Evaluation and Retrofitting of Building Foundations

Transcription

1 Evaluation and Retrofitting of Building Foundations 1. Survey on the Integrity of Building Foundations 1.1 Introduction 1.2 Survey on the ground surface 1.3 Survey on the underground foundations 1.4 Survey on Bearing Capacity 1.5 Evaluation of degree of damage 2. Restoration and reinforcement of building foundations 2.1 Outline 2.2 Repair, reinforce, settlement restoration 2.3 Restoring the settled, detached houses Annexes Annex 1 Integrity Investigation Techniques Annex 2 The Techniques of Restoration, Reinforcement and Settlement Restoration Annex 3 Countermeasure Techniques Against Differential Sedimentation 1

2 1. Survey on the integrity of foundations 1.1 Introduction Generally, in the case where any of such damages as differential sedimentation, inclination, cracks, and defects has been caused to foundations by an earthquake or consolidation settlement, a survey on the integrity of foundations is required. Great attentions should be paid to the sites, which may involve the risk of liquefaction or a settlement disaster, even if no differential settlement has been actually occurred. Nevertheless, for existing buildings, the actually-occurring phenomena such as differential settlement and inclination of the buildings and cracks in the foundation members tend to attract greater attention than the results of evaluations based on design calculation. In contrast, recently, precast piles and improved soil materials have been increasingly used in housing renewal and therefore, the supporting performance of precast piles needs to be confirmed. The evaluation items of foundation integrity may be largely classified as shown below: 1) Location of a foundation 2) Dimensions and geometry of the foundation 3) Quality of the foundation (strength/rigidity) 4) Bearing performance of the foundation Data on the location of the foundation in 1) and the dimensions and geometry of the foundation in 2) are useful at the stage of survey if design documents and construction execution reports are available. In many cases, no detailed record has been stored. The design documents used for building construction authorization may be kept by the owner but the construction execution reports have not kept anywhere in many cases except for special cases. In the survey on the earthquake damages, the foundation might have to be digging out for visual check. On the other hand, in some cases, the dimensions and geometries of foundation slabs and footings are different from those described in the design drawing in some cases and thereby, it is very important that the detail of the foundation referring not only to the design document but also to the construction execution report. The length of the pile may be roughly estimated by the IT test (PI test). For a bearing pile, its length may be different from the measured length depending on the depth of the bearing stratum and therefore, it is necessary to confirm it referring to the construction execution report. To determine the quality of the foundation, the strength test and the neutralization test are conducted using coarse samples for concrete, and the strength test as well as the check test for any corrosion are require for reinforcement. With an exception of the case where: 1) reuse of the existing foundation is required, 2) differential settlement and inclination has occurred, or 3) cracks or defects is found in the rising part of the foundation on the surface of the ground which may be visually checked, almost no survey on the quality of the foundation is conducted (Photo 1.1.1). The bearing performance of the foundation is generally evaluated based on the differential settlement or inclination if any. Thereby, the integrity is not considered in reusing the existing piles with the exception when the bearing force needs to be verify by the loading test. 2

3 Photo Method of conducting a survey on concrete integrity (general survey on concrete structures) 1) core recovery 2) core strength test 3) reinforced concrete gauge 4) reinforced concrete gauge 5) carbonation test (peeled off) 6) carbonation test (drilled out) 7) carbonation test (sampled core) 8) impact strength measurement Supplied by: Hitoshi HAMAZAKI (Building Research Institute) (excluding 2) and 4)) 3

4 In conducting the survey on the integrity of the existing foundation, the sampling test or the nondestructive test is compelling to be carried out in many cases because it is difficult to conduct in-depth survey on the entire foundation under the ground, which cannot be visually checked. In making an attempt to improve the reliability of this type of test, analysis using execution management data is useful and it is important to get deep inside into variations and differences in executed construction based on the result of execution management. At the present time, however, almost no construction execution report is prepared for evaluating variations and differences in executed construction and therefore, there is an urgent need to develop any technique for solving this problem. Building Research Institute is making efforts in developing a quality control system (3- DQC) for visualizing information on construction execution in cooperation with Koda/Satoh s Laboratory. This system provides correlated information on parameters such as excavation resistance per unit depth and material input relative to any of vertical and horizontal cross sections in the area under construction for easy identification (for example, M. Tamura, H. Sato et al, A 3-Dimensional Quality Control System in Foundation Construction, ISOPE, Toulon, France, 2004). Fig shows an example of the results from the management process of the deep mixing method of soil stabilization. Any of methods, which can automatically acquire and manage data on construction execution, may give similar views to the view shown above including the deep mixing method of soil stabilization and the penetration method of rotating piles using stirring blades. This management method provides such a function that a group of piles are managed together rather than individuals, allowing the constructor to grasp any variations in the geological stratum and any differences in construction during or immediately after construction and therefore, is expected to be useful in evaluating the integrity of the existing foundations. 4

