The bearing capacity of a granular layer on clay

Abstract

Granular layers are often placed over weaker clay soils to improve the bearing capacity of working platforms and spread foundations. Their design requires the calculation of a two-layer bearing capacity. The commonly used existing calculation models are quite empirical with imprecise input parameters and can err on the non-conservative side in some situations. Other proposed methods tend to involve multiple design charts and are suited only to either strip or circular foundations. In this paper a new and highly practical design method is proposed whose input parameters are derived directly from the shear strengths of the two layers without the need for empirical-based charts. The output can be obtained either from a single chart or a few equations that are straightforward to implement into a spreadsheet. It can be applied quite generally to both surface and shallow embedded foundations, circular and rectangular and with dry or saturated granular layers, and also to working platform design for tracked plant. The method was validated using three independent numerical studies and centrifuge testing. The results showed that, although errors can increase for foundation shapes towards square or circular and with increasing overburden, the errors are smaller than those of the commonly used existing methods and are also conservative.

Estimating the Bearing Capacity of a Square Footing on a Geogrid-Stabilised Granular Layer over Clay Using the Finite Element Method

“…Tis observation explains the rationale for using a smaller α between 11.3 °(5V : 1H) to 18.4 °(3V : 1H) as recommended in practice [12]. Te results in this study for the unstabilised granular base have good agreement with the T-value method by Lees Advances in Civil Engineering [16]. Te bearing capacities are higher when compared with the established classical methods by Meyerhof [3] and Hanna and Meyerhof [4] resulting in thinner unstabilised granular layer thickness.…”

“…Using a square area in a 2D axisymmetric model may cause mesh generation problems, especially when the radius of the model is not constant. Te square footing can be translated into circular footing of an equivalent area using the conversion factor of ����� (4/π) 􏽰 � 1.13 or vice versa [16]. It is for this reason that circular footings are used in the fnite element modelling to generate responses for the two-layer soil system.…”

“…Te processed data exhibits a strong positive linear association with points that are close to the linear trend line, as explained by higher R 2 values. Te m value for square footing can be determined by dividing the m value for circular footing by 1.13 [16]. Tis conversion factor is derived directly using the same area geometry for square and circular footings for a given applied vertical load.…”

“…Te use of the developed m and c u /p 0 design curves for square footing loaded on the surface of unstabilised and geogrid-stabilised granular layers without surface surcharge to calculate the minimum thickness of a granular layer is illustrated by a typical case. Te purpose of this case is to show the application of the design curves in the design of square footing and to compare against established design approaches, such as Meyerhof [3], Hanna and Meyerhof [4], load spread, Lees [15,16], and Lees and Matthias [39] methods to estimate the minimum thickness of an unstabilised and geogrid-stabilised granular layer. Te input soil parameters and loading conditions for the case are summarised in Table 3 and the results are plotted in Figure 17.…”

“…Tere have been extensive research studies to predict the bearing capacity of a layered soil for nearly fve decades [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Te earlier bearing capacity estimation techniques were developed from both experimental investigations and analytical approaches, which led to semiempirical methods.…”