally consolidated clay?
The stress-strain curve of an overconsolidated clay is likely to exhibit more elastic behaviour when compared with the stress-strain curve of a normally consolidated clay.
What is the degree of consolidation of clay layer?
This also can be written as follows for a clay layer. Degree of Consolidation = Settlement of the clay layer at the time t after the load is applied / Maximum consolidation settlement that the clay layer will undergo under given loading. The following soil types are prone to consolidation settlement.
What is the Poisson's ratio for overconsolidated clay?
Which method gives predictions of total settlement?
How to compare heave and settlement?
What is the proposed method?
Who said IRP plans will take longer to deliver than scrapped proposals?
Is settlement closer to the loaded boundary?
Is time dependent heave greater than time dependent settlement?
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What is a clay settlement?
Clays undergo consolidation settlement not only the action action of external loads (surcharge loads) but also under its own weight or weight of soils that exist above the clay. Clays also undergo settlement when dewatered (groundwater pumping) because the effective stress on the clay increases.
How is clay soil settlement calculated?
6:149:18Consolidation Settlement Equations - YouTubeYouTubeStart of suggested clipEnd of suggested clipGet in font in order to find the settlement I'll just multiply the strain by the thickness of theMoreGet in font in order to find the settlement I'll just multiply the strain by the thickness of the clay layer I don't show that equation here but that's all we have to do is take the strain multiplied.
Which one is more likely to cause settlement problem normally consolidated or Overconsolidated clay?
Therefore, the settlement occurring due to consolidation is greater in normally consolidated clay than when overconsolidated.
What is an Overconsolidated clay soil?
over-consolidated clay Clay that has been more compacted than would be expected from the existing overburden, e.g. it has been subjected to pressure from overburden that has subsequently been removed by erosion. Compare NORMALLY CONSOLIDATED CLAY. A Dictionary of Earth Sciences. "over-consolidated clay ."
How is settlement amount calculated?
To calculate settlement amounts, you must have a reliable total of expenses incurred as a result of the dispute....Look at your actual damages.Actual damages also may be referred to as economic damages, or as special damages. ... Typically, this amount will represent the lowest number of your settlement range.More items...•
What is the total settlement?
Total settlement refers to the overall change in vertical distance. Differential settlement involves an expected amount that the total settlement will vary between points over a horizontal distance, which can be caused by variations in the foundation soil profile and wall height over a certain distance.
What is the difference between normally consolidated and Overconsolidated clay?
Normally consolidated clays are these that are currently experiencing the maximum vertical overburden effective pressure they have ever experienced in their history. Over consolidated clays have experienced a higher overburden stress in the past.
How do you determine if a soil is normally consolidated or Overconsolidated?
Overconsolidation. If the current state of soil is on the normal compression line it is said to be normally consolidated. If the soil is unloaded it becomes overconsolidated.
What does Overconsolidated mean?
Overconsolidated definition Consolidated to an excessive degree. adjective. (of soil) Consolidated to a greater extent than that due to the overburden.
What is primary consolidation settlement?
Primary consolidation is a process of dissipation of increased pore water pressure due to the application of loads on the soil layer and increases in the effective stress in the soil skeleton. This results in a volumetric change in the soil skeleton.
What is NC and OC clay?
If the current effective stress, s', is equal (note that it cannot be greater than) to the preconsolidation stress, then the deposit is said to be normally consolidated (NC). If the current effective stress is less than the preconsolidation stress, then the soil is said to be over-consolidated (OC).
What is OCR of a soil?
The overconsolidation ratio (OCR) is a qualitative indicator of this densification or stiffening of the soil, and it is defined as the ratio of the maximum overburden stress ever experienced by the soil (i.e., with the ice sheet on top) to the present overburden stress (i.e., without the ice sheet).
How do you calculate soil settlement?
Total settlement for cohesive soils are generally estimated by the sum of immediate settlement, primary consolidation and secondary compression, where immediate settlement usually constitutes a significant portion of the total settlement.
How much does clay settle?
If left dry, clay and silt will settle rather quickly, i.e., 1-2 years. If they come in contact with water, they can settle in just a few months.
What is total settlement in soil?
The total settlement of the ground consists of 3 components: immediate settlement (commonly referred to as elastic settlement, although this is a misnomer), consolidation settlement (or primary settlement) and creep settlement (or secondary settlement).
What is allowable settlement?
The allowable settlement is defined as the acceptable amount of settlement of the structure and it usually includes a factor of safety.
Technical paper: Settlement and heave of overconsolidated clays – a ...
A S O’Brien and P Sharp, Mott MacDonald. This paper was first published in GE’s November 2001 edition.
