By digging trenches, test pits, visual examination can be done. Visual examination is the first thing to be done, for obtaining soil data and it can be obtained from walking the site, while drilling holes, or from dug trenches or test pits. In terms of cost and schedule, test pits and trenches can be much better than boring holes. Direct visual inspection of soil strata can be done, if a trench or test pit can be dug deep enough practically. This also allows recovery of large intact sections of undisturbed samples easily. Through visual tests, grain size and shape (for at least the coarse grained portion), soil color can be seen and preliminary judgment for soil classification and soil properties can be made.
If the soil has organic content, it will smell. The degree of smell is somewhat proportional to organic content. Organic soils are not suitable for building anything on them.
A very small soil is put between teeth. When bit, the sand will feel harsh between the tooth. Silt, will feel somewhat harsh. Clay will not.
Only experienced geotechnical engineers or technicians can do this usefully. They take a sample of soil in their hands and can determine the texture and consistency of the soil.
By hand, first gravels can be picked and separated from the soil volume, and this will show us the proportion of gravel, which is a valuable thing to know. After that the remaining portion will be sand plus fine grained particles. It is not possible to separate sand from fine grained particles by hand. To do this, the soil can be poured into a jar full of water, and finer particles will settle slower than coarser particles. This way, at each interval, the settled portions can be separated to obtain different grain sizes and to determine grain size distribution.
Standard Penetration Test (SPT):
A very commonly used field test to know about soil properties is called Standard Penetration Test (SPT), where a standard hammer is dropped on a standard equipment and the number of blows (expressed as N) to drive the equipment for a certain distance (such as 6 inches) is measured. The more number of blows required to drive the hammer, the stronger, denser the soil. Through extensive past research and experience, the results of SPT have been well correlated with many characteristics of soils, such as the shear strength, dry density of soil, or it stratigraphy (how many distinctive layers are there, in what thickness, in other words, strata) and more. In other words, the results of this test can be used in a variety of formulas, that were introduced by many researchers, to estimate a variety of soil properties fairly accurately. This is especially useful when there is no other reliable means of obtaining that data. Although one of the most frequently used tests, one major limitation of SPT is that it is not suitable for gravels or clays. So the soil must be sand or mainly sand, in order for this test to have meaningful results. In fact, obtaining undisturbed samples for sands are very difficult or even impossible to conduct meaningful laboratory shear strength tests, and therefore SPT is usually the only or most reliable substitute, along with CPT as described below.
Cone Penetration Test (CPT):
Suitable for sand, silt and clay, but not for gravels. Through this test, strength of sand, undrained shear strength of clay, in-situ (existing) stresses in soils, pore pressures and a more continuous information of soil stratigraphy in comparison to SPT can be obtained. As SPT, this test is also very common and its results are used in many equations introduced by researchers, to know about soil properties. So one main advantage of this test is that it can also measure clay, where SPT can not. If you paid attention, we said “undrained” shear strength of clay. This is because, when doing this test, water in clay has no time to escape from clay, and therefore the situation represents an undrained situation. Knowing undrained condition is still very useful to know for short term loading when water will not have time to escape during an actual construction in field.
Vane Shear Test:
Suitable for soft to medium clays but not sands or gravels. Undrained shear strength of clay can be measured with this.
Suitable for soft rock, dense sand, non sensitive clays, but not suitable for soft and sensitive clays, loose silts or sands. Strength, hydraulic conductivity, in-situ stresses, OCR (as in Consolidation Settlement section that we covered before), elastic shear modulus (a constant of a soil, that indicates how strong it is under shear) can be obtained with this test.
Flat Plate Dilatometer Test:
This is suitable for sand and clay but not for gravel. Soil type, OCR, undrained shear strength, elastic modulus can be measured with this.
Plate Load Test:
Also called plate bearing test, this test measures the ultimate bearing capacity of the soil and the settlement amount under a given load, and therefore very useful for calculation foundation capacity in a more direct way, in the actual project site by applying load directly to the actual soil on site. The capacity of the soil that is closer to the surface is better understood with this test, and it is especially very useful for designing temporary structures that will aid in construction of the actual structure, but it is also useful for the structure itself.
The test is done by first excavating the ground down to a depth of the actual bottom level of the future footing (which means this test in only suitable for shallow foundations, not pile foundations), and then pressing a steel plate to the ground. Since we know the area of the plate and the magnitude of the load that we apply, we can calculate the stress on the soil, while the settlement amount is constantly monitored as well.
When the settlement amount starts to suddenly increase at a greater rate, the test finishes which means the soil has failed. The area of the plate is smaller than actual foundations, therefore, same stress can be obtained with smaller loads, in comparison to actual structure loads, which makes the testing easier, but of course too small plates must not be used, to get meaningful results.
The plate sizes, loading amount and rates, what constitutes sudden settlement, test methods are defined in standards. To apply the load, the loading jack must get reaction (as in Newton’s basic action-reaction principle) support from somewhere, which are provided by heavy beams or equipment or many heavy sandbags or other weight placed near the loading equipment.
The settlement results from this test can be used to calculate the actual settlement expected from footing, by proportioning the plate size to footing size, with some modifications as needed. Then these results can be compared to a theoretical method that is used to calculate the settlement. For example for clay soils, the settlement can be simple calculated with direct proportioning of the plate and foundation area as :
where, S stands for settlement, B stands for width, and the subscript f for foundation and p for plate
For sand, another formula with similar logic but with some modifications is used.
In addition to settlement, another very important result from this test is determining the soil elasticity modulus. Just like the elasticity modulus we saw before under the structure section for structural members, soils also have elasticity modulus. Similar to the one that we described before, it is found by diving stress by strain.
This is for measuring water pressure within a soil mass. It is required for example for measuring pore water pressures under and within dam foundations to make sure that it is not too high to cause any uplift pressure to the dam, or too much reduction in effective stress of the soil (as in σT = σ’ + u that we saw before, as u gets higher, for the same total stress, effective stress σ’ decreases) or, in case of slope stability, to make sure that water pressure is within acceptable limits. Piezometers are installed at many or at least several locations throughout the soil mass, as required.
Nuclear Moisture and density test:
This test is performed to determine the soil’s moisture content and dry density.
Sand Cone Test:
This test is used to determine the density of the natural soil in place.
To determine the strength (which means shear strength) of soils in place under controlled moisture and density conditions.
Other Field Tests for soils:
There are other field tests here which can be too detailed to describe for the purpose of this book, but we will briefly mention here. These are:
- Wet shaking test (for particles passing #40 sieve)
- Roll test (for particles passing #40 sieve)
- Ribbon test (for particles passing #40 sieve)
- Dry Strength test (for particles passing #40 sieve)
- Shine Test
Groundwater Tests (observation wells):
Evaluating groundwater condition at a site is an important part for all geotechnical investigation work. Through observation wells groundwater level, permeability, seepage rates, pore pressures can be determined.
Soil testing 1
Water is pumped from test well at a constant rate, which lowers the groundwater level. By measuring water height in observation wells at different locations, permeability of the natural soil can be found, which depends on the heights of groundwater level at the observation wells with respect to testing well (h1 and h2), the distances of observation wells with respect to the test well (r1 and r2), and the flow rate of water that is pumped out (designated as q).
Stands for “Rock Quality Designation”, is a very important property for rocks. This is determined by obtaining a continuous long sample of rock from the ground. The more continuous the sample without any fractures or cracks, the better the rock quality.
In the next post of this series, we will discuss “Laboratory Soil Tests”