The National Institute for Occupational Safety and Health (NIOSH), Office of Mine Safety and Health, Ground Control Engineering Branch is investigating the use of shotcrete in weak rock mass mines with the objective of reducing fatalities and injuries resulting from rock fall accidents. When shotcrete is used as part of a multielement ground support system, there is a need to know the support characteristics that the shotcrete contributes to the overall system. To quantify the support provided by the shotcrete, flexural strength tests were conducted with two common, commercially available fiber-reinforced shotcrete (FRS) mixes using the round determinant panel (RDP) test method, ASTM C 1550. A portable RDP test machine developed by NIOSH researchers was used to determine the peak flexural load and residual load capacity for a steel FRS and a synthetic poly FRS that were sprayed using dry mix procedures and equipment. Besides the flexural strength and loading behavior determined from these tests, a method was also developed for measuring the width of cracks exposed on the underside of a shotcrete panel during a RDP test and relating these measurements to residual load values. Quantifying the peak flexural load and residual loads of a shotcrete mix through on site testing at a mine and visually assessing the loading cycle and load-carrying capability of the shotcrete applied to underground workings will improve mine safety by providing a better assessment of the stability of shotcrete supported entries.
If the crack has not opened up wide, it will be able to create a new layer within a few short years. This wood is often raised higher than all the wood around it, and the bark is smoother due to the rapid growth. This will appear as a vertical or horizontal rib.
Most cracks will occur along the long axis of the trunk or branch, and are caused by the opposing compression and extension of the outer edges of the trunk or branch. This type of crack will appear on the outer edges of the tree trunk or branch, and has a split strait through it.
These cracks will occur along the short axis of the branch. They are caused by the tension strain (usually wind higher in the tree) exceeding the strength of the wood. These types of cracks will occur on the upper side of the lean, or on the side of the tree where the wind force direction is most prominent.
These types of cracks will occur when the compression force pushes down on the wood fibers, leaving them no place to go but outwards. This will happen on the underside of a lean or where the wind force direction is most prominent.
While plastic shrinkage cracks can happen anywhere in a slab or wall, they almost always happen at reentrant corners (corners that point into the slab) or with circular objects in the middle of a slab (pipes, plumbing fixtures, drains, and manholes). Since concrete cannot shrink around a corner, stress will cause the concrete to crack from the point of that corner.
An excessively wet mix is a contributing factor to shrinkage in concrete. While water is an essential ingredient in every concrete mix, there is such a thing as too much water. When the mix contains too much water, the slab will shrink more than if the correct amount of water was used. Hot weather is another big reason for plastic shrinkage cracks.
Just like a balloon, heat causes concrete to expand. When concrete expands, it pushes against anything in its way (a brick wall or adjacent slab for example). When neither has the ability to flex, the expanding force can be enough to cause concrete to crack.
Expansion joints are used as a point of separation (or isolation), between other static surfaces. Typically made of a compressible material like asphalt, rubber, or lumber, expansion joints must act as shock absorbers to relieve the stress that expansion puts on concrete and prevent cracking.
When the ground freezes, it can sometimes lift many inches before thawing and settling back down. This ground movement brought on by the freezing and thawing cycle is a huge factor contributing to concrete cracking. If the slab is not free to move with the ground, the slab will crack.
Although concrete is a very strong building material, it does have its limits. Placing excessive amounts of weight on top of a concrete slab can cause cracking. When you hear a concrete mix has a strength of 2000, 3000, 4000, or 5000+ PSI, it is referring to the pounds per square inch it would take to crush that concrete slab.
After a heavy rain or snowmelt when the ground below is soft and wet, excessive weight on the slab can press the concrete down and result in cracks. Residential homeowners who place large recreational vehicles or dumpsters on their driveways are more likely to see this type of cracking.
Crazing cracks are very fine, surface cracks that resemble spider webs or shattered glass. When the top of a concrete slab loses moisture too quickly, crazing cracks will likely appear. While unsightly, crazing cracks are not a structural concern.
