Federal Highway Administration Design Manual: Deep Mixing for Embankment and Foundation Support

CHAPTER 4. SITE EXPLORATION PROGRAM

4.1 Introduction

The information in this chapter does not include an introduction to geotechnical site explorations in general. Instead, it provides additional guidance for DMM projects. For those not familiar with geotechnical site explorations, helpful resources include Evaluation of Soil and Rock Properties, Subsurface Investigations, Soils and Foundations Reference Manual Volume I, and Soils and Foundations Reference Manual Volume II. (See references 37-40.) This chapter focuses on specific special needs of site explorations for DMM projects. Where feasible, recommendations are given in a format compatible with the materials in Evaluation of Soil and Rock Properties.⁽³⁷⁾

The following key points are made in the exploration guidance documents: (See references37-40.)

• A phased approach to site investigation can be effective.
• Professional judgment should be applied instead of following prescriptive rules.
• In situ testing can be beneficial.
• Appropriate use of correlations can help establish soil property values.

This chapter addresses a phased approach to site exploration, general site exploration planning, and important site exploration details.

4.2 Phased Approach to Site Exploration

For highway projects in which DMM is being considered, a phased site exploration procedure is useful. Either a two- or three-phase approach may be appropriate depending on the size and complexity of the project. A three-phase approach is outlined in the following subsections. If the project is not large or complex, it may be possible to combine phases 2 and 3.

4.2.1 Phase 1—Office Studies and Site Reconnaissance

Phase 1 includes the following steps:

1. Collect project information such as alignment, grade, performance requirements, special features, etc.
2. Review geologic reports and maps, topographic maps, aerial photographs, previous subsurface investigation reports, soil survey reports, etc. (See section 3.2.3 in Evaluation of Soil and Rock Properties for more information.⁽³⁷⁾)
3. Visit the site to observe access, current land use, and surface features such as topography, outcrops, stability, drainage features, etc. (See section 3.2.4 in Evaluation of Soil and Rock Properties for more information.⁽³⁷⁾)
4. Plan the next phase(s) of the site exploration. (See section 3.2 in Evaluation of Soil and Rock Properties for more information.⁽³⁷⁾)

4.2.2 Phase 2—Preliminary Field Investigations and Laboratory Testing

In phase 2, the need for DMM may not yet be established, and other geotechnical approaches may be under consideration. Consequently, the site exploration may support more than one type of geotechnical construction. In phase 2 of a three-phase investigation, borings and soundings are made at a wide spacing, and limited laboratory testing is performed. The intent is to develop the general stratigraphy and relevant soil property values such as strength and compressibility for use in preliminary calculations to determine which technology should be considered for a more detailed field investigation, laboratory testing, and final design. Phase 2 also includes planning the phase 3 investigation.

4.2.3 Phase 3—Detailed Field Investigations and Laboratory Testing

In phase 3, it is known that a DMM design will be developed. A detailed program of field investigations and laboratory testing is conducted to provide information needed for design, construction, and preparation of bid documents.

4.3 General Site EXPLORATION Planning for DMM Projects

The planning process for geotechnical site explorations includes the following steps: (1) identify data needs, (2) gather and analyze existing site information, (3) develop a preliminary site model, (4) develop and conduct a site exploration program, and (5) develop and conduct a laboratory testing program.⁽³⁷⁾. In the phased approach, steps 1 through 3 are part of phase 1, and steps 4 and 5 are performed in phases 2 and 3.

To facilitate step 1, table 6 summarizes information needed and field and laboratory testing to be considered for support of transportation-related embankments and structures. Specifically, the table lists the engineering evaluations that should be performed, the information required for the evaluations, and the suitable field exploration procedures and laboratory tests.

