Aggregating suspensions generally separate into a clear supernatant and a Voluminous gelled sediment. The suspension remains homogeneous (i.e. a gel stable under gravity forms) only when the Volume fraction of particles exceeds a given value Phi**. The present study is devoted to the determination of Phi**. A simplified model describing the gel equilibrium has been developed and its validity is checked comparing its predictions to systematic measurements of the sediment Volume performed on calcium carbonate suspensions. Applying this model to the prediction of Phi**, we show that Phi** is not a characteristic of the physico-chemical system but depends on the height of the sample, its aspect ratio and friction between the gel and the side wall. For large (respectively small) aspect ratios, we find that Phi** is a power law function of the width (respectively the height) of the sample involving an exponent 1/(kappa - 1) which is related to the dependence of the yield stress on the volume fraction of the suspension.