1. “Code for Design of Building Foundations”
1. The center distance of friction piles should not be less than 3 times the diameter of the pile body; the center distance of enlarged bottom cast-in-place piles should not be less than 1.5 times the diameter of the enlarged bottom; when the enlarged bottom diameter is greater than 2m, the clear distance between the pile ends should not be less than 1m. When determining the pile spacing, the influence of soil squeezing and other effects on adjacent piles in the construction process should be considered.
2. The enlarged bottom diameter of enlarged bottom cast-in-place piles should not be greater than 3 times the diameter of the pile body.
3. The depth of the pile bottom entering the bearing layer should be 1 to 3 times the diameter of the pile body according to geological conditions, loads, and construction technology. When determining the depth of the pile bottom entering the bearing layer, the influence of special soil, karst, and seismic subsidence liquefaction should be considered. The minimum depth of the rock-embedded cast-in-place pile embedded in the intact and relatively intact unweathered, slightly weathered, and moderately weathered hard rock mass should not be less than 0.5m.
4. When arranging the pile position, it is advisable to make the resultant force point of the pile foundation bearing capacity coincide with the resultant force point of the vertical permanent load.
5. The concrete strength grade of precast piles should not be lower than C30, cast-in-place piles should not be lower than C20, and prestressed piles should not be lower than C40.
6. The main reinforcement of the piles should be determined by calculation. The minimum reinforcement ratio of driven precast piles should not be less than 0.8%, the minimum reinforcement ratio of static pressure precast piles should not be less than 0.6%, and the minimum reinforcement ratio of cast-in-place piles should not be less than 0.2%~0.65% (the larger value is taken for small diameter piles).
7. Reinforcement length:
1) The reinforcement length of piles subject to large horizontal loads and bending moments should be determined by calculation;
2) When there is silt muddy soil or liquefied soil layer under the pile foundation cap, the reinforcement length should pass through the silt muddy soil layer or liquefied soil layer;
3) Piles on the sloping bank, uplift piles, and rock-embedded end-bearing piles in earthquake zones of 8 degrees and above, should be reinforced throughout the length;
4) The length of the structural steel bars of bored cast-in-place piles with a pile diameter greater than 600mm should not be less than 2/3 of the pile length.
8. The length of the pile top embedded in the cap should not be less than 50mm, and the anchorage length of the main reinforcement extending into the cap should not be less than 30 times the diameter of the steel bar (grade I steel) and 35 times the diameter of the steel bar (grade II and grade III steel). For large-diameter cast-in-place piles, when one column and one pile are used, a cap can be set or the pile and column can be directly connected. The connection between the pile and the column can be selected according to the requirements of the high cup foundation in Article 8.2.6 of this Code. The length of the longitudinal reinforcement of the column inserted into the pile body should meet the requirements of the anchorage length.
9. The backfill around the cap and basement should meet the requirements of fill density.
II. “Design Code for Foundation and Foundation of Highway Bridges and Culverts”
5.1.1 Piles can be classified according to the following provisions.
1. Classification by bearing characteristics.
1) Friction piles: The load on the top of the pile is mainly borne by the pile side resistance, and the pile end resistance is taken into account.
2) End-bearing piles: The load on the top of the pile is mainly borne by the pile end resistance, and the pile side resistance is taken into account.
2. Classification by pile formation method.
1) Non-squeezing piles: divided into dry operation method bored (excavated) bored piles, mud wall method bored piles, casing wall method bored piles.
2) Partially squeezed piles: divided into punched bored piles, squeezed and expanded bored piles, pre-drilled piles, open prestressed concrete pipe piles, etc.
3) Squeezed piles: divided into sinking piles (precast piles sunk by hammering, static pressure, vibration, closed prestressed concrete pipe piles, etc.).
5.1.2 Various types of pile foundations must be adopted based on geological, hydrological, and other conditions.
1. Drilled (excavated) piles are suitable for all kinds of soil layers (including gravel soil layers and rock layers), but it should be noted that:
1) When bored piles are used in silt and soil layers where sand flow may occur, test piles should be made first.
2) Excavated piles should be used in strata without groundwater or with little groundwater.
2. Sinking piles can be used in clay, sand, and gravel soil.
5.1.3 The bottom elevation of the pedestal of various pile foundations shall meet the following requirements:
1. In frost-heaving soil areas, when the bottom surface of the pedestal is in the soil, its burial depth shall comply with the relevant provisions of Article 4.1.1.
2. For rivers with drifting ice, its elevation shall be no less than 0.25m below the bottom surface of the lowest ice layer.
3. When there is a drifting raft, other drifting objects, or a ship collision, the bottom elevation of the pedestal should ensure that the pile is not directly damaged by the impact.
4. The bottom elevation of the pedestal should be determined per the principles of Article 4.1.2.
5.1.4 For piles located in frost-heaving soil areas, if a transverse tie beam is required between the piles, its location should avoid the frost-heaving layer to avoid the effect of frost-heaving force.
5.1.5 In the same pile foundation, except for special designs, friction piles and end-bearing piles should not be used at the same time; piles with different diameters, different materials, and pile end depths that differ too much should not be used.
5.1.6 For large and extra-large bridges with the following conditions, the bearing capacity of a single pile should be determined by static load tests.
1. The depth of the pile into the soil is much deeper than that of common piles.
2. The geological conditions are complex and it is difficult to determine the bearing capacity of the pile.
3. Piles for bridges with other special requirements.
III. Design method of “Technical Specifications for Building Pile Foundations”
3.1.1 Pile foundations should be designed according to the following two limit states:
1. Bearing capacity limit state: the pile foundation reaches the maximum bearing capacity, the whole is unstable, or deformation occurs that is not suitable for continued bearing.
2. Normal use limit state: the pile foundation reaches the deformation limit specified for normal use of the building or reaches a certain limit required for durability.
3.1.2 According to the building scale, functional characteristics, adaptability to differential deformation, complexity of the site foundation and building shape, and the degree of possible building damage or impact on normal use due to pile foundation problems, the pile foundation design should be divided into three design levels listed in Table 3.1.2. When designing a pile foundation, the design level should be determined according to Table 3.1.2.
Design grade building type: Class A
1. Important buildings
2. High-rise buildings with more than 30 floors or more than 100m in height
3. High-rise and low-rise (including pure basement) connected buildings with complex shapes and a difference of more than 10 floors
4. Frame-core tube structure with more than 20 floors and other buildings with special requirements for differential settlement
5. General buildings with more than 7 floors and buildings on slopes and banks with complex site and foundation conditions
6. Buildings that have a significant impact on adjacent existing projects
Class B Buildings other than Class A and Class C
Class C General buildings with 7 floors or less with simple site and foundation conditions and uniform load distribution.
