PART V FOUNDATIONS

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7.3.2 Determination of Height and Width of Fill

[1] Height and Width of Fill Required for Soil ImprovementThe height and width of fill shall be determined by considering the strength increment of soil required forstability of sutructures to be constructed as well as the allowable settlement, influence on the surroundingarea, and others.

[Commentary]

It is desirable to set the fill top width larger thanthe width required for soil improvement (see Fig.C- 7.3.1).

[2] Height and Width of Fill Required for Stability of Fill Embankment

Stability of fill embankment itself shall beconfirmed by a circular arc analysis or otherappropriate methods for the dimensions ofheight and width of fill determined.

[Technical Notes]

The strength increment of soil and the settlementdue to fill may be determined using equations (7.3.1) and (7.3.2).

(7.3.1)

(7.3.2)where

h fill height (m) H thickness of clay layer (m)

mv coefficient of volume compressibility (m2/kN)pc preconsolidation pressure (kN/m2)S settlement (m)U degree of consolidation

coefficient of stress distribution (ratio of vertical stress distributed inside subsoil to fill pressure) ' effective unit weight of fill material (kN/m3)

increment of undrained shear strength (kN/m2) / p rate of strength increase

Since surcharge is usually applied in several stages in the vertical drain method, the degree of consolidation U to besubstituted in equations (7.3.1) and (7.3.2) differs at each surcharge stage. However, strength increment may often becalculated by assuming a uniform degree of consolidation of approximately 80%.

7.3.3 Design of Drain PilesIn designing drain piles, consolidation process shall be calculated by taking account of drain pile interval,drain pile diameter, and drainage conditions at the top and bottom of clay layer as well as thecharacteristics of drain materials and sand mat, and thickness of sand mat.

[1] Drain Piles and Sand MatDrain piles and sand mat shall have the required drainage capacity.

[Technical Notes]

(1) Consolidation Rate and Drain Pile DiameterConsolidation rate is approximately proportional to drain pile diameter and inversely proportional to the squareof drain pile interval. Generally, the amount of drain pile materials can be reduced by using sand piles of smalldiameter with a small interval rather than using those of large diameter with a large interval. However, the smalldiameter sand pile may suffer from clogging with clayey particles and breaking failure of the pile caused bydeformation due to surcharge and consolidation. According to case histories, a diameter of approximately 40 cmis prevaling, while the diameter generally varies between 30 and 50 cm. The fabri-packed drain method, inwhich sand piles wrapped by geotextile with a diameter of 12 cm are drived in, has often been used forextremely soft subsoil in land construction works. Usually, a set of four sand piles are installed simultaneously

Fig. C- 7.3.1 Fill Width for Vertical Drain Method

c c p h pc U

S mv h pc HU

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by a small size pile driving machine. In marine works, fabri-packed drains with a diameter of 40 cm or morehave been commonly used to improve extremely soft subsoil.

(2) Sand Used for Sand PilesSand used for sand piles should have high permeability as well as suitable particle sizes to prevent clogging withclayey particles. The particle size distribution of sand used in past works are shown in Fig. T- 7.3.1. Sand withslightly higher fines content have also been used in recent years.

Fig. T- 7.3.1 Examples of Sand Used in Sand Piles

(3) Materials for Plastic-Board Drain In lieu of sand piles, strip type drains with materials such as plastic-board drains are sometimes employed. In thedesign of the strip type drains, the strip is converted to a sand pile with a diameter that produces the perimeterequal to that of the strip type drain in order to apply the conventional design method. By setting the safety factorat a value higher than that of the conventional method, the design has been conducted by assuming that a striptype drain of 10 cm wide and 5 mm thick is equivalent to a 5 cm-diameter sand pile. When the drainage capacityis low, it should be considered that a possibe delay of consolidation could occur especially at the tip of thevertical drain: i.e, at the bottom of the consolidation layer.

