Description
Cone Penetrometer Technology (CPT) is a method of providing real-time data for use in characterizing the subsurface, as opposed to older methods of analyzing subsurface conditions in the laboratory. It consists of a steel cone that is hydraulically pushed into the ground at up to 40,000 pounds of pressure. Sensors on the tip of the cone collect data. Standard cone penetrometers collect information to classify soil type by using sensors that measure cone-tip pressure and friction. CPT is often used in conjunction with Hydropunch tests, which use the CPT holes to extract groundwater for laboratory analysis. An innovation of the CPT (i.e., the wireline CPT) allows multiple CPT tools to be interchanged during a single penetration, without withdrawing the CPT rod string from the ground.
Initially developed to collect information about soil characteristics, as sensor technology was developed CPT also became a platform for collecting information about a variety of contaminants. Recent advances in sensor technology have expanded cone penetrometer capabilities to detect the presence of petroleum hydrocarbons. Sensors are being tested or demonstrated for the detection of other organics, compounds, metals, radioactivity, explosives, and soil moisture.
Cone penetration testing (CPT) is the most versatile device for in situ soil testing. Without disturbing the ground, it provides information about soil type, geotechnical parameters like shear strength, density, elastic modulus, rates of consolidation and environmental properties. Further, as it can be seen as a small scale test pile, it is the best and most cost-effective device to design piled foundations and sheet piles.
Limitations and Concerns
CPT cannot be used at some sites due to high soil density. Most sensors are now used as screening tools that provide initial site characterization data. The data is confirmed by collecting samples that are analyzed in the laboratory. This is due to limitations in sensor technology, and it will likely diminish in importance as the technology improves.
CPT is useful on sites that contain unconsolidated sediments (e.g., soil and clay that are not cemented together). On the other hand, sites with large boulders, rock or cemented layers are difficult to penetrate.
CPT sensors, such as lasers, that require a lens may be hampered by fouling of the lens due to a reaction to dust. Decontamination may be necessary if the CPT comes into contact with contaminated material.
Cone Penetration Tests are conducted to obtain the cone resistance, the side friction and, if there is a piezocone, the pore pressure. The soil type can be determined by analysing these results, the values can also be used in the design of shallow foundations through the estimation of stiffness and shear strength of cohesive soils.
A 60o cone with face area 10cm2 and 150cm2 'friction sleeve' is hydraulically pushed into the ground at a constant speed (ranging form 1.5 to 2.5 cm/s). The force required to maintain this penetration rate, and the shear force acting on the friction sleeve are recorded. The friction ratio (cone resistance / side friction) gives an indication of the soil type.
Cone Resistance qc = Fc / Ac
Side Friction fs = Fs / As
Friction Ratio Rf = fs / qc
Where Fc = pushing force, Ac = cone plan area, Fs = shear force on friction sleeve, As = area of friction sleeve.
source : civcal.media.hku.hk, www.conepenetration.com, www.cpeo.org
California Bearing Ratio C.B.R
The
California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR
@ 100%). It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.)
(AASHTO, 2000). It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally. As a result, most agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.
The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05") per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.). Figure Below is a sketch of a typical CBR sample.
CBR Sample
Values obtained are inserted into the following equation to obtain a CBR value:
| where: | x | = | material resistance or the unit load on the piston (pressure)
for 2.54 mm (0.1") or 5.08 mm (0.2") of penetration |
| y | = | standard unit load (pressure) for well graded crushed stone |
|
| = | for 2.54 mm (0.1") penetration = 6.9 MPa (1000 psi) |
|
| = | for 5.08 mm (0.2") penetration = 10.3 MPa (1500 psi) |
Table below shows some typical CBR ranges.
