Wash Boring Method
1. Concept of the Test
Wash boring is a soil exploration method used to investigate subsurface conditions, primarily to obtain soil samples and determine the soil stratigraphy. The technique involves drilling a borehole by using a high-pressure water jet to erode the soil, which is then flushed to the surface along with water. This method is widely employed in geotechnical engineering due to its simplicity, cost-effectiveness, and efficiency in various soil conditions, except for very hard or dense soils and rock. Standards for wash boring include ASTM D4700.
2. Descriptions and Procedure
Apparatus Required
- Drill rig equipped with wash boring equipment
- Casing pipes to stabilize the borehole
- Drill rods and bit
- Water pump and hose for jetting water
- Recirculating tank for water
- Split-spoon sampler or thin-walled tube sampler
- Bucket or container to collect soil-water slurry
- Logbook for recording observations
Procedure
1.Site Preparation
- Selecting Test Location: Choose a site that represents the area of interest for subsurface exploration.
- Setting Up Equipment: Position the drill rig over the test location. Ensure the area is clear of obstacles and properly marked.
2.Borehole Initiation
- Starting the Borehole: Install a temporary casing at the surface to prevent collapse of the borehole. This casing typically extends a few feet into the ground.
- Initiating Jetting: Attach the drill bit and connect the drill rods to the drill rig. Begin pumping water through the drill rods to the bit, creating a jetting action that loosens the soil.
3.Drilling and Soil Sampling
- Advancing the Borehole: Continue drilling by advancing the drill rods and bit deeper into the ground. The high-pressure water jet erodes the soil, creating a slurry that is flushed to the surface.
- Collecting Samples: Periodically stop drilling to collect soil samples. Use a split-spoon sampler or thin-walled tube sampler to capture undisturbed samples from the bottom of the borehole.
- For split-spoon sampling, drive the sampler into the soil at the borehole bottom using a hammer and retrieve the sample.
- For thin-walled tube sampling, push the tube into the soil and extract it carefully.
- Recording Observations: Log the depth, soil type, color, consistency, and any other notable observations.
4.Casing Advancement
- Extending the Casing: As the borehole deepens, extend the temporary casing to prevent the walls from collapsing. This is crucial in loose or unstable soils.
5.Water Management
- Recirculating Water: Use a recirculating tank to collect the slurry, allowing the water to be reused. Ensure the water pump maintains adequate pressure for effective jetting.
- Disposing of Slurry: Properly dispose of the soil-water slurry according to environmental regulations.
6.Termination of Boring
- Reaching Desired Depth: Continue drilling and sampling until the desired depth is reached or until refusal in very dense soil or rock.
- Logging Final Observations: Record the final depth and any notable changes in soil conditions. Seal the borehole if required by local regulations.
Data Interpretation
1.Soil Stratigraphy
- Analyzing Samples: Examine the collected soil samples to determine the stratigraphy of the site. Identify different soil layers and their properties such as grain size, consistency, and color.
- Classifying Soils: Classify the soils according to standard systems like the Unified Soil Classification System (USCS) or the American Association of State Highway and Transportation Officials (AASHTO).
2.Soil Properties
- Laboratory Testing: Conduct laboratory tests on the collected samples to determine properties such as moisture content, Atterberg limits, grain size distribution, and shear strength.
- Creating Logs: Develop borehole logs that summarize the findings, including soil types, layer depths, and relevant soil properties.
Summary Table
Description
Select location, set up drill rig.
Description
Install casing, initiate jetting.
Step
Drilling and Soil Sampling
Description
Advance borehole, collect soil samples, record observations.
Description
Extend casing as borehole deepens.
Description
Recirculate water, dispose of slurry properly.
Step
Termination of Boring
Description
Reach desired depth, log final observations, seal borehole if necessary.
Description
Analyze and classify soil samples.
Description
Conduct laboratory tests, create borehole logs.
Key Points to Remember
- Purpose: Determine subsurface soil conditions and stratigraphy.
- Apparatus: Drill rig, wash boring equipment, casing pipes, water pump, samplers.
- Procedure: Prepare site, drill borehole, collect samples, manage water, and analyze data.
- Data Interpretation: Analyze soil stratigraphy and properties, conduct laboratory tests, create borehole logs.
- Standards: ASTM D4700.
Final Summary
Wash boring is an effective and economical method for investigating subsurface soil conditions. It provides valuable information about soil stratigraphy and properties, essential for geotechnical design and analysis. Proper execution and interpretation of the test ensure accurate assessment of subsurface conditions, aiding in the design of stable and safe engineering structures.
Figure 1: Wash Boring Drilling Method.