Buried Box Design to PD 6694-1:2011
(Eurocode to UK Standard)

Index

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1. Materials & Construction

PD 6694-1 'Recommendations for the design of structures subject to traffic loading to BS EN 1:2004' Clauses 9.10.1 and 10.2.3 give requirements for the materials and construction for buried concrete box structures. These structures include in-situ or precast culverts and underpasses.
The construction of the box structure will have implications on the design which need to be considered.

 

Although vertical drainage layers are required to be provided for underpasses ground water levels and the possibility of buoyancy effects needs to be considered as described in load case Table B.6. The drainage layer separates the backfill soil from the walls of the box however the back of wall friction is deemed to be included in the tabulated coefficients in the tables.

 
Preparation of the foundation for the box is important to ensure the structure is evenly supported. The definition of the existing ground material below the box is either 'hard' or 'not hard'; 'hard material' being defined as material which requires the use of blasting, breakers or splitters for its removal. The diagram above shows how either blinding concrete or granular material needs to be placed below the structure to achieve a uniform bedding layer.

Backfill to the structure is restricted to Class 6N or 6P material as defined in the Manual of Contract Documents for Highway Works Volume 1 Specification Series 600 Clause 610 and Table 6/1.

Coefficients of earth pressure for Class 6N and 6P material used in Tables B.1 to B.6 at Serviceability Limit State are:

K_a = 0.33 which equates to a ϕ' value of 30°

K_o for soil pressure = 0.6 which equates to a ϕ' value of 23.6°

K_o for traffic surcharge = 0.5 which equates to a ϕ' value of 30°

In Table B.3 a minimum value for K is given as 0.2 to consider the minimum earth pressure effects when the roof structure is heavily loaded with traffic.

The value of K_max in the Tables may be increased when considering bearing pressures, overturning or sliding providing the structure is designed for the increased pressures, and the movement required to develop the passive resistance can be accommodated (see Clauses 10.3.2 and 10.3.3).

2. Loading

Permanent loading comprises of the self-weight of the box, the weight of the carriageway construction and fill over the box, horizontal earth pressures from the backfill material, hydrostatic pressures from ground water, and settlement.
Both the minimum and maximum values of permanent loads need to be considered to assess the worst effects in the structure. The weight of the material over the box acting on the roof slab can be enhanced by the effect of negative arching.

Horizontal earth pressures can vary considerably, the range of values for the earth pressure coefficients, together with appropriate combination load factors are shown below Tables B.1 to B.6 in Annex B.
 

Live Loads consist of traffic loading (LM1, LM2 and LM3), footway loading, accidental wheel loading, construction traffic, traction and skidding loads, parapet collision loads, centrifugal loads and temperature effects. Although the list is extensive all the items need to be considered, however some may be discounted by observation.

If the cover H_c is greater than L_L then braking and acceleration forces may be neglected.
Note: the scope of the Standard limits designs to structures with the length L_L ≤ 15.0m.

When cover H_c is 0.6m or less then three vertical live loads need to be considered:

1. Load Model 1 (LM1).
   There is no dispersal allowed for the UDL System so the nominal UDL = 5.5 kN/m². Wheel loads for the Tandem System are applied uniformly over a contact area of 0.4m × 0.4m; this contact pressure may be dispersed at 1:1¹ through the surfacing and fill and concrete to the neutral axis of the roof slab in accordance with BS EN 1991-2 Clause 4.3.6.
2. Load Model 2 (LM2).
   Wheel loads for the single axle are applied uniformly over a contact area of 0.4m × 0.4m; this contact pressure may be dispersed at 1:1¹ through the surfacing and fill and concrete to the neutral axis of the roof slab in accordance with BS EN 1991-2 Clause 4.3.6.
3. Load Model 3 (LM3).
   The value of the SV model vehicle used will depend on the class of road over the box structure. Wheel loads for the SV vehicle are applied uniformly over a contact area of 0.35m × 0.35m; this contact pressure may be dispersed at 1:1¹ through the surfacing and fill and concrete to the neutral axis of the roof slab in accordance with BS EN 1991-2 Clause 4.3.6.

