Foundation engineering is an engineering discipline that explains foundation and foundation issues in the design and construction of buildings. Study the interaction between the lower structure and the rock and soil and jointly bear the various deformation and stability problems caused by the upper structure.

Basic engineering is an important part of the geotechnical engineering discipline. It is a course that uses the basic theories and methods of geotechnical engineering to solve technical problems in building foundations and basic engineering. It is also a main course in the civil engineering professional curriculum system.

Through course study, the purpose is to enable students to acquire the basic theories and calculation methods of basic engineering design, to be able to rationally select foundation plans based on the requirements of the building and foundation survey data, and to use the design principles of basic engineering to design the foundation foundation of general buildings. This will lay a good foundation for subsequent professional courses and future foundation design and construction.

1. Master the concepts related to foundations and the requirements for foundations in construction projects

2. Master the basic theories and calculation methods of shallow basic basic design;

3. Master the basic theory and calculation methods of general single pile or pile group foundation design

4. Clarify the relevant content of geotechnical engineering surveys and reports (supplementary);

5. Understand the structural forms, functions and main construction methods of the main deep foundations

6. Understand the general treatment methods of common foundations;

7. Understand the common forms and scope of application of foundation pit enclosure structures;

8. Understand the engineering characteristics of common special soil foundations and corresponding engineering measures.

Focus on the following chapters 2 and 4

Chapter 1 Introduction and Supplement (Geotechnical Investigation and Report)

Chapter 2 Shallow Basics

Chapter 3 Continuous Basics

Chapter 4 Pile Foundation

Chapter 5 Foundation Treatment

Chapter 6 Geosynthetics

Chapter 7 Retaining Wall

Chapter 8 Foundation Pit Engineering

Chapter 9 Special Soil Foundation

Chapter 10 Power Machine Foundation and Foundation Earthquake Resistance

1. The professional core courses of the civil engineering major. The quality of the basics is directly related to the success or failure of the entire project, which shows the importance of basic engineering;

2. The course content is extensive and comprehensive, involving many basic and professional courses in civil engineering (material mechanics, structural mechanics, engineering geology, soil mechanics, etc.);

3. The course involves many specifications. There is no unified foundation design specification among various industries in civil engineering. There are certain differences in relevant specifications. The regulations, terminology, calculation formulas, etc. of each industry are not consistent, which brings difficulties to learning and use. It must be difficult to come.

The course study mainly focuses on the basic knowledge of basic engineering disciplines and basic design theory. Taking into account the main contents and basic methods in basic engineering design and construction.

During study, you should pay attention to in-situ testing technology, learn to read and use foundation survey data correctly, understand the scope of application and limitations of the specifications, empirical formulas and empirical parameters used in the course, and avoid applying them in the same way regardless of industry and region. applied mechanically.

Building foundations and foundations

foundation

Base

Underground concealed engineering

The substructure of a building that transfers loads to the foundation, generally located below the ground

That part of the ground that bears the full load of a building and is affected by the building

Divided according to design and construction conditions

Geological condition

Foundation embedding depth

Ease of construction

divided into

When the foundation is made of multiple layers of soil, the soil layer in contact with the bottom surface of the foundation is called the bearing layer. The bearing layer should be a soil layer with good engineering properties as much as possible.

The foundation soil layer below the bearing layer is called the underlying layer.

foundation

Base

Basic engineering design

construction

monitor

That is: the designer first selects and determines the foundation plan for the building project (including the selection of the foundation and the design of the foundation), and then the construction and related engineering units complete the construction of the foundation foundation and the necessary inspection and monitoring work.

For example: the seepage of groundwater in the foundation soil and the rise and fall of the water level cause the foundation to deform and cause settlement; When the foundation is sandy soil or silt soil, the groundwater level is shallow and vibration liquefaction may occur, making the foundation soil liquid and losing its bearing capacity.

Survey first, then design, then construction

Geotechnical types and engineering properties

Geological structure

topography

Hydrogeological conditions

Undesirable geological phenomena

Available natural building materials

For example, the development pattern and degree of karst and its associated soil caves can be used to predict their hazards; the scope, scale, and stability of landslides can be predicted to predict their development trends and hazards; the conditions, scope, scale, and hazards of collapses; the nature of debris flows; The occurrence and its type, scale, degree of development and activity patterns; as well as underground goafs, large-area land subsidence, riverbank erosion, swamp phase sedimentation, etc.;

Project importance level

Site complexity level

Foundation complexity level

It is divided into three levels based on the scale and characteristics of the project, as well as the consequences of geotechnical problems that cause damage to the project or affect normal use.

