4.6 Lightweight Treated Soil Method

TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 4.6 Lightweight Treated Soil Method

(1) Definition and Outline of Lightweight Treated Soil Method ① The provisions in this section can be applied to the performance verification of the light weight treated soil method. ② The lightweight treated soil method is to produce artificial lightweight and stable subsoil by adding lightening materials and hardening agents to slurry-state soil in adjusting its consisting being higher than liquid limit by making use of dredged soil or excavated soil from construction sites, and then using the product as materials for landfill or backfilling. When using air foam as the lightening material, it is called the foam treated soil, and when using expanded polistyrol beads, it is called the beads treated soil. The lightweight treated soil has the following characteristics: (a) The weight is approximately one half of ordinary sand in the air and approximately one fifth in the seawater. This lightness can prevent or reduce ground settlement due to landfill or backfill. (b) Due to its light weight and high strength, the earth pressure during an earthquake is reduced. This makes it possible to create high earthquake-resistance structures or reclaimed lands. (c) Dredged soils, which are regularly produced and treated as waste in ports, or waste soils that are generated by land–based construction works, are used. Thus, employment of the lightweight treated soil method can contribute to reducing the amount of waste materials to be dealt with at waste disposal sites. ③ Refer to the “Technical Manual for the Lightweight Treated Soil Method in Ports and Airports” for further details on the performance verification of this method. (2) Basic Concept of Performance Verification ① The performance verification method described in 2 Foundations and 3 Stability of Slopes can be applied to lightweight treated soil. ② Apart from mix proportion tests, the performance verification method for lightweight treated soil is basically the same with that for other earth structure.73), 74) ③ An example of the performance verification procedure when using the lightweight treated soil method in backfilling for revetments and quaywalls is shown in Fig. 4.6.1.

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Determination of application of lightweight treated soil method

Assumption of strength and unit weight of lightweight treated soil

Assumption of area (or bounds, boundary) of improvement with lightweight treated soil

Examination of ground as a whole, including lightweight treated soil ① Evaluation of actions ② Examination of bearing capacity ③ Examination of circular slip failure ④ Examination of consolidation settlement ⑤ Examination of liquefaction of surrounding ground

Performance verification of superstructure

Determination of strength/unit weight and area of improvement with lightweight treated soil

Fig. 4.6.1 Example of Performance Verification Procedure of Lightweight Treated Soil Method

④ In performance verification, the following actions are generally considered. (a) Self weight of lightweight treated soil, and self weight of main body (caissons, etc.), backfilling material, filling material, reclaimed soil and mound materials, (considering buoyancy). (b) Earth pressure and residual water pressure (c) Surcharges including fixed loads, variable loads and repeated loads (d) Tractive force of ship and reaction of fenders (e) Actions in respect of ground motion In calculations of earth pressure and earth pressure during earthquakes, the concepts in 4.18 Active Earth Pressure of Geotechnical Material Treated with Stabilizer can be applied. ⑤ The properties of lightweight treated soil shall be evaluated by means of laboratory tests that take account of the environmental and construction conditions of the site. They may be evaluated as follows: (a) Unit weight 3 The unit weight may be set within a range of γt = 8-13 kN/m by adjusting the amount of lightening material and added water. When used in port facilities, there is a risk of flotation in case of a rise of seawater level if the unit weight is less than that of seawater. Normally, therefore, the characteristic value of the unit weight is frequently set to the following values: below water level: 3 for use uder water: γtk = 11.5-12 kN/m 3 for use in air: γtk = 10 kN/m The unit weight of lightweight treated soil will vary depending on the environmental conditions during and after placement, and particularly the intensity of water pressure. Therefore, these factors should be considered in advance in the mixture design.75), 76) (b) Strength 77) The static strength of lightweight treated soil is mainly attributable to the solidified strength due to the cement-based solidifying agent. Standard design strength is evaluated by unconfined compressive strength 2 qu and can generally be set in the range of 100–500kN/m . Because air foam or expanded beads are included in the treated soil, no increase in strength can be expected due to increased confining pressure. However, the residual strength is approximately 70% of the peak strength. The characteristic value of compressive strength

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shall be the standard design strength and be set to an appropriate value capable of satisfying performance requirements such as stability of the superstructure or the ground as a whole. As the characteristic value of shear strength, undrained shear strength cu can be used. The value of cu can be calculated using the following equation.

(4.6.1)

(4.6.2)

(d) Deformation modulus E50 When tests are conducted considering fine points such as measurement of small amounts of deformation, finishing of the ends of specimens, the test value as such is used as the deformation modulus E50. When such tests are not possible, the modulus can be estimated from the unconfined compressive strength qu using the following equation:

(4.6.3) The deformation modulus shown above corresponds to a strain level of 0.3–1.0%. (e) Poisson’s ratio Poisson’s ratio of lightweight treated soil varies depending on the stress level and the state before or after the attainment of peak strength. When the surcharge is less than the consolidation yield stress of treated soil, the following mean values may be used:

air foamed treated soil: v = 0.10 expanded beads treated soil: v = 0.15 (f) Dynamic properties The shear modulus G, damping factor h, strain dependency of G and h, and Poisson’s ratio v used in dynamic analysis should be obtained from laboratory tests. They may be estimated from the estimation method conducted for the ordinary soils as a simplified method in reference to the results of ultrasonic propagation test. (3) Examination of Area of Improvement 78) ① The area to be filled with the lightweight treated soil needs to be determined as appropriate in view of the type of structure to be built and the conditions of actions as well as the stability of the structure and the ground as a whole. ② The extent of filling area with lightweight treated soil is usually determined to meet the objective of lightening. When the method is applied to control settlement or lateral displacement, it is determined from the allowable conditions for settlement or displacement; to secure stability, it is determined from the condition of slope stability; to reduce earth pressure, it is determined from the required conditions for earth pressure reduction.79) (4) Concept of Mix Proportion ① Design of mix proportion shall be conducted to obtain the strength and the unit weight required in the field. ② Types of solidifying agents and lightening agents shall be determined after their efficiency has been confirmed in tests. ③ The target strength in laboratory mix proportion tests shall be set to a value obtained by multiplying the standard design strength by a required additional rate α, considering differences in laboratory mix proportion strength and in-situ strength and variance. The required additional rate α is expressed by the ratio of the strength in laboratory mix proportion tests and standard design strength. Normally, the following value can be used.

a = 2.2

– 520 – PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.7 Blast Furnace Granulated Slag Replacement Method

(1) Basic Concept of Performance Verification ① When using blast furnace granulated slag as backfill for quaywalls or revetments, landfill, surface covering for soft subsoil and sand compaction material, the characteristics of the materials shall be considered. ② Blast furnace granulated slag is a granular material. However, it has a latent hydraulic hardening property not found in natural sand and is a material which solidifies with lapse of time.83) When used in backfill, if its granular state and solidified state are compared, the granular state generally gives a dangerous state in the performance verification in many cases. Provided, however, that it is preferable to conduct an adequate examination, judging the individual conditions, in cases where the solidified state may pose a risk to the facilities. (2) Physical Properties ① When using granulated blast furnace slag, its physical properties are preferably to be ascertained in advance. ② Blast furnace granulated slag is in a state like coarse sand when shipped from plants. The important characteristics of physical properties of the blast furnace granulated slags are its small unit weight latent hydraulic hardening property. ③ Grain size distribution The range shown in Fig. 4.7.1 is generally standard for the grain size distribution of blast furnace granulated slag. The standard grain size of blast furnace granulated slag is 4.75 mm or less, and its fines content is extremely small. Thus, it has a stable, comparatively uniform grain size distribution. The coarse sand region accounts for the larger part of the grain sizes, with a uniformity coefficient of 2.5–4.2 and a coefficient of curvature of 0.9-1.4.

2 Percentage finer by weight (%) 0 0. 1. 10. 50. Grain size D (mm)

Fig. 4.7.1 Standard Grain Size Distribution of Blast Furnace Granulated Slag

④ Unit weight 83) Blast furnace granulated slag is lighter in weight than natural sand because its grains contain air bubbles and it has a large void ratio due to its angular shape and single grain size distribution. According to the results of studies to date, the wet unit weight of granulated slag ranges from 9-14kN/m3, and its unit weight in water is approximately 8kN/m3. ⑤ Permeability The coefficient of permeability in the granular state differs depending on the void ratio but is roughly 1×100- 1×10-1cm/s. The coefficient of permeability decreases with solidification, but even in this case is approximately 1×10-2cm/s.85) Provided, however, that when construction is conducted using methods that cause crushing of the particles, for example, in the sand compaction pile method, the coefficient of permeability becomes extremely small. Caution is required in such cases. ⑥ Compressibility The time-dependent change of compressibility of blast furnace granulated slag used for backfill, landfill, or surface covering can be ignored.

