background information of 1d consolidation analysis

8/14/2019 Background Information of 1D Consolidation Analysis

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Background Information of 1D Consolidation

Analysis Option

Revision No. : v1.0

Revision Date : Mar 4, 2013

Program Version : SoilWorks V310(v1.2)


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00. Contents

00. Overview

01. Settlement Calculation Method

1.1 Total Load Method

1.2 Individual Load Method

02. Load Increment Calculation Method in Soft Ground

2.1 Middle of Strata

2.2 Simpson Method

03. Consideration of Depth(z) to Calculate Increase in Load

3.1 Change with Depth(Z)

3.2 Fixed Depth(Z)

04. Consolidation Option

4.1 Top-Bottom Average

4.2 Top of Strata

4.3 Bottom of Strata

05. Consider Non-Consolidation Layer for Converted Layer

06. Consider Reduced Settlement due to SCP/GCP07. Settlement Calculation for Secondary Consolidation

08. Condition for Judging Soft Ground

09. Auto-calculation standard leave time of Construction Stage Consolidation

Reference Location

10. Consider Vertical/Horizontal Simultaneously

Note. Initial Water Level

Technical Document | Introduction of 1D Consolidation Analysis Option 2


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00. Overview

s ma er a exp a ns var ous op ons o onso a on na ys s w c a e e ec s on e resu s

based on the theoretical background and equation.

In this material, influence to results based on theoretical background and equation according to

analysis option in 1D consolidation analysis is instructed. 1D consolidation analysis is a method to

calculate the settlements, consolidation degree and strength increase based on the experimental

data, various calculation method and theoretical equations. Therefore, analysis should be

each option.

Analysis control of 1D consolidation analysis is shown as figure below.

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Technical Document | Background Information of I D Consolidation Analysis Option 3


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Embankment loads or external loads applied to improve soft ground are loaded by stages

considering the loading period and the loading placement period. Loading at the next stage is

applied before the consolidation due to the embankment load at the previous stage is completed.

The settlement curve after two stages of loading becomes complex because the settlements due

to the first loading and the second loading coexist. In the reality of a construction site, the total

.

boththe total load method and the incremental load method to calculate the time-settlement

relationship by stages.

1.1 Total Load Method

The total load method calculatesthe settlements over time and the total settlement for the total

accumulated load at each stage of loading, when embankment loads or external loads are

applied by stages. The ground condition at each stage is defined by the initial original ground

condition. That is, the settlements over time are calculated for the initial ground condition at the

first stage of loading, and the settlements over time are calculated for the initial ground condition

at the second stage of loading. The second stage settlement curve is then horizontally shifted

back to intersect with the first stage curve at the time of the second stage loading.


[Figure 1-1 Horizontal shift of settlement curve under staged embankments]


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. n v ua oa e o

When embankment loads or external loads are loaded by stages, the individual load method

calculatesthe settlements over time and the total settlement for the net load pertaining to each

stage, which are then accumulated to determine the overall settlement curve. The ground

condition at each stage changes with time. That is, the settlements over time are calculated for

the sta e 1 loadin based on the initial round condition. From the sta e 2 loadin the

settlement layer is recomposed to reflect the change in effective stress and the settlement taken

place in the ground. The final settlement curve is obtained by accumulating the settlement

curves of the individual stages.

Figure 1-2. Accumulation of settlement curves by individual load method



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02. Load Increment Calculation Method in SoftGround

The increase in the effective vertical stress due to a load acting on the ground surface can be

calculated by center of layer method which usesthe incremental stress at the center of each

settlement layerand Simpson method by which the vertical stresses at the top, middle and

bottom surfaces of a layer are individually calculated, andthe incremental stress at the center of

each layeris calculated by the Simpson method with equation below.

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t m bP P P P

Slight difference of increase can be shown according to analysis sections as related to accuracy of

load increase calculated.

[Figure 2-1. Calculation of incremental stress using the Simpson method]

Middle of Strata

[Total Settlement: 1.16955m]

Simpson Method




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03. Consideration of Depth(z) to Calculate

Increase in Load

load can be defined by the distance from the bottom of the embankment or from the initial ground

surface.

3.1 Change with depth(Z) - 1) Loading Distance from the Bottom of Embankment

The loading distance is taken as the distance from the bottom of embankment at each stage to

the point of calculating the increase in the ground stress.

3.2 Fixed de th Z Loadin Distance from the Initial Ground Surface

[Figure 3-1 Calculation of loading distance from the bottom of embankment]

The loading distance is taken asthe distance from the initial ground surface to the calculation

pointof the increase in the ground stress irrespective of the embankment stages.

[Figure 3-2. Calculation of loading distance from the initial ground surface]

Technical Document | Background Information of I D Consolidation Analysis Option


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03. Consideration of Depth(z) to Calculate

Increase in Load

nce oa ng s ance s eren y ca cu a e accor ng o e v s on o sur ace, erences n

settlement can be resulted from the number of surface in the model even though the same load is

acted onto the ground surface at once.

If the embankment is divided into the several areas as Figure 3-3, the settlement will decrease as

selecting Change with Depth option to calculate increase in load.

However, if the model has combined surface for the embankment as Figure 3-4, there is almost no

difference as selecting Fixed Depth option. Therefore, for the construction stage analysis that the

surface must be divided, selecting Fixed Depth option can reduce differences caused by the

modeling.

