4.6 Lightweight Treated Soil Method

Transcription

1 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

2 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 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 Evaluation of actions Examination of bearing capacity Examination of circular slip failure Examination of consolidation settlement Examination of liquefaction of surrounding ground Performance verification of superstructure Determination of strength/unit weight and area of improvement with lightweight treated soil Fig 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 The unit weight may be set within a range of γ t = 8-13 kn/m 3 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: for use uder water: γ tk = kn/m 3 for use in air: γ tk = 10 kn/m 3 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 q u and can generally be set in the range of kN/m 2. 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 519

3 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 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 c u can be used. The value of c u can be calculated using the following equation. (c) The consolidation yield stress P y may be calculated using the following equation: (4.6.1) (4.6.2) (d) Deformation modulus E 50 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 E 50. When such tests are not possible, the modulus can be estimated from the unconfined compressive strength q u using the following equation: The deformation modulus shown above corresponds to a strain level of %. (4.6.3) (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 =

4 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 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 and a coefficient of curvature of Percentage finer by weight (%) Grain size D (mm) Fig 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/m 3, and its unit weight in water is approximately 8kN/m 3. 5 Permeability The coefficient of permeability in the granular state differs depending on the void ratio but is roughly cm/s. The coefficient of permeability decreases with solidification, but even in this case is approximately cm/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. 521

5 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 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

6 4.8 Premixing Method PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 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 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). where τ f : shear strength of treated soil (kn/m 2 ) σ : effective confining pressure (kn/m 2 ) c : cohesion (kn/m 2 ) φ : angle of shear resistance (º) (4.8.1) c and φ correspond to the cohesion c d and angle of shear resistance ø d obtained by the consolidated-drained triaxial compression test, respectively. 523

7 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 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 Example of Performance Verification Procedure for Premixing Method 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 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/m 2 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/m 2. When the unconfined compressive strength of treated soil is set at less than 100 kn/m 2, 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. 524

8 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS (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 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 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 and Fig , 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 γ a 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. 525

9 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN 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 , 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 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 is disadvantageous for sliding in comparison with the shape of the treated subsoil shown in Fig , when the purpose is use as a liquefaction countermeasure, the shape of the treated subsoil shown in Fig 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 , 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. In this equation, the design values can be calculated as follows. (4.8.2) (when original subsoil under treated subsoil is sand) (when original subsoil under treated subsoil is clay) (4.8.3) where R 1 : frictional resistance of bottom surface of structure (ab) (kn/m) R 2 : frictional resistance of bottom surface of treated subsoil (bc) (kn/m) P w1 : resultant of hydrostatic water pressure acting on front of structure (af) (kn/m) P w2 : resultant of dynamic water pressure acting on front of structure (af) (kn/m) P w3 : resultant of hydrostatic water pressure acting on back of treated subsoil (cd) (kn/m) H 1 : inertia force acting on structure (abef) (kn/m) H 2 : inertia force acting on treated subsoil body (bcde) (kn/m) P h : horizontal component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kn/m) P v : vertical component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kn/m) ρ w g : unit weight of seawater (kn/m 3 ) w' : unit weight of untreated subsoil in water (kn/m 3 ) k h : seismic coefficient for verification K a : coefficient of active earth pressure during earthquake of untreated subsoil h 1 : water level at front of structure (m) h 2 : residual water level, for simplicity in this explanation, the residual water level in Fig is assumed to be the ground surface. δ : friction angle of wall between treated subsoil and untreated subsoil (cd) (º) 526

10 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS φ : angle of back of treated subsoil (cd) to vertical direction (º), counterclockwise is positive; in Fig , the value of φ is negative. f 1 : coefficient of friction of bottom of structure f 2 : coefficient of friction of bottom of treated subsoil (= 0.6) c : cohesion of original subsoil (kn/m 2 ) l bc : length of bottom of treated subsoil (bc) (m) γ a : 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 , 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 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. In this equation, the design values can be calculated as follows. (4.8.4) (when original subsoil under treated subsoil is sand) (when original subsoil under treated subsoil is clay) (4.8.5) where R 1 : frictional resistance of bottom surface of structure (ab) (kn/m) R 2 : frictional resistance of bottom surface of treated subsoil (bc) (kn/m) P w1 : resultant of hydrostatic water pressure acting on front of structure (af) (kn/m) P w2 : resultant of dynamic water pressure acting on front of structure (af) (kn/m) H 1 : inertia force acting on structure (abef) (kn/m) H 2 : inertia force acting on treated subsoil body (bcde) (kn/m) P h : horizontal component of resultant of active earth pressure during earthquake from untreated subsoil acting on back of treated subsoil (cd) (kn/m) ρ w g : unit weight of seawater (kn/m 3 ) w : unit weight of untreated subsoil in water (kn/m 3 ) k h : seismic coefficient for verification K a : coefficient of active earth pressure during earthquake of untreated subsoil h 1 : water level at front of structure (m) h 2 : water level used in calculating P h 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 , the value of φ is negative. 527

