One of the most common ways of sampling soils for geotechnical evaluation purposes is the Standard Penetration Test (SPT) as defined in ASTM D1586. The test consists of driving a 24 inch long, 2 inch outside diameter split spoon sampler with a 140 pound hammer dropping 30 inches. The number of hammer strikes (blows) required to advance the 12 inches (1 foot) for the 6 to 18 inch increment of the 24 inch sampler is defined as the “standard penetration resistance” or the “N-value” which is reported in blows per foot (bpf). This value is fundamental for many of the different types of geotechnical calculations such as bearing capacity and settlement estimates.
Refusal of the spoon sampler is defined as a total of 50 blows over any 6 inch increment, a total of 100 blows per foot (bpf), or there is no noticeable advancement of the sampler during 10 consecutive blows of the hammer.
Rock Core Drilling and Sampling
When information regarding the rock at any given site is desired rock coring can be conducted in order to evaluate the type, quality, soundness, strength and other properties of the rock. Rock coring is generally accomplished with a diamond tip rock core barrel attached to a drill rig. The coring recovers a cylindrical core of rock by rotating and advancing the hollow core barrel.
The rock samples provide significant information about the engineering nature of the rock formation. Some important properties obtained from the rock coring are the type of rock, Rock Quality Designation (RQD), percent recovery and hardness/soundness of the rock. This information is used in a wide array of engineering applications such as foundation bearing capacity and rock anchors.
Core Recovery (%)
The easiest way to characterize the amount of material recovered during rock coring is to calculate “core recovery” as the amount (i.e. length) of material divided by the total length of the core run (presented as a percentage). (Ref. FHWA Geotech Engineering Circular 5)
Rock Quality Designation
Rock Quality Designation (RQD) (ASTM D6032) is a modified core recovery percentage in which the lengths of all sound rock core pieces over 100 mm in length are summed and divided by the length of the core run. Pieces of core that are not hard and sound should not be included in the EQD evaluation even if they are at least 100 mm in length. The purpose of the soundness requirement is to downgrade rock quality where the rock has been altered and/or weakened by weathering. For the RQD evaluation, lengths must be measures along the centerline of the core. The RQD is appropriate for all core sizes except for BQ and BX core with NX and NQ core size being optimal. Core breaks caused by the drilling process should be fitted together and counted as a single piece of sound rock core. Drilling breaks are usually evidenced by rough fresh surfaces, however for laminated rocks (i.e., rocks containing horizontally oriented fracture surfaces), it may be difficult to identify core breaks caused by drilling. In this case, the RQD should be estimated conservatively: for shear strength characterization it is conservative to not count the length near horizontal breaks whereas for estimates of rock blasting requirements, it is conservative to count the length near horizontal breaks.(Ref. FHWA Geotech Engineering Circular 5)
Earth Engineering conducts Rock Coring and Cone Penetrometer testing in PA, NJ, and Delaware. Contact one of our directors or project managers at any of our 4 locations to discuss your subsurface investigation needs and how Earth Engineering Inc. can help.