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⑦ Angle of shear resistance and cohesion In the granular state, cohesion can be treated as non-existent. The angle of shear resistance in this case is 35º or greater. When solidified, shear strength is greater than in the granular state.83) In this case, the effects of both the angle of shear resistance and cohesion on maximum shear strength can be considered. However, in examining residual strength, only the effect of the angle of shear resistance should be considered. ⑧ Liquefaction during an earthquake When blast furnace granulated slag is used in backfill, it solidifies in several years because of its latent hydraulic hardening property. When solidification can be expected, liquefaction can be ignored. However, there is a risk of liquefaction for blast furnace granulated slag that has not yet solidified. Therefore in this case, the possibility of liquefaction should be examined, treating the blast furnace granulated slag as a granular material. (3) Chemical Properties ① When using blast furnace granulated slag, appropriate consideration shall be given to its chemical properties. ② The pH value of the leached water from blast furnace granulated slag is smaller than the pH of the leached water from cement and lime stabilization treatment. Furthermore, its pH is also reduced by the neutralizing and buffering action of the seawater composition and dilution by seawater. For this reason, in ordinary cases, it is not necessary to consider the effect of the pH on the environment.

– 522 – PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.8 Premixing Method 4.8.1 Fundamentals of Performance Verification

(1) Scope of Application ① The performance verification described in this section may be applied to the performance verification of the subsoil treated by the premixing method aimed at earth pressure reduction and liquefaction, prevention. ② The meanings of the terms used in connection with this method are as follows: Treated soil: Soil improved by stabilizer. Treated subsoil: Subsoil improved by filling with treated soil. Area of improvement: Area to be filled with treated soil. Stabilizer content: Weight ratio of stabilizer to dry weight of parent material, expressed as a percentage. Reduction of earth pressure: Measures to reduce earth pressure against walls (active earth pressure). ③ In the premixing method, stabilizer and antisegregation agent are added into soil for reclamation, mixed in advance and used as landfill materials to develop stable ground. The subsoil improvement is materialized as cement-based stabilizers add cohesion to the soil used in landfill by means of chemical solidification action between soil and stabilizer. This method can be applied to backfill behind quaywalls and revetments, filling of cellular-bulkhead, replacement after sea bottom excavation and refilling. ④ Soils applicable to the treatment mentioned herein are sand and sandy soils, excluding cohesive soil. This is because the mechanical properties of the treated cohesive soil differ considerably depending on the characteristic of soil. It is necessary to conduct appropriate examination according to the property of soil subject to treatment. ⑤ Besides reducing earth pressure and preventing liquefaction, this method can also be used to improve the soil strength necessary for construction of facilities on reclaimed lands. In this case, the strength of treated ground should be evaluated appropriately. ⑥ For items in connection with the performance verification and execution when using the premixing method which are not mentioned herein, Reference 1) can be used as a reference. (2) Basic Concepts ① In performance verification, it is necessary to determine the required strength of the treated soil correctly, and to determine the stabilizer content and area of improvement appropriately. ② When evaluating the earth pressure reduction effect or examining the stability of the subsoil against circular slip failure, the treated soil should be regarded as a “c-φ material”. ③ The treated subsoil may be thought to slide as a rigid body during an earthquake because the treated subsoil has a rigidity considerably greater than that of the surrounding untreated subsoil. Therefore, when determining the area of improvement, the stability against sliding of the subsoil including superstructures shall also be examined. ④ It is preferable to determine the standard design strength and area of improvement of treated subsoil by the procedure shown in Fig. 4.8.1. ⑤ In general, when the parent soil is sandy soil, the treated soil is regarded as c-ø material. Therefore, the shear strength of the treated soil can be calculated using equation (4.8.1).

(4.8.1) where 2 τf : shear strength of treated soil (kN/m ) σ’ : effective confining pressure (kN/m2) c : cohesion (kN/m2) φ : angle of shear resistance (º)

c and φ correspond to the cohesion cd and angle of shear resistance ød obtained by the consolidated-drained triaxial compression test, respectively.

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Preliminary survey and tests of untreated and treated soil

Evaluation of actions

Determination of angle of shear resistance (φ) of treated subsoil

Assumption of cohesion (c) and area of improvement of treated subsoil

Examination of liquefaction countermeasures and earth pressure reduction effect

Stability of facilities

Determination of standard design strength and area of improvement of treated subsoil

Fig. 4.8.1 Example of Performance Verification Procedure for Premixing Method

4.8.2 Preliminary Survey

(1) The characteristics of soil used in the premixing method need to be evaluated appropriately by preliminary surveys and tests. (2) Preliminary surveys and tests include soil tests on particle density, water content, grain size distribution, maximum and minimum densities of soils to be used for filling, and surveys on records of soil properties and field tests of the existing reclaimed ground nearby. (3) Because the water content, and fines content of soils used in reclamation will affect the selection of the mixing method when mixing the stabilizer and strength grain after mixing, caution is necessary. (4) The density of the treated subsoil after placement should be estimated properly in advance. Because the density of the subsoil after reclamation is basic data for determining the density for samples in laboratory mix proportion tests and has a major effect on the test results, caution is necessary.

4.8.3 Determination of Strength of Treated Soil

(1) The strength of treated soil needs to be determined in such a way to yield the required improvement effects, by taking account of the purpose and conditions of application of this method. (2) For the purpose of reducing the earth pressure, the cohesion c of treated soil needs to be determined such that the earth pressure is reduced to the required value. (3) For the purpose of preventing liquefaction, the strength of treated soil needs to be determined such that the treated soil will not liquefy. (4) There is a significant relationship between the liquefaction strength and the unconfined compressive strength of treated soils. It is reported that treated soils with the unconfined compressive strength of 100 kN/m2 or more will not liquefy. Therefore, when aiming to prevent liquefaction, the unconfined compressive strength as an index for strength of treated soil should be set at 100 kN/m2. When the unconfined compressive strength of treated soil is set at less than 100 kN/m2, it is preferable that cyclic triaxial tests should be conducted to confirm that the soil will not liquefy. (5) In determining the cohesion of treated soil, the internal friction angle φ of treated soil is first estimated. Then, the cohesion is determined by reverse calculation using an earth pressure calculation formula that takes account of cohesion and angle of shear resistance with the target reduced earth pressure and the estimated angle of shear resistance φ .

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(6) According to the results of consolidated-drained triaxial compression tests of treated soil with a stabilizer content of less than 10%, the angle of shear resistance of the treated soil is equal to or slightly larger than that of the parent soil. Accordingly, in the performance verification, to be on the safe side, the angle of shear resistance of the treated soil can be assumed to be the same as that of the untreated soil. (7) When obtaining the angle of shear resistance from a triaxial compression test, the angle of shear resistance is obtained from the consolidated-drained triaxial compression test based on the estimated density and effective overburden pressure of the subsoil after landfilling. The angle of shear resistance used in the performance verification is generally set at a value 5-10º smaller than that obtained from tests. When a triaxial test is not performed, ø can be obtained from the estimated N-value of the subsoil after landfilling. In that case, the N-value of the untreated subsoil shall be used.

4.8.4 Design of Mix Proportion

(1) Mix proportion of treated soil shall be determined by conducting appropriate laboratory mixing tests. A reduction of strength shall be taken into account because the in-situ strength may be lower than the strength obtained from laboratory mixing tests. (2) The purpose of laboratory mixing tests is to obtain the relationship between the strength of treated soil and the stabilizer content, and to determine the stabilizer content so as to obtain the required strength of treated soil. The relationship between the strength of treated soil and the stabilizer content is greatly affected by the soil type and the density of soil. Therefore, test conditions of laboratory mixing tests is preferable to be as similar to field conditions as possible. (3) For the purpose of reducing earth pressure, consolidated-drained triaxial compression tests should be carried out to obtain the relationship among the cohesion c, the angle of shear resistance φ, and the stabilizer content. For the purpose of preventing liquefaction, unconfined compression tests should be conducted to obtain the relationship between the unconfined compressive strength and the stabilizer content. (4) It is important to grasp the difference between in-situ and laboratory strengths when setting the increase factor for mix proportion design in the field. According to past experience, the laboratory strength is larger than the in-situ strength, and the increase factor of α= 1.1 to 2.2 is used. Here, the increase factor α is defined as the ratio of the laboratory to the field strengths in terms of unconfined compressive strength.