3.1.3 The pile foundation shall be subjected to the following bearing capacity calculations and stability verifications according to specific conditions:
1. The vertical bearing capacity and horizontal bearing capacity of the pile foundation shall be calculated according to the use function and stress characteristics of the pile foundation;
2. The bearing capacity of the pile body and the cap structure shall be calculated; for piles with an undrained shear strength of the pile side soil less than 10kPa and length-to-diameter ratio greater than 50, the pile body compression buckling verification shall be carried out; for precast concrete piles, the pile body bearing capacity shall be verified according to the lifting, transportation and hammering effects; for steel pipe piles, local compression buckling verification shall be carried out;
3. When there is a weak underlying layer below the pile end plane, the bearing capacity of the weak underlying layer shall be verified;
4. The overall stability of the pile foundation located on the slope and the shore shall be verified;
5. For anti-floating and anti-pullout pile foundations, the pullout bearing capacity of the base pile and the pile group shall be calculated;
6. For the pile foundation in the seismic fortification area, the seismic bearing capacity shall be verified.
3.1.4 The following building pile foundations shall be calculated for settlement:
1. Building pile foundations with non-rock embedded piles and non-thick and hard-bearing layers of design grade A;
2. Building pile foundations with design grade B that are complex in shape, have significantly uneven load distribution, or have soft soil layers below the pile end plane;
3. Composite sparse pile foundations for reducing settlement of multi-story buildings on soft soil foundations.
3.1.5 For building pile foundations that are subject to large horizontal loads or have strict restrictions on horizontal displacement, their horizontal displacement shall be calculated.
3.1.6 The crack resistance and crack width of the pile and pedestal cross-section shall be verified according to the environmental category of the pile foundation and the corresponding crack control level.
3.1.7 When designing pile foundations, the combination of action effects and the corresponding resistance shall comply with the following provisions:
1. When determining the number of piles and the arrangement of piles, the standard combination of load effects transmitted to the bottom surface of the pedestal shall be used; the corresponding resistance shall adopt the characteristic value of the bearing capacity of the foundation pile or composite foundation pile.
2. When calculating the settlement and horizontal displacement of pile foundation under load, the quasi-permanent combination of load effect should be used; when calculating the horizontal displacement of pile foundation under horizontal earthquake action and wind load, the standard combination of horizontal earthquake action and wind load effect should be used.
3. When verifying the overall stability of the pile foundation of slope and shore building, the standard combination of load effect should be used; in the seismic fortification area, the standard combination of seismic action effect and load effect should be used.
4. When calculating the bearing capacity of the pile foundation structure, and determining the size and reinforcement, the basic combination of load effect transmitted to the top surface of the pedestal should be used. When verifying the crack control of the pedestal and pile body, the standard combination of load effect and the quasi-permanent combination of load effect should be used respectively.
5. The design safety level of the pile foundation structure, the design service life of the structure, and the importance coefficient γo of the structure should be adopted under the provisions of the current relevant building structure specifications. Except for temporary buildings, the importance coefficient γo should not be less than 1.0.
6. When the pile foundation structure is subjected to earthquake resistance calculation, its bearing capacity adjustment coefficient REγ shall be adopted per the provisions of the current national standard “Code for Seismic Design of Buildings” (GB 50011).
3.1.8 The variable stiffness leveling design aimed at reducing differential settlement and internal forces of the pedestal should be implemented by the following provisions in combination with specific conditions:
1. For main-podium-connected buildings, when the high-rise main body adopts a pile foundation, the foundation or pile foundation stiffness of the podium (including pure basement) should be relatively weakened, and natural foundation, composite foundation, sparse pile, or short pile foundation can be adopted.
2. For the pile foundation of high-rise buildings with frame-core tube structure, the pile foundation stiffness of the core tube area should be strengthened (such as appropriately increasing the pile length, pile diameter, pile number, adopting post-grouting and other measures), and the pile foundation stiffness of the core tube periphery should be relatively weakened (using composite pile foundation and reducing the pile length according to the ground conditions).
3. For high-rise buildings with frame-core tube structures, when the natural foundation bearing capacity meets the requirements, it is advisable to set friction piles in the core tube area to enhance stiffness and reduce settlement.
4. For friction pile foundations of large-volume silos and storage tanks, piles should be arranged according to the principle of strong inside and weak outside.
5. For the pile foundation designed by variable stiffness leveling, it is advisable to conduct a superstructure-cap-pile-soil joint working analysis.
3.1.9 For multi-story buildings on soft soil foundations, when the natural foundation bearing capacity meets the requirements, a composite sparse pile foundation with reduced settlement can be used.
3.1.10 For building pile foundations that should be calculated for settlement as stipulated in Article 3.1.4 of this Code, systematic settlement observations should be carried out during the construction process and the use period after completion until the settlement is stable.
1. Technical Specifications for Building Pile Foundations
3.2.2 In addition to meeting the relevant requirements of the current national standard “Geotechnical Engineering Investigation Specifications” GB 50021, the detailed investigation of pile foundations shall also meet the following requirements:
1. Spacing between exploration points:
1) For end-bearing piles (including rock-embedded piles): mainly determined by the top slope of the pile end-bearing layer, preferably 12~24m. When the slope of the pile end bearing layer revealed by two adjacent investigation points is greater than 10% or the bearing layer has large fluctuations and the stratum distribution is complex, the exploration points should be appropriately increased according to the specific engineering conditions.
2) For friction piles: exploration holes should be arranged at 20~35m, but when the properties or states of the soil layer vary greatly in the horizontal direction, or there are soil layers that may affect the pile formation, the exploration points should be appropriately increased.
3) For single-pile foundations under complex geological conditions, the exploration points should be arranged according to the column line, and one exploration point should be set for each pile.
2. Exploration depth:
1) It is advisable to arrange 1/3~1/2 of the exploration holes as control holes. For building pile foundations with design grade A, at least 3 control holes should be arranged, and for building pile foundations with design grade B, at least 2 control holes should be arranged. The control holes should penetrate the thickness of the compression layer below the pile end plane; general exploration holes should be 3~5 times the pile body design diameter below the expected pile end plane, and shall not be less than 3m; for large diameter piles, it shall not be less than 5m.
