House Building Project at 60 Kangaroo Ground Road North Warrandyte

The Effluent System

Construction and Evaluation Report.

Contents

1. Synopsis
2. Background
3. Site Considerations
4. Integration with the rest of the project
5. Carrying Out the Work
6. Conclusion
7. Appendicies
Appendix 1. Record of Costs
Appendix 2. Quantity Surveyor
Appendix 3. The Specification
Appendix 4. Compliance Certificate
Appendix 5. Certificate to Use
Appendix 6. Suppliers, Contractors & Participants
Appendix 7. Invoices

1. Synopsis

The construction of the effluent treatment (septic Tank) system has taken much longer and has cost much more than estimated. It is argued that this is a result of unreliable estimates, rather than a failure of cost effective construction process. As the actual cost has proved to be the inevitable cost, that is a cost that would have to be borne by any party setting out to develop this land for residential use, it is argued that this cost represents a genuine addition to the value of the property. It is argued that any valuation of the property after the construction of this system must be greater than the valuation immediately before, by at least the magnitude of the effluent system cost.

2. Background

The Planning Authority for this property is the Shire of Nillumbik. The Shire places high value on the natural bushland value on property of this type. It fell to the Planning proposal to show how the property could be developed with a minimum of reduction in the bushland value. The same shire health department lay down requirements for the effluent treatment system. The proximity of the creek means that it is very important that the effluent be thoroughly treated. The requirements for the system are thus at the "high end".

Although the excavations for the septic tank and for the sand filter involve the movement of large amounts of earth, this is all in a concentrated area. The absorption trenches on the other hand have to cover a large area, and there is a potential for an impact on the bushland over this area. If narrow trenches are dug with a backhoe on an ordinary tractor, the bush destruction caused by the tractor would be more than caused by the actual trench. The extraction of the spoil from such a trench is a serious problem. Many drainers spread the clay over the ground. This causes more damage to the forest floor than the actual trench. In my case, to meet the requirements of the Planning permit, the spoil had to be taken away and used elsewhere on site. The planned sequence, which was actually carried out, was to take the top soil out of the bush setting and stockpile it, then remove the clay and rock and utilize it elsewhere on the project. After the absorption trench materials had been placed, the top soil was brought back from the stock pile and placed back where it had come from.

Although Melbourne and surrounding districts are well served with an industry of drainers who can build such effluent systems, there seems to be none at all who understand the requirements of such an installation in an environmentally sensitive situation. Several drainers attended the site to give a quote, and carried on a conversation like this:

Owner Builder: Many people just spread the clay over the ground, but it is a requirement of this job that it be removed for use elsewhere on the project.

Drainer: Nar! You don't have to worry about that. We just spread it around.

(Interview terminated politely but promptly, and search for a suitable drainer continues.)

It is a requirement that the Septic Tank Permit be applied for in the name of an authorized drainer, but such difficulty was experienced in the search for a suitable contractor, and the cost of the time devoted to the search was threatening the project. In addition, the permit had to be on hand before the Building Permit was applied for, and the Building Permit had to be in place for the House Loan to be established. The Shire Health Dept. Manager waived the requirement and substituted an ad hoc requirement that the inspections were to be called for by an authorized drainer. Thus the owner builder performed the earthworks under his own control, and a drainer was engaged to act as a consultant on the legal requirements and to place the pipes and liase with the authorities. This arrangement worked well.

3. Site Considerations

Apart from the bushland preservation requirements, there are other site specific factors that constrained the planning of the work.


The land is steep. The contour lines you see here are at metre intervals. The vertical drop from the front gate to the yard at the front of the house is 33 metres. The vertical drop from the front gate to the creek is 53 metres.

 


Here is a close-up of the development area. It is seen that one of the constraints is the narrowness of the land. The two buildings form an effective barrier to access from the driveway to the land behind them. This is the factor that determined that the effluent system would have to be completed before the construction of the house started.

4. Integration with the rest of the Project

Just as the gographical constraints dictated the separation of the building into two to allow both motor vehicle access to the garage and easy pedestrian access to the front door, the order in which the works were performed was tightly constrained. The house (the long narrow rectangle on the plan) could not be built before the effluent system as it would have blocked access to the works. On the other hand, it was necessary to perform the spoil extraction from the excavations at a time when some other aspect of the works was at a stage where fill was required. There is no room on the site to stockpile the quantities of material involved. The yard area in front of the barn was identified as a suitable site for the fill, so the movement of this material could not take place until the work on the Barn and the retaining walls and underground services had reached a stage where this fill could be utilized.


Fill was utilized to create the level area in front of the "garage door".

Thus the work had to be performed when the Barn was virtually complete, but when the house building had not been started at all. During the effluent system work, the house location area was occupied by an access track for the large earth moving machines.