4.8.5 Examination of Area of Improvement

(1) The area to be improved by the premixing method needs to be determined as appropriate in view of the type of structure to be constructed and the conditions of actions as well as the stability of subsoil and structures as a whole. (2) For the purpose of reducing earth pressure, the area of improvement needs to be determined in such a way that the earth pressure of treated subsoil acting on a structure should be small enough to guarantee the stability of the structure. (3) For the purpose of preventing liquefaction, the area of improvement needs to be determined in such a way that liquefaction in the adjacent untreated subsoil will not affect the stability of structure. (4) The actions and resistances to be considered on the facilities and the treated subsoil in the case that liquefaction is expected on the untreated subsoil behind the treated subsoil and in the case no liquefaction is expected are shown in Fig. 4.8.2 and Fig. 4.8.3, respectively. (5) For either reduction of earth pressure or prevention of liquefaction, it is necessary to conduct an examination of stability against sliding during action of ground motion, including the treated subsoil and the object facilities, and circular slip failure in the Permanent situation. ① Examination of sliding during action of ground motion Examination of sliding during action of ground motion is performed because there is a possibility that the treated subsoil may slide as a rigid body. As the partial factor γa which is used in this case, in general, an appropriate value of 1.0 or higher is assumed, and as the characteristic value of the coefficient of friction of the bottom of the treated subsoil, 0.6 can be used. Provided, however, that when the original subsoil in the calculation of the sliding resistance of the bottom of the treated subsoil is clay, the cohesion of the original subsoil can be used. The resultant of earth pressure in equation (4.8.2) of stability against sliding when untreated ground does not liquefy, as presented below, shows a simple case in which the residual water level is at the ground surface. When the residual water level exists underground and the untreated ground liquefies, it is considered that the subsoil above the residual water level also liquefies by propagation of excess water pressure from the lower subsoil.

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Such cases can be treated as liquefaction reaching the surface. When the purpose is reduction of earth pressure, in general, the area of improvement takes the shape of the treated subsoil as shown in Fig. 4.8.2, such that the active collapse plane is completely included in the stabilized body. On the other hand, when the purpose is a countermeasure against liquefaction, if the shape of the treated subsoil shown in Fig. 4.8.2 is adopted, liquid pressure from the liquefied subsoil will act upward on the treated subsoil, reducing the weight of the treated subsoil. Because the shape of the treated subsoil shown in Fig. 4.8.2 is disadvantageous for sliding in comparison with the shape of the treated subsoil shown in Fig. 4.8.3, when the purpose is use as a liquefaction countermeasure, the shape of the treated subsoil shown in Fig. 4.8.3 is generally used. (a) When purpose is reduction of earth pressure If the positive direction of the respective actions and resistances is defined as shown in Fig. 4.8.2, the verification of stability against sliding can be performed using equation (4.8.2). In the following, the symbol γ is the partial factor of its subscript, and the subscripts k and d denote the characteristic value and design value, respectively.

(4.8.2) In this equation, the design values can be calculated as follows.

(when original subsoil under treated subsoil is sand)

(when original subsoil under treated subsoil is clay)

(4.8.3)

where R1 : frictional resistance of bottom surface of structure (ab) (kN/m) R2 : frictional resistance of bottom surface of treated subsoil (bc) (kN/m) Pw1 : resultant of hydrostatic water pressure acting on front of structure (af) (kN/m) Pw2 : resultant of dynamic water pressure acting on front of structure (af) (kN/m) Pw3 : resultant of hydrostatic water pressure acting on back of treated subsoil (cd) (kN/m) H1 : inertia force acting on structure (abef) (kN/m) H2 : inertia force acting on treated subsoil body (bcde) (kN/m) Ph : horizontal component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kN/m) Pv : vertical component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kN/m) 3 ρwg : unit weight of seawater (kN/m ) w' : unit weight of untreated subsoil in water (kN/m3) kh : seismic coefficient for verification Ka : coefficient of active earth pressure during earthquake of untreated subsoil h1 : water level at front of structure (m) h2 : residual water level, for simplicity in this explanation, the residual water level in Fig. 4.8.2 is assumed to be the ground surface. δ : friction angle of wall between treated subsoil and untreated subsoil (cd) (º)

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φ : angle of back of treated subsoil (cd) to vertical direction (º), counterclockwise is positive; in Fig. 4.8.2, the value of φ is negative. f1 : coefficient of friction of bottom of structure f2 : coefficient of friction of bottom of treated subsoil (= 0.6) c : cohesion of original subsoil (kN/m2) lbc : length of bottom of treated subsoil (bc) (m) γa : structural analysis factor

(b) When used as liquefaction countermeasure If the positive direction of the respective actions and resistances is defined as shown in Fig. 4.8.3, verification of stability against sliding can be performed using equation (4.8.4). In the following, the symbol γ is the partial factor of its subscript, and the subscripts k and d denote the characteristic value and design value, respectively. When the untreated subsoil at the back of the treated subsoil liquefy, the static pressure and dynamic pressure from the untreated subsoil generally act on the back of the treated subsoil as shown in Fig. 4.8.3. Static pressure can be calculated by addition hydrostatic pressure to earth pressure, assuming the coefficient of earth pressure to be 1.0. Dynamic pressure can be calculated using equation (2.2.1) and equation (2.2.2) shown in Part II, Chapter 5, 2.2 Dynamic Water Pressure. Provided, however, that the unit weight of water in equation (2.2.1) and equation (2.2.2) is replaced with the unit weight of saturated soil.

(4.8.4)

In this equation, the design values can be calculated as follows.

(when original subsoil under treated subsoil is sand) (when original subsoil under treated subsoil is clay)

(4.8.5)

where R1 : frictional resistance of bottom surface of structure (ab) (kN/m) R2 : frictional resistance of bottom surface of treated subsoil (bc) (kN/m) Pw1 : resultant of hydrostatic water pressure acting on front of structure (af) (kN/m) Pw2 : resultant of dynamic water pressure acting on front of structure (af) (kN/m) H1 : inertia force acting on structure (abef) (kN/m) H2 : inertia force acting on treated subsoil body (bcde) (kN/m) Ph : horizontal component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kN/m) 3 ρwg : unit weight of seawater (kN/m ) w’ : unit weight of untreated subsoil in water (kN/m3) kh : seismic coefficient for verification Ka : coefficient of active earth pressure during earthquake of untreated subsoil h1 : water level at front of structure (m) h2 : water level used in calculating Ph due to liquefaction (This water level is assumed to be the ground surface level.) φ : angle of back of treated subsoil (cd) to vertical direction (º), counterclockwise is positive; in Fig. 4.8.3, the value of φ is negative.

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f1 : coefficient of friction of bottom of structure f2 : coefficient of friction of bottom of treated subsoil (= 0.6) c : cohesion of original subsoil (kN/m2) lbc : length of bottom of treated subsoil (bc) (m) γa : structural analysis factor

(c) Partial factors For all partial factors in the examination of sliding during action of ground motion, including the treated subsoil and the object facilities, 1.00 can be used. ② Examination of stability against circular slip failure in Permanent situation For the examination of stability against the circular slip failure in the Permanent situation, 3 Stability of Slopes can be used as a reference. (6) When it is not possible to secure the stability of the facilities or the ground as a whole, it is necessary to modify the area of improvement, or to increase the standard design strength of the treated soil, etc.

Structure Treated subsoil Untreated subsoil (not liquefied) f e d

(–) ψ (+) H2 H1 Ph h2

1 Pv h w P 2 W1 W 1' W2 W2 ' Pw1 Pw3 a b c

R1 R 2 Fig. 4.8.2 Diagram of Actions when Purpose is Reduction of Earth Pressure

Structure Treated subsoil Untreated subsoil (not liquefied) f e d Dynamic pressure ( ) (earth + water) (–) ψ + Pv H1 H2 h2

h 1 Pw2 Ph W1 W 1' W2 W2 ' Pw1 Static pressure (earth + water) a b c

R1 R 2

Fig. 4.8.3 Diagram of Actions when Used as Liquefaction Countermeasure

– 528 – PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.9 Sand Compaction Pile Method (for Sandy Soil Ground) 4.9.1 Basic Policy for Performance Verification

(1) The performance verification of the sand compaction pile method to densify sandy soils needs to be conducted appropriately after examining the characteristics of subsoil properties and construction methods, as well as by taking account of the past construction records and the results of test execution. (2) Purpose of Improvement The purpose of improving loose sandy subsoil can be classified into (a) improving liquefaction strength, (b) reducing settlement, and (c) improving the stability of slopes or bearing capacity. (3) Factors affecting compaction effect In many cases, compaction to firm ground of loose sand subsoil cannot be achieved adequately by vibration or impact from the surface. Therefore, the methods normally adopted are to construct piles of sand or gravel in the loose sandy subsoil using hollow steel pipes or to drive special vibrating rods, so as to vibrate the surrounding ground.

