Why not try to skip the line via amphibious landing, causing the enemy to pull troops back from the stalemate, perhaps weakening the line?
I answered this question a few months back, and after combing the subreddit rules to see if I was allowed to repost (and it seems this is allowed), I'll re-post the answer here;
(The question was, 'During WWI, did the British ever consider making a seaborn landing behind the German lines in Belgium to link up with a land attack to break the German flank?', which I believe covers your question.)
<This is an interesting question which really went through the British mind in two phases during the war. The first phase during which was considered was in 1914. The rout of the Allied armies and the driving of these forces back to the Marne were not even remotely considered by major authorities in French or British governments. For the French, it was always Plan XVII, by Joffre, which occupied the military train of thought - the French would send forces into Belgium, if need be and war was declared on their northern neighbour, while undertaking a grand offensive into the lusted provinces of Alsace-Lorraine. For the French, naturally, it would always be about holding the homeland. They would sacrifice anything to hold the Germans before getting to Paris, akin to the 1870 war.
For the British, this was also important. They need to get their 250 00 strong army, the British Expedtionary Force, over to France and partake in operations as quickly as possible. This was designed in pre-war secretive military meetings between Sir Henry Wilson, as Director of Military Operations, and the French General Staff, as well as in the opinion of pre-war C.I.G.S. Sir John French. Facing the exact same problems as the Allies in WW2, they also need to supply these massive forces to be militarily effective (Mind you, this all in mind of having just an elite, small B.E.F. in France, not the monstrous Kitchener's New Army!) while also committing to the defence of their tiny ally and linchpin of the entire front - Belgium.
Hence, the Channel Ports become paramount in importance beyond perhaps even Paris. These is shown, as a bit of trivial information, in two war offensives. One; the consideration by Douglas Haig in 1918 of retreating back on Calais/Dunkirk instead of holding the Western line - i.e. breaking away from Petain's French grand army. Secondly; by the legendary German 'Race to the Sea' of 1914, when the Allied rout meant a heedless sprint for the vital Channel ports.
These ports were the lifeline of sustaining British support in WWI, so the furtherest north this was considered was at Antwerp in 1914. Winston Churchill, then First Lord of the Admiralty, proposed a general landing at Antwerp to hold the Belgian defence and keep the north Belgian front open. In the end, while some Royal Marines were sent, and perished, this was not realistic in the eyes of the War Cabinet. The B.E.F. was reeling from devastating defeat at Mons, and the Germans were pushing in on the revered Channel ports while threatening Paris itself. Any manpower, already desperately short due to a tiny British army and reserve force of only 800 000 untrained boys and unmobilized, was devoted to keeping the B.E.F. from total disintegration.
This was a theme upheld even when the front did stabilize. It wasn't until 1916 that the British were equal to the grand citizen armies of Germany and France, when Kitchener's famed 'New Army' kicked in, and millions of young men entered the Western Front. But these reinforcements, since mid-1915, had always been earmarked for use by Haig in the great 28 division Somme offensive. So, as you suggest, the priority was always strategically focussed on breaking through the trenchline. This New Army was chewed up in July 1916, and capability to conduct maritime invasions dried up.
So basically - leading generals, circumstances and lack of manpower mandated that the British could never reasonably aim at opening another front in north Belgium.
The only other seaborne invasion pre-1916 was, of course, Gallipoli. But this was an effort mostly bolstered by Dominion troops and spare British/French divisions. In the end, this experiment was also abandoned due to manpower woes on the Western Front (e.g. the Australian troops ended up heading the July Somme offensive).
This leads us directly to the second phase of seaborne consideration; the Lloyd George stage. It is well known that Lloyd George generally depised Doulgas Haig, and despite strong efforts to dislodge the Field Marshall after the disastrous Passchendale battle in 1917, royal connections ultimately maintained the general in his position. And Douglas Haig had always, always believed that the war had to be won in France - hence his stubborn efforts to breakthrough in July 1916 and September 1917.
So the Prime Minister, not having the ability to interfere tactically, and not wanting to bow down to Haig, withheld millions of British reinforcements in Britain. So here was another two marks against the possibility of Belgian landings - the manpower did not exist, as in 1916, to land troops plus keep the British armies in France alive - and Lloyd George greatly disapproved of giving even more influence to an already power-hungry Haig. In fact, his position on this did not change until 1918, during the Spring Offensive when the Germans finally broke through. But, as in 1914, it was the threat against the Channel ports which gained priority over British thinking minds rather then a adventurous, Gallipoli-like expedition towards Antwerp.
There are two extra things here which need to be added in the general war which also negated this. Firstly; Gallipoli had been a disaster. It had cost politicians their careers (Mainly Churchill) and had got the Allies nowhere fast.
Secondly; Lloyd George was in favour of 'knocking out' Germany's supports. He was in the same mind as Churchill here in that he belied by taking away Germany's supports, it too would collapse. So, while the PM was not against seaborne invasions - he was simply against Douglas Haig, and hence his unwillingless to allow the field marshall to expand further into Belgium. This is supported by his strong support for expeditions into Salonika and Italy.
So, to answer your question: lack of manpower, leadership tension and Western Front priority (In Channel ports, Somme/Passchendale offensives and the Spring Offensive). I believe small scale landings were made north of the small Belgian army at the northern-most end of the Western Front, but were of little consequence for their efforts. Anything on the scale of Gallipoli or Salonika was justly impossible.
[I forgot to state my two main sources; Charles Bean's war history (frankly indisputable in referencing) and Max Hasting's Catastrophe.]>
Several points I'd like to add now:
I'm afraid this might not qualify as a top thread comment, as the only sources I could cite are general histories, but to my understanding the German High Seas Fleet had no chance to escort an amphibious fleet through the North Sea without a decisive defeat of the British Navy. Also, German war aims were about reaching Paris much more so than defeating the British.
As for the British, there really wasn't a place to land that would have mattered unless they were prepared to invade neutral Holland, which would greatly undercut their moral high ground of fighting for poor Belgium.
As a final thought, Gallipoli as it was first planned was intended as a lightning strike amphibious assault to knock the Ottomans out of the war. After its failure, I can only imagine the idea was dead on arrival when pitched for the Western Front.
Mods, feel free to delete this if I've overstepped myself.
Firstly an amphibious landing would require control of most of the English Channel, to get men to shore safely and take minimal casualties. For the German navy this was an almost impossible task after their defeat at Jutland to the British. The area was also heavily mined so British and German forces would have taken quite heavy casualties. There was also very little knowledge on how to launch a significant amphibious assault behind enemy lines. Gallipoli was a great example of this, as a huge tactical error meant that landed allied troops were left stranded and pretty much abandoned on the beaches. For these reasons, an amphibious operation would not have been as successful as a D-Day style landing in changing the flow of the war. The armies of both sides would not have been crazy about the idea of pulling troops from the trenches, as this would have resulted in a weakness which the opposition could have used to break through their front lines. Even if the trenches in France would have been abandoned, they were likely to form again deeper into France or maybe into Germany, if the allies could advance. The trenches formed as both sides failed to make any advances, reaching a stalemate. Removing the trenches may have made the war more interesting for a while, but would most likely haves formed again if the allies could not advance against the Germans. The germans may have been able to have pushed deeper into France with the allies not having a flat line of defence to hold them off with.