Settlement and heave of overconsolidated clays - ResearchGate
Download Citation | Settlement and heave of overconsolidated clays - A simplified non-linear method of calculation | Settlement at working load is usually the critical design consideration for ...
Settlement and heave of overconsolidated clays - New Civil Engineer
It is observed that compared to settlement, time dependent heave appears to develop over significantly longer periods of time. At the Shell Building, in central London, the rate of time dependent heave shows little sign of decreasing even though the excavation took place over 30 years ago, Burford 1992 11,12 .
Settlement and heave of overconsolidated clays - a simplified non ...
Title: Settlement and heave of overconsolidated clays - a simplified non-linear method of calculation (PART 2) Author: A. S. O'Brien & P. Sharp Keywords
What is settlement at working load?
Settlement at working load is usually the critical design consideration for vertically loaded foundations on stiff overconsolidated clays. Foundation settlement is usually calculated by assuming that the ground behaves as a linear elastic material. The reliability of this calculation is mainly dependent on an appropriate choice of elastic modulus.
What is the drained Young's modulus?
The drained secant Young's modulus, E9 i , is assumed to be dependent on the average mean effective stress during the load increment (or decrement) and the magnitude of vertical strain which the layer experiences. For the latter, it is necessary to iterate equation 5 until the vertical strain calculated for the layer is compatible with the strain assumed for estimating E9 i. Following a successful iteration, the final value of E9 iis the mobilised drained secant Young's Modulus for layer i, E9 i (mob) . From equation 5, e9 viis calculated following a successful iteration, and then the cumulative total vertical displacement is calculated by summing for all layers:
Who wrote the book "Numerical analysis of rectangular rafts on layered foundations"?
18. RA Fraser and LJ Wardle (1976). Numerical analysis of rectangular rafts on layered foundations. Geotechnique, Vol. 26, No. 4, pp. 613 to 630.
Abstract and Figures
Consolidation is a phenomenon where air and water in the soil skeleton (i.e clay soil) are forced out due to loading. This condition can occur when clay soil is subjected to loading resulted from pressure in laboratory test, land fill or embankment, building and other structures in the field.
References (13)
ResearchGate has not been able to resolve any citations for this publication.
Consolidation
When the construction of the foundation is done (application of additional load), pore water pressure in the saturated clay increases as the hydraulic conductivity of the clay is very small. Some time is required for excess pore water pressed to dissipate and increase the stress to be transferred to the soil skeleton.
Terminology
Consolidation of the compressible soils occurs in two stages defined previously.
Primary Consolidation
The definition of primary consolidation is discussed above under the terminology.
Secondary Consolidation
Secondary consolidation settlement occurs after the completion of dissipation of the excess pore water pressure developed immediately after the application of load.
Worked Example: Primary Consolidation Settlement Calculation in Normally Consolidated Clay
Consolidation of soil (normally consolidated) can be evaluated from the following equation as discussed in this article.
What is the overconsolidation ratio of a normally consolidated clay soil?
The overconsolidation ratio of a normally consolidated clay is unity, while the overconsolidation ratio of an overconsolidated soil is greater than unity . The nature of consolidation of a clay soil affects its behaviour in the field and when tested in the laboratory.
What is the nature of consolidation of clay soil?
Desiccation of layers close to the surface. The nature of consolidation of a clay soil affects its behaviour in the field and when tested in the laboratory. For instance, the natural moisture content of a normally consolidated clay is usually close to the liquid limit, while the natural moisture content of an overconsolidated clay is usually close ...
How to determine preconsolidation pressure?
Preconsolidation pressure can be determined from the field or in the laboratory from the e-log P curve plot (void ratio against log of pressure) after carrying out a consolidation test in the laboratory . The most widely used method was proposed by Casagrande in the year 1936 (see Figure below).
What is clay memory?
Clay soils in their natural state have memory of the magnitude of the highest pressure they have experienced in the past. This memory is locked in the soil structure and can only be broken when the clay is remoulded or reconstituted at a moisture content that is equal to or greater than the liquid limit.
What happens when clay is subjected to pressure?
When a saturated clay is subjected to external pressure, the pressure is initially taken up by the water in the pores thereby leading to excess pore water pressure. If drainage is permitted in the system, a hydraulic gradient is developed and the excess water begins to flow out of the soil mass.
How does water dissipate from the soil?
As water dissipates from the system, the pressure gradually gets transferred to the soil skeleton (soil grain) and they begin to rearrange and compress until the water is completely dissipated from the soil . This process continues until the entire pressure is transferred to the soil grains.