4.9.2 Verification of Sand Supply Rate

(1) In the verification of the sand supply rate, improvement ratio or replacement ratio, it is necessary to conduct an adequate examination of the characteristics of the object ground, necessary relative density, and N-value. (2) Setting of Target N-value It is necessary to set the N-value of the improvement target. Furthermore, when the purpose of the sand compaction pile method is a liquefaction countermeasure, it is necessary to set the N-value to a value at which it is judged that liquefaction will not occur under the object ground motion. The N-value is defined as the limit N-value. (3) Sand Supply Rate The sand supply rate is the percentage of the sand piles after improvement in the original subsoil, as shown in equation (4.9.1).

(4.9.1) (4) Determination of Sand Supply Rate when Existing Data are not available 87) The sand supply rate is determined using the relationship between the sand supply rate and the N-value after improvement shown by the following equation. Provided, however, that the existing data used in deriving the following equation (4.9.2) through equation (4.9.9) are sand supply rate FV = 0.07-0.20 and fines content Fc = 60% or less. Accordingly, caution is necessary when using conditions outside of this range.

(4.9.2) where N1 : N-value after sand supply 2 CM : coefficient; here, CM = (1/0.16) may be used. κ : coefficient; here κ = 5·10–0.01Fc may be used.

c : coefficient; here may be used.

Fc : coefficient; fines content (%) γi* : coefficient calculated using equation (4.9.3)

(4.9.3)

where N0 : N-value of original subsoil A : coefficient calculated using equation (4.9.4)

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(4.9.4) where 2 σv’ : effective overburden pressure when measuring N-value (kN/m )

Equation (4.9.2) can be solved for the sand supply rate Fv, and the sand supply rate for obtaining the target N-value can be obtained using the following equation.

(4.9.5)

Because equation (4.9.2) and equation (4.9.3) do not consider the effect of the increase in lateral pressure due to sand supply or the effect of coefficient of earth pressure at rest K0, there is a tendency to underestimate the N-value after sand supply when the sand supply rate is large. When a result is obtained in which the sand 88) supply rate exceeds FV = 0.2, a method using the following equation, which considers the effect of K0, is also available. Provided, however, that caution is necessary, as predictive accuracy deteriorates due to the large variation in the relationship between the sand supply rate and the value of K0 used in the derivation process of the following equation. Accordingly, in order to avoid dangerous results, when using the following equation, it shall be assumed that FV = 0.2, even when the results of calculation of the sand supply rate for obtaining the target N-value are less than FV = 0.2.

(4.9.6) where 2 CM : coefficient; here, CM = (1/0.16) may be used. κ : coefficient; here κ = 4・10–0.01Fc may be used.

c : coefficient; here may be used.

γi* : coefficient calculated using equation (4.9.7)

(4.9.7)

where AK1 : coefficient calculated using equation (4.9.8)

(4.9.8)

Here, α is a coefficient expressing the rate of increase in K0 relative to the sand supply rate, and can be assumed to be α = 4.

AK0 : coefficient calculated using equation (4.9.9)

(4.9.9)

2 συ’ : effective overburden pressure when measuring N-value (kN/m )

Provided, however, that when the sand supply rate for the target N-value is FV < 0.2, FV = 0.2 shall be used. (5) Setting of Sand Supply Rate, when the Existing Data are Available The increase in the N-value after execution of the sand compaction pile method is strongly affected by the subsoil characteristics and the execution method. Therefore, when abundant execution data are available for the construction site or when test execution is performed, determination based on actual records of execution is

– 530 – PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS preferable, the method in (4) notwithstanding. When the method in (4) is to be used, the resetting of the parameter κ in equation (4.9.5) should be done as follows using the existing data. When using a new compacting method, it is advisable to reset the parameter κ in equation (4.9.5) by the following method using own data. The parameter κ of equation (4.9.5) can be given by equation (4.9.10). Therefore, if data are available for the N-value after sand supply in the sand compaction pile method, the N-value before sand supply, the fines content, and the sand supply rate, κ can be calculated by using equation (4.9.10).

(4.9.10)

where γi* : coefficient calculated using equation (4.9.11)

(4.9.11)

2 CM : coefficient; here CM = (1/0.16) may be used.

c : coefficient; here may be used.

A : coefficient; here (4.9.12)

It is permissible to determine the relational equation for κ and the fines content by obtaining κ from the respective sand supply rates and N-values before and after improvement, and arranging the relationship between κ and the fines content as shown in Fig. 4.9.1. Here, it is basically assumed that the relational equation between κ and the fines content is an exponential function as shown in (4). In parameter setting, when there is a large difference in the fines content before and after improvement, and when the N-value before improvement is larger, the data for that point shall not be used. When the relationship between the value of K0 and the sand supply rate is actually measured, the parameters in equation (4.9.6) and equation (4.9.7) which consider the influence of the value of K0 can be reset. For items related to parameter setting in this case and related matters, Reference 2) can be used as reference.

Exponential regression curve of plot Approximation line at κ = 5・10-0.01Fc 20

Sand supply rate Fv = 0.7 ~ 0.20 15 κ

0 0 10 20 30 40 50 60 Fines content (%) Fig. 4.9.1 Relationship between κ and Fines Content

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(6) Other Methods of Setting Sand Supply Rate The methods of setting the sand supply rate shown in the above (4) and (5) consider compaction of the original subsoil resulting from repeated shear by sand supply under sand pile driving, and were derived by analysis of past execution data. In addition to these methods, methods referred to as A method, B method, and C method have also been proposed and have been used for some time.89) In the A method, the relationship between the N-value before and after sand supply is shown in chart form, using the sand supply rate as a parameter, and thus enables simple calculation of the sand supply rate. Provided, however, that this method has low generality in comparison with other methods because it does not consider the effect of the overburden pressure or the effect of the fines content. The B method uses empirical formulae for the relative density, N-value, effective overburden pressure, and grain size, and obtains the sand supply rate for the target N-value assuming that the ground is compacted only by the amount of the sand piles supplied. Provided, however, that this method does not consider the effect of the fines content. The C method is proposed using a concept which is basically the same as in the B method. The major difference with the B method is the fact that the effect of the fines content is considered. Thus, the C method has the highest generality of these three methods. The D method is also proposed.89) The D method considers the effect of ground rise accompanying driving of the sand piles, which is not considered in the C method. Here, the C method is described here, as this method has the highest generality and most extensive record of actual results among the three methods in conventional use.90)

① emax and emin are obtained from the fines content Fc.

(4.9.13) (4.9.14)

② The relative density Dr0 and e0 are obtained from the N-value of the original subsoil N0 and the effective surcharge pressure σv'’.

(4.9.15)

(4.9.16)

③ The reduction rate β for the increase in the N-value due to the fines fraction is obtained.

(4.9.17)

④ A corrected N-value (N1’) is obtained from the N-value (N1) calculated assuming no fines fraction, considering the reduction rate β.

(4.9.18)

⑤ e1 is obtained using equation (4.9.16) in the above ② by substituting N1’ for N0.

⑥ Sand supply rate Fv is obtained using equation (4.9.19) from e0, e1.

(4.9.19)

– 532 – PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.10 Sand Compaction Pile Method for Cohesive Soil Ground 4.10.1 Basic Policy of Performance Verification [1] Scope of Application The scope of application of the performance verification of the sand compaction pile method, SCP method, described here shall be improvement of the lower ground of gravity-type breakwaters, revetments, quaywalls, and similar structures.

