WorldWide Drilling Resource

17 WorldWide Drilling Resource ® AUGUST 2014 Why You Need to Know the Dirt Adapted from Information by Sharewell HDD When planning horizontal direction- al drilling (HDD) projects, underground conditions should be evaluated prior to bidding and beginning operations. A good knowledge of conditions to be drilled is necessary not only to plan the work ahead, but also to properly esti- mate the cost of the job. Unless you have prior knowledge of the area, the prospective drilling location should at least be paid a visit. During the visit, bid document information should be compared with your observations. Look for railroad cuts, bridges, quarries, etc. in the area. Observe the overall topography. Is it a floodplain or runoff area? Visit local water well drilling pro- fessionals. Talk to them about the depths of different formations. What types of bits do they use? What rates of penetra- tion do they achieve? What size rigs do they have? Where is the surface water table? Although a visit may be more pro- ductive, much of this information is avail- able by simply making a phone call. On larger projects where the end user has taken soil surveys or samples, study them closely. These soil studies will include three main types of infor- mation: 1. Standard Classification of Soils - This provides the drilling contrac- tor with the types of soils encountered during boring, and the elevations at every change of material. These soils include silts, sands, clays, gravel, cob- ble, and boulders. By their classifica- tion, you can determine which soil is dominant in the makeup. 2. Standard Penetration Test - This tells the drilling professional the basic density of the soil by providing the number of blows (n value) required to drive a split spoon sampler 12 inch- es. The number of blows can range from 0 to 100-plus blows per foot. Normally, anything in the range from 3 to 50 is capable of being jetted. When blows per foot exceed 50, some means of mechanically cutting the material - like a mud motor - should be consid- ered. When blows are less than 2, it may be difficult to steer because the material is too soft. 3. Sieve Analysis - This is a test performed by a geotechnical firm, and involves taking material collected in the split spoon sampler from the field to the laboratory. In the lab, material is passed over a number of sieves with different screen openings. The amount of mate- rial which passes through each sieve is measured, and a graph is constructed to show the percentage of each mate- rial. These tests are the most accurate means of determining the percentage of gravel which may be present at a given location. Significant variance of gravel con- centrations may exist in the soil sample location and the actual drilling location. The higher the percentage of gravel, cobble, and/or boulders in a crossing, the more difficult the crossing. Because these large granular materials tend to settle on the bottom of the pathway, it is difficult for drilling fluid to remove them. The first decision to make is whether the ground is capable of being jetted or if a mud motor is required. Typically, ground strengths up to around 800 psi can be jetted. This covers most clays and sands. For denser sand and clay up to about 1200 psi, a mud motor and soft formation bits for the pi lot hole must be used. Ground between 1200 and 10,000 psi will most likely have to be dri l led with a mud motor, mi l led- tooth rock bit, and milled-tooth hole open- ers. Rock with compressive strengths above 10,000 psi will probably require tungsten carbide insert bits and cutters. Modifications will need to be made based on exact circumstances. Existence of gravel will impact decisions and risk. Lengths of some sections of rock may require use of a hard formation cutter, while other sections are in softer ground. Study the charts carefully, then plan and price the job accordingly.