I'm currently in the world-building phase of my book (hence why I'm here).

To add some perspective before I ask my question. The story is based in Britain, during the year 2032. 

In my story, there is a large wall around the city of Milton Keynes (89 km²). The wall is around 300 feet tall and made of concrete. It's thick enough to keep whatever is inside, in. 

Atop the walls are guards and people, who monitor the quarantined city. Surveillance and dorms are at the top as well, so rotations can take place with little downtime. 

My question is, if this were to be built, how quickly could it be made? 

To add some more information. The wall is a matter of emergency, and the UK government has contracted all available construction companies to work on it, putting any government projects on hold to see it completed as quickly as possible. Funding isn't a problem and the cash needed to see this done realistically is available. 

Many thanks to those who reply. If you need more information, please let me know.

**Update:** 

After several comments, I'm edging toward Hydro dam levels of thickness. To add some more context to what it is to be contained, there are creatures within the city that are dangerous to human life. Killing them isn't a viable option as new ones quickly appear. They aren't from our reality so, even if the whole city were bombed, more creatures would eventually turn up. The source is within the city and building the wall would protect the rest of the country from harm.

The source of the creatures also emits an infectious airborne virus that acts similar to gamma radiation. It can travel through surfaces and skin, instead of the typical airborne method we are used to. The infection attaches itself to cells within the body and begins replicating. Within a healthy population of people, around 40% are susceptible to this form. Those infected have an incubation period varying from 1 to 4 days. They are not contagious during this stage. After this, patients will become symptomatic and begin to show signs of burn-like lesions on their skin. The Virus will continue to grow across a hosts body over a week. Lesions are infectious to touch.

It's dependent on the person etc. But during the second week, they will begin to show signs of mental and physical degradation, as the infection starts to work its way through their nervous system and brain. This happens over several days. Then, the infected individual succumbs to the virus, becoming feral and highly contagious, emitting a close proximity airborne strain of the virus. This strain allows the virus to jump to humans with a 90% infection rate. Only those who are immune are safe from its effects, although feral infected individuals will still attack them on sight.

**Second update**

Thank you to everyone who's answered so far. You've helped me figure out the logistics of how such a thing could be built, and by what means. Ideally, I'd like the wall to be made within a year, or at least completed to a stage that's containing the infection and creatures within.