It is essential to have your soil tested and a report prepared before you start building your home or any construction work. Every block of land is different and if you have a report detailing the various soil conditions, you can use this report to select the depth and type of foundation suitable for a given structure to avoid problem of collapse building. A foundation failure of a major structure is measured in millions of money; it can even lead to loss of life. An ignorant or misinformed Engineer and builder can lose millions in unanticipated costs. Such mistakes often lead to major lawsuits that drag on for years and cost everyone except the lawyers. Here, I will discuss the process of soil testing, the roles of the engineer, building contractor and what you need to know before any construction work is undertaken.
KNOWING YOUR SOIL
Knowing what type of soil you have on your site enables your engineer to make the best possible decision about the suitable type of foundation for safe and long lasting building or structure. It’s hard to know what happened to the land before your purchase. Your block of land could be very different from the one next to you and a soil test is the best way to understand your ground condition.
By understanding your soil and knowing what lies beneath, you can make good and informed decisions during the building design process. Sadly, too many people forgo soil testing, decide to gamble and let the builder start digging the foundations. This can lead to expensive-to-resolve problems, such as finding out that the foundations have to be installed much deeper than originally planned due to expansive and collapsible soil, or hitting a water table, which necessitates having the engineer design a different foundation solution. This typically falls back onto the home or structure owner as their responsibility — and as an extra cost.
The way to prevent such an unwelcome situation is to commission a geotechnical investigation, sometimes referred to as a soil test or site investigation. Geotechnical investigation reports vary depending on where the property is, what type of building is going to be built or what requirements are set by the local council authorities.
WHAT IS EXAMINED?
Geotechnical report with basic clues that must be understood to make an informed good judgment of the soil conditions for a client will include:
Soil boring equipment utilize for soil testing.
The bore hole or test pit is advanced with a variety of equipment. Bore holes advanced with an auger means that the ground can be excavated with normal earthmoving equipment, i.e. backhoes, scraper, bulldozers and the like. Bores that are made with pneumatic, carbide tipped drill or similar rock drill means that blasting or heavy ripping will be required. Test pits are usually dug with a backhoe. The size of the backhoe makes a huge difference as to how easily the ground can actually be dug. A small rubber tired backhoe-loader will show refusal (unable to dig) on soils that can be readily excavated with large track mounted backhoes or heavy equipment.
Recommendations for construction methods
If the report is concerned with settlement, liquefaction and suggests over-excavation; that usually means weak clayey soils. While these soils may be easy to dig, they may also be unsuitable for backfill and wet and sticky. Even though clayey soils may appear to be firm enough to drive on, they can start to pump with repeated heavy-wheeled traffic. This is when the moisture is worked toward the surface by the equipment pounding and turns the top layer to mud and the equipment will get stuck. If this condition is present it may be necessary to build aggregate haul roads. Rain and snowmelt will turn clays to mud and may be difficult to dry out enough to travel on.
Soil boring logs
The boring logs will detail the soil layers by depth from the surface or by elevations. The log will contain such information as: soil classification, relative denseness of the soil, sampling points, sample recovery, water content, dry unit weight, blow counts per foot and ground water depth, drill refusal and if well casing was needed. Often the soil descriptions are subjective by being based on the experience and judgment of the observing geologist. How dense or hard the soil is often based on how quickly the drill can be advanced. The description is often based on the look, feel and sometimes smell or taste of the soil. The boring logs are a good place to start to understand the soil properties. The boring logs should be plotted on the drawings and on cross sections so the relationship of the excavation, structures is scaled to the soils and water table. If casing of the hole was necessary, it usually means the ground is too weak to stand on a normal construction excavation slope.
