Bearing capacity

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Hi,

The most recognised test is the Californian Bearing Test. Comminly referred to as the CBR test. It is easily performed and gives comsistent results The test involves applyiny a progressively increasig load to a plate and measuring the deflection of the plate i.e the amount it sinks into the ground. It should be borne in mind however that all tests of this type do assume a depth of stratum beneath the test zone that is consistent over the Foundation area; therefore a series of tests will need to be carried out until an overall or average value is determined.

Rough and ready (on the move examples of the CBR technique involve jacking the plate downwards using the underframe of a large truck,not the most scientific way, but when out in the field and with an experienced operator a useful approach, you will appreciate that getting the stable reference datum is the tricky bit!

Most common method in NY is the Standard Penetration Test SPT ASTM D-1586. Drivng a 2" split spoon sampler 24 inches with a 140 lb hammer dropped 30 inches and counting the blows per 6 inches, usually take the middle two and add them up to get blows per foot ("N" value). The N value is cross-compared with the soil classification in a chart to obtain the allowable bearing capacity. Several city building codes use this method. One of the best references to find these charts is NAVFAC (Naval Facilities Design Manual) and "Geotechnical Engineering Investigation Manual "by Roy E Hunt. also most soils engineering texts. The SPT requires some type of drill rig. Other methods of determining soil bearing capacity for clay type soils are the cone penetrometer test (CPT) and a regular penetrometer. The cone penetrometer requires a special hydraulic drill rig with instrumentation, but a regular penetrometer is a hand tool with interchangeable tips. Plate load tests can be done, also but these are expensive and in most cases not cost effective. CBR tests are used here mostly for paving design. One could use a correlation chart to convert a field CBR value to a bearing capacity but this is not common here. The allowable bearing capacity has to be corrected with a couple of factors related to the depth of the water table and the size of the foundation. Big foundations will settle more for the same unit soil pressure. If ground water is closer than 5 to 10 feet, then this also necessitates a reduction in the allowable bearing capacity. An experienced geotechnical engineer familiar with the soil conditions in the area can get a pretty good estimate from visual inspection of a test pit but some type of subsurface investigation is recommended for critical, heavily loaded or high value structures. As you can see from all this, this is not a simple matter. The design soil bearing capacity has to take into account the proposed foundation loading, site geological history, the type of structure, the settlement tolerance of the proposed structure, the water table depth, the type of soil, the design frost depth and seismic considerations. Thus it is recommended that a professional be retained to perform this service. The methods and cost will depend on the structure. For a residence, a field inspection of a test pit by a geotechnical engineer for maybe a total cost of $1000. For a large and complex structure, this requires a drill rig for soil borings supplemented with laboratory tests and can run into many thousands. It is still a good investment, as the foundation is one of the most critical componenbts of any engineered structure. Sorry about the long winded explanation when probably a simple answer was wanted, but this is not a simple matter to do right and I speak from 30 years of experience in the Northeast as a licensed geotechnical engineer.

Hi JDknut.

Allow me to introduce myself. I spent my formative civils days as a soils Engineer (in the UK) and graduated from doing bore holes and performing bearing tests on both undisturbed and made up ground for the purpose of erecting high rise dwellings (up to 20 storey) to really exciting (!!) design work on earth retaining structures.

I recalled that there was literally 'no size fits all' when it comes to determining the bearing capacity of soils and you end up with a large slice over-design if you don't want to risk the firm's reputation and even PI cover.

I was inspired by your comment to look out my old faithful reference work '' Soil Mechanics in Engineering Practice'' by Karl Terzaghi and Ralph Peck and quote from Art.54 '' Year after year in almost every country a great number of load tests are performed. Yet ,most are worthless if not misleading, because the results are unfit for rational interpretation. If the subsoil consists of gravel or of sand containing large pieces of gravel, the number of blows on the sampler cannot be considered indicative of the compactness of the soil or the safe bearing capacity.

Each load test should therefore be made on a bearing plate 1ft square located at the bottom of a test pit at least 5ft square The load should be applied in increments of 200lb and increased to at least 1.5 times the estimated allowable soil pressure....." etc etc.

As you so righly say there are so many issues to consider when deciding how to go about determining the capacity of a soil before you even raise the issue of water table and annual fluctuation therof. It is no wonder that a simple rule of thumb is usually adopted unless the project involves a sophisticated structure which justifies the exotic costs attaching to a full soil investigation exercise.

Good chat look forward to the next one.

Thanks for the introduction. Terzaghi and Peck was my old textbook and still a useful reference. More than any other branch of engineering, geotechnical (or "dirt" engineering as my old boss used to call it) is a "seat-of-the-pants" field. The older and more experienced you are, the more you have seen in what works and what doesn't and the better a judgement factor you have to apply to observations, tests and calculations. Yes, when you have coarse soils, you have to use a judgement factor on SPT observations, can't just take them blindly, but in the context of other observations on the same site and the site's geological history. This comes with local knowledge which comes with experience. Field bearing tests are of limited usefulness, due to the size effect i.e. the larger the loaded area for the same unit load, the greater the settlement, however, load tests can be useful on full sized foundation elements such as piles or caissons to verify design assumptions.

The standard penatration test is the most common soil bearing test for foundations. It is an insitu test. However, it is only one of many that when taken altogether paint a true picture of the nature of the soil. There are certain soils that can behave differently when subjected to different kinds of loading. Even foundation can have different loadings. For instance silts may have a great better soil bearing test from the penatration test, but when subjected to a vibration and wet weather the may turn to jello.

The CBR is used for pavement design and is not usually applicable for building foundations, but it is a good test and is sometimes the test to run, because it tests the saturated condition on a molded sample.

Another common test is the dynamic cone penatration test. It is a test explicitly designed for footings (foundations). It is usually done in the field to confirm the comaction has been achieved.

The short answer is that there is not just one test that can always be used to determine the soil bearing capacity.

Standard Penetration Test (SPT)-ASTM-D1586