By Denis Pollak
Every day in the United States, millions of people pass over manholes, either on foot or in vehicles, without noticing them. In fact, it wasn’t until about 20 years ago that those in the water/wastewater industry paid much attention either – the old adage of “out of sight, out of mind” seemed to apply. Now the industry is recognizing just how vital manholes truly are to the country’s infrastructure and how their failure could ultimately lead to big problems throughout the U.S.
Studies have shown that much of the groundwater and surface water that enters sewer systems does so through manholes, making them the potential weak link in the system. There are definite consequences if deteriorating manholes are not repaired. From an operations standpoint, infiltration from groundwater impacts the collection system as a whole. From a structural standpoint, if there is a lot of corrosion within the manhole, cave-ins can occur.
Each component of a manhole is at a high risk of failure from normal wear and tear, not to mention factors such as exposure to hydrogen sulfide or poor construction. The manhole invert, which is the bottom of the structure, is the extension of the pipe inlets. They are particularly susceptible to leaks. The bench, the sloped floor between the invert and the wall of the barrel, can deteriorate from abrasion and corrosion. The barrel itself is prone to corrosion, which can lead to structural instability. Manhole rings are susceptible to movement from traffic loads, soil movement and the freeze/thaw cycle.
The good news is, there are resources available to rehabilitate almost any defect that can be found in a manhole. The key is well trained and experienced applicators who understand that manholes are constructed of various materials that have different structural characteristics and require different rehabilitation methods.
Manholes: A History
Before rehabilitation methods can be addressed, it is necessary to take a look back at the history of manholes. Manholes have been in existence in the United States for over 200 years, with some still in service dating back to the pre-Civil War era. At that time, bricks and mortar were the common materials used to construct manholes; however some were built with stone.
When inspecting older brick structures, many have a lot of brick still intact but very little mortar. Depending on the location and condition of the manhole, inspections can uncover a significant amount of root intrusion. In some extreme cases, soil can be seen. In the bottom area of the structure, deterioration of the bench can sometimes be observed. In this case, there may be separations or voids between penetrating pipes into the manhole structure.
Although many of these older manholes are deteriorating, there are some that have stood the test of time. The life span of brick manholes is a testament to the craftsmanship of the builders, especially considering that the bricks were hand-laid. These men were artisans – the pride they took in their work is evidenced by the very fact that many of these manhole structures still exist.
During the mid-20th century there was a shift away from brick and mortar manholes to precast concrete. Areas of spalled concrete and exposed aggregate are typical for manholes built during this time. For the most part, the typical life span of precast concrete manholes is 50 years. This means that now is the time when manholes built in the 1950s and ‘60s are beginning to fail. These manholes need to be inspected and any problems need to be addressed as soon as possible.
In the later 20th century, some manholes were constructed with cast-in place concrete. The condition of this type of manhole typically depends on the quality of the installation process. For example, if reinforcing steel was used, how much concrete cover is over the steel? Those manholes with adequate or more than adequate cover are still in good condition for the most part, and with proper inspection and maintenance can have a life cycle of 40-50 years. Those with inadequate cover are already beginning to deteriorate.
The life cycle of both cast-in place and precast concrete manholes depends primarily on two factors: the severity of H2S exposure and the quality of the construction. H2S eats away at concrete and causes corrosion and, ultimately, the failure of the manhole structure. Poor construction methods can lead to the premature failure of the concrete or lining.
Current Manhole Construction
In the world of manhole construction today, materials such as HDPE, concrete with corrosion reducing additives and plastics are used. Newly constructed concrete manholes are rarely seen without liners. Better specifications are used, and this along with improved construction practices typically means that newly constructed manholes will have a life cycle of 50+ years. However, the lasting power of T-lock and other liners depends on the contractor – a poorly installed T-lock liner can fail within the first 10 years.
Some new manholes have HDPE liners installed versus T-lock. HDPE has its merits; it is a stout material that is manufactured to be thicker than T-lock. Concrete manholes with properly installed HDPE liners can last 40-50 years without deterioration.
There are also some manholes built out of HDPE rings. There are several advantages of this type of construction: 1) It doesn’t require the installation of a liner, 2) It is relatively lightweight, which means that a lot more of it can be loaded onto a truck, translating into money saved and 3) The handling of the material is much easier – heavy equipment is not necessary. It is most likely that HDPE ring manholes can last 75-100 years.
With over 20 million manholes in the nation and many of them in poor condition, what can be done? The first step is to inspect the manholes to determine their condition. The best practice for the manhole inspection is to use a combination of technology and getting down into the trenches. An initial investigation can be conducted with the use of a CCTV camera.