Soil description (classification such as silty-sand)
This will usually identify the soil by classification of particle sizing such as cobble, gravel, sand, silt, or clay, etc. Most soils are a combination of these classifications, meaning there is a gradation of material. Sand and larger grains are often referred as cohesionless soils. Without clay or silt fines sand, gravel and cobbles will not have cohesion (glue) to bind the soil and give it shear strength. Soils with clay or silt are often referred to as cohesive soils, as they are capable of developing significant shear strength
Soil bearing capacity
How strong the bearing capacity of the soil: Soils are measured for their allowable bearing pressure. The allowable bearing pressure is the soil’s ability to carry the load of a building and its contents without excessive settlement.
Soil properties (hard, stiff, dense, loose, etc.)
Usually soils are described as loose, dense, hard, stiff, soft, etc. The relative density terms of: loose, dense are applied to sands and gravels. Terms such as soft and stiff are usually applied to clays and silts. Hard can mean rock or cemented soils, although if the soils are cemented the geologist will usually mention that fact. Loose or soft ground means that the ground may not support a slope as steep as 1H to 1V. Dense sands or gravels are no guarantee that the gravels will stand on a steep excavation slope, as they may be tightly packed but have little or no fines to cause cohesion. Cemented soils can be caliches, volcanic tuff, or pyroclastic ash. These soils can be some of the most difficult and expensive materials to excavate. They can have a relatively low density and show as being relatively soft rock. However, they tend to absorb blasting energy and almost impossible to penetrate with a ripper tooth.
Usually the boring logs are advanced well below the planned excavation depth to insure that accurate formational trends can be plotted and no weak layers are present near the foundation grade. When the borings are terminated above or just at the planned excavation depth, you must be very wary. This means you cannot be sure of what may be encountered at the bottom of the excavation. Is there going to be rock or water that must be handled? Did an obstruction halt the drilling effort?
This is usually indicated at the bottom to the boring log, if it occurs. If all refusals are well below the planned excavation depth, it will not be a major concern. If a small percentage of the borings have met refusal in the excavation limits it usually means that there are isolated obstructions. It is not possible to determine what the obstruction is unless the soils report or site history reveals the object. A small isolated boulder can stop an auger drill and be of little concern. On the other hand, it could be a ridge of solid rock that will be expensive to remove.
This is a very important measure of soil properties. This is a standard penetration test where a 140-pound weight is dropped and the number of blows to advance the point one (1) foot is counted. Obviously the more the blows the harder the soil. Loose or soft soils will show blow counts of less than 10. Blow counts of 10 to 50 blows per foot usually mean the ground will be fairly easily excavated. When the blow counts are over 50 but less than 100, ripping of the ground is very likely. When blow counts exceed 100, the ground may be very difficult to excavate and require blasting or hoe ram effort.
Soil moisture content
Optimum compaction moisture is usually between 8 and 15% of the dry weight of the soil. Ground water will show about 25 to 40% moisture by weight of the dry soil. Supersaturated clay can be 50% water. Those are oozing mud that will not support the weight of even low ground pressure tractors. Optimum water content in the soil causes some cohesion, helps to control dust, and allows easy compaction effort.
Soil dry density
Normal soils will have dry densities between 95 and 130 pounds per cubic foot (pcf). Solid rock and heavy metallic ores can have densities exceeding 150 pcf. Loose cohesionless sand will usually have a density of about 90 pcf. Volcanic cinders can be as low as 50 pcf. Soil density by itself tells little about the soil properties. Volcanic tuff can have a density of less than 100 pcf and be some of the most difficult material to excavate. Dense rock that is fractured and/or weathered can often be easily excavated.
Particle distribution curves (sieve analysis)
This test will show the grain size distribution of the various soils. A well-graded soil will have a uniform grain size distribution from gravel to clay. A poorly graded soil will have the grain concentrated around a single grain size. Dry sands and gravels that contain less than about 10% clay can be almost completely cohesionless, meaning that while they are easy to excavate, they pull apart to a slope flatter than 1H to 1V. Over sized rocks such as cobbles and boulders should be indicated. The oversized material may have to be screened out for the soil to be used as backfill or embankment. If there is more than about 25% clay, the soil may be unsuitable for backfill or embankment. Unless the soil is very dry, it is difficult to screen clay out of soils because it clogs the screen. Clay can be removed by washing but it is a slow expensive process generating a lot of very dirty water that can be difficult to get rid of.
