could you please explain how the strength and sub surface has changed
The data quoted is from the NATS ais aerodrome info website so it is current. I'm not an expert however and cannot account for the change but it seems to me that the runway is less capable than before.
The PCN number is assessed as 51, ie it is lower than it used to be so is less capable of sustaining aircraft of higher weights.
The Sub-grade strength category: Has changed from A where
A High strength: characterized by K150 Nm/m3 1 and respresenting all K values above 120 Nm/m3 for rigid pavements, and by CBR2 = 15 and representing all CBR values above 13 for flexible pavements.
to B where
B Medium strength: characterized by K80 Nm/m3 and representing a range in K of 60 to 120 Nm/m3 for rigid pavements, and by CBR = 10 and representing a range in CBR of 8 to 13 for flexible pavements.
A look at the Boeing data for a B777-200LR;
MaxTOW of 348,358Kg on a B runway, a PCN of 82 is needed.
MinTOW of 145,150Kg on a B runway, a PCN of 23 is needed.
So as long as the aircraft isn't anywhere near MaxTOW then the runway is sufficient! [Edit - a further look at the Boeing data reveals that with a PCN of 51 Rigid B runway - the MaxTOW avail on a 777-20LR is approx 290,000Kg]