1 General Provision
1.0.1 This code is formulated with the view to meet the requirements of design of ground base and foundation of highway bridges and culverts, so as to make the design keep state-of-art technology, economy and rationality, safety and usability, as well as environment protection.
1.0.2 This code is applicable to design of ground base and foundation of highway bridges and culverts. It also can be used as reference of design of other roadway bridge and culvert.
1.0.3 Design of ground base and foundation must adjust to local conditions, use local materials and save resources.
Type of foundation shall reasonably select according to hydrographic and geologic conditions, terrain, load, materials, and upper and lower structure form and execution conditions.
1.0.4 Engineering geology investigation shall be carried out at the bridge site. The investigation data shall be able to reflect specific information such as terrain, geomorphy, and stratal configuration, poor geologic conditions that may affect stability of bridge and culvert, physical mechanic properties of rock and soil and groundwater occurrence.
1.0.5 Action and effect combination of design of foundation structure shall be adopted according to the following provisions:
1 According to requirements of limit state of bearing capacity, basic combination and accidental combination of action effects shall be adopted for checking calculation of bearing capacity and stability of structural element.
1) Fundamental combination:
When carry out checking calculation for bearing capacity, the expression of action effect, importance factor of the structure, partial factor of each effect and combination factor of effects shall be carried out according to item 1 of article 4.1.6 of "General Specifications for Design of Highway Bridges and Culverts" (JTG D60-2004). When carry out checking calculation for the stability, all of the above factors shall be 1.0.
2) Accidental combination (excluding the earthquake effect):
Action effect combination may adopt the following formula:
(1.0.5)
Where,
γ0 - Importance factor of structure, γ0=1.0;
Sad - Effect combination value of action's accidental combination in the limit state of bearing capacity;
SGik - Normal value effect of the ith permanent action;
Sak - Normal value effect of accidental action;
SQlk - Normal value effect of the first changeable action besides accidental action; the normal value effect is larger than any other normal value effect of the jth changeable action;
SQjk - Normal value effect of the jth changeable action;
ψ11 - Frequent value factor of the first changeable action shall be taken according to item 1 of article 4.1.7 of "General Specifications for Design of Highway Bridges and Culverts"(JTG D60-2004); when carry out checking calculation for stability, ψ11 =1.0;
ψ2j - Quasi-permanent value factor of the jth changeable action shall be adopted according to those specified in item 2 of article 4.1.7 of "General Specifications for Design of Highway Bridges and Culverts";
, - Partial factor of corresponding action effect in the above expression shall be 1.0.
2 When the foundation structure requires design of normal limited state, the expression, frequent value factor and quasi-permanent value factor of combination for short-term action effects and those of combination for long-term action effects shall be determined according to article 4.1.7 of "General Specifications for Design of Highway Bridges and Culverts" (JTG D60-2004).
1.0.6 Checking calculation for stability of the foundation may be calculated according to the formula:
(1.0.6)
Where,
- Importance factor of the structure, =1.0;
Ssk - The combination value of normal value effect of the action which makes foundation structure unstable shall be calculated with the minimum combination value between fundamental combination and accidental combination;
Sbk - The combination value of normal value effect which makes the foundation structure stable shall be calculated with the minimal combination value between fundamental combination and accidental combination;
k - Stability factor of the foundation structure.
1.0.7 Design of durability shall be carried out for foundation structure.
1.0.8 When carry out checking calculation for vertical bearing capacity of subgrade, the action effect transferred to foundation base or cushion cap subface shall be the combination of short-term action effects in normal limited state; at the same time, accidental combination (excluding earthquake effect) of action effect also shall be considered.
Combination value of action effect shall be less than or equal to relevant allowable value of resisting force-subgrade bearing capacity or the allowable value of individual-pile bearing capacity.
1 When combination for short-term action effects is adopted, frequent value factor of changeable action shall be 1.0, and load of automobile shall be counted in impact factor.
Filling thickness (including depth of pavement) is equal to or larger than those arch bridge, culvert, and gravity type pier of 0.5m; automobile impact factor may be not counted in the calculation for subgrade.
2 When accidental combination of action effect is adopted, expression of the combination shall be adopted according to article 1.0.5 of this code, but without regard to importance factor of structure; partial factor of an action as showed in formula (1.0.5), frequent value factor and quasi-permanent value factor shall be 1.0.
1.0.9 When calculate foundation settlement, the action effect transferred to foundation base shall be adopted according to long-term action effect combination in normal limited state.
Such a combination is the effect caused by the permanent action normal value which is directly act on structure (excluding concrete shrinkage, creep action and action of foundation) and quasi-permanent value of changeable action (only refers to automobile load and crowd load).
1.0.10 Action value, effects combination, and value of related factors shall be taken according to current "General Specifications for Design of Highway Bridges and Culverts" (JTG D60), except that they are specially pointed. Calculation of foundation structure shall comply with current standards like "Code for Design of Highway Masonry Bridges and Culverts" (JTG D61) and "Code for design of reinforced concrete and prestressed concrete highway bridges and culverts (JTG D62). Aseismic design for subgrade foundation shall comply with those specified in "Specifications of Earthquake Resistant Design for Highway".
1.0.11 Design of subgrade and foundation of highway bridges and culverts shall not only comply with this code, but also comply with those specified in current national standards.
2 Terms and symbols
2.1 Terms
2.1.1 Subgrade; foundation soil
It is referred to soil body and rock body bearing the structural action.
2.1.2 Foundation
It is referred to a part of structure, which transfers all kinds of actions bear by structure to the subgrade.
2.1.3 Safe classes
It is referred to design classes classified according to severity of damage caused by broken engineering structure, which is used to endow the structure with rational safety.
2.1.4 Combination for short-term action effects
It is referred to the combination effects between standard value of permanent action and frequent value of changeable action. In which, frequent value of changeable action is the product of changeable action standard value and frequent value factor.
2.1.5 Combination for long-term action effect
It is referred to the combination effects between standard value of permanent action and permanent value of changeable action when design in the normal limited state. In which, quasi-permanent value of changeable action is the product of changeable action normal value and quasi-permanent value factor.
2.1.6 Allowable value of bearing capacity
It is referred to the pressure corresponding to a deformation in the linear deformation section of the pressure deformation curve of subgrade.
2.1.7 Joint
It is referred to crack or crevice of terrane without apparent displacement at both sides of the rock body's fracture plane.
2.1.8 Bearing stratum
It is referred to the stratum directly bear the action of foundation.
2.1.9 Underlying stratum
It is referred to the soil stratum under bearing stratum, which locates in the depth of layer may be compressed or damaged.
2.1.10 Gravimetric density (it is named density for short)
It is referred to the gravity bear by unit volume of rock and soil, which is the product of the rock and soil density and the acceleration of gravity.
2.1.11 Seasonal frozen soil
It is referred to the soil stratum freeze up in winter and fully thaws out in spring (summer).
2.1.12 Permafrost
It is referred to the soil stratum keeping frozen state for more than two years.
2.1.13 Pile foundation
It is referred to the foundation composed of pile and cushion cap or collar beam connects with pile top.
2.1.14 Negative friction
It is referred to the downward friction of soil acts on the pile's lateral surface when the settlement larger than pile body occurs around the pile body, which is caused by self consolidation, self settlement by soaking and additional load of ground.