The basic soil types are:
1. Clays and silts: These are soils where the grains are less than 0.005 inches in size (less than #200 sieve size).
2. Sands: The grain sizes are between 0.25 and 0.005 inches in size. (#4 to #200 sieve size)
3. Gravels: The grain sizes are between 4 inches and 0.25 inches in size. (4″ to #4 sieve size)
4. Cobbles: These are rounded rocks that are between 12 and 4 inches in size.
5. Boulders: These can be over 20 feet in size.
6. Rock: Massive formations that require blasting or heavy ripping to excavate.
7. Any combination of the above. Most soils usually contain more than one soil type, for instance, a soil classified as sandy-gravel is gravel-containing sand.
1. ML – Silt
2. CL – Lean Clay
3. OL – Low Plasticity Organic Soil
4. OH – High Plasticity Organic Soil
5. MH – Elastic Silt
6. CH – Fat Clay
7. GW – Well Graded Gravel
8. GP – Poorly Graded Gravel
9. GM – Silty Gravel
10. SW – Well Graded Sand
11. SP – Poorly Graded Sand
12. SM – Silty Sand
The soil classifications are often shown as the combined ones when no single classification is accurate. For example, GW-GC stands for well graded gravel with clay. The soil classifications are another step in the process, but they do not tell us how hard or soft the ground is or what excavation difficulties are to be expected. The soil classifications refer only to the grain size of the soil and little of the other ground properties.
Relative denseness of the soil:
Here we are getting to the point where the soil properties are described. Usually the terms: loose, soft, dense and hard are applied to the soil. Loose may mean running soils that will not stand on slopes greater than 1.5 Horizontal to 1.0 Vertical. Soft may mean clay with high water content and stable construction slopes may be difficult to maintain. Dense soil is usually the best material to work with because it will support the equipment, be easily excavated and steep construction slopes can be established. Hard soils probably require ripping equipment and slower excavation productions
Atterberg limits (liquid limits and plasticity index)
This is a series of tests for plasticity index and liquid limit that are applied to clays. These are a concern since the specifications may require that the backfill and embankment materials have limited plasticity and liquid limit. This requirement is designed to control swell and settlement as the moisture content of the soil changes. This can force the disposal of the excavated soil and the import of more suitable soil. Often the specifications are silent as to the suitability of the native soils for backfill and embankment, leaving it up to the contractor to determine if the soil should be used. If field tests show the soil unsuitable, then the contractor pays the extra costs. Another common requirement is the sand equivalent (SE) of the backfill material. That is a measure of what percentage of the soil acts as sand. Sometimes the soil’s report will not contain these tests, but the specifications will use them to define suitable backfill and embankment soils.
Compaction test (optimum moisture for compaction)
This is very valuable information. By comparing the optimum maximum compaction density with the in place native density, a reliable calculation can be made for how much the soil will shrink from the ground to compacted fill. Typically, the shrink is between 5 and 15% for most normal soils. Soils with high clay contents can shrink more than 25%. Blasted or ripped rock will often swell as much as 25% as it will go from a void ratio of zero to 25% or more. If the in-situ dry density of the soil is 100 pcf and the optimum density is 120 pcf, at 90% compaction the density of the embankment will be 108 pcf. The ratio of 100 pcf to 108 pcf shows an 8% shrink from in-situ to embankment. Usually it is a good idea to add a couple of percent of shrink to allow for settlement, over build and over compaction.
Direct shear tests
This is very valuable information. This test shows the friction angle and the cohesion of the soil. The friction angle is a measure of the angularity of the soil particles to resist rolling. Cohesion is the measure of the bonding of the soil particles in shear.
