Haroon,
The load on the main gear is calculated at maximum aft CG at the maximum ramp weight. For your 777-300 baseline, this is 94.84% of the aircraft weight. With two main gear legs, this is 94.84/2 = 47.4% load on one gear.
Turning to your quote, ACN is numerically equal 2 times the equivalent load in tons on a single wheel with a standard tire inflation pressure of 12.5 bar (181.3 PSI) that would require the same pavement thickness as required by the aircraft for 10,000 coverages. To get this concept to work for multi-wheel gears, ACN is based on equivalent single wheel load (ESWL), which is intended to be the single wheel load which will cause pavement damage equal to that caused by a multi-wheel gear. In practice the ESWL is defined as the load on a single tyre which will produce the same maximum deflection at subgrade level as the multi-wheel load. The ESWL is a function of pavement depth. The current method for calculating the ESWL uses the single-layer Boussinesq expression for deflection beneath, and at offsets from, a uniformly loaded circular area. Basically for each aircraft and weight, pavement designs are done for CBR 3, 6, 10, and 15 subgrades to find the required depth of pavement in each case, and the deflection at top of subgrade level. Then a single load is applied and varied until it causes the same deflection at subgrade level, and that weight is the ESWL, and ACN is twice the ESWL. That is getting well into pavement engineering and is confusing for most pavement engineers.
The load influence from a wheel spreads downwards and out in a 45 degree cone shape. The drawing below shows that for a thin pavement (which is all that is needed for a strong subgrade), there is no stress overlap from adjoining tyres and the ESWL is the wheel load. As the subgrade gets weaker, the pavement gets thicker, the stresses overlap and they add together.