If a particular area appears to be in good shape, further investigation is not required until the next round of maintenance checks. If the area appears to be in poor or questionable condition, then a confined space entry into the manhole is performed. An inspector can pound on concrete and “get a feel” for more than just what the camera shows. However, technology is extremely helpful in this part of an initial manhole inspection. Viewing the manhole via camera limits the number of confined space entries, keeping workers safe and costs down.
Manhole inspectors are like detectives – consistency is necessary in all aspects of investigation in order to make the most of the clues. The inspector needs to look closely at all defects and document his/her visual observations. This is why NASSCO’s Manhole Assessment Certification Program (MACP) is a valuable tool. This program allows for consistency of documentation and a repeatable process for evaluation. In this way, a long-term approach to manhole rehabilitation can be undertaken rather than simply reacting when problems arise.
The goals of MACP coding are to define attributes and features of the structure, document and explain defects, develop ratings for each applicable component of a manhole – structural rating, O&M rating, I/I rating – and record dimensional data that can be used for selecting rehabilitation methods. This standardized method for reporting the results of condition assessments keeps everyone on the same page, promotes cost efficiency and avoids unnecessary rehabilitation work.
In most cases, rehabilitation of the manhole is chosen over replacement due to cost and logistics. When determining what rehab methods to use for a particular manhole, it is important to note that a one-size-fits-all approach is not the best idea. No two manholes are identical. Site specific considerations such as physical location, access and right of way can be different for two manholes suffering from the same internal corrosion issues.
Fully understanding the challenges involved will help guide the rehab process. For example, where is the manhole located? Poor access to a manhole can greatly impair the ability to rehab that particular manhole. Are there any traffic control measures that need to be considered? What about flow issues, elevated H2S levels or moisture? Will it be necessary to ventilate and dehumidify? Is there root intrusion and/or infiltration? What are the cost constraints? How long does the rehab need to last? A 50-year fix isn’t necessary if manhole replacement is planned for 10 years down the road. Being armed with the answers to these questions will help ensure that the rehab process goes smoothly.
Depending on the answers to the above questions and the specific defects of the manhole, coatings, liners or a combination of both may be used to fix the damage. When it comes to coatings, spray applied epoxies, urethanes and cementitious coatings that are calcium aluminate based are all used in rehab work. Other options include repairing existing liners with a similar material. Arrowlock® is a PVC-like material that uses mechanical adhesion to repair failing T-Lock® liners. There are also other products, such as Linabond®, that uses a chemical adhesion process to achieve the same type of repair.
At times, a combination of methods may be appropriate. For example, failing T-Lock® may be cut out and the concrete beneath coated. If using a combination method such as this, the two rehab methods need to work well together, with a proper interface occurring between the coated concrete and the remaining liner.
Full replacement or installation of structural liners, such as Poly-Triplex® Liners, comes into play in situations where a coating simply isn’t enough to fix the problem. The old liner is replaced with a new structurally reinforced cure-in-place liner.
Regardless of the rehabilitation method chosen for a particular manhole, the goal is to create a corrosion barrier to prevent manhole deterioration. Although the main consequence of a failing manhole is I/I, in extreme cases dangerous conditions can result from a corroded, deteriorating manhole. WW
About the Author:
Denis Pollak is Condition Assessment Manager for V&A Engineering, an independent consulting engineering firm specializing in corrosion engineering, coatings system management, and condition assessment services. Pollak has more than 35 years experience in project management on water and wastewater projects. He was a founding member of the California Water Environment Association Southern Sections Collection System Committee and is a former President of the CWEA, San Diego Section.