2) The control drilling holes of rock-embedded piles should be not less than 3~5 times the pile body design diameter below the expected pile end plane, and the general drilling holes should be not less than 1~3 times the pile body design diameter below the expected pile end plane. When the bearing layer is thin, some of the drilling holes should penetrate the bearing rock layer. In karst and fault fracture zones, the distribution of caves, karst grooves, karst troughs, stalagmites, etc. should be ascertained, and the boreholes should penetrate the caves or fault fracture zones into the stable soil layer, and the depth of entry should meet the requirements of the above-mentioned controlled drilling and general drilling.
3. For each stratum within the exploration depth range, indoor tests should be conducted without disturbing the samples, or in-situ tests should be conducted using effective in-situ test methods according to the soil conditions to provide the parameters required for the design.
II. “Geotechnical Engineering Investigation Code”
4.9.1 Geotechnical engineering investigation for pile foundations shall include the following contents;
1. Identify the type, depth, distribution, engineering characteristics and change law of each layer of rock and soil on the site;
2. When bedrock is used as the bearing layer of the pile, the lithology, structure, rock surface changes, and weathering degree of the bedrock shall be identified,
determine its hardness, integrity, and basic quality grade, and determine whether there are caves, free surfaces, broken rock bodies, or weak rock layers;
3. Identify the hydrogeological conditions, evaluate the impact of groundwater on pile foundation design and construction, and determine the corrosiveness of water quality on building materials;
4. Identify adverse geological effects, the distribution of liquefiable soil layers and special rock and soil and their degree of harm to pile foundations, and propose suggestions for prevention and control measures;
5. Evaluate the possibility of pile formation, and demonstrate the construction conditions of piles and their impact on the environment.
4.9.2 The spacing between soil foundation exploration points shall comply with the following provisions:
1. The spacing between end-bearing piles should be 12 to 24 m, and the height difference between the bearing layers exposed by adjacent exploration holes should be controlled to 1 to 2 m;
2. The spacing between friction piles should be 20 to 35 m; when the formation conditions are complex, affecting the pile formation or there are special design requirements, the exploration points should be appropriately increased;
3. For a one-pillar-one-pile project of a complex foundation, an exploration point should be set for each column.
4.9.3 The geotechnical engineering investigation of pile foundations should be carried out by a combination of drilling and probing and other in-situ tests. For soft soil, clay soil, silt soil, and sand soil, static penetration test and standard penetration test should be used; for gravel soil, heavy or super-heavy cone dynamic probing should be used.
4.9.4 The depth of the exploration hole shall comply with the following provisions:
1. The depth of the general exploration hole shall be 3-5d below the expected pile length (d is the pile diameter), and shall not be less than 3m; for large diameter piles, it shall not be less than 5m;
2. The depth of the controlled exploration hole shall meet the verification requirements of the underlying layer; for pile foundations that need to verify settlement, it shall exceed the calculation depth of foundation deformation;
3. When encountering a weak layer when drilling to the expected depth, it shall be deepened; when encountering stable and solid rock and soil within the expected exploration hole depth, it may be appropriately reduced;
4. For rock-embedded piles, they shall be drilled 3-5d below the expected rock-embedded surface, and pass through karst caves, broken zones, and reach stable strata;
5. When there may be multiple pile length options, it shall be determined according to the longest pile option.
4.9.5 Geotechnical indoor tests shall meet the following requirements:
1. When it is necessary to estimate the lateral resistance and end resistance of the pile and verify the strength of the underlying layer, a triaxial shear test or unconfined compressive strength test should be carried out; the stress conditions of the triaxial shear test should simulate the actual situation of the project;
2. For pile foundation projects where settlement needs to be estimated, compression tests should be carried out, and the maximum pressure should be greater than the sum of the overlying deadweight pressure and the additional pressure;
3. When the bearing layer at the end of the pile is bedrock, rock samples should be taken for saturated uniaxial compressive strength tests, and softening tests should be carried out if necessary; for soft rock and very soft rock, uniaxial compressive strength tests with natural humidity can be carried out. For broken and very broken rocks that cannot be sampled, in-situ tests should be carried out.
4.9.6 The vertical and horizontal bearing capacity of a single pile should be determined based on the project grade, geotechnical properties, and in-situ test results combined with local experience. For buildings with Class A foundation design grades and areas with little experience, static load tests should be recommended. The number of tests should not be less than 1% of the number of engineering piles, and each site should be no less than 3. For piles that bear large horizontal loads, it is recommended to conduct horizontal load tests on the piles; for piles that bear upward pull forces, it is recommended to conduct pullout tests.
The survey report should provide the estimated side resistance and end resistance of the relevant rock and soil foundation piles. If necessary, it should provide the estimated vertical and horizontal bearing capacity and pullout bearing capacity.
4.9.7 For pile foundation projects that require settlement calculations, the deformation parameters of each layer of rock and soil required for the calculation should be provided, and settlement estimates should be made according to task requirements.
4.9.8 In addition to complying with the requirements of Chapter 14 of this Code and providing bearing capacity and deformation parameters under Articles 4.9.6 and 4.9.7, the geotechnical investigation report for pile foundation engineering shall also include the following contents:
1. Provide optional pile foundation types and pile end-bearing layers; propose suggestions for pile length and pile diameter schemes;
2. When there is a weak underlying layer, verify the strength of the weak underlying layer;
3. For projects with under consolidated soil and large-area loading, the possibility of negative friction on the pile side and its impact on the bearing capacity of the pile foundation shall be analyzed, and the negative friction coefficient and suggestions for reducing negative friction shall be provided;
4. Analyze the possibility of pile formation, the impact of pile formation, and the soil squeezing effect, and propose suggestions for protection measures;
5. When the bearing layer is an inclined stratum, the bedrock surface is uneven or there are caves in the rock and soil, the stability of the pile shall be evaluated and suggestions for treatment measures shall be proposed.
“Technical Specifications for Building Pile Foundations”
3.3.1 Foundation piles can be classified according to the following provisions:
1. Classification by bearing characteristics:
1) Friction piles:
Friction piles: Under the ultimate bearing capacity state, the vertical load on the pile top is borne by the pile side resistance, and the pile end resistance is so small that it can be ignored;
End-bearing friction piles: Under the ultimate bearing capacity state, the vertical load on the pile top is mainly borne by the pile side resistance.
2) End-bearing piles:
End-bearing piles: Under the ultimate bearing capacity state, the vertical load on the pile top is borne by the pile end resistance, and the pile side resistance is so small that it can be ignored;
Friction end-bearing piles: Under the ultimate bearing capacity state, the vertical load on the pile top is mainly borne by the pile end resistance.
