Performance of manhole under severe condition in Japan

Japan is prone to natural disasters such as earthquakes and torrential rains. Since those disasters have become more severe, extending the life of infrastructure is an urgent issue. Modern sewer system in Japan have been constructed since 1884, and regular inspection, cleaning, and repair are essential to prolonging the life of sewage pipes. Manholes, which are entrances to sewage systems, can be regarded to be one of the important structures that support our safe, secure, and comfortable lives.


Introduction
The first modern sewer system was constructed in Tokyo in 1884.[1] The total length of sewer pipelines and the public sewage treatment ratio in Japan has reached approximately 490,000 km and 92.6% in 2021, respectively.[2] The sewer systems over 50 years of design service life are increasing every year, and more than 3,000 road collapse accident occurred due to the aging and damage of sewer pipes.[2] The deterioration and renovation of the aged sewer infrastructures is an urgent issue.
In addition to the above, Japan is a country surrounded by the sea and also subjected to many natural disasters such as earthquake, tsunami, volcanic and hot spring action, floods and landslides caused by torrential rain.Then, the appropriate countermeasures against earthquake, salt damage and corrosion are demanded for sewer pipeline facilities.At the same time, aging population progresses and the labors shortage is getting worse.Table 1 shows the required performance for manholes [3][4] related to influence and disasters.The floating and flying off of the manhole lid due to the high internal pressure of the sewer pipe caused by a sudden rise in water level are injurious to surrounding pavements and dangerous to human life.Furthermore, three-thirds of Japan's land is mountainous and hilly, and flat land for people to live in is limited.As a result, the population is concentrated on the plains and there are the areas where roads are too narrow for large vehicles to pass.Therefore, reduced small size of sewerage facilities is desirable.
In this paper, research and developments of sewer manhole appropriated for severe natural and social conditions in Japan were studied.

Manhole cover
The manhole cover is the only gate to the sewer pipes and acts as part of road.Manhole covers that comply with Japan Sewage Works Association Standard [5] have a cast iron lid and frame connected by hinges, with an automatic lock to prevent illegal opening.In some cases, stainless steel parts are attached to the frame to prevent the falling.

Manhole bottom
The manhole bottom supports the manhole cover and provides the space for carrying in equipment and ventilation work.For different purposes and applications, manhole bottoms have been developed in various shapes and materials.There are two types of manhole bottoms: one is a regular size type where people do the inspection and cleaning work, and the other is a small size type where machines do the work instead of people.Small manholes are applied to reduce costs and to improve the workability in narrow spaces.Resin concrete manholes are superior to conventional reinforced concrete manholes in workability and corrosion resistance.Furthermore, its manholes are superior to the rigid polyvinyl chloride manholes in strength, durability and cost-performance.
Resin concrete is also known as polymer concrete.[11][12] It is a composite material consisting of thermosetting resin as a binder mixed with filler and aggregates without Portland cement and water.[13] Unsaturated polyester resin is mainly used for resin concrete products from the viewpoints of cost and workability in Japan.
The mechanical strengths of ordinary Portland cement concrete and resin concrete are compared in Table 3. Higher strength of resin concrete than that of Portland cement concrete enables cross-sectional reduction and labor saving in construction works.The small resin concrete manhole composed of glass fiber reinforced plastic (FRP) on the inner surface was proposed and manufactured to further increase the bending strength and impact resistance.This high-graded manhole can make installation and transport easier for people in narrow spaces and on roads.
Table3 Mechanical strength comparison of Portland cement concrete and resin concrete.
Table1 Required performances for manholes and related influences and disasters.3 Manhole Safety

Safety measures and challenges in Japan
Manhole covers have been improved and manufactured to withstand the severe conditions in Japan.An overview of the technological transition of manhole covers is shown in Figure 2.
The first challenge concerns materials.The grey cast iron was used for the cover before the 1960s.As the cover had to support the design vehicle load, the lids had tended to be thicker and heavier, and the installation and management of opening and closing was hard work.The cracks of the lids due to the low strength of the material caused by the impaction of vehicle traffic were often a problem.Spheroidal graphite cast iron was developed and used in the 1960s for its excellent castability, strength, ductility and toughness.As a result, the weight of the products reduced to half of conventional one, and damage accidents due to vehicle traffic the cracking problem decreased, and handling and maintenance of the cover were also improved.
The second challenge is the review of the mating structure between the iron lid and frame.The mating of the manhole cover, called a flat bearing support in which the cover was simply put on the frame, was commonly adopted before the 1970s.The problem with this type of support structure was that the iron cover rattles and makes a loud noise, which not only causes discomfort to local residents, but also shortens the life of the cover and pavement as vehicle passed through.Followingly, in the 1970s, the incline bearing support was proposed, in which the mating surface of the iron lid and frame was machined.This system could prevent the rattle and inconvenience by the properly biting and fixing the iron lid firmly into the frame.However, excessive bite induced difficulty in opening and closing the cover, and flying off risk of the iron lid.In order to solve the new problem, two steps bearing support structure was proposed as the next generation of manhole covers in the 2000s.The details and mechanism of this support are described in 3.2.1.
The third challenge is the pressure release performance.High internal pressure in the sewer pipe caused by sudden rise in water level during heavy rain resulted in the momentary release of excessive bite in the incline bearing support.The occurrence of a large impact force caused the joint failure connecting the iron lid and frame or damage of connecting parts between the frame and bottom, and furthermore, flying off of the manhole cover and floating of the bottom occurred.Manhole cover with pressure release structure was developed in the 1980s.When an internal pressure exceeding the bite force is applied to this pressure release type cover, the iron lid floats up to a certain height and the pressure is released from the gap between the parts.After that, when the internal pressure is released, the iron lid returns to its original state.

