A Slab on Grade (S.O.G) otherwise referred to as a Ground Supported Slab can give the Client substantial benefits in speed of construction, cost and performance when compared to Piled Slabs.  However the soil must be appropriate and able to support the slab loads without excessive or differential settlement.  (Traditional SOG floors were continuously reinforced with two layers of reinforcement and sometimes incorporated Saw Cut Joints.  In fact the amount of reinforcement means that the saw cut joints are redundant and will just lead to higher maintenance costs.)  Do we need to include this???   

SOG’s based on modern design codes are designed using Plastic Analysis.  Nominal reinforcement in the form of Steel Fabric or 20-25 Kg/ m3 of Steel Fibres is required to provide adequate post-crack ductility or toughness.  The slab is designed with Free Movement Joints and Induced (Saw Cut) Joints to accommodate Thermal Contraction and Drying Shrinkage.  This means that with careful design and construction the slab should have minimal or no cracks.  

Good Question!  Soil (Sub-Grade) requirements for SOG’s are often misunderstood and can lead to serious slab defects.  Generally a SOG requires soil that could be described as reasonable.  The K-Value (or Modulus of Sub-Grade Reaction) is in effect a measure of the Sub-Grades Stiffness and is used in the design of SOG’s.  For a SOG it is generally recommended that this should be a minimum of 0.035 N/ mm3, and this should ideally be proven with a Plate Bearing Test.  The actual K-Value has a small influence on the required slab thickness. 

However soil at deeper levels below the ground surface have the most significant effect on the long term settlement of the slab, therefore it is always recommended that the Client engage a Specialist Soils Engineer to review the soil conditions and if in doubt to estimate the anticipated long term or differential settlement.  It is important to note that Soil Bearing Capacity (KN/ m2) is not a factor used in the design of modern SOG’s – Bearing Capacity is used for the design of Foundations and Raft Slabs, not concrete pavements!