Propping of trenches and Deep excavations vs Pumping stations

[peteryoull123]

Hi all,

Im currently attempting to write a program to identify which type of sanitation system would be most cost effective in slums, based on a few simple input parameters about the site. As such, I am having to make some assumptions and estimates. I was hoping to get peoples proffesional opinion on a few things.

The first question I have:
I am designing a pipe sewerage network as one of the options under anaylsis. I have to make an assumption based upon how long it will take one worker to prop a trench. Whilst I am aware that it would not really be just one worker, and it will be entirely dependant upon the ground type I just need a ball park figure. The outcome of the program aims to be accurate to within 30% of the acutal costs and is only aimed at offering a suggestion as to which solutions will be most cost effective for further analysis. My initial thoughts would be that one man could prop around 2 sq.m of trench per hour, however I have no practical experience to base this on. (or a team of 2 men could manage 4 sq.m per hour). This calculation is simply to estimate the labour cost of propping a trench, and so in reality it could be a team of workers propping working over a much shorter time scale.

The second question I have, and I am aware this should be in a different area:
When excavating in undulating or hilly land, will the life cycle costs of installing very deep sewer lines (~6m) to allow the pipeline to cross trough to trough over a hillock be higher or lower than installing much shallower pipes with a pumping station to get the sewerage over the hill. It would seem logical to me that the deep lines would be more cost effective over 25 years, as you will not have to have electricity and maintenance on pumps etc, however I was wondering whether anyone has experience of this and could offer a suggestion.

Thanks in advance!, Peter

 

[MiketheEngineer]

OSHA has a lot to say about trenching and so should your soils guy. Are you really working in "slums"??

But something tells me you aren't in the US??

 

[dicksewerrat]

For trench work less than 10 feet, a single trench shield would work better than shoring. This also presupposes that a hyd. backhoe is used for excavation. Once you get deeper, you can stack shields.
going deep to go through the hills would better but connections would be difficult. Directional drilling through the hills may be a better way to go. And use shallow lines to pick up the services and run to a manhole.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[peteryoull123]

@MiketheEngineer,
Im not actually working anywhere. The program has taken 3 hypothetical sites so as to offer an approximate guide as to which solution is likely to have the lowest life cycle cost, which would then require a futher, complete design. As a result, I have no soils guy or OSHA to ask. Im not in the US, but the UK for the University of Leeds. I have taken 3 sample sites with different population densities and generated different topographical situations in order to attempt to offer pipeline designs covering all types of slum. The program is only to consider 3 population densities, so obviously a high level of error will be introduced here, likewise, there are 10 topographical options in place for each density, so futher error will be introduced here. I am only looking to give an indicative answer of which may be the lowest cost so that people can get an estimate within a few hours as opposed to undertaking a full pre-feasibility study to asses which would be best solution to be taken futher.

@Dickthesewerrat

I have assumed that a hydraulic backhoe will be used. I am asking the user of the program to input a value for "Propping of trenches per sq. m", Do you feel it would be more appropriate to ask for single trench sheild rental? I think it is unlikely that these will be readily available in all of the peri-urban sites that may be viewed, but I could be wrong. Like I said earlier, I am simply trying to estimate the man hours involved in the shoring of a trench, based upon the total square meterage which requires stabilising. Thanks for the info regarding the hill trenches, I had a strong feeling it would be that way, however its always best to get a second opinion

 

[dicksewerrat]

If you can get a hydraulic backhoe, you can get trench shields. If you can't, you are looking at very expensive excavation costs. Go to a couple of contractors near you and pick brains. you may end up with an internship or a job.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[MiketheEngineer]

I believe OSHA mandates trench shields over 4' (1.3M) or 1:1 trenching. Not sure but that comes from memory. I am sure that wherever you work - the requirements will be somewhat similar.

 

[dicksewerrat]

OSHA has a statement to the effect that trenches must be protected at 5 feet. They give a few ideas about this. .Stepping the sides sloping the sides, or trench shields.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

there can be a big difference between shoring and shielding a trench. shoring generally limits or eliminates any loss of ground. this can be especially important in areas with running sand or with existing buildings or utilities close by which could be damaged by ground movement. shielding protects workers but can allow more ground loss. OSHA is not concerned with loss of ground, only worker safety. Laying back trench sides in lieu of either shoring or shielding may not be feasible when working in tight quarters. As trenches get deeper and deeper it results in vastly larger amounts of excavation and wider trenches.

A "propping team" (for shields) likely consists of at least 3 laborers, backhoe operator to set the props and sheeting, truck to haul the props and other tools as well as to transport the laborers, and a supervisor to give orders. In order to increase production, you would need a second team. Bear in mind that most projects limit the amount of trench open and propped at one time, so one team is usually sufficient. The process would be considerably different for shoring.

 

[MiketheEngineer]

We have a bunch of "short" guys - so we use 4"

 

[dicksewerrat]

We only use short guys in the sewer tunnels.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com