Earthwork

Earthwork

Some soil types react differently to earthquake ground motions and may slide, liquify, or settle which will result in a loss of vertical support for a house or other structures. Therefore, we need to consider the earthwork below and beside our building structures. Generally speaking, bedrock (deep and unbroken rock formations) will has less earthquake damage than deep and soft sedimentary soils. However consideration of known faults and landslide prone sites is of greater importance. Visit the building site location overview page to learn more about known faults.

Lot Preparation

Many of the buildings observed were built on sloped areas. Failure to prepare the lot and establish a path for water runoff can be potentially harmful and expose the building to slope failure.In cases such as these, at some point the building strength starts being compromised and consequently the finished structure will be unable to withstand natural weather and water conditions. Seismic movements can weaken or devastate structures built in this manner, making it easier to be then swept away by tsunami. Good building practices can better deal with situations like these. From the start of a project, proper land preparation should be considered because a well-prepared and stronger surface will better beable to sustain motion as well as the weight of the structure.

Compaction

To accomplish the best settlement of the building, the lot lows should be filled with material native to the site. Materials not equivalent to those found on the site such as garbage or material that will naturally decompose like wood or any other organic material should be avoided. A clean fill material is defined as uncontaminated, non-water soluble, non-decomposable inert material. Clean fill can include soil, rock, stone, dredged material, used asphalt, brick, block or concrete from construction and demolition activities [1].

It is best to compact the infill material to achieve a high-density base soil. A highly dense soil will properly sustain the weight loads imposed by the building. The compaction method will depend on the size of the project as well as the infill to be used to grade the lot. Site developers can use tools such as walk behind compactors or rammers (jumping jacks which perform impact, vibration, and kneading).

Compaction has a relation with the soils moisture content. A way to determine a high-water content is rolling a bit of soil between both hands. If a thread ¼ inch in diameter and about 3 inches long is created, this type of soil will offer poor support. If is standard ¾-inch coarse aggregate with fines processed with water which is being placed on the lot,squeezing a bit of material between hands is important. The level of moisture would be best if the tested soil barely sticks together. Also, a good way to quickly evaluate the compaction level of the soil is hammering a steel pin into the ground. The compaction level is good if the pin is hard to drive. Sample tables for the developer are illustrated below to quick reference the equipment to be used for the material to be compacted on the site.

Fill Materials

Fill material properties:

  • Gravel -high permeability, excellent foundation support, excellent pavement subgrade, not expansive, and very easy tocompact
  • Sand -medium permeability, good foundation support, good pavement subgrade, not expansive, and easy to compact
  • Silt -medium to low permeability, poor foundation support, poor pavement subgrade, some expansion, and hard to compact
  • Clay -no permeability, moderate foundation support, poor pavement subgrade, expansive, and very hard to compactOrganic -low permeability, very poor foundation support, very poor pavement subgrade, some expansion, and very hard to compact

Equipment applications:

  • Rammers for cohesive clay
  • Vibratory plates for granular soils
  • Reversible plates for sand and clay
  • Vibratory rollers for sand, clay, and asphalt
  • Rammax rollers for cohesive clay

Learn more

References

[1] http://www.yorkccd.org/wordpress/wp-content/uploads/2009/09/DEP-Fact-Sheet-Clean-Fill-Material1.pdf

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