Monday, December 8, 2014

Design - Trenching and Back-filling

This post was written at an early planning stage whereby I intended to minimize cost by doing most of the work myself.  When it came time actually to do the trenches for the French drains and the AGS conduits, we were in the midst of the rainiest Spring in history. It became necessary to get as much work done as possible between rains so we enlisted professional help for the trenching as well as for grading for the slab floor.   It took several posts to cover the installation of the French drains and AGS conduits. Here are links to those posts:  First post on French drainsSecond post on French drainsLast post on French drains,  First post on AGS system.

Despite their irrelevancy, I am keeping the following paragraphs posted to demonstrated how ridiculously naive a DIYer can be when venturing into new areas.  

Safe Trench Work 
Nothing has kept me awake more than vacillating over the best way to dig the trenches for AGS conduits and the French drains then backfill back to floor level with enough compaction to support the house.  The French drains ideally should be about 10' below floor level in order to make sure the soil around the AGS conduits, at three to five feet below floor level, stays dry at all times.  

According to OSHA (A guide to OSHA Excavation Standard), it is not safe to work in a trench deeper than four or five feet even if it is pretty wide.  Above five feet, special precautions have to be taken that do not fit our budget.  The type and water content of the soil, the weight of nearby dirt piles and the use of heavy equipment in the vicinity are some of the variables that go into safe trench work.  To me as an amateur, this means that, if we intend to enter the trench, it should be no deeper than five feet, wider than is intuitive and vacated after heavy rains until the soil drys out. Or the trench could simply be used without entering it, which seems to be the best choice for the several reasons discussed below.


After the trenches are dug, they have to be back-filled and compacted enough to support the house.  The soil is clayey silt left behind by the glaciers.  It reliably supports structures only when left undisturbed, which is quite impossible with the amount of trenching we need to do.  Once disturbed, silt is difficult to compact even with proper equipment and ideal moisture content (US Military Field Manual FM 410, Chapter 8 in which Table 8-3 unequivocally recommends against our type of soil as back-fill under weight bearing structures).

Trenching Plan
Coming up with a plan for trenching and back-filling has involved a lot of online research, calls to equipment rental companies and consultations with a backhoe professional and a civil engineer friend (our soil engineer died before we had consulted him fully).  The outcome is that we will modify the original plan that was elaborated in Excavation and Excavation (Cont'd).  

Instead of using the track loader to cut trenches 5' wide and up to five feet deep for the AGS conduits, the trenches will be 8' wide.  Then 
DIY-friendly mini-excavator
the trenches for the French drains can be dug in the middle of the wider trenches with a 
rented mini-track-hoe.   The 8' wide trenches will serve three purposes:  (a) Provide plenty of space for the mini-track-loader, (b) allow placement of an AGS conduit at each edge of a trench thereby separating them by 8' or so and (c) allow French drain depth of no more than 5' but still positioned 10' below floor level.
However, we have decided on a radical departure from the typical design for French drains discussed in an early post Excavation (Cont'd) in order to be able to use a narrow trench safely and to minimize as much as possible the amount of backfilling with clayey silt.  

The typical drain is about a foot in cross-section, filled with gravel, a 4" perforated pipe with a geo-textile 'sock' running through it and wrapped on the outside with more geo-textile fabric. Therefore, the entire assembly can be visualized as nothing more than a crude 12" "tube" protected by fabric.

Customized French Drains
Second load of culverts

Accordingly, we have decided to use, as French drains, 8" ID corrugated plastic culverts wrapped in geo-textile fabric -- no gravel, no 4" pipe.  They will be pre-assembled by perforating the sides of the culverts, joining the 20' sections with couplers and wrapping the entire assemblies with fabric before lowering them onto a shallow beds of sand in the narrow trenches using ropes from above. The sand will allow us to use rakes from above to finesse the 1% fall for proper drainage (How to Slope a French Drain).  We will use hog rings to clamp the fabric.together after wrapping it around the culverts and before adding the ropes. Each of the seven drains will be at least 70' long.

Geo-textile Fabric Designed for Silt
A comment by the soil engineer (before he died expectantly) to the effect that our French drains would clog eventually with silt, despite the use of filter fabric around the pipes as well as around  the gravel, caused me to investigate geo-textiles online. Fortunately there was one excellent study (Research on geo-textile fabrics) that shed light on fabrics used with silt, which is the type of soil we are dealing with.  As a result, we ordered from Carthage Mills the fabric that tested best for use with silt, viz., woven fabric - 30% open area, pictured nearby as it arrived on the pallet.

Back-fill Plan
Footings and a slab floor should rest on undisturbed soil.  Unfortunately, there will not be much undisturbed soil left after excavating for the French drains and the AGS conduits.  Rather than do all of the back-filling with the excavated soil, the plan is to fill only the narrow French drain trenches with soil and, if it is too dry, attenuate it with limestone dust to draw moisture out of the air and make the soil more compactible.  The French drain fill will be compacted in "lifts" of no more than 8" using a borrowed plate compactor.  Alternatively, we may decide to backfill the narrow trenches with rock as described below for the wider trenches.

Rather than back-filling the wider trenches with the original soil and risking inadequate compaction for supporting the house, we plan to utilize the type of gravel, recycled from concrete that is used as a base under concrete highways.  It is cheaper than quarry gravel and a greener alternative.The fill will have to be built up in lifts of +/-6" and compacted with rented equipment, either a self-propelled steel roller vibratory compactor or a remote controlled roller.  Back-filling will stop at about the level of the soil that was left between the trenches initially.  Then the entire excavation will have to be prepared for the slab which is the subject of subsequent post.