Katika Marekani, mamilioni ya watu hupita kila siku kwenye mashimo ya maji, wakiwa wanatembea au wakiwa katika magari, bila kuyaona. "Hata hivyo, ni miaka 20 iliyopita kwamba wale katika sekta ya maji na maji taka walilipa tahadhari nyingi ama - msemo wa zamani wa ""nje ya macho, nje ya akili"" ilionekana kutumika." Sasa sekta ya maji inaona jinsi manholes muhimu kweli ni kwa ajili ya miundombinu ya nchi na jinsi kushindwa yao inaweza hatimaye kusababisha matatizo makubwa katika Marekani. masomo wameonyesha kwamba mengi ya maji ya chini ya ardhi na maji ya uso ambayo huingia mifumo ya mifereji ya maji ya mifereji hufanya hivyo kupitia manholes, kuwafanya uwezekano wa kiungo dhaifu katika mfumo. Kuna matokeo dhahiri ikiwa mashimo ya maji yanayoharibika hayarekebishwi. Kutoka kwa mtazamo wa shughuli, infiltration kutoka maji ya chini ya ardhi athari mfumo wa ukusanyaji kwa ujumla. Kutoka kwa mtazamo wa muundo, ikiwa kuna kutu nyingi ndani ya shimo la maji, maporomoko ya ardhi yanaweza kutokea. Kila sehemu ya shimo la mfereji ina hatari kubwa ya kushindwa kutokana na kuvaa na kupasuka kwa kawaida, bila kutaja mambo kama vile kuambukizwa na sulfidi ya hidrojeni au ujenzi duni. Invert manhole, ambayo ni chini ya muundo, ni upanuzi wa bomba inlets. Wao ni hasa nyeti kwa kuvuja. Benchi, sakafu yenye mteremko kati ya invert na ukuta wa pipa, yaweza kuharibika kutokana na kuharibika na kutu. Bomba lenyewe huwa na uwezekano wa kutu, jambo linaloweza kusababisha kutokuwa na uthabiti wa muundo. Manhole pete ni nyeti kwa harakati kutoka mzigo wa trafiki, harakati ya udongo na mzunguko wa kufungia / tha. Habari njema ni kwamba kuna njia nyingi za kurekebisha kasoro yoyote ambayo inaweza kupatikana katika shimo la maji. Mfumo wa msingi ni wa kuajiri waendeshaji wenye uzoefu na waliofundishwa vizuri ambao wanaelewa kwamba manholes hutengenezwa kwa vifaa mbalimbali ambavyo vina sifa tofauti za muundo na zinahitaji mbinu tofauti za ukarabati. Manholes: Historia Kabla ya mbinu za ukarabati zinaweza kushughulikiwa, ni muhimu kuangalia nyuma katika historia ya manholes. Manholes zimekuwapo nchini Marekani kwa zaidi ya miaka 200, na baadhi bado katika huduma dating nyuma ya kabla ya Vita vya wenyewe kwa wenyewe enzi. Wakati huo, matofali na chokaa vilikuwa vifaa vya kawaida vilivyotumiwa kujenga mashimo ya maji; hata hivyo, baadhi ya mashimo hayo yalijengwa kwa mawe. Wakati wa kukagua majengo ya zamani ya matofali, wengi wana matofali mengi bado hayameharibiwa lakini ni mawe machache sana. Ikitegemea mahali na hali ya shimo la maji, ukaguzi waweza kufunua kiasi kikubwa cha uvamizi wa mizizi. Katika visa fulani vikali, udongo waweza kuonekana. Katika eneo la chini la muundo, uharibifu wa benchi wakati mwingine unaweza kuonekana. Katika kesi hii, kunaweza kuwa na kutenganisha au voids kati ya kupenya mabomba katika muundo manhole. Ingawa mashimo mengi ya zamani ya kuvuja maji yanazidi kuwa mabaya, kuna mengine ambayo yamedumu kwa muda mrefu. Urefu wa mashimo ya matofali ni uthibitisho wa ustadi wa wajenzi, hasa kwa kuzingatia kwamba matofali hayo yaliwekwa kwa mkono. Wanaume hao walikuwa mafundi - fahari waliyokuwa nayo katika kazi yao inathibitishwa na ukweli kwamba majengo mengi ya mashimo hayo bado yapo. Katika karne ya 20, kulikuwa na mabadiliko kutoka kwa manholes ya matofali na chokaa hadi saruji iliyotengenezwa mapema. Maeneo ya saruji spalled na kufunuliwa aggregate ni ya kawaida kwa manholes kujengwa wakati huu. Kwa kawaida, maisha ya kawaida ya manholes ya saruji ya prefabricated ni miaka 50. Hii inamaanisha