Engineering evaluations that may need to be performed for DMM projects include stability, settlement, load transfer, and lateral movement of adjacent structures. The details of these stability and settlement evaluations are discussed in chapter 6. Procedures for column-supported embankments can be applied to assess load transfer from the embankment to basal geosynthetic reinforcing, if used, and to deep mixed columns. The potential for lateral movement of adjacent structures can be assessed as necessary on a case-by-case basis. Engineering evaluation of soil compatibility with binders and suitability of site soils for DMM are discussed in chapter 5.

Material parameter values and numerical performance criteria are used in the engineering evaluations. Practical construction information, such as the existence of buried utilities that might interfere with construction and suitable locations for onsite use and/or offsite disposal of spoil materials, is also required.

Table 6. Summary of evaluations, information, and testing considerations for highway applications of DMM.

Many of the field and laboratory tests listed in table 6 are standard components of geotechnical site exploration programs and need no further discussion here. The following items from table 6 are specific to DMM projects:

• Near-surface ground temperatures should not be lower than about 39 °F (4 °C) for cementation reactions to occur effectively.
• The in situ soil water content has an important impact on mixture strength, as discussed in chapter 5.
• Increasing organic content often requires higher cement content, and organic contents greater than about 10 percent may produce significant interference with cementation. Humus, which is finely divided and decomposed organic matter in soil, has more potential to interfere with cementation than fibrous organic material that is not as decomposed.
• Soil pH values should be greater than 5 for cement stabilization.
• For lime stabilization, conductivity can provide an indication of the potential for the pozzolanic reactions that are necessary to form cementitious bonds. Conductivity less than 1.02 × 10^-6 S/inch (0.4 mS/cm) indicates a non-pozzolanic soil, and conductivity greater than 3.05 × 10^-6 S/inch (1.2 mS/cm) indicates a soil that would experience pozzolanic reactions with lime. However, dissolved salts in porewater can increase conductivity without the presence of clay minerals necessary for pozzolanic reactions with lime.
• Increasing sulfate concentration indicates increasing risk that expansive minerals
  may form. Sulfate concentrations less than 3,000 ppm indicate low risk, and sulfate concentrations greater than 10,000 ppm indicate unacceptably high risk. Expansive material formation has the potential to damage the deep mixed ground and reduce
  its strength.

Table 3 of Evaluation of Soil and Rock Properties presents recommended minimum numbers and depths of investigation points for a range of different highway applications, but the table does not include entries for DMM applications.⁽³⁷⁾ Table 7 of this report has been prepared to provide that information for DMM applications.

For all geotechnical site explorations, the field crews should have a thorough understanding of the objectives of the investigation, or they should be in close contact with the design office. Field adjustments of the location and depth of borings and probes as well as sampling type and frequency should be made with consideration of the project objectives in mind.

Table 6 and table 7 provide general background information and guidance that can be used to help prepare site exploration plans for DMM projects. Important additional details are discussed in the next section.

Table 7. Guidelines for minimum numbers and depths of investigation points for highway applications of DMM.⁽³⁷⁾

4.4 Important Site exploration Details for DMM Projects

There are several details that need special attention when conducting site explorations for DMM projects. For soft ground to be improved by DMM, two parameters have an especially large impact on the ability of DMM to increase strength and decrease compressibility: the in situ water content and the organic content of the soil. For soft soils that are easy to mix, as the water content of the mixture increases, more binder is needed to achieve the same strength. Because the amount of water in the mixture has a big impact on the mixture strength, the water content of the soil is an important parameter to be characterized during site explorations for DMM projects.

Similarly, soils with high organic content may require large amounts of binder to achieve suitable strength. Organics may interfere with cementation because organic colloids can attract the calcium in cement or lime and prevent it from participating in the chemical reactions that stabilize the mixture. Humus is more detrimental to cementation than fibrous organics because organic colloids from humus can become more widely dispersed in the mixture than intact fibers from fibrous organic material. Consequently, the amount and type of organic material are key parameters that should be well characterized for a deposit.

