At rest pressures are initially developed on the back of the abutment wall during construction and whilst the backfill is compacting. Consequently the structural elements have to be designed to resist the effects of these pressures.
Any movements in the structure caused by the at rest pressure, either through rotation or deflection will reduce the pressure on the back of the wall; a state of equilibrium is reached when the pressure reduces to the active earth pressure value. Consequently the stability of the structure can be checked by using active earth pressures.
Passive pressures are developed when the structure pushes against the soil. Since movements required to develop passive pressures are considerably greater than that for active pressures, and the structure is designed to ensure that the foundations do not slide under active pressures, then it is unlikely that passive pressures will be developed in front of the abutment. The magnitude of movement required to mobilise passive pressure can be determined from EN 1997-1:2004 Clause C.3(2) and PD 6694-1:2011 Clause 7.5. There is also the chance that, at some time in the future, the soil in front of the abutment may be removed temporarily. This could happen if services, such as drainage pipes, water or gas mains, are installed or repaired in front of the abutment. Consequently the structure needs to be designed to be stable with no soil in front of the concrete footings.
If shear keys are required to prevent sliding then the key should be located under the rear half of the base and a factored value of passive pressure is used.
Integral bridges experience passive pressures on the back of the abutment wall when the deck expands. The design of integral abutments is covered in BA 42, PD 6694-1 and a number of publications, such as Integral Abutments for Prestressed Beam Bridges by B A Nicholson, and Composite Highway Bridge Design (document P356) by D C Iles give guidance and examples.
Departmental Standard BD 30 gives recommendations for the layout of backfilled cantilever retaining walls with spread footings or piled foundations.
The layout of the abutment will have implications on the design which need to be considered.
The provision of a drainage layer will allow porewater pressures to be ignored (unless there is a possibility of a large water main bursting). However the drainage layer separates the backfill soil from the wall so back of wall friction should not be included. Traffic vibration will also affect any vertical friction effects on the back of the wall.
Foundation level is usually set at least one metre below ground level to avoid deterioration of the foundation material through frost action. If services, such as gas pipes, water mains, electricity cables etc., may be installed in front of the abutment wall then the depth to foundation level may need to be increased to allow the services to be installed above the concrete footing.
It is usual to provide granular backfill to the back of the wall which limits the material to Class 6N or 6P as defined in the Manual of Contract Documents for Highway Works Volume 1 Specification Series 600 Clause 610 and Table 6/1. A typical value for the effective angle of internal friction (ϕ') for Class 6N or 6P material is 35o. This equates to serviceability limit state values of:
Ka = (1-Sinϕ') / (1+Sinϕ') = 0.27
Ko = (1-Sinϕ') = 0.43
Loading from the deck is applied to the abutment through the bearings. Maximum vertical bearing loads are obtained from the deck analysis; these loads, together with the type of restraint required to support the deck, will dictate the type of bearing provided.
Horizontal loads from the deck are produced by wind loading, temperature effects, creep movements, traction, braking and skidding loads, collision loads when high level of containment parapets are used, and centrifugal loads if the horizontal radius of curvature of the carriageway is less than 1000 metres when using BS 5400-2, or 1500 metres when using EN 1991-2.
Longitudinal loads from temperature effects in the deck will be determined according to the type of bearing used. Elastomeric bearings are effectively 'glued' in place between the deck soffit and the abutment bearing plinth so that the bearing has to distort when the deck expands and contracts. The longitudinal force produced by this distortion is proportional to the shear stiffness of the bearing and the magnitude of the movement.
Sliding bearings, on the other hand, produce a longitudinal load which is proportional to the dead(permanent) load reaction and the coefficient of friction between the sliding surfaces. The cofficient of friction (μ) varies between 0.01 and 0.08 depending on the type of bearing and bearing stress (see BS 5400 Part 9:1, Tables 2 and 3).
Free abutment with sliding bearings
Both abutments with elastomeric bearings only
Free abutment with elastomeric bearings
The longitudinal load from the temperature effect will act equally on both abutments. If sliding bearings are used then the load transmitted is equal to the friction at the bearing under dead and superimposed dead loads (permanent actions). If elastomeric bearings are used then the load transmitted is equal to the force required to distort the bearing by the distance the deck expands or contracts.
Free abutment with sliding or elastomeric bearings
Both abutments with elastomeric bearings only
The deck is very stiff in the axial direction so horizontal loads will have negligible effect on the length of the deck. Hence longitudinal loads due to traction, braking and skidding are assumed to be transmitted to the fixed abutment only. Any frictional resistance from sliding bearings at the free end of the deck would produce a relieving effect on the fixed bearing and should therefore be ignored when designing the fixed bearing. If only elastomeric bearings are used, i.e. there is no fixed abutment, then the loads due to traction, braking and skidding are shared between the two abutments.
Transverse loads on the deck will be transmitted to the abutment through the fixed and sliding-guided bearings only. These loads are unlikely to have an effect on the stability of a full height abutment, but the bearing plinths need to be designed to resist the loads. The stability of small abutments, such as bank seats, may need to be checked for these loads.
Live loading at the rear of the abutment is represented by a surcharge loading (see BS 5400 Part 2:2006 clause 5.8.2 or PD 6694-1:2011 clause 7.6). Traction, braking and skidding loads at the rear of the abutment are not required to be considered when using EN 1991-2:2003 (see clause 4.9.2). The curtain wall (also called upstand wall or ballast wall) does however need to be designed for braking forces.
Vehicle collision on abutments need not normally be considered as they are assumed to have sufficient mass to withstand the collision loads for global purposes (See BD 60/04 clause 2.2, or NA to BS EN 1991-1-7:2006 clause NA.2.13).
Stability of the abutment is determined by considering:
A comprehensive Ground Investigation Report is essential for the design of the bridge structure. Boreholes need to provide information about the nature of the ground below the foundations. Adequate sampling and testing also need to be carried out to obtain design parameters for allowable bearing pressures, together with friction and cohesion values of the soil at foundation level.
When using BD 30 sliding and overturning effects are calculated using nominal loads and active earth pressures. A factor of safety of 2.0 is used to ensure that the abutment is stable against sliding and overturning.
When using EN 1997-1:2004 stability needs to be considered at serviceability and ultimate limit states.
Several load cases need to be considered to ensure all loading conditions are catered for.
Construction sequences also need to be considered. The abutment wall will often be constructed and backfilled up to bearing shelf level; this provides good access for the deck construction. A surcharge load can be applied to the wall by the construction plant used to compact the backfill. This surcharge load, together with the active backfill earth pressures, will be acting on the back of the wall without the stabilising effects of the dead load from the deck and can result in a critical loading case.
Allowable bearing pressures are obtained from the Ground Investigation Survey. An allowable pressure is usually determined to limit settlement to about 20 to 25mm. An alternative is provided in EN 1997-1:2004 to limit the maximum SLS pressure under the foundation to a fraction of the ground strength; PD 6694-1:2011 clause 5.2.2 clarifies this fraction to be one third. As the allowable pressure will be dependent on the size of foundation and loads applied then there will need to be an initial assessment of the loads and foundation sizes before an allowable pressure can be given. This results in some redesigning until the correct base size, applied loads and allowable bearing pressures are obtained.
BS 8002 says that instability of the earth mass involving a slip failure may occur where:
If none of these conditions are present then a slip failure analysis will not be necessary.
Contact David Childs