Table 1-1 Classification of project importance levels

It is divided into three levels according to the complexity of the construction site.

There are three levels based on the complexity of the foundation.

According to the above three categories of grades, it is divided into three grades: A, B and C.

Grade A - Grade A if one or more of the project importance, site complexity and foundation complexity levels are Grade A;

Grade C 1 - The situation where the project importance, site complexity and foundation complexity are all Grade 3 is Grade C;

Class B - Survey projects other than Class A and Class C are Class B;

For first-level projects built on rocky foundations, when both the site complexity level and the foundation complexity level are level three, the geotechnical engineering survey level can also be designated as level B;

Although survey reports at different survey stages have different content emphasis, they should all include text descriptions and charts. The preparation of the survey report must be coordinated with the corresponding survey stage. Based on the geological conditions of the site, the nature and scale of the building, as well as the design and construction requirements, the basis for selecting the foundation plan and design calculation data are proposed, and existing problems and solutions are pointed out. ways and methods.

The report should be complete, true and accurate, with correct data, clear charts, well-founded conclusions, and reasonable suggestions. It should also be adapted to local conditions, highlighted, have clear project pertinence, and be easy to use.

1) Macro basic content of the report

2) Micro-specific content of the report:

When reading the report, you should first be familiar with the main contents of the survey report, understand the reliability of the survey conclusions and calculation indicators, and then judge the applicability of the recommendations in the report to the project, so as to use the survey report correctly.

When using the report, the engineering geological conditions of the site should be linked to the specific conditions and requirements of the proposed building for comprehensive analysis, so as to make full use of the favorable engineering geological conditions as much as possible.

① Be familiar with the engineering geological evaluation of construction sites, including site stability, topographic and geomorphological unit characteristics, geological structures, geotechnical conditions, geological age, adverse physical geological phenomena in the site, basic earthquake intensity, etc.;

② Be familiar with exploration points, line layout locations, borehole histograms and geological profiles, top contour maps of rock and soil layers, groundwater level maps, etc.;

③ Analyze the rationality and reliability of the rock and soil property indicators of each layer, indoor test data and their correlation;

④ Analyze the rationality of the conclusions and suggestions in the survey report, select the foundation type, reasonable burial depth, and issues that should be paid attention to in the design;

⑤Determine the foundation load capacity used in the design based on foundation strength and deformation control requirements or specifications. When there is a weak soil layer, the foundation reinforcement and treatment plan must be determined based on the results of the survey report.

Basic selection

Foundation embedding depth selection

Calculation and determination of foundation bearing capacity

How to determine the size of the foundation bottom surface

Foundation deformation and stability verification method

Extending the basic design

Main measures to reduce the hazards of uneven settlement of buildings

1. Clarify the main factors that should be considered in the design and calculation of foundation foundations and the three basic requirements that should be met;

2. Master the main types of shallow foundations, their characteristics and selection principles;

3. Master the principles for determining the burial depth of shallow foundations and clarify the main factors that affect the burial depth;

4. Master the method of determining the bearing capacity of building foundations and understand the factors that affect the bearing capacity of foundations;

5. Master the method of determining the size of the foundation base;

6. Clarify the foundation deformation and stability verification method;

7. Master the principles, methods and structural requirements of section design (structural design) of shallow foundations;

8. Clarify the main measures for buildings to reduce the hazards of uneven settlement.

(1) Shallow foundation on natural or artificial foundation (simple technology, small amount of work, convenient construction, most economical, can be preferred)

(2) Deep foundation on natural foundation

(3) Deep and shallow foundations (pile-raft, pile-box foundation)

When designing the foundation, it is necessary to fully consider the construction site and foundation geotechnical conditions according to the building's purpose and design grade, building layout and superstructure type, and comprehensively consider the impact of engineering technology, economy, construction period, environmental protection and other aspects before selecting the best. Select the type of foundation plan. When conditions permit (under the condition of ensuring the safety and reliability of the building), the design plan of shallow foundation on natural foundation should be given priority.

When conditions permit (under the condition of ensuring the safety and reliability of the building), the design plan of shallow foundation on natural foundation should be given priority.

(1) Select the material and type of foundation and lay out the foundation plane;

(2) Determine the foundation bearing layer and foundation burial depth;

(3) Determine the bearing capacity of the foundation;

(4) Determine the bottom surface size of the foundation, and conduct foundation deformation and stability verification if necessary;

(5) Carry out basic structure design (carry out internal force analysis, cross-section calculation of the foundation and meet the structural requirements of the specification);

(6) Draw basic construction drawings and propose construction instructions.