kwamba sasa ni wakati ambapo manholes kujengwa katika miaka ya 1950 na '60 ni kuanza kushindwa. Mashimo hayo yanahitaji kuchunguzwa na matatizo yoyote yanahitaji kushughulikiwa haraka iwezekanavyo. Katika karne ya 20, baadhi ya manholes walikuwa kujengwa na kutupwa katika mahali saruji. Hali ya aina hii ya manhole kawaida inategemea ubora wa mchakato wa ufungaji. Kwa mfano, ikiwa chuma cha kuimarisha kilitumiwa, ni kiasi gani cha saruji kinachofunikwa juu ya chuma hicho? Vituo vya maji vya kutosha au zaidi ya kutosha bado viko katika hali nzuri kwa sehemu kubwa, na kwa ukaguzi sahihi na matengenezo yanaweza kuwa na mzunguko wa maisha ya miaka 40-50. Wale walio na kifuniko kisichofaa tayari wanaanza kuharibika. Mzunguko wa maisha ya mifereji ya saruji ya kusukuma na ya kusukuma inategemea hasa mambo mawili: uzito wa mfiduo wa H2S na ubora wa ujenzi. H2S huathiri sana saruji na kusababisha kutu na, hatimaye, kushindwa kwa muundo wa shimo la maji. Mbinu mbaya za ujenzi zaweza kusababisha kutofaulu mapema kwa saruji au mipako. Katika ulimwengu wa ujenzi wa manhole leo, vifaa kama vile HDPE, saruji na nyongeza za kupunguza kutu na plastiki hutumiwa. Ni nadra kuona mashimo ya saruji yaliyojengwa hivi karibuni bila mipako. Kwa sababu ya ubora wa vifaa na ubora wa vifaa, viwanda vya ujenzi vinaweza kuwa na maisha ya miaka 50 au zaidi. Hata hivyo, nguvu ya kudumu ya T-kufunga na liners nyingine inategemea mkandarasi - T-kufunga liner vibaya imewekwa inaweza kushindwa ndani ya miaka ya kwanza 10. Baadhi ya manholes mpya kuwa HDPE liners imewekwa dhidi ya T-kufunga. HDPE ni nyenzo ya chuma ambayo hutengenezwa kwa unene zaidi kuliko T-Lock. Vituo vya saruji vya saruji vilivyo na mipako ya HDPE iliyowekwa vizuri vinaweza kudumu miaka 40-50 bila kuharibika. Pia kuna baadhi ya manholes kujengwa nje ya pete HDPE. Kuna faida kadhaa za aina hii ya ujenzi: 1) Haihitaji ufungaji wa liner, 2) Ni kiasi nyepesi, ambayo ina maana kwamba mengi zaidi ya inaweza kupakiwa juu ya lori, kutafsiri katika pesa kuokolewa na 3) utunzaji wa vifaa ni rahisi zaidi, vifaa nzito si muhimu. Kwa mfano, kwa mfano, viunzi vya pete za HDPE vinaweza kudumu kwa miaka 75 hadi 100. Kuna zaidi ya mashimo milioni 20 ya maji nchini humo na mengi ya hayo yamo katika hali mbaya. Hatua ya kwanza ni kuchunguza mashimo ya maji ili kujua hali yao. Mazoea bora kwa ajili ya ukaguzi manhole ni kutumia mchanganyiko wa teknolojia na kupata chini katika mitaro. Uchunguzi wa awali unaweza kufanywa kwa kutumia kamera ya CCTV. Ikiwa eneo fulani linaonekana kuwa katika hali nzuri, uchunguzi zaidi hauhitajiki hadi mzunguko ujao wa ukaguzi wa matengenezo. Kama eneo inaonekana kuwa katika hali mbaya au mashaka, basi confined nafasi kuingia katika manhole ni kufanyika. Mkaguzi anaweza kupiga juu ya saruji na "kupata hisia" kwa zaidi ya kile tu kamera inaonyesha. Hata hivyo, teknolojia husaidia sana katika sehemu hii ya ukaguzi wa kwanza wa shimo la maji. Kuangalia shimo kupitia kamera hupunguza idadi ya kuingia katika nafasi iliyofungwa, kuwahifadhi wafanyakazi salama na kupunguza gharama. Wachunguzi wa shimo la mlango ni kama wapelelezi <unk> uthabiti ni muhimu katika nyanja zote za uchunguzi ili kufanya zaidi ya vidokezo. Inspekta anahitaji kuangalia kwa karibu katika kasoro zote na kumbukumbu yake ya kuona. Hii ni kwa nini NASSCO ya Manhole Tathmini ya Cheti Programu (MACP) ni chombo muhimu. Programu hii inaruhusu uthabiti wa nyaraka na mchakato repeatable kwa ajili ya tathmini. Kwa njia hiyo, mbinu ya muda mrefu ya kurekebisha mashimo ya maji yaweza kuchukuliwa badala