STEP 1 & 2: DEMOLITION & COMPACTING THE SUB BASE

Chapter One

Soil Knowledge

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.

To determine the USDA classification of a soil, a sieve analysis is conducted on a sample.  Particle sizes are categorized as follows:

A sieve is used to test soil and base material.  There is a wire mesh at the bottom of each pan.  At the top the mesh will allow ¾” rock through, the next one down will be ½”, then ¼” all the way to the last pan that will allow only “fines”.  A fine is described as a particle that can pass through a #200 mesh. A #200 mesh has 350 openings per square inch.  This is actually like a fine cloth.  Water will not pass through a #200 mesh but fines will.  So these particles are very small!

Coarse sand 0.079 in. – 0.009 in.

Fine sand 0.009 in. – 0.003 in.

Silt 0.003 in. – 0.0002 in.

Clay Smaller than 0.0002 in.

The three-way percentage (by weight) of the particle sizes of clay, silt, and sand determine the classification.  (Note the dotted lines on the Triangular diagram.)  For example, a soil sample with 28% clay, 25% silt, and 47% sand would be classified as a clay loam soil.  This point is marked with a P on the diagram.

This classification system illustrates that soils with approximately 30% or more clay perform as a clay soil.  Clay soils hold moisture and are slow to drain, making them less desirable under pavements than other soils.  They can require more time for compaction and be slower draining. 

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.

Chapter Two

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. 

The base should be compacted and at least twice as shown below.

To achieve the desired level of compaction, 95% base should be compacted in “lifts”.  A lift is a layer of material, base, or dirt to be compacted.   For base, the lifts should be no more than 4” thick.  The compactor should be a very heavy duty machine that produces a minimum of 5,000 psi.  Smaller machines will never achieve the desired 95% compaction.

Base to be compacted well needs to have the right amount of water content.  Too dry or too wet will result in poor compaction.  To determine the right amount, a simple test would be to pick up a small handful of base and form a ball in your hand.  If the ball does not stay together in your hand you don’t have enough water.  If you drop the ball and it breaks apart, the water content is just right.  If the ball does not break apart, you have too much water.  If you can kick the base apart after you  have run the compactor over it, it is too dry.

STEP 5: INSTALLING AND SCREEDING THE SAND BED

The next step is the sand bedding.  Angular sand is used as mentioned above –a one inch sand bed is screeded in place and then the pavers are laid on top.

In the winter months it is important that the bedding sand and the joint sand is dry.  If the sand is too wet, it will be difficult for it to go between the pavers. 

For convenience, joint sand can be purchased in bags and can be broomed between the pavers.

Chapter Three

STEP 6: CUTTING THE PAVERS

The edges of the paver field are cut with a dry saw and each fit precisely in place.

The two basic methods for cutting pavers are either with a diamond saw blade or mechanical paver splitter. Because of their high strength, pavers are hard to cut. A paver splitter, with a little practice, will make a nice cut, but not as clean as a saw cut stone, and cannot make smaller precision pieces. For more precision cuts and difficult pieces around down spouts, drains, utility boxes, etc., a saw using a diamond blade is recommended.

Important note: If wet cutting, one must be careful to not drip or spray the contaminated water onto the pavers. This type of staining is extremely, if not impossible, to remove.  Pavers should be rinsed thoroughly before placing them into the laying field. It is best to use a dry cut saw.

Dry cutting is the best method of cutting pavers.  The machine is mounted on wheels and attached to a vacuum and pulled along next to the border where the pavers are cut.  The operator can pick up one stone at a time and rapidly cut it then move the next stone.  It is a very efficient procedure that gives a very clean look.

STEP 7: INSTALLING THE RESTRAINTS

Since pavers are set in sand, they need to be held together by something.  What holds them together is the borders or the outside perimeter.  This outside edge is referred to as a restraint.  Once they are finally held in place, they can not move yet the entire surface is flexible.   As compared to a poured in place concrete surface which is rigid.  As a word, flexible, best describes the interlocking concrete surface whereas rigid would describe a concrete surface.  There are several types of restraints as shown:

Chapter Four

STEPS 8, 9 & 10:  COMPACTING THE PAVERS, JOINT SAND AND CLEAN UP

Step 8: Compacting the pavers – Once the restraints are in place, the next step is to compact the pavers in doing so, the pavers are pushed into the bedding sand and the sand is forced up between the pavers.

Step 9: Sand between the pavers – Once this is done more sand, either the same bedding sand or joint sand that comes in bags, is swept between the pavers and the pavers are compacted again.  In using a vibrating plate compactor, all the joints between the pavers are filled.

Be sure to use angular washed sand with multi-sized grains. This angular sand will bind together during the compaction process. A problem with using very fine sand, even though it will fill the joints easier, it will also wash out easier causing a failure in interlock.  Actually fine silica sand will act like ball bearings and it will do the exact opposite of interlocking and will allow the pavers to move.  The bedding sand that the pavers sit on can be the same as the joint sand.

Step 10: Clean up – The final step is to sweep off the excess sand and then hose it down.