.Seismic Velocity Lines:
When the geologist finds or expects formational rock to be present, often seismic lines will be run. This test places several geophones in a long line. Then a charge or hammer blows induces sound waves into the ground. A computer analyses the time delays that are recorded at each geophone. This will show the depth to each stratum and the speed of sound through the stratums. The faster the sound travels through the ground the harder the rock. Soil with sound velocities of less than 3,000 feet per second (fps) is usually easily excavated. Rock over 10,000 fps cannot be ripped efficiently.
Most rock that can be effectively ripped lies in the 3,000 to 8,000 fps ranges. The sound velocity is a guide to ripping only. Very hard rock, such as granite, that is fractured and or weathered can sometimes be easily ripped. On the other hand, relatively soft rock that is monolithic, such as volcanic tuff and caliches, can be practically impossible to rip efficiently.
This test is run to determine how quickly ground water will travel through the soil. Clean sands and gravels can transmit huge volumes and require massive pumping to lower the water table enough to work at the bottom of an excavation. Clays are impermeable and release the water slowly. Often the ground water is transferred through aquifers that are open-graded and very permeable. These aquifers can be capped with impermeable layers. There have been cases where dewatering wells were unnecessarily extended into aquifers and forced excessive pumping. These tests aid the experienced dewatering specialist in designing an appropriate dewatering system.
Soils that contain more than 30 to 40% clay will behave like clay because the clay will more than fill the natural voids of the larger particles. This means that the larger particles do not have direct bearing on each other and the failure plane is mostly through the clay fines. Clays can be some of the most difficult and undesirable soils to try to work with. Plasticity index and liquid limit are used to define Clays. Expansive or fat clays have plastic and liquid indexes about 50 or more. These materials will significantly swell and contract with the change in moisture content. This is often the cause of pavement failure, sidewalk buckling and foundation cracking. Clays can be plastic in that they can continue to settle under a foundation over a long term, causing cracks to develop long after the structure is completed.
Clays vary from being a viscous fluid to dense and stable material. Water content and relative compaction are critical to the soil properties. If there is too much water the clay becomes mud. If there is not enough moisture the clay becomes dust. Either condition causes the clay to be difficult to handle and compact. Even when the clay is at optimum moisture, it can be difficult to achieve the required degree of compaction. To add to the problem, it can be difficult to get the clay to dry out or accept additional water. Clays that contain high moisture content, above 20% or so may be unstable. The water instead of adding cohesion now becomes a lubricant, reducing the friction angle to nearly zero. This can result in catastrophic failure, with little or no warning. Rain or irrigation water that is allowed to pond can supersaturate the soil and cause a violent slide. Sometimes warning signs of impending failure will cracks forming in the ground parallel to the bank and/or sloughing, bulging and slumping of the bank slope. In short, clay is the least desirable structural and construction soil to work with.
Well-graded sands and gravels that contain 5% to 15% clay or silt fines can be the best materials to work with. They will excavate and compact easily. There are enough fines to hold the optimum moisture but will not continue to settle under structural loading. The fines and water will add cohesion and allow steep construction slopes.
Open or poorly graded dry sand and gravel can be difficult to work with. Open or poorly graded mean the grain size is concentrated at one sieve size. Clean dry beach sand is an example of such material and can be difficult to walk and drive through. It will not easily compact and added water quickly drains away.
Rock properties are often difficult to deduce from the geotechnical report. Although you may be able to tell if the rock must be shot or ripped from the descriptions and recommendations, it usually does not describe what the rock fragments sizes will be when excavated. Sometimes, the geotechnical sampler will have cores available for inspection. These cores will give you an idea of the texture, hardness, and natural joint spacing of the rock. Try to get a sample to hold, scratch it with a knife, and see if it breaks or crumbles easily. There is no better way to understand what the rock properties are than holding samples and observing the formation in place. Any soil can be in any state of consolidation and cementation. Sandstone was once sand dunes or river deposits that were compressed by over burden and cementing minerals leached into the formation over millions of years. Slate and shale were once mud on at lake or sea bottom. Lava is heat fused inert minerals that weather very slowly and is usually very hard, requiring blasting or hoe ramming to remove. There are thousands of minerals that make up millions of soil conditions. No two sites are exactly alike.