[Figure 3-3. Dividing surface of embankment]

[Figure 3-4 Combining surface of embankment]

Dividing Surface:Total Settlement(m) Combining Surface:Total Settlement(m)

Change with Depth 1.4835m 1.5717m

Fixed Depth 1.5717m 1.5715m



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04. Consolidation Option

.

of consolidation is different for each layer, the average degree of consolidation of all the layers is

calculated using the converted thickness, and the converted consolidation coefficient. The

converted coefficient consolidation can be selected among average degree of consolidation of

individual layers, top layer and bottom layer.Although the total settlement is the same for each

option, the greater the Cv value, the less time spent on consolidation.

[Top Clay Cv : 0.014688m2/day, Bottom Clay Cv : 0.026352m2/day]The differences in the degree of consolidation after 200days of construction is as tabled below.

[Top-Bottom Average : 57.49 ]

[Top of Strata: 53.71 ]


[Bottom of Strata : 60.11 ]


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05. Consider Non-Consolidation Layer for

Converted Layer

,

unconsolidated layer may or may not be considered in the calculation of the converted thickness.

When a drainage layer exists at the bottom and a thin unconsolidated layer exists at the top, it is

not recommended to consider single face drainage for the middle consolidated layers for the

calculation of the converted thickness because of the existence of the top unconsolidated layer.

In such a case, the settlement of the thin unconsolidated layer is not calculated, but the

unconsolidated layer thickness should be included in the calculation of the converted layer

.

Even when thin unconsolidated layers exist within the multiple layers of soft clay, it is desirable to

reflect the thicknesses of the unconsolidated layers in the calculation of the converted layer

thickness.

[unconsolidated layer consideration about converted layer]

If the unconsolidated layer is relatively thicker than the consolidated layers, it is not desirable to

reflect the thickness of the unconsolidated layer thickness in the calculation of the converted layer

thickness.

[unconsolidated layer without consideration about converted layer]



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06. Consider Reduced Settlement due to

SCP/GCP

The sand compaction pile method pressure-inserts sand or gravel into the soft ground by

applying vibratory loads to form compacted piles. This method increases the density of soil,

which in turn induces an increase in bearing capacity, a decrease in consolidation settlement,

and an increase in lateral resistance, and promotes homogeneous ground and the effect of

consolidation drainage.

Stress reduction factor for cohesive soil (c) and Stress concentration factor for piles (s) can be

calculated from substitution ratio of sand (gravel) and general stress distribution ratios as figure

s own e ow.

1 1s

s

s c s

n A n

n A A n a

1

1 1

c

c

s c s

A

n A A n a

1 1s s c sa a

Considering Reduced Settlement Check)

Final settlement is calculated as below equation according to considering reduced settlement of

composite or not. Sincestress reduction factor (c) of clay is less than 1, the settlement will be

reduced compared to the case that piles are not considered.

[Total Settlement: 1.4363m]Without Considering Reduced Settlement Check Off)




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07. Settlement Calculation for Secondary

ConsolidationSecondary consolidation is referred to as the phenomenon of compression, which continues after excess pore water pressure

has completely dissipated. In Figure 7-1, the slope of the secondary consolidation is almost linear after the primary

consolidation has completed. This slope can be used to obtain the coefficient of the secondary consolidation Ca.

In general, considerable time is needed until primary consolidation is finished. And in case of considering secondary consolidation

additionally, sometimes, settlement is over calculated regarding to construction period. Therefore, secondary settlement can be

selected whether it is additionally calculated and added to the total settlement.

[Figure 7-1. Coefficient of secondary consolidation]

08. Conditions for Judging Soft Ground

vaue o san y so s use or cacuat ng mme ate sett ement. owever, vaue o c ay ayer s use asstan ar to c ass y

respond ground layer is consolidated layer or unconsolidated layer. Compare N value set in option of judging condition and N

value entered in ground property by layers, if N value entered in ground property for each layer is smaller than N value for judging

condition, ground layer is classified as consolidated layer otherwise unconsolidated layer. Settlement is not calculated in case of

unconsolidated layer and N value should be greater than 0 for consolidated layer.

0 00.4 logi

c

P P PS H

N P

Immediate settlement of sandy soil is calculated according to De Beer method using upper equation, however, when N value is 0,

o a se emen s o e n n e va ue an canno e ca cu a e . us, rea num er grea er an s ou e en ere or va ue

even actual instrumented value is 0. In case of clay layer, enter number smaller than N value for judging condition to be classified

as soft ground.

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1D consolidation in SoilWorks, when calculating idle time by construction stages, leave time can be entered oridle time can be

09. Auto-calculate standard leave time of

Construction Stage Consolidation Reference Location

calculated automatically by target degree of consolidationentered. When degree of consolidation is entered to calculate idle time,

standard position for calculating idle time should be selected. Thus, select None when leave time is entered directly. In case that

degree of consolidation is entered only, standard position can be selected among one of settlement calculation positions. When

standard position is not set, analysis can not be performed. In general, standard position is defined as center point where

embankment load is greatest.

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10. Consider Vertical/Horizontal Simultaneously

This o tion is for the case of installin draina e.Draina e basicall takes effect on acceleratin consolidation exce t method. , ,

considering strength increase such as SCP/GCP. In other words, degree of consolidation increases during construction, although

total settlement has no change. According to drainage characteristics, drainage effect can be considered in vertical direction only

or simultaneously in both vertical and horizontal direction. In case of considering vertical and horizontal simultaneously, the

average degree of consolidation regarding to total consolidation layer is calculated by equation below.

[Figure 10-1. Consider Vertical/Horizontal Simultaneously

(Check) 99.76%]

[Figure 10-2. Consider Vertical/Horizontal Simultaneously

(Check off) 99.08%]

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In terms of 1D consolidation settlement, large difference of settlements happens because initial stress differs according to whether

considering water level or not. Therefore, thewater level must be set before performing analysis.

background information of 1d consolidation analysis

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