Access track through house site

5. Carrying Out the Work

The land behind the house is as steep as any other part of the hillside. The gradient here is about 1 in 3. The small rectangle immediatly behind the house (to the right on the above drawing) is the septic tank. The larger rectangle is the sand filter. The two arc-shaped tracks are the absorption trenches. The drawing I have used here is derived from a Planning Permit application drawing. The final "in the ground" layout is much as shown, except that the septic tank is closer to the barn. (About half way down towards the word "proposed" on the drawing.)

One of the approved absorption trench configurations is thirty metres length of trench one metre wide. The smallest excavator can negotiate a track only one metre wide, and the smallest "loader" (BobCat-like machine), a "Kanger" is also this narrow. Thus it was resolved to use one metre wide trenches, and to run the earth moving machines in the trenches, thus elimenating the destruction of adjacent forest floor and flora. The use of the narrow machines also means that the access track to the trenches need only be narrow.

The required thirty metres of trench length was provided in two trenches of fifteen metres each.

Although the Kanga machines were useful in transferring soil along the trench lines, it was found that they could not reliably carry a load up the steep 1 in 3 grade. This was resolved with the use of a timber railed tramway and winch. Fortunately, some of the tramway equipment could be borrowed, and the hire of the winch cost less than the hire of a Kanga loader.

The tramway was used to transfer all the top soil to a stockpile, to transfer all the clay and rock from the trenches to the area by the garage door, to transport the scoria that was used to fill the trenches, and to return the top soil from the stockpile.


Tramway is under construction


A Turntable was used to access the Top Trench


The Top Trench had to be emptied by shovel
November 2003


The Bottom Trench was emptied by Excavator
Jan. 2004

"Reln Drain" is an approved product that is used in this context to form a continuous archway to form a space that will be occupied by the effluent. It was not mandated on this job, but was used as it saved many cubic metres of scoria, which, when the transport by tramway was factored in, was an expensive material.


Reln Drain in Place - Top Trench

After restoration of the native top soil, branches from trees that had been cut during the work were spread to cover the bare soil. These will minimize erosion, and the leaves will drop to form a natural mulch layer. When all the leaves have dropped, the branches will be removed for fire risk reasons.


After backfilling, the trenches were covered with branches

As soon as the absorption trench work was complete, the tramway was removed to make way for the large earth moving machines which were used to install the septic tank and make the sand filter. Rock was encountered just under the surface at the sand filter hole, and the twelve tonne excavator spent two days on this work. The Sand filter is six metres long by over three metres wide. It is one and a half metres deep on the low side, and two and a half metres deep on the high side.


Excavating the Sand Filter hole


The Sand Filter Hole is Huge


Pipe from Sand filter passes through a trench that is over 2m deep - mostly rock! The sand filter hole is lined with plastic and the outlet pipe installed. Then the special sand is added.


Jim Leonard the drainer - levels the slotted pipes that will distribute the effluent over the sand surface

When the Sand Filter construction was complete, it was covered with an overburden of a mixture of native top soil and sterile sand. Bush mulch was used to cover the remaining "bare earth" areas, as on this steep slope, bare earth is subject to severe water erosion.

6. Conclusion

All this work was carried out effectively and expeditiously, even though the duration was many months. Even with the wisdom of hindsight, there are no major changes to the procedures evident that could have been made to save on cost. The constraints affecting the effluent treatment works had been carefully taken into account in the project planning so that constraints for other aspects of the project did not impose additional cost onto this part of it. Virtually all the cost in performing the works described here have been inevitable cost. Other property owners might have chosen to develop this property differently. They might have arranged the buildings differently, but if the property was to be used for residential purposes, then an effluent treatment system of this scale would be unavoidable. If the property had changed hands in a completely unimproved state, except for the installation of this effluent treatment facility, it is clear that the value of the property would be increased over the unimproved value by at least the cost of the system, as any developer would have to incurr this cost at some stage anyway. This cost had not been predicted in this instance. The Quantity Surveyor, who prepared a report that was used to support the application for the house loan, used a figure of $7500, and he assured me that all costs would be within 20% of his figures. He was especially warned by me of the site constraints, but did not see a need to visit the site to help him with his report. Prices from contractor's quotations were not available, as no one could be found to quote for the work within the constraints of the planning Permit. Thus the only costing for this item that was available at the time the project was being costed for the bank loan application was the quantity surveyor's.

The actual cost of the work, $41,493 (see Appendix 1.) is a very good price indeed, as the value of my time has been thrown in for nothing. Extra time on my part was available as I used my long service leave to participate in this work.

Other costs such as the interest costs on the partly drawn-down house loan during the seven months that the project was held up by this work, and the "lost opportunity" cost resulting from the house becoming available for use seven months later than it would have been if this task had been executable promptly, are not included (about $14,000). The construction of "crib" retaining walls had to be performed concurrently with some of this work, as the spoil had to be utilized as it was removed. The cost of the wall construction is not included here.

7. Appendicies

Appendix 1. Record of Costs
Appendix 2. Quantity Surveyor
Appendix 3. The Specification
Appendix 4. Compliance Certificate
Appendix 5. Certificate to Use
Appendix 6. Suppliers, Contractors & Participants
Appendix 7. Invoices