5 Depth (m) Column Number Fig. 11 Number of mixing per 1 meter advance at X-3 Depth (m) Column Number Fig Example of the result of quality control for visualizing any change or variation in construction (3-DQC) (Example of correspondence between the number of rotations/the state of running and column numbers/depth torque or in the deep mixing method of soil stabilization) Besides, it is also effective to use any method for easy survey on the integrity of the foundations when new piles are constructed. Photo shows a marker for measuring any differential settlement installed on the rising part of the house foundation at the completion of construction. It is strongly recommended that this type of marker to be used to verify the horizontal plane prior to construction. 5

6 Marker 定 点 マーカー Photo Marker put the periphery of the house foundation for settlement monitoring Alternatively, to sense damages in addition to differential settlement and inclination, sensors and inspection windows may be prepared on the foundation members such as piles prior to construction. Several methods have been proposed for attaching optical fibers or carbon fibers to the pile bodies or winding them around the piles. Photo is an example of PHC piles with optical fibers buried inside, method still under development. It has been verified that the optical fibers can endure centrifugal fabrication. Fig shows a schematic drawing explaining this type of damage detection technique. Photo Example of PHC piles damage device survey method using optical fibers. Any cracks are checked by the bending test on piles with optical fibers attached Fig Example of pile foundation damage detection system Difficulties may arise not only in the evaluation of where responsibility for repair of damages lies or how severe of damages, but also in the determination of whether the repair work is required and in the repair work itself. Accordingly, if the damage detection system for the structures such as foundations as shown in the figure is required, this type of piles are expected to be put into practical use in future considering their importance/use and user s expectation to reusability. Unfortunately, such piles are not commonly used. In the case where any of construction methods, where piles and other members are built on site, such as the cast in-place concrete pile methods the procedure may be the following: two or more inspection windows 6

7 (steel pipes or PVC pipes) are buried inside the pile bodies along reinforced concrete baskets and then a transmitter/receiver is inserted into each of the inspection windows as in the case of nondestructive integrity surveys (see Annex 1) such as borehole sonar detection and ultrasound or gamma logging to evaluate the integrity of the pile bodies between two windows at each of predetermined depths. It is assumed that the actual state of the foundation will be confirmed at the completion of construction through this survey. The result of the confirmation may allow for easy recognition of the integrity and deterioration of the existing foundation when it is reused later. 1.2 Survey on the ground surface In the survey on the ground surface, 1) any differential settlement/inclination and 2) crack/defect in the foundation are checked. To check for any differential settlement and inclination, a level tube is used in many cases. Recently, an auto level may be used to check to see if any differential settlement and inclination have occurred in a short period of time. In an example of settlement restoration done in a foreign country introduced in Section 2.4, the leveling tube or manometer is used to manage and display the presence of differential settlement using contours. It is required that on the ground surface, not only the foundations but also the floors and columns to be checked for any inclination (Photo 1.2.1). Photo Example of method for measuring foundation integrity (simple measurement, for example, visual check or non-destructive test) Inclination measurement using a auto level or leveling tube, crack width measurement using a crack gauge, repulsive strength measurement, etc. 7

8 In most of standard buildings, the rising element of the foundation can not be visually checked. On the other hand, for small-sized houses such as detached houses, cracks or flaws of the external portion of the foundation are used as an indicator of possible defects. Accordingly, it is important that the survey on the element of the foundation, which may be visually checked, to be conducted. To check to see if defects have occurred for confirmation of the integrity of the concrete foundations on the ground surface, simple instruments such as an insert clearance gauge, as well as magnifying mirrors for check the widths of cracks, repulsive strength measuring devices, and reinforced concrete exploration devices may be used. In recent years, any soft coating material has been applied to the external elements of the foundations in some cases and therefore, it is required that the widths of cracks and other flaws to be measured fully considering the kind and properties of a finishing material. 1.3 Survey on the underground foundations One of the direct foundations structure representative damage is inclination of rigid members witch is usually evaluated to be minor, even if the building differentially settles. This type of damage is not representative for pile foundations. For direct type, the underground part of the foundation is buried near the surface and can the ground can be excavated to check the integrity of them visually. It may be difficult to visually check the foundation integrity for the old buildings because the survey require dig-out the piles and loading test. Recently for direct and piles foundations much frequently are used soil improvements technique. Accordingly it is more important than ever to evaluate the integrity of the improved soil if differential settlement or any other damage occurs. During the Miyagi Prefecture Earthquake (1978) and Southern Hyougo Earthquake (1995) many piles have been damaged. The damages were found not only at the heads but also in the middle portions of the piles. To estimate or visually checked for any damage in the middle portions and heads of piles, surveys and tests are conducted from the heads of the piles or through the hollow portions or bore holes, which have been opened in the piles. The terms of survey are classified largely into 1) those on the foundations directly beneath the building and 2) those on the underground foundations. For the direct type, the survey on the directly beneath the building is generally conducted, while any other inspection is required depending on the pile type if soil improvement or reinforcement such as the deep mixing method of soil stabilization have been employed to protect the piles. Table summarizes the individual surveys. In conducting the survey, it is always required to allow for determining whether the damages to the foundation have been caused due to insufficient bearing capacity of the foundation or due to the defect in the pile, as well as checking to see if the precast piles may be reused for selecting appropriate restoration method. To determine whether the precast piles may be reused, the survey on the integrity of the pile bodies as foundation members by the non-destructive test and the bearing capacity of the foundation by the loading test must be conducted in some cases. One of the methods for roughly examining the states and positions of the damages of the underground piles is the non-destructive test. The non-destructive methods use low-strain elastic undulation, or earthquake generating equipment installed at the top of the pile produce vibration or an impact given on the top of the pile to measure the force exerted and vibrations. Another 8