2. Classification by pile forming method:
1) Non-squeezing soil piles: dry operation method bored (excavated) cast-in-place piles, mud wall method bored (excavated) cast-in-place piles, casing wall method bored (excavated) cast-in-place piles;
2) Partially squeezed soil piles: long spiral pressure cast-in-place piles, punching cast-in-place piles, bored squeezed and expanded cast-in-place piles, mixing core piles, pre-drilled driven (static pressure) precast piles, driven (static pressure) open steel pipe piles, open prestressed concrete hollow piles and H-shaped steel piles;
3) Squeezing soil piles: sinking tube cast-in-place piles, sinking tube ramming (squeezing) expanded cast-in-place piles, driven (static pressure) precast piles, closed prestressed concrete hollow piles, and closed steel pipe piles.
3. Classification by pile diameter (design diameter d):
1) Small diameter pile: d ≤ 250mm;
2) Medium diameter pile: 250mm< d < 800mm;
3) Large diameter pile: d ≥ 800mm.
3.3.2 Pile type and pile-making process should be selected according to the principles of safety, applicability, and economic rationality based on the type of building structure, load nature, pile use function, soil layer to be penetrated, pile end bearing layer, groundwater level, construction equipment, construction environment, construction experience, pile material supply conditions, etc. The selection can be made according to Appendix A of this specification. 1. For pile raft foundations with very uneven load distribution such as frame-core tubes, it is advisable to select pile types and processes with greater adjustability of foundation pile size and bearing capacity.
2. When the soil-squeezing pipe cast-in-place pile is used in silt and silty soil layers, it should be limited to multi-story residential pile foundations.
3.3.3 The arrangement of foundation piles should meet the following conditions:
1. The minimum center distance of foundation piles should meet the requirements of Table 3.3.3-1; when reliable measures are taken to reduce the soil squeezing effect during construction, it can be appropriately reduced according to local experience.
2. When arranging foundation piles, it is advisable to make the combined force point of the pile group bearing capacity coincide with the combined force point of the vertical permanent load, and make the foundation pile have a larger bending section modulus in the direction of larger horizontal force and moment.
3. For pile box foundations and pile rafts (including flat plate and beam-slab caps) of shear wall structures, piles should be arranged under the wall.
4. For pile raft foundations of frame-core tube structures, the mutual influence should be considered according to the load distribution, and the piles should be relatively concentrated under the core tube and columns. Composite pile foundations should be used for peripheral frame columns, and the pile length should be shorter than the foundation piles under the core tube (when there is a suitable pile end-bearing layer).
5. A harder soil layer should be selected as the pile end bearing layer. The depth of the full section of the pile end entering the bearing layer should not be less than 2d for clay and silt, not less than 1.5d for sand, and not less than 1d for gravel soil. When there is a weak underlying layer, the thickness of the hard-bearing layer below the pile end should not be less than 3d.
6. For rock-embedded piles, the rock-embedded depth should be determined by comprehensive factors such as load, overlying soil layer, bedrock, pile diameter, and pile length; the full section depth embedded in inclined complete and relatively complete rock should not be less than 0.4d and not less than 0.5m. For moderately weathered rock with an inclination greater than 30%, the rock-embedded depth should be appropriately increased according to the inclination and rock integrity; the depth embedded in flat, complete hard rock and relatively hard rock should not be less than 0.2d and should not be less than 0.2m.
3.4.1 The design principles of pile foundations in soft soil should comply with the following provisions:
1. For pile foundations in soft soil, medium and low compressibility soil layers should be selected as the bearing layer at the pile end;
2. When the settlement of the soft soil around the pile due to self-weight consolidation, site filling, large-scale ground loading, lowering of groundwater level, large-scale soil extrusion, and pile sinking is greater than the settlement of the foundation pile, the influence of the negative friction resistance on the pile side on the foundation pile should be analyzed and calculated according to the specific engineering conditions;
3. When using soil extrusion piles, technical measures should be taken to reduce pore water pressure and soil extrusion effect to reduce the adverse effects of soil extrusion effect on pile quality, adjacent buildings, roads, underground pipelines, and foundation pit slopes;
4. When the pile is built first and the foundation pit is excavated later, the excavation sequence of the foundation pit must be reasonably arranged and the depth of layered excavation must be controlled to prevent the influence of soil lateral displacement on the pile.
3.4.2 The design principles of pile foundations in collapsible loess areas should comply with the following provisions: 1. The foundation piles should penetrate the collapsible loess layer, and the pile ends should be supported in low-compressible clay, silt, medium-dense, and dense sand and gravel layers;
2. In the collapsible loess foundation, the ultimate bearing capacity of a single pile of a building pile foundation with a design grade of Class A or B should be based mainly on the immersion load test;
3. The ultimate bearing capacity of a single pile in a self-weight collapsible loess foundation should be calculated based on the analysis of the influence of the negative friction on the pile side according to the specific conditions of the project.
3.4.3 The design principles of pile foundations in seasonally frozen soil and expansive soil foundations shall comply with the following provisions: 1. The depth of the pile end entering the frost depth line or the atmospheric influence sharp layer of expansive soil shall meet the requirements of pull-out stability verification, and shall not be less than 4 times the pile diameter and 1 times the diameter of the expanded end, and the minimum depth shall be greater than 1.5m;
2. To reduce and eliminate the effect of frost heave or expansion on the pile foundation of the building, it is advisable to use bored (dug) hole cast-in-place piles;
3. When determining the vertical ultimate bearing capacity of the foundation pile, in addition to not taking into account the pile side resistance within the frost heave or expansion depth range, the frost heave and expansion effect of the foundation soil shall also be considered to verify the pull-out stability of the pile foundation and the tensile bearing capacity of the pile body;
4. In order to eliminate the harm of frost heave or expansion to the pile foundation, frost and expansion isolation treatment can be performed along the pile periphery and the cap within the frost heave or expansion depth range.
3.4.4 The design principles of pile foundations in karst areas shall comply with the following provisions:
1. For pile foundations in karst areas, drilled and punched piles are recommended;
2. When the load of a single pile is large and the rock layer is shallow, rock-embedded piles are recommended;
3. When the bedrock surface is undulating and the burial depth is large, friction-type cast-in-place piles are recommended.
3.4.5 The design principles of pile foundations on the bank of a slope shall comply with the following provisions:
1. For pile foundations built on the bank of a slope, the piles shall not be supported on potential sliding bodies of the slope. The depth of the pile end entering the stable rock and soil layer below the potential slip surface should be able to ensure the stability of the pile foundation;
2. The building pile foundation and the slope should maintain a certain horizontal distance; the slope in the construction site must be completely stable. When there are adverse geological phenomena such as collapse and landslide, it should be rectified under the provisions of the current national standard “Technical Specifications for Building Slope Engineering” (GB50330) to ensure its stability;
3. The new slope and shore building pile foundation engineering should be planned and designed in a unified manner with the building slope engineering, and the construction sequence should be reasonably determined;
4. It is not advisable to use squeezed soil piles;
5. The overall stability of the pile foundation and the horizontal bearing capacity of the pile foundation under the most unfavorable load effect combination should be verified.