In addition to ensuring that the foundation structure itself has sufficient strength and stiffness, the foundation design must also It is also necessary to choose a reasonable size and layout plan for the foundation, and at the same time, it is necessary to ensure that the reaction force (strength) and settlement (deformation) of the foundation are within the allowable range.

Each building has different structural types and different usage requirements. At the same time, the sensitivity of each building to uneven settlement and the conditions of the foundation soil are also different, so different solutions are required for specific problems.

A. Main factors that should be considered in foundation design:

B. Basic requirements for foundation design

(1) Collect basic engineering design related data (aerial, underground, etc.) and analyze and study relevant data;

(2) Select the material and structure type of the foundation according to the superstructure form and load size, and lay out the foundation plane;

(3) Select the foundation bearing layer and foundation embedding depth based on the geotechnical survey report and the upper load size:

(4) Determine the foundation bearing capacity based on the geotechnical parameters recommended in the geotechnical survey report and standard methods;

(5) Determine the bottom surface size of the foundation based on the load value on the top surface of the foundation and the foundation bearing capacity of the bearing layer (if there is a weak soil layer below the foundation bearing layer, the bearing capacity of the weak underlying soil layer needs to be checked), if necessary ( For buildings with design grades of Class A, Class B and some Class C buildings), conduct foundation deformation and stability verification;

(6) Carry out foundation section and structural design (carry out internal force analysis, cross-section calculation of the foundation and meet the structural requirements of the specification)

(7) Draw basic construction drawings and propose construction instructions.

principle

Features

Applicable conditions

Design Principles

limit state

Load value regulations

foundation

Base

1. Ensure that the load of the foundation and superstructure is evenly and effectively transmitted to the foundation soil layer.

2. Protect the soil at the bottom of the pit from human disturbance and rainwater soaking

3. Improve basic construction conditions

Rigid foundation (unreinforced foundation)

Flexible foundation (reinforced concrete foundation)

Extend the base

joint basis

Strip foundation under column

Cross-strip foundation under columns (cross-strip foundation)

raft foundation

box base

Shell basics

Foundation embedding depth d

Determining the embedment depth of the foundation is an important part of foundation design. Choosing the appropriate foundation embedment depth is related to whether the foundation is reliable, the difficulty of construction, the length of the construction period and the cost of the project, and also involves the completion of the structure. The final firmness, stability and normal use issues.

When determining the depth of foundation embedding, it must be considered to set the foundation on a bearing layer with small deformation and relatively high strength to ensure that the foundation strength meets the requirements and does not cause excessive settlement or settlement difference.

On the premise of meeting the foundation stability and deformation requirements, when the bearing capacity of the upper foundation is greater than the lower soil, the upper soil should be used as the bearing layer. Except for rock foundations, the burial depth of the foundation should not be less than 0.5m, and the top surface of the foundation should be more than 0.1m lower than the designed ground to avoid exposure of the foundation.

In seismic fortification areas, except for rock foundations

The foundation should be buried above the groundwater level. When it must be buried below the groundwater level, measures should be taken to ensure that the foundation soil is not disturbed during construction. When the foundation is buried in rock formations that are easily weathered, the cushion layer should be laid immediately after excavation of the foundation pit during construction.

When there are adjacent buildings, the foundation depth of the new building should not be greater than the foundation of the original building. When the burial depth is greater than the original building foundation, a certain clear distance should be maintained between the two foundations, and the value should be determined based on the building load, foundation form and soil quality.

First consider the requirements of the building in terms of function and purpose

For example:

In order to ensure the safety of the building, a reliable bearing layer should be selected for the foundation according to the engineering geological conditions and the size and nature of the load.

When there is a weak underlying layer within the foundation stress-bearing layer (or settlement calculation depth), the bearing capacity and foundation deformation of the weak underlying layer should also meet the requirements (i.e., the verification of the weak underlying layer).

When selecting the bearing layer and burial depth, you should obtain a detailed understanding of the stratigraphic distribution of the proposed site, the physical and mechanical properties of each layer of soil, and the bearing capacity of the foundation through the geotechnical engineering survey report, and comprehensively consider and select based on other conditions.

How to distinguish good soil layer from weak soil layer in foundation

Other aspects that affect foundation burial depth

When there is groundwater, the bottom surface of the foundation should be buried above the groundwater level as much as possible (to facilitate construction and safety)

When the bottom surface of the foundation is lower than the groundwater level, issues such as foundation pit precipitation, pit wall enclosure, and possible flow or soil intrusion during construction should be considered, and measures should be taken to protect the foundation soil from being disturbed.