Latest next generation manhole suitable for harsh environments
Recently, countermeasures against torrential rain caused by global warming and a new approach to extending the life of manholes have begun in addition to above safety measurements.

Risk of floating and flying off due to excessive bite of iron lids
Indeed, the incline bearing support structure to improve the biting force was contribute to prevent the rattle, an excessive bite may occur due to repeated vehicle traffic.The excessive bite caused occasional maintenance problems, such as difficulty in opening and closing the lid, and the risk of the iron lid being scattering due to impact caused by internal pressure exceeding a certain limit.As a next generation manhole cover, the two steps bearing support structure has been developed to prevent rattling, floating and flying off at the same time by controlling the biting force.Figure 3 shows the difference in the biting force distribution obtained from Finite Element Method (FEM) analysis for the incline bearing support and two steps bearing support.The contact area between the iron lid and the frame is increased locally, because the sloped surface in the incline bearing support is a straight structure.On the other hand, the two steps bearing support is a curved structure with two sloping faces, each with a different function; the upper part controls the excessive biting and the lower part prevents rattling due to biting force, respectively.The design of the biting force is optimized by uniform contact between the iron lid and frame all around.
The legal service life of manholes in roadways is 15 years.In the meantime, the performance of the next generation manholes is inspected and evaluated regularly for prevention of rattling, noise, and flying off of iron lids at the initial state and limit state.At the initial state, the swing amount was confirmed to be below the specified value of 0.5 mm when the design load (70 kN) was applied to the specified area (250×200 mm) at both ends of the cover.At the limit state, the performance of the cover was confirmed by wheel road running test as shown in Figure 4.In this test, there was no sudden increase in the swing amount and rattling noise, even when the wheel load of 100 kN, which is twice the design load, was applied to the cover more than 500,000 times.

Floating and flying off of iron lids due to torrential rain
Flying off accidents of iron lids has decreased with the widespread use of pressure release performance manholes since 1998.However, in response to the sudden increase in internal pressure during heavy rains, which has been increasing in frequency in recent years, flying off accidents of iron lids have also occurred because the necessary exhaust volume could not be secured.
The phenomenon of generating internal pressure to float up a manhole was reproduced by experiments, and risk assessment and countermeasures were considered.[14] Several manhole cover and bottom combinations were tested.The pressure, water level and flow in the manholes, and water leakage from the joints was examined, when the amount of water in the manhole is gradually increased.It was confirmed that the grating iron cover with larger opening area and the bottom with air outlet pipe were effective to reduce manhole pressure as shown in Figure 5.
It is expected that further measures will be taken in the future when the effects of pressure reduction on the exhaust area and exhaust position are quantitatively organized and the concept of internal pressure measures during heavy rainfall is systematized.

Slip on iron lid surface due to rainfall
Frictional resistance decreases when the surface of the iron lid becomes wet due to rain and snow, making it slippery and dangerous for pedestrians and motorbikes.In addition to rain, the dry sand between the iron lid and the tire is also concerned to wear the surface of the iron lid and make it more slippery.Anti-Slipping Design (ASD), which is a special surface structure with proper grip force with tire on the iron lid surface for a long time, was investigated and developed to solve the above problem of wear and slippage.Figure 6 shows the appearance of cast iron lids with worn surfaces and the iron lid of ASD.The convex part of ASD has a three-layer structure; the upper part improves the tire grip, the middle part secures the drivability of passing vehicles and prevents sand and water from rising up, and the lower part assists drain the water that accumulates when the tires pass.The height and spacing of the convex part were determined by based on the evaluation results of the driving feeling test by motorcycle rider and friction coefficient measurement test as shown in Figure 7.This result shows that the dynamic friction coefficient of ASD is higher than that of conventional one, and the value is comparable to that of general wet asphalt in the long term.

