C.3 Facing systems

C.3.1 General specifications for facing systems

C.3.1.1 The magnitude of facing deformations during construction, and post construction deformations, will vary from system to system. Therefore, in selecting a particular system care should be taken to ensure that predicted construction and design life deformations will meet specified tolerances. Many systems are proprietary and, to achieve the required construction tolerances, it is important that construction follows recommendations made by the manufacturer.

C.3.2 Differential settlement between fill and facing

C.3.2.1 With all facing systems there is compression of the fill during, and sometimes after, construction which may lead to differential settlement between the reinforced fill mass and the facing. If the fill reinforcement is structurally connected to the facing units, and no allowance is made for such potential differential movement, then additional loads may be imposed on the fill reinforcement. Such deformations and differential movement will be mainly affected by the quality of the selected fill, and the way it is compacted.

C.3.2.2 More stringent specifications should apply to the fill materials used with less flexible facing systems. The facing system should be more flexible if the selected fill is prone to settle or not easy to compact.

C.3.2.3 For flexible facing systems made of soft units, these units deform vertically in sympathy with the settlement of the retained fill. There is therefore little concern about differential movements.

C.3.2.4 For semi-flexible systems made of partial height facing panels, or king post system with reinforcement connected to the concrete units, moderate differential movements are accommodated by the use of compressible bearing pads applied to the horizontal panel joints.

C.3.2.5 For other semi-flexible systems made of deformable facing units, low bending stiffness and vertical compressibility allow the facing units to deform vertically and to accompany moderate settlement of the retained fill.

C.3.2.6 In the case of full height facing panels, relative displacement between the fill, including the embedded fill reinforcement, and the facing may be accommodated by permitting the reinforcements to move relative to the facing panels, thus making the system semi-flexible. Various methods have been considered including the use of slots, vertical poles, compressible lugs. An effective sliding connection should permit the reinforcement to transmit horizontal load and yet slide downwards as filling progresses, without gain in load.

C.3.2.7 For rigid facing systems such as full height panels without such moving connections, and segmental blocks packed without compressible filler, fill reinforcement is usually fixed into the facing panels or blocks. With this arrangement, deformation in the region of the face connections may occur. Additional loads imposed on the connections and fill reinforcements should be mitigated by proper selection, placement and compaction of the fill material.

C.3.3 Longitudinal differential settlement

C.3.3.1 As the height of earth retaining structures may vary along their length, and the compressibility of the underlying foundation soil may also vary, so differential settlement may occur along the length of the wall or slope

C.3.3.2 Semi-flexible and flexible facing systems generally exhibit high to very high tolerance to differential longitudinal settlement.

C.3.3.3 The longitudinal flexibility of semi-flexible facing systems constructed using hard units depends on the aspect ratio of individual facing units and the degree of articulation between facing units afforded by the joint details and packing.

C.3.3.4 Precast concrete panels with an aspect ratio of near unity, e.g. the nominal width and height of the basic panels are similar, offer a high degree of articulation, and thereby a high tolerance to longitudinal differential settlement, provided this is accommodated by appropriate joint detailing and compressible bearing pads placed on horizontal joints.

C.3.3.5 Full height concrete panels, which have a high vertical aspect ratio, and precast concrete horizontal planking, which have a high horizontal aspect ratio, offer a lower tolerance to longitudinal differential settlement.

C.3.3.6 Segmental block systems are normally not provided with compressible packing material on horizontal joints, nor with closely spaced vertical slip joints. So the resistance of segmental block systems to longitudinal differential settlement is usually limited, since it can only result from the small size of the units and their potential displacements relative to each other.