Sunday, November 2, 2014

Construction - A Month's Worth Of Digging

By the Yard It's Hard, By the Inch It's a Cinch
If "by the yard it's hard, by the inch it's a cinch" capsulizes our upcoming two-year DIY home building project, excavating a big hole with a track loader and storing the dirt some distance away gives new meaning to the phase.  
The pillars of soil delineate the footprint of the house. (Click to enlarge) 
 It has taken 80 hours on the track loader hour meter and a month's time to rough in the excavation for the house shown in the picture. A layer of "hard-pan" that should have been anticipated but wasn't (see "Soil Engineers Report
" below) proved to be barely penetrable with the track loader and dealing with it has added many hours to the actual digging time. Two other factors added time as well -- three soaking rains plus the fact that the dig is larger than the actual footprint house in order to facilitate trenching for French drains, simplify batter board installation and provide access for the Ready-Mix truck at floor level.

Pre-construction Test Holes
A total of four test holes were made and fitted with piezometers prior to construction in order to monitor the water table through several seasons.  As the soil was removed from the drill, it was immediately placed in Zip-loc bags, carefully labeled as to depth and stored away with no intention of testing it.  As we bagged the samples, the drilling contractor used his experience to "questimate" soil types for the various layers, ranging from silt in the topsoil and for a short distance below and then what he called either "silty clay" or "clayey silt" for the deeper soil down to 20'. His questimate was pretty accurate.

Soil Engineer's Report
Based on a site visit and looking at the soil samples, a soil engineer reported that the soil below the topsoil was "lean clay to silty clay" for the first five feet then "transitions below this depth to a clayey silt.  He also said that "(t)he soil is a wind-blown deposit (Loess) and is known to be more than 50 feet thick in this area".  With regard to the water table, he recommended at least three more widely-spaced piezometers outside the footprint of the house in order really to know the behavior of the water table under the building site.

French Drains
After collecting data from the three new piezometers for a year and from the original one for three years, the soil engineer sketched a design for a French drain system on a napkin at a restaurant then unfortunately passed away before forwarding a formal protocol and design.   After several unsuccessful attempts to find another soil engineer who did individual residences, we opted to go ahead with a design based upon the engineer's sketch and, to compensate for any amateur errors, with some intentional over-engineering.

Contrasting Soils

The top soil has been easy to identify during excavation by its unique darker, gray-ish color in contrast to the orange-ish color of the deeper soils.
The gray-ish topsoil in the foreground,; orange-ish deeper soil beyond.
Both kinds of soil take on the consistency of flour when dry and the consistency of mayonnaise when wet . Where the track loader passes back and forth between the dig and the soil storage areas during dry periods, the soil becomes so powdery that the combined action of the loader and the wind makes it necessary to wear a mask for protection against the dust.  

The "Lean Clay" Layer

The "lean clay" layer that the soil engineer mentioned in his report is what I grew up calling "hard-pan".  It turns out to be a nearly impenetrable layer of dense clay several feet thick that nearly over-matches the loader.  It is too strong to pry up with the bucket and too hard to peel off in layers. About all I can do is to stab at it with the tooth bucket from various angles to weaken it enough to pick it apart.  Below the "hard pan", the "clayey silt" becomes manageable with the track loader.  And all of the layers below the gray-ish top soil, are orange-ish when freshly dug.  I think it is correct to say that the gray soil on top and the orange soil deeper are slightly different forms of loess but  have to be managed similarly.

Excavation Beyond Letting In the House
Earth removal for letting the house into the slope probably comprises only half of the total excavation.  A series of deep trenches  below the eventual floor level will be necessary for the French drains at a 10' depth and for the AGS conduits (AGS conduits) at mid-depth. The vacillation about the best way to do the trenching is the subject of another post (Construction - Trenching and Back-filling).

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