9 method, where various types of sensors are inserted into a bore holes formed in the hollow portions of the piles or in the piles themselves, or inspection windows previously formed during construction, may be included in the non-destructive test. Table Underground survey method (mainly pile foundations, see Annex 1) Method Description Drilling survey Visually checks the states of foundations. Leveling survey Measures inclination or differential settlement of underground foundations and others. IT test (PI test) Surveys the integrity of the pile bodies by carefully hitting the heads of piles. Borehole camera Observes piles through their hollow portions, boreholes, and gaps/cavities and others. Borehole radar Surveys the positions of damages in the piles, if any, through boreholes and others. Borehole sonar Surveys the integrity of pile bodies through the boreholes and the hollow portions of the piles. Ultrasonic measurement Surveys the integrity of pile bodies through the hollows of the pile bodies. Caliper logging Measures any variation in diameter of minute holes of the hollow portion at the cracks or cross sections of pile bodies. Used in conducting the surveys on damages/integrity through the hollow portions of piles. Gamma-ray density logging Identifies gaps, if any, in the concrete elements using the dependency of the result of measuring the gamma-ray dose density. It is difficult to detect such a variation in density that may be caused by a crack. AE measurement Detects damages to pile bodies, if any, through an elastic (acoustic emission) wave induced by a crack. Inclinometer Used in estimating the positions of the damages to pile bodies based on L-discontinuous points for inclination. Loading test Estimates bearing capacity (static, quick, impact, etc.) Others Estimates the positions and sizes of the damages through surface wave measurement. (1) Survey from the heads of piles IT test. This method uses elastic undulation in a low-strain region to estimate the lengths and damaged portions of piles based on the profile of its reflected wave. If a survey can be conducted on the piles after being removed from the footing, the result may be easily obtained. In contrast, even if the piles remain attached to the footing, this method enables the test to be conducted. With the footing attached to the tops of the piles, an impact is generally applied on the footing or the anchors installed on pile heads. With the pile heads being not open, a signal reflected from the lower portion of the pile, as well as a signal reflected from the footing or 9

10 column in the upper part is mixed into the resulting signal. Alternatively, another method (stereo measurement method) may be used. This method involves the following steps: 1) installing sensors at two test points on the piles, 2) separating a falling wave and arriving wave based on the phase contrast between elastic undulations measured at these points, and 3) evaluating the integrity of the piles based on information from the arriving wave. (2) Survey through the hollow portions of piles The non-destructive tests conducted from the heads of piles, such as the IT test, is effective as a primary method for estimating actual states of the damages to pile bodies. In some cases, however, any more direct method for grasping the damaged positions and the actual states of damages is required. Various types of measurements may be conducted through the hollow portions for precast piles and core holes formed in the pile bodies for cast-in-place piles, respectively, which gives deeper insight into the actual states of the piles. These survey methods, however, are generally effective only when the heads of piles are open. If a footing or any other member has been constructed, boreholes need to be drilled in it. In this type of measurement method, sensors (or cameras) and others are inserted into the hollow portions of piles, where measurements are conducted. In this method, 1) borehole cameras, 2) inclinometers, 3) gamma-ray densiometer, 4) caliper (hole or diameter) gauges, 5) ultrasound (acoustic intensity) measuring devices, 6) borehole sonars, etc., may be used. When the devices listed in 2), 3), and 4) are used, the sensors should be brought into contact with the sides of the holes and then slid on them for measurement (Photo 1.3.1, Annex 1). Fig Example of a survey on the underground foundation (borehole camera, IT test, and bore sonar) One, which allows for most direct measurements and give distinctive results, is observation of hole walls using 1) the borehole cameras. The borehole camera is a kind of video camera and several types have been developed including those integrating a fiber scope. Some enables measurements at an angle of 360 all at once. If the boring step is required in making measurement on the cast-in-place piles and others, the states of the concrete elements may be determined to some degree by observing core samples collected. Cracks, however, may occur in 10