Only through study of the site-specific geotechnical report and a site visit can any real judgment of construction methods be made. It is also recommended that potholing with a backhoe be performed in the presence of the estimator for the project. Review the specifications to determine if the soil is suitable for backfill. The sieve analysis may show there is oversize that must be screened out. There may be too much clay fines. The Atterburg properties or sand equivalent may make the in-situ soil unsuitable and the material must be over excavated and/or disposed of offsite and suitable material bought and imported.
Failure to understand the geotechnical report has caused many a financial disaster. The contractor must bid aggressively to win work. It is better to know the real conditions and avoid costly mistakes that can cause a project to lose money out of pocket by ignoring or misinterpreting the information contained in the report. Each geologist will present his or her findings in different ways. Some reports are more thorough than others for several reasons. It is common to find that underground conditions vary from what was discovered by the test borings. At rare times the owner will even deliberately misrepresent the facts to gain lower bids. This usually results in a long expensive lawsuit over changed conditions that only the lawyers win.
Each soils report will contain a great deal of information. There is no one test or observation that will tell the contractor what is the best construction method for each site. Each test and observation is a clue. There are always dozens or even thousands of bits of information that must be scrutinized, analyzed, and correlated. This process can take several people weeks to prepare an excavation plan for a large project such as a dam. Only then can methods and productions be reasonably estimated.
Geotechnical reports always include disclaimers and warnings that the ground conditions may vary from those found in the borings and test pits. There is good reason for the disclaimers. Geotechnical work is still as much an art as it is a science. Any extrapolations derived from the reports are individual interpretations of what that person might expect to find. No one is able to see exactly what is under the ground surface. All the tests and observations only indicate probable trends that are often subject for debate between expert geologists.
Some information will need further examination, which is done by an accredited Geotechnical laboratory. Soil samples are taken from site and sent to the laboratory to complete a comprehensive soil analysis.
ROLE OF THE ENGINEER
The design engineer will review geotechnical report to know what is under the surface to support the structures. To that end, the engineer needs to know several critical soil properties to design the most suitable foundation solution:
What is the allowable soil bearing pressure?
What is the expected foundation settlement?
What is the active soil load?
What is the passive soil loading?
What is the sliding friction factor?
What is the potential for differential settlement?
What is the soil liquefaction potential during an earthquake?
What are the seismic design accelerations?
Where is the groundwater table?
What is a permanent stable slope?
Will piling be required?
Can the native soil be used for backfill?
What are the criteria for the pavement sections?
Are hazardous wastes present?
How corrosive are the soils?
Will there be voids, obstructions or unstable soils?
ROLE OF THE BUILDING CONTRACTOR
The building contractor wants to know what the subsurface conditions are so that an accurate estimate of costs and time can be entered into a competitive bid. To that end the contractor will search the soils report to determine the following:
Where is the ground water and how much water must be pumped?
Is there rock to be drilled and shot (sometimes blasting is not allowed)?
Can the ground be ripped with a bulldozer?
What excavation equipment and methods will be most effective?
Is there enough space on the job to store backfill materials?
Can the native material be used for backfill?
Will the native material need to be processed (screened/crushed) for backfill?
How much backfill must be bought and imported?
How steep can the temporary excavation slopes be cut?
Can obstructions be expected?
What compactive effort and equipment is needed for backfilling?
Will excavation shoring be required?
What is the most effective shoring method?
Will the ground stand long enough to use trench shores or shields for pipe trenching?
The answers to the above questions are given from the Geotechnical soil report? The report is directed mostly to the design engineer and the building contractor usually must make an interpretation of the information to develop a construction plan. This interpretation is often vital to the success of the project.
Talk to us for your upcoming project in Soil Testing
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