3.4.6 The design principles of pile foundations in earthquake-resistant areas shall comply with the following provisions: 1. The length of the pile entering the stable soil layer below the liquefied soil layer (excluding the pile tip) shall be determined by calculation; for crushed stone soil, gravel, coarse and medium sand, dense silt, and hard clay soil, it should not be less than 2 to 3 times the diameter of the pile body, and for other non-rock soils, it should not be less than 4 to 5 times the diameter of the pile body; 2. The area around the cap and the side wall of the basement should be backfilled with lime soil, graded sand and gravel, and plain soil with good compaction, and compacted in layers, or plain concrete backfill;
3. When the area around the cap is liquefiable soil or soft soil with a foundation bearing capacity characteristic value of less than 40kPa (or undrained shear strength less than 15kPa), and the horizontal bearing capacity of the pile foundation does not meet the calculation requirements, the soil within the range of 1/2 of the side length of the cap on each side outside the cap can be reinforced;
4. For areas with liquefaction expansion, the stability of the pile foundation under the lateral force of soil flow should be verified.
3.4.7 The design principles of pile foundations that may have negative friction resistance should comply with the following provisions: 1. For fill construction sites, it is advisable to fill the soil first and ensure the compactness of the fill. Before filling the soft soil site, measures such as pre-installed plastic drainage boards should be taken. Piles can only be built after the settlement of the fill foundation is stable;
2. For buildings with large-area ground loads, measures should be taken to reduce the impact of ground settlement on the pile foundation of the building;
3. For self-weight collapsible loess foundations, strong tamping, compacted soil piles, and other preliminary treatments can be used to eliminate the self-weight collapsibility of the upper or all soils; for under consolidated soils, measures such as early drainage and preloading should be taken;
4. For soil-squeezing piles, measures such as reducing excess pore water pressure and controlling the pile sinking rate should be taken;
5. For piles above the neutral point, the surface can be treated to reduce negative friction resistance.
3.4.8 The design principles of pull-out pile foundations shall comply with the following provisions: 1. The crack control level of the pull-out pile shall be determined based on factors such as the environmental category and the corrosion of water and soil on the steel bars, the sensitivity of the steel bar type to corrosion, and the load action time;
2. For the first crack control level that strictly requires no cracks, prestressed tendons shall be set on the pile body; for the second crack control level that generally requires no cracks, prestressed tendons should be set on the pile body;
3. For the third crack control level, the width of the crack in the pile body shall be calculated;
4. When the pull-out bearing capacity of the foundation pile is required to be high, technical measures such as pile side post-grouting and bottom expansion can be adopted.
1. “Design Code for Foundation and Subgrade of Highway Bridges and Culverts”
5.2.1 The design diameter of bored piles should not be less than 0.8m; the diameter or minimum side width of bored piles should not be less than 1.2m; the diameter of reinforced concrete pipe piles can be 0.4-0.8m, and the minimum thickness of the pipe wall should not be less than 80mm.
5.2.2 Concrete piles.
1. Pile body concrete strength grade: bored (dig) piles and sunken piles should not be less than C25; the core concrete of pipe piles should not be less than C15.
2. The pile body of reinforced concrete sunken piles should be reinforced throughout the length according to the internal force requirements of each stage of transportation, sinking, and use. The spacing between the stirrups or spiral bars at both ends of the pile and the pile connection area must be increased, and the value can be 40-50mm.
3. Drilled (dig) piles should be reinforced in sections according to the internal force of the pile body. When the internal force calculation shows that no reinforcement is required, structural steel bars should be provided within 3.0-5.0m of the pile top.
1) The diameter of the main reinforcement in the pile should not be less than 16mm, the number of main reinforcements per pile should not be less than 8, and the net distance should not be less than 80mm and should not be greater than 350mm.
2) If there are more reinforcements, bundled reinforcement can be used. The diameter of a single steel bar that makes up the bundled reinforcement should not be greater than 36mm, and the number of single steel bars that make up the bundled reinforcement should not be more than 3 when its diameter is not greater than 28mm and should be 2 when its diameter is greater than 28mm. The equivalent diameter of the bundled reinforcement is de=√nd, where n is the number of single bundled steel bars and d is the diameter of a single steel bar.
3) The net distance of the steel bar protective layer should not be less than 60mm.
4) The diameter of the closed stirrup or spiral reinforcement should not be less than 1/4 of the main reinforcement diameter and should not be less than 8mm, and the distance should not be greater than 15 times the main reinforcement diameter and should not be greater than 300mm.
5) A reinforcing hoop with a diameter of 16-32mm is set every 2.0–2.5m on the steel cage skeleton.
6) Protruding positioning steel bars, positioning concrete blocks, or other positioning measures should be set around the steel cage.
7) The main bars at the bottom of the steel cage should be slightly bent inward as a guide. 4. The segment length of reinforced concrete precast piles should be determined according to the construction conditions, and the number of joints should be minimized. The joint strength should not be lower than the strength of the pile body, the joint flange should not protrude outside the pile body, and the joint should not loosen or crack during pile sinking and use.
5. For prestressed concrete open pipe piles with pile ends embedded in unsaturated strongly weathered rocks, effective measures should be taken to prevent water seepage and softening of the bearing layer at the pile end.
6. When the riverbed rock layer is scoured, the effective depth of the bored pile should consider the lowest scour elevation of the rock layer.
5.2.3 Steel piles.
1. Steel piles can be tubular or H-shaped, and their materials should comply with the current national specifications and standards.
2. Steel pile welding joints should be connected with equal strength. The welding rods, welding wires, and welding flux used should comply with the current national specifications and standards.
3. The end form of the steel pile should be determined based on the soil layer that the pile passes through, the nature of the bearing layer at the pile end, the size of the pile, the soil squeezing effect, and other factors.
1) The following pile end forms can be used for steel pipe piles:
① Open with reinforcement hoop (with internal baffle, without internal baffle), open without reinforcement hoop (with internal baffle, without internal baffle);
② Closed flat bottom, cone bottom.
2) The following pile end forms can be used for H-shaped steel:
① With end plate;
② Without end plate, cone bottom, flat bottom (with expanded wing, without expanded wing).
