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The shear strength of soils

Introduction

The shear strength of soils is essential for any kind of stability analysis. Therefore, it is important to determine reliable values. For this purpose triaxial tests are most appropriate. Nevertheless, direct shear tests are mostly performed to determine the shear strength of soils.

From the tests result a clearer picture of the soil strata, there will be Direct Shear Test where the data can be interpreted and correlated info soil parameters. The soil parameters will be designed accordingly to the characteristic of the soil at the site itself. This case study is about evaluating contamination for the specific site. It involves the interpretation and manipulation of data from the Shear test and chemical tests. The whole process will be determination of contamination and measure its effect on the soil and then propose some remediation practices and methods.

Part (1) Soils Contaminants

Soils are categorized according to the contaminants present (type and amount);

1. Hazardous

Soil that contains hazardous waste must be managed as such when contamination is above the nonhazardous waste limits or is otherwise classified a hazardous waste. The only options for soils that contain a hazardous waste are on-site remediation or off-site management as a hazardous waste.

2. Non-Hazardous

Contaminated soil is non-hazardous when both of the following criteria are met:

a. The contaminant levels are above the most stringent soil cleanup criteria established by NJDEP.

b. The waste is not classified as a hazardous waste.

3. Soils That Contain Contaminants Below Regulatory Concern

Soils that contain contaminants at levels that are below the most stringent site cleanup levels established by NJDEP for a specific site are not of regulatory concern with the exception of sites in the Pinelands Area Brownfield sites come in all sizes and shapes

The Brownfield Law created four different cleanup “tracks.” Cleanup levels at these tracks are based, in whole or in part, upon the site's future anticipated use. A site to be used for industrial purposes, for instance, would not have to be cleaned up as much as a site intended for commercial use. (See Appendix A for a description of each of the tracks).

Clesn Up

Site cleanups can be conducted by federal, state, or local government agencies and/or their contractors, by private parties responsible for the discharge of the hazardous substances, or by purchasers of land who did not cause the contamination but are willing to pay for and conduct the cleanup. Private parties can be directed by DEP to conduct cleanups or individuals can come to the department voluntarily and offer to do part of or all of a cleanup.

Cleanups can be conducted to allow for any future use of the land: from residences, schools, and day care centers to commercial buildings, or new industrial uses. Cleanups can also be conducted to turn formerly contaminated sites into parkland for passive and/or active recreation. The level of cleanup required to protect human health and the environment is determined by the end use envisioned for the property. Obviously, cleanup to more stringent standards is required when people will be living on the property, or when sensitive populations such as children will be using the property for schools or recreation. Protectiveness can be obtained by either totally removing the contaminants of concern, or by treating them, or by limiting access to the substances by creating barriers to contact, such as with soil or other types of capping material.

Certain cleanups can be conducted “at risk” by private individuals, with little oversight by the state. However, cleanups involving ground and surface waters must be conducted with DEP's oversight because the state is the trustee for those natural resources. Most parties conducting remediation will come to DEP for oversight if they desire a “No Further Action” (NFA) letter at the end of the process. That letter certifies that the state's regulations have been met as of a specific date, as described specifically in the letter. With the NFA comes a “Covenant Not to Sue,” which defines those parties who are no longer liable to conduct additional cleanup at the site.

Part (2) Direct Shear Test

A. Shear Strength

In the Mohr-Coulomb theory of failure, shear strength has two components:

When the direct shear test is used to investigate a cohesionless soil, successive

tests with increasing normal stress will establish a straight line that passes through the origin. The angle of inclination of the line with respect to the horizontal axis is the angle of internal friction.

Values of the angle of internal friction are given in the following table. If the soil is dense when tested, initially higher values for the angle of internal friction will be measured, but with increasing amounts of strain, the angle will decline to the approximate ranges seen in the Table.

