Armoured vehicle - launched bridge An armoured vehicle - launched bridge ( AVLB ) is a combat support vehicle , sometimes regarded as a subtype of combat engineering vehicle , designed to assist militaries in rapidly deploying tank s and other armoured fighting vehicle s across rivers . The AVLB is usually a tracked vehicle converted from a tank chassis to carry a folding metal bridge instead of weapons . The AVLB ' s job is to allow armoured or infantry units to cross craters , anti - tank ditches , blown bridges , railroad cuts , canals , rivers and ravines ) , when a river too deep for vehicles to wade through is reached , and no bridge is conveniently located ( or sufficiently sturdy , a substantial concern when moving 60 - ton tanks ) . The bridge layer unfolds and launches its cargo , providing a ready - made bridge across the obstacle in only minutes . Once the span has been put in place , the AVLB vehicle detaches from the bridge , and moves aside to allow traffic to pass . Once all of the vehicles have crossed , it crosses the bridge itself and reattaches to the bridge on the other side . It then retracts the span ready to move off again . A similar procedure can be employed to allow crossings of small chasms or similar obstructions . AVLBs can carry bridges of 60 feet ( 19 metres ) or greater in length . By using a tank chassis , the bridge layer is able to cover the same terrain as main battle tank s , and the provision of armour allows them to operate even in the face of enemy fire . However , this is not a universal attribute : some exceptionally sturdy 6x6 or 8x8 truck chassis have lent themselves to bridge - layer applications . Origins . The roots of the modern AVLB can be found in World War I , at the dawn of tank warfare . Having developed tank s , the United Kingdom and France were confronted with the problem of mounting tank advances in the face of the trenches that dominated the battlefields . Early engagements , such as at Cambrai demonstrated the tank ' s utility , but also highlighted its vulnerability to battlefield geography — many early tanks found themselves ignominiously stuck in the trenches , having insufficiently long tracks to cross them ( as at right ) . To counter this disadvantage , tanks ( especially the common British Heavy tanks ) began to go into battle with fascine s hanging over their bows , sometimes as simple as a bundle of heavy sticks . By dropping these into the trenches , they were able to create a wedge over which the tank could drive . Later , some tanks began to carry rails on their decks — the first AVLBs . By 1919 , the British Army had , at its training centre in Christchurch , a Mark V * * tank with lifting gear able to carry and place a bridge or carry out mine clearing and demolition . World War II & subsequent use . It was in the World War II era that the importance of armoured bridge layers , as well as combat engineering vehicle s and armoured recovery vehicle s , became fully clear . With the advent of Blitzkrieg warfare , whole divisions had to advance along with tanks , which were suddenly far out - pacing the speed of infantry soldiers . Besides leading to the advent of self - propelled artillery / assault gun s , mobile anti - aircraft and armoured personnel carrier s / cars , it became clear that functions like vehicle repair , mine - clearing , and the like would have to be carried out by armoured vehicles advancing along with tanks . Moreover , these forces would have to be able to cross all forms of terrain without losing speed , and without having to concentrate their thrusts over certain bridges ( and the rising weight of armoured vehicles meant that fewer and fewer bridges could support these massed crossings ) . The only feasible solution to the dilemma posed by the mobility of all - mechanised armed forces was a dedicated platform that could improvise river and obstacle crossings at short notice and in otherwise inconvenient locations . Tracked and armoured , it was capable of operating right alongside combat units , crossing rough terrain and advancing in the face of light fire . To maximize on common parts and ease maintenance complications , they were usually based on existing tank chassis . One of the earliest series - produced examples is the " Brückenleger IV " , a German AVLB based on the Panzer IV , which entered service with the Wehrmacht in 1940 . Twenty were built , but problems of excessive weight limited the vehicle ' s effectiveness , and eventually all 20 were converted back to tanks . A new scissors bridge design was brought out by the British in response to the war , sufficient to support a 24 ton load over . This was developed for the Covenanter tank . Eventually , it developed into a 30 ton capacity and therefore was carried by a turretless Valentine tank . It was used in Italy , North West Europe and Burma . The Allies developed similar equipment , mostly based on the ubiquitous Churchill infantry tank ( carrying the Small Box Girder ) and the Sherman medium tank of the British and U . S . armies , respectively . In some early designs , bridge - layers could emplace bridges , but not retract them . Other vehicles were integral to the bridge themselves ( such as the Churchill Ark ) , wading to the middle of a river or driving up against an obstacle and extending simple ramps in both directions ; following vehicles would drive directly over the bridge layer . Modern . Most modern bridge layers are based on current main battle tank chassis . Another approach to bridge laying across water is the use of amphibious vehicles which act as combination of pontoon and roadway . These enter the water and join together to form a bridge . An example is the German M3 Amphibious Rig , a bridging vehicle used by Germany , the UK , Singapore , and Taiwan .