[2] Basic Concept

(1) The SCP method for cohesive soil ground is a method in which casing pipes are driven to the required depth at a constant interval in cohesive soil ground, and the ground is compacted and sand piles are constructed simultaneously with the discharge of sand into the ground from inside the casing pipes. As features of the improved subsoil, the soil is affected in a complex manner by (a) the strength of the sand piles, (b) the sand pile replacement rate, (c) the positional relationship of the area of improvement to structures, (d) conditions related to actions such as intensity, direction, loading path and loading speed, (e) the strength of the ground between the sand piles, (f) the confining pressure applied to the sand piles by the ground between the piles, (g) the effects of disturbances inside and outside the area of improvement by sand pile driving, (h) the characteristics of the ground rise at the ground surface due to sand pile driving, and whether this rise is to be used or not. (2) Effect of Execution Because a large quantity of sand piles are driven into the ground in the SCP method, the ground is forcibly pressed out in the horizontal and upward directions, which may result in disturbance of the ground and reduction of strength in the construction area and its surroundings. This displacement of the ground, and spills of excess sand in the casing pipes on the ground surface, may also cause a heave in the ground surface. Thus, when applying the SCP method, it is necessary to examine the effect of this type of ground displacement on neighboring structures. (3) Performance Verification Method Methods of performance verification of composite ground comprising sand piles and the ground between the piles include (a) a method in which the circular slip failure calculation method is applied with corresponding changes using, as a base, an evaluation equation for mean shear strength modified to reflect the characteristics of the composite ground, and (b) a method in which the composite ground is divided for convenience into a part that behaves as sandy ground and a part that behaves as cohesive soil ground, and the actions are redistributed so that the safety of the respective parts against circular slip failure agrees.99), 100) At present, the performance verification by the former method is the general practice.

4.10.2 Sand Piles

(1) Materials for sand pile should have high permeability, low fines content of less than 75µ m, well-graded grain size distribution, ease of compaction, and sufficient strength as well as ease of discharge out of casing. When the sand piles with a low replacement area ratio are positively expected to function as drain piles to accelerate consolidation of cohesive soil layer, the permeability of the sand pile material and prevention of clogging are important. The permeability requirement is relatively less important in the case of improvement with a high replacement ratio, that is close to the sand replacement. Therefore, materials for sand pile need to be selected considering the replacement ratio and the purpose of improvement. (2) There are no particular specifications on materials to be used for the sand piles. Any sand material that can be supplied near the site may be used from the economical viewpoint as far as it satisfies the requirements. Fig. 4.10.1 shows several examples of sands used in the past. Recently, sand with a slightly higher fines content have often been used.

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0.075 0.42 2.0 5.0 20.0 Fine Medium Silt Fine sand Coarse sand gravel gravel Coarse gravel

80 Case1 70 Case2 60 Case3 50 40 Case4 30

Passing weight percentage (%) 20 Case5 10

0.01 0.05 0.1 0.5 1.0 5.0 10.0 50.0 (0.075) (0.25) (0.42) (2.0) (9.52) Grain size (mm)

Fig. 4.10.1 Examples of Grain Size Distribution of Sands Used for Sand Compaction Piles

4.10.3 Cohesive Soil Ground

(1) Estimation of Amount of Ground Heave ① The amount of ground heave accompanying sand pile driving is affected by a large number of factors, including conditions related to the original subsoil, the replacement ratio, conditions related to execution. Therefore, several estimation methods using statistical treatment of the existing measured data have been proposed.107), 108), 109) Shiomi and Kawamoto 107) proposed equation (4.10.1) , defining the ratio of the amount of ground heave to the design supply of sand piles as the ground heave ratio μ.

(4.10.1) where as : replacement ratio L : mean length of sand piles (m) V : ground heave (m3) 3 Vs : design sand supply (m ) μ : ground heave ratio

② Equation (4.10.1) was obtained by multiple regression analysis of 28 examples of execution with 6m≤L≤20m, adding supplementary data on six sites, including two examples of sand piles with lengths of 21m and one example of a length of 25.5m. As a result of the analysis, it was found that the contribution ratio to μ decreases in the order of 1/L, as, qu, the lowest contribution ratio being that of qu, namely unconfined compressive strength of original subsoil. (2) Physical Properties and Strength Evaluation of Heaved Soil Conventionally, there were many cases in which ground heave was removed. Recently, however, ground heave has been effectively utilized as part of the foundation ground in an increasing number of cases. In such cases, it is necessary to investigate the physical properties and strength of the heaved soil. Where the physical properties of heaved soil due to driving of sand piles are concerned, an example 114) has been reported in which the original subsoil was improved at a replacement rate of 70%, and the heaved soil portion was improved so as to have a replacement ratio of 40% with ø1.2m diameter of sand drain piles driven in square arrangement of 1.7m intervals with the same construction equipment without compaction. Loose sand piles with the mean N-value of 3.6 had been formed in the heaved soil area, and the height of the heaved soil in the area of improvement was 3-4m. Tests of this heaved soil immediately after sand pile driving revealed that the physical

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properties such as unit weight, moisture content, and grain size composition of the heaved soil were substantially unchanged from those of the original subsoil to a depth equivalent to the height of the heaved soil. Table 4.10.1 110) shows the results of a comparison of the unconfined compressive strength qu of the heaved soil and qu0 as the mean value of the unconfined compressive strength before improvement of the original subsoil down to a depth equal to the height of the heaved soil. In the table, the strength of heaved soil outside the area of improvement is shown separately into cases within the range of 45º or 60º from the bottom end of the sand compaction piles. The strength of the heaved soil in the improved area showed a strength decrease of approximately 50% due to driving of the sand piles, but recovered to the original level in 1.5-3.5 months. The strength reduction of the heaved soil outside the improved area was reportedly 30-40%, and recovery was slow, requiring 8 months after pile driving for attain the original subsoil level. For the final shape and physical properties of heaved soil in case of compacting in the heaved soil, the report by Fukute et al.109) provides a useful information.

Table 4.10.1 Strength Reduction and Recovery in Heaved Soil 110)

Immediately after 1.5-3.5 months after Before construction construction construction In improved area 1.00 0.46 0.93

qu / qu0 Outside improved area (45º) 1.00 0.62 0.65 Outside improved area (60º) 1.00 0.72 0.72

4.10.4 Formula for Shear Strength of Improved Subsoil

(1) Several formulae have been proposed for calculation of the shear strength of improved subsoil which is composite ground comprising sand piles and soft cohesive soil.99) However, equation (4.10.2) is the most commonly used, irrespective of the replacement ratio (see Fig. 4.10.2). When as ≥ 0.7, there are many cases in which the first term in equation (4.10.2) is ignored, and the whole area of improvement is evaluated as uniform sandy soil with ø = 30º, disregarding equation (4.10.2).

Slip line

Sand pile Cohesive soil

Fig. 4.10.2 Shear Strength of Composite Ground

(4.10.2)

where as : replacement ratio of sand pile = (area of one sand pile)/(effective cross-sectional area governed by sand pile) 2 c0 : undrained shear strength of original subsoil, when z = 0 (kN/m ) 2 c0 + kz : undrained shear strength of original subsoil (kN/m ) k : increase ratio in strength of original subsoil in depth direction (kN/m3) n : stress sharing ratio ( n = Δσ s Δσ c ) U : average degree of consolidation

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z : vertical coordinate (m) τ : average shear strength demonstrated at position of slip failure surface (kN/m2)

μs : stress concentration coefficient on sand pile (μs = Δσs Δσz = n/{1+ (n −1) as})

μc : stress reduction coefficient of clay part ( μc = Δσc/ Δσz = 1/{1+ (n −1) as}) 3 ws : unit weight of sand pile, when submerged, unit weight in water (kN/m ) φs : angle of shear resistance of sand pile (º) θ : angle of slip failure surface to horizontal (º) 2 Δσz : mean increment of vertical stress acting at position of object slip failure surface (kN/m ) 2 Δσs : increment of vertical stress acting at sand pile at position of object slip failure surface (kN/m ) Δσc : increment of vertical stress acting at cohesive soil between sand piles at position of object slip failure surface (kN/m2) Δc / Δp : strength increase ratio of original subsoil

(2) Constants used in Performance Verification In the past examples of performance verification, the constants used in equation (4.10.2) varied over a wide range. The values of the constants used in the performance verification should be set considering the strength of the original subsoil, the applicable margin of safety, the method of performance verification to be used (see 4.10.6 Performance Verification), and the speed of construction. The standard values of the stress sharing ratio and the angle of shear resistance obtained from past examples using equation (4.10.2) are as follows:

as ≤ 0.4 n = 3 φ = 30º 0.4 ≤ as ≤ 0.7 n = 2 φs = 30º-35º as ≥ 0.7 n = 1 φs = 35º

In recent years, the number of examples in which slag and similar materials were used as materials for sand piles has increased. Slag include materials which can be expected to have comparatively high angles of shear resistance. When such materials are to be used, performance verification may be performed using an angle of shear resistance close to the measured value, provided adequate caution is used in setting the stress sharing ratio. (3) Classification of Shear Strength Formulae of Composite Ground In the past examples of performance verification, in addition to equation (4.10.2), the following three equations are used.115) Equation (4.10.4) and equation (4.10.5) are those proposed as equations for shear strength of composite ground with high replacement ratios. According to the existing survey results,99) with low replacement ratios of as ≤ 0.4, almost all examples of performance verification used equation (4.10.2), and very few examples used equation (4.10.3). Similarly, when 0.4 ≤ as ≤ 0.6, the majority of examples used equation (4.10.2), and examples using equation (4.10.4) accounted for only about 1/5 of the total. When 0.6 < as, equation (4.10.4) and equation (4.10.5) were frequently used.