SOIL TYPE

ANGLE φ, DEGREES

Sand and gravel mixture

33 - 36

Well-graded sand

32 - 35

Fine to medium sand

29 - 32

Silty sand

27 - 32

Silt (non-plastic)

26 - 30

The shear strength of a cohesive soil is more complicated than a cohesionless material. The differences are due to the role of pore water in a cohesive soil. Most cohesive soils in field conditions are at or near saturation because of their tendency to hold moisture and their low permeability.

When load is applied to a soil of this type, the load is supported by an increase in

the pore-water pressure until pore-water can drain into regions of lower pressure.

At that point, soil particles are forced closer together and the strength increases,

just like a cohesionless soil. Time is an important factor however, because it takes longer for water to move out of a low permeability material.

Direct Shear Test:

Results of the direct shear tests should be evaluated statistically. The statistical assessment in the shear box test includes the values of friction angle (ϕ) and cohesion (c), derived from the Mohr-Coulomb regression line, and the (original) measured values of peak shear strength. In previous publications only the derived parameters ϕ and c were taken into account. As the most important result the investigations have shown that it makes a remarkable difference whether the pair of variables ϕ and c or peak shear strength is considered.

http://www.csus.edu/indiv/c/cornwell/engineering/9-20-06.pdf

Direct Shear Box Test Procedure:

1. Bring the shear box together

http://www.civil.mrt.ac.lk/docs/direct_shear_test

2. Compact the soil sample (which has reached the optimum moisture content)
in the mould.

3. Place the sample into the shear box

4. Record the weight of the applied load and place the loading plate on top of the upper porous plate

http://www.civil.mrt.ac.lk/docs/direct_shear_test

5. Remove the alignment screws to hold the shear box together.

6. Tighten the diagonally opposite screws to reduce the frictional force.

7. Reset the dial gauges.

7. Apply the normal load.

8. Start the motor to produce the desired constant rate of shearing

9. Take readings from the gauges,

a) Proving ring.

b) Horizontal displacement.

c) Vertical displacement.

10. When the shear load starts to reduce or remains constant stop the test

http://www.civil.mrt.ac.lk/docs/direct_shear_test

A. Data

Proving ring

Horizontal

(0.001 in)

Vertical

Time

0

9.00

0.00

00:00

5

9.11

0.00

00:30

15

9.12

0.00

01:00

15.5

9.26

0.00

01:30

18.5

9.38

0.00

02:00

20

9.63

0.00

03:00

22

9.87

0.00

04:00

23

10.12

0.00

05:00

24

10.36

0.00

06:00

23

10.62

0.00

07:00

24

10.87

0.00

08:00

26

11.12

0.00

09:00

25

11.37

0.00

10:00

26

11.62

0.00

11:00

27

11.87

0.00

12:00

27

12.12

0.00

13:00

27.5

12.37

0.00

14:00

28

12.62

0.00

15:00

28

12.87

0.00

16:00

29

13.11

0.00

17:00

28

13.37

0.00

18:00

31

13.62

0.00

19:00

29.5

13.87

0.00

20:00

Results

Shear stress Kpa

Horizontal

(0.001 in)

Time

0.000

9.00

00:00

0.951

9.11

00:30

2.862

9.12

01:00

2.950

9.26

01:30

3.528

9.38

02:00

3.812

9.63

03:00

4.194

9.87

04:00

4.381

10.12

05:00

4.577

10.36

06:00

4.381

10.62

07:00

4.577

10.87

08:00

4.959

11.12

09:00

4.763

11.37

10:00

4.959

11.62

11:00

5.145

11.87

12:00

5.145

12.12

13:00

5.243

12.37

14:00

5.331

12.62

15:00

5.331

12.87

16:00

5.527

13.11

17:00

5.331

13.37

18:00

5.527

13.62

19:00

5.625

13.87

20:00

FAILUER = 5.527 kpa.

Part (3) Contamination Assessment

PH Measurements:

PH measurements were measured against TSS results, While no apparent correlation was found between pH and salinity; samples presenting higher pH generally had higher TSS concentrations, as indicated in the figure below. The addition of cement (and its effect on turbidity) appears to have had a noticeable effect on pH.