4. The anti-corrosion treatment of steel piles shall comply with the following provisions:
1) In a seawater environment, the annual average corrosion rate of steel piles on one side can be taken according to Table 5.2.3, and can also be determined according to field measurements when conditions permit. Under other conditions, above the average low water level, the annual average corrosion rate can be taken as 0.06mm/year; below the average low water level, the annual average corrosion rate can be taken as 0.03mm/year.
2) Anti-corrosion treatment of steel piles can be carried out by coating the outer surface with an anti-corrosion layer, increasing the corrosion allowance and cathodic protection. When the inner wall of the steel pipe pile is isolated from the outside world, the inner wall anti-corrosion can be ignored.
5.2.4 Arrangement and center distance of piles.
1. The arrangement of pile groups can be symmetrical, plum blossom, or circular.
2. The center distance of piles should meet the following requirements:
1) Friction piles.
For hammering and static pressure piles, the center distance at the pile end should not be less than 3 times the pile diameter (or side length), and it should be appropriately increased for soft soil foundations; for piles sunk into sand by vibration, the center distance at the pile end should not be less than 4 times the pile diameter (or side length). The center distance of the pile at the bottom of the cap should not be less than 1.5 times the pile diameter (or side length).
The center distance of bored piles should not be less than 2.5 times the pile diameter.
The center distance of bored piles can refer to bored piles.
2) End-bearing piles.
The center distance of bored (excavated) piles supported or embedded in bedrock should not be less than 2.0 times the pile diameter.
3) Cast-in-place piles with expanded bottom.
The center distance of bored (excavated) piles with expanded bottom should not be less than 1.5 times the expanded bottom diameter or the expanded bottom diameter plus 1.0m, whichever is greater.
3 The distance between the outer side of the side pile (or corner pile) and the edge of the cap should not be less than 0.5 times the pile diameter (or side length) for piles with a diameter (or side length) less than or equal to 1.0m, and should not be less than 250mm; for piles with a diameter greater than 1.0m, it should not be less than 0.3 times the pile diameter (or side length) and should not be less than 500mm.
5.2.5 Construction of cap and transverse tie beam.
1. The thickness of the cap should be 1.0 times or more of the pile diameter, and should not be less than 1.5m, and the concrete strength grade should not be lower than C25.
2. When the top of the pile is directly buried in the cap, 1-2 layers of steel mesh should be set on the top surface of each pile. When the main reinforcement of the pile top extends into the cap, a layer of steel mesh should be set in the top plane of the pile body concrete. A steel mesh of 1200-1500mm2 should be set in each meter (in each direction). The diameter of the steel bar is 12-16mm. The steel mesh should pass through the top of the pile and should not be cut off. The top and side surfaces of the cap should be provided with surface steel mesh. The cross-sectional area of each surface in both directions should not be less than 400mm2/m, and the spacing between steel bars should not be greater than 400mm.
3. When using a horizontal tie beam to strengthen the integrity between piles, the height of the horizontal tie beam can be 0.8-1.0 times the diameter of the pile, and the width can be 0.6-1.0 times the diameter of the pile. The strength grade of concrete should not be lower than C25. The longitudinal reinforcement shall not be less than 0.15% of the cross-sectional area of the transverse tie beam; the diameter of the stirrup shall not be less than 8mm, and the spacing shall not be greater than 400mm.
5.2.6 The connection between the pile and the cap and the transverse tie beam shall meet the following requirements.
1. The top of the pile is directly embedded in the cap: when the pile diameter (or side length) is less than 0.6m, the embedded length shall not be less than 2 times the pile diameter (or side length); when the pile diameter (or side length) is 0.6-1.2m, the embedded length shall not be less than 1.2m; when the pile diameter (or side length) is greater than 1.2m, the embedded length shall not be less than the pile diameter (or side length).
2. The main reinforcement of the pile top is extended into the cap: the depth of the pile body embedded in the cap can be 100mm; the main reinforcement of the pile top extending into the cap can be made into a trumpet shape (the angle with the vertical line is about 15°). The length of the main reinforcement extending into the cap should not be less than 30 times the diameter of the steel bar (with a hook), and the length of the ribbed steel bar should not be less than 35 times the diameter of the steel bar (without a hook).
3. For large-diameter cast-in-place piles, when one column and one pile are used, a horizontal tie beam can be set or the pile and the column can be directly connected.
4. When the pipe pile is connected to the cap, if the longitudinal reinforcement extending into the cap is inserted, the number of inserted bars should not be less than 4, the diameter should not be less than 16mm, the length of the anchor into the cap should not be less than 35 times the diameter of the steel bar, and the length of the inserted into the core concrete of the top of the pipe pile should not be less than 1.0m.
5. The main reinforcement of the horizontal tie beam should extend into the pile, and its length should not be less than 35 times the diameter of the main reinforcement.
II. Technical Specifications for Building Pile Foundations I. Cast-in-place piles 4.1.1 Cast-in-place piles shall be reinforced as follows:
1. Reinforcement ratio: When the pile diameter is 300-2000mm, the cross-section reinforcement ratio can be 0.65%-0.2% (the higher value is taken for small diameter piles); for piles with particularly large loads, pull-out piles and rock-embedded end-bearing piles, the reinforcement ratio shall be determined based on calculations and shall not be less than the above-specified value;
2. Reinforcement length:
1) End-bearing piles and foundation piles located on the sloping bank shall be reinforced along the pile body with equal or variable cross-sections;
2) The reinforcement length of friction piles with a pile diameter greater than 600mm shall not be less than 2/3 of the pile length; when subjected to horizontal loads, the reinforcement length should not be less than 4.0/α,α is the horizontal deformation coefficient of the pile);
3) For foundation piles subjected to earthquakes, the reinforcement length of the pile body shall pass through the liquefiable soil layer and the soft soil layer, and the depth of entering the stable soil layer shall not be less than the depth specified in Article 3.4.6 of this Code;
4) For piles subjected to negative friction resistance and piles that rebound with the foundation soil due to pile construction before foundation pit excavation, the reinforcement length shall pass through the soft soil layer and enter the stable soil layer, and the depth of entry shall not be less than 2 to 3 times the diameter of the pile body;
5) For special pull-out piles and piles subjected to pull-out due to earthquakes, frost heave, or expansion forces, the reinforcement shall be of equal or variable cross-section throughout the length. 3. For piles subject to horizontal loads, the main reinforcement should not be less than 8φ12; for compression piles and pull-out piles, the main reinforcement should not be less than 6φ10; the longitudinal main reinforcement should be evenly arranged along the periphery of the pile body, and the net distance should not be less than 60mm;
4. The stirrups should be spiral, with a diameter of not less than 6mm and a spacing of 200-300mm; for pile foundations subject to large horizontal loads, pile foundations subject to horizontal earthquakes, and when the compressive bearing capacity of the pile body is calculated considering the main reinforcement, the stirrups within 5d below the pile top should be denser, and the spacing should not be greater than 100mm; when the pile body is located within the liquefied soil layer, the stirrups should be denser; when considering the stress of the stirrups, the stirrup configuration should comply with the relevant provisions of the current national standard “Concrete Structure Design Code” GB50010; when the length of the steel cage exceeds 4m, a welded stiffening stirrup with a diameter of not less than 12mm should be set every 2m.
