I have recently started thinking of getting into make mead and decided on a nice 15% to start. I decided to oversweeten the batch because I don't want a real dry mead. I want to add berries to the mead for a nice fruity flavour. So I suppose, rambling aside, will the yeast continue to eat the sugars even when it has reached it's tolerance? I apologize for what may seem to be a very simple question but this will be my first ever brew and want to start with a nice mead.
The tl;dr version: it depends on a number of factors (yeast health, pitching rate, aeration etc.), keeping in mind that alcohol tolerance is not a sharply defined limit but more like a "zone" where the yeast becomes increasingly inhibited when alcohol levels rise.
More details below.
Alcohol Tolerance in Brewing Yeasts
A very important aspect of any yeast strain is its alcohol tolerance. Ethanol inactivates yeast. This means that, ironically, the yeast will eventually "drown" in its own alcohol product when it ferments enough sugars.
The average lager and ale yeast tends to have an alcohol tolerance of between five and ten percent. More specialized yeasts for stronger beer styles can handle up to twelve or fifteen percent. Strains with higher alcohol tolerances do exist, but they are rare (seeing as beer styles with more than 15% of alcohol are not common) and they tend to be temperamental and difficult to use.
Note that the limit of alcohol that a yeast can handle is not a well-defined point. Firstly, the alcohol tolerance specified by the producing yeast laboratory is a general ballpark figure. Yeast that s healthy, well fed, well oxygenated and generously pitched may complete a fermentation to alcohol levels higher than the specified tolerance, whereas unhappy and underpitched yeast may throw in the towel well before the specified tolerance has been reached.
Once the yeast begins to approach its alcohol limit, the fermentation begins to slow down in spite of the fact that there are still unfermented sugars present in the wort. At the edge of its working range the yeast may still complete the fermentation eventually, or the fermentation may stall before it is complete. In order to achieve predictable and repeatable results, it is usually best to stay well below the yeast's maximum alcohol tolerance. If you need to ferment a beer close to the yeast's alcohol tolerance, make sure the yeast is healthy and pitch enough yeast. Under the proper conditions most yeasts can usually be pushed to one or two percent past their limit, but keep in mind that working this close to the edge can make for exciting home brewing, but does not guarantee reliable and repeatable results.
Also keep in mind that most yeast manufacturers specify the alcohol limits that a yeast can handle well below the actual limit so as to guarantee a reliable yeast performance. A yeast specified as having a 5% AT can frequently handle up to 8% or so in practice, but because you're past the manufacturer's guarantee, you're on you own and whatever happens is your responsibility.
I was a professional winemaker for many years. 15% is easy to hit with most wine yeasts. The upper limits of alcohol production by yeast is around 20% and is hard to hit that. In practical purposes, around 17-18% is about what you'll get before the yeast literally drown in alcohol and stop working. But adding more sugar to make it sweet will likely keep the alcohol going until it can't anymore. You'll end up with a relatively high alcohol mead with some sweetness, but unless you sterile filter to remove the yeast, it might keep trying to convert the sugar leftover and you'll end up with sparkling mead.