11 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN f 1 : coefficient of friction of bottom of structure f 2 : coefficient of friction of bottom of treated subsoil (= 0.6) c : cohesion of original subsoil (kn/m 2 ) l bc : length of bottom of treated subsoil (bc) (m) γ a : 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. f Structure Treated subsoil Untreated subsoil (not liquefied) e d H 2 H 1 ( ) ψ (+) P h h 2 h 1 Pw 1 Pw 2 W 1 W 1 ' W 2 W 2' Pv a b c Pw3 R 1 R 2 Fig Diagram of Actions when Purpose is Reduction of Earth Pressure h 1 Pw 1 Pw 2 Structure Treated subsoil Untreated subsoil (not liquefied) f e d Dynamic pressure ( ) ψ (+) (earth + water) H 1 H 2 h 2 P W 1 W 1 ' W 2 W 2' h Static pressure (earth + water) a b c Pv R 1 R 2 Fig Diagram of Actions when Used as Liquefaction Countermeasure 528

12 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.9 Sand Compaction Pile Method (for Sandy Soil Ground) 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 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 F V = and fines content F c = 60% or less. Accordingly, caution is necessary when using conditions outside of this range. where N 1 : N-value after sand supply C M : coefficient; here, C M = (1/0.16) 2 may be used. κ : coefficient; here κ = Fc may be used. (4.9.2) c : coefficient; here may be used. F c : coefficient; fines content (%) γ i * : coefficient calculated using equation (4.9.3) (4.9.3) where N 0 : N-value of original subsoil A : coefficient calculated using equation (4.9.4) 529

13 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN where σ v : effective overburden pressure when measuring N-value (kn/m 2 ) (4.9.4) Equation (4.9.2) can be solved for the sand supply rate F v, 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 K 0, 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 supply rate exceeds F V = 0.2, a method 88) using the following equation, which considers the effect of K 0, 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 K 0 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 F V = 0.2, even when the results of calculation of the sand supply rate for obtaining the target N-value are less than F V = 0.2. where C M : coefficient; here, C M = (1/0.16) 2 may be used. κ : coefficient; here κ = Fc may be used. (4.9.6) c : coefficient; here may be used. γ i * : coefficient calculated using equation (4.9.7) (4.9.7) where A K1 : coefficient calculated using equation (4.9.8) (4.9.8) Here, α is a coefficient expressing the rate of increase in K 0 relative to the sand supply rate, and can be assumed to be α = 4. A K0 : coefficient calculated using equation (4.9.9) σ υ : effective overburden pressure when measuring N-value (kn/m 2 ) (4.9.9) Provided, however, that when the sand supply rate for the target N-value is F V < 0.2, F V = 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

14 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). where γ i * : coefficient calculated using equation (4.9.11) (4.9.10) C M : coefficient; here C M = (1/0.16) 2 may be used. (4.9.11) 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 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 K 0 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 K 0 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 = Fc 15 Sand supply rate F v = 0.7 ~ 0.20 κ Fines content (%) Fig Relationship between κ and Fines Content 531

15 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN (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 e max and e min are obtained from the fines content F c. (4.9.13) (4.9.14) 2 The relative density D r0 and e 0 are obtained from the N-value of the original subsoil N 0 and the effective surcharge pressure σ v '. (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 (N 1 ) is obtained from the N-value (N 1 ) calculated assuming no fines fraction, considering the reduction rate β. (4.9.18) 5 e 1 is obtained using equation (4.9.16) in the above 2 by substituting N 1 for N 0. 6 Sand supply rate Fv is obtained using equation (4.9.19) from e 0, e 1. (4.9.19) 532

16 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 4.10 Sand Compaction Pile Method for Cohesive Soil Ground 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 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 shows several examples of sands used in the past. Recently, sand with a slightly higher fines content have often been used. 533

17 TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN Silt Fine sand Coarse sand Fine Medium gravel gravel Coarse gravel Passing weight percentage (%) Case2 Case3 Case4 Case5 Case (0.075) (0.25) (0.42) (2.0) (9.52) Grain size (mm) Fig Examples of Grain Size Distribution of Sands Used for Sand Compaction Piles 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. where a s : replacement ratio L : mean length of sand piles (m) V : ground heave (m 3 ) V s : design sand supply (m 3 ) μ : ground heave ratio (4.10.1) 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, a s, q u, the lowest contribution ratio being that of q u, 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 534

18 PART III FACILITIES, CHAPTER 2 ITEMS COMMON TO FACILITIES SUBJECT TO TECHNICAL STANDARDS 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 ) shows the results of a comparison of the unconfined compressive strength q u of the heaved soil and q u0 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 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 Strength Reduction and Recovery in Heaved Soil 110) Before construction Immediately after construction months after construction q u / q u0 In improved area Outside improved area (45º) Outside improved area (60º) 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 ). When a s 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 Cohesive soil Sand pile Fig Shear Strength of Composite Ground 535 (4.10.2) where a s : replacement ratio of sand pile = (area of one sand pile)/(effective cross-sectional area governed by sand pile) c 0 : undrained shear strength of original subsoil, when z = 0 (kn/m 2 ) c 0 + kz : undrained shear strength of original subsoil (kn/m 2 ) k : increase ratio in strength of original subsoil in depth direction (kn/m 3 ) n : stress sharing ratio ( n = Δσ s Δσ c ) U : average degree of consolidation