Exhibit 10.6

AMENDMENT NO. 5 TO CREDIT AGREEMENT

This AMENDMENT NO. 5 TO CREDIT AGREEMENT dated as of September 1, 2017 (this
“Amendment”), is among TETRA TECH, INC., a Delaware corporation (the “Company”),
TETRA TECH CANADA HOLDING CORPORATION, a Canadian corporation (together with the
Company, collectively, the “Borrowers”), BANK OF AMERICA, N.A., in its
capacities as the Administrative Agent (in such capacity, the “Administrative
Agent”) and a Lender, each of the other Lenders party hereto and each of the
Subsidiary Guarantors party hereto.

RECITALS:

A.The Borrowers, the Lenders and the Administrative Agent have entered into an
Amended and Restated Credit Agreement dated as of May 7, 2013 (as amended by
that certain Amendment No. 1 to Credit Agreement dated as of September 27, 2013,
that certain Amendment No. 2 to Credit Agreement dated as of June 23, 2014, that
certain Amendment No. 3 to Credit Agreement dated as of May 29, 2015 and that
certain Amendment No. 4 to Credit Agreement dated as of July 14, 2016, the
“Credit Agreement”). Capitalized terms used and not otherwise defined herein
shall have the meanings ascribed to them in the Credit Agreement.
B.The Subsidiary Guarantors and the Administrative Agent have entered into
Subsidiary Guaranties dated of even date with the Credit Agreement (the
“Subsidiary Guaranties”) and various Security Instruments.
C.The Borrowers have requested to amend the Credit Agreement as set forth below.
D.Subject to the terms and conditions set forth below, the parties hereto have
agreed to so amend the Credit Agreement.
In furtherance of the foregoing, the parties agree as follows:

Section 1.    Amendments to Credit Agreement. Subject to the covenants, terms
and conditions set forth herein and in reliance upon the representations and
warranties set forth herein, the Credit Agreement is amended as follows:

(a)    The definition of “Consolidated EBITDA” in Section 1.01 is amended and
restated in its entirety as follows:

“Consolidated EBITDA” means, for any period, for the Company and its
Subsidiaries on a consolidated basis, an amount equal to Consolidated Net Income
for such period plus (a) the following to the extent deducted in calculating
such Consolidated Net Income: (i) Consolidated Interest Charges for such period,
(ii) the tax expense for Federal, state, local and foreign income taxes of the
Company and its Subsidiaries for such period (net of tax benefit), (iii)
depreciation and amortization expense for such period, (iv) other non-recurring
expenses of the Company and its Subsidiaries reducing such Consolidated Net
Income which do not represent a cash item in such period or any future period,
(v) cost of employee services received in share-based payment transactions (in
accordance with FASB ASC 718) which do not represent a cash item in such period
or any future period and (vi) up to $39,000,000 of non-cash charges incurred in
connection with the wind-down of non-core construction activities in the
Company’s Remediation and Construction Management segment

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through the end of fiscal year 2019 and minus (b) to the extent included in
calculating such Consolidated Net Income, all non-cash items increasing
Consolidated Net Income for such period.

(b)    The schedules to Exhibit D to the Credit Agreement are hereby amended to
incorporate those changes that are necessary in order to conform the
calculation(s) of the financial covenants set forth therein to the amendment
contemplated by Section 1(a) above. A copy of the revised Exhibit D to the
Credit Agreement has been provided to the Company.

The amendments to the Credit Agreement are limited to the extent specifically
set forth above and no other terms, covenants or provisions of the Loan
Documents are intended to be affected hereby.

Section 2.    Conditions Precedent. The parties hereto agree that the amendments
set forth in Section 1 above shall not be effective until the satisfaction of
each of the following conditions precedent:

(a)    Documentation. The Administrative Agent shall have received (i) a
counterpart of this Amendment, duly executed and delivered by the Borrowers, the
Subsidiary Guarantors and Lenders constituting Required Lenders and (ii) such
other assurances, certificates, documents, consents or opinions as the
Administrative Agent reasonably may require.

(b)    Legal Fees and Expenses. All fees and expenses of counsel to the
Administrative Agent estimated to date shall have been paid in full (without
prejudice to final settling of accounts for such fees and expenses).
    