Note ¹: A dispersal of 1:1 does lead to an anomaly when considering load dispersal for H_c greater than 0.6m as loads are then dispersed at an angle of 30° to the vertical. This results in traffic loading having a worse effect on fill depths of between 0.6m and about 1m than those on a depth of 0.6m. It is recommended that dispersal is considered at 30° to the vertical for all depths of cover H_c.

Traffic surcharge represents vehicles positioned adjacent to the box structure and is applied as a uniformly distributed load together with two horizontal line loads to the side of the box.

Values for the UDL σ_h depend on the traffic load model being considered.
For Normal Traffic (LM1 or LM2): σ_h = 20K_dR
For LM3 (SV's): σ_h = 30K_dR
For Abnormal Loads (SOV's): σ_h = 45K_dR
Where R is 3.0/W_eff;, so for a 3m lane width R = 1.0.

Load Case Combinations to be considered:
 

 

Temperature Effects need to be considered when the depth of fill over the structure is shallow, or when the maximum clear span is more than 20% of the width L_T of the structure. The two effects that need to be analysed are:

1. Uniform Temperature.
   This is the expansion and contraction effect of the roof slab. A mid-range temperature of 10° is assumed with expansion up to T_max and contraction down to T_min. Where T_max and T_min are determined from BS EN 1991-1-5 and UK NA and are adjusted for the depth of cover using NA to BS EN 1991-1-5 Clause NA.2.2.2.
   

   
   Uniform Temperature Effects

2. Temperature Difference.
   This occurs when there is a temperature gradient through the roof slab, i.e. when there is a different temperature on the top of the slab to that on the soffit. NA to BS EN 1991-1-5 Clause NA.2.2.3 is used to determine the effects of temperature difference. The restraining moments and forces need to be distributed into the structure, and the self-equilibrating stresses added into the load effects in the roof member.

3.Stability

Stability of the box is considered in Tables B.4 to B.6 where horizontal live loads, such as traction forces, are applied to the top of the roof slab. Stability is maintained by either increasing the horizontal earth pressures on the opposite side of the box, or considering friction forces on the base of the box, or a combination of the two effects.
Sliding effects are considered in Table B.6. The out-of-balance horizontal force is resisted by friction under the base of the box and passive resistance of the soil on the side of the box. The earth pressure coefficient for the passive wall may be taken as greater than K_max for bearing, sliding and overturning.
Passive pressures can only be developed when the structure moves (lateral and/or rotational displacements). The movement required to develop full passive pressure can be significant. The magnitude of movement can be determined using PD 6694-1 Clause 7.5 and care is needed to check that the structure and surrounding services (such as gas mains and drainage systems) can accommodate the movement.

Allowable bearing pressures are obtained from the Ground Investigation Survey. An allowable pressure is usually determined to limit differential settlement to about 20 to 25mm.

4.Analysis

A unit strip method of analysis is usually suitable for most box designs.
A simple plane frame model will usually be suitable for modelling the distribution effects of the loads around the structure. Alternatively a moment distribution analysis can be used effectively.
Moments and shears are determined at critical points around the structure, i.e at corners and mid lengths of walls, roof and floor slabs. The critical section for shear in the slabs is at distance d from the end of the splay, or the corner when there is no splay, as shown.

It is important to check for combined bending and shear at these critical sections.
Special attention to reinforcement detailing is required when bending causes tension on the inside of the box at the corners. BS EN 1992-1-1 Clause J.2.3 describes strut and tie models for use in determining reinforcement requirements at corners with opening moments. Appendix D of BD 31/01 describes a method of designing for these corner 'opening' moments. An example of the calculation has been provided in the Workshop Section of this website.