(4.10.3) (4.10.4) (4.10.5)

Here, the definitions of symbols in the above equations which are different from those in equation (4.10.2) are as follows.

wm : mean unit weight (wm = wsas + wc (1− as ) 3 wc : unit weight of cohesive soil, when submerged, unit weight in water (kN/m ) φm : mean angle of shear resistance when improved subsoil with height replacement ratio is assumed to be uniform subsoil –1 φm = tan (μsas tanφs)

4.10.5 Actions

(1) The displacement of the main body during earthquake with subsoil improved by the sand compaction pile method tends to be reduced. When setting the seismic coefficient for verification of the main body in case of soil improvement by the sand compaction pile method, it is possible to set a rational seismic coefficient by appropriately evaluating this reduction effect. For the basic flow and items requiring caution when calculating the seismic coefficient for verification, Chapter 5, 2.2.2(1) Seismic coefficient for verification used in verification of damage due to sliding and overturning of wall body and insufficient bearing capacity of foundation ground in variable situations in respect of Level 1 earthquake ground motion can be used as a reference.

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The characteristic value of the seismic coefficient for verification of gravity-type quaywalls in the case of soil improvement by the sand compaction pile method with a replacement ratio of 70% or more can be calculated using equation (4.10.6) by multiplying the maximum value of corrected acceleration obtained for the unimproved soil by a reduction coefficient. In calculating the maximum value of corrected acceleration for the unimproved soil, this part, Chapter 5, 2.2.2 (1) Seismic coefficient for verification used in verification of damage due to sliding and overturning of wall body and insufficient bearing capacity of foundation ground in variable situations in respect of Level 1 earthquake ground motion can be used as a reference. It should be noted that this reduction coefficient was obtained based on a 2-dimensional nonlinear effective stress analysis for unimproved subsoil and improved subsoil with a 70% replacement ratio for gravity-type quaywalls.

(4.10.6) where kh’ : characteristic value of seismic coefficient for verification 2 αc : maximum value of corrected acceleration (cm/s ) g : gravitational acceleration ( = 980cm/s2) Da : allowable deformation (cm) ( = 10cm) Dr : standard deformation (cm) ( = 10cm) c : reduction coefficient of seismic characteristics due to improved subsoil (c = 0.75)

4.10.6 Performance Verification

(1) Examination of Circular Slip Failure ① The modified Fellenius method is frequently used in circular slip failure calculations in performance verification of improved subsoil by the sand compaction pile method. In circular slip failure calculations by the modified Fellenius method, the subsoil and superstructures are divided into several segments called slices, and the normal stress on the slip surface is calculated ignoring the statically indeterminate forces acting between slices. That is, only actions acting on the original subsoil included in a slice portion are assumed to contribute to the normal stress on the slip surface of that slice. Hereinafter, this normal calculation method is called the “slice method”. On the other hand, in actual subsoil, loads are distributed in the ground to a certain extent. In order to reflect the effects of this stress distribution in slip failure calculations, there is a method that the vertical stress increment Δσz at an arbitrary point on a slip surface obtained using Boussinesq’s equation applies to the modified Fellenius method. Hereafter, this is called the “stress distribution method”. ② In the performance verification of improved subsoil by the sand compaction pile method, either the slice method or the stress distribution method can be used. In the examination of circular slip failure, verification can be performed using equation (4.10.7). In this equation, the subscript d denotes the design value.

(4.10.7) where : sum of resistant moments (kN・N)

r : radius of slip circle (m) s : width of slice segment (m) θ : angle of slip surface to horizontal (º) : shear strength of subsoil (kN/m2)

: sum of acting moments (kN・N)

Case of quaywall:

w' : weight of slice segment (kN/m) q : surcharge on slice segment (kN/m) qRWL : buoyancy of slice segment due to difference in water level when the residual water level, RWL, at the back side of facilities is higher than the water level, LWL, at the front of the facilities ρwg (RWL - LWL) (kN/m) θ : angle of bottom of slice segment to horizontal (º) x : horizontal distance between center of gravity of slice segment and center of slip failure circle (m)

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Case of breakwater:

w' : weight of slice segment (kN/m) q : spatially-distributed load of breakwater acting on slice segment when effective weight of breakwater is divided by its width (kN/m) θ : angle of bottom of slice segment to horizontal (º)

In calculating the design values in the equation, Chapter 5, 2.2.3 (5) Examination of Sliding Failure of Ground in Permanent Situation can be used as a reference for quaywalls, and Chapter 4, 3.1.4 (5) Examination for Slip of Ground can be used for breakwaters. The shear strength of the improved subsoil can be calculated by equations (4.10.2) to (4.10.5), depending on the design conditions. For example, when using equation (4.10.2), the design value of the shear strength of the improved subsoil can be calculated by the following equation. In this case, Δσz is obtained using Boussinesq’s equation.

(4.10.8)

The design values in the equation can be calculated using the following equations. The subscript k denotes the characteristic value. For symbols, etc., equation (4.10.2) can be used as a reference.

③ Fig. 4.10.3 shows a schematic diagram of circular slip failure.

w θ τ

SCP improved subsoil

Fig. 4.10.3 Schematic Diagram of Circular Slip Failure

④ For partial factors for use in the examination of circular slip failure of improved subsoil when soil improvement is conducted by the sand compaction pile method with replacement ratios of 30% to 80%, the values shown in Table 4.10.2 can be used as a reference 116). In this case, caution is necessary, as the partial factors for circular slip failure shown in 3.2.1 Stability Analysis by Circular Slip Failure Surface cannot be used. In setting the partial factors in Table 4.10.2, the case in which the slip circle surface passes through sandy subsoil deeper than the improved subsoil is not examined. Therefore, in such cases, separate study by an appropriate method is necessary.

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Table 4.10.2 Standard Partial Factors

(a) Permanent situation (high earthquake-resistance facilities)

High earthquake-resistance facilities

Standard reliability index βT 3.1 Reliability index β used in calculation of γ 3.1

γ α µ/Xk V

Circular slip γc ' Cohesion Landfill soil 1.00 0.001 1.00 0.10 failure Original cohesive subsoil 0.95 0.092 1.00 0.10

γ Tangent of shear Mound, backfilling stones, tanφ ' 0.95 0.218 1.00 0.10 resistance etc.

SCP tanφs'=0.70 0.80 0.861 1.00 0.05

γwi Ground, caisson, etc. above level of sea bottom 1.00 –0.041 0.98 0.03 Mound, backfilling stones, etc. 1.05 –0.041 1.02 0.03 Sandy soil below sea bottom (SCP) 1.00 0.069 1.00 0.03 Cohesive soil below sea bottom 1.00 0.009 1.00 0.03

γq Surcharge 1.35 –0.270 1.00 0.40

γRWL Residual water level 1.00 –0.022 1.00 0.05

(b) Permanent situation (revetments and quaywalls)

Others

Standard reliability index βT 2.7 Reliability index β used in calculation of γ 2.7

γ α µ/Xk V

Circular slip γc ' Cohesion Landfill soil 1.00 0.001 1.00 0.10 failure Original cohesive soil 1.00 0.092 1.00 0.10

γ Tangent of shear Mound, backfilling stones, tanφ ' 0.95 0.218 1.00 0.10 resistance etc.

SCP tanφs'=0.70 0.80 0.861 1.00 0.05

γwi Ground, caisson, etc. above level of sea bottom 1.00 –0.041 0.98 0.03 Mound, backfilling stones, etc. 1.00 –0.041 1.02 0.03 Sandy soil below sea bottom (SCP) 1.00 0.069 1.00 0.03 Cohesive soil below sea bottom 1.00 0.009 1.00 0.03

γq Surcharge 1.30 –0.270 1.00 0.40

γRWL Residual water level 1.00 –0.022 1.00 0.05

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Breakwater

Standard reliability index βT 3.3 Reliability index β used in calculation of γ 3.3

γ α µ/Xk V

Circular slip γc ' Cohesion Original cohesive soil 0.90 0.484 1.00 0.10 failure γ Mound, backfilling stones, tanφ ' 1.00 0.060 1.00 0.10 Tangent of shear etc. resistance SCP tanφs'=0.70 0.90 0.664 1.00 0.05 γ Wave-dissipating works, foot protection works, etc. wi 1.05 –0.140 1.02 0.03 above sea bottom Mound 1.05 –0.140 1.02 0.03 Sandy soil below sea bottom (SCP) 1.00 –0.110 1.00 0.03 Cohesive soil below sea bottom 1.00 0.115 1.00 0.03

γq Distributed load (weight of caissons) 1.00 –0.140 0.98 0.02

(2) Examination of Consolidation ① Calculation of consolidation In performance verification of settlement, equation (4.10.9) can be used.

