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As science and technology develop, wastewater treatment has received widespread attention by people. Demand for the ecological environment has been rising, more and more sewage water can discharge after achieves the standards. Wastewater treatment is the basely public service for people provide pure water and protect the environment. Wastewater treatment plants are using more energy to treat wastewater due to the population growth. Related to more regulations, more wastewater will be considered to be reuse after treatment.
Basically there are two types of wastewater treatment systems which are a physical or chemical treatment plant and biological treatment plant. A chemical treatment is used for treat industrial purpose which means the chemical plants use chemical reactions and processes to treat the wastewater which contains chemicals can pollute environment. On the other hand, biological treatment plants can break down waste by using bacteria.
The wastewater treatment plant is consist of concrete biological trickling filter, concrete pump station, control building and screening plant complete etc. during this project, biological trickling filter is big parts of construction, because, some trickling filter is made by more than 35 meters radius and 10 meters high cycle or rectangular shape. There are three basic types of trickle filters which are used for treat rural sewage, municipal sewage and industrial wastewater. The trickling filter is consists in a bed of solid media which includes rock, plastic modules and gravel and Spraying or screened sewage over beds of rock. Most of biological tanks leads to the circle shaped reinforced concrete structure which is consist of radial walls. The roofs of the biological trickling filter are made by remove cover to make removal of the equipment.
There are two options to build the trickling filter tank; one option is the trickling filter tank is built based on deep foundation on the working site to support the heavy load of wastewater and the concrete panel of the circle wall. It needs lots of stones columns to ensure the trickling filter setting on a firm foundation. Another option is that trickling filter tank can be built on the reinforcing concrete factory when the tank is done and it will be transported to the working site and assemble to the deep foundation.
(Issues and how to solve it when the contract is done)
There is a largest infrastructure project in Gisbornes which is building a new wastewater treatment plant by HEB Construction Company. This project contains pumping station, screen facility and a biological trickling filter tank, those will be built together to treat wastewater from different industrial. The project works is done by constructing 1335 stone columns which are 10 meters deep, 600 millimetre diameter and 1.5 meters apart. Firstly the good foundation is built on site, and then the BTF tanks will be built or transported to the work site. As mentioned before, there are two options to build the BFT tanks, HEB construction choose to build the BTF on site, because the BFT tanks has 35 metres diameters which means if the tank is produced by factory, the big issue is the tank cannot be convey to the working site. Related to condition for efficient use, the tank will be constructed by concrete panels which the panels are made by reinforcing concrete with steel.
Due to this project, those building and tanks is too heavy load in the bad ground conditions and need to be good foundations to support it. Firstly HEB construction using a crane and excavators to drill 10 meters deep and then displacing silty sand, then about 4 cubic meters of gravel was poured into the hole and compacted. More than 60 truckloads of concrete were transported to the site. The one third of concrete pours to form walls for biological trickling filter tank, which the concrete is reinforced by more than 40 tonnes of steel. The concrete was poured by in different times, because the base of tank has 600mm slab of concrete and comprising 94 cubic metres.
There are two types of foundations which are Non-piled foundations and piled foundations. Foundations can become further complicated by structure having a deep basement. Non-piled foundations divide into three specific types which include strip footings, pad foundations and rafts. Piled foundations include large displacement, small displacement piles and replacement piles. In each case, the choice is related to ground conditions and the loading conditions that have to be supported. For this BTF tank construction, they use piled foundations method to build deep foundation, because stone columns is an economical method for improving physical characteristics of weak soil.
Truck, excavator and drill machine will be used during this piled foundation
Methodology of Piled foundation
Stone columns are installed using either top- or bottom-feed system. Which method can be use depending on the soil condition that is consists of unstable, fine grained soils such as silts and clays. They are formed by using a hydraulic or electric vibroflot equipped with a pressure chamber equipment. When the machine is drilling and truck will pour the granular concrete to the bottom of the system and optimum compaction of the columns. First step is Penetration, under the t combined effect of vibrations, the drilling machine will be following its own weight and the pull down force to probe penetrates. When the penetration is processing, the soil is displaced and nothing will be destroy. Next step will be compaction, during this stage; the probe is lifted, the concrete falls down based gravity in the void that is created by drilling. It is good to making continuous injection of compressed air. The concrete are inserted and compacted by extraction. Re-penetration of the vibration probe through the repeated when the probe is lifting by 300 to 500 mm, each cycle step is being repeated until the probe reaches the surface.
