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Join Date: Feb 2010
Location: Papua New Guinea
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Soak Pits

03/30/2010 7:50 PM

Hello there,

I work and live in a small town within the tropics. Every day it rains and the town is always flooding. I am given the task to identify possible soak pit locations and design soak pits for the whole town area. Since this is my first task on soak pits design and construction, can any expert in this help out?

Thank you,

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#1

Re: Soak Pits

03/30/2010 9:10 PM

You will need to know what the underground is about. Where I live is a good system. because the place is build up by coral that has been locked with limestone. It is about like a big Swiss cheese with plenty of holes in it. When you dispose to soak pits you will need to standardize the depths, because when people drain the cesspit into the ground at a given dept, and have wells at a different dept, and you need also to drain your flooding water, this all needs to be coordinated. To minimize your starting problems, you should check your deepest points on the surface where the flooding goes to. This minimizes your above ground drainage infrastructure. (water flows to the lowest point) Our land is low. Water wells are between 16 and 30 feet average. Cesspit disposal @ 40 Ft or more and drainage for rain water max 8 Feet. This is just an example!!! Again this is because our underground enables it. First you should check with the guys who drill holes there what the rules are and what works for your place.

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#2
In reply to #1

Re: Soak Pits

03/31/2010 2:24 AM

The above gives some very good points.

But here are some warnings:

- If you put the soak-away pits in the wrong locations, you can end up poisoning the town's water supply.

- If you haven't coordinated this with the town's future development including future homes and future utilities, you will be digging up your drains and the other utilities for the next 50 years at great expense

- Choose the right type of drains for the soil you are working in so that you don't have cracked drains that you need to replace every 5 years (and which can poison the town's water supply)

- Budget is all important. Make sure that you do estimates based on other similar projects, based on initial estimates from your first stab at design and then carry on updating your estimates. Make sure that you allow for contingency at each stage. Make sure that funding is in place and that those who are funding the project know what they are paying for and what they are getting.

I have assumed that you are quite new to the business and so have perhaps laid down some things that you already know. But there is no harm in saying. Hope you don't mind.

One last thing, if the town is always flooding, maybe the town is in the wrong place ? Maybe it is in a natural flood plain ? In which case it might be wise to relocate the town. I guess that depends on how big the town is at the moment and how big you think it will become.

One last last thing, if you are in the tropics, then mosquitos are going to be a problem if you let stagnant water around. The soak-away needs to be designed for this too.

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#3

Re: Soak Pits

04/01/2010 5:27 AM

http://www.rainwaterharvesting.org/urban/Design_Recharge.htm

http://www.slideshare.net/indiawaterportal/rainwater-harvesting-and-groundwater-recharge-in-urban-centres-experiences-from-the-field-biome-solutions

http://www.tn.gov.in/dtp/rainwater.htm

http://www.rainwaterharvesting.org/Urban/Components.htm

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#4

Re: Soak Pits

04/01/2010 10:21 AM

Dear Siga
Try the following: <http://www.cee.mtu.edu/peacecorps/reports/Brooke_Ahrens_Final_Report.pdf

A Comparison of Wash Area and Soak Pit Construction:

The Changing Nature of Urban, Rural, and Peri-Urban Linkages in

Sikasso, Mali

By

Brooke T. Ahrens

MICHIGAN TECHNOLOGICAL UNIVERSITY

2005

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#5

Re: Soak Pits

04/01/2010 11:32 AM

All excellent comments. Knowing the composition of the earth and the location of the pits relative to utilities and water supply is important. I would add these minor caveats:

1. Learn something about the percipitation - quantity and loading rate. This will have a direct effect on the size and number of soak pits.

2. Learn something about the surface use of each area that is tributary to a soak pit. Impervious surfaces, such as concrete, building roofs, etc. increase the rate and volume of runoff significantly. Open fields and woods tend to soak up a lot of rainfall.

3. Learn something of the surface flow patterns and position the soak pits accordingly. Try to position them so that they are equally loaded, or change the storage volume and infriltrating surface accordingly.