(4.10.9)

where Cc : compression index h : height of embankment (m) H : thickness of consolidation layer (m) 2 mv : coefficient of volume compressibility (m /kN) p’ : consolidation pressure (kN/m2) 2 p0’ : initial pressure (vertical pressure before construction) (kN/m ) 2 pc’ : preconsolidation pressure (kN/m ) Sa : allowable residual settlement (m) U : consolidation rate e0 : initial void ratio of original subsoil α : coefficient of stress distribution (ratio of distributed stress in subsoil and consolidation pressure or embankment pressure) β : settlement reduction ratio (ratio of settlement of composite ground and settlement of unimproved subsoil) γ’ : effective unit weight of embankment (kN/m3) Δe : reduction of void ratio of original subsoil Sf0 : settlement without improvement Sf : residual settlement

② Comparison of calculated settlement and measured values The residual settlement of improved subsoil is obtained by multiplying the predicted settlement of unimproved subsoil by the settlement reduction ratio β as shown in equation (4.10.9). The settlement reduction ratio β is generally expressed in a form similar to the stress reduction coeffiicient μc. An example of a comparison of the calculated settlement reduction ratio and measured values is shown in Fig. 4.10.4. Here, the values of β on the y-axis were obtained by estimating the final settlement of the improved subsoil by approximating the progress of measured settlement over time as a hyperbola, and estimating the ratio to the calculated final settlement of the original ground. The Figure also shows the settlement reduction ratio (β =1–as) which is used empirically

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with high replacement ratios and settlement reduction ratios for stress sharing ratios of n = 3, 4, and 5. From this figure, it can be understood that the reduction of settlement due to improvement is large, this effect is influenced by the replacement ratio, and although variations in the measured values are large, the values are close to those calculated assuming a stress sharing ratio of approximately 4.

1.0 Marine construction Land construction 0.8 β 0.6

0.4 n = 3

Settlement ratio n = 4 1 0.2 β = n = 5 1+(n-1)as

0 0.0 0.2 0.4 0.6 0.8 1.0

Replacement area ratio as

Fig. 4.10.4 Relationship between Settlement Reduction Ratio and Replacement Rate 109)

③ Comparison between calculated and measured consolidation time The consolidation rate of subsoil improved by the sand compaction pile method tends to be delayed compared to that predicted by Barron’s equation. Fig. 4.10.5 based on previous construction data shows the delay in consolidation in terms of the coefficient of consolidation as a major parameter. In the figure, Cv is the coefficient of consolidation reverse-analyzed from actual measurements for the time-settlement relationship, and Cv0 is the coefficient of consolidation obtained from laboratory tests. It can be seen that the time delay in consolidation becomes greater with the increase in the replacement area ratio.

Cvp : The coefficient of consolidation from actual measurements

Cv0 : The coefficient of consolidation obtained from laboratory tests 1.0 Marine constructin Land constructin 0

v 0.5 C / v p C 0.2

0.1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Replacement rate as

Fig. 4.10.5 Delay in Consolidation of Subsoil Improved by Sand Compaction Pile Method

④ Comparison of calculated and measured strength increments The increment of strength of clay between sand piles Δc can be calculated using equation (4.10.10). On the other hand, the results of a reverse calculation of μc from the measured values of the strength increment of clay 27) between sand piles are shown in Fig. 4.10.6 . The y-axis in the figure expresses the ratio (μc (Δca / Δcc)) of the measured values Δca of the strength increment in improved subsoil by the sand compaction pile method to the

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predicted values Δcc (= ΔσzΔc /ΔpU) of the strength in unimproved subsoil. The measured values of the strength increment vary, centering around stress sharing ratio n = 3–4.

(4.10.10)

where μc : stress reduction coefficient of cohesive subsoil portion (μc = Δσc Δσz =1 {1+(n −1)as}) 2 Δσz : mean value of vertical stress increment due to action at object depth (kN/m ) Δc /Δp : strength increase rate of original cohesive subsoil U : mean degree of consolidation

n = 1 1.0 cc : calculated increase of cohesion =c/p s z・U ca : increase of cohesion based on 0.8 surveys before and after construction Offshore Land

c n=2 c 0.6 /

a Kasai-oki c n=3 ( )

c μ 0.4 n=4 n=6 0.2

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Replacement area ratio as

Fig. 4.10.6 Strength Increase of Cohesive Soil between Sand Piles in Improved Subsoil 109)

4.11 Rod Compaction Method 4.11.1 Basic Policy of Performance Verification In the rod compaction method, it is necessary to conduct performance verification appropriately based on the actual records of the past execution or the result of test execution adequately considering the characteristics of the object ground and the characteristics of the execution method.

4.11.2 Performance Verification Because this improvement method is a method of compaction employing only vibration, its effect decreases exponentially with distance. Accordingly, it is preferable to determine the arrangement and spacing of the vibratory rods based on the relationship between the pitch of the vibratory rods obtained from the past examples or test execution and the N-value after execution. In application to the existing sheet pile quaywalls, the spacing of the tie rods should be considered when determining the spacing in the direction of the face line of the quaywall.

4.12 Vibro-fllotation Method 4.12.1 Basic Policy of Performance Verification In the vibro-flotation method, it is necessary to conduct performance verification appropriately based on the actual records of the past execution or the result of test execution, adequately considering the characteristics of the object ground and the characteristics of the execution method.

4.12.2 Performance Verification [1] Examination using Past Results of Execution

(1) When sufficiently reliable past results such as the characteristics of the object ground, pile driving density in the vibro-flotation method, capacity of the vibro-float, and correlation with the N-values of the ground before and after

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improvement are available, the performance verifiication of the improvement works can be conducted based on this. (2) The limits of applicability of the vibro-flotation method estimated from the examples of execution to date are as shown in Fig. 4.12.2125). Fig. 4.12.2 is prepared based on the measured values of 11 examples of execution using square and equilateral triangular patterns with pile spacings of 1.2-1.5m, together with other examples of execution, and can be used as a rough estimate of the limits of applicability of this method.

Silt Fine sand Coarse sand Gravel 100 n o ti l u ia ib r 80 r te t a is m d e p z u i e s 60 k n a i a m r s g a

e 40 Limit of effectiveness of m l vibro-floatation method u b m a i r n e i f e M r 20 p N min=8-15 N min=20-15 Percentage passing by mass (%) N min=15-20 0 0.01 0.02 0.03 0.05 0.07 0.1 0.2 0.3 0.5 1.0 2.0 3.0 5.0 Grain size (mm)

Fig. 4.12.2 Relationship between Grain Size of Original Subsoil and Minimum N-value after Compaction (Case of Sandy Soil)

4.13 Drain Method as Liquefaction Countermeasure Works In the drain method as liquefaction countermeasure works, drains using materials with good permeability are performed in ground where there is a possibility of liquefaction. These drains reduce the degree of liquefaction by increasing the permeability of the ground as a whole. Drains are frequently performed in a pile shape; however, wall shaped drains and shapes which surround the structure have also been considered. If a material with good permeability, such as sand invasion prevention sheets, is used in backfilling of quaywalls, this can also be considered a kind of drain. Crushed stone or gravel is frequently used as drain material. Recently, however, perforated pipes of synthetic resin and similar products have been developed. In short, as indicated above, a variety of drain methods are used as liquefaction countermeasure works.

4.14 Well Point Method In some cases, the well point method is used in combination with the sand drain method or plastic board drain method in order to increase effective weight of ground. Frequently, however, it is used for the purpose of reducing the water level in sand or sandy silt strata, thereby helping dry work under the ground execution. (Fig. 4.14.1) 129).