11 boring and thereby, it is difficult to correctly distinguish between the cracks caused by the earthquake and those occurring later by boring in some cases. If there is no information about the construction of pile or foot foundation, methods like the borehole radar and surface wave exploration should be used and after a rough inspection with these method, the integrity of the piles need to be checked by another method. Some of the individual survey methods are described in Annex 1. See Damage to Building Foundations and Their Restoration (Kenchikugijyutsu, 1995, Special Issue, 1) and other literatures as the need arises. 1.4 Survey on Bearing Capacity Possible causes for differential settlement of buildings and the damages to the pile bodies, is insufficient bearing capacity. Once differential settlement has occurred, the loading test may be used to ensure the direct understanding of the bearing capacity of existing piles. In some cases, before the loading test on the existing piles, the load supported by the piles must have been temporarily up borne by some way. In the commonly used method (the steel pier technique), steel pipe piles are pressed into the bearing stratum using jacks by means of reactive force from the footing to support the load applied on the footing. The reactive force will be supported by the load on the footing and the piles in the vicinity of it in the loading test. Photo shows an example of the static loading test, which is most commonly used. The loading test includes the static loading test (the reactive pile method), as well as the rapid loading test and the impact loading test. Among them, the test, which recently has attracted attention, is the rapid loading test. Photo shows an example of the rapid loading test, where almost all the load-displacement relations may be obtained. Photo is referred only for reference instead of exemplification. In this figure, a kind of pile construction method commonly used in China, by which piles are pressed in by applying static force. In the case where the pile diameter is small, it may be possible to conduct the test under press-in force for complementing the loading test. The steel pier technique commonly used in restoring the settled pile foundations is similar to this static steel pier technique, by which the bearing capacity is estimated, confirmed, and managed based on the relation between the press-in force and the amount of settlement and other parameters, if applicable. Loading equipment (5000kN class) Loading equipment (50000kN class) 載 荷 装 置 (50000kN 級 ) 載 荷 装 置 (5000kN 級 ) Fig Vertical loading test on repulsive force piles (see Annex 2-1) 11

12 Monogen モンケン Pile strain gauge 杭 体 内 歪 ゲージ Accelertor 加 速 度 計 Dynamic strain amp AD converter Cellasto buffer チェラストバッファー Load cell (1000tf) ( 緩 衝 材 ) ロードセル (1000tf) H Load cell ロードセル GL Cussion クッション 材 material 試 験 杭 Test pile Bridge フ リッシ ホ ックス box 動 歪 アンフ AD コンハ ータ Optical displacement gauge target 光 学 式 変 位 計 ターゲット AD converter ADコンハ ータ Optical 光 学 式 変 位 計 displacementgauge OD-SYSTEM CD system Exampleof 500tf 500tf 実 施 例 モンケン5tf Fig Example of the quick loading test method (see Annex 2-3) Optical 光 学 式 変 位 計 displacement gauge (ターゲット 部 ) Pile stress 杭 体 内 応 力 Fig Static steel pier technique (Push piles, Shanghai, China) 1.5 Evaluation of degrees of damages Outline The integrity of the building foundations is basically evaluated by checking for any differential settlement or inclination of buildings and for any crack or defect in their foundations. The parameters including an angle of inclination and a crack width may be used. The criteria for determining whether any defect has occurred are applicable only to the members, on which visual check may be easily done, such as the raising portions of the continuous footing foundation of a detached house. For underground piles, it is important to evaluate their integrity and the degree of damages to them depending on the type of piles. Similarly, for pile caps and foundation slabs, may need to be evaluated considering the type of piles, the location of piles, and the effects of the technique used for attaching the pile heads. As an institutional method for evaluating the degree of damages and severity of disaster damages to foundations, an evaluation method using the parameters such as the state and angle of deformation of foundation and the state of settlement in surrounding ground as indexes has been proposed for determining rapidly the risk level and severity of disaster damages of buildings after an attack of earthquake. Only a few studies have been conducted on the degree of damages to the underground slabs and piles of buildings and thereby, definitive data is almost not available. 12

13 It should be noted that it is possible to evaluate the degree of damages to building foundations and the integrity of them based on the state of piles and the result of structural calculation in a certain way, while unlike the foundations of structures constructed by public works, the necessity of restoration and recovery and the intent and degree of restoration may be dependent on a case-by-case basis in determining the severity of disaster damages to buildings and handling the result of determination. In the case where not only an earthquake but also settlement damages due to consolidation settlement have occurred, if the damages to the upper structures are not severe, inclination, if any, is perhaps restored only by replacing the floor materials with new ones to flatten in many cases because restoration of the settled foundations requires a large amount of money. Also, it should be noted that the pile heads are seldom dug out to make closer inspection unless serious building settlement or inclination occurs. The guideline 2) mentioned above assumes that the conditions (condition A) described below may be applicable to the buildings, of which foundations was damaged. For the buildings including those which satisfy the condition A, those for which settlement or inclination was detected in the rapid determination the dig-out survey it is require to be conducted. Conditions which implicitly indicate damages to foundations 2) 1. Buildings situated in the area where a geotechnical flow due to the land slide or liquefaction was observed. 2. Buildings without being damaged, which are situated in the area where a earthquake with a magnitude of VI+ or larger attacked and their surrounding buildings were seriously damaged. 3. Buildings with an aspect ratio of 2.5 or higher, which are situated in the area where an earthquake of a magnitude of V+ or larger attacked Evaluation of degree of damages to foundation slabs Table and Figure show the degrees of damages to foundation slabs. Any crack width was evaluated by ranking in four levels: 0.2 mm or less, 0.2 to 1 mm, 1 to 2 mm, and 2 mm or more. On the other hand, the severity of damages was evaluated by roughly ranking in five levels: rank I (mild), rank II (minor), rank III (moderate), rank IV (serious) and rank V (destructed). 13

