STEP 1 & 2: DEMOLITION & COMPACTING THE SUB BASE
Most contractors are familiar with the general kinds of soil in their area. However, there are variations of soil types within the range familiar to the contractor. When a soil classification such as its tendency to hold water, drain fast or slowly, compact easily or with difficulty.
There are several ways to classify soil depending on its intended use. A common method for soils under pavements is to classify them by particle size into three groups – sand, silt and clay. This is illustrated as a triangle as shown in the diagram below. It is called USDA (U.S. Dept. of Agriculture) soil classification system.
Clay particles tend to lubricate any larger particles when wet. Soils with a percentage of clay lower than 30% have greater strength and stability since the lubricating influence of clay is not as great and water drains more readily.
Unfortunately, most of the soils in North America are clay, some good and others not very good as a foundation under pavements. A simple, fast way to quickly approximate soil classification in the field is by visual appearance and feel. If coarse grains can be seen and the soil feels gritty when rubbed between the fingers, then it is sandy soil. If the grains cannot be seen with the naked eye and it feels smooth, then it is a silt or clay.
A primary factor in the performance of soil under pavement is its ability to hold water. The higher the water holding ability, the worse the soil generally performs as a foundation for pavement. Here are some easy ways for the contractor to make quick field identification and thorough assessment of the water holding capacity of soils. They are described below.
Patty Test – Evaluation of the water holding capacity of a soil:
- Mix the soil with enough water to make a putty-like consistency.
- Form the sample into a patty, let it dry completely.
- The greater the effort required to break the patty with fingers, the greater the plasticity, or ability to hold water. In other words, the more water the soil can hold, the less suitable it is under pavement. High dry-strength is characteristic of clays. Silts and silty sands will break easily.
Shake Test –
- Mix a tablespoon of water with the soil sample in the hand. The sample should be soft but not sticky.
- Shake or jolt the sample in a closed palm of the hand a few times.
- If water comes to the surface, the soil is fine sand.
- If none or a little comes to the surface, it is silt or clay.
- If squeezing the soil between the fingers causes moisture to disappear the soil is sandy.
- If moisture does not readily disappear, then the soil is silty.
- If moisture does not appear at all, the soil is clay.
Snake Test – Evaluating the thread toughness for clay content:
- A small sample of soil is moistened to the point where it is soft but not muddy or sticky.
- It is rolled into a thread or “snake” between the hands.
- The longer the thread, and the more it can be rolled without breaking, the higher the clay content.
The field tests described above are quick and easy ones to classify soils and obtain a relative measure of their water holding capacity. This can provide general guidance on which compaction equipment to use on the soil. Specific guidelines are given in a later section.
STEP 3: INSTALLING THE BASE
Class II Road Base is used. This is composed of various sizes of gravel. Refer to the earlier picture of the sieve.
Certified base has correct quantities of all the above sizes so that it will compact to at least 95%. Imagine stepping on a field of marbles – they would move or even stepping in sand at the beach – not very firm. That’s because they are all the same size.
STEP 4: COMPACTING THE BASE
Since certified base comes with the right proportions, the end result will be a very firm sturdy base.