4.1.2 The thickness of the pile body concrete and concrete protective layer shall meet the following requirements:
1. The strength grade of the pile body concrete shall not be less than C25 (in the “Code for Design of Building Foundations”, the concrete strength grade shall not be less than C20), and the strength grade of the concrete precast pile tip shall not be less than C30;
2. The thickness of the concrete protective layer of the main reinforcement of the cast-in-place pile shall not be less than 35mm, and the thickness of the concrete protective layer of the main reinforcement of the underwater cast-in-place pile shall not be less than 50mm; (the protective layer of the bridge and culvert pile foundation shall not be less than 60mm) 4.1.3 Expanded bottom pile structure
3. The thickness of the pile body concrete protective layer in the fourth and fifth environments shall comply with the relevant provisions of the current national standards “Design Code for Concrete Structures of Port Engineering” JTJ 267 and “Design Code for Anti-corrosion of Industrial Buildings” GB50046.
4.1.3 The size of the expanded bottom end of the expanded bottom cast-in-place pile shall comply with the following provisions (Figure 4.1.3):
1. For compression piles with a high bearing capacity of the bearing layer and poor overlying soil layer, and pull-out piles with a certain thickness of good soil layer above the pile end, the expanded bottom end can be used; the ratio of the expanded bottom end diameter to the pile body diameter D/d shall be determined according to the bearing capacity requirements and the soil characteristics of the expanded bottom end side and the pile end bearing layer and the expanded bottom construction method; the D/d of the bored pile shall not be greater than 3, and the D/d of the bored pile shall not be greater than 2.5;
2. The slope of the expanded bottom end side shall be determined according to the actual hole formation and soil self-supporting conditions, a/hc can be 1/4~1/2, sand can be 1/4, silt and clay can be 1/3~1/2;
3. The bottom surface of the expanded bottom end of the compression pile should be pot-bottom shaped, and the rise hb can be (0.15~0.20)D.
Ⅱ Precast concrete piles 4.1.4 The cross-sectional length of precast concrete piles should not be less than 200mm; the cross-sectional length of prestressed concrete precast solid piles should not be less than 350mm.
4.1.5 The concrete strength grade of precast piles should not be lower than C30; the concrete strength grade of prestressed concrete solid piles should not be lower than C40; the thickness of the concrete protective layer of the longitudinal reinforcement of precast piles should not be less than 30mm.
4.1.6 The reinforcement of the pile body of precast piles should be calculated and determined according to the conditions of hoisting, piling, and the force of the piles in use. When the piles are driven by hammering, the minimum reinforcement ratio of precast piles should not be less than 0.8%. When the piles are driven by static pressure, the minimum reinforcement ratio should not be less than 0.6%, the main reinforcement diameter should not be less than φ14, and the stirrups within the length of 4 to 5 times the pile body diameter below the top of the pile should be denser, and steel mesh should be set.
4.1.7 The segment length of precast piles shall be determined according to the construction and transportation conditions; the number of joints for each pile shall not exceed 3.
4.1.8 The main reinforcement of the precast pile can be welded to the auxiliary reinforcement at the pile tip. When the bearing layer is dense sand and gravel soil, it is advisable to wrap the pile tip with a steel plate pile shoe to strengthen the pile tip.
Ⅲ Prestressed concrete hollow piles
4.1.9 Prestressed concrete hollow piles can be divided into pipe piles and hollow square piles according to the cross-sectional form, and can be divided into prestressed high-strength concrete (PHC) piles and prestressed concrete (PC) piles according to the concrete strength grade. The cross-sectional dimensions, reinforcement, ultimate bending moment of the pile body, and design value of the vertical compressive bearing capacity of the centrifugally formed prestressed concrete piles can be determined according to Appendix B of this specification.
4.1.10 The pile tip type of prestressed concrete hollow piles should be selected as closed or open according to the nature of the stratum; the closed type is divided into the flat-bottomed cross-type and cone type.
4.1.11 The quality requirements of prestressed concrete hollow piles shall also comply with the current national standards GB/T13476, JC888, JG197, and other relevant standards for “Prestressed Concrete Pipe Piles by Prestressing Method”, “Prestressed Concrete Thin-wall Pipe Piles by Prestressing Method” and “Hollow Square Piles by Prestressing Method”.
4.1.12 The connection of prestressed concrete piles can be made by end plate welding, flange connection, mechanical meshing connection, and threaded connection. The number of joints per pile should not exceed 3.
4.1.13 For prestressed concrete hollow piles whose pile ends are embedded in strongly weathered rocks, fully weathered rocks, and unsaturated soils that are easily softened by water, effective anti-seepage measures shall be taken within a range of about 2m above the pile ends after the piles are sunk. Micro-expansion concrete can be used to fill the core or flexible waterproof materials can be pre-coated on the inner wall.
IV Steel Piles
4.1.14 Steel piles can be made of tubular, H-shaped, or other special-shaped steel.
4.1.15 The segment length of steel piles should be 12-15m.
4.1.16 Steel pile welded joints should be connected with equal strength.
4.1.17 The end form of steel piles should be determined based on the soil layer through which the pile passes, the nature of the bearing layer at the pile end, the size of the pile, the soil squeezing effect, and other factors, and can be adapted according to the following provisions:
1. Steel pipe piles can adopt the following pile end forms:
1) Open: with reinforcement hoop (with inner baffle, without inner baffle); without reinforcement hoop (with inner baffle, without inner baffle).
2) Closed: flat bottom; conical bottom. 2. H-shaped steel piles can adopt the following pile end forms:
1) With end plate;
2) Without end plate: conical bottom; flat bottom (with enlarged wing, without enlarged wing).