14 Degree of damage I II III IV V Table Scheme of degree of damages to foundation slabs 2) Symptoms 0.2 mm or less of fine crack occurred. No concrete material fallen off. Approx. 0.2 to 1 mm of crack occurred. No concrete material fallen down. Concrete material slightly fallen off with reinforcing steels not visible. Approx. 1 to 2 mm of crack occurred. Concrete material very slightly fallen off. Reinforcing steels may be slightly visible. 2 mm or more of crack occurred. Concrete material significantly fallen off. Reinforcing steels seriously exposed. Reinforcing steels have bent and internal concrete structure collapsed. Foundation slabs deformed in the direction of its height. Settlement and/or inclination detected. In some cases, reinforcing steels broken. 14

15 Degree Symptom 0.2 mm or less Approx mm Minor peeled surface No reinforcing steel observed Approx. 1-2 mm Minor peeled concrete No reinforcing steel 2 mm or more of reinforcing steel observed reinforcing steel not bent 2 mm or more Reinforcing steel observed Deformation in height Settlement/ inclination Broken Broken Internal concrete reinforcing reinforcing Reinforcing steel bent and internal concrete Evaluation of degree of damages completely steel steel completely separated to pile foundations broken Fig Example of degrees of damages to foundation slabs 2) 15

16 Table and Figure show the scheme for evaluating the degree of damages to cast-in-place concrete piles and examples of evaluation. As in the case of foundation slabs, crack widths were ranked at four levels: 0.2 mm or less, 0.2 to 1 mm, 1 to 2 mm, and 2 mm or more. Precast concrete piles (e.g., PHC piles), as shown in Table , were ranked at three levels: 0.1 mm or less, 0.5 mm or less, and 1 mm or less. Smaller crack widths were used in evaluating the degree of damages when the degree of damages were at the same level considering that the effects might exert on a prestressed, high-strength concrete material. Table Scheme for evaluating the degree of damages to cast-in-place concrete piles 2) Damages due to axial tension or bending (in the case where a crack has occurred at an angle of 45 to almost the horizontal line) I 0.2 mm or less of fine bending crack (horizontal crack) occurred. 0.2 mm or less of fine bending shearing crack (at an angle of 45 ) occurred. One to three cracks occurred within 1.5D on one side. No concrete material fallen off. II 1 mm of horizontal crack occurred. Approx. 1 mm of crack occurred at an angle of 45. One to three cracks occurred within 1.5D on one side. No concrete material fallen off, or only the surface material fallen off. Reinforcing steels not visible. III Approx. 1 to 2 mm of horizontal crack occurred. 1 to 2 mm of crack occurred at an angle of 45. Three or more cracks occurred within 1.5D or cracks occurred at an interval of approx. 20 to 30 cm. Surface concrete material locally fallen off (approx. 10 cm in height, or within 0.2D) Reinforcing steels may be slightly visible. Damages due to axial tension or shearing stress (in the case where a crack has occurred at an angle of 45 to almost the vertical line) 0.2 mm or less of fine crack occurred. One or more cracks occurred within 1 to 3D. No concrete material fallen off. Approx. 1 mm of fine crack occurred. One or more cracks within 1 to 3D occurred. No concrete material fallen off. Approx. 1 to 2 mm of crack occurred. One or two cracks occurred within 1 to 3D. Oblique crack occurred with concrete material fallen off from its top. Horizontal reinforcing steel not visible. Damages due to axial tension (in the case where only a horizontal crack occurred) 0.2 mm or less of fine horizontal cracks occurred. Cracks occurred at an interval of approx. 1D or more. No concrete material fallen off. 1 mm or less of fine horizontal crack occurred. Cracks occurred at an interval of 0.5 to 1D or less. No concrete material fallen off. Approx. 2 mm of horizontal crack occurred. Cracks occurred at an interval of 0.5 to 1D or less. Only 10 cm-width of concrete material fallen off along crack. Reinforcing steels are slightly visible through a gap left after concrete material was fallen off. 16

