Experience can be a brutal teacher, so we are going to try to pass on lessons we’ve learned to help you save money and limit these budget busters, hopefully preventing you from learning the hard way.
Of all the different costs associated with a project development, the most volatile and by far the hardest one to estimate are the costs of site work. Out of all components of site work costs, encountering unexpected rock can utterly destroy a project budget. One foot of rock covering an acre can cost $ 40,000 to $ 80,000 to remove, one foot over 10 acres can cost $ 400,000 to $ 800,000 to remove, and two feet of rock covering 10 acres and you are easily spending millions of dollars above projected costs. To get a better idea of what is under the surface on the property you are working with, a due diligence report or preliminary site analysis should involve subsurface investigations. Its money well spent.
The most common method used for subsurface investigations are borings. A drill rig is used to take soil borings at a specific location on the site. The borings reveal the composition and characteristics of the soil, elevation of the water table, and presence of rock. The problem with borings is that they only can tell you what is located directly in the boring hole itself. What is located 10 feet, 20 feet, or further horizontally from the initial boring hole is unknown. We can assume it is be made up of the same soil and conditions, but there is no guarantee. This is why all soil investigation reports have very specific disclaimers to this effect telling the client that conditions might greatly vary between borings.
So in order to gain a better picture of what is most likely under the ground, you need to drill additional borings. The more borings you do, the clearer the picture you have of what is under the surface. That said, borings are expensive and no one wants to, or can afford to turn their site into a piece of swiss cheese. Yet, one of the biggest mistakes developers can make is to significantly limit the number of borings to save money. This approach is quite a gamble and will increase the risk of unknown rock exposure in an effort to save money.
BREWER ENGINEERING’S APPROACH:
One possible alternative we would recommend with your subsurface investigation is to consider using Seismic Refraction. Using Seismic Refraction in combination with borings is a great economical option to increase the understanding of what is under ground and reduce the number of borings required.
Seismic Refraction is a method that uses a set of geophones arrayed in a line to record seismic waves generated by an explosion (like a hammer strike on a steel plate – See Figure 1). The geophones record the reflective and refractive waves generated by the hammer strike. From that information the velocity of the waves travelling through the soil can be measured and the density of the materials can be calculated.