Section 3.    Representations and Warranties.

(a)    In order to induce the Administrative Agent and the Lenders to enter into
this Amendment, each Borrower represents and warrants to the Administrative
Agent and the Lenders as follows:

(i)    The representations and warranties of the Borrowers and the other Loan
Parties contained in Article V of the Credit Agreement and in each other Loan
Document are true and correct on and as of the date hereof, except to the extent
that such representations and warranties specifically refer to an earlier date,
in which case they are true and correct as of such earlier date, and except that
for purposes of this Amendment, the representations and warranties contained in
Sections 5.05(a) and (b) of the Credit Agreement shall be deemed to refer to the
most recent statements furnished pursuant to Sections 6.01(a) and (b) of the
Credit Agreement, respectively.

(ii)    Since the date of the Audited Financial Statements, there has been no
event or circumstance, either individually or in the aggregate, that has had or
could reasonably be expected to have a Material Adverse Effect.

(iii)    No Default has occurred and is continuing or will exist after giving
effect to this Amendment.

(b)    In order to induce the Administrative Agent and the Lenders to enter into
this Amendment, each Borrower and each Subsidiary Guarantor represents and
warrants to the Administrative Agent and the Lenders that this Amendment has
been duly authorized, executed and delivered by it and constitutes its legal,
valid and binding obligation.

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Section 4.    Miscellaneous.

(a)    Ratification and Confirmation of Loan Documents. Each Borrower and each
Subsidiary Guarantor hereby consents, acknowledges and agrees to the amendments
set forth herein and hereby confirms and ratifies in all respects the Loan
Documents to which such Person is a party (including without limitation, with
respect to each Subsidiary Guarantor, the continuation of its payment and
performance obligations under the Subsidiary Guaranty to which it is a party
and, with respect to each Borrower and each Subsidiary Guarantor, the
continuation and extension of the liens granted under the Security Instruments
to secure the Secured Obligations), in each case upon and after the
effectiveness of the amendments contemplated hereby.

(b)    Fees and Expenses. The Company shall pay on demand all reasonable costs
and expenses of the Administrative Agent in connection with the preparation,
negotiation, execution, and delivery of this Amendment and any other documents
prepared in connection herewith, including, without limitation, the reasonable
fees and out-of-pocket expenses of counsel for the Administrative Agent.

(c)    Headings. Section and subsection headings in this Amendment are included
herein for convenience of reference only and shall not constitute a part of this
Amendment for any other purpose or be given any substantive effect.

(d)    Governing Law; Jurisdiction; Waiver of Jury Trial; Etc. This Amendment
shall be governed by and construed in accordance with the laws of the State of
New York, and shall be further subject to the provisions of Sections 10.14 and
10.15 of the Credit Agreement.

(e)    Counterparts. This Amendment may be executed in any number of
counterparts, each of which when executed and delivered shall be deemed to be an
original, and all of which when taken together shall constitute one and the same
agreement. Delivery of an executed counterpart of a signature page of this
Amendment by facsimile or electronic transmission (including .pdf file) shall be
effective as delivery of a manually executed counterpart hereof.

(f)    Entire Agreement. This Amendment, together with all the Loan Documents
(collectively, the “Relevant Documents”), sets forth the entire understanding
and agreement of the parties hereto in relation to the subject matter hereof and
supersedes any prior negotiations and agreements among the parties relating to
such subject matter. No promise, condition, representation or warranty, express
or implied, not set forth in the Relevant Documents shall bind any party hereto,
and no such party has relied on any such promise, condition, representation or
warranty. Each of the parties hereto acknowledges that, except as otherwise
expressly stated in the Relevant Documents, no representations, warranties or
commitments, express or implied, have been made by any party to the other in
relation to the subject matter hereof or thereof. None of the terms or
conditions of this Amendment may be changed, modified, waived or canceled orally
or otherwise except in writing in accordance with Section 10.01 of the Credit
Agreement.

(g)    Enforceability. Should any one or more of the provisions of this
Amendment be determined to be illegal or unenforceable as to one or more of the
parties hereto, all other provisions nevertheless shall remain effective and
binding on the parties hereto.

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(h)    Successors and Assigns. This Amendment shall be binding upon and inure to
the benefit of the parties hereto and their respective successors and assigns
(subject to Section 10.06 of the Credit Agreement).