Sand Clay Silt Gravel Fine sand Coarse sand 100 Vacuum Gravity drainage drainage 80 Sumping Well method Vacuum well 60 method Electro- method 40 osmosis

Percentage passing by mass (%) 0 0.001 0.005 0.01 0.05 0.1 0.5 1.0 5 Grain size (mm)

Fig. 4.14.1 Applicability of Methods in respect of Soil Grain Size

– 543 – TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 4.15 Surface Soil Stabilization Method Surface soil stabilization methods are widely used for purposes such as securing trafficability for construction equipment in advance of actual soil improvement and increasing the bearing capacity of extremely soft subsoil, for example, in reclaimed land which has been reclaimed using soft cohesive or extremely soft cohesive soil, and to prevent residents from falling into the reclaimed land, prevent foul odors, prevent breeding of disease-bearing insects in standing water, and seal harmful industrial wastes in reclaimed land near residential areas.130), 131)

4.16 Liquefaction Countermeasure Works by Chemical Grouting Methods 4.16.1 Basic Policy of Performance Verification

(1) The following describes the method of performance verification when using chemical grouting methods for the purpose of liquefaction countermeasure works. As grouting methods for liquefaction countermeasure works, the permeation grouting method, multiple permeation grouting method, grouting method, and others have been developed.132), 133), 134) (2) Regarding applicable soil quality, based on past records, it can be assumed that the fines content generally comprises no more than 40% of the subsoil. (3) In the examination of stability against circular slip failure safety side examination results should be adopted by evaluating the improved subsoil as c material or c–ø material. (4) As a guideline, the improved strength for preventing liquefaction of soil with solution-type chemicals is an unconfined compressive strength of 80–100 kN/m2. This improved strength is equivalent to a high liquefaction resistance on the order of RL20 = 0.4 of cyclic shearing stress ratio in the cyclic undrained triaxial test. Here, soil improved by solution-type chemical grout, even when its unconfined compressive strength is 100kN/m2, is not always regarded as a material which does not liquefy due to such as its deformation characteristic under cyclic motions. Therefore, it is necessary to specify the improved strength by calculating actions in accordance with the performance criteria of the facilities. On the contrary, even with very low improved strength, such as an unconfined compressive strength of the order of 16kN/m2, it has been reported that dilatancy characteristics change from loosely filled sand to dense sand, in that flluid liquefaction like that in loose sand is not observed, and liquefaction potential is greatly improved.

4.16.2 Setting of Improvement Ratio In principle, the improvement ratio shall be 100%, namely the entire area subject to the improvement shall be improved. In cases where the improvement ratio is to be reduced, a careful examination should be made, for example, by confirming that settlement and deformation which are detrimental to facilities will not occur by conducting model tests, etc.

4.17 Pneumatic Flow Mixing Method 4.17.1 Basic Policy of Performance Verification

(1) It is necessary to conduct performance verification of the pneumatic flow mixing method by appropriately setting the necessary strength of the treated subsoil, area of improvement, etc. based on surveys and test results of the soil which is to be improved, and the stabilized soil, and the conditions of application. (2) In the pneumatic flow mixing method, stabilizer is added to the soil being improved, for example, dredged soil, during pneumatic transportation. The object soil and stabilizer are mixed using the turbulence effect of the plug flow generated in the transport pipe, and the mixture is then placed at the designated location. For the principle and features of this execution method, Manual on Pneumatic Flow Mixing Technology 135), 136) can be used as a reference.

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(1) This section describes fundamentals of performance verification for calculation of active earth pressure when using geotechnical materials solidified by stabilizers such as cement as backfill materials. Solidifying agents considered in this section include those that harden naturally and others that are hardened artificially by adding cement or other stabilizer. Materials developed to date are listed below. The variety of materials tend to increase in future. ① Premixed soil (treated soil by premixing method) ② Lightweight treated soil ③ Cement-mixed soils other than the above two ④ Solidified coal ash ⑤ Self-hardening coal ash ⑥ Blast furnace granulated slag used for solidifying

4.18.2 Active Earth Pressure [1] Outline

(1) When using solidified geotechnical materials, the material properties and the characteristics of earthquake motion should be appropriately taken account in calculations of active earth pressure on a structure. (2) When calculating active earth pressure during an earthquake, the seismic coefficient method may generally be used. When detailed examination of earth pressure during an earthquake is required, however, response analysis and others must be carried out. Methods to calculate earth pressure using the seismic coefficient method considering material properties are described in 4.18.2 [2] Strength Constants. (3) Generally, when solidifying agents are judged to have sufficiently large cohesion, liquefaction in the treated area need not be considered. Although depending on actions due to ground motion, if the unconfined compressive 2 strength qu is greater than approximately 50–100kN/m , excess pore water pressure in the area of improvement during action of ground motion may be ignored.

[2] Strength Constants The method of determining strength constants for geotechnical materials will differ depending on the material used. It is necessary to consider cohesion and the angle of shear resistance in accordance with the properties of the respective materials used. In general, deep mixed soil, lightweight treated soil, and soil solidifiied with coal ash are assumed to be c materials. Premixed soil can be considered to be a material of both the c and ø type. Granulated slag is usually treated as ø material, but it may also be treated as a c material in cases where its solidification property is positively employed.

[3] Calculation of Active Earth Pressure

(1) Generally, the earth pressure may be evaluated based on the provisions in Part II, Chapter 5, 1 Earth Pressure. The principle for calculation of earth pressure may be the same as the Mononobe-Okabe principle. In this method, the earth pressure is calculated by an equilibrium of forces in accordance with Coulomb’s concept of earth pressure by assuming that the subsoil fails while forming a wedge. (2) Many factors remain unknown about the earth pressure during an earthquake. This is particularly significant on the earth pressure during an earthquake in submerged subsoils. Nevertheless, the principle of earth pressure in Part II, Chapter 5, 1 Earth Pressure has so far been adopted in the performance verification of many structures with satisfactory results. (3) Equation (4.18.1), an expansion of the earth pressure equation in Part II, Chapter 5, 1 Earth Pressure, can be applied to materials having both the cohesion c and angle of shear resistance φ (see Fig. 4.18.1).

– 545 – TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN

(4.18.1)

where 2 pai : active earth pressure intensity acting on wall by the i-th layer (kN/m ) 2 ci : cohesion of soil in the i-th layer (kN/m ) φi : angle of shear resistance in the i-th layer (°) 3 γi : unit weight of the i-th layer (kN/m ) hi : thickness of the i-th layer (m) ψ : angle of wall to the vertical (°) β : angle of ground surface to the horizontal (°) δ : angle of wall friction (°) ζi : angle of failure surface of the i-th layer to the horizontal (°) ω : surcharge per unit area of ground surface (kN/m2) θ : resultant seismic angle (°)θ=tan–1k or θ=tan–1k' k : seismic coefficient k' : apparent seismic coefficient

ξ1

ξ 2 P2

h2 Pi-1

Pi Piv

+8) ( ψ δ

hi Pi

Pih ξi

Fig. 4.18.1 Earth Pressure

(4) Equation (4.18.1) is an extension of Okabe’s equation.142) This extension lacks such rigorousness that Okabe solved the equilibrium of forces. However, when the soil is exclusively granular material with no cohesion or exclusively cohesive material with no angle of shear resistance φ, it is consistent with the equations in Part II, Chapter 5, 1 Earth Pressure.

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(5) The earth pressure and the angle of failure surface should be calculated separately at each soil layer with different soil properties, while the earth pressure distribution and the failure line inside each layer are treated as linear. Actually within a soil layer, the earth pressure and the failure line sometimes become curved when calculated for divided sublayers. This contradicts the original assumption in Okabe’s equation that is based on a linear slip on the premise of Coulomb’s earth pressure. (6) When using the equations above, the existence of cracks sometimes has to be considered in accordance with the characteristics of the geotechnical materials used.

[4] Cases where Improvement Width is Limited When the area treated with solidified geotechnical materials is limited and Mononobe-Okabe’s equation cannot be applied simply, the earth pressure is evaluated by a suitable method that allows the influence of the treated area to be assessed. When the treated area is limited, the earth pressure can be evaluated by the slice method143). ① With the slice method, three modes of failure are examined (see Fig. 4.18.2). ② The earth pressure distribution is calculated by assuming that the difference between the resultant earth pressures at adjacent depths is the earth pressure intensity for the corresponding depth Mode 1: when a uniform slip surface is formed in the whole backfill (shear resistance mode) Mode 2: when a cracks down to the bottom of the solidified soil layer is developed (crack failure mode) Mode 3: when a slip surface is formed along the edge line of the solidified range (friction resistance mode) Note: Among Mode 1, the case in which the slip surface does not pass the solidified body is categorized as Mode 0.

Mode 2 Mode 1 Mode 3

Mode 0

Fig. 4.18.2 Three Failure Modes Considered in the Slice Method

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