The mineralogy of the soils to be treated is also important. For example, silty and sandy soils without a significant amount of clay minerals respond better to cement treatment than to lime treatment, whereas clay soils are treatable with lime/cement mixtures. Atterberg limits tests provide a useful indication of the presence and amount of clay minerals in a soil.

The site exploration should also identify conditions that can interfere with the mixing process such as the presence of boulders, cobbles, layers of dense sands/gravels, construction debris, abandoned foundations, or underground utilities.

Bench-scale treatability studies should be performed to assess the effect of different binders and binder factors and to investigate the range of strengths that can be achieved (see chapter 10). The strengths of laboratory-prepared specimens and field-mixed material are not the same due to differences in mixing, curing, and loading conditions. Nevertheless, the strength of laboratory-prepared specimens provides a useful indication of the potential of DMM to improve the strength of soft ground. Laboratory testing is discussed in more detail in chapter 5.

A significant quantity of soil sample must be obtained to perform laboratory tests. It was found that 6.5 lb (3 kg) of soil is enough to make a batch of eight 2- by 4-inch (50- by 100-mm) specimens for curing and strength testing for most mix designs. Two specimens can be tested at each of four curing times, which permits a smooth curve to be drawn through the data to determine the 28-day strength. Multiple batches are ordinarily prepared to investigate different mix designs. For investigating lime-cement-soil mixtures by the dry method or binder-water-soil mixtures by the wet method, a minimum of five batches are necessary, but the use of twice that many is common. Such testing would require 35 to 70 lb (15 to 30 kg) of soil.

The critical soil layer (i.e., the wettest and softest soil layer) may drive the mix design, but there are many circumstances for which more than one layer should be tested. For example, consider a soil profile including a layer of highly organic soil about 6 ft (2 m) thick and an underlying layer of soft clay that is 33 ft (10 m) thick. In this case, it may be necessary to perform laboratory mix design studies for both layers, which would require about 35 to 70 lb (15 to 30 kg) of each soil for each treatment method to be investigated.

Although relatively undisturbed samples are necessary for many performance-related tests such as consolidation, shear strength, and permeability of fine-grained soils, disturbed soil samples are suitable for mix design studies for DMM projects.

The soil for mix design testing can be obtained using a variety of sampling techniques as follows:

• Thin-wall tube samples: A 24-inch (600-mm) push of a 3-inch (75-mm) thin-wall sampler with 100 percent recovery produces about 9 lb (4 kg) of soil. Thus, 4 full samples of this size are needed for 5 batches, and 8 are needed for 10 batches. Larger samplers can obtain enough material with fewer samples.
• Thick-walled split-spoon samples: The sampler used for SPT has a 1.4-inch (35-mm) inside diameter, which is generally too small to obtain sufficient sample for mix design studies. Larger split-spoon samplers may be suitable.
• Backhoe test pits: It is easy to obtain large samples using a backhoe, but the exploration depth is limited.
• Bucket augers: A bucket auger consists of a short barrel with a hinged drop bottom that has cutting teeth and slots for soil entry. The bucket auger is attached to the drill string and is used to obtain samples from the bottom of a borehole. It may be necessary to clean the bottom of the borehole before obtaining a sample.
• Auger cuttings: Samples for laboratory mix design studies can be obtained from auger cuttings, but the depth of sampling by this method is not certain.

Samples should be carefully sealed to prevent drying during transportation and storage prior to testing. Thin-walled samples can be sealed in the field immediately after sampling. Disturbed samples should be placed in sealable plastic bags with as much air removed as possible to prevent oxidation reactions prior to laboratory testing. Household vacuums have even been used to help remove air. The sealed bag should be placed inside another sealed container such as a second bag or a plastic pail with a sealable lid to help protect the first plastic bag against damage. A wet sponge can be placed outside the first plastic bag and inside the second container to help create a humid atmosphere that reduces diffusion of moisture through the first plastic bag. The samples should be protected from warm temperatures. If the samples are not tested within 1 to 2 days after sampling, they should be stored in a humid room.