2. Detailed Design

You have been asked to undertake the detailed design and assessment of the following elements:

Task 1: Mechanically Stabilised Earth (MSE) wall retaining solution

This task involves the geotechnical design of the solution selected to retain the transition from the

embankment section to the viaduct. The face of the MSE wall will be vertical, to satisfy space

requirements, and the backslope of the wall can be taken as horizontal. Only one cross-section needs

to be considered in the design, at Ch. 650m perpendicular to the road axis.

The retaining solution needs to be designed while considering the following at minimum:

The MSE wall must be designed according to, and comply with the specifications listed in the U.S.

Federal Highway Administration (FHWA) Manuals FHWA-NHI-10-024 and FHWA-NHI-10-025.

These manuals are freely available from GEC 11 Design and Construction of Mechanically

Stabilized Earth Walls and Reinforced Soil Slopes - Earth Retaining Structures - Geotech - Bridges

& Structures - Federal Highway Administration (dot.gov)

The design life of the retaining system is 75 years.

The contractor has indicated that they wish to use a retaining system comprising segmental precast

concrete panels paired with linear unidirectional ribbed steel reinforcement strips. Common

reinforcement strips are galvanised, ribbed along their top and bottom, are 50 mm wide and 4 mm

thick, and are made from steel of yield stress 450 MPa. You are advised to consider the above

properties in your design, however these properties can be varied (e.g. use of higher grade steel)

if required by the project conditions./nThe reinforced fill will comply with the FHWA requirements. You must consider the following

parameters for the reinforced fill: Drained friction angle ' = 34°, effective cohesion c' = 0 kPa and

unit weight y = 20 kN/m³. In addition, you must consider the following parameters for the retained

fill: Drained friction angle '= 30°, effective cohesion c' = 0 kPa and unit weight y = 20 kN/m³.

Traffic, pavement and vehicle impact loads must be considered in the design, in accordance with

the FHWA requirements. Earthquake loads need not be considered. Traffic and pavement loads

can be treated as uniform surcharge of 0.6 m of fill (12 kPa), in line with FHWA requirements.

Vehicular impact loads on the traffic barriers must also be taken according to FHWA (45 KN

horizontal force applied at 810 mm above the crest of the wall).

The design must satisfy all the stability and durability checks outlined in the FHWA Manuals, while

considering the relevant load factors and combinations.

If ground improvement is required in order to comply with the FHWA requirements, the contractor

has indicated that they wish to explore the feasibility of preloading the subsoil, as it is the most

economical improvement solution. The contractor has advised that the MSE wall does not fall on

the critical path of the project, hence the duration of preloading is irrelevant. However, the height

of the preloading fill cannot exceed 6 m without jeopardising global instability and subsequent

project delays, and the unit weight of the preloading fill cannot exceed y = 20 kN/m³.

If preloading (subjected to the above restrictions) cannot by itself increase the subsoil strength to

the level required to satisfy the FHWA stability checks, you must propose alternative, appropriate

ground improvement solutions (one or two solutions), that could achieve the required increase in

soil strength. You must describe this/these solution(s) in brief (1-2 paragraphs). You must also

provide the target subsoil shear strength parameters that must be achieved to satisfy all the FHWA

stability checks, assuming that ground improvement will result in homogeneous soil of better

strength and compressibility characteristics. The design of the ground improvement measures to

achieve the target subsoil characteristics will be performed by the soil improvement sub-contractor,

and is not within the scope of the design at hand.

Long-term serviceability of the road requires settlement to be less than 50 mm over 10 years past

the end of construction. If this requirement is not satisfied, you must provide the target subsoil

stiffness characteristics that must be achieved by means of the ground improvement method you

proposed above, to satisfy this criterion. It is recommended to use a simple model for the calculation

of MSE wall settlements i.e. assume that the time required for the construction of the retaining

solution and of the fill is negligible; assume 1-D settlement conditions; ignore soil creep; ground

improvement will result in homogeneous soil of better strength and compressibility characteristics.

Provide drawings that depict the layout and geometry of your technical solution (cross-section and

plan view).

Connection strength calculations are not required: The structural strength between metallic

reinforcements and concrete panels will be checked according to AASHTO by the structural

engineer.

If you are enrolled in CIVL6571, you need to consider 2D settlements conditions and provide an

estimate of settlement in time obtained from Plaxis simulations.