Countermeasures for hydrogeological conditions

Seasonally frozen soil is a layer of soil that freezes in winter and thaws when the weather warms. The thickness of seasonally frozen soil is generally about 0.5-3m

Classification of foundation soil (according to frost heave properties)

The frost heave properties of different soils vary, and are closely related to the level of groundwater levels. High (shallow) water levels result in higher frost heave properties.

Generally, fine-grained soil has the characteristics of frost heaving.

Usually climate change or tree growth will cause the expansion and contraction of the foundation soil, and other biological activities may harm the foundation of the building. Therefore, when determining the foundation burial depth taking into account the site environmental conditions, attention should be paid to:

(1) Generally, except for rock foundations, the minimum burial depth of the foundation should not be less than 0.5m, and the top surface of the foundation should generally be 0.1m lower than the design ground;

(2) When there are pipelines, trenches, pits and other facilities near the foundation, the bottom surface of the foundation should generally be lower than the bottom surface of these facilities;

(3) For buildings facing water, in order to prevent erosion by running water or waves, the bottom surface of the foundation should be located below the erosion line;

(4) When there are adjacent buildings, the foundation burial depth of the new building should not be greater than the foundation burial depth of the original building (to avoid safety problems caused by foundation pit construction)

(5) When the foundation of a new building is buried deeper than the original building, a certain clear distance should be maintained between the two foundations. The value should be determined according to the load size of the original building, the foundation type and the soil condition. Generally, the foundation 1 to 2 times the height difference of the bottom surface. (Twice the difference, double the good one)

If the above requirements cannot be met, construction measures such as segmented construction, temporary reinforcement supports, sheet piling, and underground diaphragm walls should be adopted, or the foundation of the original building should be reinforced.

That is, the ability of the foundation soil to bear load per unit area (KN/m² or KPa)

Under the condition of ensuring the stability of the foundation (in terms of strength), the foundation bearing capacity that prevents the settlement (deformation) of the building from exceeding the allowable value is called the characteristic value of the foundation bearing capacity, represented by fa

The determination of fa depends on two conditions

The characteristic value of foundation bearing capacity can be expressed as:

fa

Note: For weakly permeable soil, its foundation bearing capacity is also closely related to the loading rate, and its short-term bearing capacity is much lower than its long-term bearing capacity!

Generally, the bearing capacity of alluvial and diluvial soil is greater than that of slope soil, while aeolian soil has the smallest bearing capacity. The older the same kind of soil is accumulated, the higher the bearing capacity of the foundation.

The most important factor

For example

When the groundwater rises, the foundation soil is buoyed by the groundwater, and the natural weight of the soil is reduced to the buoyancy weight. At the same time, the water content of the soil increases, and the bearing capacity of the foundation decreases.

Especially for collapsible loess, rising groundwater will cause collapse; expansive soil expands when exposed to water and shrinks due to water loss, which has a great impact on the bearing capacity of the foundation.

The buoyant weight of the soil = the effective weight of the soil

If the superstructure is simple in shape, has high overall stiffness, and has good adaptability to uneven settlement of the foundation, the foundation bearing capacity will be high.

If the foundation width is large and the burial depth is deep, the bearing capacity of the foundation will be increased.

1. Calculate according to the shear strength index of the soil using a theoretical formula (theoretical fraction method)

2. Determine according to the p-s curve of on-site load test (load test method)

3. Determine according to the standard bearing capacity table (standard table method)

4. When the soil quality of the foundation is basically the same, it shall be determined with reference to the engineering experience of nearby buildings (engineering experience method).

The calculated value of building foundation deformation should not be greater than the allowable value of foundation deformation

△≤[△]

Classification

Compressive modulus (E): The ratio of the vertical compressive stress to the total vertical strain of the soil when it is completely unable to deform laterally. It is an important indicator to measure the compressibility of soil and an important parameter used to calculate foundation settlement.

If △>[△], you can first consider appropriately adjusting the size of the base bottom surface (such as increasing the base area or adjusting the position of the base centroid) or burial depth;

If the requirements of △≤[△] are still not met, effective measures can be taken from the aspects of architecture, structure, and construction to prevent damage to buildings due to uneven settlement, or other foundation design solutions can be used.

Note: The settlement amounts of foundations with different structural forms often vary greatly. Therefore, the shape of the foundation structure within the same structural unit of the building should be unified to avoid excessive uneven settlement.

If necessary, it is necessary to separately estimate the foundation deformation value of the building during construction and use in order to reserve clearance between relevant parts of the building, select the connection method and construction sequence.