17 IV 2 mm or more of horizontal crack occurred. 2 mm or more crack occurred at a angle of 45. Five or more cracks occurred within 1.5D. Cracks occurred at an interval of approx. 20 to 30 cm. Surface concrete material fallen off. Approx. 20 to 30 cm of crack occurred or crack occurred within approx. 0.5D. Concrete material remains inside reinforcing steel members. Local buckling found in reinforcing steels. Vertical crack occurred. V Pile axially compressed. Concrete material broken down and buckling found in all the reinforcing steels. Reinforcing steels broken down. 2 m or more crack occurred. Two or three cracks occurred within 1 to 3D. Concrete material fallen off along oblique crack. Reinforcing steels are visible along oblique crack. Buckling not found in reinforcing steels. Buckling found in reinforcing steels along oblique crack. Vertically compressed. Reinforcing steels broken down. Note) D indicates the diameter of a pile in the table. Concrete material fallen off along crack (approx. 10 cm in width). Reinforcing steels exposed along gap left after concrete material was fallen off. Clearance left between pile head and footing, through which fixed concrete material is visible. Buckling found in reinforcing steels. Axially compressed. Pile clinched. Reinforcing steels broken down. 17

18 Degree A Foundation slab Foundation slab Foundation slab Foundation slab Foundation slab 3 cracks within Peeled surface 1.5D concrete 1-3 cracks within 1.5D 0.2 mm or less 1-3 cracks within 1.5D App. 1mm Peeled surface, No reinforcing steel observed Approx. 10 cm or 0.2D Approx.1-2 mm Local peeled concrete, reinforcing steel partially observed Vertical crack Reinforcing steel partially bucked Vertica lly contrac ted All the reinforcing steel bucked B Foundation slab Foundation slab Foundation slab Foundation slab Foundation slab Pile foundation 0.2mm 1mm or less Peeled concrete, No reinforcing steel observed Approx. 1-2mm 2 mm or more Peeled concrete Reinforcing steel observed, not buckled Axially contracted Reinforcing steel buckled, reinforcing steel broken C Foundation slab Foundation slab Foundation slab Foundation slab Foundation slab Reinforcing Separated 0.5-1D or steel slightly from footing, 0.5-1D or less observed fixed Axially less reinforcing contracted 0.2 mm or less 1 mm or less steel Approx. 1D Peeled concrete Approx. 10 mm (minor) Approx 2mm Peeled concrete Exposed reinforcing steel Bent pile Reinforcing steel buckled Fig Example of evaluation of degrees of damages to cast in place concrete piles 2) A=Damages due to axial force and bending force B=Damages due to axial force and shear force C=Damages due to axial force 18

19 Table Scheme for evaluating the degree of damages to precast concrete piles (PC, PHC, PRC) 2) Degree of damage I III V Damages due to axial tension or bending stress. (in the case where crack occurred at an angle of horizontal line to almost 45 ) 0.1 mm or less of fine bending crack (horizontal crack) occurred. 0.1 mm or less of fine bending shearing crack occurred (at an angle of 45 ). Two or three cracks occurred within 1.5D on one side. No concrete material fallen off. Approx. 1 mm or less of horizontal crack occurred. Approx. 1 mm or less of crack occurred at an angle of 45. Three or more cracks occurred within 1.5D on one side. Or, cracks occurred at an angle of approx. 20 to 30 cm or less. Local surface concrete material may be fallen off (10 cm in height or within 0.2D). Steel material may be slightly visible. 1 mm or more of horizontal crack occurred. 1 mm or more of crack occurred at an angle of 45. Type of damage Damages due to axial\tension or shearing stress. (in the case where crack occurred at an angle of 45 to almost vertical line) 0.1 mm of fine crack occurred. One or more cracks occurred within 3D on one side. No concrete material fallen off. 0.5 mm or less of fine crack occurred. Three or less cracks occurred within 3D on one side. No concrete material fallen off. 0.5 mm or more crack occurred. Three or more cracks occurred within 3D on one side. Damages due to axial tension. (in the case where only horizontal cracks occurred) 0.1 mm of fine horizontal crack occurred. Cracks occurred at an interval of approx. 0.5D or more. No concrete material fallen off. Approx. 1 mm of horizontal crack occurred. Cracks occurred at an interval of 0.5D or less. Concrete material slightly fallen off along crack (10 cm in width). Concrete material fallen off along crack (10 cm in width). Steel material exposed along gap left after 19

