Settlement is a consideration for specifiers of Recon walls and geotechnical engineers tasked with determining the allowable bearing pressure. The settlement can be uniform along the length of the wall or differential. Differential settlement means that depth of settlement varies along the wall due to changes in foundation soil properties and wall height.
Recon walls are modular and suited to handle some differential settlement. NCMA and FHWA have suggested differential settlement limits 1%(L/100) or 0.5%(L/200), respectively. The limits reduce the likelihood of cracking of the Recon blocks.
Aesthetically, walls experiencing differential settlement develop gaps between units at the bounds of differential settlement. Blocks near the base of the wall can sometimes become cracked from the flexural stresses. Recon blocks are solid concrete blocks and are far less likely to develop cracks versus cored precast concrete blocks.
Settlement can be divided into three categories:
- Immediate Settlement – Elastic deformation of dry, moist or saturated soils without any changes in the moisture content.
- Primary Consolidation Settlement – A volume change in saturated cohesive soils due to expulsion of water occupying void spaces.
- Secondary Consolidation Settlement – Long term creep due to deformation of the soil particles.
In saturated sands, immediate settlement and consolidation (expulsion of water) occur simultaneously. In saturated clays, the expulsion of water is slow and primary consolidation can take years. Secondary consolidation takes even longer than primary consolidation but for clays is a lesser part of the total consolidation (Note: organic and in-organic highly compressible soils experience high secondary consolidation).
Geotechnical engineers and engineers monitoring preparation of the subgrade should be mindful of foundation soils. Removal and replacement (or other improvement methods) may be required in locations. Pre-consolidation could also be considered.