In addition to the various testing that was performed as part of this study, two additional studies beyond the original scope were conducted:

(a) an evaluation of the effect that organic content found within silt sediments would have on soil-cement hydration and strength gain, and

(b) an assessment of whether the cement stabilization/solidification of contaminated sediments immobilizes organic and inorganic contaminants within the sediment.

Part (4) Remediation.

Chemical Fixation And Solidification

Taking into consideration to select the best fit remediation method and conserve ecosystem and trying not to undergo major landscape disruption.

One alternative to dredging and disposing of contaminated sediment is to solidify the sediment in-situ using CDSM, and cover the stabilized sediment with a cap. In this scenario, the mixing is a viable way to reduce the potential for highly contaminated sediments to be eroded over time by natural hydrologic events, vessel traffic or dredging.

In situations where decisions regarding removal of sediment have not yet been made or if management alternatives (decontamination) are not yet fully developed, the CDSM technology can be used to stabilize sediment for later removal.

Chemical fixation and Solidification, CFS, also commonly referred to as Solidification/Stabilization Treatment, S/S, is a widely used treatment for the management of a broad range of wastes, especially those classified as hazardous. The initial application of the technology in the United States dates back to the 1950s when it was fruitfully used to treat and dispose of radioactive waste.

The USEPA considers S/S an established treatment technology, and has identified it as the best demonstrated available technology, BDAT, for 57 RCRA-listed wastes. It is anticipated that 30% of the CERCLA (Superfund) remediation sites include the use of S/S.

There is an ample body of technical literature that documents the fundamentals for applying S/S, as well as the practical experience using the technology in a wide variety of wastes, contaminants and chemical matrices. The applicability of using S/S to immobilize the principal contaminants in the Passaic River sediments was evaluated.

The following sections briefly summarize those findings.

There are several additives mentioned in the literature as potential enhancers of the PCB immobilization. These depend on the particular conditions of the application:

1. Organically modified clays

2. Activated carbon

3. Ferric hydroxide

4. Rubber particulate

A well-documented case where S/S was used for the immobilization of PCBs in contaminated soils is provided in the Yellow Water Road Dump in Duval County, Florida. Where In that case, a superfund site that started remedial action in 1984 was removed from the NPL in 1999. During the remedial activities, 4472 cubic yards of PCB contaminated soils were excavated and treated with S/S and placed back within the former operations area of the site.

A groundwater monitoring program was established, and monitoring wells were installed to evaluate the future need for contingent pump-and-treat remedy. The site was monitored as part of the EPA five-year review program. In September 2000, the five-year-review report established that the selected remedy remained protective of human health and the environment. Site inspections and groundwater monitoring continue to ensure long-term protectiveness.

Out-Of-State Recycling

For recycling soils out-of-state, a written determination from NJDEP is required as to the non-applicability of the solid waste management regulations set forth in N.J.A.C. 7:26-1 et seq. For sites without a Site Remediation Program lead (i.e., Industrial Site Recovery Act, Bureau of Underground Storage Tanks, Bureau of Field Operations) and for the recycling of soil as a solid process waste, contact the Bureau of Resource Recovery and Technical Programs (609/984-6985).

Sites with a Site Remediation Program lead must send this information in lieu of a Soil Reuse Proposal to the assigned case manager for an approval. The following are the standard requirements pursuant to (N.J.A.C. 7:26-1) for approval to send soils out-of-state:

a. A letter, sent to the Bureau of Resource Recovery and Technical Programs from the generator of soil, certifying that the soil in question has been analyzed or is known in accordance with N.J.A.C. 7:26G-5.1 not to contain a hazardous waste. This also must include any necessary test results documenting that the soil contains constituents and hazardous waste characteristics below their regulatory levels.

b. A letter sent to the Solid and Hazardous Waste Division from the receiving a facility stating that they agree to accept the specified amount of soil, indicating intention and method to beneficially use or reuse the soil and the time frame for such activity from the date of receipt at the facility.