Typical CBR Ranges
Typical CBR Ranges
| General Soil Type | USC Soil Type | CBR Range |
| Coarse-grained soils | GW | 40 - 80 |
GP | 30 - 60 |
GM | 20 - 60 |
GC | 20 - 40 |
SW | 20 - 40 |
SP | 10 - 40 |
SM | 10 - 40 |
SC | 5 - 20 |
| Fine-grained soils | ML | 15 or less |
CL LL < 50% | 15 or less |
OL | 5 or less |
MH | 10 or less |
CH LL > 50% | 15 or less |
OH | 5 or less |
Standard CBR test methods are:
AASHTO T 193: The California Bearing Ratio
ASTM D 1883: Bearing Ratio of Laboratory Compacted Soils
Source : training.ce.washington.edu
Other source :
The
California bearing ratio (
CBR) is a penetration test for evaluation of the mechanical strength of road subgrades and basecourses. It was developed by the California Department of Transportation.
The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The CBR test is described in ASTM Standards D1883-05 (for laboratory-prepared samples) and D4429 (for soils in place in field), and AASHTO T193.
The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR over 80. The standard material for this test is crushed California limestone which has a value of 100.
 | = CBR [%] |
 | = measured pressure for site soils [N/mm²] |
 | = pressure to achieve equal penetration on standard soil [N/mm²] |
source : wikipedia
CBR Component :
- Swell Plate : Contact end of the stem is easily locked in place with a knurled nut.
- Swell Tripod Attachment : Metal tripod supports dial gauge for measuring the amount of swell during soaking. Attachment is used with swell plate. Order dial indicator separately.
- Dial Indicator : Dial indicator has 1.000" operating range, graduated in 0.001" divisions, clockwise movement and revolution counter.
- Cutting Edge : Machined from seamless tubing with a sharpened edge to enable undisturbed samples to be taken in the field, cutting edge is plated for rust resistance. Cutting edge has 6" (152mm) ID and is 2" (51mm) high. Recess in upper section allows edge to be mounted at either end of the Compaction Mold or CBR Mold with Perforated Base mold to facilitate sample removal in the field.
- Filter Screen :
- Filter Paper : used to separate spacer disc and soil in the CBR mold during compaction operation or over the top surface of the soil when the compaction operation is completed.
- Surcharge Weight : Used in the application of surcharged loads on the soil‘s surface during soaking and penetration. Rust-resistant, plated annular disc weighs 5 lbs. (2.3kg), 5-7/8" (149mm) OD with a 2-1/8" (54mm) ID hole in center.
- Slotted Surcharge Weight
- Spacer Disc : Disc is used as a false bottom in a soil mold during the compaction process.
- Penetration Piston : CBR Penetration Piston has 3 sq. in. (19.35cm2) base area and is about 7-1/2" (191mm) long. Designed for use in conjunction with weights Surcharge Weight and Slotted Surcharge Weight to apply penetration surcharge loads.
- Dial Indicator Bracket : Bracket used to attach a dial indicator to the penetration piston.
- Swivel Base : for mechanical jack
source : humbolt
TYPE OF CALIFORNIA BEARING RATIO
1.
1. SCOPE
Description of Test
This method describes the sampling of the subgrade for California Bearing Ratio (C.B.R.). The resulting information is used for pavement design thickness.
2. APPARATUS AND MATERIALS
Equipment Required
- A suitable rig equipped with a 150 mm auger.
- Large cotton bags (with tags).
3. PROCEDURE
Test Procedure
- Take a 35 kg sample at every third group index sample location, spacing may be increased on projects where the material is found to be very uniform. The district materials engineer or his designate will advise on the spacing of sampling. Extreme care must be taken to ensure the sample is not contaminated with prepared Subgrade material (top 5 - 15 cm).
- Take a group index sample and C.B.R. sample from the same hole.
- The group index sample is taken first, then enlarge the hole to obtain the C.B.R. sample.
- Ensure neither sample is contaminated from loose surface materials.
- Record control section, station and offset at which the sample was taken on two tags.
- Place one tag inside the bag and attach the other to the outside of the bag.
- Keep samples dry while stored in the field and transport to the district lab at earliest opportune time. A special trip is not required.
4. RESULTS AND CALCULATIONS
Reporting Results
Record control section, hole number, stationing and offset for further testing as proctor density is completed in the district lab and the C.B.R. testing is completed at the central lab in Regina.
5. ADDED INFORMATION
General
Refer to diagram in STP 104-5 for sampling procedure.
C.B.R. sampling is required on all subgrade types with the exception of clean sands