4.1.18 The anti-corrosion treatment of steel piles shall comply with the following provisions:
1. When there is no measured data, the corrosion rate of steel piles can be determined according to Table 4.1.18;
2. The anti-corrosion treatment of steel piles can be achieved by coating the outer surface with an anti-corrosion layer, increasing the corrosion allowance and providing cathodic protection; when the inner wall of the steel pipe pile is isolated from the outside world, the inner wall anti-corrosion can be ignored.
4.2.1 The construction of pile foundation caps shall meet the requirements of anti-shear, anti-shear, and anti-bending bearing capacity and superstructure, and shall also meet the following requirements:
1. The minimum width of the pile foundation cap under the independent column shall not be less than 500mm, and the distance from the center of the side pile to the edge of the cap shall not be less than the diameter or side length of the pile, and the distance from the outer edge of the pile to the edge of the cap shall not be less than 150mm. For the strip cap beam under the wall, the distance from the outer edge of the pile to the edge of the cap beam shall not be less than 75mm. The minimum thickness of the cap shall not be less than 300mm.
2. The minimum thickness of the flat plate and beam-slab raft caps of high-rise buildings shall not be less than 400mm, and the minimum thickness of the raft cap of the shear wall structure with piles under the wall shall not be less than 200mm.
3. The construction of the box cap of a high-rise building shall comply with the provisions of “Technical Specifications for Raft and Box Foundations of High-Rise Buildings” JGJ6.
4.2.2 The concrete material and strength grade of the cap shall meet the requirements of durability and impermeability of structural concrete.
4.2.3 The steel bar configuration of the cap shall meet the following requirements:
1. The longitudinal force-bearing steel bars of the independent pile foundation cap under the column shall be configured throughout the length (Figure 4.2.3-a). For caps with more than four piles (including four piles), they should be evenly arranged in two directions. For triangular caps with three piles, they should be evenly arranged in three directions, and the triangle formed by the innermost three steel bars should be within the column section (Figure 4.2.3-b). The anchorage length of the longitudinal steel bar is calculated from the inside of the side pile (when it is a round pile, its diameter should be multiplied by 0.8 to be equivalent to a square pile), and should not be less than 35dg (dg is the steel bar diameter); if it does not meet the requirements, the longitudinal steel bar should be bent upwards. At this time, the length of the horizontal section should not be less than 25 kg, and the length of the bent section should not be less than 10 kg. The diameter of the longitudinal force-bearing steel bar of the cap should not be less than 12mm, and the spacing should not be greater than 200mm. The minimum reinforcement ratio of the independent pile foundation cap under the column should not be less than 0.15%.
2. The independent two-pile cap under the column should be equipped with longitudinal tensile steel bars, and horizontal and vertical distribution steel bars under the deep bending members in the current national standard “Code for Design of Concrete Structures” (GB50010). The anchorage length and structure of the ends of the longitudinal force-bearing steel bars of the cap should be the same as those of the multi-pile cap under the column. Regarding the provisions on the minimum reinforcement ratio (Figure 4.2.3-c), the diameter of the main reinforcement should not be less than 12mm, the diameter of the frame reinforcement should not be less than 10mm, and the diameter of the stirrups should not be less than 6mm. The anchorage length and structure of the longitudinal force-bearing steel bars at the ends of the cap beam should be the same as those of the multi-pile cap under the column.
3. When only the local bending moment is considered in the calculation of the raft cap or box cap, considering the influence of the overall bending, the reinforcement ratio of the lower steel bars in the longitudinal and transverse directions should not be less than 0.15%; the upper steel bars should be fully connected according to the calculated reinforcement ratio. When the thickness of the raft slab is greater than 2000mm, a bidirectional steel mesh with a diameter of not less than 12mm and a spacing of not more than 300mm should be set in the middle of the slab thickness.
4. The thickness of the concrete protective layer of the steel bars at the bottom of the cap should not be less than 50mm when there is a concrete cushion layer, and should not be less than 70mm when there is no cushion layer; in addition, it should not be less than the length of the pile head embedded in the cap.
4.2.4 The connection structure between the pile and the cap should comply with the following provisions:
1. The length of the pile embedded in the cap should not be less than 50mm for medium-diameter piles; it should not be less than 100mm for large-diameter piles.
2. The longitudinal main reinforcement of the pile top of the concrete pile should be anchored in the cap, and its anchoring length should not be less than 35 times the diameter of the longitudinal main reinforcement. For pull-out piles, the anchorage length of the longitudinal main reinforcement at the pile top should be determined under the current national standard “Code for Design of Concrete Structures” (GB50010).
3. For large-diameter cast-in-place piles, when one column and one pile are used, a cap can be set or the pile and column can be directly connected.
4.2.5 The connection structure between the column and the cap shall comply with the following provisions:
1. For one column and one pile foundation, when the column and the pile are directly connected, the length of the column longitudinal main reinforcement anchored into the pile body shall not be less than 35 times the longitudinal main reinforcement diameter.
2. For multi-pile caps, the column longitudinal main reinforcement shall be anchored into the cap for not less than 35 times the longitudinal main reinforcement diameter; when the cap height does not meet the anchoring requirements, the vertical anchoring length shall not be less than 20 times the longitudinal main reinforcement diameter, and be bent 90° toward the column axis.
3. When there are seismic fortification requirements, for columns with first and second seismic resistance levels, the longitudinal main reinforcement anchorage length shall be multiplied by a coefficient of 1.15; for columns with third seismic resistance levels, the longitudinal main reinforcement anchorage length shall be multiplied by a coefficient of 1.05.
4.2.6 The connection structure between the caps shall comply with the following provisions:
1. When there is one column and one pile, a connecting beam shall be set in the two main axis directions of the pile top. When the ratio of the cross-sectional diameter of the pile to the column is greater than 2, a connecting beam may not be set.
2. For the cap of the two-pile foundation, a connecting beam shall be set in its short direction.
3. For the cap of the column-under-pile foundation with seismic protection requirements, a connecting beam should be set along the two main axis directions.
4. The top surface of the connecting beam should be at the same elevation as the top surface of the cap. The width of the connecting beam should not be less than 250mm, and its height can be 1/10~1/15 of the center distance of the cap, and should not be less than 400mm.
5. The reinforcement of the connecting beam should be determined by calculation, and the upper and lower reinforcement of the beam should not be less than 2 steel bars with a diameter of 12mm; the longitudinal reinforcement of the connecting beam located on the same axis should be arranged throughout the length.
4.2.7 The gaps between the outer walls of the pedestal and basement and the side walls of the foundation pit should be poured with plain concrete, or compacted in layers using lime soil, graded sand and gravel, or plain soil with good compaction properties, and the compaction coefficient should not be less than 0.94.
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