Issue and addressed
When the stone columns are installed in the week ground soils, it may be failure to install because it will lose the cohesionless soils. In order to solve this problem, the stone columns essentially need to be increased the bearing capacity; the consolidation settlement of the ground under the loading is reduced. Stone column designs made without proper concept that will be cause the foundation of failure as well.
Errors in construction
As mentioned before in the vibro technique above, compaction of surrounding soil created a cavity, when the granular material is filled into that cavity, which has to be cohesionless. If the soil has cohesion, the disturbance reduces the surrounding soil strength, resulting in no densification. As a results ground improvement by stone column cannot be achieved in clays with sensitivity great than 3. Due to this errors, when the stone columns is used that is be testing to achieve the capacity by the resistance.
The Biological trickling filter is being made by precast panels off site; the concrete is compacted as it is poured and holding by wooden shutters. Concrete elements, Casted concrete tanks and Steel sandwich elements normally are using for the BTF wall material. For the BTF tank, HEB construction use 7 pre-cast concrete panels-lifted into place around the 32m circumference. All the work done It need 40 panels, each of them 8.8 metres tall and 8.5 tonnes and the panels are erected before concrete is poured in the vertical gaps. The structure is putting the tensioning steel cables in ducts within the wall panels. The tank also has covered which make the final height to 15 metres above the ground. Those panels are used cranes to lifting and combined to the foundation columns.
The advantage of pre-cast reinforcing concrete
It can minimize the erection time
Does not need formwork and false-work on site
High quality control
The whole construction can be pre-fabricated off site and delivered when need and saving working space and avoiding to storage.
Installation method of pre-cast reinforcing concrete
Precast concrete structure is widely used and also plays an important role in commercial industrial and training use. There are two types of pre-cast concrete structure which are individual components erected in place and volumetric units. The precast concrete panels are then transported to the construction site and lifted into place. The construction time is import and needed, it is reduced due to the fact that large forms are not constructed on site and it does not become essential to wait for the curing of the concrete. When install the pre-cast reinforcing concrete panels, first need suitable site access and safe unloading areas and corrected arranged to delivery of the units. The BFT tank columns should be measured and unites allocated to each columns. The concrete panels are lifted using suitable telehander with 2 leg, tested chain with safety hooks onto tested. Move panels into position with panels and once sides have been fitted. For panels positioned between the columns. Position panels between stone columns, pack to ensure even bearing at each end. Fixing packing material to ensure panels cannot possible shift out of position. After this, apply joint sealant to horizontal and vertical joints.
Crane is equipped with mechanical means for raising or lowering panels suspended by means of a hook or other load-handling device and it can move suspended loads both vertically and horizontally.
Healthy and safety
During the install the pre-cast concrete panels on site, health and safety should be considered all the times, because each panel has 8.5 tonnes. The main hazards include working with the cranes, working at heights and falls. Employees need to be trained and safety plan need to be set up to provide a safety environment.
Some failures of connections between precast concrete panels and columns, as a result of the earthquake, with the most commonly observed failures being bolted connections between panel to roof, panel-to-column and panel-to-panel. How to avoid this problem to be happened? When the design the pre-cast concretes the insufficient edge distance and in sufficient tensile capacity should be considered.
During the construction of the Gisborne Waste Water Treatment Plant was built, all the work is done be higher standard than previously. All domestic wastewater is processing through the new BFT system that enables fine wastewater solids to be treated. The total $39.5 million project came in under budget and on time. From this project, the methodology and civil engineering issue was be determined and making plan to studied and addressed.