Ground Improvement Requirements When Working With Sandy Soil
Sandy soil can cause structural failure if not dealt with properly. But a well thought out building and contingency plan can make building on sandy soil feasible.
Building a structure on sand is often considered foolish. Sandy soil is not the most stable site and can result in damage or total structure loss. However, despite this soil type being the least popular foundation, in-depth analysis and planning of the project can mitigate potential problems.
Consider This Before Beginning Construction on Sandy Soil
Testing the ground before even beginning the planning phase of construction is vital to project success, especially if your project site consists of mostly sandy soil. Each of the following items helps construction supervisors understand the composition of the ground he or she is working on and how to compensate for any issues.
Before beginning work on a sandy site, a soil proctor compaction test is necessary to determine the degree of density and help predict future ground performance. The test requires a sample of the site soil. This sample is divided; each new pile gets different amounts of water added. After moistening, each sample has a 5.5-pound weight dropped on it from 12-inches. This pounding occurs twenty times per sample, compacting it and making it denser. From this, testers calculate the density to determine what is the optimal moisture content for the site.
In addition to the proctor test, understanding the soil classification on the site is important. The soil classification includes grain size and cohesion.
Grain size is simple to determine. Project developers must grab a handful and look to identify the different sizes of soil particles. Grained soils, such as sand, have different course classifications based on how large the particles are. For sand, these particles range from rock salt size down to sugar sized grains. In non-grained soil, the particles are not distinguishable from one another; clay is an excellent example of non-grained soil.
Cohesion is another part of the classification process. Sandy soil is non-cohesive, meaning when you attempt to form a ball out of a handful, it will not stick together. Clay and silt, on the other hand, have good cohesive qualities.
Soil slope is difficult to calculate and can be inexact due to land’s naturally non-homogenous nature. Despite slopes often considered a problem of the construction supervisor, knowing a few general rules about them can make it easier to understand why there may be a failure.
Slopes fail for two reasons: increased stress and reduced strength. Pressure on the slope can increase by the addition of loads, steepness of the slope, and water increasing the weight. Water and additional loads can also cause the slope to reduce in strength. Freezing and thawing, as well as general weather conditions, also reduce the overall strength of the slope.
Armed with this knowledge a construction supervisor or planner can develop contingency plans for the project.
Things to Include in Contingency Plans
Once the site director and construction supervisor have a feel for the soil conditions, development of a contingency plan begins. Some things to consider are how to deal with flooding issues, regulations concerning excavation and backfill of the site, and potential soil stabilization techniques for working on sandy soil.
The results of the proctor test give the planner and supervisor a good idea of the optimal moisture saturation content for the site. A site which regularly receives more rain than is optimal requires flooding plans. These plans may include levee designs to help control nearby bodies of water and foundation seepage control. Flood prevention may include designing and implementation of sheet pile walling to help brace soil and prevent landslides. (Learn more in "When Ground Improvement is Needed During Trenchless Rehabilitation.")
As part of a flood contingency plan, excavating and backfilling the site might be an option. However, these options have specific guidelines that must be followed to ensure a safe work place. If excavation is in the plans, the excavation site must undergo analysis for stability. Additionally, OSHA regulations forbid unsupported or vertical side slopes.
Backfill also has regulations to which it must adhere. Avoid impact loading of pipelines, structures, appurtenances and shafts when refilling trenches. If compacting, use layers of four to six-inches in depth, compacting with a 1000-pound compactor in between each layer. Flow fill technique requires the cement-soil fill to be cake batter consistency during the filling process. For sandy soil, a final backfill method is a water-jetting to move material and bedding into place.
A final consideration in the contingency plan is potential soil stabilization. Sandy soil is notoriously hard to build on, so many crews consider changing the consistency with stabilization methods. The type of stabilization depends on the location of the job site and project type. Common choices for sandy conditions are cement, lime and bitumen, each aid in cohesion albeit in different manners. In some cases, use of multiple methods, such as cement and lime, offer the best results with the least damage to the ecosystem.
What Can Go Wrong Building on Sandy Soil?
While a good planning goes a long way to mitigate some of the issues, there are still things that can go wrong. Contingency plans help to address some of the problems. Others are not always foreseeable.
Drainage issues are one problem that a proper contingency plan should cover. Sandy soil is free draining, meaning it does not hold much water. Flooding of a sandy area can cause erosion of the ground making the building foundation weak. This natural characteristic can lead to other issues.
Settlement occurs when the soil gets too wet, causing an increase in pore pressure. With no friction between the grains, sand smoothly shifts out of place. This shifting causes structures built on it to settle; and we all know settling foundations do crack. Areas where foundation settling is common often employ piers to correct the problem.
Sand has a natural consistency to shift and slide around. While drainage issues and using piers driven down into firmer substrate help, there is always a concern that the soil will freely move under the proper circumstances. If this natural characteristic is not considered, structures built on these sites can sink or fall.
Sandy soil is not the best place to build on. However, with proper planning and consideration, completing the perfect project is possible. It is important for you to know what to expect with sand and ways to mitigate problems that may arise.