20 Five or more cracks occurred within 1.5D on one side. Cracks occurred at an interval of 20 to 30 cm or less. Local buckling or breakage found in copper material. Vertical crack occurred. Pile axially compressed. Concrete material broken down. Concrete material fallen off along oblique crack. Buckling or breakage found in steel material along oblique crack. Pile axially compressed. concrete material was fallen off. Clearance formed between pile and footing, through which fixed reinforcing steels are visible. Buckling or breakage found in steel material. Pile axially compressed. Pile clinched. 2. Restoration and reinforcement of building foundations 2.1 Outline To restore and reinforce foundations, first of all it is necessary to determine whether the damaged elements will be restored for reusing, whether they are replaced with new ones, and whether the damaged elements will be left with no restoration for retrofitting using additional piles. If differential sedimentation occurred, needs to be corrected. For the foundations, the restoration of damages are usually done in parallel with the correction of differential sedimentation without an exception of repairing works on the raising elements of foundations and cracks and defects on foundation slabs because the differential sedimentation occurs in most cases. Generally, the foundation members (composed mainly of concrete) are repaired in the same way commonly used as that for the structural members on the ground. On the other hand, at deep points under the ground, usually, repair works are not easily done and thereby, the use of additional piles may be basically useful in repairing when the members have been apparently damaged. If no other methods are available, such a method may be used that the surrounding area around the damages member is compacted by improving the ground (e.g., the grouting technique). This method is difficult to apply to structural computation and usually, is considered to be a quick fix or reserve-capacity one. Resin injection (the automatic low-pressure grouting technique) may be essentially used in repairing cracks in concrete materials and cross-section repairing with high-strength mortar or concrete in repairing defects. In some cases, however, steel pipes are attached to the damaged piles to restore or reinforce depending on the degree of damages and the type of piles. The methods for restoring settlements may be classified mainly into two: jack up and grouting. Herein, the outline and basics of settlement restoring methods will be described. The 20

21 individual rearing, reinforcing, and settlement restoring methods are introduced in Annex 3. In addition, they are also described in References 1 and Repair, retrofitting, settlement restoration To restore damaged foundations, various types of methods are used depending on the factors, such as the foundation form, building size, and desired restoration level, especially mainly on the foundation form (pile foundation/direct foundation). It is unlikely that broken direct foundations lead to functionality deterioration even if the foundation members themselves incline to the same extent as in the case of pile foundations. In many cases, insufficient bearing capacity tends to incline the entire building together with its foundation and therefore, differential sedimentation needs to be restored from the standpoint of the functionality and dwelling performance. Expectation on restoration considerably varies on a case by case basis. To restore the settled buildings, the most commonly used methods are jacking-up or grouting and the level is adjusted to use the existing bearing layer with no modification. In some cases new piles may be used, depending on the state of the ground. In the case where the buildings incline with minor damage, a simple method, by which the upper part above the foundation of its settled portion is jacked up and mortar is filled in clearances, or a method, by which differential sedimentation is restored using the grouting technique. On the other hand, in the case where settlement or differential sedimentation is severe with many cracks in the foundation, the upper side of the foundation may be jacked up construct a new foundation. If it is difficult to jack up the foundation due to the site condition or any other factor, it may be jacked down to adjust the level. Note that methods for restoring settlement commonly used in foreign countries are introduced in Section 2.5. Among them, one of the methods used in China involves digging out soil under the foundation on the raised side (on the not-settled side) by boring to restore to the horizontal level. To prevent middle size of detached houses and RC buildings from settling in the future, new piles are pressed into the ground to modify the form of the foundation. To press piles into the ground for stabling the entire ground under the foundation by means of improvement, various methods are used; for example, 1) actual piles are used, 2) mortar is injected into the ground to form pile-like cement bodies, smalldiameter of steel pipes are pressed into the ground, or 3) post ground improvement (in the methods 1) to 3), virtual piles are used). The jack-up and grouting techniques are described below Jack-up technique The jack-up technique, by which buildings are lifted, is the most used method for restoring the settled buildings and may be classified into several groups depending on the size of a building, site conditions, actual damages, and actual factors. The jack-up technique involves lifting up the settled portion of a building or the entire building using jacks literally. Hydraulic jacks are commonly used. The jacks with capacity two to three times the building load should be correctly inserted so that the same level of post load is applied on each of jacks. The jacks are usually inserted beneath the foundation footing supporting the posts. In some cases, however, they may be inserted beneath the underground beams. A special important factor is the capacity and arrangement of jacks to be used. 21

22 For jacking-up the building, reaction force is requiring. The reaction force can be obtained in three ways described below depending on the size of a building and the ground state around it: 1) The existing foundation is used as reaction force as it is. 2) A mechanical jacking is used to ensure reaction force. 3) New piles are pressed into the ground to ensure reaction force. The method 1) is used to easily and speedy restore settlement in the relatively small-sized buildings (detached houses, steel-structured warehouses, etc.) with minor damages. This technique does not restore substantially the settled building and thereby, the building may settle again depending on the cause of the initial settlement. The method 2) is useful in the case where the ground around the settled building is relatively stable and the possibility of resettlement is low. The method 3) is used when reaction force can not be ensured on the existing foundation or ground, or when future settlement needs to be prevented in any way possible. The most commonly used method for restoring differential sedimentation of buildings using jacks is the steel pier technique. This technique involves a process, in which steel pipes with φ 200 to 400 mm in diameter, 1 m in length, are pressed into the ground one after another, up to the bearing layer using the building load as reaction force. It may be assumed that 1/2 to 1/3 times the maximum press-in force is set for long-term permissible bearing capacity. When the piles are pressed into the ground, the pressing-in force can be read using a manometer. This means that the technique has an advantage in that most of bearing force may be verified as in the load test (note that it is not complete unlike the standard load test). Moreover, construction is made only under the foundation and thereby, the building can be used as usual. The working space under the foundation is about 1.5 m. Photos and show the states of the steel pier technique and the building restored by this technique in Niigata Earthquake in Photo shows the building restored by the mechanical jacking technique. In addition to the steel pier technique, typical jack-up techniques include the techniques of mechanical jacking, saddle technique, shed restoration technique, and nekagami technique, of which outlines are described in Fig For more information, refer to Annex 3 if necessary. Photo Example of construction by steel pipe press-in technique 22