The comment was made in a previous reply that I concur with. If the flooding is occuring because a tributary tops its banks every time it rains, then protection from this type of flooding is different than water that pools in the area because it is raining faster than it can soak into available pervious ground.

Good Luck. It sounds like a beneficial and fun project to work on.

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#6

Re: Soak Pits

04/01/2010 1:26 PM

The things you need to do-

1. prepare a concept level study, this study should identify a number of conceptual level alternative locations where percolation facilities may be viable. The study should review availability of lands, any available soils, geologic and hydrogeologic mapping and information, local groundwater mapping and information, general area topography and drainage features (both municipal and natural). The Study should determine vaibility of alternatives based on this preliminary information, on the basis of estimates of percolation rates and required sizing of concept facilities (how much land is necessary and how deep), induced flooding, groundwater mounding impacts, water quality impacts and separation requirements, cost of lands aquisition and development, connections to local municipal drainage conveyance, proximity and risk from or impacts to local natural drainage features.

2. conduct site specific studies to evaluate the alternatives for viability, such studies might include preliminary geotechnical and hydrologic/geologic studies, preliminary land surveys, and planning/land use and title reviews.

3. reduce the alternatives to those sites deemed viable under the specific alternatives studies. There should be a very few alterntative sites at this point, for each percolation facility (less than 3, possibly even only 1)

4. Prepare a design concept for each facility at each viable alternative.

4. conduct site specific environmental review studies of the remaining viable alternatives and the concept design.

5. conduct concept-specific site-specific studies, particularly geotechnical, hydrologic/geologic and land surveying that are specific to the design concept as modified with environmental mitigations

6. prepare a design based on the design concept, findings of studies and resultant engineering parameters, and environmental mitigations. Prepare construction contract documents

7. negotiate land acquisitions and acquire lands.

8. go to bid and retain contractors.

9. build facilities

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#7

Re: Soak Pits

04/01/2010 2:50 PM

Hello

I can advise that first of all you should obtain topographical map of the town. and determine the lowest points on the map, here are the possible location for the soak pits. beside that if you can find geological map check out the layer of clay substrade. The soak pits are to be constructed on that substrade....

I hope I helped you

Sincerely

Selim Cosgun

Civil Engineer

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#8
In reply to #7

Re: Soak Pits

04/01/2010 3:04 PM

Actually, unlike a wastewater treatment plant, the lowest point in a municipality is not particularly the best location for percolation facilities as these tend to represent ponding depressions frequently, where fine grained lacustrine materials historically tend to have gathered in sometimes extrensive contiguous strata (features such as these depressions can sometimes be discerned early on depending surface modifications based on historical topography and aerial photographs). Additionally, it is generally better for municipal storm drainage to develop such facilities at a variety of decentralized locations, rather than one large centralized location at the end of the storm drainage system. One central location at the loest point will typically lead to a location where groundwater is the shallowest and presents the greatest impacts, and large discharges can exacerbate mounding and water quality related issues. Decentralized locations can spread the discharge load out presenting less of a localized mounding issue. It can allso allow a substanital reduction in the sizing of conveyance facilities over substantial portions of the conveyance lengths as the discharge points into the percoaltion facilities will be located closer to the points of collection. Also, something to consider is the application of dual-use facilities, such that they can provide other uses for local districts within the municipality such as recreational facilities, parks, etc, if properly located and designed.

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#9

Re: Soak Pits

04/01/2010 3:10 PM

First instruction set: The best places are these spots in the puddles where after the rain your feet (dis)appear the most under water. Plant a nice flag there to drill later.

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#10

Re: Soak Pits

04/05/2010 9:03 PM

Do not use soak pits in this situation. Use Evapotransipration with flEXible pipes with 0.5 mm diam holes, WUT PIES PLACED on the ground. Use in heaVILY TREED AREAS TO ALLOW THE WIND, LEAF DEMAND AND ROOT DEMAND TO TAKE UP THE MOISTURE

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