Is clay soil consolidated?
A clay soil is said to be normally consolidated if the effective overburden pressure that it is currently experiencing is the maximum it has ever experienced in its history. On the other hand, it is said to be overconsolidated if the present overburden pressure is less than the effective overburden pressure it has experienced in the past.
What is the Poisson's ratio for overconsolidated clay?
For drained loading, when the Poisson's ratio is less than 0.5 (generally between 0.1 and 0.2 for overconsolidated clays), the horizontal stress changes given by conventional theories are significantly in error.
Which method gives predictions of total settlement?
Burland et al 1977 27 demonstrated that the one-dimensional method gives predictions of total settlement (for vertically loaded foundations on overconsolidated clays) which are as good as and usually better than more sophisticated methods, for example Figure 7 after Nyaoro 1983 28 .For elastic soils, the one-dimensional method gives total (ie undrained plus time dependent) settlement if the drained elastic modulus is used.
How to compare heave and settlement?
Heave compared with settlement Table 2 compares the heave calculated for a net reduction in stress of 160kN/m 2with the settlement calculated for a net increase of stress of 160kN/m 2 (at the centre of a 20m wide, infinitely long strip). The calculated total heave is about 150mm compared with 100mm total settlement, and the time dependent heave is 110mm compared with 60mm time dependent settlement. The ratio R = d u/d td is significantly greater for settlement than for heave, similar to that observed in practice. This effect is purely due to the differences in average mean effective stress which are prevalent during drained unloading compared with drained loading, and which, in turn, influence the mobilised drained Young's modulus.Hence, application of the proposed method assists in explaining much of the differences which have been observed in the way heave and settlement develop.Stress path and recent stress history effects have not been taken into account, although in practice these may also lead to differences in the magnitude of heave and settlement for a given change in net bearing pressure. In this comparison, it has been assumed that there has been no change in equilibrium pore water pressure. It should be noted that the magnitude of settlement or heave is sensitive to this assumption. If it is believed that the drainage boundary conditions may change significantly as a result of foundation construction, then it may be prudent to carry out seepage analyses, so that the change in equilibrium pore water pressure can be quantified and incorporated within the analysis.
What is the proposed method?
The proposed method is considered to be a practical means of overcoming some of the difficulties which are currently encountered by foundation engineers. These difficulties include conventional methods which assume unrealistic ground behaviour (ie linear elastic) and complex non-linear finite element techniques which can be difficult to understand and validate, and time consuming to use.In overcoming these difficulties, it is believed that the proposed method is particularly suited to the fields of design and build, and value engineering where there is a need for cost-effective, rapid (ie simple) and realistic evaluations of ground movements.
Who said IRP plans will take longer to deliver than scrapped proposals?
John Porter on NPR boss claims IRP plans will take longer to deliver than scrapped proposals: ‘I think the IRP has mostly come to the right decisions, but... ‘
Is settlement closer to the loaded boundary?
settlement is concentrated closer to the loaded boundary than linear elastic methods predict; as bearing pressure is increased, settlement increases by a proportionately greater amount; as bearing pressure is increased, normalised settlements become concentrated closer to the loaded boundary.
Is time dependent heave greater than time dependent settlement?
for a given net change in effective pressure, time dependent heave due to unloading is significantly greater than time dependent settlement due to loading .
Consolidation
Terminology
- Primary Consolidation
Consolidation of the compressible soils occurs in two stages defined previously. Primary consolidation is a process of dissipation of increased pore water pressure due to the application of loads on the soil layer and increases in the effective stress in the soil skeleton. This results in a … - Secondary Consolidation
Secondary consolidation occurs after the primary consolidation that is with the dissipation of the increased pore water pressure. Secondary consolidation occurs with the rearrangement of the soil skeleton with the dissipation of the pore water pressure and with the reduction of the volume. Se…
Primary Consolidation
- The definition of primary consolidation is discussed above under the terminology. Let’s the method of primary Consolidation settlement calculation There are two different stages of soil. 1. Normally Consolidated Soil 2. Over Consolidated Soil It is required to assess the consolidation of soil is done based on the above states. In simple terms, the normally consolidated soil is experiencing …
Secondary Consolidation
- Secondary consolidation settlement occurs after the completion of dissipation of the excess pore water pressure developed immediately after the application of load. The settlement caused due to the plastic adjustment of the soil fabrics at the end of the primary consolidation is called secondary consolidation. Secondary Consolidation also can be explained as slippage and reorientation of s…
Types of Tests to Find Consolidation Settlement
- One dimensional consolidation test
- Oedometer Test
- Incremental Loading Oedometer