23 Photo Examples of settlement restoration by steel pier technique and lifting technique Examples of settlement restoration of direct foundation type buildings settled by an attack of Niigata Earthquake by steel pier technique while being used. As a part of construction management in settlement restoration, any vertical and horizontal displacement was automatically measured using a slide meter and a seismometer. Photo Example of settlement restoration by pressure board technique The ground in the vicinity of the periphery of the building was dug out and concrete was cast on a suitable natural ground. Then, jacks were inserted between the concrete board and the bracket attached to the side wall of the periphery of the foundation to lift the foundation. 23

24 Steel pier Pressure board Sandle Support jack Hydraulic jack Hydraulic jack Pressure board Steel pipe piles are cast to reinforce the foundation and use as a repulsive force in restoration. Prevents resettlement from occurring. Concrete board (pressure board) is cast under the foundation to use as a repulsive force in restoration. In many cases, used for prefabricated and reinforced concrete houses. Uses the ground as a repulsive force in restoration. Shed restoration Negarami Base Bracket Reinforcing steel post Negarami steel Hydraulic jack Often used for buildings constructed by conventional methods such as timbered axis. Note that it is prerequisite that the ground is stable. Often used for large-scale reinforced concrete buildings (factories and warehouses). It is also prerequisite that the ground is stable. Fig Outline of jack-up technique (Supplied by: Mase Construction) 24

25 2.2.2 Grouting technique Most of restoring techniques basically involves a mechanical process for lifting the foundation up using jacks, ensuring high certainty. In some cases, the grouting technique, by which grout is injected into the ground for raising it, lifting the building, is also used for direct foundation buildings. This technique has a reduce reliability and certainty; however, it may be useful when easy and speedy restoration is required. The grouting technique is largely classified into two types: in one type, the grout is permeated into the ground and in the other type it is solidified by itself without permeating. To improve safety of the entire foundation ground, the former may be used under the foundation or around piles. To restore differential settlement, the latter is suitable. Cement grout is used as grout because of its excellent durability (Photo ). The chemical grouting technique involves a process for injecting a chemical (for jacking up, cement flash-set chemical), which requires a given time for curing when injected, to compact the ground (the chemical can not permeate into the viscous soil layer and thereby, it enters into the ground in the form of nervation). Recently, a new grouting technique has been put into practical use by which highly illiquid grout with a slump of almost zero is pushed into the ground under high pressure (Photo ). To restore differential settlement by grouting, it is required that the impermeable grout be pressed into the ground on the depressed side to increase the volume of the ground causing the ground to rise. This results in the raising ground. In the chemical grouting technique, several grouting works are used. To restore settlement, the simple rod technique or the double packer technique, which allows for close construction management, is used. The chemical is in a liquid state and has high fluidity, when injected, even if an impermeable one is used and therefore, it tends to travel in the form of nervation or layer. Accordingly, it is difficult to artificially control the degree to which the ground is raised. Depending on the ground condition, no effect of grouting is observed. On the other hand, unlike chemical grouting, in compaction grouting, illiquid (slump being almost zero) cement mortar is pushed into the ground under high pressure (approx. 100 kgf/cm2 of max. discharge pressure) and so, it is unlikely that the grout travels in the form of nervation or layer and a mass (bulb-like) of cemented bodies are usually formed. Note that if the ground, into which the grout is injected, is heterogeneous, the injected grout deforms. Since for the grout is hard to travel out from a given area, the degree to which the ground is raised can be easily controlled. Accordingly, the settled building may be restored if construction is carefully made while the lifting condition and effects of lifting on the periphery of the building are being monitored. Determining from the past results, the foundation form, to which compaction grouting is applicable, is the direct foundation (especially, raft foundation). For a larger size of building, the vertical load under the foundation is also large. This means that the ground tends to expand laterally when the foundation is raised. In this case, simply the ground around the building may be raised without the building itself being lifted. This technique is only applicable to a moderate size of buildings. It should be sufficiently noted that compared with the jacking up technique, the grouting technique has an advantage in time for completion and construction cost, while depending on the ground condition/foundation form/size of building, no effect of grouting is expected. It is required to consider the ground environment because: 1) the grout may enter the neighboring sites across the boundary; 2) a water survey is conducted when a grouting work is made in the 25