In addition, a copy of this information must be sent directly to the solid waste coordinator of the county of the soil's origin.

c. A letter sent to the Solid and Hazardous Waste Division from the proper regulatory agency of the receiving state or a copy of a current facility permit verifying that facility is operating in accordance with applicable rules and regulations and can accept the soils for the declared use/reuse.

d. Once the soil is delivered to the identified use/reuse facility, a letter from the facility or a bill of lading stating the date and amount of soil received must be sent to Solid and Hazardous Waste Division and the solid waste coordinator of the county of the soil's origin.

Operational Landfill Cover

Operating landfills that are permitted to accept ID 27 waste may use non-hazardous soil for daily landfill cover with approval from the Bureau of Landfill and Recycling Management. Fine grained soils which may create erosion problems or are easily windblown are prohibited for use as daily cover.

Brownfield sites are found in both urban and rural settings and the present challenges that make the cleanup and redevelopment of these sites unique as compared with other real estate projects.

Brownfield sites deal with four key issues, including:

Despite these challenges significant opportunities exist for brownfield redevelopment which can economically revitalize an area and improve the quality of life for communities.

Brownfield redevelopment is also an ideal time to integrate a number of sustainability features that can result in improved storm water management, reduced air emissions and energy consumption, and preserve the history and culture of our communities.

The purpose of this guide is to provide a starting point for information about brownfield redevelopment planning, regulatory considerations and resources available from the federal Environmental Protection Agency (EPA) and the Iowa Departments of Economic Development and Natural Resources.

6. References

1- NJDEP. Discussion Paper on Landfill Closure and Remediation Issues.(1993).

2- Environmental Protection Agency. Handbook for Remedial Action at Waste Disposal Sites.

3- Bujang B.K. Huat et al., (2007) Modified shear box test apparatus for measuring shear strength of unsaturated residual soil , Faisal Hj. Ali, S.Hashim , Thomson Gale (December 27, 2007)

4- Marcel van der Perk, (2009), Soil and Water Contamination: From Molecular to Catchment Scale, T&F Books UK; 1 edition (January 28, 2009)

Appendix A

Description Of The Brownfield Cleanup Tracks

The statute establishes four different “tracks” that a developer can follow in remediating a site.

Under Track 1, a remedial program “shall achieve a cleanup level that will allow the site to be used for any purpose without restriction and without reliance on the long-term employment of institutional or engineering controls.” ECL 27-1415(4). With respect to soil remediation, the statute directs that Track 1 cleanups “shall achieve” the generic SCOs designed to allow for unrestricted future use of the property. Id. A developer who remediates a site to Track 1 standards receives a greater tax credit than is available for remediation under the other three tracks.

Under Track 2, a remedial program “may include restrictions on the use of the site or reliance on the long-term employment of engineering and/or institutional controls.” ECL 27-1415(4). With respect to soil remediation, however, the statute directs that a Track 2 cleanup “shall achieve” the generic SCOs appropriate for the future use of the property “without the use of institutional or engineering controls to reach such objectives.” Id.

Under Track 3, the developer does not need to achieve the generic SCOs, but instead “may use site specific data to determine” soil remediation objectives. ECL 27- 1415(4). Those sitespecific objectives must “conform with the criteria used to develop” the generic SCOs. Id. Like the generic SCOs, soil remediation objectives developed by the applicant pursuant to Track 3 “shall not exceed an excess cancer risk of one in one million for carcinogenic end points and a hazard index of one for non-cancer end points,” except where rural soil background contamination exceeds that risk level. ECL 27-1415(6)(b).

A Track 4 remedial program “shall achieve a cleanup level that will be protective for the site's current, intended or reasonably anticipated residential, commercial, or industrial use with restrictions and with reliance on the long-term employment of institutional or engineering controls to achieve such level.” ECL 27-1415(4). The statute instructs that “[f]or Track 4, exposed surface soils shall not exceed the generic contaminant-specific [SCOs] developed for unrestricted, commercial, or industrial use pursuant to this subdivision which conforms with the site's current intended, or reasonably anticipated future use.” ECL 27-1415(6)(d). The statute defines “exposed surface soils” as “two feet for sites used for residential use and one foot for

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