Generally, the settlement amount completed by a multi-story building during the construction period is:

Refers to the settlement value of the midpoint of an independent foundation or the average settlement value of the entire building foundation.

control target

Note: Class A and B buildings and foundations with poor soil quality must carry out this verification (s＜[s])

Refers to the difference in settlement between the midpoints of two adjacent column bases in the same building.

control target

Refers to the ratio of the settlement difference between the two end points in the direction of foundation inclination to its distance.

control target

Note: The tilt of high-rise buildings or tall structures mainly depends on the sensitivity of people's vision. When the tilt reaches a clearly visible level, the tilt value is approximately 1/250. Structural damage generally begins when the tilt value reaches 1/150.

Refers to the ratio of the settlement difference between two points of the foundation within 6-10m in the longitudinal direction of the masonry load-bearing structure and its distance.

control target

sustaining layer

Calculation of the bearing capacity of the weak underlying layer (the underlying layer far away from the bearing layer)

For some buildings, it is also necessary to consider

Select basic materials and types and carry out basic layout;

Determine the foundation burial depth (foundation bearing layer);

Determine the bearing capacity of the foundation;

Determine the bottom size of the foundation (check foundation deformation and stability if necessary);

Perform infrastructure design;

Draw basic construction drawings.

Generally, rectangular or strip foundations are used as foundations for columns and walls.

Circular structures often use circular or annular foundations

According to the eccentricity of the superstructure load to the centroid of the base, the load of the superstructure on the base surface is

Assuming that the base reaction force is uniformly distributed, it is required that the average pressure of the base does not exceed the bearing capacity characteristic value of the bearing layer soil, that is

Single base under the pillar

rectangular base

foundation under wall

Calculation process

choose

Under the action of eccentric load (the combined action of axial load, bending moment or horizontal force), the foundation reaction force presents a trapezoidal distribution. If fa is determined according to the load test or specification table, the following requirements should be met.

The correction formula requires two conditions

When eccentric load acts

Common one-way eccentric moment

Description of eccentricity

The additional pressure caused by the foundation near the slope surface does not affect the stability of the slope.

Not satisfied: Calculate the stability of the soil slope according to the arc sliding surface method based on the average pressure of the base to determine the distance between the foundation and the edge of the slope top and the depth of the foundation.

It is to extend the load of the upper wall or column laterally into the soil so that it can meet the requirements of the bearing capacity and deformation of the foundation.

Design requirements

design method

Structural requirements for unreinforced extended foundations of different materials

design steps

definition

Features: Good shear resistance and bending resistance, not limited by aspect ratio, flexible and free form, wide application range, large amount of steel bars, construction technology and engineering cost are higher than rigid foundations.

Applicable: It can be used when the vertical load is high, the bearing capacity of the foundation is not high, and it can withstand horizontal forces and moments, etc., "wide foundation and shallow burial".

Design requirements: Use the method of expanding the bottom area of ​​the foundation to meet the requirements of the foundation's bearing capacity, but it is not necessary to increase the burial depth of the foundation; select appropriate foundation materials, heights and reinforcements to meet the foundation's shear and bending resistance requirements.

design content

design steps

Base widthb

Base height h

Foundation floor reinforcement As

strength

stiffness

Durability

independent foundation under column

Calculate the design value of the net reaction force of the foundation

Shear resistance calculation

Bending resistance calculation

strength

stiffness

Durability

Construction requirements

1 Determine the base width

2 Calculate the design value of foundation net reaction force Pj

3 Shear resistance calculation

4Bending resistance calculation

tapered

Equal thickness plate

Play the role of isolating water

Design ideas

1 Since the upper structure is a wall, reinforced concrete strip foundation under the wall is used

production conditions

Divided according to intensity

Steel bar grades and their meanings

Single base under the pillar

Strip foundation under wall

1. The shape of the building

1 Reduce the weight of the building

2. Set up ring beams

3Set the foundation beam

4. Reduce

deep base type

platform effect

1. Collection and analysis of design data

two. Select pile type, pile cross-section size and pile length

3. Determine the characteristic value Ra of the vertical bearing capacity of a single pile

4. Determine the number of piles and the plane layout of the piles

5. Pile foundation bearing capacity verification

6. Pile structure calculation

7. Pile foundation cap design

20 points

Structural requirements for shallow foundations

step height

Common measures to mitigate the hazards of uneven settlement

For general small and medium-sized buildings, the last measure should be given priority, and other foundation solutions should be used only when necessary.

Cross-sectional shape

2 lanes

Vertical bearing reaction force

There are tendons