If you decide that entering into Crack in the Ground is too technical, you may opt to follow an above-ground trail that looks down into the fissure. Just be mindful of where you step, as the fissure cracks can be seen from below but may not be visible from above. The trail in Crack in the Ground is approximately 2 miles long, with a 70ft elevation gain. The trail is moderately difficulty with sand and uneven rocky surfaces. Keep in mind that temperatures in the crack are about 20 degrees cooler than the surrounding area. 4-wheel drive vehicles are recommended, high-clearance vehicles not needed in dry weather. We opted to camp after a full day of exploration, and we found a perfect little spot at the Green Mountain campsite.
Fissured means a soil material that has a tendency to break along definite planes of fracture with little resistance, or a material that exhibits open cracks, such as tension cracks, in an exposed surface.
(iii) Observe the side of the opened excavation and the surface area adjacent to the excavation. Crack-like openings such as tension cracks could indicate fissured material. If chunks of soil spall off a vertical side, the soil could be fissured. Small spalls are evidence of moving ground and are indications of potentially hazardous situations.
(B) Samples that dry without cracking are to be broken by hand. If considerable force is necessary to break a sample, the soil has significant cohesive material content. The soil can be classified as an unfissured cohesive material and the unconfined compressive strength should be determined.
Safety should be the top priority when assessing trees for defects or performing any kind of tree work. If the work to be done is higher than you can reach from the ground, or the tree needs to be removed completely, we strongly recommend contacting a professional tree care company. If the tree and its branches are over or adjacent to power lines contact your local power company first. They may be responsible for the tree if it is within a right of way or easement. Special training is needed for trees near power lines and if the power company will not perform the work then you must find a professional tree care company. Visit our care and maintenance page for advice on finding an arborist or professional tree care company.
If you see dead, broken, or loose branches in the tree, remove them as soon as possible if it is safe to do so. They could fall at any time and cause injury to people or damage to property, which means they pose a substantial liability to the land owner. If the branch is higher than you can reach from the ground, contact a professional tree care company to have the dead, broken, or loose branch removed. When removing the damaged or loose branch, make sure to follow best pruning practices.
Look for holes and cavities in the trunk and large branches of your tree. Cavities may be caused by broken branches, injury, or animal and insect activity. If the holes are visible or accessible from the ground, look for signs of moisture and decay inside them and notice how deep the cavity goes. Like wounds in your skin, cavities allow decay and disease into the tree which will weaken it over time. However trees can heal around injuries and remain stable for many years with cavities in them. There are two rules you can use to determine if the tree or branch is at high risk of failing due to a cavity:
Trees with split trunks will likely fail completely in a later storm and should be removed. Vertical cracks can be caused by a variety of reasons, may be associated with decay and will eventually fail. Horizontal cracks indicate that the tree is already failing and should be removed as soon as possible.
Another thing to look for is co-dominant stems - when a tree trunk splits into 2 or more stems of are approximately the same size and arise from the same point. Co-dominant stems and narrow "V" shaped attachments often are weakly attached, you may notice cracks or splits at these connections. These are the points at which failure will occur when the tree is under stress from high winds, snow or ice. The best time to correct a these weak attachments is while the tree is young.
Symptoms of branch canker and dieback include exuding reddish sap that dries to a brown and white powder. Bark may be cracked, darkly discolored, or slightly sunken. With older cankers, the bark may be friable (crumbly) and easily removed from the damaged area. Under the canker, the inner bark and wood is reddish brown to brown instead of the normal pale color. When the branch is cut transversely, a characteristic wedge-shaped canker extending deep into the xylem may be visible. If much of the xylem becomes infected, limbs may collapse and leaves quickly turn brown, but remain attached.
Branch cankers closely resemble Phytophthora trunk canker. Branch cankers usually occur higher above the ground, beginning around the first main branch crotch or higher. Branch canker can affect twigs and smaller branches, as well as the upper trunk and large limbs. Branch cankers sometimes extend deep into wood, whereas Phytophthora cankers only discolor a shallow layer of outer wood. Except when trees are young, branch canker is usually not as serious as diseases caused by Phytophthora spp. 2b1af7f3a8