[Remainder of page intentionally left blank; signature pages follow]

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The following parties have caused this Amendment to be executed as of the date
first written above.

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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BORROWERS:

TETRA TECH, INC.

By: /S/ DAN L. BATRACK            
Name: Dan L. Batrack
Title: Chairman and Chief Executive Officer

TETRA TECH CANADA HOLDING CORPORATION 

By: /S/ DAN L. BATRACK            
Name: Dan L. Batrack
Title: President

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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SUBSIDIARY GUARANTORS:

ADVANCED MANAGEMENT TECHNOLOGY, INC.
AMERICAN ENVIRONMENTAL GROUP, LTD.
ARD, INC.
ARDAMAN & ASSOCIATES, INC.
COSENTINI ASSOCIATES, INC.
PRO-TELLIGENT, LLC
ROONEY ENGINEERING, INC.
TETRA TECH CONSTRUCTION, INC.    
TETRA TECH EC, INC.
TETRA TECH ES, INC.
TETRA TECH TESORO, INC.
WESTERN UTILITY CONTRACTORS, INC.
BPR INC.
BPR-ÉNERGIE INC.
BPR-INFRASTRUCTURE INC.
PARKLAND PIELINE CONTRACTORS LTD.
PARKLAND PIPELINE EQUIPMENT LTD.
PARK L PROJECTS LTD.
TETRA TECH CANADA INC.
TETRA TECH IN INC.
TETRA TECH QC INC.
TETRA TECH QE INC.
TETRA TECH QI INC.
COFFEY INTERNATIONAL, INC.
INDUS CORPORATION
MANAGEMENT SYSTEMS INTERNATIONAL, INC.
TETRA TECH UTILITY CONSTRUCTION, INC.

By: /S/ DAN L. BATRACK            
Name:     Dan L. Batrack
Title:     Vice President

TETRA TECH EXECUTIVE SERVICES, INC.

By: /S/ DAN L. BATRACK            
Name:    Dan L. Batrack     
Title:    Chief Executive Officer     

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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TETRA TECH HOLDING LLC

By:    TETRA TECH, INC., its Sole Member

By: /S/ DAN L. BATRACK            
Name:     Dan L. Batrack
Title: Chairman and Chief Executive Officer

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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ADMINISTRATIVE AGENT:

BANK OF AMERICA, N.A., as Administrative Agent

By: /S/ MAURICE WASHINGTON            
Name: Maurice Washington
Title: Vice President

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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LENDERS:

BANK OF AMERICA, N.A.

By: /S/ ARTI DIGHE                    
Name: Arti Dighe
Title: Vice President

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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.S. BANK NATIONAL ASSOCIATION 

By: /S/ GLENN LEYRER                
Name: Glenn Leyrer                    
Title: Vice President                    

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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WELLS FARGO BANK, N.A.

By: /S/ CATHERINE ABE                
Name: Catherine Abe
Title: Senior Vice President

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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BANK OF MONTREAL 

By: /S/ JOHN ARMSTRONG            
Name: John Armstrong                    
Title: Managing Director                

By: /S/ HELEN ALVAREZ                
Name: Helen Alvarez                    
Title: Managing Director                

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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HSBC BANK USA, NATIONAL ASSOCIATION

By: /S/ ANDREW W HIETALA            
Name: Andrew W Hietala
Title: Senior Vice President

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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MUFG UNION BANK, N.A.
By: /S/ RYAN PARKER                
Name: Ryan Parker
Title: Vice President

UNION BANK, Canada Branch
By: /S/ ANNE COLLINS                
Name: Anne Collins
Title: Director

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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SUMITOMO MITSUI BANKING CORPORATION 

By: /S/ JAMES D. WEINSTEIN                
Name: James D. Weinstein
Title: Managing Director

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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THE BANK OF NOVA SCOTIA 

By: /S/ MICHAEL GRAD                
Name: Michael Grad
Title: Director

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page

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THE NORTHERN TRUST COMPANY 

By: /S/ MOLLY DRENNAN                
Name: Molly Drennan                    
Title: Senior Vice President                

Tetra Tech, Inc.
Amendment No. 5 to Credit Agreement
Signature Page