(4) Sand MatThickness of sand mat is usually set to be approximately 1.0 m to 1.5 m for marine works and 0.5 m to 1.0 m forland works. A thick layer of sand mat may cause troubles in drain pile driving, while a thin layer of sand matmay lose its permeability partially due to infiltration of clayey particles. When the drainage capacity of sand matis low, a delay in consolidation may occur due to the head loss within drains. In such a case, the delay inconsolidation is more noticeable around the center of the area spreaded with the sand mat. Thus, the sand matmaterials must have high permeability. For a case that the delay is anticipated because of low permeability ofsand mat or large area of soil improvement work, an approximate solution may be employed to evaluate thedelay.

[2] Interval of Drain PilesInterval of drain piles shall be so determined that the required degree of consolidation can be obtainedduring a given construction period.

[Technical Notes]

(1) GeneralThe vertical drain method is applied when the rate of one-dimensional consolidation by the preloading method istoo small under the constraint of time limit of the construction period. Figure T- 7.3.2 shows the relationshipbetween the required consolidation time t80 (day), drainage distance H (m), and coefficient of consolidation cv(cm2/min), which is calculated for the condition of 80% consolidation of a clayey layer for the preloadingmethod or the vacuum consolidation method without vertical drains.

(2) Determination of Interval of Drain PilesThe interval of drain piles should be determined by means of Fig. T- 7.3.3 and equation (7.3.3). If the interval ofdrain piles is too small, consolidation may be delayed due to the effect of smear (the disturbance of clay layer bythe process of drain pile driving) and others 2).

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Fig. T- 7.3.2 Required Days for 80% Consolidation of Clay Layer

(7.3.3)where

D : interval of drain piles (cm): factor ( = 0.886 for square grid pattern, and = 0.952 for triangular grid pattern)

n : diameter ratio of De/Dw (n is read off from Fig. T- 7.3.3)De : effective diameter of drain area (cm)Dw : diameter of drain pile (cm)Th : parameter similar to time factor (Th = cvh t/Dw

2)cvh : horizontal coefficient of consolidation (cm2/min)

t : consolidation time (min)

Note: the time t used in Figs. T- 7.3.3 and 7.3.4 is expressed in the units of days.

Fig. T- 7.3.3 Calculation Chart for n

(3) Calculation of the Degree of ConsolidationAfter determining the interval of drain piles, the accurate value of the degree of consolidation Uh is calculated byequations (7.3.4) and (7.3.5) and Fig. T- 7.3.4.

D nDw

TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN

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(7.3.4)

(7.3.5)

whereTh : time factor of horizontal consolidationcvh : horizontal coefficient of consolidation (cm2/min)

t : elapsed time from start of consolidation (min)De : effective diameter of drain area (cm) Dw : diameter of drain pile (cm)

Fig. T- 7.3.4 Horizontal Consolidation Calculation Chart

(4) Effective DiameterThe effective diameter of drain area De is the diameter of an equivalent circle that has the same area as the soilbeing drained by a sand pile. The relationship between De and interval of the drain pile D is as follows:

De = 1.128D for square grid pattern.

De = 1.050D for equilateral triangular grid pattern.

7.4 Deep Mixing Method

7.4.1 Principle of Design

[1] Scope of Application

(1) The design method described in this section shall be applied to improvement of subsoil beneathgravity type structures such as breakwaters, quaywalls, and revetments.

(2) The design method shall be applied to the block type and the wall type improvement works.

[Commentary]

(1) The deep mixing method dealt with in this section is the one in which the soil in situ is mixed mechanically withcement.

(2) Large-scaled soil improvement with the deep mixing method in port and harbor construction works have beenmostly applied to the subsoils for caisson type breakwaters, quaywalls, or revetments. There are few cases ofapplication to other types of structures. Thus, the sope of this section is set as specified above.

(3) When applying the deep mixing method to port and harbor structures, a rigid underground structure is formed byoverlapping stabilized soil columns treated by mixing machines. The pattern of improvement is selecteddepending on the superstructure or the properties of the subsoil. The block type and the wall type shown in Fig.C- 7.4.1 are typical improvement patterns in port and harbor construction works. In this section, these twopatterns are discussed.

Thcvht

De2

nDeDw