Deterioration of sewer manhole in harsh environments
Generally, electrodeposition coating is applied to the manhole cover in order to provide corrosion resistance in atmospheric environments.Sometimes actual sewerage facilities are exposed to corrosive environments such as sulfide gas or sulfuric acid occurred from bacterial activities, and dew condensation water, and corrosion may progress.Furthermore, corrosion of the manhole cover is accelerated by paint damage due to opening and closing of the lid, dragging an iron lid on the road, and collision between iron lid and frame.Particularly, corrosion of the inclined surface between the iron lid and frame cause lifting the iron lid, sticking between the iron lid and frame, and occurrence of steps on the frame, as shown in Figure 8.
Steps cause tripping and falling of pedestrians.Sticking makes it difficult to open and close, and due to malfunction of the pressure release performance, flying off of the cover are worried.It is desirable to avoid such risks of the accidents.
As a surface treatment for severe corrosive environments, anti-corrosive treatments were investigated.Corrosion protection was performed by spraying a metal that acts as a sacrificial anode on a cast iron substrate and then applying an electrodeposition coating as a topcoat.The comparison between the untreated lid and the anti-corrosive lid exposed to the corrosive environments is shown in Figure 9.The anti-corrosive effectiveness was confirmed for anticorrosive lid.The anti-corrosive effectiveness varies depending on the installation environment and driving frequency of opening and closing of the lid, and research on anti-corrosion method suitable for site conditions continues.[15][16]   Reinforced concrete manhole has been used in Japan from the viewpoints of installation performance and material cost, however durability issues of the manhole such as frost damage in cold region, salt damage in coastal area, and chemical erosion due to acid water and hydrogen sulfide in hot spring area arose and became apparent.Resin concrete has almost no water absorption and no salt penetration, and thus products have excellent durability in these severe conditions.Figure 10 shows the remarkable results of the freezing and thawing resistance test of resin concrete.As the water absorption rate of resin concrete is 0.3% or less, it hardly absorbs water, and there is no decrease in relative dynamic elastic modulus and mass change rate even after 300 cycles.In order to clarify the resistance of resin concrete to erosion in hot spring, diskshaped resin concrete specimens of 50 mm diameter and 10 mm thickness were immersed in an acidic hydrogen sulfide spring of pH1.8 and 55-70°C for 3 months.The appearance of the specimens after 3 months of immersion is shown in Figure 11.The surface of the resin concrete was slightly whitened, but there was no change in mass.On the other hand, the cement mortar specimens used for comparison became brittle throughout and its mass was decreased by 50%.The results of this immersion test show that resin concrete can be used under corrosive environments such as hot springs and sewage treatment facilities.

Conclusions
Research and development of manhole in line with the natural and social environment of Japan was introduced and investigated.The main results related to materials and structures are summarized in the following: 1. Manhole lid of spheroidal graphite cast iron and manhole bottom of resin concrete were newly developed in Japan.The small size manhole, which is significantly superior in safety and maintenance than conventional ones, could be manufactured with these excellent lid and bottom parts.2. The biting force was controllable by changing the mating method between the lid and manhole frame from incline bearing support to two steps bearing support, and the desirable manhole structure as to suppress rattle and to improve a pressure release performance could be obtained.3. Grating iron lid and bottom box with air outlet pipe can withstand rapid increase in internal pressure due to unexpected torrential rainstorm.4. The design of the proper height and spacing of irregularities on the surface of the iron lid was proposed based on the wet road surface friction coefficient to prevent the slip on the iron cover. 5. Corrosion of manholes caused by hydrogen sulfide gas and acid hot springs in the sewer environment is a serious problem.Based on the immersion test, preferable materials with high corrosion resistance are selected and recommended.

Figure 1
Figure 1 Composition of Manhole for sewerage.

Figure 2
Figure 2 Transition of manhole covers.

Figure 3
Figure 3 Difference of biting force distribution by FEM analysis.

Figure 5
Figure 5 Appearance of the grating iron cover (a) and the bottom with air outlet pipe (b).

Figure 6
Figure 6 Appearance of cast iron lids with worn surfaces (a) and antislip structure (b).

Figure 7
Figure 7 Ageing of dynamic friction coefficients.

Figure 8
Figure 8 Example of corroded cast iron lids.(a) Corrosion on the back of the lid.(b) Corrosion on the mating surface between the lid and frame.

Figure 9
Figure 9 Result of installation in corrosive environments.

Figure 10
Figure 10 Result of freeze thaw test.

Table 2
Japan Sewage Works Association Standards for manhole bottoms.