ya kuitikia tu matatizo yanapotokea. Lengo la MACP ni kufafanua sifa na sifa za muundo, kumbukumbu na kuelezea kasoro, kuendeleza viwango kwa kila sehemu inayotumika ya shimo la mfereji, kiwango cha muundo, kiwango cha O&M, kiwango cha I&M, na rekodi ya data ya vipimo ambayo inaweza kutumika kwa kuchagua mbinu za ukarabati. Njia hii ya kawaida ya kuripoti matokeo ya tathmini ya hali inaweka kila mtu kwenye ukurasa mmoja, inakuza ufanisi wa gharama na kuepuka kazi isiyo ya lazima ya ukarabati. Katika visa vingi, ukarabati wa manhole ni kuchaguliwa juu ya uingizwaji kutokana na gharama na vifaa. Wakati wa kuamua ni njia gani za kurekebisha kutumia kwa manhole maalum, ni muhimu kutambua kwamba mbinu moja-ukubwa-fits-wote si wazo bora. Hakuna mashimo mawili yanayofanana. Mahitaji maalum ya tovuti kama vile eneo la kimwili, upatikanaji na haki ya njia inaweza kuwa tofauti kwa manholes mbili wanaoteseka kutokana na matatizo sawa ya kutu ya ndani. Kuelewa kikamili magumu yanayohusika kutasaidia kuongoza mchakato wa kurekebisha tabia. Kwa mfano, shimo hilo liko wapi? Upatikanaji mbaya wa shimo la mfereji waweza kudhoofisha sana uwezo wa kurekebisha shimo hilo hususa. Je, kuna hatua zozote za kudhibiti trafiki zinazohitaji kuzingatiwa? Je, ni kwa sababu ya hali ya hewa, viwango vya juu vya H2O, au hali ya hewa? Je, ni lazima hewa na dehumidify? Je, kuna uvamizi wa mizizi na/au kupenya? Ni nini vizuizi vya gharama? Ugonjwa huo unahitaji kudumu kwa muda gani? Utaratibu wa miaka 50 sio muhimu ikiwa kubadilisha shimo la mfereji umepangwa kwa miaka 10 chini ya barabara. Kuwa na majibu ya maswali hayo kutasaidia kuhakikisha kwamba mchakato wa kurekebisha tabia unapita vizuri. Kulingana na majibu ya maswali ya juu na kasoro maalum ya manhole, mipako, liners au mchanganyiko wa wote inaweza kutumika kurekebisha uharibifu. Kwa upande wa vitambaa, epoksidi, urethane na vitambaa vya saruji vinavyotokana na aluminati ya kalsiamu hutumiwa katika kazi ya ukarabati. Njia nyingine ni kurekebisha vitambaa vilivyopo kwa kutumia vifaa kama hivyo. Arrowlock ni PVC-kama nyenzo ambayo hutumia adhesion mitambo kurekebisha kushindwa T-Lock liners. Kuna pia bidhaa nyingine, kama vile Linabond®, ambayo hutumia mchakato wa kushikamana kwa kemikali kufikia aina sawa ya ukarabati. Nyakati nyingine, huenda njia mbalimbali zikahitajika. Kwa mfano, kushindwa T-Lock® inaweza kukatwa nje na saruji chini ya coated. Ikiwa unachagua kutumia mbinu ya mchanganyiko, mbinu mbili za ukarabati zinahitaji kufanya kazi pamoja, na kiunganishi sahihi kinachotokea kati ya saruji iliyofunikwa na liner iliyobaki. Kubadilisha kamili au ufungaji wa liners muundo, kama vile Poly-Triplex® Liners, huja katika kucheza katika hali ambapo mipako tu si kutosha kurekebisha tatizo. Layer ya zamani ni kubadilishwa na mpya muundo imara kuimarisha tiba-katika-mahali liner. Bila kujali njia ya ukarabati iliyochaguliwa kwa manhole maalum, lengo ni kuunda kizuizi cha kutu ili kuzuia uharibifu wa manhole. Ingawa matokeo kuu ya shimo la kuanguka ni I-I, katika hali mbaya hali hatari inaweza kutokea kutokana na shimo la kuharibika, kuharibika. "Wikipedia: ""Wikipedia:Mwanzilishi wa V&A Engineering, kampuni ya uhandisi ya ushauri ya kujitegemea inayohusika na uhandisi wa kutu, usimamizi wa mfumo wa mipako, na huduma za tathmini ya hali." Pollack ana uzoefu wa miaka 35 katika usimamizi wa miradi ya maji na maji taka. Yeye ni mwanachama mwanzilishi wa California Water na Mazingira Association Kusini Sehemu Mkusanyiko System Kamati na ni Rais wa zamani wa CWEA San Diego Sehemu.