Japanese Pacific Island Defenses 1941–45

Principles of construction

In common with other armies, the Japanese had developed a defensive doctrine along with prescribed principles and methods of construction, many based on British World War I manuals (though as noted local adaptation was common). For the most part defensive positions were dug as deep as possible and kept low to present a low profile. This was not always possible because of high water tables, swampy ground, or shallow bedrock; or because the roof had to be thick to protect from heavy artillery fire; or because the position's firing port had to be set higher in order to cover its field of fire effectively, especially if firing downhill. Positions dug into the sides of hills, ridges, gorges as well as cave positions were usually built flush with the surface making them difficult to detect if well camouflaged with foliage or rocks. Unless made of concrete, interior walls were built of logs, planks, sand-filled boxes or drums, sandbags, or some form of shoring to prevent collapse when hit by artillery or bombs. Positions built of rock were sometimes lined with a layer of sandbags to prevent flying rock fragments. Rock pillboxes had to be shored with logs as they could easily collapse. We know of some instances where pillboxes were built of stacked coral stones. Once roofed over, concrete was poured over the structure and allowed to run down the sides to form a concrete cap. Concrete parapets for some large, open-topped coast defense and AA gun positions were constructed by building a wooden structure in the desired angular shape, stacking layers of coral stone, pouring concrete over each subsequent layer, and then pouring more into the form to give its sides a finished appearance. Even though built without rebar, such positions were quite robust.
Most pillboxes, bunkers, and other positions were built of coconut or hardwood logs, laid horizontally or dug-in vertically. Horizontal log walls were supported by vertically-piled logs with the ends often held together by steel staples. Wire and cable were sometimes used to lash logs together. Gaps were left between horizontal logs for firing ports or alternatively a section was cut out. Open-topped fighting positions often had similar log walls to reinforce the sides and prevent collapse from a near miss.

Concrete pillboxes, walls for cave openings, and other concrete structures contained as much rebar as available. Even wire and rope were used for reinforcement. Wooden frames were put in place and concrete poured into them: they were also used to shape firing ports and doors, and were reused before being taken apart and incorporated into other fighting positions. Close examination of these fortifications reveals that the Japanese were not always adept at pouring concrete: seams can be detected between different batches along with air voids, weakening the structure. The failure to overlap and tie-in rebar connections, the use of crushed coral or shell rather than gravel, a high water-to-cement ratio and the use of seawater also made for low-density and weak concrete. Typical compressive strength of Japanese concrete was 0.180kg/m2 while US military concrete standards were twice that. Firing tests on Kwajalein pillboxes revealed that the US 37mm AP round penetrated 785mm (31in.) of Japanese concrete, but only 457mm (18in.) of US-standard concrete. The US 75mm AP round penetrated 1,060mm (42in.) of Japanese concrete, but only 609mm (24in.) of US concrete. Overhead cover of fighting positions was critical to provide protection from naval gunfire, artillery, mortars, aerial rockets, and bombs. Log roofs were laid in solid layers with alternating layers perpendicular to the others. Occasionally the log layers were laid in the same direction. Anywhere from one to six layers of 4-18in. diameter logs could be used, but two or three layers were the most common. Different-sized logs might be used in each layer and rocks may have been placed between some layers as a burster layer to detonate projectiles or deform AP rounds. Even green coconuts were used for this. Between the layers of logs and rock sand or earth was placed. If available the position had corrugated sheet metal placed between layers to provide protection from rainwater. In larger bunkers and covered positions vertical support logs were necessary to shore up the roof. Stinger logs, beams, or rails supporting the roof were usually positioned 2-3ft apart. More sand or earth was piled over the position and banked against the sides. This created a significant hillock, but it was often contoured to blend into the surrounding terrain and camouflaged. US 60mm mortars did not possess the power to penetrate most bunkers. US 81mm heavy HE rounds and the 4.2in. mortar were more effective.
Firing ports were located to cover the position's assigned sector of fire, and placed very low (if not flush) to the ground. A small ditch might be dug immediately in front of the firing port. This prevented falling debris from blocking the port as well as making it more difficult for grenades to be rolled in. Some positions had only one firing port, others had additional ports to cover wider areas or supplementary sectors to the flanks and rear. Sometimes there was a single large firing port for the position's main weapon and smaller ports for rifles and LMGs. These were occasionally positioned to the side and higher than the main port so that a soldier could spot targets and direct the fire of the main weapon, if the crew had their vision obscured by smoke and dust. Firing ports were typically small to make them difficult to detect and hit, though size affected the weapon's field of fire: occasionally, wider ports were created to allow a weapon to cover a large sector. Plank or log closures were sometimes provided for firing ports in an attempt to block fire: steel shutters were rare, the few examples being in concrete pillboxes.

The width of a firing port was determined by the field of fire the position was assigned: the height of the opening though was usually kept as narrow as possible. Firing ports were of two basic types. The most common had a wide outer opening with a smaller inner opening. This allowed a weapon pivoting on a fixed mount a wide traverse. The disadvantage of this type of port was that its larger outer opening was easier to detect, presented a large target, and allowed bullets striking the angled sides to ricochet into the position. Wood embrasures sometimes absorbed bullets, but some could still find their way in. Concrete embrasures could be constructed in a ""stepped"" manner, which helped deflect bullets. The second type was built in the opposite manner, with a small outer opening and a large inner aperture. This allowed non-fixed weapons (such as rifles and LMGs) to be traversed by the firer shifting his position, and the opening was more difficult to detect.

Entrances to positions were of course normally in the rear, but in some instances they might be on the side of a position, depending on the protection and concealment afforded by surrounding terrain. Entrances were often protected to prevent direct-fire, blast, fragmentation, grenades, demolition charges, and flamethrower fire from entering: this might be a blast barrier inside the position or a similar barrier or wall on the outside. Entry may have been gained by a trench or tunnel with one or more right-angle turns. Many positions though had only a straight, unprotected entryway. Doors were seldom provided other than on some concrete fortifications. This often proved to be the weakest point of attack as they were usually protected by fire from adjacent positions.
Some positions were built as two-story structures with firing ports on both levels covering different areas. Often only the upper level had a firing port and the lower provided shelter for the crew when receiving heavy fire. The troops manning the weapon could duck into the lower level through a hatch and possibly survive a demolition or flame attack. Larger positions were often compartmented as well. An interior log wall separated the weapon firing position from the rear portion where ammunition was stored. It also offered the crew some protection from explosions and flames entering through the embrace. If there were two or more crew-served weapons in a bunker they were separated by a wall so that explosives thrown through one port would not knock out the other weapon(s). In larger positions the rear compartment also served as living quarters. Often a well-protected bomb shelter lay to the rear connected by a short trench or tunnel: this might double up as living quarters. Grenade wells were sometimes dug in the floor of covered positions. If a grenade was thrown in there may have been time to kick it into the deep but narrow hole. This also served as a drain water collection sump.